Calcium Isotope Analytical Method And Geological Applications

Calcium is the most abundant alkaline earth element in the Earth's crust.It is one of the major rock-forming elements,and one of the most important mobile elements,and an essential nutrient element for biological organisms,which can migrate easily among major geochemical reservoirs.Significant Ca isotopic fractionation has been observed in both high and low temperatures,and there is about⁶‰variation in Ca isotopic compositions(expressed as?^44/40Ca)in nature.These features suggest that Ca isotopes can provide new insight into understanding cosmochemistry,geochemistry,bioscience,archaeology,etc.In the study,we first make detailed investigations on how to choose the optimal double-spike and the optimal mixture ratios of the two single spikes in the double-spike,and on how to determinate the optimal sample/spike ratio range during measurement by multi-collector thermal ionization mass spectrometer?MC-TIMS?.Then we present Ca isotopic compositions of a lot of geological reference materials,including igneous,sedimentary and metamorphic rocks,and mainly felsic rocks are reported for the first time here.And then we take Tengchong volcanic rocks as an example to explore the potential possibility of Mg-Ca isotopes of tracing deep carbon cycle.Besides,Ca isotopic compositions of carbonate weathering crust are systematically measured,in order to have a better understanding on Ca isotopic fractionation during weathering processes,which can be important to study global Ca cycle.Compared with the ⁴²Ca-⁴⁸Ca and ⁴³Ca-⁴⁸Ca double-spikes which are currently common used in geochemistry,⁴²Ca-⁴³Ca double-spike has its distinctive advantages:?1?although the error on the fractionation factor?is relatively larger when using ⁴²Ca-⁴³Ca double-spike,it remains stable at a much wider range of sample/spike ratio in spiked samples,which is benefit for practical operation;?2?when a lower amount of sample was loaded and/or the large fractionation on the instrument occurred during mass determination,errors of fractionation correction produced by ⁴²Ca-⁴³Ca double spike will be minimized;?3?⁴²Ca and ⁴³Ca isotopes can be easily collected simultaneously on TIMS without significant ion optic effect,which can lower/remove the uncertaintity caused by signal's and instrument's fluctuatation,and thus can improve the data quality.On the other hand,the long-term mean and intermediate precision of independent measurements over a period of 5 years for?^44/40Ca of IAPSO seawater and NIST SRM 915a were 0.01±0.12‰?2SD,2 standard deviation,n=441?and 1.82±0.12‰?2SD,n=267?,respectively.We also report high-precision calcium isotopic compositions of 35 geological references materials,including sedimentary rocks,metamorphic rocks and igneous rocks.It should be noted that it is the first time to report Ca isotopic compositions of many felsic rocks.Felsic rock values of?^44/40Ca ranged from 0.13‰to 1.17‰,which is probably correlated with ⁴⁰K decay or fractionation crystallization during magma evolution.?^44/40Ca of ultramafic rocks ranging from 0.74‰to 1.51‰,were positively correlated with MgO and negatively with CaO contents,possibly owing to Ca isotopic fractionation during partial melting.?^44/40Ca of intermediate-mafic rocks were around 0.78‰and displayed limited variation,suggesting Ca isotopic fractionation is insignificant during magma evolution processes.?^44/40Ca of sedimentary and metamorphic rocks varied widely due to complex geological processes,indicating that Ca isotopic fractionation will occur during low and high temperature processes.Generally,carbon is in the form of carbonate during deep carbon cycling,so tracking the fingerprint of recycled carbonate in the Earth's mantle is important for constructing deep carbon cycle.Carbon isotopes are generally efficient to identify recycled organic carbon but not sensitive to inorganic carbon.However,the subducted carbon is dominated by inorganic carbonate.Therefore,new tracers are required to better identify inorganic carbonate.The CaO contents of the subducted carbonates are usually higher than that of the mantle,while?