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Article

Multistage Diagenetic Fluid Shaping Miocene Island Dolostones on One Isolated Atoll in the South China Sea: Insights from LA-ICP-MS U–Pb Dating and Geochemical Characterization

by
Yun Luo
1,
Gang Li
1,*,
Xiyang Zhang
1,
Weihai Xu
1,
Xiaowei Zhu
1,
Wanqiu Zhou
1,2,
Huiwen Huang
1,
Wen Yan
1,2,* and
Fuchang Zhong
1
1
Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
*
Authors to whom correspondence should be addressed.
Minerals 2024, 14(2), 157; https://doi.org/10.3390/min14020157
Submission received: 19 December 2023 / Revised: 24 January 2024 / Accepted: 29 January 2024 / Published: 31 January 2024
(This article belongs to the Section Mineral Geochemistry and Geochronology)
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Abstract

:
Cenozoic dolomitization of reefal carbonates has been widely found on many tropical islands worldwide. However, most ages and geochemical data obtained from bulk samples prevent a clear understanding of the previous complex diagenetic processes of these island dolostones due to a lack of in situ age and fluid composition. In this study, one deep borehole penetrated Cenozoic carbonates on Meiji Island in the southern South China Sea and massive dolostones with thicknesses over 400 m were uncovered. The in situ U–Pb geochronology and elemental analysis were conducted on the lower Nanwan Formation (upper Miocene) comprising undolomitized calcite (bioclast), replacive dolomite, and dolomite cement. Strontium isotope ages and U–Pb dates show that the penecontemporaneous replacive dolomitization occurred at 11.0–8.5 Ma, close to the deposition of precursor limestone. The dolomite cement precipitated at 8.5–6.0 Ma. In situ elemental analyses indicate that the formation of replacive dolomite and dolomite cement in the Nanwan Formation was probably controlled by seawater. The higher Mg/Ca ratio and lower Mn and Sr contents in dolomite cements show that their fluid underwent more evaporation. The dolomite content is positively related to the porosity of reefal limestones in the Nanwan Formation, suggesting that primary voids play an important role in fluid transportation during following dolomitization. Coralline algae and lime mud with algal fragments are beneficial for the rapid nucleation of dolomite. This study demonstrates that in situ elemental analysis using laser ablation has great potential for identifying the source of multistage dolomitizing fluids and can help refine the existing dolomitization model of isolated atolls.

1. Introduction

Cenozoic island dolostones are widely distributed on many carbonate platforms and isolated islands in tropical oceans around the world, such as the Bahamas Bank [1], Grand Cayman Island [2,3], the Enewetak Atoll [4], and the Xisha Islands in the South China Sea (SCS) [5]. The global dolomitization of island carbonates during the Cenozoic was found to have occurred synchronologically and might have been connected with global eustacy and seawater evolution [1]. Compared with ancient dolostones, these Cenozoic island dolostones have advantages in probing the “dolomite problem” [1,6] due to their shallow burial and weak diagenetic modification [1,7].
Dolomitizing fluids of Cenozoic dolostones in various genetic models mainly include normal seawater, brine or hypersaline water, slightly evaporated seawater, and mixed water [1,2,8,9,10,11,12]. The dissolution of precursor minerals was also proposed to supply magnesium-rich fluid for carbonate dolomitization [1]. Geochemical compositions of dolomites contain information on diagenetic processes and fluid sources [1,13,14,15]. The δ13C values (0.5‰–3.2‰) and δ18O values (0.5‰–4.5‰) of most Cenozoic island dolostones indicate dolomitizing fluids derived from normal seawater or slightly modified seawater [1,7]. The decoupling of δ13C and δ18O indicates that atmospheric freshwater or mixed water does not significantly participate in dolomitization [1,14]. The low Sr content (152–306 ppm) and low Fe (<300 ppm) and Mn (<35 ppm) contents of most island dolostones also show that dolomitizing fluids were composed of normal seawater [1,2,7,11]. There are also some cases of covariant trends in δ13C and δ18O and high Sr content, tending to support the mixed-water origin of island dolostones, but examples with these attributes are rare [16]. The dolomitizing fluid of dolostones on the Xisha Islands (SCS) is interpreted as high-salinity and concentrated seawater based on evidence of gas compositions in fluid inclusions [10]. However, since the δ18O value of dolostone depends on temperature and fluid δ18O, using only the δ18O proxy cannot distinguish between normal seawater and slightly modified seawater [5,14]. Different types of dolomites (e.g., low-Ca dolomite and high-Ca dolomite) show different temperatures and fluid δ18O ratios in the Bahama Bank and in the SCS [5,17,18], which are closely related to the mixing of brine and normal seawater [18].
Island dolostones are the result of long-term and multistage dolomitization [19], and these multiple stages of dolomites coexist on a crystal scale, making it difficult to obtain their in situ chemical compositions [20]. Most chemical studies of Cenozoic dolostones work on whole rocks, such as carbon/oxygen isotopes, elemental, and other metal isotope data. However, these bulk chemical data obscure the crucial differences among multiple stages of dolomitization. Therefore, the in situ microanalytical technique is highly expected to probe into the complex evolution of these multiple-stage island dolostones.
One deep borehole, named Well NK-1, penetrated Cenozoic carbonates on the Meiji Island in southern South China Sea, and massive dolostones with a thickness of over 400 m were uncovered. The Nanwan Formation is located at the bottom of the thick dolostones in Well NK-1 (Figure 1). The lower part of the Nanwan Formation (upper Miocene) is composed of reefal limestone, dolomitic limestone, and dolostone (Figure 1 and Figure 2a) [21,22]. It can be divided into a partly dolomitized unit and a pervasively dolomitized unit. These reefal carbonates with varying degrees of dolomitization in the lower Nanwan Formation provide an ideal opportunity to explore the early dolomitization processes of island carbonate. In this study, in situ U–Pb dating and elemental analyses, along with petrological and bulk geochemical data, were employed to accomplish the following: (1) identify the stages of dolomitization, (2) recognize fluid sources, and (3) evaluate multiple-stage dolomitization models for these isolated atolls.

