Calcium Isotope Fractionation Research Articles

Calcium isotope fractionation during tropical weathering of granites

Calcium (Ca) is a highly soluble and mobile element during silicate weathering, and its isotopes could be a promising tracer for continental weathering processes. However, it remains controversial whether Ca isotopes fractionate during granitoids’ weathering. Here we investigated Ca isotope fractionation (measured by δ44/42Ca) behaviours on the weathering profile of the late middle Jurassic Fogang granite (165 ± 2 Ma) under a tropical climate with the help of radiogenic anomaly data (i.e., ε40/44Ca) (both relative to the NIST standard SRM 915a), as well as its implications for the Ca isotopic composition in the hydrosphere. A total of 15 saprolite samples, 2 fresh parent rocks, 3 fresh individual minerals of one granite weathering profile from Fogang county, South China, and 8 stream water samples nearby the profile have been analysed for their Ca isotopic compositions by thermal ionisation mass spectrometry using a double spike method. Results reveal extreme radiogenic and stable Ca isotopic variations among saprolites. The saprolites have ε40/44Ca ranging from 0.8 to 55.6, whereas fresh granite rocks exhibit uniform ε40/44Ca close to 0. The large variation of ε40/44Ca in the saprolites can be attributed to the preferential dissolution of plagioclase and exsolved albite in K-feldspar or the variation of K/Ca molar ratios in K-feldspar. In general, saprolites are enriched in lighter Ca isotopes compared to the parent rocks (0.39 ± 0.04 ‰) and have δ44/42Ca values ranging from −0.45 ‰ to 0.43 ‰. This suggests that newly formed clay minerals preferentially absorb the lighter Ca isotopes (e.g., 40Ca). However, the δ44/42Ca of stream waters draining highly weathered granite catchments reflect δ44/42Ca of bedrock, rather than δ44/42Ca of the saprolites. One possible reason is that too little Ca was adsorbed by secondary clay minerals in the saprolite to shift the Ca isotope composition of dissolved Ca from the bedrock. Our study shows that a combination of radiogenic and stable Ca isotope data is a promising tracer for the sources, migration, and transformation processes of this key element during continental weathering processes. This research contributes to a broader understanding of Ca isotopes as a tracer for tropical weathering and their implications for global biogeochemical cycles.

Calcium isotope fractionation by intracellular amorphous calcium carbonate (ACC) forming cyanobacteria.

The formation of intracellular amorphous calcium carbonate (ACC) by various cyanobacteria is a widespread biomineralization process, yet its mechanism and importance in past and modern environments remain to be fully comprehended. This study explores whether calcium (Ca) isotope fractionation, linked to ACC-forming cyanobacteria, can serve as a reliable tracer for detecting these microorganisms in modern and ancient settings. Accordingly, we measured stable Ca isotope fractionation during Ca uptake by the intracellular ACC-forming cyanobacterium Cyanothece sp. PCC 7425. Our results show that Cyanothece sp. PCC 7425 cells are enriched in lighter Ca isotopes relative to the solution. This finding is consistent with the kinetic isotope effects observed in the Ca isotope fractionation during biogenic carbonate formation by marine calcifying organisms. The Ca isotope composition of Cyanothece sp. PCC 7425 was accurately modeled using a Rayleigh fractionation model, resulting in a Ca isotope fractionation factor (Δ44Ca) equal to -0.72 ± 0.05‰. Numerical modeling suggests that Ca uptake by these cyanobacteria is primarily unidirectional, with minimal back reaction observed over the duration of the experiment. Finally, we compared our Δ44Ca values with those of other biotic and abiotic carbonates, revealing similarities with organisms that form biogenic calcite. These similarities raise questions about the effectiveness of using the Ca isotope fractionation factor as a univocal tracer of ACC-forming cyanobacteria in the environment. We propose that the use of Δ44Ca in combination with other proposed tracers of ACC-forming cyanobacteria such as Ba and Sr isotope fractionation factors and/or elevated Ba/Ca and Sr/Ca ratios may provide a more reliable approach.

