Characterisation of primary and secondary carbonates in hypabyssal kimberlites: an integrated compositional and Sr-isotopic approach

Carbonates in fresh hypabyssal kimberlites worldwide have been studied to understand their origin [i.e. primary magmatic (high T) versus deuteric (‘low T’) versus hydrothermal/alteration (‘low T’)] and identify optimal strategies for petrogenetic studies of kimberlitic carbonates. The approach presented here integrates detailed textural characterisation, cathodoluminescence (CL) imaging, in situ major- and trace-element analysis, as well as in situ Sr-isotope analysis. The results reveal a wide textural diversity. Calcite occurs as fine-grained groundmass, larger laths, segregations, veins or as a late crystallising phase, replacing olivine or early carbonates. Different generations of carbonates commonly coexist in the same kimberlite, each one defined by a characteristic texture, CL response and composition (e.g., variable Sr and Ba concentrations). In situ Sr isotope analysis revealed a magmatic signature for most of the carbonates, based on comparable ⁸⁷Sr/⁸⁶Sr values between these carbonates and the coexisting perovskite, a robust magmatic phase. However, this study also shows that in situ Sr isotope analysis not always allow distinction between primary (i.e., magmatic) and texturally secondary carbonates within the same sample. Carbonates with a clear secondary origin (e.g., late-stage veins) occasionally show the same moderately depleted ⁸⁷Sr/⁸⁶Sr ratios of primary carbonates and coexisting perovskite (e.g., calcite laths-shaped crystals with ⁸⁷Sr/⁸⁶Sr values identical within uncertainty to those of vein calcite in the De Beers kimberlite). This complexity emphasises the necessity of integrating detailed petrography, geochemical and in situ Sr isotopic analyses for an accurate interpretation of carbonate petrogenesis in kimberlites. Therefore, the complex petrogenesis of carbonates demonstrated here not only highlights the compositional variability of kimberlites, but also raises concerns about the use of bulk-carbonate C-O isotope studies to characterise the parental melt compositions. Conversely, our integrated textural and in situ study successfully identifies the most appropriate (i.e. primary) carbonates for providing constraints on the isotopic parameters of parental kimberlite magmas.

     

Mineralogy of primary carbonate-bearing hypabyssal kimberlite, Lac de Gras, Slave Province, Northwest Territories, Canada

A suite of fresh, Late Cretaceous to Eocene hypabyssal kimberlites from the Lac de Gras field were studied in order to understand better carbonate, silicate and oxide paragenesis. The samples have excellent preservation of textures and primary mineralogy and are archetypal or Group 1 kimberlite. Five kimberlite localities are identified as calcite-bearing based on the presence of high Sr–Ba calcite as phenocrysts, microphenocrysts and in segregations. Three kimberlite localities are identified as dolomite-bearing based on the presence of mixed calcite–dolomite segregations containing oscillatory and banded textures of calcite–dolomite solid solution and dolomite (±magnesite). Sr–Ba calcite are characterized by high X_Ca (>0.95) and are enriched in Sr (4900–11,100 ppm) and Ba (3200–14,200 ppm). The calcite–dolomite and dolomite–magnesite solid solution compositions span the X_Ca range from 0.42 to 0.95, and typically have Sr and Ba contents in the range of 1000–4000 ppm. The carbonate, silicate and oxide mineral compositions suggest that the origin of the calcite-bearing versus dolomite-bearing kimberlites studied is related to subtle differences in parent magma composition, in particular, the CO₂/H₂O ratio. Formation of the carbonates reflects the latter part of a protracted magmatic crystallization sequence, in which Sr–Ba calcite precipitates from an evolved kimberlite melt. Subsequently, calcite–dolomite solid solution and dolomite is precipitated from localized, Mg-rich carbonate fluids at relatively high temperatures (higher than serpentine stability).     

Anomalous unradiogenic 87Sr//86Sr ratios in ultrahigh‐pressure crustal carbonates – evidence for fluid infiltration during deep subduction?

Marbles from Changpu (Dabie Shan, eastern China), subducted to 4.4 GPa, have ⁸⁷Sr/⁸⁶Sr values < 0.7040. These low ⁸⁷Sr/⁸⁶Sr values, which would imply a sedimentation age > 2 Ga if considered as primary signature, reflect fluid–rock interaction with a fluid from a low‐⁸⁷Sr/⁸⁶Sr source. The introduction of low‐⁸⁷Sr/⁸⁶Sr was paralleled by introduction of Mg and loss of Si, K and Na in such a way that carbonates from the purest marbles have the least evolved Sr isotopic composition. Introduction of Mg is also indicated by the distribution of calcite and dolomite. Calcite forms inclusions in garnet, whereas dolomite is restricted to the matrix. These chemical changes, inferred from the mineralogy, in combination with textural evidence require a mobile metamorphic fluid. P–T–X constraints for fluid generation and for permeability increase related to mineral reactions and phase transitions suggest that the marbles acquired their anomalous Sr‐isotopic composition during subduction below 60 km. The marbles with the least radiogenic Sr isotopic composition demonstrate that crustal rocks may lose their isotopic fingerprint during deep subduction.     