2. Geological Setting

The Nansha Islands (also known as the Dangerous Grounds) (Figure 1a), with an area of ~900,000 km2, are located in the southern SCS. Numerous isolated coral atolls and carbonate platforms have existed on elevated seamounts and on submarine plateaus since the upper Oligocene [23,24]. Seismic stratigraphic studies showed that shallow-water carbonates in the Nansha Islands were more widely distributed during the early Neogene [25,26].
Meiji Island is located west of the Liyue Bank (also known as the Reed Bank). Meiji Island is an isolated coral atoll in deep water (over 1500 m) with a ring-shaped reef flat enclosing a lagoon. Reefal carbonates with a thickness of 997.7 m were deposited over the upper Triassic dacite basement [27]. Carbonate successions on the Meiji Island were initiated from the upper Oligocene (~25.8 Ma) to the Quaternary and were divided into the Liyue Formation, Meiji Formation, Nanwan Formation, Yongshu Formation, Nansha Formation, and Nanhai Formation according to the chronological frame in Well NK1 [21]. Meiji Island is influenced mainly by the East Asian monsoon [28]. The annual precipitation is approximately 1700 mm, and the annual surface seawater temperature ranges from 26.9 to 29.8 °C.
The Nanwan Formation of the upper Miocene has a thickness of ~240 m and is composed of dolostones and partially dolomitized limestones. The upper Nanwan Formation at a depth of 290–430 m is composed entirely of dolostones with rudstones and grainstones. The lower Nanwan Formation (at 430–538 m) in Well NK-1 is lithologically variable and is mainly composed of grainstones, packstones, rudstones/floatstones, and framestones [21].

3. Materials and Methods

3.1. Sampling

A total of 36 carbonate samples were collected at 3–5 m intervals from the lower part of the Nanwan Formation (at 430–538 m in Well NK-1). All samples were collected on the fresh surface of the rock and washed with ultrapure water 3 times to remove salt and contaminants and were dried in an oven at 45 °C for 3 days. The dried samples were ground into powder finer than 200 mesh using an agate mortar and pestle. These samples were prepared for bulk chemical analysis and thin section preparation.

3.2. Petrologic Features and XRD Analysis

Petrologic and diagenetic features were examined based on the observation of hand specimens and thin sections. The thin sections were stained with Alizarin red and observed by using a Leica DM2700P polarization microscope (Leica DM2700P, Leica Microsystems Inc., Wetzlar, Germany) equipped with a cathodoluminescence source (CL, BII CLF-2, Beacon Innovation International Inc., Ontario, Canada). The percentage of dolomite cement was estimated based on the content of different components in microscopic photographs. Microscopic observation of diagenetic structures and energy spectrum analysis were conducted on a scanning electron microscope (Hitachi, SU3500, Tokyo, Japan). The above experiments were carried out at the South China Sea Institute of Oceanology, Chinese Academy of Sciences. The porosity data at a resolution of 1 m were calculated via well logging data.
XRD analyses were conducted using a Bruker D8 Advance diffractometer (Bruker AXS GmbH, Karlsruhe, Germany), following the protocol proposed by Jones [29]. The reflection peak positions of d(104) and the intensities of d(015) and d(110) in the dolomite were collected from the diffraction spectra using the DIFFRACT.EVA software (5.2.0.5). The molar %CaCO3 content of dolomite (hereafter referred to as %Ca) was determined using the procedures proposed by Jones [29]. The cation ordering value of dolomite was calculated using the intensity ratio of the d(015) and d(110) reflection peaks.

3.3. Carbon and Oxygen Isotopes and Mg-Ca Content Analysis

Thirty-six samples were analyzed for carbon and oxygen isotopes. Powdered samples of ~50 to 70 μg were reacted with 100% phosphoric acid at 90 °C, and the extracted CO2 was analyzed on a Finnigan MAT-253 stable isotopic mass spectrometer. The results were corrected by using the NBS-19 standard and converted to values relative to the Vienna Pee Dee Belemnite (V-PDB) standard. The standard deviation of all measurements was lower than 0.1‰ for δ18O and δ13C. The Ca and Mg contents of 36 bulk samples were measured on an X-ray fluorescence spectrometer (Thermo Fisher-ARL Perform X4200, Thermo Fisher Scientific, Waltham, MA, USA) at the Institute of Geochemistry, Chinese Academy of Sciences. The 10% duplicate samples and a carbonate standard (GBW07120) were used to monitor the analytical reproducibility.

3.4. In Situ Elemental Analysis

Among the thirty-six analyzed samples, eight thin sections of dolomitic limestones were selected for in situ elemental analysis using a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS). Before the scanning, the calcite (bioclast without cement), replacive dolomite, and dolomite cement were located through Alizarin red staining. In situ elemental analysis was conducted using a Thermo Scientific Quatraple iCap TQ inductively coupled plasma mass spectrometer (Q-ICP-MS) (Thermo Fisher Scientific, Waltham, MA, USA) coupled with an ASI Resolution LR 193 nm ArF excimer laser ablation system at the Micro-Origin and Spectrum Laboratory (Sichuan Chuangyuan Weipu Analytical Technology Co., Ltd., Chengdu, China). The laser ablation time for the experiment was set as 3 s for surface cleaning, 7 s for washout, 15 s for background, 15 s for ablation, and 5 s for washout at a 10 Hz repetition rate with a fluence of 2.5 J/cm2 and sample spots of 67 µm. The NIST 614 glass standard was used as a primary reference material bracketing carbonate samples, and the NIST 612 was used as the monitoring standard for trace element fractionation correction.

