Pleistocene Basaltic Volcanism on the Fly Platform in Papua New Guinea

Abstract

Papua New Guinea (PNG) remains one of the least studied regions in the Southwest Pacific, and the tectono-magmatic evolution of extensive areas such as the Fly Platform remains poorly constrained. This study presents the first investigation of basaltic volcanism on the Fly Platform based on the discovery of two previously unrecognized volcanic centres, Meahill and Yemsigi, located approximately 13 km apart along a northeast–southwest alignment. Integrated field observations and petrographic studies reveal contrasting eruptive and emplacement histories. Yemsigi is characterized by coherent porphyritic basalts, whereas Meahill comprises highly vesicular porphyritic to micro-porphyritic basalts indicative of explosive volcanism. Mineral chemistry and whole-rock geochemistry indicate that both basalt suites were derived from a common magma source. The K–Ar age of 0.57 ± 0.09 Ma for the Meahill basalts suggests that volcanism on the Fly Platform represents a late-stage magmatic event that post-dates the Maramuni Arc and Central Highlands magmatism on the PNG mainland. These findings provide the first evidence for Middle Pleistocene volcanism on the Fly Platform and place new constraints on the tectono-magmatic evolution of southern PNG.

1. Introduction

Papua New Guinea (PNG) occupies one of the most tectonically dynamic regions on Earth, situated at a complex triple junction involving the Australian continental plate, the Pacific oceanic plate, and the Caroline oceanic sub-plate. Convergence between the northeastward-moving Australian Plate (~63 mm/yr) and the west-northwestward-moving Pacific Plate (~67 mm/yr) produces exceptionally high convergence rates of ~10–11 cm/yr (Holm et al., 2016) [1], driving rapid crustal deformation, uplift, and magmatism along the New Guinea Orogenic Belt. This active plate boundary system has made PNG a natural laboratory for understanding plate interactions, arc magmatism, basin evolution, and continental growth in the southwest Pacific (e.g., Weissel & Watts, 1979; Abbott, et al., 1994; Hill & Hall, 2002; Hall, 2002; Crowhurst et al., 2005; Baldwin et al., 2012; Bortford et al., 2012; Webb, et al., 2014; Holm et al., 2015; Holm et al., 2016; Bulois et al., 2018; Holm et al., 2019; Holm et al., 2020; Ponyalou et al., 2023; Brandl et al., 2024) (e.g., [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]).

Despite extensive regional investigations, the tectonic and magmatic evolution of the Fly Platform in southwestern PNG remains poorly constrained. Existing interpretations are largely derived from regional syntheses (Dow, 1977; Williamson & Hancock, 2005; Sheppard & Cranfield, 2012; Davies, 2012) [16,17,18,19] and subsurface datasets generated through hydrocarbon and mineral exploration (e.g., Corbett & Leach, 1997; Callot et al., 2017; Conolly et al., 2020) [20,21,22]. Direct field-based petrological, geochemical, and geochronological data from the Fly Platform remain scarce. Although Pleistocene basaltic volcanism has previously been recognized (Wilmott, 1973) [23], our recent investigations confirmed basaltic eruptive activity on Daru Island (Lunge et al., 2026) [24].

This study examines two basaltic exposures on the Fly Platform that were discovered approximately 37 and 47 km, respectively, north to northeast of Daru Island. The objective is to document these occurrences and evaluate their petrogenesis through field mapping, sampling, petrography, mineral chemistry, whole-rock geochemistry and potassium-argon (K-Ar) dating.

2. Geology of the Fly Platform

The Fly Platform forms the southwestern portion of the Late Mesozoic–Cenozoic Papuan Basin, one of the most extensive sedimentary basins in New Guinea, occupying the northern margin of the Australian continental crust (Sheppard and Cranfield, 2012; Davies, 2012) [18,19]. The northern boundary of the basin is defined by the Papuan Fold and Thrust Belt. The Cross-section A-A′ on Figure 1 shows the tectonic architecture and crustal configuration extending from the Fly Platform across the Fold and Thrust Belt to the northern coast.

The evolution of the Papuan Basin is closely linked to the breakup of Gondwanaland and the subsequent northward drift of the Australian Plate (Hill & Hall, 2002; Hall, 2002; Crowhurst, et al., 2005; Holm, 2016) [1,4,5,6]. Australia began rifting away from Gondwanaland during the Late Jurassic to Early Cretaceous (ca. 160–130 Ma), initiating the development of passive continental margins along northern Australia. By the Eocene (~45 Ma), Australia had completely separated from Antarctica, and its northern continental margin experienced significant subsidence, forming a broad sedimentary basin. Within this basin, Jurassic to Miocene sandstones, siltstones, shales, and carbonates accumulated under marine conditions. Continued northward drift of the Australian Plate led to collision with the Pacific Plate during the Miocene, generating compressional stresses that uplifted and deformed sedimentary sequences north of the Fly Platform foreland, resulting in the development of the Papuan Fold and Thrust Belt.

Numerous studies recognize two principal magmatic arc events, the Miocene Maramuni Arc and the Pliocene–Quaternary arc, that contributed to the formation of major ore deposits along the New Guinea Orogenic Belt. This belt, also referred to as the New Guinea Mobile Belt, covers both the Papuan Fold and Thrust Belt and the New Guinea Thrust Belt (Davies, 2012; Holm et al., 2015; Holm et al., 2016) [1,10,19]. Relics of these magmatic events are preserved within the Fly-Highlands region along the Papuan Fold and Thrust Belt, which hosts a chain of dormant Pleistocene volcanoes, including Mount Bosavi, Mount Murray, and Mount Hagen (Johnson et al., 1978; MacKenzie & Johnson, 1984; Hamilton et al., 1983) [25,26,27].

In contrast, the Fly Platform has long been interpreted as a relatively stable foreland region, in comparison to the tectonically active Papuan Fold and Thrust Belt to the north (Sheppard & Cranfield, 2012; Davies, 2012) [18,19]. On the mainland, most rock units remain concealed beneath thick sedimentary and alluvial cover and are primarily known from subsurface data acquired through petroleum exploration drilling.

Pleistocene basaltic volcanism was reported on Daru Island (Willmott, 1972) [23] and our recent studies also confirmed evidence for basaltic volcanism and carbonate magma mingling leading to a phreatomagmatic eruption (Lunge et al., 2026) [24]. Our field reconnaissance survey to the north of Daru Island on the mainland of Fly Platform led to the discovery of two isolated basaltic exposures at Yemsigi and Meahill, suggesting that the region was not tectonically and magmatically inactive.

3. Mapping and Sampling

The basalt exposures are located at Meahill and Yemsigi on the mainland, west of the Fly River Delta, and are separated by approximately 13 km along a northeastsouthwest alignment (Figure 2). Yemsigi lies ~37 km northwest of Daru Island, whereas Meahill is situated ~47 km northeast of Daru Island. Both localities were accessed by motorboat from Daru Island to the Tabio and Mur villages, followed by overland travel on foot.

The areas are generally flat lying, with elevations reaching up to ~50 m in the central part. Access from Tabio to Meahill is through low-lying marshland and swampy ground, whereas the route from Mur follows an old logging track that is now largely overgrown with vegetation.

At Yemsigi, the unit is preserved as large, dense, dark subrounded basalts along the Kiwikopa Creek (see Figure 3a). The area is strongly oxidized with weathered basalt clasts weathered to reddish-brown hematitic surfaces (Figure 3c). This unit displays a porphyritic texture and contains euhedral to subhedral, lenticular-shaped olivine phenocrysts set within a fine- to medium-grained groundmass composed predominantly of clinopyroxene, plagioclase, and opaque minerals (Figure 3e).

At Meahill the main basaltic exposure is along the Aleale Creek (143.273402° E, 8.652976° S, see Figure 3b). Basaltic rocks collected along Aleale Creek are typically dark grey to black on fresh surfaces and range in texture from micro-porphyritic to porphyritic and highly vesicular (see Figure 3f). Phenocrysts are predominantly olivine and clinopyroxene, occurring as pale green to black, euhedral to subhedral crystals. The groundmass is microcrystalline and forms a dense interstitial matrix composed of plagioclase, clinopyroxene, and opaque minerals. Vesicles are abundant and variable in size.

One exposure (TM2; 143.355556° E, −8.633557° S; see Figure 3d) is a 4-m-high section of light brown, welded, fine-grained tuff showing low-angle cross-stratification wit pebble- to cobble-sized dark basaltic clasts. A second exposure (TM3; 143.355556° E, −8.633557° S) of basaltic clasts remain well preserved in clay altered matrix.

4. Analytical Methods

4.1. Petrography

The four least altered samples from the Meahill (TM9 and TM13) and Yemsigi (MY4 and MY6) were prepared for petrography and mineralogical study. The preparation process involved cutting the rock samples and mounting them onto glass slides, followed by grinding and polishing to a standard thickness of 30 µm. Petrographic analysis was conducted using a Nikon Eclipse LV100 POL microscope at Earth Resource Science Department of Akita University, under both plane-polarized light (PPL) and cross-polarized light (XPL) conditions to document mineral assemblages, textural relationships, and alteration features.

Mineral identification and verification were complemented by Scanning Electron Microscopy (SEM) (Akita University) using a JEOL JSM-IT300LV operated at 15 kV and 2.2 nA. For SEM analysis, the polished thin sections were carbon-coated to ensure electrical conductivity and improve imaging contrast. Backscattered Electron (BSE) images were obtained to enhance mineral phase distinction.

Vesicle abundance and size distribution were estimated directly from the thin sections. Vesicle frequency was determined as the proportion of vesicle cross-sections relative to total field area, employing point-count and digital image analysis methods. Individual vesicle diameters were measured from the calibrated microscope scale.

4.2. Mineral Chemistry

The major element compositions of olivine, pyroxene, feldspar and titatanomagnetite were analyzed by a JEOL JXA-8230 Electron Probe Micro-Analyzer (EPMA) at Akita University, with a 10 μm defocused beam at 15 kV and 20 nA, with peak counting times of 10 to 20 s. Analytical accuracy and precision were monitored using internal standards [e.g., albite (Na), wollastonite (Ca), rutile (Ti), hematite (Fe), nickel oxide (Ni), and forsterite (Mg)]. Elements included SiO₂, TiO₂, Al₂O₃, Cr₂O₃, FeO, MnO, MgO, NiO, CaO, Na₂O, and K₂O. The mineral compositions were calculated using MagMin_PT: an excel-based mineral classification and geothermobarometry program for magmatic rocks (Gündüz & Kürşad, 2023) [28].

4.3. Whole-Rock Geochemistry

The twelve least altered rock samples from Meahill (n = 6) and Yemsigi (n = 6) were pulverized with a tungsten carbide mill, and a 3 g aliquot was used to determine Loss on Ignition (LOI) by sequential heating to 900 °C. The whole-rock major elements were measured using a ZSX Primus II XRF spectrometer (Rigaku Co., Akishima, Japan) at Akita University. The trace elements and Rare Earth Element (REE) concentrations were determined by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) at Akita University. A 0.3 g portion of each sample was digested in a three-acid mixture (HClO₄, HNO₃, HF) and heated at 135 °C for 1–2 days. The solutions were then diluted, acidified, filtered, and stored. Quality control includes blanks, duplicates, and certified reference materials (JG-2). Analytical accuracy and precision were monitored through repeated analysis of internal standards with uncertainty generally better than ±1%–2%.

4.4. Potassium–Argon (K–Ar) Age Dating

Due to budgetary constraints, K–Ar geochronological analysis was conducted on a less altered basalt sample from Meahill (TM5). The K–Ar geochronology was performed at the Hiruzen Geochronology Research Institute Co., Ltd., in Okayama, Japan. A sample weighing 0.95 g was sent to the laboratory where feldspar grain sizes ranging between 51 and 73 µm were extracted and analyzed using the K–Ar geochronology method. The feldspars contained a potassium content of 1.654 wt% ± 0.033, providing a suitable basis for radiometric dating. The potassium content was measured using flame photometry or an atomic absorption spectrophotometer. A standard reference material was analyzed alongside the sample to ensure accuracy. The results were then corrected for any potential interferences or systematic errors. The sample was heated in a vacuum system to release argon gas. The extracted gas was then purified to remove contaminants such as atmospheric argon or other interfering gases. The amount of radiogenic Ar was determined using a mass spectrometer (Nagao, Nishito, et al., 1984; Itaya, et al., 1991; Yagi, Okada, et al., 2015) [29,30,31].

5. Results

Table 1. Mineral model % in thin sections (microliths, microphenocrysts, phenocrysts).

Mineral	Micro-Porphyritic Basalt (TM9)	Porphyritic Basalt (TM13)	Porphyritic Basalt (MY4)	Porphyritic Basalt (MY6)
Microliths (<100 µm)–up to 60%
Pl	30	30	30	30
Op	15	15	15	15
Oli	5	5	5	5
Cpx	10	10	10	10
Subtotal	60	60	60	60
Microphenocrysts (100–500 µm)–up to 20%
Pl	5	5	5	5
Op	2	2	2	2
Oli	8	8	8	8
Cpx	5	5	5	5
Subtotal	20	20	20	20
Phenocrysts (>500 µm)–up to 20%
Oli	15	15	15	15
Cpx	5	5	5	5
Subtotal	20	20	20	20
Total Model %	100	100	100	100
Other Components
Alteration minerals
Cb	pervasive > 5%	pervasive > 5%	minor < 1%	minor < 1%
Idd	–	–	pervasive > 5%	pervasive > 5%
Vesicles in Groundmass	High (>5%)	High (>5%)	Low (<2.5%)	Low (<2.5%)

Mineral abbreviations: Primary Minerals: Cpx (clinopyroxene), Oli (olivine), Op (opaque minerals), Pl (plagioclase); Seceondary Alteration minerals: Cb (carbonates), Idd (iddingsite).

summarizes the petrographic characteristics of the basalts from Meahill and Yemsigi. Photomicrographs are presented in Figure 4, with BSE images shown in Appendix A (Figure A1 and Figure A2).

5.1. Petrography

Yemsigi and Meahill basalts have similar mineralogical compositions dominated by olivine + clinopyroxene + plagioclase ± titanomagnetite. However, differ particularly in texture and vesicularity.

Yemsigi Basalts (MY4 and MY6)

Yemsigi samples display porphyritic textures characterized by olivine phenocrysts enclosed within a fine-grained microlitic groundmass of plagioclase, clinopyroxene, and titanomagnetite. Olivine is dominant, occurring as subhedral to anhedral grains (typically 600–1500 μm in MY4 and 300–700 μm in MY6). Clinopyroxene is smaller (300–1000 μm), while plagioclase appears as microphenocrysts. The groundmass is fine-grained and dense consisting of plagioclase laths (30–80 μm) intergrown with clinopyroxene (3–60 μm) and minor opaque oxides (<30 μm). Vesicles are sparse (150–600 μm), occasionally featuring microlithic rims or thin oxide linings. Iddingsite alteration is a common feature in olivine and is more prominent in MY6, where alteration halos and reddish-brown zones are pervasive. Titanomagnetite is euhedral to anhedral and occurs interstitially with silicate minerals.

Meahill Basalts (TM9 and TM13)

Meahill samples range from micro-porphyritic (TM9) to porphyritic (TM13) textures and are notably more vesicular than Yemsigi samples. The samples contain olivine and clinopyroxene phenocrysts. TM9 comprises smaller, subhedral olivine crystals (120–700 μm), while TM13 exhibits larger, euhedral to subhedral olivine (800–1200 μm). Clinopyroxene ranges from 150 to 900 μm, and plagioclase occurs as micro-phenocrysts and microlithic laths. The groundmass is intergranular to microlitic, predominantly of plagioclase (<80 μm) and interstitial clinopyroxene (<50 μm), with opaque oxides. The TM13 sample is densely packed with plagioclase microliths surrounding the larger phenocrysts. Vesicles are abundant, measuring 400–700 μm, and are often elongated and irregular, lined or bordered by microlithic plagioclase and opaque minerals. Carbonates occur as secondary alteration, particularly along the fracture surfaces and on vesicle walls. Titanomagnetite occurs irregularly forming aggregates intergrown with the silicates.

5.2. Mineral Chemistry

The chemical compositions of olivine, clinopyroxene, plagioclase, and titanomagnetite are presented in the Appendix A (

Table A1. EPMA olivine chemistry results for the Yemsigi basalt sample (MY4).

