Ca Isotopes Research Articles

Magnitude and timescales of Ca isotope variability in human urine: implications for bone mass balance monitoring.

Calcium (Ca) isotopes in blood/urine are emerging biomarkers of bone mineral balance (BMB) in the human body. While multiple studies have investigated Ca isotopes in patients suffering from diseases affecting BMB, comparatively little effort has been devoted to understanding the homeostasis of Ca isotopes in healthy individuals. Here, we report on a longitudinal study of the urine Ca isotope composition (δ44/42CaUrine) from 22 healthy participants (age 19 to 60) over timescales ranging from days to months. Data from a single participant collected over a 30-day period show that morning urine is an excellent proxy for 24 hr pooled urine fractions. Data from all participants reveal large inter-individual variability in δ44/42CaUrine (up to 2.2 ‰), which is partly due to anthropometric differences, as shown by a correlation between the participants' body mass index (BMI) and δ44/42CaUrine values. In contrast, intra-individual data reveal encouraging stability (within ∼ ±0.2-0.3 ‰) over timescales>160 days, indicating that self-referencing approaches for BMB monitoring hold greater promise than cross sectional ones. Our data confirm that intra-individual δ44/42CaUrine variations are mainly a function of Ca reabsorption in the kidney, but also reveal the impact of other (and at times equally important) drivers, such as diet, alcohol consumption, physical exercise, or fasting. We also find that a magnetic resonance imaging contrast agent (gadolinium) can lead to artifacts during Ca isotope analysis. Based on our results, a series of practical considerations for the use of Ca isotopes in urine as tracers of BMB is presented.

Understanding Water–Rock Interaction in Crystalline Shield Fluids Using Calcium Isotopes

Calcium isotopes provide a potentially robust tool for understanding the evolution of crystalline shield fluids, but previous applications have focused on near surface groundwaters. The tendency of Ca isotopes to be affected by mass-dependent fractionation during processes such as water–rock interaction provides a powerful tool for studying the evolution and origin of groundwaters hosted in crystalline rocks. We report Ca isotope ratios (δ44/40Ca) of deep fluids (>300m) from across the Canadian Shield and integrate these with Sr and Br isotope values to understand long-term fluid–rock interactions in crystalline shield environments. Ca isotope ratios have a wide range of values from 0.07‰ to 0.86‰. At individual sites, δ44/40Ca values are variable whereas 87Sr/86Sr ratios are relatively constant. Sr isotope ratios (87Sr/86Sr) have a negative relationship with the Ca vs. Na content of the fluid indicating different host-minerals contributing Sr to the fluid. Ca isotope fractionation was caused by metamorphic reactions and by the growth and dissolution of Ca-rich fracture-filling minerals. The δ44/40Ca signatures of these processes are overprinted by radiogenic ingrowth of 40Ca by decay of 40K, which is expected to affect older and more K-rich rocks. At one site, δ44/40Ca and δ81Br variability reflects gas-generating reactions in the fluid and/or water–rock interaction processes. These new results demonstrate the strength of combining multiple isotope analysis to elucidate the sources of groundwater salinity and decipher the complex long-term processes that occur in crystalline shield environments.

Fore-arc metasomatism by hybrid slab fluids during subduction initiation: Sr–Mg–Ca isotopes of rodingite, western Yarlung Zangbo suture zone