Mg-Sr isotopes of low-δ26Mg basalts tracing recycled carbonate species: Implication for the initial melting depth of the carbonated mantle in Eastern China

ABSTRACT

Recent studies show that crustal carbonates recycled into the mantle can be traced using Mg isotopes of basalts. However, the species of recycled carbonates are poorly constrained. Carbonates have lower δ²⁶Mg values and higher ⁸⁷Sr/⁸⁶Sr ratios relative to the mantle, but different carbonate species display different mixing curves with the mantle in the Mg-Sr isotopic diagram because of differences in their Sr and Mg contents. Thus a combined study of Mg-Sr isotopes can constrain the species of deeply recycled carbonates. Here, we present newly determined ⁸⁷Sr/⁸⁶Sr ratios of the <110 Ma basalts from Eastern China, and together with published Mg isotopic data we evaluate the species of recycled carbonates in the mantle and discuss their implication. The <110 Ma basalts display low δ²⁶Mg values of ?0.60 to ?0.30‰ and relatively low initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70328 to 0.70537, suggesting that their mantle source was hybridized by recycled carbonates with a light Mg isotopic composition which had more significant effects on Mg than Sr isotope ratios. Mg-Sr isotopic data indicate that the recycled carbonates consist of magnesite and aragonite, but the possibility of calcite and dolomite cannot be eliminated. Based on the carbonated peridotite solidus, the equilibrium line between dolomite and magnesite + aragonite, as well as the mantle adiabat, the initial melting depth of the carbonated mantle, the source region of the studied basalts, was constrained at ~300–360 km. Thus, the subducted depth of the west Pacific slab underlying the carbonated mantle and supplying recycled carbonates should be greater than ~300–360 km, consistent with the seismic tomography result that the west Pacific slab now stagnates in the mantle transition zone.     

Reconciling the elemental and Sr isotope composition of Himalayan weathering fluxes: insights from the carbonate geochemistry of stream waters

Determining the relative proportions of silicate vs. carbonate weathering in the Himalaya is important for understanding atmospheric CO₂ consumption rates and the temporal evolution of seawater Sr. However, recent studies have shown that major element mass-balance equations attribute less CO₂ consumption to silicate weathering than methods utilizing Ca/Sr and ⁸⁷Sr/⁸⁶Sr mixing equations. To investigate this problem, we compiled literature data providing elemental and ⁸⁷Sr/⁸⁶Sr analyses for stream waters and bedrock from tributary watersheds throughout the Himalaya Mountains. In addition, carbonate system parameters (P_CO2, mineral saturation states) were evaluated for a selected suite of stream waters. The apparent discrepancy between the dominant weathering source of dissolved major elements vs. Sr can be reconciled in terms of carbonate mineral equilibria. Himalayan streams are predominantly Ca²⁺-Mg²⁺-HCO₃⁻ waters derived from calcite and dolomite dissolution, and mass-balance calculations demonstrate that carbonate weathering contributes ∼87% and ∼76% of the dissolved Ca²⁺ and Sr²⁺, respectively. However, calculated Ca/Sr ratios for the carbonate weathering flux are much lower than values observed in carbonate bedrock, suggesting that these divalent cations do not behave conservatively during stream mixing over large temperature and P_CO2 gradients in the Himalaya.The state of calcite and dolomite saturation was evaluated across these gradients, and the data show that upon descending through the Himalaya, ∼50% of the streams evaluated become highly supersaturated with respect to calcite as waters warm and degas CO₂. Stream water Ca/Mg and Ca/Sr ratios decrease as the degree of supersaturation with respect to calcite increases, and Mg²⁺, Ca²⁺, and HCO₃⁻ mass balances support interpretations of preferential Ca²⁺ removal by calcite precipitation. On the basis of patterns of saturation state and P_CO2 changes, calcite precipitation was estimated to remove up to ∼70% of the Ca²⁺ originally derived from carbonate weathering. Accounting for the nonconservative behavior of Ca²⁺ during riverine transport brings the Ca/Sr and ⁸⁷Sr/⁸⁶Sr composition of the carbonate weathering flux into agreement with the composition of carbonate bedrock, thereby permitting consistency between elemental and Sr isotope approaches to partitioning stream water solute sources. These results resolve the dissolved Sr²⁺ budget and suggest that the conventional application of two-component Ca/Sr and ⁸⁷Sr/⁸⁶Sr mixing equations has overestimated silicate-derived Sr²⁺ and HCO₃⁻ fluxes from the Himalaya. In addition, these findings demonstrate that integrating stream water carbonate mineral equilibria, divalent cation compositional trends, and Sr isotope inventories provides a powerful approach for examining weathering fluxes.     

