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• Ca isotopes do not fractionate during magmatic differentiation of carbonate-rich magmas. • Carbonated silicate melts have δ44/40Ca lower than MORB. • K-richterite and carbonate in mantle sources produce low δ44/40Ca values of magmas. • Ca isotopes are a robust tracer of volatile-induced mantle lithological variation. The volatile components CO 2 and H 2 O induce mantle melting and thus exert major controls on mantle heterogeneity. Primitive intraplate alkaline magmatic rocks are the closest analogues for incipient mantle melts and provide the most direct method to assess such mantle heterogeneity. Given the considerable Ca isotope differences among carbonate, clinopyroxene, garnet, and orthopyroxene in the mantle (up to 1 ‰ for δ44/40Ca), δ44/40Ca of alkaline rocks is a promising tracer of lithological heterogeneity. We present stable Ca isotope data for ca. 1.4 Ga lamproites, 590–555 Ma ultramafic lamprophyres and carbonatites, and 142 Ma nephelinites from Aillik Bay in Labrador, eastern Canada. These primitive alkaline rock suites are the products of three stages of magmatism that accompanied lithospheric thinning and rifting of the North Atlantic craton. The three discrete magmatic events formed by melting of different lithologies in a metasomatized lithospheric mantle column at various depths: (1) MARID-like components (mica-amphibole-rutile-ilmenite-diopside) in the source of the lamproites; (2) phlogopite-carbonate veins were an additional source component for ultramafic lamprophyres during the second event; and (3) wehrlites at shallower depths were an important source component for nephelinites during the final event. The Mesoproterozoic lamproites show lower δ44/40Ca values (0.58 to 0.66 ‰) than MORBs (0.84 ± 0.03 ‰, 2se). This cannot be explained by fractional crystallization or melting of the clinopyroxene-dominated source but can be attributed to a source enriched in the alkali amphibole K-richterite, which has characteristically low δ44/40Ca. The δ44/40Ca values of the ultramafic lamprophyre suite during the second rifting stage are remarkably uniform, with overlapping ranges for primary carbonated silicate melts (aillikite: 0.67 to 0.75 ‰), conjugate carbonatitic liquids (0.71 to 0.82 ‰) and silicate-dominated damtjernite liquid (primary damtjernite: 0.68 to 0.72 ‰). This suggests negligible Ca isotope fractionation during liquid immiscibility of carbonate-bearing magmas. Combined with previously reported δ44/40Ca values for carbonatites and kimberlites, our data suggest that carbonated silicate melts in Earth's mantle have δ44/40Ca compositions resolvably lower than those for MORBs (0.74 ± 0.02 ‰ versus 0.84 ± 0.03 ‰, 2se). The δ44/40Ca values of the Cretaceous nephelinites (0.72 to 0.78 ‰) are homogenous and similar to those of the 590–555 Ma ultramafic lamprophyres, suggesting that the wehrlitic source component for the nephelinites formed by mantle metasomatism during interaction with rising aillikite magmas during the second rifting stage. Our results highlight that both K-richterite and carbonate components in mantle sources can result in the systematically low δ44/40Ca values of alkaline magmas, which may explain previously reported low δ44/40Ca values of alkaline rocks and some carbonatites. Our study indicates that Ca isotopes are a robust tracer of lithological variation caused by volatiles in the Earth's upper mantle. [ABSTRACT FROM AUTHOR]

The calcium (Ca) isotopic composition in urine during bed rest has been demonstrated to be systematically light, indicating a negative bone mineral balance (i.e., bone loss). Here we present new Ca isotope data on urine during the “nutritional countermeasures” (NUC) bed rest study. We analyzed the Ca isotopic composition of 24 h pooled urine samples from seven healthy male subjects during baseline data collection (BDC), head-down-tilt bed rest and recovery. Additionally, we analyzed urine from two follow-up examinations after the regeneration phase. We observed a change in Ca isotopic composition during the bed rest phase, indicative of bone loss with a time delay of 10 to 21 days. We also observe that the Ca isotopic composition of urine is strongly dependent on the individual Ca metabolism and varies between subjects. We relate this individuality in Ca metabolism to differences in the amounts of Ca being recycled in the kidneys. Previous studies have shown that the more Ca is reabsorbed in the kidneys the more enriched the urine becomes in heavy isotopes of calcium. The Ca isotopic composition of urine is thus modified by more than one process and cannot be used in a straightforward manner to monitor net bone mineral balance. To overcome this problem, we propose a new baseline approach for using Ca isotopes, which effectively cancels out the effects of individual renal Ca reabsorption. This allows us to detect bone loss in patients without ambiguity by combining measurements of the Ca isotopic composition of urine and daily Ca excretion rate and comparing these to data collected on healthy individuals with a normal steady-state bone balance.

Abstract We report Ca isotope fractionation among a co-genetic suite of samples from the Mauritanian Guelb el Azib ultramafic-mafic-anorthosite complex (GAC), which represents the fractional crystallization sequence of an igneous magma chamber. We investigate how the composition of an evolving crystallizing liquid and the resultant mineralogy of co-crystallizing assemblages affects inter-phase Ca isotope fractionation. Because the GAC is an Archean metamorphosed complex, we first investigate the resistance of the Ca isotope signature to secondary hydrothermal alteration and to metamorphism. While we cannot conclude here that Ca isotopes are undisturbed at the mineral scale, we show that they are preserved at the bulk rock scale. This is adequate for our study where several samples are almost monomineralic at the bulk scale level. The δ44/40Ca (relative to NIST standard SRM 915a) of GAC layers range from −1.53 to 1.61‰, with the earliest-formed ultramafic cumulate layer being isotopically heaviest and the later, more felsic layers being isotopically lighter. For the first time, we show that plagioclase is much more enriched in light Ca isotopes compared to olivine and (Ca-) pyroxene. Monte Carlo simulations suggest that Ca isotopes are fractionated among co-existing silicate minerals during fractional crystallization, possibly in relation with the residual liquid composition. In qualitative agreement with computational models based on first principles lattice dynamics, we observe that Ca isotope fractionation is mineralogically controlled and importantly, that the degree of fractionation can vary according to the CaO composition of the residual liquid. While previous studies have aimed to understand partial melting as a source of Ca isotope fractionation, our results suggest that fractional crystallization is also a source of Ca isotope variability among co-genetic samples. Highlights • Co-genetic samples from the Guelb el Azib ultramafic-mafic-anorthosite complex (GAC) show Ca isotope fractionation. • The Ca isotopic compositions of GAC rocks span 3.18‰; ultramafic cumulates are isotopically heavier than more felsic layers. • Fractional crystallization could be a source of Ca isotope variability among co-genetic samples. • Plagioclase is much more enriched in light Ca isotopes compared to olivine and (Ca-) pyroxene. [ABSTRACT FROM AUTHOR]

Abstract The Earth's mantle has a complex history of partial melting and melt-peridotite reaction that have redistributed Ca and other elements between residues and melting products. Given the considerable Ca isotopic variation reported in mantle rocks, evaluation of the fractionation of stable Ca isotopes in magmatic processes in the mantle is critical to decode mantle evolution and the effect of recycled materials. We have performed precise and accurate Ca isotopic analyses on a series of well-characterized spinel-facies mantle peridotites (lherzolite, harzburgite and dunite, n = 29), pyroxenites (websterite, clinopyroxenite and orthopyroxenite, n = 15) and their mineral constituents (n = 8) from the Balmuccia and Baldissero peridotite massifs of the Ivrea Zone in the Italian Alps. These peridotites underwent variable degrees of melting and melt-peridotite reaction, whereas the pyroxenites are mainly the products of melt-peridotite reaction and crystallization of migrating basic melts from the asthenosphere. The lherzolites from Balmuccia and Baldissero show δ44/40Ca values of 0.94 ± 0.11‰ (2sd, n = 22), which are uniform within long-term external reproducibility (±0.14‰, 2sd). The δ44/40Ca values of the harzburgites (0.83‰ to 0.92‰) do not differ from those of lherzolites, including those with a history of intensive melt-peridotite reaction to form replacive dunites. The websterites and spinel clinopyroxenites display δ44/40Ca of 0.86 ± 0.10‰ (n = 14), within the range of the lherzolites and harzburgites. The indistinguishable δ44/40Ca among these very diverse mantle rocks is the consequence of the overwhelming control of stable Ca isotopes by clinopyroxene in the magmatic processes involved, because clinopyroxene dominates the budget of Ca (>90% for harzburgites; 93% to 99% for lherzolites, websterites and clinopyroxenites). Only the clinopyroxene-poor (<3 wt.%) dunites and orthopyroxenite show higher δ44/40Ca (e.g., 1.11‰ to 1.81‰ and 1.13‰, respectively). This reflects the signatures of olivine and orthopyroxene which display higher δ44/40Ca than clinopyroxene. These results and modeling suggest that negligible Ca isotope fractionation (<0.12‰) occurs during <25% of partial melting, silicate melt-peridotite reaction, or magmatic differentiation in the upper mantle. Only highly depleted harzburgite residues that formed by >25% melting and replacive dunites tend to display slightly heavier Ca isotopic compositions. Consequently, irrespective of their magmatic history, most fertile mantle rocks from different geological settings display a homogenous Ca isotope composition, summarized as, δ44/40Ca of 0.94 ± 0.10‰ (2sd, n = 47) for the Earth's mantle. The deviations in Ca isotopic variations observed in other mantle rocks may be attributed to kinetic isotope fractionation and metasomatism by melts with isotopic compositions influenced by recycled crustal materials. [ABSTRACT FROM AUTHOR]