A dolomite-based record of seawater calcium isotope composition over the Neogene

The calcium isotope composition of seawater (δ44/40Casw) records important information on the evolution of the calcium and carbon cycles over geologic time. Over the past two decades, many attempts have been made to reconstruct the Neogene δ44/40Casw variation using various calcium isotope records of seawater or seawater-like pore fluids. Nonetheless, large discrepancies still exist among these Neogene δ44/40Casw records. Here we report δ44/40Ca values in a suite of Neogene island dolomites (20.8–3.4 Ma, n = 18) from the South China Sea. Combining multiple geochemical proxies (δ44/40Ca, δ13C, and δ18O, this study; Sr/Ca, ∑REY, δ7Li, δ26Mg, 87Sr/86Sr, and Δ47, previous studies) and evidence from petrography, fluid inclusions, and magnetostratigraphy, we suggest that the dolomitization for these dolomites was buffered by seawater-like fluids in the shallow marine burial domain or even marine diagenetic domain (<180 m) over a considerable period of time (<2 Myr). Then, we apply the equilibrium calcium isotope fractionation factor between dolomite and seawater of −0.52 ± 0.20 ‰ (2SD, n = 13) estimated from the literature to these fluid-buffered dolomites and reconstruct a new Neogene δ44/40Casw record. After using the Locally Weighted Regression (LOWESS) curve fitting method, our dolomite-based δ44/40Casw record is consistent with the reconciled seawater record from other calcium isotope archives, including shark teeth, corals, foraminifera, brachiopods, barites, phosphates, and bulk pelagic carbonates. Our results support the use of fabric-retentive early marine diagenetic dolomites to trace the evolution of δ44/40Casw through Earth’s history.

Calcium isotope fractionation during melt immiscibility and carbonatite petrogenesis

Calcium isotope fractionation during melt immiscibility and carbonatite petrogenesis

Calcium isotopic fractionation during aragonite and high-Mg calcite precipitation at methane seeps

The formation of authigenic carbonate in marine environments represents a process that buffers ocean chemistry and atmospheric carbon dioxide levels. The isotopic composition of calcium (δ44/40Ca) in authigenic carbonate can be used to investigate the calcium (Ca) cycle, seawater chemistry, and diagenesis of carbonate rocks, archiving key information on the evolution of the Earth's surface environments. Laboratory experiments have shown that Ca isotopic fractionation during carbonate precipitation – the basis of the δ44/40Ca proxy – is mainly determined by both mineral phase and precipitation rate. However, the precipitation rate of submarine authigenic carbonate is much lower than rates in laboratory experiments. This study investigated the Ca isotopic composition of pore water and carbonate nodules collected from two gravity piston cores and one push core from the Haima seeps in the northern South China Sea. Methane fluxes calculated for the three cores range from 12 to 134 µmol cm−2 yr−1. The core with intermediate methane flux showed evidence of aragonite precipitation, while high-Mg calcite was the dominant mineral phase in the other two cores as indicated by Mg/Ca and Sr/Ca ratios of pore water. Numerical modeling of pore water geochemistry revealed that: (1) precipitation rates of carbonate per unit reactive surface area range from 0.05 to 21 µmol m−2 h−1, which is one to three orders of magnitude lower than rates in precipitation experiments; (2) Ca isotopic fractionation for aragonite precipitation is –1.8 ± 0.3 ‰, close to the reported equilibrium isotopic fractionation in precipitation experiments; and (3) Ca isotopic fractionation for high-Mg calcite precipitation is similar for the two cores (−0.9 ± 0.3 ‰ and −0.8 ± 0.3 ‰), although the modeled precipitation rates differed by two orders of magnitude. These results indicate that mineralogy rather than precipitation rate is the primary control on Ca isotopic fractionation during carbonate precipitation at seeps, resulting in lower δ44/40Ca values of aragonite (0.91±0.06 ‰, N = 4) than of high-Mg calcite (1.41±0.04 ‰, N = 9) reported relative to NIST SRM915a. This study documents distinguishable Ca isotopic fractionation during the formation of methane-derived high-Mg calcite and aragonite, providing fundamental parameters for further exploration of the calcium isotopic composition of marine authigenic carbonates in the geological record.

Calcium isotope fractionation associated with adsorption and desorption on/from δ-MnO2