3.5. Sr Isotopes and In Situ LA-ICP-MS U–Pb Dating Analysis

Bulk Sr isotope analyses were conducted on eleven samples (two dolomitic limestones, two limestones, and seven dolostones). For two dolomitic limestone samples at 493.0 m and 494.3 m, Alizarin red staining and microscopic observation were performed first to highlight the dolomite and calcite. Samples for Sr isotope analyses were obtained using micro-drilling to target the calcite and dolomite in the two thin sections. XRD analysis was used to further determine mineral compositions before the Sr isotope analyses. Acetic acid with a concentration of 1 N was used to dissolve powder samples prior to column chemistry using Sr-spec resin. The specific Sr isotope extraction and testing procedures followed the scheme of Jiang [30]. Sr isotope data were measured on a high-resolution multi-collector (MC)-ICP-MS (Nu plasma 2, Nu Instruments Ltd., Wrexham, UK) at the Radiogenic Isotope Facility, University of Queensland, Australia. Afterward, the raw Sr isotope ratio was normalized to 86Sr/88Sr = 0.1194 and the 87Sr/86Sr ratio was corrected to SRM987 = 0.710249. All Sr isotope ages were obtained from the LOWESS 6 lookup table [31].
In situ U–Pb dating of two thin sections (at depths of 493.0 m and 494.3 m) was conducted on LA-ICP-MS at the Micro-Origin and Spectrum Laboratory (Sichuan Chuangyuan Weipu Analytical Technology Co. Ltd., Chengdu, China), following the same procedure as [21]. In situ prescreening of the U/Pb ratio and U–Pb dating was conducted using a Q-ICP-MS (Thermo Scientific) coupled with an ASI Resolution LR 193 nm ArF excimer laser ablation system. The beam diameter was 67 μm, and the frequency was 8 Hz with a fluence of 2.5 J/cm2. The laser ablation time was set to 3 s for surface cleaning, 7 s for washout, 15 s for background, and 16–20 s for ablation. The NIST 614 glass standard was used to correct for the 207Pb/206Pb fractionation and for instrumental drift of the 238U/206Pb ratio [32]. The 238U/206Pb ratios of calcite samples were then further calibrated following in-house standards (AHX-1d, 238.2 ± 0.9 Ma) and cross-checked with the ID-MS-calibrated calcite standard PTKD-2 (153.7 ± 1.7 Ma [33]) and an in-house intensive LA-calibrated standard (LD-5, 72.5 ± 1.0 Ma, Xiao et al., in preparation) for age reliability and accuracy. Raw 238U/206Pb and 207Pb/206Pb ratio data were regressed on Tera–Wasserburg plots using Isoplot 3.75 to determine the ages.

4. Results

4.1. Lithology and Mineralogy of the Lower Nanwan Formation

Corals and large red algae dominate the biota along with foraminifera and echinoid plates in the lower Nanwan Formation (Figure 3b,c,e,f). Many exposed surfaces, dissolved voids, and reddish brown or rusty yellow nodules are found in this Formation (Figure 3a). The porosity of the samples varies from 0.4% to 56.5%, with an average value of 15.7%. Overall, limestone has a higher porosity than dolostone in Unit II (Figure 2a). The stratum is mainly composed of primary calcite, replacive dolomite, and dolomite cement at the edge of the void (Figure 3). Most well-preserved coral skeletons and algal fragments are primary calcites with rare sparry calcite cement.
Minerals 14 00157 g002
Figure 2. Composite plots. (a) Stratum, mineral composition, and porosity. Note that the red stars represent the sampling locations for in situ LA-ICP-MS analysis, and the black curve represents the location of the exposure surface. (b) Sr isotopic composition. Sr isotope data (solid blue circle) is from Li [21] and new Sr isotope data are shown using red square. (c) δ13C and δ18O. (d) MgO and CaO contents. (e) Sr content.
Figure 2. Composite plots. (a) Stratum, mineral composition, and porosity. Note that the red stars represent the sampling locations for in situ LA-ICP-MS analysis, and the black curve represents the location of the exposure surface. (b) Sr isotopic composition. Sr isotope data (solid blue circle) is from Li [21] and new Sr isotope data are shown using red square. (c) δ13C and δ18O. (d) MgO and CaO contents. (e) Sr content.
Minerals 14 00157 g002
The cation order of dolomite ranges from 0.34 to 0.43, with an average of 0.37 in Unit II, which seems to be positively correlated with the content of sparry dolomite cement (Table 1). Dolomite crystals in this unit are typically homogeneous and easily distinguishable from calcite based on backscatter images and EDS in situ analyses (Figure 4). Following Jones [29], dolomite can be divided into high-calcium dolomite (HCD, %Ca of 55%–62%) and low-calcium dolomite (LCD, %Ca of 50%–55%). All the dolostones from Unit II are composed of nonstoichiometric HCD with the %Ca of 56.2%–59.3% (Table 1).

4.2. Dolomite Types

Dolostones in Unit I and III are composed of replacive dolomites with well-preserved primary fabric, texture, and microstructure, such as corals and red algal remnants (Figure 3c). A small amount of limpid and interlocking anhedral to subhedral dolomite crystals (20 to 50 μm long) are embedded at the edge of the voids. Crystals nearest to the voids are more euhedral than those away from the voids (Figure 3c).
Selective dolomitization occurred in carbonate rocks of Unit II. These dolomites are mainly composed of replacive dolomites and dolomite cements on the lining of voids (Figure 3). Most of the primary texture has been obliterated (Figure 3d–h), except for small amounts of bioclasts such as coral, foraminifera, and algal fragments (Figure 3e,f). Replacive dolomites are microcrystalline (<10 μm). Some crystals show cloudy cores and clear rims (Figure 3e,h). A large amount of dolomite cement is embedded at the voids of limestone, showing subhedral to euhedral rhomb, which is called sucrosic dolomite. The individual crystals of these sucrosic dolomites are 100–200 μm long (Figure 3d,g,h). Several subhedral to euhedral dolomite crystals float in the limestone groundmass (Figure 3d). Dolomite crystals generally become coarser toward the void centers (Figure 3g,h). In brief, dolomite crystal size in this unit has a bimodal distribution of microcrystal and fine crystalline.
Replacive dolomites exhibit homogeneous reddish orange luminescence, whereas dolomite cements exhibit dull bluish gray luminescence or zoned luminescence under the CL (Figure 5). Replacive dolomites are mostly microcrystalline dolomite and have reddish orange luminescence (Figure 5d). In the dolomite cements, cloudy cores have the same reddish orange luminescence as replacive dolomites, whereas the rims show moderately bright reddish orange zones alternating with dull bluish gray zones (Figure 5b). These sucrosic dolomite cements can be divided into DC-I (bright red orange) and DC-II (dull bluish gray or no luminescence) based on their CL characteristics (Figure 5b).