The boron isotope compositions (δ11B) of biogenic carbonates have proven to be an invaluable tool for investigating changes in ocean carbonate chemistry, especially the impacts of declining seawater pH due to rising levels of atmospheric CO2. Accurate, high precision boron isotopic measurements, however, remain analytically challenging with conventional approaches being extremely time consuming and labour intensive. Here we present a new analytical approach for high precision accurate measurements of boron isotope compositions in biogenic carbonates. We show that a laser ablation MC-ICPMS system equipped with 1013 Ω amplifiers provides a powerful tool for determining micro-scale δ11B values with an internal precision of ≤0.15‰ (2SD) and a total long-term accuracy of better than 0.22‰, which is equivalent to or better than those of conventional solution-based methods. This accuracy is achieved by careful subtraction of baseline interference around the masses of 10B and 11B, which arises from matrix-specific scattered Ca, C, and O ions. We also report detailed protocols for time-response corrections of transient signals in biogenic carbonates, essential for high precision boron isotopic measurements when using 1013 Ω amplifiers. The overall veracity of our laser ablation MC-ICPMS method is further demonstrated by the isotopic variability observed at macro- (mm) and micro-scales (μm) in both calcite and aragonite biogenic minerals. An aragonite deep-sea coral, Desmophyllum dianthus, is shown to preserve large (up to 6‰) variations in δ11B accompanied by near synchronous changes in B/Ca. This variability is excessive compared to that expected for the relatively stable carbonate ion system of deep-ocean water and is therefore attributed to isotopic and elemental fractionation of B during coral biomineralisation. In contrast, cyclic 2.5‰ variability is found in the sclerosponge Ceratoporella nicholsoni, which can be directly linked to seasonal changes in seawater pH. These examples highlight the power of laser ablation MC-ICPMS and its ability to accurately and precisely recover small environmental signals in boron isotopes. We thus show that these significant improvements in both the internal precision and now long-term accuracy and reproducibility of laser ablation MC-ICPMS, together with its microscale analytical capabilities and relatively rapid ease of operation, have the potential to revolutionise the study of boron isotope systematics in biogenic carbonates. [ABSTRACT FROM AUTHOR]

Abstract Ca isotopes can be strongly fractionated at the Earth's surface and thus may be tracers of subducted carbonates and other Ca-rich surface materials in mantle rocks, magmas and fluids. However, the δ44/40Ca range in the mantle and the scope of intra-mantle isotope fractionation are poorly constrained. We report Ca isotope analyses for 22 mantle xenoliths: four basalt-hosted refractory peridotites from Tariat in Mongolia and 18 samples from the Obnazhennaya (Obn) kimberlite on the NE Siberian craton. Obn peridotites are Paleoproterozoic to Archean melting residues metasomatised by carbonate-rich and/or silicate melts including unique xenoliths that contain texturally equilibrated carbonates. δ44/40Ca in 15 Obn xenoliths shows limited variation (0.74–0.97‰) that overlaps the value (0.94 ± 0.05‰) inferred for the bulk silicate Earth from data on fertile lherzolites, but is lower than δ44/40Ca for non-metasomatised refractory peridotites from Mongolia (1.10 ± 0.03‰). Bulk δ44/40Ca in four Obn peridotites containing metasomatic carbonates ranges from 0.81 ± 0.08‰ to 0.83 ± 0.06‰, with similar values in acid-leachates and leaching residues, indicating isotopic equilibration of the carbonates with host rocks. We infer that (a) metasomatism tends to decrease δ44/40Ca values of the mantle, but its effects are usually limited (≤0.3‰); (b) Ca isotopes cannot distinguish "carbonatite" and "silicate" types of mantle metasomatism. The lowest δ44/40Ca value (0.56‰) was obtained for a phlogopite-bearing Obn peridotite with a very high Ca/Al of 8 suggesting that the greatest metasomatism-induced Ca isotope shifts may be seen in rocks initially low in Ca that experienced significant Ca input leading to high Ca/Al. Two Obn peridotites, a dunite (melt channel material) and a veined spinel wehrlite, have high δ44/40Ca values (1.22‰ and 1.38‰), which may be due to isotope fractionation by diffusion during silicate melt intrusion and percolation in the host mantle. Overall, we find no evidence that recycling of crustal carbonates may greatly affect Ca isotope values in the global mantle or on a regional scale. [ABSTRACT FROM AUTHOR]

Our understanding of the evolution of the shell structure in nuclei far from stability is based on the study of some key nuclei. Nuclei at or next to double shell closures play a special role in this. Presently, a lot of discussion is concentrated on the neutron-rich calcium isotopes, which provide a rich testing ground for various nuclear models with several traditional and new magic numbers. 60Ca is now almost within reach with the most advanced radioactive beam facilities. In order to investigate the evolution of the shell gap at N = 40, the configuration of states in the odd-even titanium isotopes up to N = 37 (59Ti) have been studied. In order to experimentally access the shell gap at N = 40, it is nowadays within the reach of the most advanced facility the investigation of neutron hole configuration states in odd-even titanium isotopes up to N = 37, in the 59Ti nucleus. Such states correspond to relatively simple configurations that constitute a challenging testing ground for effective nuclear interactions. The new data obtained in our experiment allows to place the present predictions concerning the shell closure at N = 40 in the calcium region on a more solid ground. [ABSTRACT FROM AUTHOR]

This study was designed to optimize calcium-45 (45Ca) sample preparation method in a Liquid Scintillation Counter (LSC) for use with vegetables and soil samples. Different tests were conducted on fruit and soil samples to determine the best conditions related to drying, time and centrifuge speed (rpm) after calcination at 450°C and 550°C for vegetable and soil samples, respectively. The homogeneity of samples, linearity, detection and quantification limits were also determined. The quench curve was calculated to establish the parameters to program the LSC. Six 45Ca sample replicates were measured with a coefficient of variation (CV) of 1.74%, which indicates good sample homogeneity. The method is linearity over the range of activity studied for 0-30,000 disintegrations per minute (dpm). Our results suggest that the optimum homogenized sample preparation for calcination purposes was lyophilization. Techniques using 45Ca applied to soil and plant tissues are promising tracers of Ca in plants, which can be very useful for the study of Ca kinetics in plants and can serve to guide efficient fertilization programs. [ABSTRACT FROM AUTHOR]

Abstract Dealing with the 41K interference during 41Ca measurement with low energy AMS systems is challenging. The detection of 39K to correct the measured 41Ca/40Ca ratios is a powerful tool that improves precision and accuracy. However, the study of the sources of this interference can help to reduce it to its minimum. In this work, it is shown that iron is a factor enhancing the production of the 41K interference, since the (41K57Fe)− ion has the same mass as (41K19F 3)−. To validate this observation, we measured 57Fe2+ together with 41M2+ (all ions with mass 41) and 39K2+ in a blank sample and an aluminum target at the 600 kV AMS system at ETH Zurich. We also show the temporal evolution of 41M/40Ca and 39K/40Ca ratios in two blank samples during a measurement on the 1 MV AMS system at CNA Seville. Even when unstable behavior for these ratios is observed for one of the blanks, the relationship between both, 41M/39K, is relatively stable over time. This supports the accuracy of the K–correction method, providing that it is applied sequence by sequence. Two programs, one for each of these compact AMS systems, were written in FORTRAN code to deal with the complexity of the data analysis due to the K-correction. [ABSTRACT FROM AUTHOR]

Abstract The analysis of 41Ca can be quantified by measuring the ratio 41Ca/40Ca. At present, ratios of 41Ca/40Ca ≤ 10−11 can only be measured by accelerator mass spectrometry (AMS). The sensitivity is mainly limited by the interference of the isobar 41K. With compact AMS (≤1 MV), abundance sensitivities of 10−11 to 10−12 have been demonstrated, but this may necessitate the measurement of 39K to correct for the 41K background. With a 3 MV AMS, the dE/dx method can still be used to partially separate 41Ca from 41K. In this work, a new capability for 41Ca analysis has been tested at the Xi'an-AMS. Ca4+ was chosen to achieve a balanced performance between 41Ca/41K separation and transmission efficiency. A 41Ca/40Ca background level of near 10−13 was achieved without the need for 39K correction. This new analytical capability at our facility was then applied to 41Ca tracer samples from our studies on the "White Jade" land snail shells, in which 41Ca was used as a tracer for studying the absorption rate of calcium. [ABSTRACT FROM AUTHOR]

The spectra of highly excited 40Ca atoms prepared using a magneto-optical trap and an ultraviolet laser are experimentally studied. The dependence of the atomic density in the magneto-optical trap on the detuning frequency of cooling radiation and on the magnetic field gradient is determined. By using the technique based on the reduction of resonance fluorescence, the energy of the 761S0 Rydberg state for 40Ca atoms is determined. A new scheme for stabilization of the laser frequency to create highly excited Rydberg atoms is demonstrated. This scheme employs the saturated absorption spectrum in a cell with rubidium atomic gas. [ABSTRACT FROM AUTHOR]