(a) Yemsigi Basalt (MY4)
wt%	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20
SiO2	37.92	41.44	41.04	39.78	39.74	39.70	40.64	40.00	40.48	38.19	39.84	37.62	39.04	39.07	39.52	40.04	38.58	39.45	39.48	38.67
TiO2	0.00	0.06	0.00	0.03	0.00	0.00	0.03	0.05	0.03	0.00	0.05	0.03	0.11	0.00	0.00	0.00	0.03	0.01	0.03	0.00
Al2O3	0.00	0.19	0.18	0.00	0.00	0.19	0.00	0.00	0.00	0.01	0.09	0.15	0.04	0.11	0.00	0.12	0.00	0.03	0.12	0.07
Cr2O3
FeO	25.38	16.83	16.58	16.09	15.90	15.83	15.37	16.01	16.54	23.53	18.00	21.84	22.26	18.03	16.55	17.51	21.76	17.26	17.43	16.67
MnO	0.27	0.13	0.21	0.18	0.17	0.24	0.16	0.17	0.18	0.30	0.23	0.35	0.33	0.18	0.22	0.24	0.31	0.25	0.23	0.21
MgO	36.07	40.81	41.67	43.70	43.88	43.74	43.58	43.42	42.60	37.64	41.51	39.17	37.90	42.20	43.47	41.69	38.97	42.65	42.39	44.05
NiO
CaO	0.26	0.35	0.25	0.14	0.22	0.21	0.19	0.34	0.16	0.25	0.25	0.66	0.20	0.27	0.22	0.26	0.23	0.20	0.22	0.31
Na2O	0.05	0.01	0.02	0.00	0.05	0.03	0.00	0.00	0.00	0.03	0.02	0.06	0.05	0.00	0.00	0.04	0.07	0.00	0.06	0.00
K2O	0.05	0.01	0.03	0.00	0.04	0.03	0.00	0.00	0.00	0.03	0.02	0.06	0.05	0.00	0.00	0.04	0.07	0.00	0.06	0.00
P2O5	0.04	0.06	0.00	0.02	0.03	0.02	0.02	0.00	0.00	0.04	0.01	0.07	0.00	0.06	0.00	0.06	0.05	0.01	0.09	0.04
SO3	0.01	0.08	0.03	0.03	0.01	0.03	0.00	0.00	0.01	0.00	0.01	0.02	0.02	0.00	0.00	0.04	0.03	0.03	0.00	0.00
F	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Cl	0.00	0.04	0.03	0.02	0.01	0.00	0.01	0.00	0.00	0.00	0.00	0.02	0.02	0.02	0.02	0.00	0.00	0.03	0.00	0.00
Total	100.05	99.99	100.03	99.98	100.04	100.03	100.00	99.99	100.00	100.02	100.02	100.04	100.01	99.94	100.00	100.04	100.11	99.92	100.11	100.02
Si	1.00	1.04	1.03	1.00	1.00	1.00	1.02	1.01	1.02	1.00	1.01	0.98	1.01	1.00	1.00	1.02	1.00	1.00	1.00	0.98
Ti	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Al	0.00	0.01	0.01	0.00	0.00	0.01	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Cr	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Fe2+	0.56	0.35	0.35	0.34	0.34	0.33	0.32	0.34	0.35	0.52	0.38	0.48	0.48	0.38	0.35	0.37	0.47	0.37	0.37	0.35
Mn	0.01	0.00	0.00	0.00	0.00	0.01	0.00	0.00	0.00	0.01	0.00	0.01	0.01	0.00	0.00	0.01	0.01	0.01	0.00	0.00
Mg	1.42	1.53	1.56	1.64	1.65	1.64	1.63	1.63	1.60	1.47	1.57	1.52	1.47	1.61	1.64	1.58	1.51	1.62	1.60	1.67
Ca	0.01	0.01	0.01	0.00	0.01	0.01	0.01	0.01	0.00	0.01	0.01	0.02	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01
Na	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
K	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Ni	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
TOTAL	3.00	2.95	2.96	3.00	3.00	3.00	2.98	2.99	2.98	3.00	2.99	3.02	2.99	3.00	3.00	2.98	3.00	3.00	3.00	3.02
Fo	71.49	81.10	81.57	82.73	82.96	82.91	83.33	82.71	81.95	73.80	80.24	75.89	74.94	80.51	82.21	80.72	75.89	81.28	81.06	82.31
Fa	28.21	18.76	18.20	17.08	16.86	16.83	16.49	17.11	17.85	25.87	19.51	23.73	24.69	19.30	17.56	19.01	23.77	18.45	18.70	17.47
Tp	0.30	0.15	0.23	0.19	0.18	0.26	0.18	0.19	0.20	0.33	0.25	0.38	0.37	0.20	0.23	0.27	0.35	0.27	0.24	0.22
XFeO(l)/XMgO(l)	1.32	0.77	0.74	0.69	0.68	0.68	0.66	0.69	0.73	1.17	0.81	1.04	1.10	0.80	0.71	0.79	1.04	0.76	0.77	0.71
(b) Yemsigi Basalt (MY4)
wt%			21	22	23	24	25	26	27	28	29	30	34	35	36	37	38	39	40	41
SiO2			39.32	39.82	37.45	39.59	38.27	38.23	37.06	38.83	37.61	39.84	37.40	37.00	40.00	39.07	39.60	39.74	39.74	39.34
TiO2			0.00	0.02	0.00	0.00	0.00	0.01	0.06	0.02	0.03	0.01	0.14	0.04	0.10	0.00	0.00	0.00	0.00	0.01
Al2O3			0.00	0.00	0.00	0.00	0.02	0.00	0.00	0.00	0.00	0.07	0.04	0.10	0.04	0.00	0.20	0.00	0.17	0.00
Cr2O3
FeO			17.41	17.03	18.89	17.58	22.47	23.78	23.05	20.10	26.72	17.23	26.49	28.76	18.07	19.71	16.23	16.80	16.82	18.36
MnO			0.29	0.22	0.24	0.15	0.19	0.24	0.26	0.23	0.34	0.21	0.29	0.36	0.22	0.20	0.29	0.25	0.14	0.24
MgO			42.67	42.54	43.08	42.32	38.67	37.43	39.25	40.45	34.83	42.35	35.24	33.25	41.23	40.61	43.32	42.96	42.85	41.79
NiO
CaO			0.27	0.27	0.28	0.23	0.29	0.25	0.32	0.29	0.30	0.16	0.29	0.35	0.28	0.31	0.18	0.19	0.16	0.25
Na2O			0.01	0.00	0.00	0.10	0.01	0.00	0.00	0.00	0.00	0.03	0.00	0.02	0.01	0.03	0.06	0.00	0.05	0.00
K2O			0.01	0.00	0.00	0.10	0.01	0.00	0.00	0.00	0.00	0.03	0.00	0.02	0.01	0.04	0.05	0.01	0.00	0.00
P2O5			0.02	0.01	0.06	0.05	0.03	0.06	0.06	0.01	0.07	0.17	0.02	0.04	0.10	0.04	0.06	0.10	0.04	0.05
SO3			0.00	0.03	0.00	0.00	0.01	0.01	0.00	0.00	0.02	0.03	0.05	0.00	0.01	0.00	0.02	0.01	0.03	0.02
F			0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Cl			0.00	0.00	0.02	0.01	0.00	0.00	0.00	0.01	0.00	0.01	0.02	0.00	0.00	0.00	0.01	0.01	0.01	0.00
Total			100.01	99.94	100.01	100.12	99.96	100.00	100.05	99.93	99.91	100.14	99.98	99.95	100.07	100.01	100.01	100.06	100.00	100.05
Si			1.00	1.01	0.96	1.01	1.00	1.00	0.97	1.00	1.00	1.01	1.00	1.00	1.02	1.00	1.00	1.01	1.01	1.00
Ti			0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Al			0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.00	0.01	0.00
Cr			0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Fe2+			0.37	0.36	0.41	0.37	0.49	0.52	0.51	0.43	0.60	0.37	0.59	0.65	0.38	0.42	0.34	0.36	0.36	0.39
Mn			0.01	0.00	0.01	0.00	0.00	0.01	0.01	0.01	0.01	0.00	0.01	0.01	0.00	0.00	0.01	0.01	0.00	0.01
Mg			1.62	1.61	1.65	1.60	1.50	1.46	1.53	1.55	1.38	1.60	1.40	1.33	1.56	1.56	1.63	1.62	1.62	1.59
Ca			0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.00	0.01	0.01	0.01	0.01	0.00	0.01	0.00	0.01
Na			0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
K			0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Ni			0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
TOTAL			3.00	2.99	3.04	3.00	3.00	3.00	3.03	3.00	3.00	2.99	3.00	3.00	2.98	3.00	3.00	2.99	2.99	3.00
Fo			81.12	81.47	80.06	80.97	75.26	73.53	75.01	78.00	69.65	81.23	70.11	67.06	80.07	78.43	82.38	81.79	81.83	80.02
Fa			18.57	18.29	19.69	18.87	24.53	26.20	24.71	21.74	29.97	18.54	29.56	32.53	19.69	21.35	17.31	17.94	18.02	19.72
Tp			0.31	0.24	0.26	0.17	0.20	0.27	0.28	0.25	0.38	0.23	0.33	0.42	0.24	0.22	0.31	0.27	0.15	0.26
XFeO(l)/XMgO(l)			0.76	0.75	0.82	0.78	1.09	1.19	1.10	0.93	1.43	0.76	1.41	1.62	0.82	0.91	0.70	0.73	0.73	0.82

,

Table A2. EPMA olivine chemistry results for the Yemsigi basalt sample (MY6).

(a) Yemsigi Basalt (MY6)
wt%		1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20
SiO2		38.85	39.07	38.77	38.51	39.20	37.54	38.76	38.34	37.82	38.84	37.10	38.88	38.40	39.22	38.54	39.14	38.64	38.71	38.49	39.24
TiO2		0.00	0.06	0.00	0.02	0.00	0.02	0.05	0.00	0.06	0.04	0.01	0.02	0.05	0.00	0.00	0.00	0.01	0.00	0.03	0.03
Al2O3		0.08	0.04	0.05	0.02	0.05	0.05	0.00	0.05	0.03	0.05	0.03	0.06	0.04	0.00	0.00	0.01	0.07	0.04	0.00	0.05
Cr2O3		0.06	0.06	0.03	0.06	0.01	0.02	0.02	0.03	0.02	0.04	0.02	0.04	0.07	0.03	0.02	0.05	0.09	0.08	0.03	0.06
FeO		15.90	15.61	17.56	16.93	13.73	16.62	16.97	18.70	21.39	16.55	16.44	16.74	16.81	15.39	15.65	16.48	16.96	17.12	20.45	19.85
MnO		0.17	0.18	0.19	0.27	0.15	0.17	0.23	0.29	0.31	0.20	0.19	0.20	0.20	0.19	0.21	0.14	0.21	0.22	0.30	0.23
MgO		45.00	45.22	43.90	43.94	46.75	42.93	44.47	42.71	41.03	44.58	43.13	44.39	44.30	45.40	45.70	44.99	44.61	44.15	41.86	41.15
NiO		0.19	0.32	0.19	0.26	0.29	0.18	0.22	0.11	0.17	0.25	0.18	0.18	0.22	0.33	0.29	0.26	0.20	0.20	0.08	0.13
CaO		0.18	0.22	0.27	0.20	0.06	0.20	0.25	0.29	0.29	0.20	0.21	0.24	0.22	0.16	0.07	0.18	0.22	0.23	0.27	0.36
Na2O		0.00	0.03	0.01	0.02	0.00	0.01	0.00	0.03	0.00	0.01	0.00	0.00	0.00	0.00	0.03	0.01	0.01	0.00	0.04	0.00
K2O		0.00	0.01	0.00	0.00	0.01	0.01	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.01	0.00	0.02	0.00	0.01
P2O5		0.04	0.04	0.05	0.03	0.04	0.00	0.04	0.10	0.07	0.00	0.00	0.00	0.02	0.00	0.00	0.00	0.04	0.02	0.04	0.02
Total		100.46	100.85	101.02	100.28	100.29	97.74	101.02	100.64	101.18	100.77	97.30	100.75	100.34	100.72	100.51	101.27	101.05	100.78	101.58	101.12
Si		0.98	0.98	0.98	0.98	0.98	0.98	0.98	0.98	0.97	0.98	0.97	0.98	0.97	0.98	0.97	0.98	0.97	0.98	0.98	1.00
Ti		0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Al		0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Cr		0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Fe2+		0.34	0.33	0.37	0.36	0.29	0.36	0.36	0.40	0.46	0.35	0.36	0.35	0.36	0.32	0.33	0.35	0.36	0.36	0.43	0.42
Mn		0.00	0.00	0.00	0.01	0.00	0.00	0.00	0.01	0.01	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.00
Mg		1.69	1.69	1.65	1.66	1.74	1.67	1.67	1.62	1.57	1.67	1.68	1.67	1.67	1.70	1.72	1.68	1.68	1.66	1.59	1.56
Ca		0.00	0.01	0.01	0.01	0.00	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.00	0.00	0.00	0.01	0.01	0.01	0.01
Na		0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
K		0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Ni		0.00	0.01	0.00	0.01	0.01	0.00	0.00	0.00	0.00	0.01	0.00	0.00	0.00	0.01	0.01	0.01	0.00	0.00	0.00	0.00
TOTAL		3.02	3.01	3.02	3.02	3.01	3.02	3.02	3.02	3.02	3.01	3.02	3.01	3.02	3.01	3.02	3.01	3.02	3.02	3.02	3.00
Fo		83.31	83.62	81.52	81.99	85.73	82.00	82.17	80.03	77.12	82.59	82.22	82.37	82.27	83.86	83.71	82.83	82.25	81.94	78.25	78.51
Fa		16.52	16.19	18.29	17.72	14.12	17.81	17.59	19.66	22.55	17.20	17.57	17.42	17.51	15.94	16.07	17.02	17.54	17.82	21.44	21.24
Tp		0.17	0.19	0.20	0.29	0.16	0.18	0.25	0.31	0.34	0.21	0.21	0.21	0.21	0.20	0.22	0.15	0.22	0.23	0.31	0.25
XFeO(l)/XMgO(l)		0.66	0.65	0.75	0.72	0.55	0.72	0.71	0.82	0.97	0.69	0.71	0.70	0.71	0.63	0.64	0.68	0.71	0.73	0.91	0.90
(b) Yemsigi Basalt (MY6)
wt%	21	22	23	24	25	26	27	28	29	30	31	32	33	34	35	36	37	38	39	40	41
SiO2	40.33	39.51	40.20	39.38	39.99	40.06	40.02	39.79	40.11	39.75	39.50	39.86	40.00	40.08	40.06	39.91	40.13	40.05	39.14	38.98	39.68
TiO2	0.06	0.02	0.03	0.03	0.06	0.04	0.04	0.00	0.00	0.05	0.03	0.04	0.03	0.00	0.06	0.03	0.03	0.01	0.05	0.04	0.04
Al2O3	0.00	0.04	0.03	0.06	0.02	0.04	0.05	0.03	0.03	0.04	0.01	0.01	0.05	0.01	0.06	0.04	0.06	0.00	0.05	0.03	0.00
Cr2O3	0.05	0.00	0.00	0.00	0.02	0.03	0.01	0.03	0.00	0.01	0.01	0.00	0.03	0.03	0.04	0.01	0.01	0.06	0.03	0.03	0.02
FeO	16.09	15.90	16.04	15.69	15.97	15.98	16.12	16.74	16.36	16.41	16.86	17.10	16.10	16.44	16.02	15.79	15.95	16.06	15.83	16.17	17.52
MnO	0.22	0.24	0.25	0.22	0.25	0.24	0.19	0.27	0.27	0.24	0.22	0.18	0.26	0.24	0.25	0.22	0.24	0.26	0.24	0.22	0.25
MgO	45.12	43.33	45.14	45.36	44.77	44.81	44.59	44.17	44.36	44.67	44.13	43.86	44.51	45.84	44.69	44.48	44.54	44.58	46.00	44.64	42.97
NiO	0.19	0.14	0.17	0.15	0.17	0.18	0.20	0.21	0.22	0.22	0.18	0.14	0.16	0.19	0.19	0.19	0.16	0.19	0.20	0.15	0.19
CaO	0.32	0.32	0.31	0.31	0.32	0.34	0.32	0.34	0.35	0.33	0.33	0.32	0.36	0.33	0.33	0.33	0.32	0.33	0.32	0.33	0.34
Na2O	0.02	0.03	0.00	0.03	0.03	0.04	0.04	0.05	0.00	0.00	0.03	0.00	0.00	0.03	0.00	0.02	0.00	0.00	0.01	0.03	0.00
K2O	0.00	0.00	0.00	0.01	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.00	0.01	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
P2O5	0.01	0.02	0.01	0.04	0.01	0.02	0.03	0.02	0.00	0.02	0.00	0.02	0.01	0.03	0.05	0.00	0.00	0.01	0.03	0.02	0.04
Total	102.42	99.55	102.18	101.27	101.60	101.78	101.61	101.65	101.69	101.73	101.31	101.54	101.51	103.21	101.74	101.00	101.43	101.55	101.87	100.63	101.05
Si	0.99	1.00	0.99	0.98	0.99	0.99	1.00	0.99	1.00	0.99	0.99	1.00	1.00	0.98	0.99	1.00	1.00	1.00	0.97	0.98	1.00
Ti	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Al	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Cr	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Fe2+	0.33	0.34	0.33	0.33	0.33	0.33	0.34	0.35	0.34	0.34	0.35	0.36	0.34	0.34	0.33	0.33	0.33	0.33	0.33	0.34	0.37
Mn	0.00	0.01	0.01	0.00	0.01	0.01	0.00	0.01	0.01	0.01	0.00	0.00	0.01	0.01	0.01	0.00	0.01	0.01	0.01	0.00	0.01
Mg	1.66	1.64	1.66	1.69	1.66	1.66	1.65	1.64	1.64	1.66	1.65	1.63	1.65	1.68	1.65	1.66	1.65	1.65	1.70	1.68	1.61
Ca	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01
Na	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
K	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Ni	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
TOTAL	3.00	2.99	3.00	3.01	3.00	3.00	3.00	3.00	3.00	3.00	3.01	3.00	3.00	3.01	3.00	3.00	3.00	3.00	3.02	3.01	3.00
Fo	83.14	82.72	83.16	83.56	83.11	83.12	82.98	82.24	82.63	82.71	82.16	81.90	82.90	83.04	83.05	83.21	83.06	82.97	83.61	82.93	81.17
Fa	16.63	17.02	16.58	16.21	16.62	16.62	16.82	17.48	17.09	17.04	17.61	17.91	16.82	16.71	16.69	16.57	16.68	16.76	16.14	16.85	18.56
Tp	0.23	0.26	0.26	0.23	0.27	0.25	0.20	0.28	0.28	0.26	0.24	0.19	0.28	0.25	0.26	0.23	0.26	0.27	0.25	0.23	0.27
XFeO(l)/XMgO(l)	0.67	0.69	0.66	0.65	0.67	0.67	0.68	0.71	0.69	0.69	0.71	0.73	0.68	0.67	0.67	0.66	0.67	0.67	0.64	0.68	0.76

,

Table A3. EPMA olivine chemistry results for the Meahill basalt sample (TM9).