4.3. Sr Isotopic Ages and U–Pb Ages

The Sr isotope ages of primary calcite (coral skeleton) and two U–Pb dates of replacive dolomite are shown in Table 2. In situ U–Pb dating of calcite (coral skeleton) and dolomite cements was performed, but no reliable U–Pb ages were obtained due to the low U/Pb ratios. The 87Sr/86Sr values of primary calcite and replacive dolomite within the sample S7 (493.0 m in depth) are 0.708862 and 0.708865, corresponding to mean ages of 10.9 Ma and 10.8 Ma, respectively. One in situ U–Pb age of replacive dolomite is 10.7 Ma on the sample S7, which is very close to the Sr isotope age of dolomite (Figure 6a,b). The 87Sr/86Sr values of primary calcite (coral skeleton) and replacive dolomite from sample S4 (494.3 m in depth) are 0.708860 and 0.708861, respectively. The 87Sr/86Sr age of 11.0 Ma is also close to the U–Pb age of replacive dolomite (Figure 6a,c).

4.4. Carbon and Oxygen Isotopes

The δ13C and δ18O values for the lower Nanwan Formation from Well NK1 varied with different mineral components (Figure 2a,c). The δ18O values in completely dolomitized rock from Unit I and Unit III vary between 1.6‰ and 4.0‰ with an average of 2.8‰, and the δ13C values range from 1.3‰ to 3.4‰ with an average of 2.6‰. In contrast, limestone or dolomitic limestone in Unit II has negative carbon and oxygen isotope values. The δ13C and δ18O values fluctuate greatly between −6.5‰ and 0.9‰ (average of −2.2‰) and from −6.5‰ and −1.1‰ (average of −4.6‰) (Figure 2c). In addition, there is a weak correlation between carbon and oxygen isotopes in limestones, but not in dolostones (Figure 7b).

4.5. Elemental Geochemistry

4.5.1. MgO, CaO and Sr Contents of the Bulk Rock

The MgO contents in the lower Nanwan Formation range from 0.4% to 19.2%, and CaO contents range from 33.4% to 55.7% (Figure 2d). The contents of MgO and CaO in each sample have a strong negative correlation (R2 = 1, n = 38). The Sr content of the limestones varies from 140 ppm to 976 ppm, with an average of 365 ppm. The Sr content of dolostone is 169–224 ppm, with an average of 192 ppm (Figure 2e).

4.5.2. In Situ Fe, Mn, and Sr Concentrations and Mg/Ca Ratio

The primary calcites (bioclast) contain 93 to 294 ppm Fe with an average of 135 ppm (n = 47). There are no significant differences in Fe content between replacive dolomite and dolomite cement, with the averages of 108 ppm (56 to 150 ppm, n = 55) and 100 ppm (64 to 139 ppm, n = 31), respectively. The Mn contents of different mineral phases are mostly less than 40 ppm (Figures 9c,d and 10; Table 3), with average values of 14 ppm, 11 ppm, and 6 ppm in calcite, replacive dolomite, and dolomite cement, respectively. No correlation is found between the Fe and Mn contents in calcite, replacive dolomite, or dolomite cement. The Sr content in calcites ranges from 155 to 4103 ppm (average 697 ppm, n = 47), whereas replacive dolomite and dolomite cement have contents of 65 to 277 ppm (average 216 ppm, n = 55) and 112 to 285 ppm (average 207 ppm, n = 31), respectively. Overall, the average Fe, Mn, and Sr contents in the replacive dolomite are higher than those in the dolomite cement at the void edge (Figure 9). The average Mg/Ca ratios in the replacive dolomite are lower than those in the dolomite cement (Table 3, Figure 8 and Figure 9).

4.5.3. REY Characteristics

The REE contents in calcite, replacive dolomite and dolomite cement are low (averages of 2.98 ppm, 2.34 ppm and 2.41 ppm, respectively) (Figure 7a). The PAAS-normalized REY patterns [36] of different mineral phases and bulk carbonates are similar to the pattern of modern oxic seawater [35] and modern coral from Meiji Island [34], which are characterized by LREE depletion, HREE enrichment, positive Eu anomalies and negative Ce anomalies (Figure 7a).

5. Discussion

5.1. Ages and Stages of Dolomitization

Recently, strontium isotope and in situ U–Pb dating methods have been used to study Cenozoic reefal carbonates [21,37]. The 87Sr/86Sr ratios of seawater are identical worldwide due to much longer Sr residence time than mixing time in seawater [38]. The 87Sr/86Sr ratio of global seawater has monotonically increased since 40 Ma, which makes it widely used for dating marine carbonate rocks in the Cenozoic [39]. In this study, the new Sr isotope results match well with the previous data [21]. The 87Sr/86Sr age ranges from 11.5 to 6.0 Ma in the lower Nanwan Formation. The replacive dolomite at 493.0 m is dated to 10.8 Ma, which is close to the ages of limestone (10.9 Ma). Similar results were also obtained at 494.3 m. The close ages between primary calcite and replacive dolomite (Figure 6, Table 2) show that the replacive dolomitization might have occurred soon after the deposition of the Nanwan Formation. Uzelman (2009) measured the 87Sr/86Sr isotopes of partially dolomitized island carbonates from Cayman Brac and determined that the fluctuations in Sr isotope ages of bulk rocks were caused by the mixing of calcite and dolomite rather than by different dolomitization events [40]. However, the micro-drilling dating method in the Well NK-1 rules out the possibility of mixed calcite and dolomite. Although we did not obtain the effective age of dolomite cement, given the close age between the primary calcite and the replacive dolomite, it can be inferred that the age difference between replacive dolomite and dolomite cement should be responsible for the Sr isotope age fluctuations of bulk rocks.
The bimodal 87Sr/86Sr histogram of dolostones also shows that two stages of dolomitization existed in the lower Nanwan Formation at 11.5–8.5 Ma and 8.5–6.0 Ma (Figure 10 and Figure 11b). Two stages of dolomitization have also been found in the Cayman Islands [2,13] and Xisha Islands [5]. The different CL colors of replacive dolomite (bright reddish orange) and dolomite cement (dull bluish gray) also proved the existence of two distinct fluids with different chemical compositions (Figure 5). Available evidence indicates that the two dolomitization events created two types of dolomites in the lower Nanwan Formation. Massive replacive dolomitization produced replacive dolomites (stage 1), and the cementation created dolomite cement around the void edges (stage 2).

