Laser In Situ U–Pb Isotope Dating of Carbonate Rocks in Weijia Guyot in the Western Pacific Ocean and Its Geological Significance

Abstract

Shallow-water carbonate rocks constitute a crucial component of large guyots, arising in distinct environments and harboring valuable insights into the evolutionary stages of seamount islands as well as the tectonic conditions of the underlying oceanic plate. Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) was used to conduct in situ U–Pb isotope dating of carbonate minerals with low uranium content collected from Weijia Guyot. This dating approach yielded crucial evidence for the vertical development of the seamount. Our study indicates that shallow-water carbonate rocks in Weijia Guyot had a temporal range between 91 My and 137 My. The carbonate rocks underwent two growth phases, Hauterivian to Barremian and Cenomanian to Turonian, with a hiatus of approximately 20 My. Since the Hauterivian age, the shield volcano of Weijia Guyot is essentially complete, with its seamount top exposed at or near sea level and receiving its first stage of shallow-water carbonate sedimentation. Based on the dating of both shallow-water carbonate rocks and hawaiite within the Weijia Guyot, it is inferred that approximately 10 My elapsed from shield-building volcanism to late alkalic volcanism. During the Turonian age, the main reason for the second phase of shallow-water carbonate rocks in the seamounts was the regional tectonic uplift triggered by the drift of the Weijia Guyot along with the Pacific Plate toward the Society hotspot.

1. Introduction

Seamounts are widely distributed in various ocean basins worldwide [1,2,3,4,5,6,7,8,9]. Some have abundant Co-rich crust and fishery resources and are repositories for marine life and microorganisms [10,11,12,13,14,15,16,17,18]. In addition, seamounts are key points of intersection between the biosphere, hydrosphere, and lithosphere, and are globally relevant [19].

Seamount formation and evolution begin due to submarine volcanism [19,20] and expand as magmatic activity continues. When an evolving seamount reaches a water depth of 700 m or less, eruption intensity increases sharply due to the relatively low seawater pressure. Eventual emergence above sea level provides substrates for carbonate reefs, ranging from minor shoreline reefs to massive coral reefs, that may cover the entire subsiding volcano. Once the volcanism ceases, the volcanic island will eventually drown due to erosion and subsidence [19]. When coral reefs grow slower than the subsidence of volcanic islands, coral reefs sink below the photic zone and eventually die. Former islands and coral reefs have become seamounts with flat tops (namely guyots) through these geological processes [20]. When a seamount reaches a subduction zone, or when the ocean basin it is located in closes due to the collision of two continental plates on both sides of the ocean, its life cycle ends.

In addition to volcanic bases, carbonate reefs, and their related sedimentary rocks are also important seamount components. These rocks originated from distinct environments and serve as repositories for crucial data pertaining to seamount evolutionary stages and the tectonic conditions of the underlying ocean plate. The isotopic chronology of carbonate minerals helps elucidate the evolutionary process of seamounts. U–Pb isotope dating is the most widely used method in current geochronology. However, the uranium content of carbonate minerals is typically two to four orders of magnitude lower than that of zircon, and common Pb is variable. Cheng et al. [21] and Kendrick et al. [22] presented an in situ U–Pb isotope dating method for low uranium content carbonate minerals using Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICPMS) and developed a suitable carbonate mineral standard AHX-1a. This method allows for the quick and accurate determination of carbonate rock formation in seamounts. This study applies this method for conducting laser in situ U–Pb isotope dating of carbonate rock samples collected from Weijia Guyot (originally called Ita Mai Tai) in the Western Pacific Ocean. Research results provide chronological constraints for the island stage evolution of the seamount. They also provide an important basis for discussing the vertical evolution of the seamount.