Hydrologic controls on radiogenic Sr in meltwater from an alpine glacier system: Athabasca Glacier,Canada

Filtered subglacial meltwater samples were collected daily during the onset of melt (May) and peak melt (July) over the 2011 melt season at the Athabasca Glacier (Alberta, Canada) and analyzed for strontium-87/strontium-86 (⁸⁷Sr/⁸⁶Sr) isotopic composition to infer the evolution of subglacial weathering processes. Both the underlying bedrock composition and subglacial water–rock interaction time are the primary influences on meltwater ⁸⁷Sr/⁸⁶Sr. The Athabasca Glacier is situated atop Middle Cambrian carbonate bedrock that also contains silicate minerals. The length of time that subglacial meltwater interacts with the underlying bedrock and substrate is a predominant determining factor in solute concentration. Over the course of the melt season, increasing trends in Ca/K and Ca/Mg correspond to overall decreasing trends in ⁸⁷Sr/⁸⁶Sr, which indicate a shift in weathering processes from the presence of silicate weathering to primarily carbonate weathering.Early in the melt season, rates of carbonate dissolution slow as meltwater approaches saturation with respect to calcite and dolomite, corresponding to an increase in silicate weathering that includes Sr-rich silicate minerals, and an increase in meltwater ⁸⁷Sr/⁸⁶Sr. However, carbonate minerals are preferentially weathered in unsaturated waters. During the warmest part of a melt season the discharged meltwater is under saturated, causing an increase in carbonate weathering and a decrease in the radiogenic Sr signal. Likewise, larger fraction contributions of meltwater from glacial ice corresponds to lower ⁸⁷Sr/⁸⁶Sr values, as the meltwater has lower water–rock interaction times in the subglacial system. These results indicate that although weathering of Sr-containing silicate minerals occurs in carbonate dominated glaciated terrains, the continual contribution of new meltwater permits the carbonate weathering signal to dominate.     

Relative contributions of silicate and carbonate rocks to riverine Sr fluxes in the headwaters of the Ganges

Exhumation of the Himalayan-Tibetan orogen is implicated in the marked rise in seawater ⁸⁷Sr/⁸⁶Sr ratios since 40 Ma. However both silicate and carbonate rocks in the Himalaya have elevated ⁸⁷Sr/⁸⁶Sr ratios and there is disagreement as to how much of the ⁸⁷Sr flux is derived from silicate weathering. Most previous studies have used element ratios from bedrock to constrain the proportions of silicate- and carbonate-derived Sr in river waters. Here we use arrays of water compositions sampled from the head waters of the Ganges in the Indian and Nepalese Himalaya to constrain the end-member element ratios. The compositions of tributaries draining catchments restricted to a limited range of geological units can be described by two-component mixing of silicate and carbonate-derived components and lie on a plane in multicomponent composition space. Key elemental ratios of the carbonate and silicate components are determined by the intersection of the tributary mixing plane with the planes Na = 0 for carbonate and constant Ca/Na for silicate. The fractions of Sr derived from silicate and carbonate sources are then calculated by mass-balance in Sr-Ca-Mg-Na composition space. Comparison of end-member compositions with bedrock implies that secondary calcite deposition may be important in some catchments and that dissolution of low-Mg trace calcite in silicate rocks may explain discrepancies in Sr-Ca-Na-Mg covariation. Alternatively, composition-dependent precipitation or incongruent dissolution reactions may rotate mixing trends on cation-ratio diagrams. However the calculations are not sensitive to transformations of the compositions by incongruent dissolution or precipitation processes provided that the transformed silicate and carbonate component vectors are constrained. Silicates are calculated to provide ∼50% of the dissolved Sr flux from the head waters of the Ganges assuming that discrepancies between Ca-Mg-Na covariation and the silicate rock compositions arise from addition of trace calcite. If the Ca-Mg-Na mixing plane is rotated by composition-dependent secondary calcite deposition, this estimate would be increased. Moreover, when ⁸⁷Sr/⁸⁶Sr ratios of the Sr inputs are considered, silicate Sr is responsible for 70 ± 16% (1σ) of the ⁸⁷Sr flux forcing changes in seawater Sr-isotopic composition. Since earlier studies predict that silicate weathering generates as little as 20% of the total Sr flux in Himalayan river systems, this study demonstrates that the significance of silicate weathering can be greatly underestimated if the processes that decouple the water cation ratios from those of the source rocks are not properly evaluated.     

Major-ion chemistry, ��13C and 87Sr/86Sr as indicators of hydrochemical evolution and sources of salinity in groundwater in the Yuncheng Basin, China

Processes controlling hydrogeochemistry in the Yuncheng Basin, China, were characterised using major-ion chemistry, ⁸⁷Sr/⁸⁶Sr ratios and ??¹³C values. Evapotranspiration during recharge increased solute concentrations by factors of ??5?C50 in deep palaeowaters, while higher degrees of evapotranspiration have occurred in shallow, modern groundwater. Aquifer sediments (loess) contain approximately 15 weight% calcite; trends in groundwater HCO₃ concentrations and ??¹³C values (ranging from ?16.4 to ?8.2??) indicate that carbonate weathering is a significant source of DIC. Groundwater ⁸⁷Sr/⁸⁶Sr ratios (0.7110?C0.7162, median of 0.7116) are similar to those in both loess carbonate (0.7109?C0.7116) and local rainfall (0.7112), and are significantly lower than Sr in aquifer silicates (0.7184?C0.7251). Despite evidence for substantial carbonate dissolution, groundwater is generally Ca-poor (<?10% of total cations) and Na-rich, due to cation exchange. Saturation with respect to carbonate minerals occurs during or soon after recharge (all calcite and dolomite saturation indices are positive). Subsequent carbonate dissolution in the deep aquifer must occur as a second-stage process, in response to Ca loss (by ion exchange) and/or via incongruent dissolution of dolomite and impure calcite. The latter is consistent with positive correlations between ??¹³C values and Mg/Ca and Sr/Ca ratios (r ²?=?0.32 and 0.34).     