(a) Meahill Basalts (TM9)
wt%	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20
SiO2	38.43	38.49	38.58	38.51	39.38	38.83	38.58	39.09	38.93	39.20	39.01	39.94	39.32	39.53	39.66	39.68	38.93	39.62	39.51	38.46
TiO2	0.00	0.02	0.02	0.00	0.06	0.03	0.00	0.05	0.02	0.05	0.02	0.04	0.07	0.02	0.00	0.02	0.03	0.03	0.02	0.09
Al2O3	0.00	0.05	0.04	0.02	0.32	0.07	0.03	0.04	0.06	0.01	0.03	0.09	0.02	0.00	0.04	0.00	0.02	0.00	0.01	0.04
Cr2O3	0.02	0.07	0.03	0.03	0.04	0.05	0.04	0.03	0.02	0.01	0.04	0.02	0.07	0.05	0.00	0.06	0.02	0.06	0.00	0.01
FeO	17.65	19.52	19.86	17.68	16.77	17.17	17.46	17.27	17.13	16.72	17.53	15.49	18.09	18.12	16.82	16.74	20.37	16.28	17.09	21.42
MnO	0.26	0.36	0.34	0.28	0.23	0.20	0.23	0.22	0.21	0.26	0.24	0.23	0.30	0.27	0.23	0.30	0.32	0.25	0.23	0.30
MgO	44.04	42.60	42.16	43.38	42.44	44.48	44.18	44.12	44.37	44.21	44.17	44.85	42.62	42.80	43.78	43.82	40.89	44.02	43.26	39.54
NiO	0.19	0.12	0.16	0.14	0.16	0.15	0.18	0.21	0.19	0.17	0.17	0.17	0.13	0.15	0.15	0.16	0.13	0.21	0.17	0.10
CaO	0.25	0.15	0.21	0.31	0.33	0.22	0.27	0.24	0.26	0.24	0.23	0.29	0.36	0.37	0.33	0.35	0.40	0.34	0.36	0.39
Na2O	0.01	0.00	0.01	0.05	0.04	0.00	0.00	0.04	0.05	0.04	0.00	0.01	0.00	0.00	0.00	0.03	0.02	0.02	0.03	0.04
K2O	0.01	0.00	0.01	0.00	0.00	0.02	0.00	0.00	0.00	0.02	0.01	0.00	0.00	0.00	0.00	0.01	0.00	0.00	0.00	0.00
P2O5	0.00	0.04	0.01	0.04	0.03	0.04	0.00	0.00	0.00	0.00	0.00	0.06	0.01	0.02	0.02	0.01	0.02	0.03	0.03	0.04
Total	100.86	101.41	101.42	100.43	99.78	101.27	100.97	101.29	101.24	100.94	101.45	101.17	100.97	101.34	101.03	101.17	101.14	100.86	100.69	100.42
Si	0.97	0.98	0.98	0.98	1.00	0.98	0.97	0.98	0.98	0.99	0.98	0.99	0.99	1.00	1.00	0.99	0.99	0.99	1.00	0.99
Ti	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Al	0.00	0.00	0.00	0.00	0.01	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Cr	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Fe2+	0.37	0.41	0.42	0.38	0.36	0.36	0.37	0.36	0.36	0.35	0.37	0.32	0.38	0.38	0.35	0.35	0.43	0.34	0.36	0.46
Mn	0.01	0.01	0.01	0.01	0.00	0.00	0.00	0.00	0.00	0.01	0.01	0.00	0.01	0.01	0.00	0.01	0.01	0.01	0.00	0.01
Mg	1.66	1.61	1.60	1.65	1.61	1.67	1.66	1.65	1.66	1.66	1.65	1.67	1.61	1.61	1.64	1.64	1.56	1.65	1.63	1.52
Ca	0.01	0.00	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01
Na	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
K	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Ni	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
TOTAL	3.02	3.02	3.02	3.02	2.99	3.02	3.02	3.01	3.02	3.01	3.02	3.00	3.00	3.00	3.00	3.00	3.00	3.00	3.00	3.00
Fo	81.43	79.25	78.81	81.15	81.66	82.03	81.66	81.81	82.02	82.27	81.59	83.57	80.52	80.57	82.07	82.09	77.89	82.60	81.66	76.44
Fa	18.30	20.37	20.83	18.55	18.10	17.76	18.10	17.96	17.76	17.45	18.16	16.19	19.17	19.14	17.68	17.59	21.76	17.14	18.10	23.23
Tp	0.28	0.38	0.37	0.30	0.25	0.21	0.24	0.23	0.22	0.27	0.25	0.24	0.32	0.29	0.25	0.32	0.35	0.27	0.24	0.32
XFeO(l)/XMgO(l)	0.75	0.86	0.88	0.76	0.74	0.72	0.74	0.73	0.72	0.71	0.74	0.65	0.79	0.79	0.72	0.71	0.93	0.69	0.74	1.01
(b) Meahill Basalts (TM9)
wt%	21	22	23	24	25	26	27	28	29	30	31	32	33	34	35	36	37	38	39	40
SiO2	39.92	39.57	39.76	39.64	40.05	40.14	40.08	39.72	39.77	40.04	40.06	40.00	39.61	39.57	39.40	37.02	37.06	37.57	37.29	38.06
TiO2	0.01	0.02	0.01	0.03	0.00	0.02	0.02	0.05	0.02	0.04	0.07	0.04	0.00	0.03	0.03	0.08	0.05	0.06	0.06	0.04
Al2O3	0.04	0.07	0.05	0.01	0.04	0.04	0.05	0.00	0.04	0.02	0.05	0.03	0.00	0.00	0.08	0.03	0.01	0.04	0.04	0.01
Cr2O3	0.11	0.03	0.00	0.00	0.06	0.05	0.00	0.00	0.00	0.06	0.04	0.03	0.02	0.00	0.00	0.00	0.00	0.04	0.00	0.00
FeO	16.06	15.33	16.46	17.70	15.35	15.40	15.95	17.54	15.66	15.91	16.15	17.20	17.99	18.64	19.72	31.79	30.76	27.50	28.69	25.74
MnO	0.30	0.26	0.23	0.27	0.23	0.23	0.27	0.25	0.22	0.24	0.24	0.25	0.28	0.25	0.27	0.43	0.48	0.41	0.51	0.40
MgO	44.33	44.28	43.94	43.03	44.75	44.85	44.44	42.74	44.52	44.76	44.38	43.28	42.91	42.10	41.32	31.09	32.16	33.83	32.84	35.85
NiO	0.22	0.18	0.16	0.09	0.18	0.17	0.18	0.14	0.19	0.28	0.19	0.18	0.16	0.12	0.10	0.07	0.05	0.04	0.00	0.07
CaO	0.29	0.30	0.36	0.36	0.29	0.29	0.32	0.37	0.32	0.31	0.33	0.32	0.35	0.36	0.36	0.44	0.42	0.58	0.54	0.53
Na2O	0.01	0.01	0.00	0.04	0.00	0.00	0.00	0.01	0.03	0.03	0.00	0.00	0.00	0.00	0.02	0.00	0.04	0.00	0.02	0.01
K2O	0.00	0.00	0.00	0.01	0.01	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
P2O5	0.01	0.01	0.01	0.06	0.03	0.03	0.02	0.01	0.01	0.03	0.00	0.04	0.02	0.05	0.02	0.05	0.04	0.05	0.05	0.00
Total	101.29	100.06	100.99	101.24	100.99	101.21	101.32	100.83	100.78	101.70	101.51	101.36	101.35	101.12	101.32	100.99	101.06	100.13	100.04	100.72
Si	1.00	1.00	1.00	1.00	1.00	1.00	1.00	1.00	1.00	0.99	1.00	1.00	1.00	1.00	1.00	1.00	1.00	1.00	1.00	1.00
Ti	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Al	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Cr	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Fe2+	0.34	0.32	0.34	0.37	0.32	0.32	0.33	0.37	0.33	0.33	0.34	0.36	0.38	0.39	0.42	0.72	0.69	0.61	0.65	0.57
Mn	0.01	0.01	0.00	0.01	0.00	0.00	0.01	0.01	0.00	0.00	0.00	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01
Mg	1.65	1.66	1.64	1.61	1.66	1.66	1.65	1.61	1.66	1.66	1.65	1.62	1.61	1.59	1.56	1.25	1.29	1.35	1.32	1.41
Ca	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.02	0.02	0.01
Na	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
K	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Ni	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
TOTAL	3.00	3.00	3.00	3.00	3.00	3.00	3.00	2.99	3.00	3.00	3.00	2.99	3.00	3.00	3.00	3.00	3.00	2.99	3.00	3.00
Fo	82.84	83.51	82.43	81.02	83.66	83.65	83.01	81.06	83.32	83.17	82.84	81.55	80.72	79.89	78.65	63.23	64.72	68.36	66.72	70.97
Fa	16.84	16.21	17.32	18.69	16.10	16.11	16.71	18.67	16.44	16.58	16.91	18.18	18.98	19.84	21.06	36.27	34.72	31.17	32.69	28.58
Tp	0.32	0.28	0.25	0.29	0.24	0.24	0.29	0.27	0.23	0.25	0.25	0.27	0.30	0.27	0.29	0.50	0.55	0.48	0.59	0.45
XFeO(l)/XMgO(l)	0.68	0.65	0.70	0.77	0.64	0.64	0.67	0.77	0.66	0.66	0.68	0.74	0.78	0.83	0.89	1.91	1.79	1.52	1.63	1.34

,

Table A4. EPMA olivine chemistry results for the Meahill basalt sample (TM13).

Meahill Basalts (TM13)
wt%	1	2	3	4	5	6	7	8	9	13	14	15	17	19	20	21	22	23	25	26
SiO2	37.46	37.72	37.27	38.62	37.74	37.24	38.23	38.53	38.21	37.28	37.74	37.45	36.26	37.96	38.30	37.67	37.58	37.47	38.14	37.85
TiO2	0.03	0.03	0.04	0.00	0.05	0.07	0.00	0.02	0.01	0.06	0.00	0.02	0.11	0.02	0.01	0.04	0.06	0.04	0.00	0.04
Al2O3	0.04	0.07	0.03	0.05	0.05	0.00	0.03	0.03	0.03	0.01	0.04	0.06	0.49	0.03	0.06	0.03	0.02	0.00	0.03	0.08
Cr2O3	0.01	0.00	0.00	0.06	0.00	0.00	0.04	0.00	0.03	0.01	0.02	0.01	0.03	0.02	0.04	0.03	0.00	0.00	0.01	0.05
FeO	23.37	22.81	25.21	18.67	23.20	26.59	20.83	18.94	21.13	25.31	21.95	23.54	25.47	20.08	19.38	23.86	24.42	24.40	22.85	22.71
MnO	0.32	0.29	0.34	0.24	0.28	0.35	0.26	0.20	0.30	0.33	0.26	0.30	0.37	0.27	0.21	0.35	0.31	0.32	0.32	0.24
MgO	39.47	39.81	37.85	43.03	39.68	36.56	41.38	42.65	41.76	37.71	40.07	39.08	36.09	41.97	42.58	39.11	38.84	38.44	39.50	40.35
NiO	0.15	0.15	0.11	0.18	0.13	0.11	0.17	0.15	0.11	0.12	0.16	0.15	0.03	0.19	0.21	0.10	0.11	0.11	0.12	0.24
CaO	0.27	0.28	0.32	0.22	0.27	0.30	0.20	0.22	0.25	0.34	0.19	0.23	0.81	0.18	0.17	0.30	0.27	0.30	0.24	0.22
Na2O	0.02	0.01	0.00	0.03	0.00	0.00	0.04	0.00	0.03	0.03	0.02	0.00	0.04	0.02	0.02	0.01	0.01	0.02	0.04	0.01
K2O	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.00	0.00	0.00	0.01	0.01	0.00	0.00	0.01	0.01	0.00	0.00
P2O5	0.01	0.02	0.01	0.00	0.02	0.00	0.00	0.05	0.03	0.07	0.03	0.06	0.05	0.01	0.00	0.00	0.07	0.00	0.03	0.04
Total	101.13	101.19	101.17	101.09	101.42	101.21	101.18	100.78	101.91	101.25	100.47	100.91	99.75	100.76	100.98	101.51	101.70	101.11	101.27	101.82
Si	0.97	0.98	0.98	0.98	0.98	0.98	0.98	0.98	0.97	0.98	0.98	0.98	0.97	0.97	0.98	0.98	0.97	0.98	0.98	0.97
Ti	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Al	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.02	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Cr	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Fe2+	0.51	0.49	0.55	0.40	0.50	0.59	0.45	0.40	0.45	0.55	0.48	0.51	0.57	0.43	0.41	0.52	0.53	0.53	0.49	0.49
Mn	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.00	0.01	0.01	0.01	0.01	0.01	0.01	0.00	0.01	0.01	0.01	0.01	0.01
Mg	1.53	1.53	1.48	1.63	1.53	1.43	1.58	1.62	1.59	1.47	1.55	1.52	1.44	1.60	1.62	1.51	1.50	1.49	1.52	1.54
Ca	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.02	0.00	0.00	0.01	0.01	0.01	0.01	0.01
Na	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
K	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Ni	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
TOTAL	3.02	3.02	3.02	3.02	3.02	3.02	3.02	3.02	3.02	3.02	3.02	3.02	3.02	3.02	3.02	3.02	3.02	3.02	3.01	3.02
Fo	74.82	75.45	72.54	80.22	75.08	70.75	77.76	79.90	77.65	72.39	76.28	74.50	71.35	78.62	79.49	74.22	73.68	73.48	75.25	75.81
Fa	24.84	24.25	27.10	19.53	24.62	28.87	21.96	19.89	22.04	27.25	23.44	25.17	28.24	21.10	20.29	25.40	25.99	26.17	24.41	23.94
Tp	0.34	0.31	0.37	0.25	0.31	0.38	0.28	0.21	0.31	0.36	0.28	0.32	0.41	0.28	0.22	0.38	0.33	0.35	0.34	0.26
XFeO(l)/XMgO(l)	1.11	1.07	1.25	0.81	1.09	1.36	0.94	0.83	0.95	1.25	1.02	1.13	1.32	0.89	0.85	1.14	1.18	1.19	1.08	1.05
Fe2+/(Fe2+ + Mg)	0.25	0.24	0.27	0.20	0.25	0.29	0.22	0.20	0.22	0.27	0.24	0.25	0.28	0.21	0.20	0.25	0.26	0.26	0.24	0.24
Mg/(Fe2+ + Mg)	0.75	0.76	0.73	0.80	0.75	0.71	0.78	0.80	0.78	0.73	0.76	0.75	0.72	0.79	0.80	0.75	0.74	0.74	0.76	0.76

,

Table A5. EPMA pyroxene chemistry results for the Yemsigi basalt sample (MY6).