The study area is located in Weijia Guyot, a feature of the Magellan Seamount Trail located in the Western Pacific Ocean (Figure 1). The Western Pacific Ocean is a region with a high concentration of seamounts due to its age and complex tectonic history. The formation of Weijia Guyot occurred between about 118 My and 120 My [23], which is associated with hotspot activity in the South Pacific Isotope and Thermal Anomalies Area (SOPITA) of the French Polynesian islands during the Cretaceous period [24,25,26]. Weijia Guyot is approximately 150 km long and has an irregular shape with an overall NE orientation, typical of a summit platform. The water depth at the edge of the platform ranges from 1600 m to 2200 m and reaches 5500 m at its base.

Figure 1. Structure sketch map of the Magellan seamounts and samples location.

2. Materials and Methods

2.1. The Studied Samples

Carbonate rock samples (Figure 2) were obtained by deep-sea shallow drilling at the edge of the platform or the slope of Weijia Guyot. The length of the cores ranges from 62 cm to 102 cm. These stations, including MCSD165, MCSD170, and MCSD181, are located at the edge of the mountaintop platform with water depths ranging from 1594 m to 1630 m. MCSD174 and MCSD175 are located on a sloping platform with relatively flat terrain with water depths ranging from 1985 m to 2029 m. MCSD146 is located on steep slope areas with a water depth ranging from 2343 m to 2937 m (Figure 1).

Figure 2. Photos of carbonate rock cores.

2.2. Microscopic Observation and Chemical Analysis

A polarizing microscope (Leica DM4 P, Leica Microsystems Inc., Deerfield, IL, USA) was used in this study to identify minerals in carbonate rock thin sections and capture micrographs. The MNR Key Laboratory of Marine Mineral Resources of GMGS conducted X-ray fluorescence analysis of major elements (XRF; Axios, PANalytical B.V., Almelo, The Netherlands), with a detection limit of 0.01–0.1% and a relative standard deviation (RSD) of less than 2%.

2.3. Laser In Situ U–Pb Isotope Dating

The samples were first sliced into 0.5 cm thick sections and carefully cleaned with water. Next, the clean part was selected, avoiding any parts mixed with clay or iron staining, to create circular sample targets with a diameter of 2.5 cm. The sample targets were then sanded, polished, and cleaned with anhydrous ethanol to avoid interference from common Pb [27].

Carbonate U–Pb dating was performed using laser ablation-multi-collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) employing a Nu Plasma II MC-ICP-MS coupled with a 193 nm ArF excimer laser system (RESOlution SE, ASI) at the Radiogenic Isotope Facility at the University of Queensland (Brisbane, Australia). The carbonate samples were screened using LA-ICP-MS (iCAP RQ, Thermo Scientific, Waltham, MA, USA) before U–Pb dating to confirm the laser spot position where the U/Pb ratio was highest and common Pb was lowest. To ensure accurate data, we tuned the instrument to glass standard NIST614 to reduce isotopic fractionation and oxidation levels before performing the experiments. The analysis began when the instrument signal was tuned to ²⁰⁶Pb/²³⁸U ≈ 0.7 and ThO⁺/Th⁺ ≈ 0.8%.

For laser ablation analyses, laser beams with diameters of 100 μm were used and set to a fluence of 2.5 J/cm² and a frequency of 10 Hz. Each analytical process began with 15 s, 20 s, and 15 s measurements for the background, sample ablation, and washout, respectively. Standard sample bracketing was used with NIST 614 glass as the primary reference material to correct ²⁰⁷Pb/²⁰⁶Pb fractionation and instrumental drift in ²³⁸U/²⁰⁶Pb rations [28,29]. Carbonate reference AHX-1a (the recommended age is 209.8 ± 0.48 My) was used for the matrix-biased calibration of ²³⁸U/²⁰⁶Pb fractionation. Additionally, secondary reference material ASH15D (the recommended age is 2.965 ± 0.011 My) was used to assess the accuracy of the dating results. During the experiment, three carbonate reference materials were inserted between each of the five analyses of unknown samples in measurement sequences. NIST614 was inserted every five spots throughout the whole sequence. Following the data reduction routine in Iolite v3 software [28], the calculated ²⁰⁷Pb/²⁰⁶Pb versus ²³⁸U/²⁰⁶Pb ratios were plotted in a Tera–Wasserburg diagram. The formation ages were determined from lower intercepts on inverse isochron using Isoplot 3.75 software in MS EXCEL [30].