Determination of Sr/Sr mass-dependent isotopic fractionation and radiogenic isotope variation of Sr/Sr in the Neoproterozoic Doushantuo Formation

We measured both mass-dependent isotope fractionation of δ⁸⁸Sr (⁸⁸Sr/⁸⁶Sr) and radiogenic isotopic variation of Sr (⁸⁷Sr/⁸⁶Sr) for the Neoproterozoic Doushantuo Formation that deposited as a cap carbonate immediately above the Marinoan-related Nantuo Tillite. The δ⁸⁸Sr and ⁸⁷Sr/⁸⁶Sr compositions showed three remarkable characteristics: (1) high radiogenic ⁸⁷Sr/⁸⁶Sr values and gradual decrease in the ⁸⁷Sr/⁸⁶Sr ratios, (2) anomalously low δ⁸⁸Sr values at the lower part cap carbonate, and (3) a clear correlation between ⁸⁷Sr/⁸⁶Sr and δ⁸⁸Sr values. These isotopic signatures can be explained by assuming an extreme greenhouse condition after the Marinoan glaciation. Surface seawater, mixed with a large amount of freshwater from continental crusts with high ⁸⁷Sr/⁸⁶Sr and lighter δ⁸⁸Sr ratios, was formed during the extreme global warming after the glacial event. High atmospheric CO₂ content caused sudden precipitation of cap carbonate from the surface seawater with high ⁸⁷Sr/⁸⁶Sr and lighter δ⁸⁸Sr ratios. Subsequently, the mixing of the underlying seawater, with unradiogenic Sr isotope compositions and normal δ⁸⁸Sr ratios, probably caused gradual decrease of the ⁸⁷Sr/⁸⁶Sr ratios of the seawater and deposition of carbonate with normal δ⁸⁸Sr ratios. The combination of ⁸⁷Sr/⁸⁶Sr and δ⁸⁸Sr isotope systematics gives us new insights on the surface evolution after the Snowball Earth.     

Nd,Sr, Pb,Ar, and 0 isotopic systematics of Sturgeon Lake kimberlite,Saskatchewan, Canada: constraints on emplacement age,alteration, and source composition

Rb-Sr isotopic dating of phlogopite megacryst samples separated from Sturgeon Lake kimberlite, Saskatchewan, yields a crystallization age of 98±1 Ma (2 , MSWD=1.2; ⁸⁷Sr/⁸⁶Sr(t)=0.7059). The ⁴⁰Ar/³⁹Ar analyses of a phlogopite megacryst sample indicate the presence of large amounts of excess ⁴⁰Ar and yield an excessively old age of 410 Ma. Assessment of the Ar data using isotope correlation plots indicates clustering of the data points about a mixing line between the radiogenic ⁴⁰Ar component at 98 Ma and a trapped component with uniform ³⁶Ar/⁴⁰Ar and Cl/⁴⁰Ar. Values of {ie212-1} as high as +20%. (VSMOW) for calcite from the groundmass and a whole-rock sample indicate pervasive lowtemperature alteration. The {ie212-2} of matrix carbonate is-11.3%. (PDB), slightly lighter than typical values from the literature. The {ie212-3} values of about +5%. (VSMOW) for brown phlogopite megacrysts may be primary, green phlogopites are interpreted to be an alteration product of the brown variety and are 2%. heavier. Initial Nd-Sr-Pb isotopic ratios for a whole-rock sample {ie212-4}; ⁸⁷Sr/⁸⁶Sr=0.7063, ²⁰⁶Pb/²⁰⁴Pb=18.67, ²⁰⁷Pb/²⁰⁴Pb=15.54, ²⁰⁸Pb/²⁰⁴Pb=38.97) suggest an affinity with group I kimberlites. Initial {ie212-5} values of +1.7 and +0.5 (⁸⁷Sr/ ⁸⁶Sr(t)=0.7053 and 0.7050) for eclogitic and lherzolitic garnet megacryst samples, and values of 0.0 for two phlogopite megacryst samples reflect an origin from an isotopically evolving melt due to assimilation of heterogeneous mantle. Lilac high-Cr lherzolitic garnet megacrysts give an unusually high {ie212-6} of +28.6 (⁸⁷Sr/⁸⁶Sr=0.7046) indicating a xenocrystic origin probably from the lithospheric mantle. The very radiogenic ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios of the kimberlite are consistent with melting of EM II (enriched) mantle components.     

