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Article

Styles of Alteration of Ti Oxides of the Kimberlite Groundmass: Implications on the Petrogenesis and Classification of Kimberlites and Similar Rocks

by
Jingyao Xu
1,*,
Joan Carles Melgarejo
1 and
Montgarri Castillo-Oliver
2
1
Department of Mineralogy, Petrology and Applied Geology, Faculty of Earth Sciences, University of Barcelona, 08028 Barcelona, Spain
2
ARC Centre of Excellence for Core to Crust Fluid Systems and GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2019, Australia
*
Author to whom correspondence should be addressed.
Minerals 2018, 8(2), 51; https://doi.org/10.3390/min8020051
Submission received: 27 November 2017 / Revised: 22 January 2018 / Accepted: 1 February 2018 / Published: 6 February 2018
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Abstract

The sequence of replacement in groundmass perovskite and spinel from SK-1 and SK-2 kimberlites (Eastern Dharwar craton, India) has been established. Two types of perovskite occur in the studied Indian kimberlites. Type 1 perovskite is found in the groundmass, crystallized directly from the kimberlite magma, it is light rare-earth elements (LREE)-rich and Fe-poor and its ΔNNO calculated value is from −3.82 to −0.73. The second generation of perovskite (type 2 perovskite) is found replacing groundmass atoll spinel, it was formed from hydrothermal fluids, it is LREE-free and Fe-rich and has very high ΔNNO value (from 1.03 to 10.52). Type 1 groundmass perovskite may be either replaced by anatase or kassite along with aeschynite-(Ce). These differences in the alteration are related to different f(CO2) and f(H2O) conditions. Furthermore, primary perovskite may be strongly altered to secondary minerals, resulting in redistribution of rare-earth elements (REE) and, potentially, U, Pb and Th. Therefore, accurate petrographic and chemical analyses are necessary in order to demonstrate that perovskite is magmatic before proceeding to sort geochronological data by using perovskite. Ti-rich hydrogarnets (12.9 wt %–26.3 wt % TiO2) were produced during spinel replacement by late hydrothermal processes. Therefore, attention must be paid to the position of Ca-Ti-garnets in the mineral sequence and their water content before using them to classify the rock based on their occurrence.

1. Introduction

Ti-rich minerals from kimberlite such as ilmenite, spinels, rutile and perovskite are important carriers of petrogenetic information. Xenocrystic Ti-rich oxides in kimberlitic rock, such as Cr-rich rutile, Ti-rich spinel and ilmenite, provide information about the metasomatic processes in the cratonic lithospheric mantle [1,2]. In addition, Ti-rich oxides from kimberlite groundmass, such as Ti-rich spinel [3] and perovskite [4], could supply information about the evolution of kimberlitic magmas. Finally, perovskite [5,6] and rutile [7,8] could also be used to determine the kimberlite emplacement age. However, in many cases these minerals undergo complex alteration processes during the hydrothermal or supergene late stages of the kimberlite crystallization sequence that could disturb the petrogenetic interpretations based on geochemical data.
Perovskite is a principal host of LREE in SiO2-undersaturated ultramafic and alkaline rocks [9] and is a mineral that may be produced along different crystallization stages of these magmas. Therefore, the chemical and textural study of these generations can provide information about several stages of magmatic crystallization [10,11]. The crystallization of magmatic perovskite is produced later than that of macrocrystal spinel; and simultaneously with “reaction” Fe-rich spinel and groundmass spinels of the magnesian ulvöspinel—magnetite series in kimberlite [12].
However, perovskite may be unstable in CO2-rich fluid environments characteristic of the final stage of some carbonatites and kimberlites worldwide [12,13,14]. Products of perovskite replacement in kimberlites may include kassite, anatase and titanite along with calcite, ilmenite and unidentified LREE-Ti oxides [13,15]. Similarly, carbonatitic perovskite is replaced by anatase with calcite and finally by ilmenite and ancylite [13]. In consequence, only pristine unaltered perovskite has been used to establish U-Pb age [16,17,18] and Sr-Nd-Pb isotopic composition [19,20,21] from different worldwide kimberlites.
However, perovskite cannot be formed only by magmatic processes. A metasomatic origin has been inferred for perovskite from carbonatites [22,23] and perovskite from skarns is also widely described as a hydrothermal product [9].
Primary Ti-rich garnet is considered as a key mineral for the classification of ultramafic lamprophyres, such as aillikites [24] and orangeites [25]. Ti-rich garnets classified as andradite, schorlomite, zirconian schorlomite and kimzeyite occur in the Torngat aillikite dykes [24]; kimzeyite and Ti-andradite from Aillik Bay are considered as primary magmatic minerals and are indicators of the aillikitic affinity of these rocks [26]. A primary magmatic origin has also been suggested for the Ca-Ti-Fe rich garnets found in orangeite from Swartruggens [27]. However, Ti-rich hydrogarnets can be produced by subsolidus reactions in a wide span of environments, as in metapyroxenites [28], serpentinites [29], basalts from the oceanic seafloor [30] and magmatic alkaline rocks and carbonatites [31].
Therefore, the Ti-rich minerals can supply information of petrological or economic interest. However, these minerals can be easily altered by subsolidus processes. In addition, the hydrothermal occurrences of these minerals in other geological environments suggest that these minerals can be produced by the reaction of the existing magmatic minerals with late hydrothermal or supergene fluids. In the present work, we describe the different alteration styles of groundmass Ti-rich oxide (perovskite and spinel) from SK-1 and SK-2 kimberlites (Eastern Dharwar craton, India), including the neoformation of pristine perovskite and Ti-rich hydrogarnets by subsolidus processes. The petrogenetic implications are discussed.