3. Results

3.1. Microstructure and Mineral Characteristics of Carbonate Rocks

Based on field descriptions and microscopic analyses (Figure 3), carbonate rock grains mainly consist of carbonate bioclasts and coral reef clasts formed in a reef environment. MCSD146 is a carbonate intraclast sandstone with a yellow color and high porosity. The rock grains consist of carbonate bioclasts, basalt clasts, palagonite clasts, and ooids, with bioclasts dominating. Basalt clasts, palagonite clasts, and ooids are dispersed in strips, with sparry calcite cementing the grains. MCSD165, MCSD170, MCSD174, MCSD175, and MCSD181 are mineralogically similar, being pure white with a slight yellowish tinge. They are porous and consist mainly of carbonate bioclasts with some micritic limestone clasts. The cement is predominantly micritic, with some sparry calcite present.

Figure 3. Micrographs of carbonate rocks (with plane polarized light). (a) MCSD146; (b) MCSD165; (c) MCSD170; (d) MCSD174; (e) MCSD175; (f) MCSD181.

3.2. Characteristics of Major Elements in Carbonate Rocks

Table 1 shows analysis results for major elements in carbonate rocks. All samples have a high loss on ignition (LOI), ranging from 29.95% to 43.41%. CaO content ranges from 40.08% to 56.37%; SiO₂ content ranges from 0.01% to 15.05%; Al₂O₃ content ranges from 0.01% to 6.11%, and Fe₂O₃ content ranges from 0 to 3.69%. MCSD146 contains basaltic clasts, so SiO₂, Al₂O₃, and Fe₂O₃ content are relatively high. The rest of the samples have high LOI with CaO content greater than 50%.

Table 1. Oxide content of major elements in carbonate rocks from Weijia Guyot (wt.%).

Samples	SiO2	Al2O3	Fe2O3	MgO	CaO	Na2O	K2O	MnO	TiO2	P2O5	LOI	Total
MCSD146	15.05	6.11	3.69	0.72	40.08	1.31	1.30	0.11	0.92	1.32	29.95	100.56
MCSD165	0.08	0.02	0.01	0.51	55.67	0.33	0.01	0.01	0.02	3.35	40.21	100.21
MCSD170	0.01	0.01	0.01	0.61	56.08	0.25	0.01	0.00	0.02	0.02	43.29	100.29
MCSD174	0.03	0.05	0.00	0.66	55.74	0.22	0.01	0.00	0.03	0.09	43.41	100.23
MCSD175	0.01	0.01	0.00	0.61	55.77	0.26	0.01	0.01	0.03	0.42	43.40	100.52
MCSD181	1.70	0.06	0.05	0.46	56.37	0.43	0.02	0.00	0.02	0.84	39.66	99.61

3.3. Formation Age of Carbonate Rocks

Table A1 in Appendix A shows the U–Pb isotope dating analysis results for all carbonate rock samples. Table A2 lists the analysis results for standards AHS15D and AHX-1a. Figure A1 shows the Tera–Wasserburg concordia results for standards AHS15D and AHX-1a. The U content of carbonate rock samples is about two orders of magnitude lower than that of zircon and other uranium-rich minerals, ranging from 0.0102 to 6.5380 μg/g, with most of the analyzed points having a U content of less than 1 μg/g (accounting for 91%). On the contrary, although Pb content of samples is low, ranging from 0.0063–1.4320 μg/g, it is two to three orders of magnitude higher than that of zircon. This finding indicates a high level of common Pb content in the samples, which is difficult to subtract during data processing. Thus, the Tera–Wasserburg concordia diagram method, which does not require common Pb subtraction, was used to calculate U–Pb ages.

The Tera–Wasserburg concordia results (Figure 4) indicate that sample ages can be divided into two ranges: 91–96.8 My (MCSD165, MCSD174, and MCSD181) and 120–137 My (MCSD146, MCSD170, and MCSD175), suggesting that shallow-water carbonate rocks in the Weijia Guyot accumulated during two distinct events in the Early and Late Cretaceous periods respectively, with a hiatus of approximately 20 My.