Abstract A sub-surface dwelling planktonic foraminifer, Pulleniatina obliquiloculata , in particular its shell's geochemical properties such as stable oxygen isotopic composition and Mg/Ca ratios, are widely used to reconstruct past changes in the thermocline of tropical Indo-Pacific. However, the depth range that P. obliquiloculata population generally dwells or calcifies has been shown to significantly vary, depending on the regions and the seasons. In this study, δ18O and Mg/Ca of core-top P. obliquiloculata from the tropical Indo-Pacific are analyzed by means of comparison with modern hydrographic data. We apply a δ18O-based approach to decipher the varying apparent calcification depth (ACD) of this species for the tropical Indo-Pacific, and find that P. obliquiloculata 's ACD varies between 50 and 150 m regionally, in connection with local oceanographic settings. At given ACD, exponential relationships between temperature and P. obliquiloculata Mg/Ca are determined as Mg/Ca = 0.245 ± 0.041 exp. (0.088 ± 0.009 T) for these core-top samples. The sensitivity of P. obliquiloculata Mg/Ca to temperature is in good accordance with most previous findings, particularly the recent work using core-top samples from the western tropical Pacific. Taken together, our results not only improve the understanding of the calcification conditions of P. obliquiloculata , but also make it possible to better estimate the sub-surface temperature of the Indo-Pacific Warm Pool (IPWP) using Late-Quaternary sedimentary materials. Highlights • The average ACD of P. obliquiloculata ranges 50–100 m water depth in tropical Indo-Pacific • We show how ACD of P. obliquiloculata varies with regional oceanographic settings • New Mg/Ca-T relationship of P. obliquiloculata is defined for tropical Indo-Pacific [ABSTRACT FROM AUTHOR]

Abstract Calcium isotope analyses show a depletion of heavy calcium isotopes in vertebrates, compared to food sources along each trophic step. Recent studies show considerable variability of the calcium isotopic composition of bone and teeth in modern mammals, leading to inconclusive interpretations regarding the utility of Ca isotopes for trophic inference in mammal-dominated terrestrial ecosystems. Here, we analyzed modern enamel samples from the Tsavo National Park (Kenya), and fossil enamel samples dated from ca. 4 Ma to 1.6 Ma from the Turkana Basin (Kenya). We found a constancy of taxa ordering between the modern and fossil datasets, suggesting that the diagenesis of calcium isotopes is minimal in fossils. In modern herbivore samples using similar digestive physiologies, browsers are enriched in 44Ca compared to grazers. Both grazer and browser herbivore tooth enamel is enriched in 44Ca relative to carnivores by about +0.30‰. Used together, carbon and calcium isotope compositions may help refine the structure of the C 3 and C 4 trophic chains in the fossil record. Due to their high preservation potential, combining both carbon and calcium isotope systems represent a reliable approach to the reconstruction of the structure of past ecosystems. Highlights • Calcium isotopes are measured in modern and Plio-Pleistocene mammal teeth from Kenya. • The impact of diagenesis on calcium isotopes in fossil tooth enamel is minimal. • We observe a constancy of taxa ordering between our modern and fossil dataset. • Calcium isotopes provide a promising tool to discuss the paleoecology of extinct taxa. [ABSTRACT FROM AUTHOR]

Abstract Growth pathways of calcium (Ca) phosphate minerals are still under debate, but non-classical growth pathways, which encompass any mode of growth involving attachment of primary particles instead of monomer ions, are currently thought to dominate over classical mechanisms under a wide range of growth conditions. Cation desolvation during ion-by-ion growth is associated with the preferential uptake of isotopically light Ca in Ca-bearing phases, so the Ca isotope composition of Ca-bearing minerals can help to elucidate the dominant growth mechanism. Here, we combine stable Ca isotope analysis with mineralogical characterization and nanoscale imaging of growth features to determine for the first time the rate-dependent Ca isotope fractionation during seeded growth of hydroxyapatite (HAP) involving an octacalcium phosphate (OCP) precursor. Our data reveal that growth rates are strongly attenuated by pH, and that Ca isotope fractionation between the solid and growth solution is independent of growth rate between 1.9 × 10−9 and 2.8 × 10−8 mol Ca m−2 s−1. Moreover, nanoscale images of surface topography reveal direct deposition of primary particles on the HAP seed surface following sustained growth, providing visual evidence of a non-classical growth pathway. Together, these findings support the hypothesis that hydroxyapatite growth is dominantly non-classical. The process of cluster nucleation from the bulk solution and attachment of these clusters to the seeded HAP apparently does not involve a significant kinetic isotope effect, which implies that abiogenically formed Ca-phosphates likely preserve the Ca isotope composition of the fluid from which the crystals originated. [ABSTRACT FROM AUTHOR]

A method for calcium isotope separation in a plasma centrifuge, with a hot wall and axial circulation, is examined. The rotation of the weakly ionized calcium plasma is provided by a rotating magnetic field. An increase in the radial separation effect in the axial direction is achieved by the incorporation of axial circulation, which is induced by a traveling magnetic field. The product and waste flows are induced by means of cooled discs at both ends of a column. The operation of the plasma centrifuge with a product flow is considered. The dependence of the separation factor on the product flow rate and an axial flow is calculated. [ABSTRACT FROM AUTHOR]

A brief survey of self-consistent models used to perform global calculations of β-decay properties of neutron-rich nuclei is given. These models include the continuum quasiparticle randomphase approximation (CQRPA) based on the energy density functional (DF) proposed by Fayans and his colleagues (DF + CQRPA), relativistic quasiparticle random-phase approximation (RQRPA), and the finite-amplitude method (FAM). These models take into account allowed Gamow-Teller transitions and first-forbidden transitions. Models that allow for complex configurations beyond the QRPA framework are also analyzed. The β-decay properties of heavy calcium, potassium, and scandium isotopes in the vicinity of the N = 32 and 34 neutron subshells, which are new magic subshells for neutrons, are calculated on the basis of the self-consistent DF + CQRPA approach. The predicted high probability for two-neutron emission is found to be correlated with the anomalous nuclear radii measured for potassium and calcium isotopes in the region around N = 32. The results ofDF3 + CQRPA calculations are compared with their counterparts obtained within the self-consistent models implemented with the SkO' Skyrme functional and the D3C* relativistic functional. [ABSTRACT FROM AUTHOR]

We report the calcium isotopic composition (δ44Ca) of primary aragonite laminae, primary gypsum, and secondary gypsum in sediments deposited from Lake Lisan, the last glacial cycle of the Dead Sea (70–14.5 ka). The δ44Ca of primary gypsum varies between 0.17‰ and 0.71‰ versus bulk silicate earth, with an average of 0.29‰, whereas the aragonite δ44Ca varies between −0.68‰ and −0.16‰ with an average of −0.4‰. The secondary gypsum δ44Ca is close to the calcium isotope composition of the aragonite, averaging at −0.3‰. The aragonite δ44Ca shows small variations temporally in sync with lake level fluctuations, suggesting the aragonite δ44Ca reflects changes in the lake calcium balance, which in turn reflects changes in the local hydrological cycle. The secondary gypsum calcium isotope composition (−0.3‰) overlaps with that of coeval aragonite, suggesting the calcium for secondary gypsum was derived from the aragonite through quantitative, or near‐isotopic equilibrium, recrystallization of the aragonite to gypsum after the lake desiccation and exposure of sediments during the Holocene. A numerical box model is used to explore the effect of changing lake water levels on the calcium isotope composition of the aragonite and gypsum in the lake. The relatively low variability in the δ44Ca over the lake's history suggests that a high‐concentration calcium‐rich brine buffers the calcium cycle. Key Points: The calcium isotope composition of aragonite and primary gypsum can be linked to the changing hydrological cycle in Lake LisanThe disseminated gypsum calcium isotope composition confirms its secondary precipitation during the oxidation of sulfide minerals [ABSTRACT FROM AUTHOR]

Abstract Titanium and calcium are both refractory lithophile elements. Significant stable isotopic variations on Ti and Ca have been documented within calcium, aluminum-rich inclusions (CAIs) in carbonaceous chondrites. To trace the condensation history of Ti in the solar nebula, we conducted a high-precision double-spike Ti stable isotopic study on a large set of chondrites. The studied chondrites have a homogeneous bulk Ti stable isotopic composition (δ 49/47Ti IPGP-Ti = −0.069 ± 0.018‰, 2se, n = 22, i.e., the per mil deviation of the 49Ti/47Ti ratios relative to the IPGP-Ti reference material). The homogeneity across eleven chondrite groups implies that chondrites have acquired, through the condensation sequence at equilibrium, the average stable isotopic composition of Ti in the refractory solids that condensed early in the solar nebula. In contrast, the light Ca stable isotopic compositions of bulk chondrites can be attributed to either the presence of CAIs (CV-, CM- and CO-type) or parent-body aqueous alteration (CR- and CI-type). [ABSTRACT FROM AUTHOR]

Wide application of stable Ca isotopes requires reliable production of high-precision data and in recent years multiple collector inductively coupled plasma mass spectrometers (MC-ICP-MS) have been used as an alternative to traditional thermal ionization mass spectrometers. The main challenges in using MC-ICP-MS to measure stable Ca isotopes are scattered ions and peak tailing of high level 40Ar+ and 40Ca+ ion beams, along with variable interferences. In this study we optimize Ca isotopic measurements on a large geometry high resolution MC-ICP-MS (Nu Plasma 1700) operating in conventional hot Ar-plasma mode. 40Ar+ and 40Ca+ were trapped by a specially designed dummy bucket, to prevent scattering of ions that might affect the Ca isotopic measurement. Strontium and matrix elements in samples were eliminated to blank levels by chemical separation. Instrumental mass bias of Ca isotopes was corrected by a sample-standard bracketing method using a purified sea water Ca isotope standard. Potential effects of Ar-related interferences, Ca-hydride, acid molarity, concentration mismatch and matrix elements were evaluated. The results from both measurements and modeling suggest that 40Ar+ peak tailing and Ca-hydride cause negligible Ca isotopic offset under normal MC-ICP-MS operating conditions. The precision and accuracy were validated using the measurement of twelve well-characterized international reference materials and a pure Ca standard solution. The long-term external precisions are better than ±0.07‰ (2SD) for δ44/42Ca and ±0.10‰ (2SD) for δ43/42Ca. Calcium isotopic compositions of all reference materials measured in this study agree with previously published data within uncertainties. [ABSTRACT FROM AUTHOR]