(a) Yemsigi Basalt (MY6)—Pyroxene Chemistry
wt%		1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
	SiO2	55.79	54.08	50.50	49.90	58.31	49.70	51.43	51.27	51.63	49.09	49.02	51.38	48.34	51.03	49.46	48.67	51.65	46.61
	TiO2	1.47	0.94	1.44	1.27	1.16	2.15	1.37	1.79	1.56	2.55	2.29	1.63	2.52	1.93	2.24	2.51	1.55	4.25
	Al2O3	3.33	7.09	2.36	14.21	3.62	3.34	3.47	3.24	3.20	5.23	5.43	3.30	6.08	3.42	4.89	6.04	3.29	6.15
	Cr2O3	-	-	-	-	-	-	0.36	0.15	0.22	0.08	0.39	0.25	0.41	0.00	0.34	0.34	0.25	0.01
	FeO	8.32	4.61	10.85	5.84	3.59	9.44	6.45	6.47	6.40	7.13	6.68	6.37	6.74	7.43	6.56	6.84	6.32	9.55
	MnO	0.08	0.06	0.22	0.03	0.15	0.19	0.19	0.16	0.14	0.13	0.10	0.18	0.13	0.13	0.12	0.10	0.18	0.16
	MgO	11.03	7.30	11.52	8.44	8.52	12.61	15.11	14.94	15.19	13.56	13.83	15.04	13.50	14.47	14.08	13.69	15.05	11.16
	NiO	-	-	-	-	-	-	0.00	0.01	0.02	0.02	0.00	0.05	0.00	0.00	0.01	0.02	0.01	0.04
	CaO	18.52	12.98	22.44	15.72	13.22	22.42	22.07	22.31	22.14	22.33	22.22	22.14	22.22	22.12	22.03	22.52	22.33	21.77
	Na2O	2.71	3.52	0.48	3.37	5.75	0.47	0.30	0.35	0.39	0.43	0.40	0.32	0.41	0.39	0.36	0.45	0.36	0.59
	K2O	0.10	0.27	0.07	0.37	0.67	0.02	0.00	0.01	0.00	0.01	0.00	0.00	0.00	0.00	0.01	0.00	0.01	0.04
	P2O5	0.05	0.07	0.05	0.02	0.03	0.03	0.01	0.03	0.03	0.01	0.02	0.00	0.04	0.02	0.02	0.02	0.03	0.06
	Total	101.40	90.92	99.93	99.16	95.02	100.36	100.75	100.74	100.90	100.57	100.37	100.66	100.38	100.93	100.10	101.21	101.03	100.37
	Cations
T (IV)	Si	2.04	2.18	1.91	1.83	2.21	1.86	1.89	1.88	1.89	1.81	1.81	1.89	1.79	1.87	1.83	1.78	1.89	1.75
	Al	0.00	0.00	0.09	0.17	0.00	0.14	0.11	0.12	0.11	0.19	0.19	0.11	0.21	0.13	0.17	0.22	0.11	0.25
	Sum T (IV)	2.04	2.18	2.00	2.00	2.21	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00
M1 (VI)	Ti	0.04	0.03	0.04	0.04	0.03	0.06	0.04	0.05	0.04	0.07	0.06	0.04	0.07	0.05	0.06	0.07	0.04	0.12
	Al	0.14	0.34	0.01	0.45	0.16	0.00	0.03	0.02	0.03	0.04	0.05	0.03	0.05	0.02	0.04	0.04	0.03	0.03
	Cr	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.00	0.01	0.00	0.01	0.01	0.01	0.00	0.01	0.01	0.01	0.00
	Fe3+	0.00	0.00	0.04	0.00	0.00	0.05	0.02	0.02	0.02	0.03	0.03	0.01	0.04	0.02	0.02	0.06	0.02	0.02
	Fe2+	0.23	0.11	0.29	0.14	0.10	0.22	0.15	0.15	0.15	0.16	0.15	0.16	0.15	0.18	0.16	0.13	0.15	0.25
	Mg	0.54	0.32	0.62	0.36	0.45	0.66	0.74	0.74	0.75	0.68	0.69	0.74	0.67	0.72	0.70	0.68	0.74	0.57
	Sum M1 (VI)	0.96	0.80	1.00	0.98	0.75	1.00	0.99	0.99	0.99	0.99	0.99	0.99	0.99	0.99	0.99	0.99	0.99	1.00
M2 (VI)	Fe2+	0.02	0.05	0.01	0.04	0.01	0.02	0.03	0.03	0.03	0.02	0.03	0.03	0.03	0.03	0.03	0.02	0.03	0.02
	Mn	0.00	0.00	0.01	0.00	0.00	0.01	0.01	0.00	0.00	0.00	0.00	0.01	0.00	0.00	0.00	0.00	0.01	0.01
	Mg	0.06	0.12	0.03	0.10	0.03	0.05	0.09	0.08	0.08	0.07	0.07	0.08	0.07	0.08	0.08	0.07	0.08	0.05
	Ca	0.72	0.56	0.91	0.62	0.54	0.90	0.87	0.88	0.87	0.88	0.88	0.87	0.88	0.87	0.87	0.88	0.87	0.88
	Na	0.19	0.28	0.04	0.24	0.42	0.03	0.02	0.03	0.03	0.03	0.03	0.02	0.03	0.03	0.03	0.03	0.03	0.04
	K	0.00	0.01	0.00	0.02	0.03	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
	Sum M2 (VI)	1.00	1.02	1.00	1.02	1.04	1.00	1.01	1.01	1.01	1.01	1.01	1.01	1.01	1.01	1.01	1.01	1.01	1.00
	TOTAL	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00
	FeT/FeT+Mg	0.30	0.26	0.35	0.28	0.19	0.30	0.19	0.20	0.19	0.23	0.21	0.19	0.22	0.22	0.21	0.22	0.19	0.32
	Fe2+/Fe2+ + Mg	0.30	0.26	0.32	0.28	0.19	0.26	0.18	0.18	0.18	0.20	0.19	0.18	0.19	0.20	0.19	0.17	0.18	0.31
	Pyroxene Components (%)
	Wo	45.89	48.55	47.81	49.10	47.41	47.37	45.86	46.34	45.86	47.76	47.61	46.09	48.03	46.03	47.13	48.01	46.32	48.65
	En	38.03	37.98	34.14	36.67	42.53	37.07	43.68	43.17	43.79	40.35	41.22	43.57	40.59	41.91	41.91	40.61	43.44	34.69
	Fs	16.08	13.47	18.05	14.23	10.06	15.56	10.46	10.49	10.35	11.89	11.16	10.35	11.38	12.06	10.96	11.38	10.24	16.66

,

Table A6. EPMA pyroxene chemistry results for the Meahill basalt sample (TM9).

Meahill Basalt (TM9)—Pyroxene Chemistry
		1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30
	SiO2	51.37	51.11	49.18	47.28	50.51	48.42	48.19	49.35	49.73	51.21	48.82	50.27	50.94	47.55	51.22	48.76	47.79	49.35	51.37	51.11	49.18	47.28	50.51	48.42	48.19	49.35	49.73	51.21	48.82	50.27
	TiO2	1.70	1.79	2.24	3.30	1.93	2.63	3.16	2.53	2.42	1.74	2.88	2.25	1.76	3.03	1.59	2.60	3.14	2.07	1.70	1.79	2.24	3.30	1.93	2.63	3.16	2.53	2.42	1.74	2.88	2.25
	Al2O3	3.46	3.56	3.66	6.35	4.27	5.58	5.29	3.75	3.72	3.32	5.40	3.03	3.49	6.27	3.50	5.85	5.88	4.86	3.46	3.56	3.66	6.35	4.27	5.58	5.29	3.75	3.72	3.32	5.40	3.03
	Cr2O3	0.03	0.13	0.00	0.05	0.31	0.19	0.00	0.00	0.01	0.12	0.01	0.00	0.11	0.23	0.15	0.24	0.04	0.33	0.03	0.13	0.00	0.05	0.31	0.19	0.00	0.00	0.01	0.12	0.01	0.00
	FeO	6.58	6.58	11.88	8.25	6.27	7.24	8.32	9.08	8.16	6.71	7.65	10.12	7.02	7.04	6.63	7.19	8.11	6.99	6.58	6.58	11.88	8.25	6.27	7.24	8.32	9.08	8.16	6.71	7.65	10.12
	MnO	0.16	0.12	0.29	0.15	0.11	0.17	0.20	0.14	0.15	0.15	0.13	0.22	0.11	0.12	0.13	0.14	0.12	0.10	0.16	0.12	0.29	0.15	0.11	0.17	0.20	0.14	0.15	0.15	0.13	0.22
	MgO	14.81	14.68	10.38	12.39	14.56	13.14	12.49	12.76	13.54	14.77	13.11	12.10	14.37	13.13	14.74	13.16	12.57	13.87	14.81	14.68	10.38	12.39	14.56	13.14	12.49	12.76	13.54	14.77	13.11	12.10
	NiO	0.03	0.00	0.07	0.00	0.02	0.00	0.01	0.01	0.00	0.00	0.00	0.00	0.03	0.00	0.04	0.04	0.00	0.03	0.03	0.00	0.07	0.00	0.02	0.00	0.01	0.01	0.00	0.00	0.00	0.00
	CaO	22.47	22.58	21.99	22.23	22.37	22.37	22.12	21.91	22.24	22.50	22.22	21.98	22.36	22.52	22.69	22.33	22.45	22.22	22.47	22.58	21.99	22.23	22.37	22.37	22.12	21.91	22.24	22.50	22.22	21.98
	Na2O	0.40	0.38	0.65	0.58	0.39	0.42	0.49	0.52	0.45	0.41	0.46	0.43	0.38	0.48	0.40	0.51	0.52	0.45	0.40	0.38	0.65	0.58	0.39	0.42	0.49	0.52	0.45	0.41	0.46	0.43
	K2O	0.01	0.02	0.05	0.01	0.01	0.01	0.05	0.01	0.00	0.01	0.01	0.01	0.02	0.01	0.00	0.01	0.01	0.01	0.01	0.02	0.05	0.01	0.01	0.01	0.05	0.01	0.00	0.01	0.01	0.01
	P2O5	0.01	0.02	0.01	0.01	0.02	0.01	0.01	0.04	0.03	0.02	0.01	0.03	0.04	0.01	0.02	0.01	0.03	0.01	0.01	0.02	0.01	0.01	0.02	0.01	0.01	0.04	0.03	0.02	0.01	0.03
	Total	101.02	100.96	100.39	100.59	100.77	100.18	100.33	100.10	100.46	100.98	100.68	100.42	100.62	100.41	101.09	100.84	100.64	100.29	101.02	100.96	100.39	100.59	100.77	100.18	100.33	100.10	100.46	100.98	100.68	100.42
	Cations
T (IV)	Si	1.88	1.87	1.86	1.76	1.85	1.80	1.80	1.85	1.85	1.88	1.81	1.89	1.88	1.76	1.87	1.80	1.77	1.82	1.88	1.87	1.86	1.76	1.85	1.80	1.80	1.85	1.85	1.88	1.81	1.89
	Al	0.12	0.13	0.14	0.24	0.15	0.20	0.20	0.15	0.15	0.12	0.19	0.11	0.12	0.24	0.13	0.20	0.23	0.18	0.12	0.13	0.14	0.24	0.15	0.20	0.20	0.15	0.15	0.12	0.19	0.11
	Sum T (IV)	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00	2.00
M1 (VI)	Ti	0.05	0.05	0.06	0.09	0.05	0.07	0.09	0.07	0.07	0.05	0.08	0.06	0.05	0.08	0.04	0.07	0.09	0.06	0.05	0.05	0.06	0.09	0.05	0.07	0.09	0.07	0.07	0.05	0.08	0.06
	Al	0.03	0.03	0.02	0.03	0.04	0.04	0.03	0.01	0.01	0.02	0.04	0.02	0.03	0.03	0.02	0.05	0.03	0.04	0.03	0.03	0.02	0.03	0.04	0.04	0.03	0.01	0.01	0.02	0.04	0.02
	Cr	0.00	0.00	0.00	0.00	0.01	0.01	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.00	0.01	0.00	0.01	0.00	0.00	0.00	0.00	0.01	0.01	0.00	0.00	0.00	0.00	0.00	0.00
	Fe3+	0.03	0.03	0.04	0.07	0.02	0.04	0.03	0.04	0.04	0.04	0.02	0.00	0.02	0.07	0.04	0.04	0.06	0.05	0.03	0.03	0.04	0.07	0.02	0.04	0.03	0.04	0.04	0.04	0.02	0.00
	Fe2+	0.16	0.16	0.33	0.18	0.15	0.17	0.21	0.23	0.20	0.16	0.20	0.30	0.18	0.14	0.15	0.17	0.18	0.15	0.16	0.16	0.33	0.18	0.15	0.17	0.21	0.23	0.20	0.16	0.20	0.30
	Mg	0.74	0.73	0.55	0.63	0.72	0.67	0.64	0.65	0.69	0.74	0.66	0.62	0.72	0.67	0.74	0.66	0.64	0.70	0.74	0.73	0.55	0.63	0.72	0.67	0.64	0.65	0.69	0.74	0.66	0.62
	Sum M1 (VI)	1.00	1.00	1.01	1.00	1.00	1.00	1.00	1.01	1.00	1.00	1.00	1.01	1.00	1.00	1.00	1.00	1.00	1.00	1.00	1.00	1.01	1.00	1.00	1.00	1.00	1.01	1.00	1.00	1.00	1.01
M2 (VI)	Fe2+	0.02	0.01	0.01	0.01	0.02	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.02	0.01	0.01	0.01	0.01	0.01	0.02	0.01	0.01	0.01	0.02	0.01	0.01	0.01	0.01	0.01	0.01	0.01
	Mn	0.01	0.00	0.01	0.00	0.00	0.01	0.01	0.00	0.00	0.00	0.00	0.01	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.00	0.01	0.00	0.00	0.01	0.01	0.00	0.00	0.00	0.00	0.01
	Mg	0.07	0.07	0.03	0.05	0.07	0.06	0.06	0.06	0.06	0.07	0.06	0.05	0.07	0.06	0.07	0.06	0.05	0.07	0.07	0.07	0.03	0.05	0.07	0.06	0.06	0.06	0.06	0.07	0.06	0.05
	Ca	0.88	0.89	0.89	0.88	0.88	0.89	0.88	0.88	0.88	0.88	0.88	0.88	0.88	0.89	0.89	0.88	0.89	0.88	0.88	0.89	0.89	0.88	0.88	0.89	0.88	0.88	0.88	0.88	0.88	0.88
	Na	0.03	0.03	0.05	0.04	0.03	0.03	0.04	0.04	0.03	0.03	0.03	0.03	0.03	0.03	0.03	0.04	0.04	0.03	0.03	0.03	0.05	0.04	0.03	0.03	0.04	0.04	0.03	0.03	0.03	0.03
	K	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
	Sum M2 (VI)	1.00	1.00	0.99	1.00	1.00	1.00	1.00	0.99	1.00	1.00	1.00	0.99	1.00	1.00	1.00	1.00	1.00	1.00	1.00	1.00	0.99	1.00	1.00	1.00	1.00	0.99	1.00	1.00	1.00	0.99
	TOTAL	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00	4.00
	FeT/FeT + Mg	0.20	0.20	0.39	0.27	0.19	0.24	0.27	0.29	0.25	0.20	0.25	0.32	0.21	0.23	0.20	0.23	0.27	0.22	0.20	0.20	0.39	0.27	0.19	0.24	0.27	0.29	0.25	0.20	0.25	0.32
	Fe2+/Fe2+ + Mg	0.18	0.18	0.37	0.22	0.18	0.20	0.24	0.26	0.22	0.17	0.23	0.32	0.20	0.17	0.17	0.20	0.22	0.18	0.18	0.18	0.37	0.22	0.18	0.20	0.24	0.26	0.22	0.17	0.23	0.32
	Pyroxene Components (%)
	Wo	46.61	46.90	48.11	48.43	47.08	48.32	48.09	46.87	46.87	46.59	47.86	47.05	46.74	48.67	46.90	48.27	48.53	47.30	46.61	46.90	48.11	48.43	47.08	48.32	48.09	46.87	46.87	46.59	47.86	47.05
	En	42.74	42.43	31.61	37.54	42.63	39.47	37.79	37.97	39.71	42.56	39.28	36.04	41.81	39.46	42.41	39.60	37.79	41.09	42.74	42.43	31.61	37.54	42.63	39.47	37.79	37.97	39.71	42.56	39.28	36.04
	Fs	10.65	10.67	20.28	14.03	10.30	12.20	14.12	15.16	13.42	10.85	12.86	16.91	11.45	11.88	10.69	12.13	13.68	11.61	10.65	10.67	20.28	14.03	10.30	12.20	14.12	15.16	13.42	10.85	12.86	16.91

,

Table A7. EPMA feldspar chemistry results for Meahill basalt (TM9).