Small mineral particles present in soils, such as clay minerals and some oxyhydroxides, constitute important nutrient reservoirs. This behavior is due to the negative charges and high specific surface areas of these particles allowing them to adsorb cations such as calcium (Ca), a macronutrient that occupies key physiological and structural functions in plant metabolism. Although the chemical reactivity of clay minerals is rather well-known in the literature, especially toward macronutrients such as Ca, that of oxyhydroxides such as phyllomanganate minerals remains largely unexplored.To enhance our understanding of the mechanisms at the origin of the storage/release of the different isotopes of Ca in a soil solution, the possible fractionation between 40Ca and 44Ca during adsorption and desorption of Ca on a synthetic phyllomanganate, abiotically precipitated in the laboratory [synthetic analog of vernadite (δ-MnO2)], was studied. Experiments were performed in batch (closed system), and several parameters (time, pH of the solution, Ca concentration and nature and concentration of the desorbent) were tested to cover a large range of physicochemical conditions.This study demonstrated that the light 40Ca isotope is preferentially adsorbed on δ-MnO2, with Δ44/40Ca (the apparent fractionation of the aqueous solution at the stationnary state of adsorption compared to the initial one) of the adsorbed Ca that can reach 1.19 ± 0.15‰. The results showed that this isotopic fractionation occurs at chemical equilibrium in a closed system and that the isotopic fractionation measured in this study during Ca adsorption on phyllomanganate is significantly higher than that reported in the literature during Ca adsorption on other soil constituents such as clay minerals. At pH below 4 Ca occupies only the interlayer/basal sites whereas above this pH Ca fills interlayer/basal sites (∼86%) and edges sites (∼14%). By combining the experimental data obtained at different pH values, initial Ca concentrations and interaction times, our results suggest that isotopic signature of the Ca adsorbed on δ-MnO2 is dependent on the nature of the site involved in the adsorption step (i.e., enriched in 40Ca in the interlayer with Δ44/40Ca equal to −0.43‰ and enriched in 44Ca when bound to the edges with Δ44/40Ca equal to +3.5‰). As revealed by surface complexation modeling, such contrasting behavior between the two types of adsorption sites could be due to the bidendate nature of the Ca adsorption occurring on edges and to ion exchange of Ca2+ with H+ in the interlayer sites. Finally, desorption experiments point to total but not instantaneous Ca desorption, probably due to a partial collapse of the interlayer with some ions used to desorb Ca2+ (K+, NH4+, hexaamine-cobalt). This suggests that the amount of bioavailable Ca in soils by simple ion-exchange reactions is highly dependent on the nature of the ions which could desorb Ca present in the soil solution.

A dual role of methane seepage intensity on calcium isotopic fractionation

A dual role of methane seepage intensity on calcium isotopic fractionation

Calcium isotope ratios of malformed foraminifera reveal biocalcification stress preceded Oceanic Anoxic Event 2

Ocean acidification causes biocalcification stress. The calcium isotope composition of carbonate producers can archive such stress because calcium isotope fractionation is sensitive to precipitation rate. Here, we synthesize morphometric observations of planktic foraminifera with multi-archive calcium isotope records from Gubbio, Italy and the Western Interior Seaway spanning Cretaceous Ocean Anoxic Event 2 (~94 million years ago). Calcium isotope ratios increase ~60 thousand years prior to the event. The increase coincides with foraminiferal abnormalities and correlates with existing proxy records for carbon dioxide release during large igneous province volcanism. The results highlight Ocean Anoxic Event 2 as a geologic ocean acidification analog. Moreover, decreasing calcium isotope ratios during the event provide evidence for ocean alkalinization, which could have shifted air-sea carbon dioxide partitioning. These data offer an explanation for the Plenus Cold Event and further have implications for refining ocean alkalinity enhancement, a leading anthropogenic carbon dioxide removal strategy.

Calcium isotope fractionation during the partial dissolution of artificial calcite

We studied the calcium (Ca) isotope fractionation during the partial dissolution of artificial calcite in pure water and in artificial seawater at 20 °C. Calcite (1.00 g) was stirred in 500 ml solution while bubbling it with air rich in CO2 gas (400 and 500 ppm). Depending on the composition of the dissolving medium, the measured pH close to equilibrium decreased with increasing concentration of CO2 in air. The dissolution rate (R*; μmol/m2.h) increased with the concentration of CO2 in air. The Ca isotope composition (δ44/40Ca) of the dissolved Ca in pure water during different intervals of partial dissolution process was almost constant and very close to that of calcite (1.06‰ ± 0.11‰). This phenomenon indicated that partial dissolution occurred without Ca isotope fractionation. By contrast, the isotope composition of Ca in artificial seawater (0.92‰ ± 0.17‰) was almost constant during the partial dissolution of calcite (<10% of calcite dissolved) because the Ca isotope composition in both calcite and in the artificial seawater were close to each other and the change in [Ca2+] ions due to dissolution was not >9%.