Subduction zone metasomatism is critical for Earth's material exchanges, yet the details of slab dehydration, particularly deserpentinization beneath fore-arcs, remain poorly understood. Here, we present Sr–Mg–Ca isotopic data for Purang rodingites from the western Yarlung Zangbo suture zone (YZSZ), a remnant of the Neo-Tethys Ocean that was subducted during the collision between the Indian and Eurasian Plates. The rodingites, occurring as centimeter- to meter-sized veins and blocks in serpentinized harburgite, are dominated by amphibolite- to greenschist-facies minerals like tremolite, magnesiohornblende, and chlorite. Their cumulate textures and rare earth element patterns resemble troctolite or gabbronorite, presumably formed beneath a seafloor spreading center. The rodingites are enriched in large ion lithophile elements and depleted in high field strength elements, with higher Sr/Nb and Ba/La and lower Nb/U and Ce/Pb ratios than mid-ocean ridge basalts (MORB) and rodingites formed through seawater or serpentinizing fluid alteration of MORB-like protoliths. They also exhibit higher initial 87Sr/86Sr ratios (0.7067–0.7075) and elevated δ26Mg values (−0.22 ± 0.07 ‰ to −0.13 ± 0.02 ‰) compared to unaltered oceanic basalts, while their δ44/40Ca values (0.72 ± 0.02 ‰ to 0.87 ± 0.03 ‰) remain similar to MORB. These signatures point to a metasomatic process beyond seafloor alteration, suggesting modification of a MOR-derived protolith in the fore-arc mantle at slab depths of <40 km, driven by Sr- and Mg-rich fluids from clay-rich sediments and serpentinitized mantle. The measured Sr, Mg and Ca isotope compositions can be reproduced by mixing a MORB-like protolith with hybrid fluids derived from varying proportions of sedimentary clays and serpentinitized peridotite. Combined with previous studies on YZSZ metamorphic soles, we propose that this fore-arc metasomatism occurred during the early Cretaceous, concurrent with the incipient subduction of the Neo-Tethys oceanic rocks. These results highlight the significance of deserpentinization at shallow fore-arc mantle settings during subduction initiation, suggesting that slab dehydration is more complex than previously recognized.

Trace metal evolution of the Late Cretaceous Ocean

The Cenomanian-Turonian boundary (Late Cretaceous) witnessed the last spectacular manifestation of Mesozoic Anoxic Events (OAE 2, ~94 Ma), marked by a prominent carbon isotope excursion (CIE) and burial of organic-matter-rich sediments under high atmospheric CO2 concentrations. But the Late Cretaceous generally was a time of profound environmental change. OAE 2 was preceded by other CIEs, including the Mid-Cenomanian Event (MCE), and was punctuated by a short re‑oxygenation and cooling event (the Plenus Cold Event, PCE). Extensive previous studies, including many trace metal studies, have focused on OAE 2, but there is still debate concerning the degree of drawdown of oceanic trace metal reservoirs during OAE 2, whether this drawdown is global or local, its causes and consequences for ocean ecology. Here, we present records of eight trace metals, over about 5 Myr of the Late Cretaceous, from the Tarfaya Basin in the proto-North Atlantic. The long records from a core preserving a continuous sedimentary succession allow us to set changes occurring across OAE 2 in the broader context of Late Cretaceous, including the lead up to OAE 2. Moreover, the multiple trace metal dataset allows us to broadly investigate the oceanographic setting in the context of recent studies of multiple trace metals in modern organic-rich sediments aimed at refining the proxies.Trace metal enrichments in these organic-rich sediments are discussed on three different timescales. Firstly, comparison of these Late Cretaceous sediments with modern organic-rich sediments are consistent with deposition in an open ocean upwelling margin in the Late Cretaceous, very like the modern Peru or Namibian Margin, although the deep proto-North Atlantic was probably partially restricted. Secondly, in common with previous studies, metal/TOC ratios often show sharp drops in the early part of OAE 2. Thirdly, however, this sharp drop occurs within a framework of pseudo-cyclical variations in metal/TOC, with a period of about 143 ±19 kyr (1 SD), that is a feature of these long records well before OAE 2, including across the MCE. Different metals respond differently to the perturbation in the early part of OAE 2 itself. Simple mass balance considerations suggest that trace metal drawdown with organic carbon must be at least partially compensated by changes in the rate of chemical weathering on the continents, as previously inferred from Li and Ca isotopes. Moreover, changes in the patterns of variation between different metals, as well as covariation of metal/TOC ratios and Os isotopes, hint at changes in the pattern of chemical weathering, most prominently in the contribution of mafic rocks to the chemical weathering flux.