The stable isotopes of calcium have tremendous potential for constraining the evolution of the mantle as well as fingerprinting the recycling of carbon in the mantle via subduction. In order to better identify possible contributions of subducted carbonate-bearing materials to upper mantle heterogeneity, we present Ca isotope data for peridotite xenoliths from Fanshi, northern North China Craton. These peridotites have previously been studied for major and trace element, Sr-Nd isotopic compositions. Two metasomatic events in the lithosphere mantle caused by subduction of carbonated sediments from the Pale-Asian oceanic slab, were previously identified: the first by carbonatite melt and the second by carbonate-rich silicate melt. δ 44/40 Ca (=(( 44 Ca/ 40 Ca) SAMPLE /( 44 Ca/ 40 Ca) SRM915a  − 1) × 1000) of clinopyroxenes (Cpx) in the Fanshi peridotites varies from +0.84‰ to +1.17‰ and δ 44/40 Ca of orthopyroxenes (Opx) from +0.82‰ to +1.22‰. δ 44/40 Ca of the clinopyroxenes show a larger variation than that could be attributed to partial melting and display positive and negative trends with (La/Yb) N and Ti/Eu, respectively, suggesting the effect of two distinct metasomatic events. A harzburgite, modified only by the early carbonatite metasomatism, has the highest δ 44/40 Ca (+1.17‰). The lherzolites affected by the second metasomatic melt show lower Δ 44/40 Ca Opx-Cpx (=δ 44/40 Ca Opx  − δ 44/40 Ca Cpx , −0.11‰ to 0‰) than the un-metasomatized lherzolites (+0.11‰ to +0.24‰), implying that the second metasomatism triggered Ca isotope disequilibrium between the clinopyroxene and co-existing orthopyroxene. This is consistent with positive trends between δ 44/40 Ca and 87 Sr/ 86 Sr, (La/Yb) N in the clinopyroxenes of the lherzolites, indicating that the second metasomatism also resulted in high δ 44/40 Ca in the clinopyroxenes of the lherzolites (up to +1.09‰). Furthermore, the harzburgites and lherzolites converge toward the second metasomatic event in diagrams of 87 Sr/ 86 Sr versus δ 44/40 Ca, suggesting an extensive effect of the second metasomatism. The heavy Ca isotope signature in these metasomatic clinopyroxenes applies to both first carbonatite melt and second carbonate-rich silicate melt derived from carbonated sediments (>+1.17‰ and ∼+1.09‰, respectively). However, heavy Ca isotope composition is inconsistent with low δ 44/40 Ca in carbonated sediments from the Paleo-Asian ocean, but could be the characteristic of an evolved sediment-derived hydrous carbonate melt. This implies significant Ca isotope fractionation during subduction of carbonated sediments. While currently there is plenty of debate regarding the exact significance of Ca isotope heterogeneity in mantle rocks, our results show at a minimum that Ca isotopes are rather heterogeneous in supra-subduction mantle rocks and could be used to deciphering mantle metasomatic models and detecting recycled crustal materials. [ABSTRACT FROM AUTHOR]

In neutron star mergers, neutron excess nuclei and the r-process are important factors governing the production of heavier nuclear systems. An evaluation of calcium nuclei suggests that the heaviest Z¼20 nucleus will have mass 60 with filling of the 1f5/2 neutron shell. A¼57-60 calcium isotopes have limited experimental half-life data, but the model predicts beta decay halflives in the range of 30-40 ms. Based on comparisons to lighter Z¼20 systems, these results likely overestimate the half-lives of these neutron excess nuclei. [ABSTRACT FROM AUTHOR]

Abstract: Dedolomitization is a common diagenetic process in shallow burial environments and is often associated with sulphates in mixed carbonate‐evaporite successions. In these settings, elevated Ca2+/Mg2+ ratios necessary for dedolomitization result from the dissolution of sulphate phases by the incursion of undersaturated groundwater. Reported dedolomite textures from other studies are varied, but the most prevalent is a rhombic texture interpreted to result from the partial to complete pseudomorphic replacement of secondary dolomite rhombs formed in the burial diagenetic realm. In this study of primary cryptocrystalline to finely crystalline dolomicrites in the Prairie Evaporite Formation of north‐eastern Alberta, dedolomitization has resulted in sutured to loosely packed mosaics of dedolomite that range from subhedral to distinctly euhedral (rhombic) crystal fabrics; however, no prior aggrading neomorphism producing dolomite rhombs is evident in the precursor dolomicrites. Non‐pseudomorphic dedolomitization of the dolomicrites results in textures that include rhombic dedolomite crystals with cloudy cores comprising remnant dolomicrite and clear rims. These textures are similar to those observed in the pseudomorphic dedolomitization of secondary dolomite rhombs. The Prairie Evaporite Formation of north‐eastern Alberta has experienced extensive karstification near the erosional margin of the sedimentary succession. Dedolomitization of dolomicrites occurs in marker beds within the Prairie Evaporite succession associated with evaporite karstification. Along with stratigraphic and petrographic considerations, stable isotope results support the interpretation of a shallow dedolomitization event influenced by meteoric waters derived from the basin margin. Negative δ18O and low δ13C values (averages of −13·6‰VPDB and 0·5‰VPDB, respectively) of the dedolomite, compared with those of the primary dolomicrite (averages of −6·0‰VPDB and 1·2‰VPDB, respectively), point to isotopically light diagenetic fluids. These results show that rhombic dedolomite textures can form through shallow, non‐pseudomorphic dedolomitization of dolomicrites by meteoric fluids in the presence of sulphates, with resulting textures that are similar to the pseudomorphic dedolomitization of secondary dolomite rhombs. [ABSTRACT FROM AUTHOR]

Shallow-water carbonate sediments constitute one of the most abundant and widely used archives of Earth’s surface evolution. One of the main limitations of this archive is the susceptibility of the chemistry of carbonate sediments to post-depositional diagenesis. Here, we develop a numerical model of marine carbonate diagenesis that tracks the elemental and isotopic composition of calcium, magnesium, carbon, oxygen, and strontium, during dissolution of primary carbonates and re-precipitation of secondary carbonate minerals. The model is ground-truthed using measurements of geochemical proxies from sites on and adjacent to the Bahamas platform (Higgins et al., 2018) and authigenic carbonates in the organic-rich deep marine Monterey Formation (Blättler et al., 2015). Observations from these disparate sedimentological and diagenetic settings show broad covariation between bulk sediment calcium and magnesium isotopes that can be explained by varying the extent to which sediments undergo diagenesis in seawater-buffered or sediment-buffered conditions. Model results indicate that the covariation between calcium and magnesium isotopes can provide a semi-quantitative estimate of the extent and style (fluid-buffered vs. sediment-buffered) of early marine diagenesis. When applied to geochemical signatures in ancient carbonate rocks, the model can be used to quantify the impact of early marine diagenesis on other geochemical proxies of interest (e.g. carbon and oxygen isotopes). The increasing recognition of early marine diagenesis as an important phenomenon in shallow-water carbonate sediments makes this approach essential for developing accurate records of the chemical and climatic history of Earth from the chemical and isotopic composition of carbonate sediments. [ABSTRACT FROM AUTHOR]

We present calcium isotope data from pore fluids and solids from Ocean Drilling Program Leg 175: Sites 1081 and 1086 (off the coast of West Africa). These sites are similar with respect to geographic location, sediment age (from modern to 8 Myr), and water depth (800 m), but Site 1081 is carbonate-poor, whereas Site 1086 is carbonate-rich. Therefore, these sites are suited for the exploration of the influence of sediment type on carbonate dissolution, precipitation, and recrystallization. We use two numerical modelling approaches to explore the rates of carbonate dissolution and precipitation in the sediment column. The first is the standard diffusion-reaction approach, using the strontium concentration within the pore fluid to ascertain a dissolution rate for the carbonate, which is then applied to a second model of calcium isotopes within the pore fluid to calculate precipitation rates. Given the high sedimentation rates we also apply an advection-reaction model (Huber et al., 2017) which results in the same depth distribution of carbonate precipitation but significantly higher overall rates, which is discussed. Calcium isotope ratios in pore fluid calcium increase in zones where our model predicts carbonate precipitation, and approach isotopic equilibrium with the solid in zones where our model predicts equivalent rates of dissolution and precipitation, similar to previous findings. Contrary to previous findings in marine sediments, our model requires a calcium isotope fractionation on carbonate precipitation to fit the data, as there is an offset between the δ 44 Ca of the fluid and the solid. Using the zones of carbonate precipitation determined from the models and previously published carbon isotope profiles of the dissolved inorganic carbon from these sites, we suggest that the δ 13 C of the authigenic carbonate is uniformly lower than biogenic carbonate but by a wider range than was previously suggested. [ABSTRACT FROM AUTHOR]