New insights into Mn2+ and Mg2+ inhibition of calcite growth

Impurity ion and isotope partitioning into carbonate minerals provide a window into the molecular processes occurring at the fluid-mineral interface during crystal growth. Here, we employ calcium isotope fractionation together with process-based modeling to elucidate the mechanisms by which two divalent cations with starkly contrasting compatibility, magnesium and manganese, inhibit calcite growth and incorporate into the mineral lattice. Calcite growth inhibition by Mg2+ is log-linear and KMg is on the order of 0.02–0.03 throughout the range of {Mg2+}/{Ca2+} studied here (0.01–2.6). Mn2+ exhibits much stronger log-linear growth rate inhibition at low Mn2+ concentrations (fluid {Mn2+}/{Ca2+} = 0.001–0.02). Mn2+ is readily incorporated into the calcite lattice to form a calcite-rhodochrosite solid solution, with large partition coefficients (KMn 4.6–15.6) inversely correlated to growth rate. For both Mn2+ and Mg2+, calcium isotope fractionation is found to be invariant with {Me2+}/{Ca2+} despite more than an order of magnitude decline in growth rate. This invariant Δ44/40Ca suggests that the presence of Mn2+ or Mg2+ does not significantly change the relative rates of Ca2+ attachment and detachment at kink sites during growth, indicative of a dominantly kink blocking inhibition mechanism. Because the partitioning behavior dictates that Mn2+ must attach to the surface significantly faster than Ca2+, attachment of Mn2+ is likely to be as a non-monomer species such as an ion pair or possibly a larger polynuclear cluster. We propose that calcite growth rate inhibition by Mn is determined by the kinetics of carbonate attachment at Mn-occupied kink sites, potentially due to slow re-orientation kinetics of carbonate ions that have formed an inner-sphere complex with Mn2+ at the surface but must reorient to incorporate into the lattice. We demonstrate that patterns in Mg2+ partitioning and inhibition behavior are broadly consistent with growth inhibition driven by slow Mg2+-aquo complex dehydration relative to Ca2+ but argue that this mechanism likely represents one endmember scenario, seen in Mg-calcite growth at low supersaturations and net precipitation rates. During growth at faster net precipitation rates, some portion of Mg2+ is likely incorporated as a partially hydrated or otherwise complexed species, but calcite growth remains significantly inhibited by the kinetics of CO32− attachment at Mg2+ kink sites. These findings suggest a hybrid classical/nonclassical growth mechanism whereby Ca2+ incorporates largely as a free ion at kink sites while Mn2+ and some portion of Mg2+ are incorporated via non-monomer attachment. This pattern may be generalizable; trace constituent cations with aquo-complex desolvation rates significantly slower than the mineral growth rate preferentially incorporate as a non-monomer species during otherwise classical crystal growth.

Pressure and concentration effects on intermineral calcium isotope fractionation involving garnet

To better understand the behaviors of calcium (Ca) isotopes during igneous and metamorphic processes that differentiate the Earth and other rocky planets, we investigated the effects of pressure and Ca/Fe concentration on the average CaO bond length and the reduced partition function ratio of 44Ca/40Ca (103ln44/40Caβ) in garnet using first-principles calculations. Our calculations show that (1) the average CaO bond length increases and 103ln44/40Caβ decreasing (approximately 0.41‰ at 1000 K from a Ca/(Fe + Ca + Mg) of 1/24 to 12/12) with increasing Ca concentrations. In contrast to orthopyroxene and forsterite, whose CaO bond lengths and 103ln44/40Caβ are sensitive to Ca concentrations only in narrow range, the garnet CaO bond length and 103ln44/40Caβ vary with Ca concentrations in wide ranges. (2) The average CaO bond length increases and 103ln44/40Caβ decreases with increasing Fe concentrations (by 0.12‰ at 1000 K, FeO content of 12.4 wt%). (3) The average CaO bond length decreases by ~0.014 Å, and 103ln44/40Caβ increases by 0.1‰–0.2‰ at 1000 K when the pressure increases from 0 to 3 GPa. (4) However, the pressure effect on 103ln44/40Caαgarnet–clinopyroxene is insignificant.Using our new results of the effects of pressure and Ca/Fe concentration on 103ln44/40Caαgarnet–clinopyroxene, we evaluated the observed difference in Ca isotope compositions between garnet and clinopyroxene in natural samples. Most have reached equilibrium, whereas some samples exceed our prediction, which might reflect the majorite effect or kinetic fractionation.