	Meahill Basalt—TM9 Feldspar
Sample	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25
SiO2	53.83	57.24	53.98	57.41	53.59	54.89	48.60	53.20	53.43	53.70	54.11	54.09	54.03	53.57	53.34	52.98	52.49	53.07	52.36	56.44	54.31	54.56	52.40	53.28	52.64
Al2O3	29.40	27.07	29.46	27.01	29.26	28.19	8.15	29.47	29.79	29.35	29.21	29.40	29.01	29.78	29.51	29.89	30.04	29.83	30.29	27.06	29.23	28.47	30.29	29.56	30.02
CaO	11.83	8.82	11.77	8.77	11.83	11.03	21.76	12.44	12.22	11.88	11.53	11.61	11.60	12.36	12.29	12.53	13.10	12.69	12.96	9.32	11.33	11.03	12.77	12.35	12.70
Na2O	4.76	6.33	4.76	6.28	4.62	5.20	0.98	4.61	4.52	4.71	4.90	4.99	4.86	4.30	4.44	4.25	4.02	4.19	3.99	5.95	4.91	4.97	4.02	4.33	4.07
K2O	0.37	0.57	0.44	0.63	0.40	0.46	0.04	0.32	0.32	0.35	0.39	0.34	0.47	0.45	0.44	0.42	0.41	0.43	0.37	0.70	0.40	0.44	0.40	0.41	0.40
Cr2O3	0.01	0.04	0.04	0.00	0.02	0.04	0.05	0.00	0.00	0.04	0.03	0.00	0.01	0.02	0.00	0.06	0.06	0.08	0.00	0.00	0.02	0.02	0.00	0.00	0.00
FeO	0.58	0.40	0.58	0.41	0.64	0.63	7.57	0.64	0.63	0.63	0.64	0.61	0.58	0.52	0.49	0.58	0.51	0.55	0.51	0.42	0.59	0.84	0.52	0.50	0.51
MnO	0.00	0.01	0.03	0.02	0.00	0.01	0.10	0.04	0.01	0.02	0.01	0.00	0.00	0.02	0.01	0.02	0.02	0.00	0.01	0.01	0.00	0.00	0.01	0.00	0.00
MgO	0.05	0.01	0.03	0.02	0.13	0.11	11.62	0.02	0.03	0.04	0.02	0.04	0.03	0.05	0.05	0.02	0.07	0.05	0.07	0.07	0.04	0.12	0.09	0.04	0.06
NiO	0.00	0.03	0.00	0.01	0.00	0.02	0.01	0.00	0.05	0.01	0.03	0.03	0.00	0.01	0.00	0.00	0.00	0.02	0.02	0.02	0.00	0.03	0.00	0.00	0.01
P2O5	0.02	0.03	0.02	0.06	0.06	0.25	0.19	0.04	0.01	0.00	0.02	0.03	0.03	0.03	0.00	0.03	0.01	0.00	0.02	0.06	0.00	0.09	0.01	0.00	0.00
TiO2	0.19	0.20	0.15	0.11	0.15	0.22	2.94	0.16	0.13	0.16	0.17	0.21	0.20	0.16	0.09	0.15	0.15	0.12	0.06	0.18	0.17	0.16	0.12	0.16	0.19
	101.0	100.8	101.3	100.7	100.7	101.0	102.0	100.9	101.1	100.9	101.1	101.3	100.8	101.3	100.7	100.9	100.9	101.0	100.7	100.2	101.0	100.7	100.6	100.6	100.6
Si	0.90	0.95	0.90	0.96	0.89	0.91	0.81	0.89	0.89	0.89	0.90	0.90	0.90	0.89	0.89	0.88	0.87	0.88	0.87	0.94	0.90	0.91	0.87	0.89	0.88
Al	0.58	0.53	0.58	0.53	0.57	0.55	0.16	0.58	0.58	0.58	0.57	0.58	0.57	0.58	0.58	0.59	0.59	0.59	0.59	0.53	0.57	0.56	0.59	0.58	0.59
Ca	0.21	0.16	0.21	0.16	0.21	0.20	0.39	0.22	0.22	0.21	0.21	0.21	0.21	0.22	0.22	0.22	0.23	0.23	0.23	0.17	0.20	0.20	0.23	0.22	0.23
Na	0.15	0.20	0.15	0.20	0.15	0.17	0.03	0.15	0.15	0.15	0.16	0.16	0.16	0.14	0.14	0.14	0.13	0.14	0.13	0.19	0.16	0.16	0.13	0.14	0.13
K	0.01	0.01	0.01	0.01	0.01	0.01	0.00	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.01
O_total	2.95	2.97	2.96	2.97	2.93	2.94	2.26	2.94	2.95	2.94	2.95	2.96	2.94	2.95	2.94	2.94	2.93	2.94	2.93	2.94	2.95	2.94	2.93	2.94	2.93
Norm_factor	2.71	2.70	2.71	2.69	2.73	2.72	3.54	2.72	2.71	2.72	2.71	2.71	2.72	2.71	2.72	2.72	2.73	2.72	2.73	2.72	2.71	2.73	2.73	2.72	2.73
Si_8O	2.43	2.57	2.43	2.57	2.43	2.48	2.86	2.41	2.41	2.43	2.44	2.44	2.45	2.41	2.42	2.40	2.38	2.40	2.38	2.55	2.45	2.47	2.38	2.41	2.39
Al_8O	1.56	1.43	1.56	1.43	1.56	1.50	0.57	1.57	1.59	1.57	1.55	1.56	1.55	1.58	1.58	1.60	1.61	1.59	1.62	1.44	1.55	1.52	1.62	1.58	1.61
Ca_8O	0.57	0.42	0.57	0.42	0.58	0.53	1.37	0.60	0.59	0.58	0.56	0.56	0.56	0.60	0.60	0.61	0.64	0.62	0.63	0.45	0.55	0.54	0.62	0.60	0.62
Na_8O	0.42	0.55	0.42	0.55	0.41	0.46	0.11	0.40	0.40	0.41	0.43	0.44	0.43	0.38	0.39	0.37	0.35	0.37	0.35	0.52	0.43	0.44	0.35	0.38	0.36
K_8O	0.02	0.03	0.03	0.04	0.02	0.03	0.00	0.02	0.02	0.02	0.02	0.02	0.03	0.03	0.03	0.02	0.02	0.02	0.02	0.04	0.02	0.03	0.02	0.02	0.02
An_%	56.68	42.09	56.31	42.01	57.24	52.54	92.26	58.81	58.80	57.06	55.27	55.20	55.34	59.79	58.97	60.50	62.79	61.04	62.84	44.53	54.72	53.65	62.26	59.73	61.83
Ab_%	41.23	54.67	41.16	54.42	40.45	44.84	7.54	39.39	39.36	40.91	42.50	42.90	41.97	37.62	38.54	37.09	34.87	36.51	35.00	51.47	42.95	43.79	35.44	37.90	35.86
Or_%	2.10	3.24	2.53	3.57	2.30	2.62	0.21	1.80	1.84	2.02	2.23	1.91	2.69	2.59	2.49	2.41	2.34	2.45	2.16	4.01	2.32	2.56	2.30	2.37	2.31
Si + Al_check	4.00	4.00	4.00	4.00	4.00	3.99	3.43	3.99	4.00	3.99	4.00	4.00	3.99	4.00	3.99	4.00	3.99	3.99	4.00	3.99	4.00	4.00	4.00	3.99	4.00
Ca + Na + K_check	1.01	1.01	1.01	1.00	1.00	1.02	1.49	1.03	1.01	1.01	1.01	1.01	1.02	1.00	1.01	1.01	1.01	1.01	1.00	1.01	1.00	1.00	1.00	1.00	1.00

,

Table A8. EPMA feldspar chemistry results for Yemsigi basalt (MY6).

	Yemsigi Basalt—MY6 Feldspar
Sample	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17
SiO2	52.61	53.97	58.09	52.76	52.42	53.20	55.05	52.85	52.86	55.09	57.23	38.56	38.05	38.28	52.76	52.36	53.04
Al2O3	29.92	29.35	26.61	29.96	30.16	29.73	24.30	30.22	30.41	28.84	26.92	0.06	0.01	0.12	30.33	30.40	29.81
CaO	12.57	11.71	8.31	12.56	12.82	12.20	12.08	12.67	12.56	10.65	8.78	0.47	0.48	0.52	13.00	13.10	12.58
Na2O	4.26	4.66	6.37	4.24	4.04	4.16	4.94	3.90	4.25	5.18	6.16	0.05	0.03	0.00	4.05	3.98	4.22
K2O	0.31	0.40	0.84	0.34	0.41	0.40	0.38	0.36	0.26	0.57	0.83	0.00	0.00	0.01	0.37	0.33	0.38
Cr2O3	0.02	0.00	0.00	0.05	0.03	0.00	0.00	0.05	0.00	0.00	0.00	0.01	0.00	0.00	0.00	0.00	0.01
FeO	0.57	0.61	0.43	0.64	0.54	0.70	1.81	0.56	0.70	0.56	0.49	25.82	26.56	25.99	0.56	0.55	0.58
MnO	0.00	0.00	0.04	0.01	0.02	0.04	0.03	0.00	0.00	0.01	0.01	0.38	0.35	0.42	0.00	0.00	0.03
MgO	0.05	0.04	0.00	0.10	0.08	0.08	2.14	0.07	0.02	0.04	0.03	33.60	35.29	35.57	0.06	0.07	0.04
NiO	0.03	0.00	0.01	0.00	0.00	0.00	0.03	0.03	0.01	0.01	0.04	0.06	0.11	0.10	0.02	0.00	0.06
P2O5	0.03	0.01	0.03	0.06	0.03	0.02	0.03	0.00	0.03	0.01	0.12	0.16	0.18	0.06	0.01	0.00	0.00
TiO2	0.17	0.18	0.16	0.16	0.11	0.18	0.55	0.15	0.08	0.17	0.18	0.03	0.10	0.12	0.11	0.13	0.12
	100.5	100.9	100.9	100.9	100.7	100.7	101.3	100.8	101.2	101.1	100.8	99.2	101.2	101.2	101.3	100.9	100.9
Si	0.88	0.90	0.97	0.88	0.87	0.89	0.92	0.88	0.88	0.92	0.95	0.64	0.63	0.64	0.88	0.87	0.88
Al	0.59	0.58	0.52	0.59	0.59	0.58	0.48	0.59	0.60	0.57	0.53	0.00	0.00	0.00	0.59	0.60	0.58
Ca	0.22	0.21	0.15	0.22	0.23	0.22	0.22	0.23	0.22	0.19	0.16	0.01	0.01	0.01	0.23	0.23	0.22
Na	0.14	0.15	0.21	0.14	0.13	0.13	0.16	0.13	0.14	0.17	0.20	0.00	0.00	0.00	0.13	0.13	0.14
K	0.01	0.01	0.02	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.02	0.00	0.00	0.00	0.01	0.01	0.01
O_total	2.93	2.95	2.98	2.93	2.93	2.93	2.85	2.94	2.95	2.96	2.96	1.29	1.28	1.29	2.95	2.94	2.94
Norm_factor	2.73	2.71	2.69	2.73	2.73	2.73	2.81	2.72	2.71	2.70	2.70	6.18	6.27	6.22	2.71	2.72	2.72
Si_8O	2.39	2.44	2.60	2.39	2.38	2.41	2.58	2.39	2.39	2.48	2.57	3.97	3.97	3.96	2.38	2.37	2.40
Al_8O	1.60	1.56	1.40	1.60	1.62	1.59	1.34	1.61	1.62	1.53	1.43	0.01	0.00	0.01	1.61	1.62	1.59
Ca_8O	0.61	0.57	0.40	0.61	0.62	0.59	0.61	0.61	0.61	0.51	0.42	0.05	0.05	0.06	0.63	0.64	0.61
Na_8O	0.38	0.41	0.55	0.37	0.36	0.37	0.45	0.34	0.37	0.45	0.54	0.01	0.01	0.00	0.35	0.35	0.37
K_8O	0.02	0.02	0.05	0.02	0.02	0.02	0.02	0.02	0.02	0.03	0.05	0.00	0.00	0.00	0.02	0.02	0.02
An_%	60.92	56.79	39.88	60.90	62.14	60.37	56.29	62.87	61.08	51.46	41.99	84.68	91.09	98.43	62.59	63.31	60.86
Ab_%	37.32	40.88	55.34	37.17	35.47	37.27	41.63	34.98	37.40	45.24	53.28	14.67	8.91	0.00	35.28	34.79	36.96
Or_%	1.76	2.33	4.78	1.93	2.39	2.36	2.08	2.15	1.52	3.30	4.73	0.64	0.00	1.57	2.13	1.90	2.17
Si + Al_check	4.00	4.00	4.00	4.00	4.00	4.00	3.91	4.01	4.00	4.00	4.00	3.97	3.97	3.97	3.99	4.00	3.99
Ca + Na + K_check	1.01	1.00	1.00	1.00	1.00	0.98	1.08	0.98	0.99	1.00	1.01	0.06	0.06	0.06	1.00	1.00	1.00

,

Table A9. EPMA titanomagnetite chemistry results for Meahill basalt (TM9).

Meahill Basalt (TM9)—Titanomagnetite Chemistry
No.	1	2	3	4	5	6	7	9	10	11	12	13	14	15	16	17
SiO2	0.0	0.0	0.0	0.0	0.0	0.0	0.0	52.7	0.0	2.0	0.0	0.0	0.0	3.3	0.0	0.1
TiO2	25.6	25.2	26.0	24.3	27.0	26.9	26.8	0.2	26.0	25.9	26.4	26.0	25.8	25.2	27.6	26.4
Al2O3	2.3	2.3	2.3	2.2	2.2	2.5	2.6	30.1	2.1	1.8	1.8	1.5	1.6	3.5	2.6	1.3
Cr2O3	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
FeO	67.8	67.5	67.6	68.7	67.2	66.8	66.4	0.6	68.1	64.7	67.4	67.7	68.3	63.5	65.3	67.6
MnO	0.8	0.8	0.8	0.7	0.6	0.6	0.6	0.0	0.9	0.9	0.8	0.8	0.8	0.7	0.6	0.6
MgO	1.3	1.3	1.4	1.1	1.2	1.4	1.4	0.0	1.0	0.9	1.3	0.9	1.0	1.3	1.9	1.0
NiO	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.1	0.1	0.0	0.0	0.0	0.0	0.0	0.0
CaO	0.1	0.1	0.1	0.0	0.1	0.0	0.1	12.8	0.1	0.2	0.1	0.1	0.1	0.5	0.1	0.1
Na2O	0.0	0.0	0.0	0.0	0.0	0.0	0.0	4.1	0.0	0.8	0.0	0.1	0.1	0.4	0.1	0.0
K2O	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.3	0.0	0.1	0.0	0.1	0.1	0.1	0.0	0.0
P2O5	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.1	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
Total	97.9	97.3	98.2	97.2	98.4	98.2	98.0	101.0	98.3	97.5	97.9	97.2	97.6	98.6	98.1	97.2
Cations
Si	0.0	0.0	0.0	0.0	0.0	0.0	0.0	1.5	0.0	0.1	0.0	0.0	0.0	0.1	0.0	0.0
Ti	0.7	0.7	0.7	0.7	0.7	0.7	0.7	0.0	0.7	0.7	0.7	0.7	0.7	0.7	0.8	0.7
Al	0.1	0.1	0.1	0.1	0.1	0.1	0.1	1.0	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1
Fe3+	0.5	0.5	0.5	0.5	0.4	0.4	0.4	-2.1	0.5	0.3	0.5	0.5	0.5	0.2	0.4	0.5
Fe2+	1.6	1.6	1.6	1.6	1.7	1.6	1.6	2.1	1.6	1.7	1.6	1.7	1.6	1.7	1.6	1.7
Cr	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
Mn	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
Mg	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.0	0.1	0.1	0.1	0.0	0.1	0.1	0.1	0.1
Ca	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.4	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
Na	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.2	0.0	0.1	0.0	0.0	0.0	0.0	0.0	0.0
K	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
Ba	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
Zn	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
V	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
Ni	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
Nb	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
TOTAL	3.0	3.0	3.0	3.0	3.0	3.0	3.0	3.1	3.0	3.0	3.0	3.0	3.0	3.0	3.0	3.0
Ulvospinel	2.2	2.2	2.2	2.2	2.2	2.2	2.2	1.7	2.2	2.2	2.2	2.2	2.2	2.2	2.2	2.2
FeO (%)	52.2	51.7	52.7	51.2	53.9	53.6	53.4	87.6	53.1	54.8	52.8	52.9	52.7	55.8	53.5	53.2
Fe2O3 (%)	17.3	17.6	16.6	19.5	14.7	14.7	14.5	-96.7	16.7	11.0	16.2	16.5	17.3	8.6	13.1	16.0
TOTAL	99.6	99.0	99.9	99.1	99.9	99.7	99.5	91.3	100.0	98.6	99.5	98.8	99.3	99.4	99.4	98.8

and

Table A10. EPMA Titanomagnetite Chemistry Results for Meahill basalt (MY6).

Yemsigi Basalt (MY6)—Titanomagnetite Chemistry
No.	1	2	3	4	5	6	7	8	9	10	11	12	13
SiO2	0.00	0.00	0.03	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.05	0.00
TiO2	26.13	26.46	26.32	26.62	26.56	26.41	26.23	26.20	27.83	26.82	27.64	27.20	26.38
Al2O3	2.33	2.38	2.40	2.44	2.58	2.35	2.31	2.36	2.45	1.98	2.00	2.09	2.46
Cr2O3	0.00	0.00	0.02	0.01	0.00	0.02	0.00	0.07	0.01	0.00	0.00	0.03	0.01
FeO	66.94	67.32	67.29	67.03	67.28	67.19	66.87	67.11	65.49	67.24	66.83	66.78	66.87
MnO	0.70	0.69	0.65	0.68	0.58	0.66	0.64	0.73	0.76	0.66	0.68	0.61	0.70
MgO	1.62	1.56	1.59	1.59	1.46	1.46	1.37	1.43	1.69	1.28	1.28	1.31	1.55
NiO	0.00	0.02	0.00	0.04	0.00	0.00	0.00	0.02	0.00	0.02	0.00	0.00	0.00
CaO	0.04	0.00	0.00	0.00	0.02	0.08	0.02	0.05	0.08	0.07	0.04	0.05	0.01
Na2O	0.10	0.03	0.05	0.06	0.00	0.00	0.06	0.04	0.00	0.03	0.12	0.07	0.06
K2O	0.05	0.00	0.00	0.01	0.00	0.04	0.02	0.02	0.02	0.05	0.03	0.05	0.01
P2O5	0.02	0.00	0.04	0.01	0.04	0.02	0.00	0.00	0.00	0.03	0.01	0.01	0.00
Total	97.93	98.46	98.39	98.48	98.52	98.23	97.53	98.03	98.33	98.17	98.65	98.25	98.06
Cations
Si	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Ti	0.72	0.73	0.73	0.73	0.73	0.73	0.73	0.73	0.77	0.74	0.76	0.75	0.73
Al	0.10	0.10	0.10	0.11	0.11	0.10	0.10	0.10	0.11	0.09	0.09	0.09	0.11
Fe3+	0.45	0.44	0.44	0.43	0.43	0.44	0.44	0.44	0.36	0.42	0.38	0.40	0.43
Fe2+	1.61	1.62	1.62	1.62	1.63	1.63	1.63	1.62	1.65	1.65	1.67	1.66	1.62
Cr	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Mn	0.02	0.02	0.02	0.02	0.02	0.02	0.02	0.02	0.02	0.02	0.02	0.02	0.02
Mg	0.09	0.09	0.09	0.09	0.08	0.08	0.08	0.08	0.09	0.07	0.07	0.07	0.08
Ca	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Na	0.01	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.00	0.00
K	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Ba	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Zn	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
V	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Ni	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Nb	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
TOTAL	3.01	3.00	3.00	3.00	3.00	3.00	3.00	3.00	3.00	3.00	3.01	3.00	3.00
Ulvospinel	2.21	2.20	2.20	2.20	2.20	2.21	2.22	2.21	2.20	2.22	2.21	2.21	2.21
FeO (%)	52.31	52.96	52.84	53.08	53.35	52.95	52.76	52.68	53.78	53.49	54.35	53.93	52.77
Fe2O3 (%)	16.25	15.95	16.06	15.50	15.48	15.83	15.68	16.03	13.01	15.27	13.88	14.27	15.67
TOTAL	99.54	100.05	99.94	100.01	100.04	99.77	99.10	99.57	99.62	99.68	100.02	99.64	99.62

). Forsterite contents (Fo = 100 × Mg/(Mg + Fe)) for olivine range from Fo63 to Fo84 (n = 61) for the Meahill basalts and ranges from Fo67 to Fo86 in the Yemsigi basalt (n = 79). These compositions fall within the hyalosiderite to chrysolite fields (Figure 5a). Clinopyroxene compositions from both Meahill and Yemsigi basalts have compositions ranging from Wo47 to Wo49 and plot within diopside field (Figure 5b). Plagioclase compositions range from An39 to An63 and plot between andesine to labradorite (Figure 5c). Titanomagnetite analyses show variations in TiO₂ and FeO consistent with the magnetite-ulvöspinel solid-solution series (Figure 5d).