2. Geological Setting

Kimberlites occur in the Bastar craton (central India) and the Eastern Dharwar craton (EDC, southern India) in India. Kimberlite intrusions (around 100 bodies) found in the EDC of southern India are distributed in three fields: (1) the southern Wajrakarur kimberlite field; (2) the northern Narayanpet kimberlite field; and (3) the Raichur kimberlite field (RKF) [17,32]. This work studied two sequences of alteration in samples from two nearby pipes (SK-1 and SK-2, about 1100 m of distance) from Siddanpalli, RKF, EDC, southern India (Figure 1). Emplacement of these two kimberlites took place during the Mesoproterozoic around 1.1 Ga [17].

3. Methods

Minerals have been identified in situ on thin polished section by SEM-BSE-EDS, EMPA and Raman spectroscopy at the Scientific and Technological Centers of the University of Barcelona (CCiTUB), Barcelona, Spain.
Petrographic and textural studies were carried out using optical and scanning electron microscopy (SEM), the latter employing an E-SEM-Quanta 200 FEI-XTE-325/D8395 (FEI, Hillsboro, OR, USA) with a BSE detector and coupled to a Genesis EDS microanalysis system. The operating conditions were 20–25 kV, 1 nA beam current and 10 mm distance to detector.
The compositional study was carried out using an electron microprobe (EMPA) JEOL JXA-8230 (JEOL, Tokyo, Japan), equipped with five wavelength dispersive spectrometers (WDS) and an energy dispersive spectrometer (EDS), at the CCiTUB. The operating conditions were: accelerating voltage of 20 kV combined with a beam current 15 nA. Calibration standards and analytical crystals used for the analyses of perovskite and kassite were: wollastonite (Si, TAP, Kα), corundum (Al, TAP, Kα), rutile (Ti, PETJ, Kα), CeO2 (Ce, PETJ, Lα), LaB6 (La, PETJ, Lα), periclase (Mg, TAPH, Kα), albite (Na, TAPH, Kα), Ta (Ta, LIFH, Lα), Fe2O3 (Fe, LIFH, Kα), rhodonite (Mn, LIFH, Kα), REE-1 (Pr, LIFH, Lb), REE-4 (Nd, LIFH, Lα), barite (Ba, LIFH, Lα), wollastonite (Ca, PETL, Kα), orthoclase (K2O, PETL, Kα), Nb (Nb, PETL, Lα), ZrSiO4 (Zr, PETL, Lα), celestine (Sr, PETL, Lα).
Calibration standards and analytical crystals used for the analyses of aeschynite were the following: celestine (Sr, PETJ, Lα), YAG (Y2O3, PETJ, Lα), ZrO2 (Zr, PETJ, Lα), Nb (Nb, PETJ, Lα), wollastonite (Ca, PETJ, Kα), UO2 (U, PETJ, Mb), ThO2 (Th, PETJ, Mα), albite (Na, TAPH, Kα), periclase (Mg, TAPH, Kα), corundum (Al, TAPH, Kα), wollastonite (Si, TAPH, Kα), fluorite (F, TAPH, Kα), barite (Ba, LIFH, Lα), rutile (Ti, LIFH, Kα), LaB6 (La, LIFH, Lα), CeO2 (Ce, LIFH, Lα), REE-4 (Nd, LIFH, Lb), Cr2O3 (Cr, LIFH, Kα), REE-1 (Pr, LIFH, Lb), rhodonite (Mn, LIFH, Kα), REE-3 (Sm, LIFL, Lb), Fe2O3 (Fe, LIFL, Kα), REE-3 (Gd, LIFL, Lb), REE-1 (Er, LIFL, Lα), REE-1 (Dy, LIFL, Lb), Hf (Hf, LIFL, Lα), Ta (Ta, LIFL, Lα), PbS (Pb, LIFL, Lα).
Calibration standards and analytical crystals used for the analyses of Ti-rich garnets were: wollastonite (Si, TAP, Kα), corundum (Al, TAP, Kα), Cr2O3 (Cr, PETJ, Kα), rutile (Ti, PETJ, Kα), periclase (Mg, TAPH, Kα), albite (Na, TAPH, Kα), barite (Ba, LIFH, Lα), rhodonite (Mn, LIFH, Kα), Fe2O3 (Fe, LIFH, Kα), celestine (Sr, PETL, Lα), ZrSiO4 (Zr, PETL, Lα), orthoclase (K2O, PETL, Kα), wollastonite (Ca, PETL, Kα).
Micro Raman analyses were obtained at the CCiTUB by using a HORIBA Jobin Yvon LabRam HR 800 dispersive spectrometer (HORIBA, Kyoto, Japan), equipped with an Olympus BXFM optical microscope. Non-polarized Raman spectra were obtained by applying a 532 nm laser, the pixel size was 1 µm. The exposure time was 5 s with 3 scans and laser power at sample was 2.5 mW for anatase. The exposure time was 5 s with 10 scans and laser power at sample was 5 mW for kassite. The exposure time was 20 s with 5 scans and laser power at sample was 1.25 mW for garnet.