Assessing Sedimentary Boundary Layer Calcium Carbonate Precipitation and Dissolution Using the Calcium Isotopic Composition of Pore Fluids

We present pore fluid geochemistry, including major ion and trace metal concentrations and the isotopic composition of pore fluid calcium and sulfate, from the uppermost meter of sediments from the Gulf of Aqaba (Northeast Red Sea) and the Iberian Margin (North Atlantic Ocean). In both the locations, we observe strong correlations among calcium, magnesium, strontium, and sulfate concentrations as well as the sulfur isotopic composition of sulfate and alkalinity, suggestive of active changes in the redox state and pH that should lead to carbonate mineral precipitation and dissolution. The calcium isotope composition of pore fluid calcium (δ44Ca) is, however, relatively invariant in our measured profiles, suggesting that carbonate mineral precipitation is not occurring within the boundary layer at these sites. We explore several reasons why the pore fluid δ44Ca might not be changing in the studied profiles, despite changes in other major ions and their isotopic composition, including mixing between the surface and deep precipitation of carbonate minerals below the boundary layer, the possibility that active iron and manganese cycling inhibits carbonate mineral precipitation, and that mineral precipitation may be slow enough to preclude calcium isotope fractionation during carbonate mineral precipitation. Our results suggest that active carbonate dissolution and precipitation, particularly in the diffusive boundary layer, may elicit a more complex response in the pore fluid δ44Ca than previously thought.

Calcium isotopic fractionation during magma differentiation: Constraints from volcanic glasses from the eastern Manus Basin

To better constrain Ca isotopic fractionation during volcanic magma differentiation, we report Ca isotope data for a series of volcanic glasses from the eastern Manus Basin that represent a continuum of compositions comprising basaltic andesite, andesite, dacite, and rhyolite. Our results show that these volcanic glasses exhibit ~0.25‰ variation in Ca isotopic compositions, with δ44/40Ca ranging from 0.81 ± 0.04 to 1.05 ± 0.04‰. The similar trace element patterns and Sr-Nd isotopic compositions exclude source heterogeneity as the cause of the varied Ca isotopic compositions. Due to their low K/Ca ratios and young age, the Ca isotopic variations cannot be attributed to 40K decay. Instead, the observed Ca isotopic variations are primarily controlled by their lithology. Specifically, δ44/40Ca of basaltic andesite (0.81 ± 0.04) and andesite (0.82 ± 0.01) are similar to the averages of reported mid-ocean ridge basalts (MORBs; 0.84 ± 0.10; 2SD, N = 26) and back-arc basin basalts (BABBs; 0.80 ± 0.08; 2SD, N = 21) in literature, but do not correlate with SiO2 or MgO contents, indicating the Ca isotopic fractionation is insignificant during intermediate-mafic magma evolution. As magma differentiation continues, δ44/40Ca of dacite and rhyolite evolves from 0.85 ± 0.01 to 1.05 ± 0.04‰. Moreover, δ44/40Ca of felsic glasses are negatively correlated with Eu/Eu*, which suggests that the increasing δ44/40Ca of felsic glasses is most likely related to plagioclase (Pl) fractional crystallization. Therefore, we conclude that Ca isotopes can behave differently at different stages of volcanic magma differentiation. Modeling of fractional crystallization using rhyolite-MELTS shows that the relative Ca fractions in crystallized minerals (e.g., Pl vs. Cpx) and the Ca isotopic fractionation factor between the residual melt and crystallized Pl (αmelt-Pl, which is likely related to the An number of Pl, magma temperature and kinetic effects during crystallization), provide a plausible explanation for the behavior of Ca isotopes during volcanic magma differentiation.

Calcium isotope fractionation by osteoblasts and osteoclasts, across endothelial and epithelial cell barriers, and with binding to proteins.

Timely and accurate diagnosis of osteoporosis is essential for adequate therapy. Calcium isotope ratio (δ44/42Ca) determination has been suggested as a sensitive, noninvasive, and radiation-free biomarker for the diagnosis of osteoporosis, reflecting bone calcium balance. The quantitative diagnostic is based on the calculation of the δ44/42Ca difference between blood, urine, and bone. The underlying cellular processes, however, have not been studied systematically. We quantified calcium transport and δ44/42Ca fractionation during in vitro bone formation and resorption by osteoblasts and osteoclasts and across renal proximal tubular epithelial cells (HK-2), human vein umbilical endothelial cells (HUVECs), and enterocytes (Caco-2) in transwell systems and determined transepithelial electrical resistance characteristics. δ44/42Ca fractionation was furthermore quantified with calcium binding to albumin and collagen. Calcified matrix formed by osteoblasts was isotopically lighter than culture medium by -0.27 ± 0.03‰ within 5 days, while a consistent effect of activated osteoclasts on δ44/42Ca could not be demonstrated. A transient increase in δ44/42Ca in the apical compartment by 0.26‰ occured across HK-2 cells, while δ44/42Ca fractionation was small across the HUVEC barrier and absent with Caco-2 enterocytes, and with binding of calcium to albumin and collagen. In conclusion, δ44/42Ca fractionation follows similar universal principles as during inorganic mineral precipitation; osteoblast activity results in δ44/42Ca fractionation. δ44/42Ca fractionation also occurs across the proximal tubular cell barrier and needs to be considered for in vivo bone mineralization modeling. In contrast, the effect of calcium transport across endothelial and enterocyte barriers on blood δ44/42Ca should be low and is absent with physiochemical binding of calcium to proteins.