5.3. Whole-Rock Geochemistry

Whole-rock major, trace, and rare earth element (REE) data obtained from XRF and ICP-MS analyses are presented in Appendix A (

Table A11. XRF whole-rock chemistry results for Yemsigi and Meahill basalts.

SN#	MY1	MY4	MY5	MY6	MY7	MY8	MY10	TM3	TM4	TM7	TM8A	TM8B	TM9	TM11	TM12	TM13	TM14	TM15	TM16
Major elements measured by XRF (wt %)
SiO2	46.52	46.34	47.71	46.54	46.10	46.39	46.82	47.60	47.58	45.20	49.32	48.16	47.07	47.51	47.97	47.59	47.90	48.18	48.42
TiO2	2.34	2.26	2.29	2.29	2.31	2.33	2.35	2.30	2.29	2.51	2.35	2.29	2.24	2.33	2.26	2.32	2.33	2.33	2.33
Al2O3	13.41	13.18	14.02	13.28	13.24	13.34	13.51	14.13	14.15	15.90	14.69	14.31	13.83	14.04	14.23	14.15	14.19	14.30	14.35
FeOt	10.12	10.42	9.75	10.23	10.68	10.41	10.19	10.09	9.45	9.55	9.26	9.05	9.58	9.68	9.18	9.65	9.61	9.33	9.27
MnO	0.16	0.24	0.16	0.21	0.31	0.26	0.16	0.14	0.14	0.14	0.14	0.14	0.15	0.15	0.15	0.17	0.15	0.15	0.16
MgO	8.48	8.50	8.06	8.49	8.19	8.51	8.57	7.46	8.17	8.18	8.29	8.02	8.16	7.90	8.31	7.91	7.88	7.69	7.76
CaO	9.58	10.07	8.71	9.96	9.25	9.13	9.63	9.08	8.56	8.18	9.07	8.70	8.99	8.70	8.47	8.37	8.26	8.55	8.44
Na2O	2.43	2.58	2.78	2.59	2.81	2.61	2.86	1.79	2.75	1.90	2.72	2.60	2.76	3.04	3.27	3.06	3.32	3.28	3.37
K2O	2.02	1.90	1.98	1.93	1.99	2.03	2.08	1.13	1.87	2.22	1.70	1.63	1.79	1.96	1.98	1.98	2.00	2.03	2.01
P2O5	0.51	0.50	0.59	0.50	0.54	0.53	0.52	0.52	0.62	0.71	0.65	0.63	0.58	0.60	0.62	0.62	0.62	0.63	0.64
LOI	0.79	1.29	0.65	0.95	1.31	1.08	0.20	2.80	1.45	2.35	1.73	1.73	1.27	0.54	0.42	0.61	0.32	0.33	0.09
Recalculated to Dry 100%
SiO2	48.85	48.19	49.62	48.40	48.41	48.71	48.23	50.46	49.96	47.91	50.31	50.57	49.43	49.41	49.89	49.50	49.82	50.11	49.87
TiO2	2.46	2.35	2.38	2.38	2.43	2.45	2.42	2.44	2.40	2.66	2.40	2.41	2.36	2.42	2.35	2.41	2.42	2.42	2.40
Al2O3	14.08	13.71	14.58	13.81	13.90	14.01	13.91	14.98	14.86	16.85	14.98	15.02	14.52	14.61	14.80	14.71	14.76	14.88	14.78
FeOt	10.62	10.83	10.14	10.64	11.21	10.93	10.50	10.70	9.93	10.12	9.45	9.50	10.06	10.07	9.55	10.04	10.00	9.70	9.55
MnO	0.16	0.25	0.16	0.22	0.32	0.28	0.16	0.15	0.14	0.15	0.14	0.14	0.15	0.16	0.15	0.17	0.16	0.15	0.16
MgO	8.90	8.84	8.38	8.83	8.60	8.93	8.83	7.91	8.58	8.67	8.45	8.42	8.57	8.22	8.64	8.23	8.20	8.00	7.99
CaO	10.06	10.47	9.06	10.35	9.71	9.58	9.91	9.63	8.99	8.67	9.25	9.14	9.44	9.05	8.81	8.71	8.59	8.89	8.69
Na2O	2.56	2.68	2.89	2.69	2.95	2.74	2.94	1.89	2.89	2.02	2.77	2.73	2.90	3.16	3.40	3.18	3.45	3.41	3.47
K2O	2.12	1.98	2.06	2.00	2.09	2.13	2.14	1.20	1.96	2.35	1.74	1.71	1.88	2.04	2.06	2.06	2.08	2.11	2.07
P2O5	0.54	0.52	0.61	0.52	0.57	0.56	0.53	0.55	0.65	0.76	0.66	0.66	0.61	0.62	0.64	0.64	0.65	0.66	0.65
Total	100.34	99.82	99.88	99.85	100.19	100.32	99.57	99.90	100.35	100.17	100.15	100.31	99.91	99.76	100.29	99.64	100.13	100.32	99.65
Mg#	0.49	0.49	0.47	0.49	0.46	0.47	0.49	0.47	0.48	0.46	0.49	0.49	0.48	0.47	0.48	0.46	0.46	0.48	0.48
H2O−	0.07	0.03	0.07	0.03	0.03	0.03	0.03	0.12	0.07	0.14	0.04	0.04	0.04	0.03	0.03	0.03	0.02	0.05	0.03
H2O+	0.04	0.04	0.03	0.03	0.04	0.03	0.01	0.14	0.07	0.11	0.05	0.05	0.04	0.02	0.02	0.02	0.01	0.02	0.00
Trace elements measured by XRF (ppm)
Cr	356.00	318.00	273.00	325.00	321.00	318.00	305.00	298.00	264.00	211.00	284.00	274.00	270.00	271.00	253.00	260.00	248.00	272.00	279.00
Ni	223.00	273.00	192.00	280.00	230.00	208.00	204.00	223.00	179.00	183.00	208.00	202.00	192.00	187.00	183.00	179.00	165.00	219.00	205.00

and

Table A12. ICPMS whole-rock trace element chemistry results for Yemsigi and Meahill basalts.

Trace Elements Measured by IC-PMS (ppm)
	MY1	MY4	MY5	MY6	MY7	MY8	MY10	TM3	TM4	TM7	TM8A	TM8B	TM9	TM11	TM12	TM13	TM14	TM15	TM16
La	19.4	18.4	na	18.4	19.3	18.3	18.8	19.9	20.1	24.3	21.0	20.8	20.2	20.7	20.6	20.2	20.5	20.5	20.3
Ce	38.7	37.2	na	37.3	38.5	36.8	37.9	40.7	40.8	48.7	42.8	42.7	41.2	42.4	42.2	40.9	42.0	42.2	41.4
Pr	4.8	4.6	na	4.7	4.8	4.6	4.7	5.3	5.3	6.0	5.5	5.4	5.2	5.4	5.4	5.1	5.3	5.3	5.3
Nd	21.4	20.5	na	20.6	21.2	20.4	21.0	23.9	23.5	25.9	24.9	24.7	23.7	24.6	24.0	23.9	24.1	24.1	24.1
Sm	5.5	5.2	na	5.2	5.4	5.1	5.4	6.1	6.0	6.1	6.1	6.2	6.0	6.1	6.0	5.9	6.2	6.1	6.1
Eu	1.9	1.8	na	1.8	1.9	1.8	1.8	2.1	2.1	2.2	2.3	2.3	2.1	2.1	2.0	2.2	2.1	2.1	2.1
Gd	5.9	5.7	na	5.7	5.8	5.5	5.5	6.0	5.8	6.1	6.3	6.2	5.9	6.2	6.1	5.8	6.0	6.0	5.9
Tb	0.8	0.8	na	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.7
Dy	4.5	4.4	na	4.3	4.5	4.3	4.4	4.1	4.0	4.1	4.2	4.2	4.0	4.1	4.0	4.0	4.1	4.0	4.1
Ho	0.8	0.8	na	0.7	0.8	0.8	0.8	0.7	0.6	0.7	0.7	0.7	0.6	0.7	0.7	0.6	0.7	0.6	0.6
Er	2.2	2.1	na	2.0	2.2	2.0	2.1	1.7	1.6	1.7	1.7	1.8	1.7	1.8	1.7	1.7	1.7	1.7	1.6
Tm	0.2	0.2	na	0.2	0.2	0.2	0.2	0.2	0.2	0.2	0.2	0.2	0.1	0.2	0.2	0.2	0.2	0.1	0.2
Yb	1.5	1.4	na	1.5	1.5	1.5	1.4	1.1	1.0	1.2	1.2	1.1	1.0	1.1	1.1	1.0	1.2	1.1	1.1
Lu	0.1	0.2	na	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1
Cs	0.5	0.2	na	0.3	0.3	0.4	0.4	0.3	0.3	0.9	0.3	0.3	0.3	0.3	0.5	0.5	0.2	0.5	0.4
Hf	3.9	3.7	na	3.9	3.8	3.8	3.8	4.1	4.0	4.5	4.1	4.0	3.9	4.0	4.0	4.0	3.9	3.9	4.0
Nb	39.4	6.4	na	39.0	9.5	43.8	41.9	12.9	11.9	57.8	37.9	8.4	13.3	10.9	13.3	9.3	4.9	8.9	10.7
Ta	14.5	3.2	na	10.3	2.5	8.7	8.6	2.4	2.6	8.3	9.6	2.3	2.5	2.8	3.0	2.0	1.8	1.7	2.1
Th	2.1	2.0	na	2.0	2.1	2.0	2.0	2.4	2.2	2.6	2.2	2.3	2.2	2.3	2.3	2.1	2.2	2.1	2.2
Y	22.1	20.9	na	20.9	22.2	20.5	21.0	18.0	17.9	18.8	19.0	19.1	18.0	18.5	18.3	17.8	18.3	18.6	18.1
Zr	168.2	162.9	na	168.9	167.9	166.4	167.8	182.3	177.8	210.8	185.4	184.8	181.1	184.2	181.0	178.4	177.7	183.7	180.5
Ba	397.1	266.3	na	272.7	394.5	360.5	268.8	377.7	380.5	488.8	439.3	439.5	291.1	331.1	293.0	652.3	331.0	332.8	323.9
Sc	20.4	20.3	na	20.5	20.1	19.9	20.0	16.6	15.5	19.6	17.9	17.0	16.5	16.2	16.4	15.6	16.4	15.9	15.8
Ni	248.9	310.0	na	317.1	266.1	229.7	231.1	245.0	201.4	193.4	230.7	230.0	218.0	210.7	203.5	193.4	184.7	244.9	232.4

). The LOI values are generally low for both Meahill and Yemsigi, ranging from 0.2 to 1.3 wt% for the Yemsigi basalts and from 0.09 to 2.8 wt% for the Meahill basalts (Figure 6). To minimize the effects of secondary alteration, the major element data was recalculated to anhydrous values for geochemical interpretation. Normalized geochemical data values are included in

Table A13. REE Chondrite-normalized values for the Yemsigi and Meahill basalt samples using normalization factors from McDonough and Sun (1995) [40].

Chondrite-Normalized
REE	MY1	MY4	MY6	MY7	MY8	MY10	TM3	TM4	TM7	TM8A	TM8B	TM9	TM11	TM12	TM13	TM14	TM15	TM16
Cs	2.41	1.10	1.61	1.68	2.36	2.37	1.46	1.57	4.90	1.74	1.69	1.44	1.45	2.73	2.50	1.19	2.87	2.33
Ba	164.77	110.50	113.15	163.69	149.59	111.54	156.72	157.88	202.82	182.28	182.37	120.79	137.39	121.58	270.66	137.34	138.09	134.40
Th	71.28	69.59	69.45	71.17	68.28	70.03	82.83	76.52	91.10	77.41	78.17	74.28	78.03	80.41	71.24	74.34	72.66	76.38
Nb	164.21	26.65	162.29	39.40	182.38	174.42	53.88	49.58	240.71	158.04	35.09	55.25	45.54	55.42	38.62	20.57	37.14	44.38
Ta	1064.71	237.87	760.29	180.22	637.13	633.38	179.56	192.94	608.60	702.79	166.54	184.93	205.37	218.60	144.63	134.49	122.65	154.19
La	81.94	77.72	77.68	81.27	77.17	79.11	84.14	84.81	102.36	88.40	87.59	85.15	87.38	86.96	85.27	86.33	86.62	85.44
Ce	63.18	60.69	60.80	62.82	60.00	61.86	66.46	66.59	79.40	69.89	69.69	67.19	69.09	68.87	66.67	68.50	68.79	67.46
Pr	51.97	49.64	50.32	51.42	49.83	50.43	56.73	56.89	64.62	58.80	58.69	55.84	57.72	58.04	55.48	57.49	56.95	56.86
Nd	46.74	44.77	45.14	46.35	44.57	45.84	52.19	51.31	56.67	54.42	53.98	51.82	53.92	52.45	52.28	52.65	52.65	52.71
Sm	37.18	35.35	34.89	36.74	34.65	36.47	41.33	40.76	41.35	40.90	41.74	40.54	41.35	40.47	40.03	41.71	41.20	40.91
Zr	44.03	42.64	44.21	43.95	43.56	43.93	47.72	46.54	55.18	48.53	48.38	47.41	48.22	47.38	46.70	46.52	48.09	47.25
Hf	38.34	35.82	38.12	37.06	36.69	36.96	40.12	38.75	43.83	39.53	39.21	37.52	39.29	38.68	38.73	37.73	38.27	39.04
Eu	34.01	32.84	32.06	33.59	32.59	32.59	38.10	36.91	38.65	40.44	40.04	37.62	38.03	35.95	39.41	37.46	37.89	37.44
Gd	29.75	28.44	28.60	29.00	27.47	27.83	29.95	29.38	30.46	31.81	31.04	29.49	30.97	30.62	29.32	30.12	30.20	29.89
Tb	22.57	21.52	21.53	22.37	21.76	21.66	21.34	21.46	21.87	22.61	21.86	21.26	21.71	21.47	21.09	21.22	21.47	20.59
Dy	18.47	17.84	17.62	18.23	17.52	18.03	16.71	16.45	16.74	17.06	16.92	16.17	16.85	16.43	16.26	16.75	16.40	16.49
Y	14.10	13.31	13.31	14.11	13.03	13.38	11.46	11.38	11.97	12.09	12.14	11.45	11.80	11.62	11.32	11.68	11.82	11.50
Ho	14.51	13.77	13.64	14.28	13.81	14.25	12.12	11.50	12.42	12.67	12.61	11.80	12.08	12.18	11.52	12.05	11.85	11.62
Er	13.73	13.15	12.75	13.51	12.45	12.95	10.53	10.30	10.54	10.84	11.06	10.73	11.09	10.79	10.57	10.73	10.77	9.98
Tm	9.36	8.81	8.68	9.04	8.70	8.62	6.16	6.79	7.05	6.76	6.61	6.06	6.61	7.10	6.20	6.45	5.78	6.10
Yb	9.55	8.87	9.39	9.22	9.01	8.57	6.77	6.37	7.27	7.27	6.71	6.40	6.61	7.00	6.48	7.22	6.57	6.86
Lu	5.97	6.46	5.72	5.77	5.27	5.43	3.77	3.77	3.53	4.07	4.04	3.48	3.84	3.77	3.75	4.07	3.37	3.09

,

Table A14. REE Primitive-Mantle-normalized values for the Yemsigi and Meahill basalt samples using normalization factors from Sun and McDonough (1989) [39].

Primitive Mantle-Normalized
REE	MY1	MY4	MY6	MY7	MY8	MY10	TM3	TM4	TM7	TM8A	TM8B	TM9	TM11	TM12	TM13	TM14	TM15	TM16
Cs	21.82	9.93	14.59	15.23	21.36	21.40	13.19	14.24	44.31	15.76	15.30	13.02	13.14	24.68	22.62	10.78	25.98	21.04
Ba	60.17	40.35	41.32	59.77	54.62	40.73	57.23	57.65	74.06	66.56	66.59	44.11	50.17	44.39	98.83	50.15	50.42	49.08
Th	26.00	25.38	25.33	25.96	24.91	25.55	30.21	27.91	33.23	28.24	28.52	27.09	28.47	29.33	25.99	27.12	26.50	27.86
Nb	59.89	9.72	59.19	14.37	66.52	63.62	19.65	18.09	87.80	57.64	12.80	20.15	16.61	20.21	14.09	7.50	13.55	16.19
Ta	391.35	87.43	279.46	66.24	234.19	232.81	66.00	70.92	223.70	258.32	61.22	67.97	75.49	80.35	53.16	49.43	45.08	56.68
La	29.97	28.43	28.41	29.72	28.23	28.94	30.77	31.02	37.44	32.33	32.04	31.14	31.96	31.81	31.19	31.57	31.68	31.25
Ce	23.12	22.21	22.25	22.99	21.96	22.64	24.32	24.37	29.06	25.58	25.50	24.59	25.28	25.21	24.40	25.07	25.18	24.69
Pr	18.99	18.14	18.39	18.79	18.20	18.43	20.73	20.78	23.61	21.48	21.44	20.40	21.09	21.20	20.27	21.00	20.81	20.78
Nd	17.09	16.37	16.50	16.94	16.30	16.76	19.08	18.76	20.72	19.90	19.74	18.94	19.71	19.18	19.11	19.25	19.25	19.27
Sm	13.55	12.89	12.72	13.39	12.63	13.30	15.07	14.86	15.07	14.91	15.21	14.78	15.07	14.75	14.59	15.20	15.02	14.91
Zr	16.02	15.51	16.09	15.99	15.85	15.98	17.36	16.93	20.08	17.66	17.60	17.25	17.54	17.24	16.99	16.92	17.50	17.19
Hf	13.95	13.04	13.87	13.49	13.35	13.45	14.60	14.10	15.95	14.39	14.27	13.66	14.30	14.08	14.10	13.73	13.93	14.21
Eu	12.44	12.01	11.72	12.28	11.92	11.92	13.93	13.49	14.13	14.79	14.64	13.75	13.90	13.14	14.41	13.69	13.85	13.69
Gd	10.88	10.40	10.46	10.61	10.05	10.18	10.96	10.75	11.14	11.64	11.35	10.79	11.33	11.20	10.73	11.02	11.05	10.94
Tb	8.23	7.85	7.85	8.16	7.94	7.90	7.78	7.83	7.97	8.25	7.97	7.75	7.92	7.83	7.69	7.74	7.83	7.51
Dy	6.74	6.51	6.43	6.65	6.39	6.58	6.10	6.00	6.11	6.23	6.18	5.90	6.15	6.00	5.93	6.11	5.99	6.02
Y	5.15	4.86	4.86	5.15	4.76	4.89	4.19	4.16	4.37	4.41	4.43	4.18	4.31	4.24	4.13	4.27	4.31	4.20
Ho	5.32	5.05	5.00	5.23	5.06	5.22	4.44	4.21	4.55	4.64	4.62	4.32	4.43	4.46	4.22	4.42	4.34	4.26
Er	5.02	4.80	4.66	4.94	4.55	4.73	3.84	3.76	3.85	3.96	4.04	3.92	4.05	3.94	3.86	3.92	3.93	3.65
Tm	3.40	3.20	3.15	3.29	3.16	3.13	2.24	2.47	2.56	2.45	2.40	2.20	2.40	2.58	2.25	2.34	2.10	2.22
Yb	3.49	3.24	3.43	3.37	3.29	3.13	2.47	2.32	2.66	2.66	2.45	2.34	2.41	2.56	2.37	2.63	2.40	2.50
Lu	2.18	2.36	2.08	2.10	1.92	1.98	1.37	1.38	1.29	1.48	1.47	1.27	1.40	1.37	1.37	1.48	1.23	1.13

,

Table A15. REE N-MORB-normalized values for the Yemsigi and Meahill basalt samples using normalization factors from White and Klein (2014) [38].