Impact of correlations on nuclear binding energies

A strong effort will be dedicated in the coming years to extend the reach of ab initio nuclear-structure calculations to heavy doubly open-shell nuclei. In order to do so, the most efficient strategies to incorporate dominant many-body correlations at play in such nuclei must be identified. With this motivation in mind, the present work analyses the step-by-step inclusion of many-body correlations and their impact on binding energies of Calcium and Chromium isotopes. Employing an empirically-optimal Hamiltonian built from chiral effective field theory, binding energies along both isotopic chains are studied via a hierarchy of approximations based on polynomially-scaling expansion many-body methods. More specifically, calculations are performed based on (i) the spherical Hartree–Fock–Bogoliubov mean-field approximation plus correlations from second-order Bogoliubov many-body perturbation theory or Bogoliubov coupled cluster with singles and doubles on top of it, along with (ii) the axially-deformed Hartree–Fock–Bogoliubov mean-field approximation plus correlations from second-order Bogoliubov many-body perturbation theory built on it. The corresponding results are compared to experimental data and to those obtained via valence-space in-medium similarity renormalization group calculations at the normal-ordered two-body level that act as a reference in the present study. The spherical mean-field approximation is shown to display specific shortcomings in Ca isotopes that can be understood analytically and that are efficiently corrected via the consistent addition of low-order dynamical correlations on top of it. While the same setting cannot appropriately reproduce binding energies in doubly open-shell Cr isotopes, allowing the unperturbed mean-field state to break rotational symmetry permits to efficiently capture the static correlations responsible for the phenomenological differences observed between the two isotopic chains. Eventually, the present work demonstrates that polynomially-scaling expansion methods based on unperturbed states that possibly break (and restore) symmetries constitute an optimal route to extend ab initio calculations to heavy closed- and open-shell nuclei.

Determination of Calcium Isotopes Using Inductive Coupled Plasma-Tandem Mass Spectrometry with Ozone-Induced Oxidation.

In this study, we propose a new method for determination of calcium (Ca) isotopes using inductive coupled plasma-tandem mass spectrometry (ICP-MS/MS). The most abundant isotope of Ca, 40Ca, has a spectrum identical with that of argon (Ar), which is often used as the carrier gas in ICP-MS analysis. The detection capability for 40Ca is thus significantly reduced in the presence of 40Ar. To avoid interference from 40Ar during the 40Ca measurements, ozone (O3) was used as the reaction gas in the ICP-MS/MS analysis. The reaction of Ca+ with O3 in the collision cell preferentially produced CaO3+. In contrast, the reaction between Ar+ and O3 produced ArO+, which further reacted with the oxidized O3 to produce Ar+. Because Ar+ does not produced ArO3+ as the main product of the reaction with O3, the measurement of Ca+ as CaO3 via O3-induced oxidation resulted in considerably less interference from Ar+. The detailed mechanism of CaO3+ formation was investigated via quantum chemistry calculations by using density functional theory. The interaction between Ca+ and O3 yielded CaO3+, which immediately dissociated into either Ca+ and O3 or CaO+ and O2 because the pressure in the collision cell was approximately 1 Pa in the MS/MS experiments. Consequently, CaO3+ was formed by the consecutive reaction of Ca+ with O3, with CaO+ and CaO2+ as intermediates. The present method achieved the detection of less abundant Ca isotopes in certified reference materials such as 43Ca, 44Ca, 46Ca, and 48Ca, thereby allowing the determination of Ca isotopes with high sensitivity.

Ca–Zn isotope fractionation during fluid–rock interaction in subduction zones and its implication for deep carbon cycle

Subducting slabs transport Earth's surface carbon into the deep mantle, with a portion of this carbon potentially recycled through volcanic eruptions at mid-ocean ridges or ocean islands. Non-traditional stable isotopes such as calcium (Ca) and zinc (Zn) are increasingly used to trace this deep carbon cycle. However, the potential for Ca and Zn isotope fractionation in the subduction zone, critical for the successful application of these tracers, remains insufficiently explored. In this study, we investigate high-pressure carbonated metamafic rocks and enclosed vein systems from a Paleo-oceanic subduction complex (SW Tianshan) to explore the possible fractionation of Ca and Zn isotope during subduction-related metamorphism and fluid–rock interaction. The δ44/40Ca (0.77 ‰–0.90 ‰) and δ66Zn isotope compositions (0.20 ‰–0.39 ‰) of greenschist-, blueschist- and eclogite-facies metabasites are consistent with their protoliths (OIBs and MORBs). We observe no systematic fractionation of either Ca and Zn isotopes across prograde metamorphism and associated dehydration. However, the precipitation of carbonate in surrounding rock along fluid pathways leads to an increase in the δ44/40Ca of residual fluids (>0.90 ‰). More significantly, we find detectable Zn isotope fractionation (ΔA-H = ẟ66Znaltered - ẟ66Znhost of 0.06–0.10 ‰) during fluid-mediated metasomatism of some blueschists/eclogites. Light Zn isotopes are preferentially released into the infiltrating fluid, resulting in an elevated δ66Zn isotope composition in the immediate carbonated eclogites along the vein. The modified slab fluids during migration are likely charactered by heavy δ44/40Ca and light δ66Zn compositions, which may play a crucial role in determining the low δ66Zn values found in some arc lavas. However, the MORB-like δ44/40Ca compositions observed in global arc magmatic rocks suggest that the slab-released Ca (possibly along with some carbon) is probably sequestered and stored along the migration path of the fluid, thereby hindering the slab–arc carbon recycling. Our study emphasizes that intraslab fluid–rock interaction results in similar δ44/40Ca and δ66Zn signature in carbonate-bearing eclogites compared to sedimentary carbonates. Consequently, deeply subducted carbonate-bearing eclogites –not necessarily sedimentary carbonates– might carry light δ44/40Ca and heavy δ66Zn signatures into the deep mantle, potentially influencing the corresponding Ca-Zn isotope compositions of some mantle-derived basalts.