The influence of Ca:CO3 stoichiometry on Ca isotope fractionation: Implications for process-based models of calcite growth

The solution stoichiometry dependence of calcium isotope fractionation during calcite precipitation was investigated as a direct test of the conceptual model of calcium isotope discrimination driven by Ca exchange at surface sites during growth. Classical ion-by-ion models of calcite growth predict a strong solution stoichiometry influence on Δ44/40Cacalcite-fluid: In low Ca2+:CO32–solutions,Δ44/40Cais predicted to approach a kinetic limit (∼−2 to −4‰), while in high Ca2+:CO32–solutions, exchange at dominantly Ca-occupied kink sites drives Δ44/40Catowards the equilibrium fractionation (near 0‰). To test thisprediction,a series ofseeded and unseeded constant composition calcite growth experiments were performed in which all aspects of solution chemistry were held constant and the Ca2+:CO32– activity ratio was varied. Experiments were performed at pH 8.5, ionic strength 0.1 M (adjusted with KCl), and calcite saturation index (SI = log10({Ca2+}{CO32–}/Ksp)) of either 0.5 or 0.8.Calcium isotope fractionation is found to be weakly stoichiometry dependent. The expected trend of larger magnitude fractionations at lower Ca2+:CO32– is observed, but the magnitude of change in Δ44/40Ca over the solution stoichiometries studied (Ca2+:CO32– = 1–250) is only ∼ 0.4‰. Similar trends in Δ44/40Ca with Ca2+:CO32– are observed at SI = 0.5 and 0.8, with smaller magnitude fractionations at lower supersaturation. This yields an inverse correlation between Δ44/40Ca and growth rate, confirming the Δ44/40Ca-rate relationship for inorganic calcite growth observed by Tang et al. (2008). The ion-by-ion model framework captures measured Δ44/40Ca only when a surface complexation model is incorporated, highlighting the role of surface speciation in dictating Ca attachment/detachment dynamics. The model captures observed trends with Ca2+:CO32– using best-fit kinetic and equilibrium fractionations consistent with end-members observed in natural systems (αkinetic ∼ 0.9978, αeq ∼ 0.9998). This result implies a total possible range in Δ44/40Ca of 2‰ and suggests that for most carbonate precipitating environments, solution supersaturation will be a stronger determinant of Δ44/40Ca than stoichiometry. The demonstrated importance of surface speciation, however, implies a strong pH influence on Δ44/40Ca, independent of its influence on carbonate ion activity, that requires further investigation.The results of this study provide strong evidence supporting the model of kink-exchange driven Ca isotope fractionation and suggest that calcite grows by a dominantly classical mechanism over the solution conditions investigated. Model predictions regarding the relationship between Δ44/40Ca and growth inhibition in the presence of impurity ions lay the foundation for the use of Ca isotopes as molecular tracers of carbonate crystal growth pathways.

Small calcium isotope fractionation at slow precipitation rates in methane seep authigenic carbonates