4. Results

4.1. Mineral Textures

Both kimberlites have similar xenocrysts, mainly consisting of Ti oxides such as rutile and ilmenite, scattered in a fine-grained groundmass. Primary groundmass minerals are also similar in both kimberlites and the dominant minerals include calcite, apatite, perovskite and altered atoll-shaped spinel-group minerals. The abundance of minerals of the serpentine group in the groundmass suggests that olivine was also common in these kimberlites.
Groundmass perovskite (type 1 perovskite) from SK-1 is euhedral, zoned and slightly replaced by anatase following grain borders and small cracks (Figure 2a). Ti-rich spinel-group minerals in groundmass are strongly altered to serpentine, calcite, magnetite and cryptocrystalline Ti-rich andradite (Figure 2b,c).
Groundmass perovskite (type 1 perovskite) from SK-2 is euhedral to subhedral and it has oscillatory zoning. This perovskite is partially or nearly totally replaced by kassite [CaTi2O4(OH)2] accompanied by abundant aeschynite-(Ce), ideally [(Ce,Ca,Fe,Th)(Ti,Nb)2(O,OH)6] (Figure 2d–f). The ensemble may also be replaced by Mn-rich ilmenite along small cracks (Figure 2f). In addition, a second perovskite generation (type 2 perovskite, euhedral) replaces the groundmass atoll spinel along with Ti-rich hydrogarnets, calcite and serpentine (Figure 2g,h). Type 2 perovskite often shows a geode-like texture into the pseudomorphized atoll spinel (Figure 2h). Ilmenite and magnetite xenocrysts from SK-2 may also be replaced by a sequence of typical groundmass minerals and sometimes by type 2 perovskite (Figure 2i,j); on its turn, this type 2 perovskite may be replaced by anatase.

4.2. MicroRaman Study

Identity of anatase was confirmed by microRaman spectroscopy and its spectrum was compared with that of submicroscopic anatase, mixed with calcite, altering perovskite in carbonatites [14,15] (Figure 3a). Calcite has a peak at 1088 cm−1 which corresponds to the vibration of the CO3 group [15]. However, calcite is absent from the products of alteration of the perovskite from Siddanpalli.
Kassite [CaTi2O4(OH)2] has a very similar chemical composition to cafetite (CaTi2O5·H2O). The identification of kassite from the SK-2 kimberlite was confirmed by comparing the kassite Raman spectrum with the available spectrum of kassite standards [15] (Figure 3b).
Raman spectra of Ti-rich garnet from SK-2 kimberlite is also recorded (Figure 3c) and shows a peak at 3576 cm−1 which corresponds to the OH vibration [30]. The spectrum was compared with those of other hydrogarnets [33,34].
The microRaman study also confirmed the identification of the second generation of perovskite (type 2) by comparison with the spectrum of the magmatic perovskite (type 1) (Figure 3d). Minor differences in the position and intensity of the bands can also be related to changes in the chemical composition or to different orientation.
Finally, the Raman analysis was helpful to establish the identity of aeschynite-(Ce). In this case, most of the bands have similarities with those from the published standards of members of the aeschynite group. Minor differences in the positions of the bands can be explained because the studied aeschynite is Ta-poor and La- and Nd-rich when compared with the standard aeschynites [35,36].