Analysis of level structure and monopole effects in Ca isotopes

Analysis of level structure and monopole effects in Ca isotopes

The potential of Ca isotopes to trace subducted marine carbonates in deep mantle

The potential of Ca isotopes to trace subducted marine carbonates in deep mantle

Theoretical comparison of transition rate B(E2) and deformation parameter with experimental data for calcium (20Ca) isotopes using shell model theory

A theoretical comparison has been made for some calcium isotopes ([Formula: see text]Ca) which are even–even nuclei and have the atomic mass (Z = 20) with its previous experimental data. Theoretical calculations of some [Formula: see text]Ca isotopes (A = 42, 44, 46, 48, 50, 52) adopted by the shell model theory were performed to calculate the transition rate B(E2), theoretical intrinsic quadruple moments ([Formula: see text] and theoretical deformation parameters ([Formula: see text], [Formula: see text] were calculated by two methods by using different effective interactions for each isotope such as, su3fp, fpbm, fprkb, fpd6, kb3. Through code NuShellX@MSU, the single-body density matrix was calculated. The effects of the core polarization were neglected by adopting various effective charges that were employed, effective charges of conventional (Con-E), effective charges of standard (St-E) and effective charges of Bohr and Mottelson (B-M-E) which were calculated. The theoretical values of the B(E2)Th, the [Formula: see text] and the ([Formula: see text], [Formula: see text] were then compared with the previous experimental data where values of the transition rate B(E2)Th, theoretical intrinsic quadrupole moments [Formula: see text] and theoretical deformation parameter ([Formula: see text], [Formula: see text], using the fpbm, the fpd6 and the kb3 interactions were the best.

HIDALGO: A FUN object from the earliest epoch of the solar system’s history

Chemical and isotopic measurements of HIDALGO, a stoichiometrically pure hibonite inclusion found in the matrix of the Dar al Gani 027 meteorite, were conducted by secondary ion mass spectrometry to investigate its origin and evolution. HIDALGO is characterized by large mass-dependent isotope fractionations in O, Ca, and Ti, as well as large negative anomalies in neutron-rich 48Ca and 50Ti, making it the newest member of the HAL-type FUN inclusions. The highly fractionated Ca and Ti isotopes but unfractionated Mg isotopes are consistent with HIDALGO being a residue from an extensive evaporation event, during which large fractions of initial Ca and Ti, and essentially all the initial Mg, in the precursor material were lost. HIDALGO appears to have incorporated live 26Al at a higher level than other HAL-type inclusions, but still at a lower amount compared to the Solar System’s initial 26Al abundance typically found in non-FUN CAIs. Interestingly, the inferred 10Be abundance in HIDALGO is comparable to the values observed in the majority of CV3 CAIs but ∼ 2.5 times higher than those in HAL-type samples. HIDALGO’s unusual 26Al/27Al and 10Be/9Be ratios, together with the 48Ca-50Ti anomalies, can be best explained by the formation of its precursor material in the isotopically heterogeneous solar nebula. Finally, large 7Li excesses correlating with Be/Li were found in HIDALGO, a behavior that can be interpreted as due to in-situ decay of live 7Be. Charged particle spallation of initially Li-free HIDALGO can simultaneously account for the inferred 7Be abundance and the measured Li elemental concentration. The consistency between the measurement and spallation calculation results provides support for the prior existence of 7Be in HIDALGO, possibly produced by irradiation close to the Sun

Calcium isotope composition in serum and urine for the assessment of bone mineral balance (BMB) – The Osteolabs post-market follow-up study