5.2. Sources and Characteristics of the Dolomitizing Fluids

The stoichiometric variation of island dolostones can reflect the diagenetic evolution and trace chemical composition (e.g., salinity and Mg/Ca ratios) of fluid [41,42,43,44]. Most island dolostones are heterogeneous and composed of HCD and LCD, such as at the Cayman Formation of the Cayman Islands [3] and at the Huangliu Formation of the Xisha Islands in the SCS [45]. Replacive dolomites and dolomite cements in Unit II of the lower Nanwan Formation are entirely HCDs with a low cation order (Figure 4 and Table 1), which is suggestive of the dolomitizing fluid with low Mg/Ca. The elevated salinity and Mg/Ca ratio in dolomitizing fluid can promote the transformation of nonstoichiometric dolomites to near-stoichiometric dolomites [43]. The HCDs are thought to be mediated by the fluid with low Mg/Ca ratios and salinity, and the LCDs are always related to the high Mg/Ca and more saline fluid [41]. In addition, unstable HCDs can be liable to transform to LCDs during recrystallization or post-diagenesis based on thermodynamic theory [11,27,42,46]. Therefore, the dolomites composing entirely of HCDs in the lower Nanwan Formation on Meiji Island and in the Brac Formation on the Cayman Islands [2] were interpreted to be associated with the early dolomitization before the recrystallization.
Carbon and oxygen isotopes are one of the chemical proxies for dolomitized fluids. Carbon and oxygen isotopes of the dolostones in the lower Nanwan Formation (Figure 7b) are similar to those obtained from other island dolostones (δ13C, 0.5‰~3.2‰; δ18O, 2.0‰~3.5‰) mediated by normal seawater and slightly evaporated seawater [1,10,47]. The negative δ13C of limestones indicates meteoric diagenesis. Like dolostones in the Brac Formation, Cayman Islands [2], the positive δ13C values of dolostones in the lower Nanwan Formation suggest that dolomitizing fluids have completely reset the δ13C of precursor limestone. The decoupling of δ13C and δ18O in dolostone further excludes the influence of freshwater during dolomitization [9,48]. It indicates that dolomitizing fluids might be oxic and have low Fe and Mn concentrations [1,2,11,49].
Trace elemental contents can supply information for different diagenetic processes and fluid sources [1,7,14,15]. Mn/Sr ratios can be used to evaluate the degree of carbonate diagenesis [50], and carbonates with little or no diagenesis have a Mn/Sr ratio of less than two [51]. In this study, the average Mn/Sr ratios of replacive dolomite and dolomite cement are less than 0.04 (Table 3), showing no post-dolomitization alterations in the Nanwan Formation. In addition, all dolomites (replacive dolomite and dolomite cement) in Unit II tend to have low Fe (<140 ppm) and Mn concentrations (<20 ppm) (Figure 9), which is also consistent with the results of many studies on other island dolostones mediated by seawater [1,10,47]. The lack of correlation between Fe and Mn suggests that small amounts of Mn and Fe were inherited from precursor carbonates [49]. The in situ Sr contents of replacive dolomites and dolomite cements in the Nanwan Formation are almost identical to that of bulk rock. The low Sr concentration in island dolostones globally (70–300 ppm [1,52]) was suggested to be associated with the open normal seawater or slightly modified seawater [1,7,47]. The REY patterns of replacive dolomite and dolomite cement are also consistent with that of coral and seawater (Figure 7a). Given the content of REY in seawater is very low compared with precursor limestone, dolomites mediated by seawater will not significantly change the REY composition [34,53,54]. The Mg/Ca ratios of replacive dolomites are slightly lower than those of dolomite cements surrounding voids in the lower Nanwan Formation (Figure 8a,b and Figure 9). The higher Mg/Ca ratio and the lack of evaporate minerals suggests that the dolomitizing fluids of cement are slightly evaporated magnesium-rich fluids [39,55].