The elemental and isotopic mass balance of Ca and Sr between seawater and the oceanic crust at mid-ocean ridge (MOR) hydrothermal systems integrates various physiochemical processes in the subseafloor, such as dissolution of primary silicate minerals, formation of secondary minerals, and phase separation in the subseafloor. In particular, the precipitation and recrystallization of anhydrite are recognized as important processes controlling the Ca and Sr elemental and isotope composition of high temperature vent fluids and coexisting ocean crust, and yet, little experimental data exist to constrain the mechanism and magnitude of these critical geochemical effects. Thus, this study experimentally examines Sr/Ca partitioning, Ca isotope fractionation, and the rate of exchange between anhydrite and dissolved constituents. These experimental constraints are then compared with Sr/Ca and Ca isotope compositions of anhydrite and vent fluids sampled from the TAG hydrothermal system. Accordingly, anhydrite precipitation and recrystallization experiments were performed at 175, 250, and 350 °C and 500 bar at chemical conditions characteristic of active MOR hydrothermal systems. Experimental data suggest that upon entrainment and recharge of seawater into MOR hydrothermal systems anhydrite will rapidly precipitate with a Ca isotopic composition that is depleted in the heavy isotope compared to the hydrothermal fluid. The magnitude of the Ca isotope fractionation, Δ 44/40 Ca (Anh-Fluid) , is temperature dependent, −0.45, −0.22, and −0.02‰, for 175, 250, and 350 °C, respectively, but likely indicative of kinetic effects. Utilization of a 43 Ca spike in solution was implemented to quantify the time-dependent extent of isotope exchange during anhydrite recrystallization at chemical equilibrium. These data indicate that the rate of exchange is a function of temperature, where 12, 46, and 45% exchange occurred within 1322, 867, 366 h at 175, 250, and 350 °C, respectively. The partitioning of Sr/Ca between anhydrite and constituent dissolved species during precipitation depends greatly on the saturation state of the hydrothermal fluid with respect to anhydrite at each experimental temperature, K D(Anh-Fluid)  = 1.24–0.55 at 175–350 °C, broadly similar to results of earlier experimental observations by Shikazono and Holland (1983). Equilibrium K D(Anh-Fluid) values were estimated by taking explicit account of time dependent magnitude of exchange, yielding values of 0.43, 0.36, 0.29 at 175, 250, and 350 °C, respectively. Coupling these experimental constraints with the temperature gradient inferred for high temperature MOR hydrothermal systems suggests that the Ca isotope and Sr elemental composition of anhydrite formed near the seafloor will retain the composition derived upon initial formation conditions, which is indicative of disequilibrium. In contrast, at greater depths and at higher temperatures, anhydrite will reflect close to equilibrium Sr/Ca partitioning and Ca isotope fractionation conditions. The experimental and natural data presented in this study can be used to further understand the effect of anhydrite precipitation during hydrothermal circulation in the oceanic crust and on the chemical and isotopic composition of seawater on geologic timescales. [ABSTRACT FROM AUTHOR]

Ediacaran Cloudina and Namacalathus are among the earliest shell-forming organisms. The debated carbonate phase of their skeletons, high-magnesium calcite or aragonite, has been linked to seawater chemistry and pCO2, yet independent constraints on the original mineralogy are lacking. We present a new method to distinguish primary skeletal mineralogy using δ44/40Ca values and trace element compositions of the skeletons and associated cements. Ca isotopes are useful because they are relatively insensitive to diagenetic alteration during burial, and they vary with the primary mineralogical phase of carbonate. We applied this method to microdrilled carbonate and cements associated with both Namacalathus and Cloudina skeletons from the Ediacaran Omkyk Member of the Nama Group in southern Namibia. These data demonstrate that both organisms originally produced aragonitic skeletons, which later underwent diagenetic conversion to calcite. We suggest that calcium isotopes can be used to further constrain unknown skeletal mineralogies through time and to reassess the relationship between seawater chemistry and the mineralogy of biocalcifiers. [ABSTRACT FROM AUTHOR]

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A negative shift in the calcium isotopic composition of marine carbonate rocks spanning the end-Permian extinction horizon in South China has been used to argue for an ocean acidification event coincident with mass extinction. This interpretation has proven controversial, both because the excursion has not been demonstrated across multiple, widely separated localities, and because modeling results of coupled carbon and calcium isotope records illustrate that calcium cycle imbalances alone cannot account for the full magnitude of the isotope excursion. Here, we further test potential controls on the Permian-Triassic calcium isotope record by measuring calcium isotope ratios from shallow-marine carbonate successions spanning the Permian-Triassic boundary in Turkey, Italy, and Oman. All measured sections display negative shifts in δ44/40Ca of up to 0.6‰. Consistency in the direction, magnitude, and timing of the calcium isotope excursion across these widely separated localities implies a primary and global δ44/40Ca signature. Based on the results of a coupled box model of the geological carbon and calcium cycles, we interpret the excursion to reflect a series of consequences arising from volcanic CO2 release, including a temporary decrease in seawater δ44/40Ca due to short-lived ocean acidification and a more protracted increase in calcium isotope fractionation associated with a shift toward more primary aragonite in the sediment and, potentially, subsequently elevated carbonate saturation states caused by the persistence of elevated CO2 delivery from volcanism. Locally, changing balances between aragonite and calcite production are sufficient to account for the calcium isotope excursions, but this effect alone does not explain the globally observed negative excursion in the δ13C values of carbonate sediments and organic matter as well. Only a carbon release event and related geochemical consequences are consistent both with calcium and carbon isotope data. The carbon release scenario can also account for oxygen isotope evidence for dramatic and protracted global warming as well as paleontological evidence for the preferential extinction of marine animals most susceptible to acidification, warming, and anoxia. [ABSTRACT FROM AUTHOR]

Previous studies have shown that CV and CM chondrites incorporated Ca, Al-rich inclusions (CAIs) with different isotopic characteristics, which may represent different snapshots in the isotopic evolution of the early Solar System. To better understand how the isotopic characteristics of CAIs vary between different chondrite groups, we have studied calcium and titanium isotopes in CAIs from CM, CO, and CR chondrites. We show that all three chondrite groups contain CAIs with large anomalies in 48 Ca and/or 50 Ti (10s of ‰ or 100s of ε -units) as well as CAIs with no anomalies resolved beyond measurement uncertainties. Isotopically, the anomalous CO and CR chondrite CAIs resemble the platy hibonite crystals (PLACs) from CM chondrites, but they are more mineralogically complex. The new data are consistent with the well-established mutual exclusivity relationship between incorporation of 26 Al and the presence of large anomalies in 48 Ca and 50 Ti. The two highly anomalous CO chondrite CAIs have correlated anomalies in 46 Ti and 50 Ti, while most other highly anomalous CAIs do not. This result could indicate that the reservoir with coupled 46 Ti and 50 Ti that was sampled by bulk meteorites and CV chondrite CAIs already existed before arrival and/or homogeneous distribution of 26 Al in the protoplanetary disk. Among the studied CM chondrite CAIs are ten spinel-hibonite inclusions (SHIBs) with known oxygen isotopic compositions. Our results show that these objects sampled a reservoir that was well-mixed in oxygen, calcium, and titanium isotopes. We further show that SHIBs tend to be slightly enriched in the heavy calcium isotopes, suggesting that their formation history was different from CV chondrite CAIs. [ABSTRACT FROM AUTHOR]

Calcium stable isotopic compositions (δ 44/40 Ca SRM 915a ) as well as selected major and trace element concentrations are reported for micro-drilled samples of a hand-specimen sized spheroidally weathered diabase (synonymously used as dolerite or microgabbro) from southern India. A sample of the ~2.37 Ga old un-weathered diabase (UW) dike was also analyzed. X-ray micro-CT imaging of a representative portion of the weathered sample shows the presence of two dominant mineral phases, plagioclase and clinopyroxene, with minor proportions of an iron-bearing phase (possibly haematite or ilmenite). Two different generations of cross-cutting micro-fractures are identified from the micro-CT image. The older fracture is sealed with secondary mineral deposits. The δ 44/40 Ca values of the weathered samples range from 0.42‰ to 0.84‰ with samples showing both higher and lower values than that of the un-weathered diabase dike which shows a δ 44/40 Ca value of 0.65‰. The variation in δ 44/40 Ca in the weathered samples is significantly higher than the external reproducibility of our measurements (<0.1‰) based on multiple measurements of Ca isotopic standards SRM 915a, SRM 915b and seawater (NASS-6). Mass balance calculations based on Sr/Ca and δ 44/40 Ca values rule out the contributions of secondary silicate as well as carbonate minerals in causing the variability of δ 44/40 Ca in the weathered samples. The positive correlations between δ 44/40 Ca and Mg/Al, Mg/Na in the weathered samples further negate the possible contribution of secondary carbonates in causing the variability in δ 44/40 Ca, as Al and Na are unlikely to be present in carbonates. The samples with lower δ 44/40 Ca show higher CIA (Chemical Index of Alteration) values and Al/Ca than the UW diabase while samples having higher δ 44/40 Ca show lower CIA and Al/Ca than the UW diabase. The weathered sample having the lowest δ 44/40 Ca (close to the δ 44/40 Ca of plagioclase) exhibits the highest value of europium anomaly (Eu/Eu*). These observations are explained by the presence of variable relative proportions of residual clinopyroxene and plagioclase in the weathered diabase, due to selective weathering of these minerals. Since plagioclase and clinopyroxene could not be separated from the diabase, we measured the Ca isotopic compositions of a clinopyroxene (δ 44/40 Ca = 1.06‰) from the San Carlos mantle peridotite and plagioclase from the Chilka anorthosite complex (δ 44/40 Ca = 0.40‰) from the Eastern Ghats and used these isotopic values as representative of the compositions of these minerals in the diabase. Based on mixing models, using δ 44/40 Ca, Al/Ca and Sr/Ca of the plagioclase and clinopyroxene end-members, we estimate that the δ 44/40 Ca of the UW diabase can be explained by 31% clinopyroxene and 69% plagioclase, which is consistent with the modal proportion of these minerals in an average unweathered diabase. Based on mixing calculations, the δ 44/40 Ca, Sr/Ca and Al/Ca variability of the micro-drilled samples of the weathered diabase are explained by the presence of varying proportions of residual clinopyroxene (20–65%) and plagioclase (80–35%) in the weathered rock. This study demonstrates that selective weathering of major rock-forming minerals in nature can result in significant variation in δ 44/40 Ca in weathered rocks and that selective weathering of rock-forming minerals should be considered as an additional mechanism to explain the δ 44/40 Ca variability in rivers. [ABSTRACT FROM AUTHOR]