4.3. Mineral Chemistry

Spinel-group minerals from groundmass of both kimberlites have compositions in the ulvöspinel-titanomagnetite domain. However, they are strongly altered and the restitic cores may be seldom enriched in Zn (0.1 wt %–2.0 wt % ZnO in SK-1 and 0 wt %–7.5 wt % ZnO in SK-2).
Compositional trends of type 1 groundmass perovskite are similar for both kimberlites (Table 1, Figure 4). Hence, the cores of the perovskite crystals from SK-1 are slightly enriched in REE (4.9 wt %–8.4 wt % ∑LREE2O3), whereas their rims have only 1.9 wt %–3.7 wt % ∑LREE2O3; similarly, perovskite crystals from SK-2 have 4.0 wt %–5.5 wt % ∑LREE2O3 in the centres and 0.9 wt %–2.6 wt % ∑LREE2O3 in the borders. Nb is also slightly enriched in the cores compared to the rims (0.6 wt %–1.1 wt % Nb2O5 in the cores and 0.4 wt %–0.6 wt % Nb2O5 in the rims in SK-1; 0.5 wt %–0.8 wt % Nb2O5 in the cores and 0.4 wt %–0.6 wt % Nb2O5 in the rims in SK-2). Type 1 perovskite in both kimberlites has Fe2O3 contents ranging 1.0 wt %–1.7 wt % (Table 1, Figure 4).
Type 2 perovskite from the SK-2 kimberlite has different composition than type 1 perovskite. It is depleted in LREE (<1 wt % ∑LREE2O3) and Nb (<0.1 wt % Nb2O5) but it has higher Fe contents (2.0 wt %–4.0 wt % Fe2O3) than type 1 perovskite (Figure 4, Table 1).
Frequency histograms of log fO2 expressed relative to the NNO buffer (ΔNNO) calculated using the perovskite oxybarometer developed by Bellis and Canil [4] are shown in Figure 5. Type 1 perovskite in SK-1 and SK-2 has similar ΔNNO value (from −0.73 to −3.40 in SK-1 and from −1.07 to −3.82 in SK-2) but type 2 perovskite has very high ΔNNO value (from 1.03 to 10.52) and was formed in a highly oxidizing environment.
Kassite replacing type 1 perovskite in SK-2 has a stoichiometric composition (Table 1), whereas the associated aeschynite-(Ce) tends to concentrate LREE and, to a lesser extent, Nb. The aeschynite-(Ce) produces the next average structural formula: (Ca0.39Ce0.33La0.13Nd0.12Pr0.04)∑1.01(Ti1.82Nb0.07Fe3+0.02Zr0.02)∑1.93(O,OH)6 (Table 1). Y, Er, Dy and Pb contents in aeschynite-(Ce) have been analysed but they are below detection limit. Therefore, the aeschynite-group minerals from SK-2 kimberlite are poor in Nb, U and Th when compared to similar minerals typically occurring as metamictic phases in carbonatites [37] and metasomatised rocks [38]. However, their compositions are similar to those of the late Ti-REE minerals described in the Iron Mountain kimberlite field [13].
Ti-rich hydrogarnets from the SK-2 kimberlite were analysed by EMPA, while those from the SK-1 pipe are too small to be analysed (Table 2). According to the IMA nomenclature for garnet group minerals [39], schorlomite end member has 2 apfu Ti in Y position, while andradite end member has 2 apfu Fe3+ in Y position. The Ti-rich garnets (12.9 wt %–26.3 wt % TiO2) studied in the present work could correspond to a theoretical hydrous andradite (when it has <1 apfu Ti) and hydrous schorlomite (when it has >1 apfu). However, Ti-rich hydroandradite from the SK-2 kimberlite returns low total (88 wt %–96 wt %) and Si is also very low (1.6–1.9 apfu), thus suggesting the substitution of Si by OH in Z position and the existence of H2O molecules. They plot inside the field of Ti andradites from ultramafic lamprophyres [25] (Figure 6).

5. Discussion

Perovskite is a valuable mineral recorder of the crystallization conditions of the kimberlites. It commonly crystallizes directly from the kimberlite magma [40]. Therefore, pristine primary perovskite grains in both kimberlites and carbonatites are often used for geochemical investigations and, in particular for U-Pb dating [5,6,10,12,13,16,17,18,19,20,21,41,42]. However, it has been proved that different perovskite generations can occur in the same kimberlite. Simultaneous occurrence of two populations of primary perovskite has been explained by magma mingling [5]. In addition, primary magmatic perovskite can be altered [12,13] during subsolidus processes to secondary minerals that may redistribute REE and potentially, U, Pb and Th [43]. Our petrographic data shows that two types of texturally fresh (i.e., pristine) perovskite occur in the studied Indian kimberlites. Groundmass type 1 crystals may be interpreted as primary magmatic perovskite. However, type 2 perovskite occurs along with calcite and serpentine filling porosity produced by replacement of Ti-rich spinels. This assemblage suggests that type 2 perovskite could be produced by subsolidus hydrothermal phenomena and thus not necessarily related to the primary perovskite. Similar pristine secondary hydrothermal perovskites have been described in carbonatites and cannot be used to obtain the age of the intrusive [23]. Therefore, our observations further restrict the use of groundmass perovskite for geochronological purposes, since they show for the first time that pristine perovskite can be also formed in kimberlites by hydrothermal processes. Therefore, an accurate petrographic study is necessary to exclude perovskite affected by subsolidus processes. Hence, we suggest taking additional cautions when using perovskite grains in concentrates.
The alteration of perovskite is strongly dependent upon pH, f(CO2) and temperature. It is expected to occur in late-stage hydrothermal alteration processes and in the subaerial weathering environment [9]. The replacement of perovskite occurs at the late stage of groundmass formation, resulting from a decrease in f(O2) and temperature (<350 °C) at low pressure (P < 2 kbars) and over a wide range of a(Mg2+) values [12]. The replacement of olivine by serpentine as well as the replacements of perovskite and spinel suggest that P (CO2) and P (H2O) remained relatively high [13]. The two Indian kimberlites studied here have significant differences regarding their subsolidus history, mainly represented by the higher complexity of perovskite alteration in SK-2. The alteration process took place under different fluid/rock ratios in each kimberlite, in a relatively closed system. Under these conditions, Ti-rich minerals are unstable and, in particular, Ti-rich spinels are easily replaced in both kimberlites by mixtures of Ti-rich hydrogarnets, calcite and serpentine. A relatively low SiO2 and high water activities were necessary to avoid the crystallization of titanite and to favour the crystallization of hydrogarnets. Slight replacement of perovskite by anatase in SK-1 could be indicative of a decrease of temperature under conditions of medium to high f(CO2), following the thermodynamic calculations data [15]. However, the same experimental data suggest that the strong replacement of perovskite by kassite in SK-2 needs a high f(H2O) and a low activity in alkalis. The formation of kassite or anatase during perovskite replacement may also involve different rates of Ca-leaching, as kassite formation is favoured by lower Ca-leaching [13]. The LREE-rich perovskite is more unstable during these processes than pure end-member perovskite. Therefore, aeschynite-(Ce) inherits the composition of the replaced LREE-bearing perovskite cores and it is Nb-poor because the cores were also Nb-poor.
The occurrence of abundant Ti-rich garnets in the groundmass of the rock could suggest an aillikitic affinity during a preliminary examination, based on the International Union of Geological. Sciences (IUGS) rock classification [44]. Ti-rich garnets in groundmass from Indian kimberlites have been used to classify the rocks as orangeites [25]. Those Ti-rich garnets have similar composition to the Ti-rich garnets studied in the current work, which also plot inside the field of high Ti-andradite from kimberlite-UML rocks (Figure 6). However, hydrogarnets from SK-1 and SK-2 kimberlites replace Ti-rich oxides and are accompanied by hydrothermal minerals such as serpentine and type 2 perovskite, thus indicating that they were produced by late hydrothermal processes. Therefore, these garnets cannot be representative of the parental magma composition. In fact, Ti-rich hydrogarnets have also been found in ophiolite sequences as a result of hydrothermal alteration [33]. Hence, attention must be paid to the position of Ca-Ti-rich garnet in the mineral sequence before using it to classify the rock based on its occurrence.