5.3. Multistage Dolomitization Models for Isolated Atolls

The porosity and permeability of precursor carbonates are thought to have important influences on the flow patterns of dolomitizing fluids and the types of dolostones [1,56,57,58]. In the lower Nanwan Formation, the undolomitized limestone has higher porosity, and dolomites are patchily distributed in limestones (Figure 2a and Figure 3b), showing that seawater fluids preferentially intruded carbonates through voids and induced selective dolomitization in limestone. This interpretation is also supported by petrologic evidence that calcite is always distributed away from voids in partially dolomitized limestone (Figure 3d–h). The dolomitization temperature of the carbonate rocks in Well NK-1 is 18–34 °C based on clumped isotope geochemistry [17], so the dolomitization is considered to have occurred in shallow water and has not undergone burial diagenesis. This is a common diagenetic characteristic of island dolostone [5,18].
Tectonic activities and sea level changes are important factors influencing the growth and dolomitization of coral reefs [59,60,61]. Tectonic activity in the southern SCS was stable after ~11 Ma, and coral reefs thrived on the Meiji Island [21]. The dolomitization of island carbonates in the SCS is generally considered to be related to sea level changes [28]. Available petrological evidence indicates that the periodic exposure of coral reefs during sea level oscillations (Figure 3a and Figure 12c,d) [28,62,63] resulted in calcification during the Miocene and created dissolved voids in limestones (Figure 12a,d). Subsequently, the intrusion of seawater quickly transformed the low-Mg calcite to dolomite (stage 1: replacive dolomitization, Figure 12b,e) which was followed by the formation of dolomite cement (stage 2, Figure 12c,f).
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Figure 11. Dolomitization stages of the Nanwan Formation and sea level history. (a) The mineralogy and Sr isotope of the lower Nanwan Formation in Well NK-1. Sr isotope data (solid blue circle) is from Li [21] and new Sr isotope data are shown using red square. (b) Dolomitization age of the Nanwan Formation. There are two age intervals of 11.5–8.5 Ma (blue dot) and 8.5–6.0 Ma (red plus), corresponding to two stages of dolomitization. (c) Global sea level curve [62]. (d) The sea level variation in the SCS inferred from the organic proxy (BIT) from the Well XK-1 drilled in the Xisha Islands [28].
Figure 11. Dolomitization stages of the Nanwan Formation and sea level history. (a) The mineralogy and Sr isotope of the lower Nanwan Formation in Well NK-1. Sr isotope data (solid blue circle) is from Li [21] and new Sr isotope data are shown using red square. (b) Dolomitization age of the Nanwan Formation. There are two age intervals of 11.5–8.5 Ma (blue dot) and 8.5–6.0 Ma (red plus), corresponding to two stages of dolomitization. (c) Global sea level curve [62]. (d) The sea level variation in the SCS inferred from the organic proxy (BIT) from the Well XK-1 drilled in the Xisha Islands [28].
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In stage 1, dolomite crystals are extremely small (mainly micrite) (Figure 5c,d) and the primary structures are well preserved. Microcrystalline grains always have a larger surface area and are thought to be conducive to the rapid nucleation of dolomite [64,65]. Further, dolomites are found in living coralline algae [66,67], indicating that algae may contain many “seeds” conducive to dolomite nucleation. However, such “dolomite seeds” are too small to be identified using standard X-ray diffraction analysis [49]. Mg2+ provided by algae alone are not sufficient to explain the formation of massive dolostones on isolated islands [49]. The huge supply of seawater during the sea level rise is still considered to be the requisite for the massive dolomitization on coral islands [7,47]. Stage 1 represents the early dolomitization of island carbonates when the sea level rose [49].
In stage 2, the falling sea levels might block seawater exchange between the lagoon and open ocean (Figure 12c). When the slightly evaporated seawater of the lagoon intrudes into the voids in partially dolomitized limestones, fluids might be unsaturated compared to the replacive dolomites which formed in stage 1 and antecedent dolomites near the voids might be partially dissolved. Sucrosic dolomite precipitates as cement around the dissolved voids (Figure 12c,f). The core of sucrosic dolomite has the same cathodoluminescence as that of replacive dolomite (Figure 5b), which further proves that they might be inherited from partially dissolved replacive dolomites [2]. This process creates fabric-destructive textures in partially dolomitized limestones [2]. These sucrosic dolomite cements were commonly found beneath unconformities and were also thought to be probably related to evaporated seawater during sea level fall [5,10,52]. Similar dolomitization processes were also found on some other islands, such as Kitadaitojima [68,69], Grand Cayman [13], Funafuti [70], New Providence [71], and Xisha Island [11]. The two-stage evolution model of massive dolomitization on these islands might be related to the enhancement of seawater activity (e.g., stronger waves, tides or currents) due to the reorganization of ocean circulation in the Pacific Ocean during the Miocene [17,72,73,74], which promoted the migration of Mg2+ in precursor carbonates and the generation of dolostones.

6. Conclusions

We draw the following conclusions based on the combined results of petrological data, in situ U–Pb ages, trace element analysis, and whole-rock geochemistry of the lower Nanwan Formation in Well NK-1 on Meiji Atoll in the South China Sea.
(1)
Undolomitized calcite, replacive dolomite, and dolomite cement coexisted in the lower Nanwan Formation in the Well NK-1. Replacive dolomite and dolomite cements are nonstoichiometric HCDs.
(2)
Fluids of replacive dolomite and dolomite cement in the lower Nanwan Formation are seawater. Higher Mg/Ca ratios are found in dolomite cements, which tended to be mediated by slightly evaporated seawater.
(3)
Strontium isotope ages and in situ U–Pb ages suggest that replacive dolomitization occurred soon after carbonate deposition. The age fluctuation of dolostones shows two-stage dolomitization occurred in the lower Nanwan Formation.
(4)
Coralline algae and lime mud with algal fragments is beneficial for the rapid nucleation of dolomite. Meteoric diagenesis before atoll dolomitization might have created important channels for diagenetic fluids.

Author Contributions

G.L. and W.Y. initiated and designed the research. Y.L. contributed to the field work, LA-ICP-MS and Sr isotopic analyses, interpreted data, and wrote the mauscript draft. X.Z. (Xiyang Zhang), G.L. and W.X. contributed to review and editing. X.Z. (Xiaowei Zhu) and F.Z. contributed to field work. H.H. and W.Z. contributed to petrological analyses. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the National Key Research and Development Program of China (No. 2021YFC3100600), the Natural Science Foundation of Guangdong Province (No. 2022A1515110407; No. 2022A1515111023), the National Natural Science Foundation of China (No. 41976063; No. 41976062 and No. 42376079), and the special fund of South China Sea Institute of Oceanology of the Chinese Academy of Sciences (No. SCSIO2023QY05).