The isotopic composition of strontium in some South African kimberlites

Strontium isotopic studies of kimberlites reveal no significant differences between the respective whole-rock Sr⁸⁷/Sr⁸⁶ ratios of fissure and pipe kimberlites. Kimberlites from the Swartruggens fissure (calcareous micaceous kimberlite) have Sr⁸⁷/Sr⁸⁶ ratios of from 0.709 to 0.716, whilst those from the Wesselton pipe have Sr⁸⁷/Sr⁸⁶ ratios of from 0.708 to 0.715. Other kimberlites range from 0.706 to 0.715. Samples are considered to be late Cretaceous to early Tertiary and thus the ratios are approximately initial ratios. The Sr⁸⁷/Sr⁸⁶ ratios bear no relation to the Rb or Sr content of individual kimberlite bodies. The high initial ratios are not due to bulk assimilation of granitic material in either a kimberlite or carbonatitic magma. Rb-Sr data for garnet peridotites and eclogite xenoliths in kimberlite are not compatible with production of kimberlite by eclogite fractionation from a melt derived from garnet lherzolite. The Sr isotopic composition of kimberlite is compatible with partial melting of garnet mica peridotite. The isotopic composition of liquids formed by partial melting of this rock can be modified by (i) gross contamination with material of low Sr⁸⁷/Sr⁸⁶ ratio or (ii) selective diffusion of material of high Sr⁸⁷/Sr⁸⁶ ratio into kimberlitic fluids.     

Ca/Sr and Sr isotope systematics of a Himalayan glacial chronosequence: carbonate versus silicate weathering rates as a function of landscape surface age

We explored changes in the relative importance of carbonate vs. silicate weathering as a function of landscape surface age by examining the Ca/Sr and Sr isotope systematics of a glacial soil chronosequence located in the Raikhot watershed within the Himalaya of northern Pakistan. Bedrock in the Raikhot watershed primarily consists of silicate rock (Ca/Sr ≈ 0.20 μmol/nmol, ⁸⁷Sr/⁸⁶Sr ≈ 0.77 to 1.2) with minor amounts of disseminated calcite (Ca/Sr ≈ 0.98 to 5.3 μmol/nmol, ⁸⁷Sr/⁸⁶Sr ≈ 0.79 to 0.93) and metasedimentary carbonate (Ca/Sr ≈ 1.0 to 2.8 μmol/nmol, ⁸⁷Sr/⁸⁶Sr ≈ 0.72 to 0.82). Analysis of the exchangeable, carbonate, and silicate fractions of seven soil profiles ranging in age from ∼0.5 to ∼55 kyr revealed that carbonate dissolution provides more than ∼90% of the weathering-derived Ca and Sr for at least 55 kyr after the exposure of rock surfaces, even though carbonate represents only ∼1.0 wt% of fresh glacial till. The accumulation of carbonate-bearing dust deposited on the surfaces of older landforms partly sustains the longevity of the carbonate weathering flux. As the average landscape surface age in the Raikhot watershed increases, the Ca/Sr and ⁸⁷Sr/⁸⁶Sr ratios released by carbonate weathering decrease from ∼3.6 to ∼0.20 μmol/nmol and ∼0.84 to ∼0.72, respectively. The transition from high to low Ca/Sr ratios during weathering appears to reflect the greater solubility of high Ca/Sr ratio carbonate relative to low Ca/Sr ratio carbonate. These findings suggest that carbonate weathering controls the dissolved flux of Sr emanating from stable Himalayan landforms comprising mixed silicate and carbonate rock for tens of thousands of years after the mechanical exposure of rock surfaces to the weathering environment.     

Strontium isotope geochemistry of groundwater affected by human activities in Nandong underground river system, China

The Nandong Underground River System (NURS) is located in a typical karst area dominated by agriculture in SE Yunnan Province, China. Groundwater plays an important role in the social and economical development in the area. The effects of human activities (agriculture and sewage effluents) on the Sr isotope geochemistry were investigated in the NURS. Seventy-two representative groundwater samples, which were collected from different aquifers (calcite and dolomite), under varying land-use types, both in summer and winter, showed significant spatial differences and slight seasonal variations in Sr concentrations and ⁸⁷Sr/⁸⁶Sr ratios. Agricultural fertilizers and sewage effluents significantly modified the natural ⁸⁷Sr/⁸⁶Sr ratios signature of groundwater that was otherwise dominated by water-rock interaction. Three major sources of Sr could be distinguished by ⁸⁷Sr/⁸⁶Sr ratios and Sr concentrations in karst groundwater. Two sources of Sr are the Triassic calcite and dolomite aquifers, where waters have low Sr concentrations (0.1-0.2 mg/L) and low ⁸⁷Sr/⁸⁶Sr ratios (0.7075-0.7080 and 0.7080-0.7100, respectively); the third source is anthropogenic Sr from agricultural fertilizers and sewage effluents with waters affected having radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.7080-0.8352 for agricultural fertilizers and 0.7080-0.7200 for sewage effluents, respectively), with higher Sr concentrations (0.24-0.51 mg/L). Due to the overlapping ⁸⁷Sr/⁸⁶Sr ratios, it is difficult to distinguish the sources of Sr in groundwater samples contaminated by agricultural fertilizers or sewage effluents based only on their ⁸⁷Sr/⁸⁶Sr ratios. However, ⁸⁷Sr/⁸⁶Sr ratios do provide key information for natural and anthropogenic sources in karst groundwater.     