6. Conclusions

Two types of perovskite occur in the SK-1 and SK-2 Indian kimberlites. The first type crystallized directly from the kimberlite magma, whereas a hydrothermal origin was inferred from the second. Additionally, two different replacement trends of groundmass perovskite have been identified. Type 1 groundmass perovskite is replaced by anatase in SK-1 and by kassite along with aeschynite-(Ce) in SK-2. The different sequences are related to alteration under different f(CO2) and f(H2O) conditions. In some cases, perovskite may be strongly altered to secondary minerals, resulting in a redistribution of REE and potentially, U, Pb and Th. Therefore, U-Pb dating studies involving perovskite require a detailed petrographic characterisation to confirm its primary (i.e., magmatic) origin.
Ti-rich hydrogarnets in SK-1 and SK-2 replace groundmass atoll spinel and could correspond to “hydroandradite” and “hydroschorlomite.” They were produced by hydrothermal processes. Therefore, attention must be paid to the position of Ca-Ti-garnet in the mineral sequence and to its water content before using it to classify the rock based on its occurrence.

Acknowledgments

This research was supported by the CGL2006-12973 and CGL2009-13758 projects of the Ministerio de Ciencia e Innovación of Spain, the AGAUR 2014SGR01661 project of the Generalitat de Catalunya and by a FI grant to Jingyao Xu (coded FI_B 00904) sponsored by the Secretaria d’Universitats i Recerca del Departament d’Economia i Coneixement de la Generalitat de Catalunya. The authors also acknowledge the thin section preparation laboratory and the Scientific and Technological Centers of the University of Barcelona (CCiTUB) for the assistance with SEM-BSE-EDS study (Javier García-Veigas, David Artiaga) and EMP analyses (Xavier Llovet). The authors are also very grateful to the two anonymous reviewers for their critical comments.