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors are grateful to anonymous reviewers for their critical and constructive reviews, which greatly improved the paper.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Map showing the Nansha Islands in deep southern South China Sea. The red star indicates the location of Well NK-1. The figure was drawn using GMT 6 software (GMT 6.4.0) and the bathymetry and topography data are from the database hosted by the NOAA (https://www.ncei.noaa.gov/products/etopo-global-relief-model, accessed on 28 January 2024). (b) Stratigraphic information regarding Well NK1, which includes the formation, chronology, lithology, and mineralogy (adapted from Li [21]). The red box represents the stratigraphy of the Lower Nanwan Formation.
Figure 1. (a) Map showing the Nansha Islands in deep southern South China Sea. The red star indicates the location of Well NK-1. The figure was drawn using GMT 6 software (GMT 6.4.0) and the bathymetry and topography data are from the database hosted by the NOAA (https://www.ncei.noaa.gov/products/etopo-global-relief-model, accessed on 28 January 2024). (b) Stratigraphic information regarding Well NK1, which includes the formation, chronology, lithology, and mineralogy (adapted from Li [21]). The red box represents the stratigraphy of the Lower Nanwan Formation.
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Figure 3. Core photographs and photomicrographs from the Nanwan Formation in Well NK-1. (a) Core photos showing numerous moldic voids around the exposed surface. (b) Grainstone with calcite (white) and dolomite (brown). Red solid circle represents the micro-drilling position for Sr isotope analysis. (c) Replacive dolomite (RD) with coralline algae (CA). Precursor bioclasts structure is well preserved, and there is no dolomite cement at the edge of the pore (P). (d) Dolomitic limestone stained with Alizarin red S showing dolomite cements (DC) floating in the calcite (C). (e) Lime dolostone stained with Alizarin red S showing undolomitized coral skeletons (CS) and fine and dirty replacive dolomite (RD) with cloudy cores and clear rims. (f) Dolomitic limestone stained with Alizarin red S contains undolomitized foraminifera (F). (g,h) Lime dolostone showing replacive dolomite (RD), dolomite cement (DC) at the edge of the void (P), and undolomitized calcite (C). The depths specified in the top right of each image are below the present-day sea level.
Figure 3. Core photographs and photomicrographs from the Nanwan Formation in Well NK-1. (a) Core photos showing numerous moldic voids around the exposed surface. (b) Grainstone with calcite (white) and dolomite (brown). Red solid circle represents the micro-drilling position for Sr isotope analysis. (c) Replacive dolomite (RD) with coralline algae (CA). Precursor bioclasts structure is well preserved, and there is no dolomite cement at the edge of the pore (P). (d) Dolomitic limestone stained with Alizarin red S showing dolomite cements (DC) floating in the calcite (C). (e) Lime dolostone stained with Alizarin red S showing undolomitized coral skeletons (CS) and fine and dirty replacive dolomite (RD) with cloudy cores and clear rims. (f) Dolomitic limestone stained with Alizarin red S contains undolomitized foraminifera (F). (g,h) Lime dolostone showing replacive dolomite (RD), dolomite cement (DC) at the edge of the void (P), and undolomitized calcite (C). The depths specified in the top right of each image are below the present-day sea level.
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Figure 4. Backscatter images and EDS spot analysis of dolostones within the Nanwan Formation. (a,b) Backscatter images of dolostones at depths of 455.5 m and 492.0 m. Light gray = calcite; dark gray = dolomite; black = porosity. Dolomite is homogeneous, and foraminifera (F) is undolomitized. (c,d) EDS data of the spot from subplot (a). (e,f) EDS data of the spot from subplot (b).
Figure 4. Backscatter images and EDS spot analysis of dolostones within the Nanwan Formation. (a,b) Backscatter images of dolostones at depths of 455.5 m and 492.0 m. Light gray = calcite; dark gray = dolomite; black = porosity. Dolomite is homogeneous, and foraminifera (F) is undolomitized. (c,d) EDS data of the spot from subplot (a). (e,f) EDS data of the spot from subplot (b).
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Figure 5. Thin section photomicrographs and the corresponding cathodoluminescence images. (a,b) Fabric-destructive dolostone showing replacive dolomite with reddish orange luminescence and limpid dolomite cements with zoned luminescence. DC-I = dolomite cement I with reddish orange luminescence located in the core of the rhombs, DC-II = dolomite cement II with dark luminescence. Note that DC-I (the core of dolomite cement) has similar luminescence to that of the replacive dolomite. Depth = 443.1 m. (c,d) Fine and dirty replacive dolomites. Microcrystalline dolomite shows reddish orange luminescence, whereas dolomite cement shows darker luminescence. RD = replacive dolomite, DC = dolomite cement. Depth = 492.0 m.
Figure 5. Thin section photomicrographs and the corresponding cathodoluminescence images. (a,b) Fabric-destructive dolostone showing replacive dolomite with reddish orange luminescence and limpid dolomite cements with zoned luminescence. DC-I = dolomite cement I with reddish orange luminescence located in the core of the rhombs, DC-II = dolomite cement II with dark luminescence. Note that DC-I (the core of dolomite cement) has similar luminescence to that of the replacive dolomite. Depth = 443.1 m. (c,d) Fine and dirty replacive dolomites. Microcrystalline dolomite shows reddish orange luminescence, whereas dolomite cement shows darker luminescence. RD = replacive dolomite, DC = dolomite cement. Depth = 492.0 m.
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Figure 6. (a) Cross plot of Sr isotope ages and in situ U–Pb ages at depths of 493.0 m and 494.3 m in Well NK-1. Two U–Pb ages of dolomite have a good fit with 87Sr/86Sr isotope ages. (b,c) Harmonic diagram of U–Pb dating in the dolomite zone from two samples. The insert in the bottom left corner of each image shows ablation points of U–Pb dating.