Strontium isotope composition of the lower proterozoic carbonate concretions: The Zaonega Formation,Southeast Karelia

The middle part of the volcanosedimentary Zaonega Formation of the Ludikovian Suprahorizon (approximately 2.0 Ga) includes large carbonates concretions and lenses in shungite layers. Carbonate lenses and concretions are primarily elongated and flattened, and their thickness varies from tens of centimeters to a few meters. Some lenses retain relicts of lamination. Concretions are composed of calcite or dolomite. They contain abundant organic matter, as well as mica, talc, chlorite, quartz, and pyrite crystals. The calcite concretions contain some dolomite admixture (Mg/Ca = 0.011?0.045) and differ from sedimentary limestones by a low Fe/Mn value (0.3–2.1). The Sr content is as much as 385–505 μg/g in most samples and is low (86 μg/g) only in one sample. The Rb-Sr systematics of carbonate concretions was studied with the stepwise dissolution procedure, which included processing with the ammonium acetate solution (AMA fraction) to partially remove the secondary carbonate material, with dissolution of the residue in acetic acid (ACA fraction). In individual calcite samples, discrepancy between the measured ⁸⁷Sr/⁸⁶Sr values in the AMA and ACA calcite fractions shows a variation range of 0.0008–0.0033. The initial ⁸⁷Sr/⁸⁶Sr ratio in the ACA fractions of the studied samples varies from 0.7053 to 0.7162. The ratio shows a positive correlation with Mg/Ca and the proportion of siliciclastic admixture and negative correlation with the Mn content. The concretions were formed when the sediments subsided, probably, during the transition from a zone with “mild” reductive conditions to zones with active sulfate reduction and methanogenesis. In the sulfate reduction zone, where most pyrite-bearing concretions were formed, the sediment was not geochemically exchaged with the bottom water and was evolved into a closed or semiclosed system. Processes of diagenesis in this zone promoted the release of the radiogenic ⁸⁷Sr from the associated siliciclastic minerals, resulting in growth of the initial ⁸⁷Sr/⁸⁶Sr in concretions up to 0.7108–0.7162. Some calcite concretions, which lacked pyrite (or contained its minimal amount) were likely formed in a thin surficial sediment layer located above the sulfate reduction zone. Therefore, they precipitated Sr in isotope equilibrium with Sr of the bottom water. However, large concretions and carbonate lenses with an insignificant siliciclastic admixture could retain the signature of early diagenesis or even sedimentation. The initial ⁸⁷Sr/⁸⁶Sr ratio in one of such samples with the siliciclastic admixture of 6.2% makes it possible to estimate the maximal value of this ratio (0.7053) in the Ludikovian paleobasin.     

Ion microprobe measurement of strontium isotopes in calcium carbonate with application to salmon otoliths

The ion microprobe has the capability to generate high resolution, high precision isotopic measurements, but analysis of the isotopic composition of strontium, as measured by the ⁸⁷Sr/⁸⁶Sr ratio, has been hindered by isobaric interferences. Here we report the first high precision measurements of ⁸⁷Sr/⁸⁶Sr by ion microprobe in calcium carbonate samples with moderate Sr concentrations. We use the high mass resolving power (7000 to 9000 M.R.P.) of the SHRIMP-RG ion microprobe in combination with its high transmission to reduce the number of interfering species while maintaining sufficiently high count rates for precise isotopic measurements. The isobaric interferences are characterized by peak modeling and repeated analyses of standards. We demonstrate that by sample-standard bracketing, ⁸⁷Sr/⁸⁶Sr ratios can be measured in inorganic and biogenic carbonates with Sr concentrations between 400 and 1500 ppm with ∼2‰ external precision (2σ) for a single analysis, and subpermil external precision with repeated analyses. Explicit correction for isobaric interferences (peak-stripping) is found to be less accurate and precise than sample-standard bracketing. Spatial resolution is ∼25 μm laterally and 2 μm deep for a single analysis, consuming on the order of 2 ng of material. The method is tested on otoliths from salmon to demonstrate its accuracy and utility. In these growth-banded aragonitic structures, one-week temporal resolution can be achieved. The analytical method should be applicable to other calcium carbonate samples with similar Sr concentrations.     

Sr isotopes in the Orgueil CI meteorite: Chronology of early solar system hydrothermal activity

New Sr isotopic analyses and calculated formation ages of carbonates from the Orgueil CI meteorite are reported. Among the samples analyzed in this work, dolomites give the youngest formation ages and may have been deposited intermittently starting near the time of parent body formation and continuing for at least 30 Ma. The Sr isotope data also suggest that breunnerites (Fe-Mn-Mg carbonates) crystallized after dolomite formation. Leaching experiments on bulk meteorite samples provide evidence for a very mobile, water soluble Sr reservoir in Orgueil that is characterized by extremely radiogenic Sr (⁸⁷ Sr/⁸⁶ Sr≈ 0.81-0.82). This unsupported Sr reflects recent element redistribution, possibly at the time of parent body breakup recorded by the ∼ 10 Ma exposure age of Orgueil. The carbonate data in particular corroborate earlier indications that hydrothermal processes were among the earliest events to affect the CI parent body.     