Figure 4. Laser in situ U–Pb isotope dating results of carbonate rocks.

4. Discussion

4.1. The Formation Time of the Initial Shield Volcano

According to analyses from ODP Sites 143 and 144, guyots in the Western Pacific Ocean are covered by Cretaceous shallow-water carbonate platforms [31]. After submergence, extinct seamounts remain intact until they are consumed by subduction or ocean basin closure [19]. The samples in this study showed no trace of undergoing late-stage alteration or metamorphism. The results of in situ U–Pb isotope dating represent the formation age of seamount carbonate rocks, which formed during the Hauterivian to Barremian and Cenomanian to Turonian ages.

The duration of volcanic activity on large seamounts can be substantial. Basalt samples from seamounts are prone to alteration due to prolonged seawater exposure. For these two reasons, the ⁴⁰Ar/³⁹Ar dating of seamount basalts is somewhat uncertain [5]. In addition, determining the main formation time of seamounts can be challenging due to the coverage of early shield-building basalt by younger alkaline lavas. Weijia Guyot evolved into its current form after multiple volcanic eruptions and underwent a long evolutionary process from shield volcanoes to its final stable stage. An age of 118–120 My (⁴⁰Ar/³⁹Ar) was obtained from the hawaiites of Weijia Guyot [26] and exhibited multiple late-stage small volcanic cones on its southwestern edge (Figure 1). According to U–Pb dating, the shallow-water carbonate cover of Weijia Guyot formed between 137 and 120 My, before the eruption of the hawaiites. Thus, the hawaiites at 118–120 My represent the seamount’s subsequent activity. Due to uncertain dating, 10 My elapsed between the end of shield volcano-building and younger alkaline activity.

4.2. Genesis of the Second Stage Carbonate Rocks

The growth and demise of tropical carbonate platforms are influenced by numerous factors, including biota and skeletal particle production, sea level fluctuations, tectonic activity, volcanic eruptions, and environmental conditions [32]. The deposition of shallow-water carbonates on seamounts in the Northwestern Pacific Ocean was llinked more closely with volcanic activities and tectonic movements [33]. Based on the dating results of shallow-water carbonate rocks, the Weijia Guyot entered the second stage of carbonate rock growth around 97 My (Cenomanian), approximately 20 My after the first stage of growth ceased. Weijia Guyot formed around 120 My in the South Pacific Isotope and Thermal Anomaly Zone (SOPITA) [24,25,26] and subsequently drifted northwestward with the Pacific Plate to its current location. According to Seton’s global tectonic evolution model [34], Weijia Guyot was approximately 180 km away from the Society hotspot at 90 My [35]. Caiwei Guyot, located about 350 km northwest of Weijia Guyot, was also formed during the same period. During this period, there may have been multiple small-scale volcanic activities occurring in late stages at the southwestern edge of the seamount. Furthermore, it is postulated that the thermal uplift resulting from regional magmatic activity served as the principal factor responsible for the formation of the second stage shallow-water carbonate deposition in the Weijia Guyot.

5. Conclusions

(1) The results of laser in situ U–Pb isotope dating conducted on carbonate minerals reveal that the shallow-water carbonate rocks of the Weijia Guyot underwent two distinct formation phases: Hauterivian to Barremian followed by Cenomanian to Turonian, with a hiatus of approximately 20 My between them;

(2) Since the Hauterivian age (ca. 130 My), the shield volcano-building stage has been completed and exposed at or near the surface, allowing the deposition of the first carbonate platform. A temporal gap of approximately 10 My separates the cessation of shield volcanism and more recent alkaline volcanic activity;

(3) During the Turonian age (ca. 90 My), the regional tectonic uplift caused by the drift of the Weijia Guyot with the Pacific Plate toward the Society hotspot was the primary factor driving the formation of the second stage of shallow-water carbonate rocks.