To further explore the clinical applicability of the calcium (Ca) isotope marker (CIM), we determined the 44Ca/42Ca isotope ratio in blood serum and urine. This ratio is expressed in the conventional δ-notation (as defined in the text below) specifically as CIM-serum for serum and as CIM-urine for urine. Our study tested the hypothesis that CIM values can differentiate between positive and negative bone mineral balance (BMB) across a diverse clinical population considering variables such as age, gender, and diet. The threshold values (CIM-serum: −0.85 ± 0.06 ‰ and CIM-urine: 0.23 ± 0.06 ‰) established in the OsteoGeo study (NCT02967978, Eisenhauer et al., 2019) were evaluated in 2320 participants as part of a surveillance study referred to as Osteolabs study. The earlier study revealed women with osteoporosis had an average CIM-serum value of −0.91 ± 0.21 ‰ (N = 24) and a CIM-urine value of 0.18 ± 0.33 ‰ (N = 71) that are significantly below the threshold values (p = 0.02 for urine, one-sided Wilcoxon rank test, p < 0.001 for serum, one-sided Student's t-test). Diseases affecting BMB such as osteoporosis, acute and chronic kidney disease (CKD), hyperthyroidism, breast cancer, prostate cancer, and myeloma were associated with significantly lower average CIM values, falling below the equilibrium thresholds and indicating negative BMB. In contrast, patients receiving osteoprotective treatments such as denosumab, Romosozumab, bisphosphonates, or hormone replacement therapy for certain diseases, had CIM values above the equilibrium thresholds indicating a positive BMB. Additionally, Ca supplements taken by some of the patients ((N = 22 (serum), N = 49 (urine), median dose: 500 mg) showed a Ca isotope composition approximately 1 ‰ higher than that from a normal diet. Consequently, their CIM values need to be adjusted to account for the amount and duration of supplementation to be comparable to those with a normal diet. Participants taking vitamin D (237 women; 58 men) showed no significant difference from the average values of the study group. Counterintuitively, the possible impact of malnutrition on individual BMB was most pronounced in vegans, who exhibited the highest average CIM-urine values compared to patients on a normal diet (p < 0.001, N = 17). The results of this study were consistent with the registered OsteoGeo study (NCT02967978) and other earlier published Ca isotope-based studies on BMB. We confirm that the CIM threshold values determined in the OsteoGeo study are generally valid for this much larger and diverse surveillance study group covering a diverse population encompassing various medical conditions and therapies.

Weakening of N = 28 shell gap and the nature of 02+ states

The work reports a novel application of the interacting boson model in light-mass region around 48Ca, that takes into account intruder states and configuration mixing. The model is shown to provide a reasonable description of the observed low-lying yrast and yrare states of the N = 28 even–even isotones from Si to Fe, and even–even Ca isotopes. The nature of 02+ states is addressed in terms of the competition between spherical and deformed intruder configurations, and the rigidity of the N = 28 shell gap is tested, particularly for 44S. The 02+ isomer in 44S is shown to arise from a weak mixing between the two configurations and called as shape isomer. The unresolved low-lying spectra in 46Ar is also approached using the core-excitations across N = 28 shell. The SU(3) structure in 42Si is supported by the present calculation. The nearly spherical nature of 50Ti, 52Cr, 54Fe and 42,44,46Ca isotopes is found, while the core excitations are found to be essential for the description of yrare states. Shell model calculation is also performed for comparison.

Symmetry energy from two-nucleon separation energies of Pb and Ca isotopes

Symmetry energy from two-nucleon separation energies of Pb and Ca isotopes

Experimental determination of Si, Mg, and Ca isotope fractionation during enstatite melt evaporation