Geochemical constraints on the modes of carbonate precipitation in peridotites from the Logatchev Hydrothermal Vent Field and Gakkel Ridge

The ultramafic-hosted Logatchev Hydrothermal Field (LHF) at 15°N on the Mid-Atlantic Ridge and the Arctic Gakkel Ridge (GR) feature carbonate precipitates (aragonite, calcite, and dolomite) in voids and fractures within different types of host rocks. We present chemical and Sr isotopic compositions of these different carbonates to examine the conditions that led to their formation. Our data reveal that different processes have led to the precipitation of carbonates in the various settings. Seawater-like ⁸⁷Sr/⁸⁶Sr ratios for aragonite in serpentinites (0.70909 to 0.70917) from the LHF are similar to those of aragonite from the GR (0.70912 to 0.70917) and indicate aragonite precipitation from seawater at ambient conditions at both sites. Aragonite veins in sulfide breccias from LHF also have seawater-like Sr isotope compositions (0.70909 to 0.70915), however, their rare earth element (REE) patterns show a clear positive europium (Eu) anomaly indicative of a small (< 1%) hydrothermal contribution. In contrast to aragonite, dolomite from the LHF has precipitated at much higher temperatures (~ 100 °C), and yet its ⁸⁷Sr/⁸⁶Sr ratios (0.70896 to 0.70907) are only slightly lower than those of aragonite. Even higher temperatures are calculated for the precipitation of deformed calcite veins in serpentine–talc fault schists form north of the LHF. These calcites show unradiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.70460 to 0.70499) indicative of precipitation from evolved hydrothermal fluids. A simple mixing model based on Sr mass balance and enthalpy conservation indicates strongly variable conditions of fluid mixing and heat transfers involved in carbonate formation. Dolomite precipitated from a mixture of 97% seawater and 3% hydrothermal fluid that should have had a temperature of approximately 14 °C assuming that no heat was transferred. The much higher apparent precipitation temperatures based on oxygen isotopes (~ 100 °C) may be indicative of conductive heating, probably of seawater prior to mixing. The hydrothermal calcite in the fault schist has precipitated from a mixture of 67% hydrothermal fluid and 33% seawater, which should have had an isenthalpic mixing temperature of ~ 250 °C. The significantly lower temperatures calculated from oxygen isotopes are likely due to conductive cooling of hydrothermal fluid discharging along faults. Rare earth element patterns corroborate the results of the mixing model, since the hydrothermal calcite, which formed from waters with the greatest hydrothermal contribution, has REE patterns that closely resemble those of vent fluids from the LHF. Our results demonstrate, for the first time, that (1) precipitation from pure seawater, (2) conductive heating of seawater, and (3) conductive cooling of hydrothermal fluids in the sub-seafloor all can lead to carbonate precipitation within a single ultramafic-hosted hydrothermal system.     

Sr isotopic and elemental characteristics of calcites in the Chinese deserts: Implications for eolian Sr transport and seawater Sr evolution

Calcite content, Sr concentrations, and isotopes of calcites in the Chinese deserts are systematically studied in this paper. Calcite contents, which are calculated according to acid-soluble Ca contents in the deserts, are generally higher in the sandy deserts than in the sandy lands and decrease roughly from northwest to northeast of China. Acid-soluble Sr is well correlated with calcite in the Chinese deserts, implying acid-soluble Sr comes mainly from the calcite dissolution. Sr concentrations in calcites, calculated on the basis of calcite contents and acid-soluble Sr concentrations in the deserts, have an inverse relation to calcite contents, essentially mirroring the degree of Sr substitution for Ca in the calcite development. Desert calcites have regional variations in Sr isotopic ratios. Calcite Sr isotopic ratios depend on geological settings and chemical weathering. The Badain Jaran, and Tengger deserts are probably affected by additional factors such as the remote groundwater cycle or overturning of underlying sand deposits.Only four deserts (Taklimakan, Qaidam, Badain Jaran, and Tengger) appear to be potential sources of eolian deposits in the Chinese Loess Plateau (CLP). Isotopic signatures of calcite Sr and silicate Nd further indicate that the Tengger desert was not an important source for eolian deposits in the CLP. Eolian calcite was probably enriched due to wind sorting from the potential sources to the CLP and suffered weathering–leaching after it accumulated in the CLP. Sr isotopic compositions and Ca/Sr molar ratios of calcites are different between the deserts and the Lingtai profile, due to the integrated effect of wind sorting and weathering–leaching.It is essential to calculate accurately the ⁸⁷Sr/⁸⁶Sr ratio and Sr concentration of eolian calcite entering the oceans according to geochemical data of the Chinese deserts, because of the importance of the Chinese deserts in the global dust cycle. The calculated Sr concentration and ⁸⁷Sr/⁸⁶Sr ratio of eolian calcite entering the North Pacific Ocean, are 11.75 μmol/g and 0.71032, respectively. The calculated values in this study are close to the recommended values by Jacobson [Jacobson A. D. (2004) Has the atmospheric supply of dissolved calcite dust to seawater influenced the evolution of marine ⁸⁷Sr/⁸⁶Sr ratios over the past 2.5 million years? Geochem. Geophys. Geosyst. 5(12), 1–9, Q12002. doi:10.1029/2004GC000750]. Using the same model as that of Jacobson (2004), the effect of Asia dust on the evolution of seawater Sr isotopes is evaluated. (⁸⁷Sr/⁸⁶Sr)_seawater increases by 0.3 × 10⁻⁵ if the lower dust flux of 2.34 × 10⁸ mol Sr/yr is used in the model, suggesting the little effect of Asian dust on the seawater Sr record in the Quaternary. The increase in (⁸⁷Sr/⁸⁶Sr)_seawater is 1.5 × 10⁻⁵ if the higher value of 1.17 × 10⁹ mol Sr/yr is used, as observed in the Quaternary Sr record. These results further support the suggestions of Jacobson (2004).     