Natural calcium carbonate minerals express a range of calcium isotope fractionations, with the precipitated mineral typically enriched in the lighter isotopes of calcium relative to source fluids. Experimental and theoretical evidence shows a strong dependence on precipitation rate, although this relationship has not been well quantified over the range of precipitation rates observed in natural settings. Endmember cases show that average marine carbonate precipitation expresses a large fractionation (δ44/40Ca values lower than seawater by approximately 1‰), while diagenetic carbonate phases assumed to have precipitated or recrystallized at very slow rates show negligible fractionation. The limited examples of quantified precipitation rates in natural settings with measurable, non-zero fractionation represents a barrier for applying mechanistic models of calcium isotope fractionation to geological applications. This study examines a methane seep system in the northern Barents Sea south of Svalbard where authigenic carbonate minerals are precipitating, driven by anaerobic oxidation of methane, and where the apparent calcium isotope fractionation factor and precipitation rate can be constrained by measuring properties of the pore fluids. Pore fluid profiles are analyzed in two shallow cores, and authigenic carbonate nodules are analyzed in one of these cores. The pore fluid profiles point to a transitional, non-steady state which approximates a closed system, where the elevation of pore fluid calcium isotope ratios through carbonate precipitation can be modeled as a Rayleigh distillation system. The apparent fractionation factors for 44Ca/40Ca ratios at these sites are α = 0.99985 and 0.9996, although the carbonate nodules suggest a different calcium isotope fractionation factor may have been expressed under past conditions. Precipitation rates at the two sites are estimated to be 1.4 and 3.5 μmol/m2/h, intermediate between those of typical laboratory experiments and the much slower rates of marine diagenesis. Trace element analyses of the nodules (Mg/Ca and Sr/Ca ratios) suggest that both precipitation rate and mineralogy affect nodule composition. These results provide new constraints for the relationship between precipitation rate and calcium isotope fractionation and can inform modeling efforts leading towards mechanistic understanding of calcium isotope fractionation and trace element distributions in carbonate minerals.

Constraints on Archean crust recycling and the origin of mantle redox variability from the δ44/40Ca – δ18O – fO2 signatures of cratonic eclogites

The nature of the deep calcium geochemical cycle through time is unresolved, in part due to the dearth of information about the calcium isotope composition of Archean recycled oceanic crust. Remnants of such ancient oceanic crust are preserved in the form of cratonic mantle eclogites, brought to surface as xenoliths in kimberlite magma eruptions. The δ44/40Ca of fresh mantle-derived eclogite xenoliths (i.e., garnet and omphacite mineral separates) from the Bellsbank kimberlite on the Kaapvaal craton in South Africa are presented here in combination with their trace element compositions, garnet Fe3+ contents and δ18O values.The studied Bellsbank eclogite xenoliths have geochemical compositions that indicate oceanic crustal protoliths, with bulk Al2O3 from 15 to 27 wt.%, Eu anomalies from 0.8 to 2.6 and, significantly, garnet δ18O values from +2.7 to +6.2‰. Garnet Fe3+/ΣFe contents yield logfO2(ΔFMQ) values between −4.0 and −1.2 for a depth range of 110–180 km, recording strong redox heterogeneity of the eclogite component within the Archean Kaapvaal mantle lithosphere. Reconstructed bulk eclogite MgO contents correlate negatively with fO2, suggesting that the redox compositions are related to magmatic differentiation during oceanic crust formation, excluding secondary metasomatic overprints. These data may thus emphasize that Archean basaltic oceanic crust had a similarly variable redox composition to modern MORB-type crust.Reconstructed bulk δ44/40Ca values for the Bellsbank eclogites range from +0.28 to +1.56‰. Although some of the xenoliths have δ44/40Ca values that overlap with the average mantle composition and modern MORB (+0.94 ± 0.1 and +0.83 ± 0.05‰), half of our dataset shows excursions to more extreme Ca isotopic compositions. Both higher and lower δ44/40Ca relative to mantle compositions are recorded by the eclogites, with a general negative correlation with δ18O suggestive of seawater-alteration of oceanic crust. The combined low δ44/40Ca (+0.28‰) and δ18O (+3.4‰) measured for one eclogite xenolith may record a subtle imprint by carbonate-rich mantle melts, which are known to contain isotopically light calcium contributed by recycled sediments. In contrast, the high δ44/40Ca of up to +1.56‰ for some eclogite xenoliths, coupled with strong LREE depletion, can be explained by calcium isotope fractionation during partial melting.The protracted history of recycled oceanic crust as probed by cratonic mantle eclogites is recorded by their highly variable δ44/40Ca–δ18O–fO2 signatures. Whereas some of this heterogeneity can be linked to processes that operated on the Archean ocean floor such as seawater-alteration of basaltic crust, other sources of compositional variability are introduced by loss and addition of melts during subduction recycling and mantle residence. The observed δ44/40Ca complexity of ancient recycled oceanic crust components at the scale of a single mantle-derived eclogite xenolith suite implies that mantle plume sourced intraplate magmas should reveal similarly strong calcium isotope variations contributed by apparently essential recycled crust components – as observed in the global oceanic island basalt record.