^44/40Ca of ancient marine carbonates are lower than 0.5‰,so Ca isotopes can be a tool to trace deep carbon cycle.To better unstandard the process of subduction and recycling,we apply Mg and Ca isotopes to trace carbonate recyclying in Tengchong volcanic rocks located in the southeastern Tibetan Plateau.The low?²⁶Mg?-0.31±0.03‰to-0.38±0.03‰?and?^44/40Ca?0.67±0.07‰to 0.80±0.04‰?values of these volcanic rocks compared to those of the mantle?-0.25±0.07‰and 0.94±0.05‰,respectively?indicate the incorporation of isotopically light materials into the mantle source,which may be carbonate-bearing sediments with low?²⁶Mg and?^44/40Ca values.Several geochemical observations suggest that the Tengchong mantle source incorporates recycled marine sediments:?1?Tengchong volcanic rocks have ultra-high Th/U ratios?vary from 8.5 to 10.5?,even higher than Gaussberg lavas with ultra-high Th/U ratios of 7.55±0.05;?2?Tengchong volcanic rocks have high Th/Yb ratios?5–13?,low Ba/La ratios?11–15?,high Pb contents?12.9–28.9 ppm?and low Ce/Pb ratios?4.7–6.3?,indicating the input of sediment or sediment melts rather than slab-derived fluids into the mantle source;?3?the good negative relationships between Sr/Ce and Th and the good hyperbolic relationships between Ba/Rb and Rb/Sr,indicating the existence of phlogopite produced by the reaction between depleted mantle and melts derived from subducted clay-rich sediments;?4?the good negative relationships between?_Ndd and Th/Nd and Th/Nb illustrate that sediments are recycled into the mantle.In addition,no correlations of?²⁶Mg and?^44/40Ca with either SiO₂ contents or trace element abundance ratios?e.g.,Sm/Yb and Ba/Y?are observed,suggesting that limited Mg and Ca isotopic fractionation occurred during cal-alkaline magmatic differentiation.Since recycled sediments contain a fraction of carbonates with light Mg and Ca isotopes,we suppose that light Mg and Ca components are derived from subducted marine carbonates.A binary mixing model using Mg–Ca isotopes shows that 5–8%carbonates dominated primarily by dolostone were recycled back into the mantle.The continental weathering plays an important role in Earth's surface calcium cycle.The atmospheric CO₂ will be consumed by the weathering of silicate and carbonate,and at the meanwhile Ca will be released,and then they will be stored as calcium carbonate in the ocean.Therefore,calcium is a key element that links atmosphere,hydrosphere,biosphere and crust.Although carbonate rocks only account for 10-14%of the continental crust,at a global scale it is thought that Ca is mainly derived from carbonate weathering and an average contribution of carbonate weathering to Ca total flux comprised between 66%and 90%.However,many work are focused on silicate weathering processes,while studies on carbonate weathering are rare.Thus,we report Ca and Sr isotopic compositions of saprolites from a weathering profile developed on argillaceous carbonate rocks in northern Guangdong,South China,to explore the geochemical behavior of Ca isotopes during weathering processes and then make new implications of carbon cycle.This profile is an in-situ weathering crust instead of deposits from outer space.Radiogenic ⁸⁷Sr/⁸⁶Sr ratios of the whole rocks are depth-dependent with 0.710086±6?2SE?at the parent rock and 0.722164±8?2SE?at the top-soil,possibly related with the enrichment of residual silicates.In the middle stage of weathering,Ca and Sr are decoupled and Ca/Sr ratios vary much?up to 1871?,suggesting rapid dissolution and re-precipitation of secondary carbonates.Interestingly,Ca isotopic compostions of the whole rock in this process vary small with?^44/40Ca ranging from 0.59 to 0.65‰,suggesting that limited Ca isotopic fractionation will occur during rapid dissolution and re-precipitation of secondary carbonates.In the late stage of weathering,?^44/40Ca of the whole rock decrease by 0.08‰,possibly related with sorption or cation exchange of clay and organic matter.The drop of?^44/40Ca in the late stage reflects that heavy isotopes prefer to liquid phase and light isotopes enriched at the situ,which will result into heavier isotopes in rivers than the parent rocks.