Silicate and carbonate weathering in the drainage basins of the Ganga-Ghaghara-Indus head waters: Contributions to major ion and Sr isotope geochemistry

The role of silicate and carbonate weathering in contributing to the major cation and Sr isotope geochemistry of the headwaters of the Ganga-Ghaghara-Indus system is investigated from the available data. The contributions from silicate weathering are determined from the composition of granites/ gneisses, soil profiles developed from them and from the chemistry of rivers flowing predominantly through silicate terrains. The chemistry of Precambrian carbonate outcrops of the Lesser Himalaya provided the data base to assess the supply from carbonate weathering. Mass balance calculations indicate that on an average ∼ 77% (Na + K) and ∼ 17% (Ca + Mg) in these rivers is of silicate origin. The silicate Sr component in these waters average ∼40% and in most cases it exceeds the carbonate Sr. The observations that (i) the⁸⁷Sr/⁸⁶Sr and Sr/Ca in the granites/gneisses bracket the values measured in the head waters; (ii) there is a strong positive correlation between⁸⁷Sr/⁸⁶Sr of the rivers and the silicate derived cations in them, suggest that silicate weathering is a major source for the highly radiogenic Sr isotope composition of these source waters. The generally low⁸⁷Sr/⁸⁶Sr (< 0.720) and Sr/Ca (∼ 0.2 nM/ μM) in the Precambrian carbonate outcrops rules them out as a major source of Sr and⁸⁷Sr/⁸⁶Sr in the headwaters on a basin-wide scale, however, the high⁸⁷Sr/⁸⁶Sr (∼ 0.85) in a few of these carbonates suggests that they can be important for particular streams. The analysis of⁸⁷Sr/⁸⁶Sr and Ca/Sr data of the source waters show that they diverge from a low⁸⁷Sr/⁸⁶Sr and low Ca/Sr end member. The high Ca/Sr of the Precambrian carbonates precludes them from being this end member, other possible candidates being Tethyan carbonates and Sr rich evaporite phases such as gypsum and celestite. The results of this study should find application in estimating the present-day silicate and carbonate weathering rates in the Himalaya and associated CO₂ consumption rates and their global significance.     

Strontium distribution and origins in a natural clayey formation (Callovian-Oxfordian, Paris Basin, France): A new sequential extraction procedure

Strontium is a good monitor of geochemical processes in natural clayey formations. In the Callovian-Oxfordian formation of Bure in France, strontium is sorbed on clay minerals and carried by carbonates, detrital minerals and accessory celestite. In order to determine the strontium distribution among these different phases, four-step sequential extractions (1. cobalt hexamine trichloride, 2. acetic acid, 3. EDTA and 4. tri-acid) were performed on samples from different levels of the clayey formation. The leachates were also analyzed for strontium isotopes, in order to determine the strontium origins. This sequential procedure is well suited to determining strontium distribution in claystones, although it is less efficient in clay-rich limestones and in celestite-rich samples. The carbonates (38-47% of the total strontium) show ⁸⁷Sr/⁸⁶Sr ratios (0.7070-0.7071) that have recorded the isotopic composition of the Callovian-Oxfordian seawater. Diagenetic carbonates (dolomite, ankerite and siderite) have almost not incorporated any strontium, which has been trapped by celestite during the late diagenesis. The major part of the celestite shows ⁸⁷Sr/⁸⁶Sr ratios (0.7069-0.7070) quite close to the primary carbonates. However, a second generation of celestite (0.7074) shows a slight ⁸⁷Sr-enrichment and is isotopically in equilibrium with the exchangeable strontium (27-48% of the total strontium with a mean ⁸⁷Sr/⁸⁶Sr value of 0.70745) and the present-day porewater (0.7074). This very low ⁸⁷Sr-enrichment could be explained by the partial destabilisation of detrital minerals (feldspars, micas, clays) which exhibit ⁸⁷Sr/⁸⁶Sr ratios consistent with their pristine Hercynian origin (0.7229-0.7350). Diffusion of strontium from the subjacent Dogger aquifers (0.7076-0.7082) could also be invoked to explain the slight ⁸⁷Sr-enrichment.