Large predators are overabundant in mid-Cretaceous continental dinosaur assemblages of North Africa. Such unbalanced ecosystem structure involves, among predatory dinosaurs, typical abelisaurid or carcharodontosaurid theropods co-occurring with long-snouted spinosaurids of debated ecology. Here, we report calcium (Ca) isotope values from tooth enamel (expressed as δ44/42Ca) to investigate resource partitioning in mid-Cretaceous assemblages fromNiger (Gadoufaoua) and Morocco (KemKemBeds). In both assemblages, spinosaurids displaya distinct isotopic signature, themost negative inour dataset. This distinct taxonomic clustering in Ca isotope values observed between spinosaurids and other predators provides unambiguous evidence for niche partitioning at the top of the trophic chains: spinosaurids foraged on aquatic environments while abelisaurid and carcharodontosaurid theropods relied almost exclusively on terrestrial resources. [ABSTRACT FROM AUTHOR]

Full recovery from column separation and matrix effects are the two factors that need to be considered for the high-precision analysis of stable Ca isotopes, but generally they are difficult to balance. In many cases, to get a pure Ca fraction, the interference of the matrix elements is reduced at the cost of discarding a fraction of Ca overlapping with other elements (e.g. Sr and K). However, quantitative evaluation using this approach is challenging but greatly needed. Our study investigates the influence of low Ca recovery on δ44/40Ca using thermal ionization mass spectrometry (TIMS) with the double spike technique. δ44/40Ca of IAPSO seawater, ML3B-G and BHVO-2 in different Ca subcuts (e.g. 0–20, 20–40, 40–60, 60–80 and 80–100%), display limited variation after iterative correction by 42Ca–43Ca double spike with the exponential law. Notably, δ44/40Ca of each Ca subcut with ∼20% recovery is consistent with that of the Ca cut with full recovery, within a margin of error. Our results indicate that 42Ca–43Ca double spike technique can simultaneously correct Ca isotopic fractionation, occurring during column separation, and TIMS determination, because both follow the exponential fractionation law well. Therefore, a “peak cut” procedure of column separation for Ca isotope measurement using the double spike technique on TIMS is proposed. Briefly, we can mix the double spike with the sample solution well before column separation, then collect the peak of the Ca cut and abandon both sides of the Ca eluate that may overlap with other elements. This procedure would eliminate matrix effects efficiently, especially for samples with low CaO contents which typically must be passed through the column twice (e.g. peridotite and dunite). [ABSTRACT FROM AUTHOR]

The Ca isotope composition of meteorites and their components may vary due to mass-dependent and/or -independent isotope effects. In order to evaluate the origin of these effects, five amoeboid olivine aggregates (AOAs), three calcium aluminum inclusions (CAIs), five chondrules (C), a dark inclusion from Allende (CV3), two dark fragments from North West Africa 753 (NWA 753; R3.9), and a whole rock sample of Orgueil (CI1) were analyzed. This is the first coupled mass-dependent and -independent Ca isotope dataset to include AOAs, a dark inclusion, and dark fragments. Where sample masses permit, Ca isotope data are reported with corresponding petrographic analyses and rare earth element (REE) relative abundance patterns. The CAIs and AOAs are enriched in light Ca isotopes (δ 44/40 Ca −5.32 to +0.72, where δ 44/40 Ca is reported relative to SRM 915a). Samples CAI 5 and AOA 1 have anomalous Group II REE patterns. These REE and δ 44/40 Ca data suggest that the CAI 5 and AOA 1 compositions were set via kinetic isotope fractionation during condensation and evaporation. The remaining samples show mass-dependent Ca isotope variations which cluster between δ 44/40 Ca +0.53 and +1.59, some of which are coupled with unfractionated REE abundance patterns. These meteoritic components likely formed through the coaccretion of the evaporative residue and condensate following Group II CAI formation or their chemical and isotopic signatures were decoupled (e.g., via nebular or parent-body alteration). The whole rock sample of Orgueil has a δ 44/40 Ca +0.67 ± 0.18 which is in agreement with most published data. Parent-body alteration, terrestrial alteration, and variable sampling of Ca-rich meteoritic components can have an effect on δ 44/40 Ca compositions in whole rock meteorites. Samples AOA 1, CAI 5, C 2, and C 4 display mass-independent 48/44 Ca anomalies (ε 48/44 Ca +6 to +12) which are resolved from the standard composition. Other samples measured for these effects (AOA 5, CAI 1, CAI 2, C 3, D 1, D 2, D 3) possess the same 48/44 Ca isotope composition as the standard within analytical uncertainty. These data indicate a heterogeneous distribution of 48 Ca in the early solar nebula during formation of CAIs, AOAs, and chondrules. In a ε 48/44 Ca vs. δ 44/40 Ca plot, no strong correlation is evident which suggests that the thermal processing event which caused a heterogeneous distribution of ε 48/44 Ca in the solar nebula is unlikely to be directly related to the thermal processing event that caused coupled REE and Ca mass-dependent isotopic fractionation in meteoritic components. [ABSTRACT FROM AUTHOR]

Two sets of hydrothermal experiments were performed to explore Ca isotope fractionation and exchange rates at hydrothermal conditions (410–450 °C, 31.0–50.0 MPa). The first set of experiments determined the magnitude of vapor-liquid Ca isotope fractionation and anhydrite solubility in the CaSO 4 -NaCl-H 2 O system. The data indicate no statistical difference between the Ca isotopic composition of coexisting vapor and liquid. The second set of experiments utilized an anomalous 43 Ca spike to determine the rate of Ca exchange between fluid and anhydrite as a function of total dissolved Ca concentration. Results show that the rate of exchange increases with dissolved Ca concentrations (12–23 mM/kg), but no change in exchange rate is observed when the Ca concentration increases from 23 to 44 mM/kg Ca. 74–142 days are required to achieve 90% anhydrite-fluid Ca isotope exchange at the conditions investigated, while only several hours are necessary for vapor-liquid isotopic equilibrium. The lack of vapor-liquid Ca isotope fractionation in our experiments is consistent with δ 44 Ca of mid-ocean ridge hydrothermal vent fluids that remain constant, regardless of chlorinity. Moreover, the narrow range of end member fluid δ 44 Ca, −0.98 to −1.13‰ (SW), is largely indistinguishable from MORB δ 44 Ca, suggesting that neither phase separation nor fluid-rock interactions at depth significantly fractionate Ca isotopes in modern high-temperature mid-ocean ridge hydrothermal systems. [ABSTRACT FROM AUTHOR]

To understand the characteristics of Ca isotope composition and fractionation in silicate-dominated Quaternary aquifer system, hydrochemical and isotope studies ( 87 Sr/ 86 Sr, 13 C DIC and 44/40 Ca) were conducted on groundwater, sediment and rock samples from the Datong basin, China. Along the groundwater flow path from the basin margin to the center, groundwater hydrochemical type evolves from Ca-HCO 3 to Na-HCO 3 /Na-Cl type, which results from aluminosilicate hydrolysis, vertical mixing, cation exchange between CaX 2 and NaX, and calcite/dolomite precipitation. These processes cause the decrease in groundwater Ca concentration and the associated modest fractionation of groundwater Ca isotopes along the flowpath. The groundwater δ 44/40 Ca value varies from −0.11 to 0.49‰. The elevated δ 44/40 Ca ratios in shallow groundwater are attributed to vertical mixing involving addition of irrigation water, which had the average δ 44/40 Ca ratio of 0.595‰. Chemical weathering of silicate minerals and carbonate generates depleted δ 44/40 Ca signatures in groundwater from Heng Mountain (east area) and Huanghua Uplift (west area), respectively. Along the groundwater flow path from Heng Mountain to central area of east area, cation exchange between CaX 2 and NaX on clay mineral results in the enrichment of heavier Ca isotope in groundwater. All groundwater samples are oversaturated with respect to calcite and dolomite. The groundwater environment rich in organic matter promotes the precipitation of carbonate minerals via the biodegradation of organic carbon, thereby further promoting the elevation of groundwater δ 44/40 Ca ratios. [ABSTRACT FROM AUTHOR]