Abstract Evaporation of silicate materials from Earth or its precursors may be important in shaping their primordial compositions represented by undifferentiated meteorites, e.g., enstatite chondrites; however, the conditions under which evaporation occurs and the extent of evaporation-induced elemental and isotope fractionation remain uncertain. Here, we experimentally determine the volatility and isotope fractionation of Si, Mg, Ca, Nb, and Ta during enstatite melt evaporation at 2423–2623 K using a high-temperature conical nozzle levitator. Homogenous glasses are recovered after experiments; then we use EPMA and LA-ICP-MS to measure the elemental compositions, MC-ICP-MS to measure the Si and Mg isotopes, and TIMS to measure the Ca isotopes. Our results show that the evaporation rates of Si are larger than Mg, and the mean vapor/melt isotope fractionation factors (α = Rvapor/Rmelt; R = isotope ratio) are 0.99585 ± 0.00002 for 29Si/28Si and 0.98942 ± 0.00130 for 25Mg/24Mg. However, neither evaporative loss of Ca, Nb, and Ta nor Ca isotope fractionation was observed within analytical uncertainty. In conjunction with previous studies, we find that in an evaporation experiment the saturation degree (partial vapor pressure/equilibrium vapor pressure) of Si (SSi) is larger than SMg when Si is more volatile than Mg, and vice versa. If the Mg/Ca and Si/Ca ratios and isotopes in the bulk silicate Earth are attributed to the evaporation of enstatite chondrite-like precursors, evaporation temperatures &amp;gt;5000 K and SSi &amp;lt; SMg are required.

Spectroscopy of deeply bound orbitals in neutron-rich Ca isotopes

The calcium isotopes are an ideal system to investigate the evolution of shell structure and magic numbers. Although the properties of surface nucleons in calcium have been well studied, probing the structure of deeply bound nucleons remains a challenge. Here, we report on the first measurement of unbound states in 53Ca and 55Ca, populated from 54,56Ca(p,pn) reactions at a beam energy of around 216 MeV/nucleon at the RIKEN Radioactive Isotopes Beam Factory. The resonance properties, partial cross sections, and momentum distributions of these unbound states were analyzed. Orbital angular momentum l assignments were extracted from momentum distributions based on calculations using the distorted wave impulse approximation (DWIA) reaction model. The resonances at excitation energies of 5516(41)keV in 53Ca and 6000(250)keV in 55Ca indicate a significant l =3 component, providing the first experimental evidence for the ν0f7/2 single-particle strength of unbound hole states in the neutron-rich Ca isotopes. The observed excitation energies and cross-sections point towards extremely localized and well separated strength distributions, with some fragmentation for the ν0f7/2 orbital in 55Ca. These results are in good agreement with predictions from shell-model calculations using the effective GXPF1Bs interaction and ab initio calculations and diverge markedly from the experimental distributions in the nickel isotones at Z=28.

Calcium isotope fractionation during weathering of argillaceous carbonates in a humid climate

The continental weathering is a key process that controls calcium (Ca) transportation from the continental crust to the waters. To elucidate the behavior of Ca isotopes during carbonate weathering, the concentrations and δ44/40Ca (relative to NIST SRM 915a) of bulk saprolites, exchangeable, acid-leachable and residual phases of a weathering profile developed on the marine carbonates, Guangdong province, South China, were investigated. Upwards the profile, δ44/40Ca values of the bulk saprolites systematically decrease from 0.77 ± 0.12 ‰ to −0.44 ± 0.12 ‰, suggesting that significant Ca isotope fractionation occurred during chemical weathering. The exchangeable fractions have δ44/40Ca values higher than those of the bulk saprolites with Δ44/40Caexchangeable-saprolite varying from −0.01 ‰ to 0.73 ‰, suggesting that heavy isotopes are preferentially adsorbed onto the clays. The acid-leachable phases display a relatively narrow δ44/40Ca range from 0.52 ‰ to 0.74 ‰ with Ca fractions varying from 7.4 % to 100.3 %, potentially indicating that limited Ca isotopic fractionation occurs during the dissolution of primary carbonates. The residual Ca pool is strongly fractionated with δ44/40Ca ranging from 0.64 ± 0.08 ‰ to −0.98 ± 0.02 ‰, systematically lower than their bulk saprolites, perhaps indicating light Ca isotopes are preferentially incorporated into the clay lattices. Altogether, it seems that the Ca isotopic fractionation directions are opposite between clay structural incorporation and adsorption. Our study provides important insight of Ca behavior and Ca isotopic fractionation during chemical weathering, which is critical to shape Ca isotopic compositions of the upper continental crust and trace the global biogeochemical cycle of Ca.