Figure 6. (a) Cross plot of Sr isotope ages and in situ U–Pb ages at depths of 493.0 m and 494.3 m in Well NK-1. Two U–Pb ages of dolomite have a good fit with 87Sr/86Sr isotope ages. (b,c) Harmonic diagram of U–Pb dating in the dolomite zone from two samples. The insert in the bottom left corner of each image shows ablation points of U–Pb dating.
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Figure 7. Composite plots. (a) PAAS-normalized REY distribution patterns from calcite, replacive dolomite, and dolomite cement in partially dolomitized carbonates in Unit II using laser ablation. The reference REY data include coral skeletons from the Meiji Atoll [34] and surface seawater in the southern SCS [35]. (b) Carbon and oxygen isotopic compositions of limestone and dolostone. Note that there is a certain correlation between carbon and oxygen isotopes in limestone but not in dolostone.
Figure 7. Composite plots. (a) PAAS-normalized REY distribution patterns from calcite, replacive dolomite, and dolomite cement in partially dolomitized carbonates in Unit II using laser ablation. The reference REY data include coral skeletons from the Meiji Atoll [34] and surface seawater in the southern SCS [35]. (b) Carbon and oxygen isotopic compositions of limestone and dolostone. Note that there is a certain correlation between carbon and oxygen isotopes in limestone but not in dolostone.
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Figure 8. In situ element composition using laser ablation. (a,b) Mg/Ca ratio (mol/mol), (c,d) Mn content, (e,f) Sr content. RD = replacive dolomite. DC = dolomite cement.
Figure 8. In situ element composition using laser ablation. (a,b) Mg/Ca ratio (mol/mol), (c,d) Mn content, (e,f) Sr content. RD = replacive dolomite. DC = dolomite cement.
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Figure 9. In situ elemental compositions (Mg/Ca ratios and Mn, Fe and Sr contents) of replacive dolomites (RD) and dolomite cements (DC) in partially dolomitized carbonates.
Figure 9. In situ elemental compositions (Mg/Ca ratios and Mn, Fe and Sr contents) of replacive dolomites (RD) and dolomite cements (DC) in partially dolomitized carbonates.
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Figure 10. The distribution curves of 87Sr/86Sr values and ages of dolostones in the Nanwan Formation using class intervals of 0.00001 (a) and 0.00002 (b).
Figure 10. The distribution curves of 87Sr/86Sr values and ages of dolostones in the Nanwan Formation using class intervals of 0.00001 (a) and 0.00002 (b).
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Figure 12. Schematic diagrams show the dolomitization process in the lower Nanwan Formation on the Meiji Atoll. (a–c) Large-scale diagenetic evolution. (d–f) Microscopic evolution pathway.
Figure 12. Schematic diagrams show the dolomitization process in the lower Nanwan Formation on the Meiji Atoll. (a–c) Large-scale diagenetic evolution. (d–f) Microscopic evolution pathway.
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Table 1. Mineral compositions and dolomite stoichiometry of the partially dolomitized carbonates from the lower Nanwan Formation in Well NK-1.
Table 1. Mineral compositions and dolomite stoichiometry of the partially dolomitized carbonates from the lower Nanwan Formation in Well NK-1.
Sample No.Depth/mCalcite/%Dolomite/%Cation Order of Dolomited(104)CaCO3
(mol%)		Dolomite TypeDolomite Cement (%)
S1489.877.822.20.432.9080658.2HCD40
S2495.571.928.10.432.9041956.9HCD20
S3503.770.329.70.362.9052957.3HCD10
S4494.348.551.50.362.9053857.3HCD30
S5455.538.961.10.342.9019756.2HCD10
S6468.620.479.60.332.9028856.5HCD20
S7493.020.379.70.342.9111959.3HCD20
S8492.018.981.10.382.9101958.9HCD30
Table 2. Sr isotope ages and in situ U–Pb ages of calcite (coral skeleton) and replacive dolomite from two samples.
Table 2. Sr isotope ages and in situ U–Pb ages of calcite (coral skeleton) and replacive dolomite from two samples.
No.Depth/mMineralogy/%87Sr/86SrMean Sr Age/MaIn Situ U–Pb Age/Ma
S7-C493.0primary calcite, 100.0%0.708862 ± 510.9-
S7-Dreplacive dolomite, 99.2%0.708865 ± 510.810.7 ± 0.8
S4-C494.3primary calcite, 100.0%0.708860 ± 511.0-
S4-Dreplacive dolomite, 98.7%0.708861 ± 411.011.0 ± 0.3
Note: “-” means no data.
Table 3. In situ elemental composition (average Mg/Ca ratio and Mn, Fe, and Sr contents, Mn/Sr) using the laser ablation technique.
Table 3. In situ elemental composition (average Mg/Ca ratio and Mn, Fe, and Sr contents, Mn/Sr) using the laser ablation technique.
Sample No.Depth/mMineralNumber of PointsAverage Mg/Ca (mol/mol)Average Mn/ppmAverage Fe/ppmAverage Sr/ppmAverage Mn/Sr
S1489.8Primary calcite20.0315.72556400.02
Replacive dolomite------
Dolomite cement90.536.71181730.04
S2495.5Primary calcite80.0413.31428880.03
Replacive dolomite90.4810.31302510.04
Dolomite cement40.625.21022040.02
S3503.7Primary calcite130.0517.61225190.04
Replacive dolomite150.5319.11102220.09
Dolomite cement------
S5455.5Primary calcite70.010.31271523<0.01
Replacive dolomite130.614.5931970.02
Dolomite cement30.644.6961740.03
S6468.6Primary calcite30.010.9118494<0.01
Replacive dolomite20.635.6902230.03
Dolomite cement50.664.6781920.02
S7493.0Primary calcite120.0322.61342390.11
Replacive dolomite70.537.01092410.03
Dolomite cement130.627.0991900.04
S8492.0Primary calcite20.024.61271300<0.01
Replacive dolomite70.5810.1992590.04
Dolomite cement60.673.9892120.02
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Luo, Y.; Li, G.; Zhang, X.; Xu, W.; Zhu, X.; Zhou, W.; Huang, H.; Yan, W.; Zhong, F. Multistage Diagenetic Fluid Shaping Miocene Island Dolostones on One Isolated Atoll in the South China Sea: Insights from LA-ICP-MS U–Pb Dating and Geochemical Characterization. Minerals 2024, 14, 157. https://doi.org/10.3390/min14020157

AMA Style

Luo Y, Li G, Zhang X, Xu W, Zhu X, Zhou W, Huang H, Yan W, Zhong F. Multistage Diagenetic Fluid Shaping Miocene Island Dolostones on One Isolated Atoll in the South China Sea: Insights from LA-ICP-MS U–Pb Dating and Geochemical Characterization. Minerals. 2024; 14(2):157. https://doi.org/10.3390/min14020157

Chicago/Turabian Style

Luo, Yun, Gang Li, Xiyang Zhang, Weihai Xu, Xiaowei Zhu, Wanqiu Zhou, Huiwen Huang, Wen Yan, and Fuchang Zhong. 2024. "Multistage Diagenetic Fluid Shaping Miocene Island Dolostones on One Isolated Atoll in the South China Sea: Insights from LA-ICP-MS U–Pb Dating and Geochemical Characterization" Minerals 14, no. 2: 157. https://doi.org/10.3390/min14020157

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