Preserved biomineral chemical compositions serve as valuable tools in constructing proxies for reconstructing past climates. A persistent effort exists to enhance and innovate temperature proxies, ensuring a comprehensive understanding of historical temperature variability—a vital Earth System parameter. Calcium isotopes (δ44/40Ca) in foraminifera have been debated to be sensitive to temperature and requires further refinement. In this study we present a comprehensive assessment of δ44/40Ca temperature sensitivity in foraminifera by utilizing concurrent δ44/40Ca, δ18O, Mg/Ca, and Sr/Ca ratios data on same samples. These coupled isotopic and elemental ratios were determined across various foraminifera species, including benthic (C. wuellerstorfi), thermocline planktic (G. truncatulinoides, G. inflata), and mixed layer planktic (G. ruber, G. bulloides, O. universa, G. siphonifera) from core-top and sediment trap samples from the Atlantic Ocean. The δ44/40Ca -T calibration included calcification temperatures derived from both δ18O and Mg/Ca, utilizing established δ18O -T and Mg/Ca-T relationships. The temperature sensitivity of δ44/40Ca in foraminifera is 0.042–0.058 ‰/°C using all species data from published work and this study (except data from <3 °C habitat temperature). We report foraminifera δ44/40Ca and temperature relationships: δ44/40Ca (‰) = 0.042 × T (°C) + 0.289 (r2 = 0.80, p < 0.0001), using temperatures derived from δ18O and Mg/Ca, respectively. Furthermore, we introduce a novel paleotemperature equation for foraminifera, employing a multi-regression approach that combines two temperature proxies, δ44/40Ca and Mg/Ca: SST (°C) = (9 ± 2.6) × δ44/40Ca + (3.28 ± 0.63) × Mg/Ca - (2.89 ± 1.09). This multi-proxy approach can mitigate the uncertainty associated with individual proxy measurements and allows cross-validation. Application of this multi-proxy relationship to an East Atlantic Site (GeoB1112) yields a glacial-interglacial temperature change of 4.2 ± 0.93 °C between MIS 1 and MIS 3 which is in agreement with published work. [ABSTRACT FROM AUTHOR]

We present high-precision Ca isotopic data for 21 Cenozoic basaltic lavas from eastern China to evaluate the potential of Ca isotopes as a tool to trace the recycling of surface carbonates. These samples were derived from a variety of peridotite to pyroxenite mantle sources hybridized by different slab-derived agents. The δ44/42Ca of eastern China Cenozoic basaltic lavas ranges from 0.24 ± 0.03‰ (2SE, if not specified) to 0.39 ± 0.03‰, with an average of 0.31 ± 0.09‰ (2SD, N = 21), systematically lower than mid-ocean ridge basalts (MORB, 0.39 ± 0.07‰, 2SD, N = 30). No convincing correlation has been observed with δ26Mg and δ66Zn, indicating that metasomatism by recycled carbonates may not always yield Ca isotopically distinct mantle sources. δ44/42Ca shows correlations with parameters reflecting degree of partial melting (e.g., K 2 O, Nb, LREE, U, Th, Rb, and Sr) and residual garnet signature (i.e., (Dy/Yb) N). Nevertheless, modeling results suggest that Ca isotope fractionation during mantle and/or slab melting with the presence of garnet can partially account for the observed variable δ44/42Ca. Recycled carbonates with low δ44/42Ca were likely involved in the origins of the lightest lavas. Particularly, the nephelinitic lightest lavas also have the lowest δ26Mg, Ti/Ti⁎ and Hf/Hf⁎ among all the studied samples, reflecting a role of carbonatitic metasomatism in their sources. This study illustrates that a strongly light Ca isotope anomaly (e.g., δ44/42Ca ∼ 0.24‰) observed in basaltic lavas is likely associated with the recycling of carbonates into the mantle. • Cenozoic basaltic lavas from eastern China have δ44/42Ca variable from a MORB-like value (∼0.39‰) to ∼0.24‰. • The observed δ44/42Ca variation can be partially attributed to isotope fractionation during melting processes. • Involvement of light carbonates in the sources appears to be rather likely for the lightest lavas. [ABSTRACT FROM AUTHOR]

The deep carbon cycle requires tracers to understand carbon sequestration and mobility in the deep Earth. Recently, the use of metal stable isotopes has emerged as a promising and innovative approach to address element cycles, such as the use of magnesium and zinc isotopes to trace the magnesium carbonate cycle. However, despite the prevalence of calcium carbonate as the dominant sedimentary carbonate during the Phanerozoic era, its tracking through the use of metal stable isotopes has proven challenging. Here we show that deep subduction of Phanerozoic calcium carbonates can significantly decrease the δ44/40Ca value, while simultaneously increasing the δ66Zn value and 87Sr/86Sr ratio of the mantle. The Neogene Dalihu basalts in Inner Mongolia have high 87Sr/86Sr ratios, ranging from 0.7058 to 0.7063, which are positively correlated with Sr/Nd ratio, indicating the presence of sedimentary carbonate within their mantle source. The Dalihu basalts exhibit significantly lower δ44/40Ca values (0.51 to 0.70‰) but higher δ66Zn values (0.35 to 0.45 ‰) than the mantle. These low δ44/40Ca values cannot be attributed to partial melting, even when considering the jadeite effect. Our calculations indicate that adding 5–10% recycled calcium carbonate into the mantle source explains the high 87Sr/86Sr ratio and δ66Zn values but low δ44/40Ca values of the Dalihu basalt. Therefore, recycled calcium carbonate directly influences correlated Ca-Zn-Sr isotope variation, thereby providing compelling evidence for its role in the deep carbon cycle. Our findings suggest that combining metal stable isotopes with radiogenic Sr isotope is a powerful tracer of the deep carbon cycle. [ABSTRACT FROM AUTHOR]

The Mesozoic-Cenozoic transition is a period of biogeochemical cycle perturbations. The strongest of them is the Cretaceous-Paleogene boundary (K-Pg) crisis, characterized by one of the most important extinctions of planktonic marine calcifiers in Earth's history. One of the primary drivers of this biocalcification crisis is thought to be the increase in atmospheric CO 2 concentration and ocean acidification triggered by the Chicxulub Impact, and/or Deccan volcanism. Because it reflects changes of the calcium cycle and/or depends on parameters of the carbonate system, the Ca isotope composition of carbonate minerals precipitated from seawater (δ 44 / 40 Ca) offers the potential to reconstruct some of the environmental changes that occurred. Here we present new high-resolution planktonic and benthic foraminiferal δ 44 / 40 Ca, δ 18 O, δ 13 C, and Sr/Ca records across the K-Pg transition from Shatsky rise (Leg 198; ODP Site 1209, Hole C). The δ 44 / 40 Ca record displays a succession of rapid shifts of ca. − 0.4 ‰ across the K-Pg transition. They are similar though not identical between the planktonic and benthic records. These shifts took place on a timescale significantly shorter than the residence time of Ca in the oceans and are therefore unlikely to result from global disequilibrium in the oceanic Ca budget. Instead, changes in the fractionation factor between carbonate minerals and seawater in response to changes in precipitation rates may explain the observed δ 44 / 40 Ca and Sr/Ca record. The benthic and planktonic δ 44 / 40 Ca records show a late Maastrichtian and an early Danian negative excursions best explained by a succession of episodes of ocean alkalinity increase related to increased continental weathering caused by CO 2 emissions from Deccan volcanism and the aftermath of the K-Pg biocalcification crisis. Carbonate compensation via carbonate sediment dissolution, biological carbonate compensation via reduction of biocalcification, and/or an increase in continental weathering must have occurred to offset the excess CO 2 , ultimately resulting in rapid changes in ocean carbonate chemistry, in combination with reduced surface alkalinity export in response to the early Paleogene planktonic biomineralization crisis. • Ca isotopes (δ 44 / 40 Ca) measured from planktonic and benthic foraminifera. • Ca records reflect episodes of ocean alkalinity increase related to Deccan volcanism. • Deccan volcanism induces changes in ocean carbonate chemistry across K-Pg transition. [ABSTRACT FROM AUTHOR]

A new simple and rapid method was developed for the purification of Ca from geological and biological samples. The proposed separation protocol is based on DGA resin and consists of only three elution steps and two types of eluents (6.8 mL 4 mol L−1 HNO3 and 3 mL DI-water), which afford a straightforward separation of Ca with a high yield (95–99%), good purity and low blank level. Moreover, a vacuum box was employed which enables Ca purification for a batch of twelve samples within two hours, with greatly improved efficiency compared to conventional gravity flow separation methods. Ca isotopic ratios were measured by multi-collector inductively coupled plasma mass-spectrometry (MC-ICP-MS) using a standard–sample bracketing method. Based on repeated measurements of NIST SRM 915a and Alfa Ca, the long-term reproducibility was evaluated at ±0.07‰ (2SD) for δ44/42Ca. The validity of the proposed method was demonstrated by the analysis of thirteen international geological and biological reference materials with satisfactory results which are in good agreement with published values. Therefore, the proposed method can be used for a rapid and efficient determination of stable Ca isotopic ratios in geological and biological samples. [ABSTRACT FROM AUTHOR]

High resolution microwave spectra for the somewhat unstable compound 1-chloroborepin were measured in the 5-10 GHz range using a pulsed beam Fourier transform microwave spectrometer. Transitions were assigned and measured for three isotopologues, which include the most abundant isotopologue, 11B35Cl, and the less abundant 10B35Cl and 11B37Cl isotopologues. The molecular parameters (MHz) determined for the 11B35Cl isotopologue are A = 3490.905(35), B = 1159.38520(79), C = 870.59492(56), 1.5 χaa (11B) = -0.220(22), 0.25(χbb - χcc) (11B) = -1.5300(99), 1.5χaa (35Cl) = -54.572(33), and 0.25(χbb - χcc) (35Cl) = 4.7740(79). The inertial defect is calculated to be Δ = -0.174 amu Ų from the experimental rotational constants, indicating a planar structure with some out of plane vibrational motion. An extended Townes-Dailey analysis was performed on the 11B and 35Cl nuclei to determine the electron occupations in the valence hybridized orbitals using the experimental quadrupole coupling strengths. From the analysis it was determined that Cl is sharing some electron density with the empty p-orbital on B. The B-Cl bond length determined from the data is 1.798(1) Å, and the B-C bond lengths are 1.533(10) Å. The structural parameters and electronic structure properties of 1-chloroborepin are consistent with an aromatic boron-containing molecule. [ABSTRACT FROM AUTHOR]