Author Contributions

Jingyao Xu analysed the data; Joan Carles Melgarejo devised the project and guided analysis and interpretation; Montgarri Castillo-Oliver contributed samples. Jingyao Xu also wrote the paper, assisted by Joan Carles Melgarejo and Montgarri Castillo-Oliver.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Simplified geological map of Southern India showing locations of Narayanpet kimberlite field (NKF), Raichur kimberlite field (RKF) and Wajrakarur kimberlite field (WKF), adapted from Dongre [32]; (b) Geological setting of the Siddanpalli kimberlites in the RKF, showing the studied kimberlite samples, adapted from Chalapathi Rao et al. [17].
Figure 1. (a) Simplified geological map of Southern India showing locations of Narayanpet kimberlite field (NKF), Raichur kimberlite field (RKF) and Wajrakarur kimberlite field (WKF), adapted from Dongre [32]; (b) Geological setting of the Siddanpalli kimberlites in the RKF, showing the studied kimberlite samples, adapted from Chalapathi Rao et al. [17].
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Figure 2. Representative SEM (BSE) images of the groundmass from SK-1 and SK-2 kimberlites. (a) Type 1 perovskite (Prv1) with an ulvöspinel inclusion (Usp) from SK-1 kimberlites being replaced by anatase (Ant) along borders and fractures; (b) Euhedral atoll spinel from SK-1, replaced by serpentine (Srp), calcite (Cal) and cryptocrystalline Ti-rich garnets (Grt), with a relict of ulvöspinel (Usp) in the centre of the crystal; (c) Atoll spinel from SK-1, altered to serpentine, calcite, magnetite (Mag) and cryptocrystalline Ti-rich garnets, with a relict of titanomagnetite (Ti-Mag) in the centre; (d) Type 1 groundmass perovskite from SK-2 kimberlite, replaced by kassite (Kas) with aeschynite (Aes); (e) Type 1 groundmass perovskite from SK-2, replaced by kassite with aeschynite; (f) Type 1 perovskite from SK-2, altered to kassite and aeschynite (Kas + Aes) in grain borders; the ensemble is replaced by Mn-rich ilmenite (Mn-ilm) along fractures and grain borders; (g) Groundmass atoll spinel from SK-2 altered to type 2 perovskite (Prv2), Ti-rich garnets and serpentine, showing ulvöspinel relicts; (h) Atoll groundmass spinel from SK-2 altered to type 2 euhedral perovskite, Ti-rich garnets and calcite, showing relicts of ulvöspinel and type 1 perovskite; (i) Mg-rich ilmenite (Mg-ilm) from SK-2, replaced by ulvöspinel, which on its turn is replaced by type 2 perovskite, calcite and serpentine; (j) Magnetite from SK-2, replaced by type 2 euhedral perovskite, serpentine and calcite.
Figure 2. Representative SEM (BSE) images of the groundmass from SK-1 and SK-2 kimberlites. (a) Type 1 perovskite (Prv1) with an ulvöspinel inclusion (Usp) from SK-1 kimberlites being replaced by anatase (Ant) along borders and fractures; (b) Euhedral atoll spinel from SK-1, replaced by serpentine (Srp), calcite (Cal) and cryptocrystalline Ti-rich garnets (Grt), with a relict of ulvöspinel (Usp) in the centre of the crystal; (c) Atoll spinel from SK-1, altered to serpentine, calcite, magnetite (Mag) and cryptocrystalline Ti-rich garnets, with a relict of titanomagnetite (Ti-Mag) in the centre; (d) Type 1 groundmass perovskite from SK-2 kimberlite, replaced by kassite (Kas) with aeschynite (Aes); (e) Type 1 groundmass perovskite from SK-2, replaced by kassite with aeschynite; (f) Type 1 perovskite from SK-2, altered to kassite and aeschynite (Kas + Aes) in grain borders; the ensemble is replaced by Mn-rich ilmenite (Mn-ilm) along fractures and grain borders; (g) Groundmass atoll spinel from SK-2 altered to type 2 perovskite (Prv2), Ti-rich garnets and serpentine, showing ulvöspinel relicts; (h) Atoll groundmass spinel from SK-2 altered to type 2 euhedral perovskite, Ti-rich garnets and calcite, showing relicts of ulvöspinel and type 1 perovskite; (i) Mg-rich ilmenite (Mg-ilm) from SK-2, replaced by ulvöspinel, which on its turn is replaced by type 2 perovskite, calcite and serpentine; (j) Magnetite from SK-2, replaced by type 2 euhedral perovskite, serpentine and calcite.
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Figure 3. Representative Raman spectrum analysed in: (a) anatase from SK-1 kimberlite (present work) compared with anatase reference from Martins et al. [15], the last presents a peak at 1088 cm−1 which corresponds to calcite; (b) kassite from SK-2 kimberlite (present work) compared with kassite from Martins et al. [15]; (c) Ti-rich garnet from SK-2 kimberlite, compared with grossular-hydroandradite from Ghosh et al. [33] and andradite from the RRUFF Project database (R060350) [34]; (d) type 2 perovskite replacing spinel and type 1 groundmass perovskite from SK-2 (present work).
Figure 3. Representative Raman spectrum analysed in: (a) anatase from SK-1 kimberlite (present work) compared with anatase reference from Martins et al. [15], the last presents a peak at 1088 cm−1 which corresponds to calcite; (b) kassite from SK-2 kimberlite (present work) compared with kassite from Martins et al. [15]; (c) Ti-rich garnet from SK-2 kimberlite, compared with grossular-hydroandradite from Ghosh et al. [33] and andradite from the RRUFF Project database (R060350) [34]; (d) type 2 perovskite replacing spinel and type 1 groundmass perovskite from SK-2 (present work).
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Figure 4. CaO-∑LREE2O3-Fe2O3 (wt %) plot of type 1 perovskite from SK-1 and SK-2 and type 2 perovskite from SK-2.
Figure 4. CaO-∑LREE2O3-Fe2O3 (wt %) plot of type 1 perovskite from SK-1 and SK-2 and type 2 perovskite from SK-2.
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Figure 5. Frequency histograms of log fO2 expressed relative to the NNO buffer (ΔNNO) calculated using the perovskite oxybarometer developed by Bellis and Canil [4]. (a) type 1 perovskite from SK-1; (b) type 1 perovskite from SK-2; (c) type 2 perovskite from SK-2.