N-MORB-Normalized
REE	MY1	MY4	MY6	MY7	MY8	MY10	TM3	TM4	TM7	TM8A	TM8B	TM9	TM11	TM12	TM13	TM14	TM15	TM16
Cs	24.12	10.98	16.12	16.83	23.61	23.66	14.57	15.74	48.98	17.42	16.91	14.39	14.52	27.28	25.01	11.92	28.71	23.26
Ba	27.01	18.12	18.55	26.84	24.52	18.29	25.69	25.88	33.25	29.88	29.90	19.80	22.52	19.93	44.37	22.52	22.64	22.03
Th	11.11	10.85	10.83	11.10	10.65	10.92	12.91	11.93	14.20	12.07	12.19	11.58	12.17	12.54	11.11	11.59	11.33	11.91
Nb	16.91	2.75	16.72	4.06	18.79	17.97	5.55	5.11	24.79	16.28	3.61	5.69	4.69	5.71	3.98	2.12	3.83	4.57
Ta	75.81	16.94	54.14	12.83	45.37	45.10	12.79	13.74	43.34	50.04	11.86	13.17	14.62	15.57	10.30	9.58	8.73	10.98
La	5.73	5.43	5.43	5.68	5.40	5.53	5.88	5.93	7.16	6.18	6.12	5.95	6.11	6.08	5.96	6.04	6.06	5.97
Ce	3.80	3.65	3.65	3.78	3.61	3.72	3.99	4.00	4.77	4.20	4.19	4.04	4.15	4.14	4.01	4.12	4.13	4.05
Pr	2.77	2.65	2.68	2.74	2.66	2.69	3.03	3.03	3.45	3.14	3.13	2.98	3.08	3.10	2.96	3.07	3.04	3.03
Nd	2.32	2.22	2.24	2.30	2.21	2.27	2.59	2.54	2.81	2.70	2.68	2.57	2.67	2.60	2.59	2.61	2.61	2.61
Sm	1.73	1.64	1.62	1.70	1.61	1.69	1.92	1.89	1.92	1.90	1.94	1.88	1.92	1.88	1.86	1.94	1.91	1.90
Zr	1.89	1.83	1.90	1.89	1.87	1.89	2.05	2.00	2.37	2.09	2.08	2.04	2.07	2.04	2.01	2.00	2.07	2.03
Hf	1.67	1.56	1.66	1.62	1.60	1.61	1.75	1.69	1.91	1.73	1.71	1.64	1.71	1.69	1.69	1.65	1.67	1.70
Eu	1.67	1.61	1.57	1.64	1.60	1.60	1.87	1.81	1.89	1.98	1.96	1.84	1.86	1.76	1.93	1.83	1.85	1.83
Gd	1.38	1.32	1.33	1.35	1.27	1.29	1.39	1.36	1.41	1.48	1.44	1.37	1.44	1.42	1.36	1.40	1.40	1.39
Tb	1.01	0.96	0.96	1.00	0.97	0.97	0.95	0.96	0.97	1.01	0.97	0.95	0.97	0.96	0.94	0.95	0.96	0.92
Dy	0.86	0.83	0.82	0.85	0.82	0.84	0.78	0.77	0.78	0.80	0.79	0.76	0.79	0.77	0.76	0.78	0.77	0.77
Y	0.69	0.65	0.65	0.69	0.64	0.65	0.56	0.55	0.58	0.59	0.59	0.56	0.58	0.57	0.55	0.57	0.58	0.56
Ho	0.69	0.66	0.65	0.68	0.66	0.68	0.58	0.55	0.59	0.61	0.60	0.57	0.58	0.58	0.55	0.58	0.57	0.56
Er	0.66	0.64	0.62	0.65	0.60	0.63	0.51	0.50	0.51	0.52	0.53	0.52	0.54	0.52	0.51	0.52	0.52	0.48
Tm	0.47	0.44	0.44	0.46	0.44	0.43	0.31	0.34	0.36	0.34	0.33	0.31	0.33	0.36	0.31	0.32	0.29	0.31
Yb	0.48	0.45	0.47	0.46	0.45	0.43	0.34	0.32	0.37	0.37	0.34	0.32	0.33	0.35	0.33	0.36	0.33	0.35
Lu	0.30	0.32	0.29	0.29	0.26	0.27	0.19	0.19	0.18	0.20	0.20	0.17	0.19	0.19	0.19	0.20	0.17	0.16

,

Table A16. REE Chondrite-normalized values for the Central Highlands (Mackenzie and Johnson, 1982) [26] and Northeast Queensland (Georoc database) [45] using normalization factors from McDonough and Sun (1995) [40].

Chondrite-Normalized
Sample No.	5030E	5047B	5030G	0002D	0009A	4.00	0015B	samp. TOM	samp. LKD13	samp. CK1	samp. LKD2	samp. CK40	samp. LKD9	samp. CK19	samp. CK25
REE	Mt Murry (1)	Mt Murry (2)	Mt Murry (3)	Mt Bosavi (1)	Mt Bosavi (2)	Mt Bosavi (3)	Mt Bosavi (4)	NEQld (1)	NEQld (2)	NEQld (3)	NEQld (4)	NEQld (5)	NEQld (6)	NEQld (7)	NEQld (8)
Cs	0.95	2.42	1.89	3.68	2.26	0.84	3.79	2.53	3.99	6.43	2.48	1.99	4.45	1.93	1.26
Ba	0.00	0.00	0.00	0.00	0.00	0.00	0.00	314.11	324.48	241.08	224.48	310.37	197.10	203.32	96.68
Th	41.38	75.86	200.00	103.45	89.66	117.24	168.97	378.62	418.97	282.76	130.00	383.45	135.86	164.14	60.34
Nb	0.00	0.00	0.00	0.00	0.00	0.00	0.00	612.50	579.17	470.83	191.67	662.50	175.00	241.67	75.00
Ta	0.00	0.00	0.00	0.00	0.00	0.00	0.00	661.76	610.29	505.88	178.68	711.76	169.12	221.32	77.21
La	59.07	92.83	139.24	63.29	84.39	135.02	105.49	328.69	344.73	256.54	115.19	324.89	0.00	148.95	57.38
Ce	37.52	60.36	106.04	58.73	60.36	102.77	83.20	225.94	234.09	183.85	85.64	228.55	81.89	107.99	41.11
Pr	37.72	57.11	98.06	52.80	44.18	76.51	57.11	202.59	206.90	164.87	73.28	210.13	72.20	91.59	38.79
Nd	35.01	48.14	76.59	48.14	39.39	59.08	50.33	1.12	0.00	1.12	1.12	1.12	1.12	1.12	1.12
Sm	25.68	30.41	38.51	33.78	29.05	31.76	29.05	80.41	72.30	68.92	33.78	87.84	33.11	39.86	23.65
Zr	0.00	0.00	0.00	0.00	0.00	0.00	0.00	89.01	70.42	68.06	37.96	93.98	37.96	42.41	26.96
Hf	31.07	33.98	57.28	34.95	28.16	36.89	43.69	78.16	65.34	63.20	33.40	82.72	35.92	37.86	28.93
Eu	21.31	28.42	31.97	26.64	24.87	24.87	23.09	70.34	61.63	60.75	32.15	77.80	31.79	36.94	23.27
Gd	19.10	22.61	25.13	22.11	20.10	18.59	16.08	52.76	48.74	48.24	24.62	58.29	24.62	28.64	20.60
Tb	17.17	18.28	19.39	18.84	17.17	15.24	13.02	38.78	36.84	35.73	19.39	42.94	20.22	23.27	18.01
Dy	15.04	15.04	15.85	15.45	13.82	12.20	11.79	25.20	25.20	24.39	14.23	28.05	15.45	17.89	15.85
Y	0.00	0.00	0.00	0.00	0.00	0.00	0.00	18.15	19.49	18.98	11.59	20.45	12.48	15.22	15.03
Ho	13.74	14.65	13.00	13.00	12.27	12.09	10.99	18.50	19.60	19.05	11.72	21.25	13.00	15.20	14.10
Er	12.50	12.50	11.88	11.88	10.63	11.25	10.00	13.94	15.75	15.38	9.69	16.19	11.00	13.00	12.75
Tm	10.93	10.93	11.34	10.12	9.31	9.72	9.31	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Yb	11.18	11.18	13.04	9.94	9.32	9.94	9.94	8.45	10.43	10.37	7.14	10.00	8.45	9.69	9.94
Lu	0.00	0.00	0.00	0.00	0.00	0.00	0.00	6.91	9.35	8.94	6.50	8.54	7.72	8.94	9.35

,

Table A17. REE Primitive-Mantle-normalized values for the Central Highlands (Mackenzie and Johnson, 1982) [26] and Northeast Queensland (Georoc database) [45] using normalization factors from Sun and McDonough (1989) [39].

Primitive Mantle-Normalized
Sample No.	5030E	5047B	5030G	0002D	0009A	4.00	0015B	samp. TOM	samp. LKD13	samp. CK1	samp. LKD2	samp. CK40	samp. LKD9	samp. CK19	samp. CK25
REE	Mt Murry (1)	Mt Murry (2)	Mt Murry (3)	Mt Bosavi (1)	Mt Bosavi (2)	Mt Bosavi (3)	Mt Bosavi (4)	NEQld (1)	NEQld (2)	NEQld (3)	NEQld (4)	NEQld (5)	NEQld (6)	NEQld (7)	NEQld (8)
Cs	8.57	21.90	17.14	33.33	20.48	7.62	34.29	22.86	36.10	58.14	22.48	18.00	40.29	17.43	11.43
Ba	0.00	0.00	0.00	0.00	0.00	0.00	0.00	114.70	118.48	88.03	81.97	113.33	71.97	74.24	35.30
Th	15.09	27.67	72.96	37.74	32.70	42.77	61.64	138.11	152.83	103.14	47.42	139.87	49.56	59.87	22.01
Nb	0.00	0.00	0.00	0.00	0.00	0.00	0.00	223.40	211.25	171.73	69.91	241.64	63.83	88.15	27.36
Ta	0.00	0.00	0.00	0.00	0.00	0.00	0.00	243.24	224.32	185.95	65.68	261.62	62.16	81.35	28.38
La	21.60	33.95	50.93	23.15	30.86	49.38	38.58	120.22	126.08	93.83	42.13	118.83	0.00	54.48	20.99
Ce	13.73	22.09	38.81	21.49	22.09	37.61	30.45	82.69	85.67	67.28	31.34	83.64	29.97	39.52	15.04
Pr	13.78	20.87	35.83	19.29	16.14	27.95	20.87	74.02	75.59	60.24	26.77	76.77	26.38	33.46	14.17
Nd	12.80	17.60	28.00	17.60	14.40	21.60	18.40	0.41	0.00	0.41	0.41	0.41	0.41	0.41	0.41
Sm	9.36	11.08	14.04	12.32	10.59	11.58	10.59	29.31	26.35	25.12	12.32	32.02	12.07	14.53	8.62
Zr	0.00	0.00	0.00	0.00	0.00	0.00	0.00	32.38	25.62	24.76	13.81	34.19	13.81	15.43	9.81
Hf	11.31	12.37	20.85	12.72	10.25	13.43	15.90	28.45	23.78	23.00	12.16	30.11	13.07	13.78	10.53
Eu	7.79	10.39	11.69	9.74	9.09	9.09	8.44	25.71	22.53	22.21	11.75	28.44	11.62	13.51	8.51
Gd	6.99	8.27	9.19	8.09	7.35	6.80	5.88	19.30	17.83	17.65	9.01	21.32	9.01	10.48	7.54
Tb	6.26	6.67	7.07	6.87	6.26	5.56	4.75	14.14	13.43	13.03	7.07	15.66	7.37	8.48	6.57
Dy	5.49	5.49	5.79	5.64	5.04	4.45	4.30	9.20	9.20	8.90	5.19	10.24	5.64	6.53	5.79
Y	0.00	0.00	0.00	0.00	0.00	0.00	0.00	6.63	7.12	6.93	4.23	7.47	4.56	5.56	5.49
Ho	5.03	5.37	4.77	4.77	4.50	4.43	4.03	6.78	7.18	6.98	4.30	7.79	4.77	5.57	5.17
Er	4.57	4.57	4.34	4.34	3.88	4.11	3.65	5.09	5.75	5.62	3.54	5.91	4.02	4.75	4.66
Tm	3.97	3.97	4.12	3.68	3.38	3.53	3.38	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Yb	4.08	4.08	4.76	3.63	3.40	3.63	3.63	3.08	3.81	3.79	2.61	3.65	3.08	3.54	3.63
Lu	0.00	0.00	0.00	0.00	0.00	0.00	0.00	2.52	3.41	3.26	2.37	3.11	2.81	3.26	3.41

and

Table A18. REE N-MORB-normalized values for the Central Highlands (Mackenzie and Johnson, 1982) and Northeast Queensland (Georoc database) [45] using normalization factors from White and Klein (2014) [38].

N-MORB-Normalized
Sample No.	5030E	5047B	5030G	0002D	0009A	4.00	0015B	samp. TOM	samp. LKD13	samp. CK1	samp. LKD2	samp. CK40	samp. LKD9	samp. CK19	samp. CK25
REE	Mt Murry (1)	Mt Murry (2)	Mt Murry (3)	Mt Bosavi (1)	Mt Bosavi (2)	Mt Bosavi (3)	Mt Bosavi (4)	NEQld (1)	NEQld (2)	NEQld (3)	NEQld (4)	NEQld (5)	NEQld (6)	NEQld (7)	NEQld (8)
Cs	9.47	24.21	18.95	36.84	22.63	8.42	37.89	25.26	39.89	64.26	24.84	19.89	44.53	19.26	12.63
Ba	0.00	0.00	0.00	0.00	0.00	0.00	0.00	51.50	53.20	39.52	36.80	50.88	32.31	33.33	15.85
Th	6.45	11.83	31.18	16.13	13.98	18.28	26.34	59.03	65.32	44.09	20.27	59.78	21.18	25.59	9.41
Nb	0.00	0.00	0.00	0.00	0.00	0.00	0.00	63.09	59.66	48.50	19.74	68.24	18.03	24.89	7.73
Ta	0.00	0.00	0.00	0.00	0.00	0.00	0.00	47.12	43.46	36.02	12.72	50.68	12.04	15.76	5.50
La	4.13	6.49	9.73	4.42	5.90	9.44	7.37	22.98	24.10	17.94	8.05	22.71	0.00	10.41	4.01
Ce	2.25	3.63	6.37	3.53	3.63	6.18	5.00	13.58	14.07	11.05	5.15	13.74	4.92	6.49	2.47
Pr	2.01	3.05	5.23	2.82	2.36	4.08	3.05	10.80	11.03	8.79	3.91	11.21	3.85	4.89	2.07
Nd	1.74	2.39	3.80	2.39	1.95	2.93	2.49	0.06	0.00	0.06	0.06	0.06	0.06	0.06	0.06
Sm	1.19	1.41	1.79	1.57	1.35	1.47	1.35	3.73	3.35	3.20	1.57	4.08	1.54	1.85	1.10
Zr	0.00	0.00	0.00	0.00	0.00	0.00	0.00	3.83	3.03	2.93	1.63	4.04	1.63	1.82	1.16
Hf	1.36	1.48	2.50	1.53	1.23	1.61	1.91	3.41	2.85	2.76	1.46	3.61	1.57	1.65	1.26
Eu	1.04	1.39	1.57	1.30	1.22	1.22	1.13	3.44	3.02	2.97	1.57	3.81	1.56	1.81	1.14
Gd	0.89	1.05	1.17	1.03	0.93	0.86	0.75	2.45	2.26	2.24	1.14	2.70	1.14	1.33	0.96
Tb	0.77	0.81	0.86	0.84	0.77	0.68	0.58	1.73	1.64	1.59	0.86	1.91	0.90	1.04	0.80
Dy	0.70	0.70	0.74	0.72	0.65	0.57	0.55	1.18	1.18	1.14	0.66	1.31	0.72	0.83	0.74
Y	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.89	0.95	0.93	0.57	1.00	0.61	0.74	0.73
Ho	0.66	0.70	0.62	0.62	0.59	0.58	0.53	0.89	0.94	0.91	0.56	1.02	0.62	0.73	0.68
Er	0.60	0.60	0.57	0.57	0.51	0.54	0.48	0.67	0.76	0.74	0.47	0.78	0.53	0.63	0.62
Tm	0.55	0.55	0.57	0.51	0.47	0.49	0.47	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Yb	0.56	0.56	0.66	0.50	0.47	0.50	0.50	0.43	0.53	0.52	0.36	0.50	0.43	0.49	0.50
Lu	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.35	0.47	0.45	0.33	0.43	0.39	0.45	0.47

.