Abstract: Mantle peridotites show that Ca is isotopically heterogeneous in Earth's mantle, but the mechanism for such heterogeneity remains obscure. To investigate the effect of partial melting on Ca isotopic fractionation and the mechanism for Ca isotopic heterogeneity in the mantle, we report high‐precision Ca isotopic compositions of the normal Mid‐Ocean Ridge Basalts (N‐MORB) from the southern Juan de Fuca Ridge. δ44/40Ca of these N‐MORB samples display a small variation ranging from 0.75 ± 0.05 to 0.86 ± 0.03‰ (relative to NIST SRM 915a, a standard reference material produced by the National Institute of Standards and Technology), which are slightly lower than the estimated Upper Mantle value of 1.05 ± 0.04‰ and the Bulk Silicate Earth (BSE) value of 0.94 ± 0.05‰. This phenomenon cannot be explained by fractional crystallization, because olivine and orthopyroxene fractional crystallization has limited influence on δ44/40Ca of N‐MORB due to their low CaO contents, while plagioclase fractional crystallization cannot lead to light Ca isotopic compositions of the residue magma. Instead, the lower δ44/40Ca of N‐MORB samples compared to their mantle source is most likely caused by partial melting. The offset in δ44/40Ca between N‐MORB and BSE indicates that at least 0.1–0.2‰ fractionation would occur during partial melting and light Ca isotopes are preferred to be enriched in magma melt, which is in accordance with the fact that δ44/40Ca of melt‐depleted peridotites are higher than fertile peridotites in literature. Therefore, partial melting is an important process that can decrease δ44/40Ca in basalts and induce Ca isotopic heterogeneity in Earth's mantle. [ABSTRACT FROM AUTHOR]

The external reproducibility (2σSD) of Ca stable isotope ratio measurements (δ44/40Ca) using double-spike thermal ionization mass spectrometry (TIMS) shows a large range from ＜0.1‰ to 0.5‰. We demonstrate that using a 43Ca–48Ca double spike, which allows simultaneous measurements of δ44/40Ca and δ44/42Ca, and analyses at moderate signal strengths (40Ca ranging from 6 to 10 V), high precision δ44/40Ca (external reproducibility better than ±0.08‰, 2σSD) as well as δ44/42Ca can be obtained if samples and standards are analyzed under similar fractionation conditions. To monitor the fractionation conditions, the usage of a parameter β is proposed, which measures the deviation of the 43Ca/48Ca ratio of the sample–double spike mixture from that of the pure double spike. A novel, low-cost sample loading technique using a combination of Re and Ta filaments and a Ta2O5 activator is presented which results in a steady signal. We report the δ44/40Ca and δ44/42Ca values, calculated w.r.t. NIST SRM 915a and reported as deviations in parts per mil (‰), of NIST standards SRM 915a (0.01, −0.02) and SRM 915b (0.73, 0.32), NASS 6 (seawater, 1.80, 0.89), USGS silicate rock standards BHVO-2 (0.86, 0.47) and BCR-2 (0.89, 0.48), Geological Survey of Japan carbonate standards JCp-1 (coral, 0.85, 0.45) and JCt-1 (clam shell, 0.83, 0.46) and ECRM 752-1 (limestone, alternative name BCS-CRM 393) (0.83, 0.46) from the Bureau of Analyzed Samples Ltd. UK. The Ca stable isotopic compositions of JCt-1 and ECRM 752-1 are reported for the first time. [ABSTRACT FROM AUTHOR]

High-precision on-line procedure for measurement of calcium isotopic ratio by coupling ion chromatography to multicollector inductively coupled plasma mass spectrometry was developed. Calcium separation from the sample matrix was achieved on an ion chromatography column--IonPac CS16--ID 3 mm connected with CERS 500 2 mm suppressor and followed by multicollector inductively coupled plasma mass spectrometry calcium isotopic ratio determination. Dry plasma mode was used with Aridus II desolvation system. To sustained samples with high level of total dissolved salts as well as account capacity of applied analytical column, the method has been optimized regarding calcium isotope ratio measurements with low-resolution mass spectrometry. Mass discrimination and instrument drift were corrected by sample-standard bracketing method using the 44Ca/42Ca isotope ratio of SRM 915a as a standard. Good accuracy and reasonable precision of calcium isotope ratio (generally 0.20‰ [2SD]) were achieved, which are comparable to off-line Ca separation and continuous measurement. The reproducibility of the proposed analytical procedure was verified by measuring the SRM 915a standard as a sample randomly over 3 months (n = 56). Applicability of the protocol was demonstrated for matrix-rich natural water samples, coral samples and bone standard reference materials. [ABSTRACT FROM AUTHOR]

Early Neoproterozoic tectonics of the Yangtze block remains poorly understood because very limited igneous records are available from the time interval of ∼1000–870 Ma. In this paper, our new SIMS U–Pb dating results demonstrate that the Liushudian mafic intrusion and Pinghe alkaline complex in the northwest Yangtze block were emplaced at 888 ± 6 Ma and 891 ± 7 Ma, respectively, representing the products of a ∼890 Ma igneous event. Gabbros from the Liushudian intrusion have rather depleted zircon ɛ Hf( t ) (mean = 10.4) and normal mantle-like zircon δ 18 O (mean = 5.97‰). Their parental magma was thus probably derived from asthenospheric mantle. Geochemically, these mafic rocks have an affinity to continental flood tholeiitic basalts rather than ocean island basalts, as previously thought. In contrast, an ijolite sample from the Pinghe complex has less depleted zircon ɛ Hf( t ) (mean = 5.7) and anomalously high zircon and apatite δ 18 O (mean = 13.76‰ and 13.80‰, respectively). Such a characteristic δ 18 O signal, among the highest yet known for igneous zircons, could be either inherited from a magma source in metasomatized lithospheric mantle or acquired by assimilation of high-δ 18 O supracrustal materials (e.g., limestone, chert) during magma evolution. An intra-plate extensional environment is suggested for the ∼890 Ma igneous event in the northwest Yangtze block, although it is as yet unclear whether this igneous event is related to a mantle plume or not. It could be concluded that magmatism on the western periphery of the Yangtze block was not shut down between ∼1000 and ∼870 Ma, and the ∼890 Ma intra-plate igneous event may mark either the onset of Neoproterozoic continental rifting or the ending of Late Mesoproterozoic to Early Neoproterozoic lithospheric extension. [ABSTRACT FROM AUTHOR]

In this study, we linked Ca speciation with isotope composition in plants. To do this, we performed leachate experiments to access the soluble Ca, structurally bound Ca and insoluble Ca (i.e., water and weak acid resistant) within beech tree organs ( Fagus sylvatica L.). Ca isotopic measurements were combined with infrared spectroscopy and calcium oxalate biomineralization identification. The results from our study indicate that bark and leaves are the most enriched in monohydrated calcium oxalate crystals (whewellite), which are observable in parenchyma and sclerenchyma tissues, whereas roots and wood are enriched in structurally bound Ca. Our leaching experiments also show decreasing δCa isotopic signatures in the order of soluble Ca > structurally bound Ca > insoluble Ca. This finding implies that because leaves degrade faster than wooden organs and because Ca linked to pectate decomposes faster than Ca linked to oxalate crystals, differential Ca isotopic signatures are expected to be observed during litter degradation. [ABSTRACT FROM AUTHOR]

Coccoliths comprise a major fraction of the global carbonate sink. Therefore, changes in coccolithophores' Ca isotopic fractionation could affect seawater Ca isotopic composition, affecting interpretations of the global Ca cycle and related changes in seawater chemistry and climate. Despite this, a quantitative interpretation of coccolith Ca isotopic fractionation and a clear understanding of the mechanisms driving it are not yet available. Here, we address this gap in knowledge by developing a simple model (CaSri–Co) to track coccolith Ca isotopic fractionation during cellular Ca uptake and allocation to calcification. We then apply it to published and new δ 44 / 40 Ca and Sr/Ca data of cultured coccolithophores of the species Emiliania huxleyi and Gephyrocapsa oceanica . We identify changes in calcification rates, Ca retention efficiency and solvation–desolvation rates as major drivers of the Ca isotopic fractionation and Sr/Ca variations observed in cultures. Higher calcification rates, higher Ca retention efficiencies and lower solvation–desolvation rates increase both coccolith Ca isotopic fractionation and Sr/Ca. Coccolith Ca isotopic fractionation is most sensitive to changes in solvation–desolvation rates. Changes in Ca retention efficiency may be a major driver of coccolith Sr/Ca variations in cultures. We suggest that substantial changes in the water structure strength caused by past changes in temperature could have induced significant changes in coccolithophores' Ca isotopic fractionation, potentially having some influence on seawater Ca isotopic composition. We also suggest a potential effect on Ca isotopic fractionation via modification of the solvation environment through cellular exudates, a hypothesis that remains to be tested. [ABSTRACT FROM AUTHOR]