Figure 5. Frequency histograms of log fO2 expressed relative to the NNO buffer (ΔNNO) calculated using the perovskite oxybarometer developed by Bellis and Canil [4]. (a) type 1 perovskite from SK-1; (b) type 1 perovskite from SK-2; (c) type 2 perovskite from SK-2.
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Figure 6. Compositional variation of TiO2, FeOT and CaO for Ti-rich garnet from SK-2 kimberlite pipe compared with Ti-rich andradites from ultramafic lamprophyres [25].
Figure 6. Compositional variation of TiO2, FeOT and CaO for Ti-rich garnet from SK-2 kimberlite pipe compared with Ti-rich andradites from ultramafic lamprophyres [25].
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Table 1. Representative electron microprobe (EMPA) analysis of type 1 perovskite from SK-1 and SK-2 kimberlites, type 2 perovskite, aeschynite-(Ce) and kassite from SK-2 kimberlite.
Table 1. Representative electron microprobe (EMPA) analysis of type 1 perovskite from SK-1 and SK-2 kimberlites, type 2 perovskite, aeschynite-(Ce) and kassite from SK-2 kimberlite.
MineralType 1 PerovskiteType 2 PerovskiteAeschynite-(Ce)Kassite
SK-1SK-2SK-2 SK-2 SK-2 SK-2 SK-2 SK-2 SK-2
(wt %)CentreCentreBorderBorderCentreCentreBorderBorder
SrO0.340.180.230.250.200.190.200.24bdlbdl0.13bdlbdlbdlbdl
ZrO20.100.210.230.380.080.140.190.280.090.460.740.250.430.400.08
Nb2O50.600.980.730.610.540.630.440.500.10bdl3.100.990.630.460.62
CaO34.6834.6437.1137.6335.3635.2738.0638.1939.9538.207.248.0022.4922.1221.32
ThO2----------0.37bdl---
Na2O0.570.570.350.2236.530.600.280.22bdlbdlbdlbdlbdlbdlbdl
MgO0.06bdl0.050.090.08bdl0.050.050.140.080.040.050.260.05bdl
Al2O30.330.300.240.310.250.290.230.28bdlbdlbdlbdlbdl0.03bdl
SiO2bdlbdlbdl0.04bdlbdlbdlbdl0.610.600.38bdl0.050.060.07
BaO0.190.12-0.08-0.130.180.12--0.460.15---
TiO254.3654.2654.1356.3054.9954.7557.1056.6755.4953.4848.6152.2062.4264.7465.26
La2O30.971.020.670.500.600.840.620.53bdlbdl7.7911.11bdlbdl0.15
Ce2O33.073.031.790.882.182.840.890.89bdlbdl18.3417.990.230.480.58
Nd2O31.561.501.080.351.181.500.340.41bdlbdl6.883.990.240.220.28
Cr2O3----------bdl0.05---
Pr2O30.450.430.170.190.080.360.190.17bdlbdl1.971.450.120.08bdl
MnO0.020.050.04bdl0.03bdl0.020.05bdl0.03bdl0.190.150.371.28
Sm2O3----------0.510.19---
Fe2O31.281.361.741.310.991.291.161.012.004.030.590.661.290.930.51
Gd2O3----------0.240.13---
HfO2----------0.11bdl---
Ta2O5bdlbdlbdlbdlbdlbdlbdlbdl0.07bdl0.060.11bdl0.04bdl
K2O0.040.030.020.000.030.030.020.020.000.02--bdl0.01bdl
Total98.6398.7498.5799.1698.2598.9299.9999.6598.8097.1997.5397.7988.3790.0590.19
ΣLREE2O36.045.983.701.934.045.542.041.990.140.1535.4934.730.590.811.03
(apfu)O = 3O = 3Σcations = 3Σcations = 3
Sr0.0050.0030.0030.0030.0030.0030.0030.0030.0000.0000.0040.0000.0000.0000.000
Y----------0.0000.000---
Zr0.0010.0020.0030.0040.0010.0020.0020.0030.0010.0050.0180.0060.0010.0010.001
Nb0.0060.0110.0080.0060.0060.0070.0050.0050.0010.0000.0700.0220.0120.0080.011
Ca0.8850.8830.9380.9320.9240.8940.9360.9420.9850.9600.3870.4170.9860.9570.926
Th----------0.0040.000---
Na0.0260.0260.0160.0100.0220.0270.0120.0100.0000.0010.0000.0000.0000.0000.000
Mg0.0020.0010.0020.0030.0030.0000.0020.0020.0050.0030.0030.0040.0160.0030.000
Al0.0090.0080.0070.0080.0070.0080.0060.0080.0000.0000.0000.0000.0000.0010.000
Si0.0000.0000.0000.0010.0000.0000.0000.0000.0140.0140.0190.0000.0020.0020.003
Ba0.0020.0010.0000.0010.0000.0010.0020.001--0.0090.003---
Ti0.9740.9700.9600.9790.9770.9740.9850.9810.9600.9431.8231.9081.9221.9671.990
La0.0080.0080.0050.0040.0050.0070.0050.0040.0000.0000.1320.1840.0000.0000.002
Ce0.0270.0260.0150.0070.0190.0250.0070.0070.0000.0000.3350.3200.0040.0070.009
Nd0.0130.0130.0090.0030.0100.0130.0030.0030.0000.0000.1230.0690.0030.0030.004
Cr----------0.0000.002---
Pr0.0040.0040.0010.0020.0010.0030.0020.0010.0000.0000.0360.0260.0020.0010.000
Mn0.0000.0010.0010.0000.0000.0000.0000.0010.0000.0000.0000.0060.0040.0100.036
Sm----------0.0090.003---
Fe0.0230.0240.0310.0230.0180.0230.0200.0170.0350.0710.0220.0240.0400.0280.016
Gd----------0.0040.002---
Hf----------0.0020.000---
Ta0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0010.0010.0000.0000.000
K0.0010.0010.0010.0000.0010.0010.0010.0010.0000.000--0.0000.0000.000
ΣLREE0.0520.0510.0310.0160.0340.0470.0170.0160.0010.0010.5120.5330.0090.0120.015
Table 2. Representative EMPA analysis of Ti-rich garnet from SK-2 kimberlites.
Table 2. Representative EMPA analysis of Ti-rich garnet from SK-2 kimberlites.
(wt %)#1#2#4#5#15#17#19#20#21
SiO220.2025.4421.6221.8621.5123.1820.6921.2420.00
Al2O30.630.690.980.980.850.890.850.840.92
Cr2O30.320.831.230.772.810.372.631.252.22
TiO226.3323.3914.8014.6015.5013.3015.6414.4616.02
MgO0.801.460.080.200.170.470.140.100.68
Na2Obdl0.04bdlbdlbdlbdlbdlbdlbdl
BaO0.100.11bdlbdlbdlbdlbdl0.100.11
MnO0.130.090.070.090.060.080.100.080.22
FeO13.6211.8814.3415.1613.0716.0412.4215.1314.95
SrObdlbdlbdlbdlbdlbdlbdlbdlbdl
ZrO20.790.770.200.280.180.300.540.400.35
K2O0.010.010.020.010.010.010.01bdlbdl
CaO30.6130.2434.9234.8235.1934.1235.2434.9432.91
Total93.8695.2288.2688.7789.3588.7788.2588.5388.39
Recalculated Analyses
Fe2O315.1413.2015.9416.8514.5217.8313.8016.8116.61
FeO0.000.000.000.000.000.000.000.000.00
Total95.1296.2889.8890.4790.8190.5589.6690.2690.06
(apfu)Cation for 3(Ca + K + Na + Sr + Ba + Mn + Mg)
Si1.77302.19411.72411.73921.69711.86261.63091.68831.6432
Al0.06550.07040.09210.09200.07930.08420.07860.07880.0895
Cr0.02220.05630.07780.04850.17530.02380.16390.07840.1442
Ti1.73851.51750.88780.87380.91990.80390.92730.86460.9901
Mg0.10490.18780.00970.02400.02010.05620.01590.01190.0838
Na0.00000.00630.00000.00000.00000.00000.00000.00000.0000
Ba0.00350.00370.00000.00000.00000.00000.00000.00300.0036
Mn0.00970.00630.00480.00600.00420.00570.00660.00520.0151
Fe3+0.99960.85670.95621.00850.86221.07770.81861.00561.0271
Sr0.00000.00000.00000.00000.00000.00000.00000.00000.0000
Zr0.03400.03250.00770.01090.00680.01180.02080.01560.0141
K0.00110.00130.00150.00100.00090.00080.00090.00000.0000
Ca2.87842.79422.98332.96792.97442.93732.97592.97542.8968
“bdl”: below detection limit.