The major elements and trace element results are presented in the hacker variation diagram in Figure 7. The Meahill samples exhibit a wider compositional spread with SiO₂ ranging from 49.41 to 56.11 wt%. Na₂O varies from 0.66 to 3.47 wt%, and K₂O ranges from 0.73 to 2.08 wt%, giving total alkalis of 1.39 to 5.55 wt%. The MgO varies from 4.20 to 8.57 wt%, while the FeOt ranges from 9.45 to 11.07 wt%, Al₂O₃ from 14.52 to 15.24 wt%, and CaO from 8.59 to 9.63 wt%. The TiO₂ ranges from 2.16 to 2.44 wt%, while MnO (0.14 to 0.17 wt%) and P₂O₅ (0.55 to 0.86 wt%) remain low.

The Yemsigi basalts display relatively narrow major-oxide compositional ranges with SiO₂ from 47.91 to 48.85 wt%. The Na₂O ranges from 2.02 to 2.95 wt%, and K₂O ranges from 1.98 to 2.35 wt%, giving total alkalis a range of 4.00 to 5.30 wt%. The MgO ranges from 8.60 to 8.93 wt%, while FeOt ranges from 10.12 to 11.21 wt%, Al₂O₃ from 13.71 to 16.85 wt%, and CaO from 8.67 to 10.47 wt%. The TiO₂ ranges from 2.35 to 2.66 wt%, while MnO (0.15 to 0.32 wt%) and P₂O₅ (0.52 to 0.76 wt%) are consistently low. Among the trace elements measured, Ni, Cr, Sr, and Nb show pronounced decreases with increasing SiO₂, while Zr concentrations remain low. Ba concentrations are elevated in both suites, with the Meahill basalts displaying much higher Ba concentration.

On the total alkali-silica (TAS) diagram of (Figure 8a), both the Meahill and Yemsigi basalt samples plot within the basalt field and fall predominantly in the alkaline field. On the potassium oxide versus silica (K₂O versus SiO₂) diagram (Figure 8b), the samples plot within the medium- to high-potassium calc-alkaline field. The Meahill samples show slightly greater dispersion, whereas the Yemsigi samples cluster more tightly.

Figure 9 shows chondrite-normalized (Figure 9a), Primitive Mantle-normalized (Figure 9b), and N-MORB-normalized (Figure 9c) REE spider diagrams for Meahill and Yemsigi basalts. Both basalt groups exhibit broadly similar patterns. Notably, Nb exhibits both positive and negative anomalies relative to Ta. Mt Bosavi and Mt Murry sample data of the Fly Highlands Quaternary volcanics (MacKenzie & Johnson, 1984) [26] are plotted for comparison.

The Nb/Yb versus Th/Yb diagram (Figure 10) shows that the Meahill and Yemsigi basalt samples plot within the MORB-OIB mantle array, between the N-MORB, E-MORB, and OIB compositional fields. Most samples are characterized by relatively high Nb/Yb ratios and moderate Th/Yb ratios, indicating derivation from an enriched mantle source with affinities to E-MORB- and OIB-type magmatism. The samples do not plot within the volcanic arc array above the mantle array, suggesting that subduction-related fluid enrichment was not a significant factor in magma genesis. Instead, their elevated Th/Yb values relative to N-MORB indicate varying degrees of mantle enrichment and possible contributions from deep crustal recycling processes, as indicated by the upward trend toward higher Th/Yb ratios. The clustering of most samples within the OIB–E-MORB field suggests that Meahill and Yemsigi magmas were generated from a heterogeneous enriched asthenospheric mantle source, with only minor evidence of magma-crust interaction during ascent and emplacement.

The tectonic setting of the magmas can also be inferred using the immobile trace element discrimination diagrams in Figure 11, after Saccani (2015) [43] and Pearce and Gale (1977) [44] in Rollinson and Pease (2021) [42]. Meahill and Yemsigi samples fall within the Within-Plate Basalt field (Figure 11a), plot adjacent to the Convergent Plate Margin field, and generally plot within the Divergent Plate Setting field corresponding to non-subduction oceanic and rift-margin environments (Figure 11b).

Volcanic samples from the PNG Central Highlands (Mt Murry, Mt Hagen, Mt Bosavi) in the Fold and Thrust Belt, along with those from the active volcanic arc (Rabaul), and samples from Northeast Queensland taken from the Georoc database [45] are also plotted for comparison. Due to incomplete data sets, some samples are not shown on Figure 11b.

5.4. KAr Age Dating

The Meahill basalt (TM7) yielded an age of 0.57 ± 0.09 Ma from the K–Ar dating (

Table 2. KAr Results.

Sample No#	Particle Size	Potassium Content (wt%)	Radioactive Origin 40ar (10-8cc STP/g)	K-Ar Age (Ma)	Non-Radioactive Origin 40Ar (%)
TM5	Feldspar (51–73 µm)	1.654 ± 0.033	3.6439 ± 0.5656	0.5676 ± 0.0888	90.5

). This suggests that the timing of the magmatic activity at Meahill took place in the Middle Pleistocene.

6. Discussion

6.1. Volcanism and Magmatic Evolution

Meahill and Yemsigi basalts exhibit similar magma composition yet record different emplacement styles (Figure 12). At Yemsigi, basaltic rocks occur as massive, dense blocks with relatively low vesicularity and pervasive post-magmatic alteration. Extensive replacement of olivine by iddingsite reflects prolonged fluid-rock interaction after emplacement under low-temperature conditions (e.g., Eggleton, 1984) [46]. The coarse porphyritic texture suggests relatively slow cooling and progressive degassing, compatible with emplacement as subaerial lava flows or shallow intrusive bodies (Gill, 2010; Burchardt, 2018) [47,48].

At Meahill, the presence of fine volcanic tuffs containing basaltic clasts and vesicular basalts (Figure 3d–f) confirms explosive volcanism. Abundant vesicles, fractured olivine crystals, and disequilibrium textures (Figure 4d–f) are evidence of the syn-eruptive degassing and fragmentation that occurs when magma interacts with water (e.g., Lunge et al. 2026) [24].

The mineral compositions record the magmatic history of the rocks (Gill, 2010; Burchardt, 2018) [47,48]. Olivine compositions reflect crystallization from relatively primitive magmas (reaching >Fo80 for both samples suites), followed by progressive evolution toward more fractionated compositions evident from pyroxene and plagioclase feldspars (e.g., Roeder & Emslie, 1970; Kushiro & Walter, 1998; Rasmussen, et al., 2020; Namur et al., 2010; Filiberto & Dasgupta, 2011; Latypov et al., 2020) [49,50,51,52,53,54]. Titanomagnetite compositions reflect moderate oxidation conditions during magmatic differentiation and lie along the magnetiteulvöspinel solid-solution series (Buddington and Lindsley, 1964) [35].

6.2. Geochemical Constraints on Magma Source and Tectonic Setting

High field strength elements (HFSEs) are generally considered immobile during alteration, making them reliable indicators for interpreting magma sources (Rollinson & Pease, 2021) [49]. The geochemical signatures, particularly rare earth element (REE) anomalies such as Eu and Nd, provide important insights into magmatic differentiation processes and magma source characteristics. Europium (Eu) is unique among the REEs because it can occur in both divalent (Eu²⁺) and trivalent (Eu³⁺) oxidation states, whereas most other REEs are typically trivalent. This dual valence allows Eu to behave differently during magmatic processes. A negative Eu anomaly is commonly interpreted as evidence of plagioclase fractionation during basaltic magma crystallization. Under mildly reducing conditions, Eu²⁺ becomes stable and behaves similarly to Ca²⁺, allowing it to substitute readily into the crystal structure of plagioclase feldspar. As plagioclase crystallizes and is removed from the melt, it preferentially incorporates Eu²⁺, leaving the residual melt depleted in Eu and producing a negative Eu anomaly in the REE pattern. However, in the samples studied, both suites exhibit weak positive Eu anomalies, suggesting that plagioclase fractionation was not a dominant process during magma differentiation and instead played only a minor role.

Meanwhile, Nd typically behaves as Ce in subduction-related volcanic systems (Bellot et al., 2018) [55]. It is generally considered an incompatible element in basaltic systems, remaining preferentially enriched in the melt; therefore, a negative Nd anomaly is unusual and suggests more complex petrogenetic processes. One possible explanation is that the mantle source experienced prior episodes of partial melting, resulting in a residually depleted source with reduced Nd content. Alternatively, assimilation of crustal material during magma ascent may have influenced the REE signature, particularly if the assimilated components were relatively Nd-depleted or exhibited negative Nd anomalies, thereby modifying the overall REE pattern of the of the final rock.

In contrast, both the Meahill and Yemsigi samples exhibit a distinct positive Ta anomaly coupled with a negative Nb anomaly. This decoupling of the normally coherent high field strength elements (HFSEs) is unusual, as Nb and Ta typically behave similarly during mantle melting and fractional crystallization. The observed relative enrichment of Ta over Nb suggests source-related processes or selective fractionation, potentially involving Ti-bearing oxide phases (particularly rutile), crustal contamination, or metasomatic modification of the mantle source. The systematic NbTa decoupling, expressed by variable negative Nb anomalies and consistently positive Ta anomalies, further indicates high-pressure partial melting in the presence of residual rutile, which preferentially retains Nb relative to Ta (Foley et al., 2000; Kamber et al., 2003; Schmidt et al., 2004; Klemme et al., 2005) [56,57,58,59]. This HFSE fractionation pattern implies the involvement of recycled lithospheric or crustal components in the mantle source, which is consistent with a modified or metasomatized mantle domain rather than a simple depleted MORB mantle.

This geochemical interpretation is also supported by the Nb/Yb versus Th/Yb classification diagram in Figure 10. The Meahill and Yemsigi samples plot above and along with the MORB-OIB mantle array, indicating deviation from a simple depleted mantle source and enrichment in incompatible elements. Their positions between the E-MORB and OIB fields reflect derivation from an enriched mantle source. The slight elevation in Th/Yb relative to Nb/Yb further suggests the addition of a subduction-modified or crustal component, consistent with limited magma-crust interaction during ascent.

When compared to the Central Highlands volcanism (e.g., Mount Bosavi and Mount Murray), they show weaker arc signatures and lack the pronounced HFSE depletion typical of active subduction systems. However, when compared to Northeastern Queensland, the samples consistently plot together in the intra-plate basalt field or within-plate basalts field (refer to Figure 11).

The mantle structure beneath the region provides additional insight into the origin of the magma source (Figure 13). A regional seismic tomography study by Hall and Spakman (2002) [60] identified several mantle anomalies, with both the Fly Platform and northeastern Queensland magmatism spatially coinciding with the same lower-mantle anomaly (designated A8). This anomaly was interpreted as a fragment of an older subducted slab that has continued to sink into the mantle. The presence of this slab fragment may have influenced regional mantle flow patterns, generating localized zones of mantle upwelling capable of sustaining intraplate basaltic volcanism.

6.3. Tectono-Magmatic Implications

The Middle Pleistocene age (0.57 ± 0.09 Ma) obtained from KAr dating of a single Meahill basalt sample is insufficient to constrain the full duration and evolution of volcanism in the Meahill–Yemsigi area. Nevertheless, it provides an important temporal constraint, demonstrating that this volcanic episode occurred during the Middle Pleistocene and clearly postdated both the Miocene magmatism of the Maramuni Arc and the PliocenePleistocene volcanism of the Fly Highlands (Mackenzie and Johnson, 1984; Holm et al., 2016) [1,26].

Lithospheric thinning related to crustal delamination beneath the Central Highlands may have provided an additional source of mantle melting. From approximately 6 Ma to the present, dense lower crust is inferred to have detached and sunk into the mantle, inducing adiabatic mantle upwelling and partial melting (Johnson et al., 1978; Mackenzie and Johnson, 1984; Hamilton et al., 1983) [25,26,27]. This process has been linked to renewed magmatism throughout the Papuan Fold and Thrust Belt and to the development of major magmatic–hydrothermal systems (Holm et al., 2015; Holm et al., 2019) [10,12].

The tectonic evolution model presented in Figure 14 is adapted from Johnson, et al., (1978) [25] and explains the evolution of the Central Highlands from Eocene to the Fly Highlands and Fly Platform in the Pliocene to Pleistocene.

(A) Eocene Melanesian Trench subduction (~45 Ma). Southward subduction of the Pacific oceanic lithosphere beneath northern Australian continental margin-initiated arc magmatism along the Melanesian Arc. Collision of the Ontong Java Plateau with the trench at approximately 26 Ma terminated subduction and triggered a major metallogenic event, resulting in the formation of porphyry Cu–Au and epithermal deposits between 24 and 20 Ma, including Simuku, Plesyumi, Mt Nakru, and Esis in New Britain (Holm et al., 2019) [12].

(B) Middle Miocene subduction at the Pocklington Trough (~15 Ma). Renewed subduction along the Pocklington Trough led to the development of the Maramuni Arc. Subsequent collision of the Australian continental margin at approximately 12 Ma caused crustal thickening and regional uplift. Magmatism associated with this event generated Cu–Au porphyry deposits throughout the New Guinea Highlands and epithermal gold systems across the Papuan Peninsula between approximately 12 and 6 Ma.

(C) Late Miocene–Pleistocene Central Highlands magmatism (6 Ma to Pleistocene). Following cessation of subduction beneath the Central Highlands, lower-crustal delamination resulted in the detachment and sinking of dense crustal material into the mantle, promoting adiabatic mantle melting (Cloos et al., 2005) [62]. This process generated widespread volcanism, including Mt Bosavi, Mt Murray, Mt Hagen, and Mt Giluwe, and contributed to the formation of major porphyry and epithermal deposits, such as Ok Tedi, Porgera, and Crater Mountain, within the Papuan Fold and Thrust Belt.

(D) Middle Pleistocene Fly volcanism (~0.57 Ma). The relatively young age of the Meahill volcanism reflects localized mantle processes beneath the Fly Platform, potentially associated with tectonic reorganization following Coral Sea spreading and regional lithospheric extension (Bulois et al., 2017) [11]. One possible mechanism involves small-scale asthenospheric upwelling along the margins of a deeper mantle anomaly (Hall and Spakman, 2002) [60]. Seismic tomographic models identify a deep high-velocity zone beneath the Coral Sea–Fly Platform region that has been interpreted as older subducted slab material (Zhou et al., 2023; Baques et al., 2023) [63,64]. These slab remnants may have promoted edge-driven convection and localized decompression melting within the overlying mantle lithosphere, generating small-volume alkaline magmatism.

7. Conclusions

This study documents the first integrated field observations combined with analytical studies of two distinct volcanic centers on the Fly Platform in Papua New Guinea. Field observations reveal distinct differences in eruptive style and volcanic facies between the two centers. The Yemsigi is dominated by coherent basalts exhibiting porphyritic textures, in contrast, Meahill comprises highly vesicular basalts with textures ranging from porphyritic to micro-porphyritic. Yemsigi and Meahill basalts have similar mineralogical compositions dominated by olivine + clinopyroxene + plagioclase ± titanomagnetite. Field and petrographic evidence suggests more explosive volcanism at Meahill and much quieter volcanism at Yemsigi.

Olivine compositions (Fo62-86) support crystallization from Mg-rich magmas or Primitive-Mantle-derived melts. The compositions of clinopyroxene, plagioclase, and titanomagnetite suggest that magma evolution was primarily controlled by fractional crystallization. However, particularly at Meahill, magma recharge, volatile exsolution, and shallow crustal processes modified this evolutionary trend, producing localized disequilibrium crystallization and more explosive eruptive behavior.

Whole-rock major, trace, and rare earth element geochemistry indicates derivation from an enriched, metasomatized mantle source. Enrichment in incompatible trace elements, REE patterns, and HFSE, including NbTa behavior and residual-rutile signatures, indicate partial melting of a heterogeneous mantle source inherited from earlier subduction-related metasomatism. Weak Eu anomalies suggest only limited plagioclase fractionation. Geochemical discrimination diagrams classify the basalts as within-plate compositions with affinities ranging from E-MORB to OIB, comparable to basaltic provinces of northeastern Queensland. These characteristics indicate magma generation through localized decompression melting of enriched lithospheric or asthenospheric mantle rather than active subduction-zone magmatism.

Meahill volcanism occurred during the Middle Pleistocene from the K–Ar age of 0.57 ± 0.09 Ma, which confirms that and clearly postdates both the Miocene Maramuni Arc and Pliocene–Pleistocene Central Highlands magmatism. The Fly Platform basalts represent a late-stage, intraplate volcanic episode sourced from a modified mantle source influenced by long-lived subduction inheritance and deep mantle dynamics beneath the region.