Compositional and pressure controls on calcium and magnesium isotope fractionation in magmatic systems

Stable isotope fractionation in magmatic systems depends on equilibrium isotope fractionation between different phases. However, current equilibrium stable isotope theory mostly assumes ideal crystal structures and simple chemical compositions, but it is unclear how the pressure and complex compositional variations in natural microscopic mineral structures affect inter-mineral stable isotope fractionation and thus control stable isotope fractionation in macroscopic magmatic systems. Here, we calculate the CaO and MgO bond lengths controlled by pressure and compositional variations of coexisting garnet (Grt) and clinopyroxene (Cpx) in the Roberts Victor eclogites from the Kaapvaal Craton and use these data as a proof of concept to interpret their inter-mineral Ca and Mg isotopic compositions (Δ44/40CaGrt-Cpx and Δ26MgGrt-Cpx). Our results show that the CaO difference between Grt and Cpx (ΔCaOGrt-Cpx) shows a significant increase with CaO content from 3.4 to 13.6 wt.% in Grt and with pressure from 2.9 to 6.9 GPa. ΔCaOGrt-Cpx has an excellent negative correlation with inter-mineral Ca isotope fractionation corrected for temperature effect (Δ44/40CaGrt-Cpx × T2/106), indicating that inter-mineral Ca isotope fractionation is controlled by pressure and compositional variations of the Grt through effects on the bond lengths. Inter-mineral Mg isotope fractionation corrected for temperature effect (Δ26MgGrt-Cpx × T2/106) in these eclogites shows a negative correlation with pressure but no obvious correlations with the mineral compositions, suggesting the dominant role of pressure effect in addition to temperature. The MgO bond length of Grt increases by about 0.02 Å with increasing CaO content in Grt of these eclogites, implying a mild compositional effect on inter-mineral Mg isotope fractionation. The results suggest that pressure and compositional variations in minerals control the equilibrium stable isotope fractionation between minerals. Utilizing the Ca isotope fractionation factor controlled by crystal chemistry of garnet, our modelling indicates that partial melting of eclogite in the mantle could not significantly fractionate Ca isotopes and, therefore, that low δ44/40Ca values in previously reported basalts cannot be attributed to the involvement of eclogite in their sources. This suggests that crystal chemistry exerts major controls on isotope fractionation in magmatic systems in addition to temperature.

Calcium isotopes in high-temperature terrestrial processes

This chapter explores the growing body of stable and radiogenic Ca isotope measurements in high temperature terrestrial materials and covers the emerging applications for Ca isotope variability in igneous and metamorphic rocks and minerals. Calcium isotope fractionation at high temperature has been found to lead to larger effects than initially theorized, sometimes even exceeding those found in low temperature near-surface environments, yet many of the underlying fractionation mechanisms still remain poorly understood. Igneous whole-rocks span a δ44Ca range of ~2‰ (−0.9 to +0.9‰, relative to bulk-silicate Earth) and their constituent minerals have a range of ~2.5‰ (−1.2 to +1.3‰). Metamorphic whole rocks span a larger δ44Ca range of ~6‰ (−2.5 to +3.3‰), while metamorphic mineral separates span ~8‰ (−2.2 to +5.8‰). Observed Ca isotope variations can stem from a variety of sources including: (i) isolation of isotopically-distinct mineral components, such as during melting or crystallization, (ii) mixing of isotopically-distinct reservoirs into magmatic sources, such as during mantle metasomatism or assimilation of distinct crustal components, and (iii) isotopic rate differences (kinetic effects) during melting, crystallization, transport, and metamorphism, which are often governed by Ca diffusion. The emerging picture is that Ca isotopes are sensitive to a large number of high-temperature processes and can be used to understand the evolution of crust and mantle reservoirs, along with mechanisms leading to the formation of igneous, metamorphic, and hydrothermal rocks and minerals, through geologic time.

Calcium isotope fractionation during microbially induced carbonate mineral precipitation

We report the calcium isotope fractionation during the microbially-induced precipitation of calcium carbonate minerals in pure cultures of the marine sulfate-reducing bacterium Desulfovibrio bizertensis. These data are used to explore how the calcium isotope fractionation factor during microbially-induced carbonate mineral precipitation differs from the better-constrained calcium isotope fractionation factors during biogenic or abiotic carbonate mineral precipitation. Bacterial growth was then modulated with antibiotics, and the evolution of δ44Ca in solution was monitored under different microbial growth rates. The faster the microbial growth rate, the larger the calcium isotope fractionation during carbonate mineral precipitation, with Δ44Ca(s-f) ranging from −1.07‰ to −0.48‰. The reported calcium isotope fractionation can help us understand the link between calcium isotope fractionation and microbial metabolism in carbonate minerals precipitated during sedimentary diagenesis.