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Xu, J.; Melgarejo, J.C.; Castillo-Oliver, M. Styles of Alteration of Ti Oxides of the Kimberlite Groundmass: Implications on the Petrogenesis and Classification of Kimberlites and Similar Rocks. Minerals 2018, 8, 51. https://doi.org/10.3390/min8020051

AMA Style

Xu J, Melgarejo JC, Castillo-Oliver M. Styles of Alteration of Ti Oxides of the Kimberlite Groundmass: Implications on the Petrogenesis and Classification of Kimberlites and Similar Rocks. Minerals. 2018; 8(2):51. https://doi.org/10.3390/min8020051

Chicago/Turabian Style

Xu, Jingyao, Joan Carles Melgarejo, and Montgarri Castillo-Oliver. 2018. "Styles of Alteration of Ti Oxides of the Kimberlite Groundmass: Implications on the Petrogenesis and Classification of Kimberlites and Similar Rocks" Minerals 8, no. 2: 51. https://doi.org/10.3390/min8020051

APA Style

Xu, J., Melgarejo, J. C., & Castillo-Oliver, M. (2018). Styles of Alteration of Ti Oxides of the Kimberlite Groundmass: Implications on the Petrogenesis and Classification of Kimberlites and Similar Rocks. Minerals, 8(2), 51. https://doi.org/10.3390/min8020051

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