The Genesis of A‐Type Granites in Lower Yangtze River Belt: Evidence From Calcium Isotopes

AbstractA‐type granite is a favorable rock for the study of crustal evolution, crust‐mantle interaction and metallogenesis. However, the origin of A‐type granite remains controversial. In this study, we report high‐precision Ca isotopic compositions of a co‐genetic suite of A‐type granites from the Lower Yangtze River Belt (LYRB) in eastern China. Our results show that the δ44/40Ca of the A‐type granites ranges from 0.55 ± 0.11‰ to 1.44 ± 0.06‰, and is negatively correlated with CaO, Sr and Ba contents and Eu/Eu*, indicating that the Ca isotope fractionation of A‐type granites is dominated by plagioclase and potassium feldspar. There is kinetic fractionation of Ca isotopes during A‐type granite magma evolution. Sample BSL‐7, the least evolved sample, revealed that it had a low δ44/40Ca of 0.59‰, which is significantly lower than that of the upper continental crust (δ44/40Ca = 0.71–0.72‰) and the bulk silicate earth (δ44/40Ca = 0.94‰). The low δ44/40Ca can be best explained by partial melting of the enriched mantle metasomatized by subduction material. Combined with the geodynamic background of the LYRB, we propose that the formation of A‐type granites in the LYRB originates from partial melting of the enriched mantle, triggered by early Cretaceous ridge subduction of the Pacific and Izanagi plates.

Calcium isotopic composition of the bulk silicate Earth: A komatiite perspective

Calcium (Ca) isotopic composition of the mantle has been previously estimated as δ44/40Ca. However, δ44/40Ca of the mantle records both its stable isotopic composition and excess/deficit in radiogenic 40Ca that can be produced by the decay of 40K. Here, we present the high-precision Ca isotopic compositions of 16 Archean komatiites from five representative localities (Barberton, Munro, Abitibi, Yilgarn, and Taishan). Due to the high degrees of partial melting that led to their formation, these komatiites are used here to simultaneously estimate ε40/44Ca, δ44/40Ca, and δ44/42Ca of the bulk silicate Earth. The komatiites exhibit a resolvable negative ε40/44Ca relative to SRM915a, with an average of −0.6 ± 0.1 (2SE, 95 c.i.% if not specified). Given the lack of significant differences among the samples and the non-zero value of ε40/44Ca, we suggest using δ44/42Ca to report the stable Ca isotopic composition in this study. Excluding one sample (SD6/400), which may have been affected by alteration, the remaining komatiites display relatively homogeneous δ44/42Ca, ranging from 0.39 ± 0.02‰ (2SE, N = 16) to 0.49 ± 0.02‰ (2SE, N = 10), defining a mean of 0.44 ± 0.02‰ (2SE, N = 15). The absence of correlations between δ44/42Ca and LOI, Sr/Nd, (Dy/Yb)N, and (La/Sm)N among the komatiites indicates that post-eruption alteration, the influence of residual garnet, as well as prior melt extraction from the mantle source have had a negligible influence on their stable Ca isotopic compositions. Based on available experimental petrological data and isotope fractionation factors, it is demonstrated that komatiites were insignificantly fractionated from their mantle sources (i.e., <0.02% in δ44/42Ca). Their δ44/42Ca represents the ambient mantle at the time of segregation. Accordingly, we suggest that the average δ44/42Ca and ε40/44Ca of the komatiites measured here are representative of the bulk silicate Earth. Our new estimate can serve as a reference point for Ca isotopes as a two-dimensional tracer for sources and geological processes.

Quantification of classical and non-classical crystallization pathways in calcite precipitation

Crystal precipitation from aqueous solution occurs through multiple pathways. Besides the classical ion-by-ion addition, non-classical crystallization mechanisms, such as multi-ion polymer and nano-particle attachment, could be significant. These non-classical crystallization processes have been observed with advanced microscopy, yet detailed quantification of their contributions remains challenging. Building from paired Ca and Sr isotope observations, we develop a theoretical framework to quantify the contributions of classical and non-classical crystallization pathways on the precipitation of the calcium carbonate mineral calcite, a common precipitate in nature. We demonstrate that the classical crystallization pathway alone is insufficient to account for the observed isotope behaviors and, thus, the entire calcite precipitation process. We further present a surface kinetic model that incorporates non-classical crystallization pathways. This model enables the characterization of the roles of classical and non-classical crystallization mechanisms in calcite precipitation. The results suggest that the relative contribution of non-classical crystallization pathways increases with saturation state and can, under high supersaturation levels, be comparable to or greater than the classical pathway. The presented theoretical framework readily explains observed trace element partitioning and isotope fractionation behaviors during calcite precipitation and can be further expanded onto other mineral systems to gain insights into crystal growth mechanisms.