Carbon isotope fractionation in synthetic magnesian calcite

Mg-calcite was precipitated at 25 °C in closed system, free-drift experiments, from solutions containing NaHCO₃, CaCl₂ and MgCl₂. The carbon stable isotope composition of bulk solid and solution were analyzed from subsamples collected during time course experiments of 24 h duration. Considering only the Mg-content and δ¹³C values for the bulk solid, the carbon isotope fractionation factor for the Mg-calcite-HCO_3(aq)⁻ system (as ) increased with average mol percentage of Mg (X_Mg) in the solid at a rate of (0.024 ± 0.011) per mol% MgCO₃. Extrapolation of this relationship to the pure calcite end member yields a value of 0.82 ± 0.09, which is similar to published values for the calcite-HCO_3(aq)⁻ system. Although did not vary for precipitation rates that ranged from 10^3.21 to 10^4.60 μmol m⁻² h⁻¹, it was not possible to hold Mg-content of the solid constant, so kinetic effect on 10³ ln α could not be evaluated from these experiments.     

Calcium isotope fractionation in calcite and aragonite

Calcium isotope fractionation was measured on skeletal aragonite and calcite from different marine biota and on inorganic calcite. Precipitation temperatures ranged from 0 to 28°C. Calcium isotope fractionation shows a temperature dependence in accordance with previous observations: 1000 · ln(α_cc) = −1.4 + 0.021 · T (°C) for calcite and 1000 · ln(α_ar) = −1.9 + 0.017 · T (°C) for aragonite. Within uncertainty the temperature slopes are identical for the two polymorphs. However, at all temperatures calcium isotopes are more fractionated in aragonite than in calcite. The offset in δ^44/40Ca is about 0.6‰. The underlying mechanism for this offset may be related to the different coordination numbers and bond strengths of the calcium ions in calcite and aragonite crystals, or to different Ca reaction behavior at the solid-liquid interface. Recently, the observed temperature dependence of the Ca isotope fractionation was explained quantitatively by the temperature control on precipitation rates of calcium carbonates in an experimental setting (Lemarchand et al., 2004). We show that this mechanism can in principle also be applied to CaCO₃ precipitation in natural environments in normal marine settings. Following this model, Ca isotope fractionation in marine Ca carbonates is primarily controlled by precipitation rates. On the other hand the larger Ca isotope fractionation of aragonite compared to calcite can not be explained by different precipitation rates. The rate control model of Ca isotope fractionation predicts a strong dependence of the Ca isotopic composition of carbonates on ambient CO₃²⁻ concentration. While this model is in general accordance with our observations in marine carbonates, cultured specimens of the planktic foraminifer Orbulina universa show no dependence of Ca-isotope fractionation on the ambient CO₃²⁻ concentration. The latter observation implies that the carbonate chemistry in the calcifying vesicles of the foraminifer is independent from the ambient carbonate ion concentration of the surrounding water.     

Oxygen isotope fractionation in synthetic magnesian calcite

Mg-bearing calcite was precipitated at 25°C in closed system free-drift experiments from solutions containing NaHCO₃, CaCl₂ and MgCl₂. The chemical and isotope composition of the solution and precipitate were investigated during time course experiments of 24-h duration. Monohydrocalcite and calcite precipitated early in the experiments (<8 h), while Mg-calcite was the predominant precipitate (>95%) thereafter. Solid collected at the end of the experiments displayed compositional zoning from pure calcite in crystal cores to up to 23 mol% MgCO₃ in the rims. Smaller excursions in Mg were superimposed on this chemical record, which is characteristic of oscillatory zoning observed in synthetic and natural solid-solution carbonates of differing solubility. Magnesium also altered the predominant morphology of crystals over time from the {104} to {100} and {110} growth forms.The oxygen isotope fractionation factor for the magnesian-calcite-water system (as 10³lnα_Mg-cl-H2O) displayed a strong dependence on the mol% MgCO₃ in the solid phase, but quantification of the relationship was difficult due to the heterogeneous nature of the precipitate. Considering only the Mg-content and δ¹⁸O values for the bulk solid, 10³lnα_Mg-cl-H2O increased at a rate of 0.17 ± 0.02 per mol% MgCO₃; this value is a factor of three higher than the single previous estimate (Tarutani T., Clayton R.N., and Mayeda T. K. (1969) The effect of polymorphims and magnesium substitution on oxygen isotope fractionation between calcium carbonate and water. Geochim. Cosmochim. Acta 33, 987-996). Nevertheless, extrapolation of our relationship to the pure calcite end member yielded a value of 27.9 ± 0.02, which is similar in magnitude to published values for the calcite-water system. Although no kinetic effect was observed on 10³lnα_Mg-cl-H2O for precipitation rates that ranged from 10^3.21 to 10^4.60 μmol · m⁻² · h⁻¹, it was impossible to disentangle the potential effect(s) of precipitation rate and Mg-content on 10³lnα_Mg-cl-H2O due to the heterogeneous nature of the solid.The results of this study suggest that paleotemperatures inferred from the δ¹⁸O values of high magnesian calcite (>10 mol% MgCO₃) may be significantly underestimated. Also, the results underscore the need for additional experiments to accurately characterize the effect of Mg coprecipitation on the isotope systematics of calcite from a chemically homogeneous precipitate or a heterogeneous material that is analyzed at the scale of chemical and isotopic zonation.     

Magnesium stable isotope fractionation in marine biogenic calcite and aragonite

This survey of magnesium stable isotope compositions in marine biogenic aragonite and calcite includes samples from corals, sclerosponges, benthic porcelaneous and planktonic perforate foraminifera, coccolith oozes, red algae, and an echinoid and brachiopod test. The analyses were carried out using MC-ICP-MS with an external repeatability of ±0.22‰ (2SD for δ²⁶Mg; n = 37), obtained from a coral reference sample (JCp-1).Magnesium isotope fractionation in calcitic corals and sclerosponges agrees with published data for calcitic speleothems with an average Δ²⁶Mg_{calcite-seawater} = −2.6 ± 0.3‰ that appears to be weakly related to temperature. With one exception (Vaceletia spp.), aragonitic corals and sclerosponges also display uniform Mg isotope fractionations relative to seawater with Δ²⁶Mg_{biogenic aragonite-seawater} = −0.9 ± 0.2.Magnesium isotopes in high-Mg calcites from red algae, echinoids and perhaps some porcelaneous foraminifera as well as in all low-Mg calcites (perforate foraminifera, coccoliths and brachiopods) display significant biological influences. For planktonic foraminifera, the Mg isotope data is consistent with the fixation of Mg by organic material under equilibrium conditions, but appears to be inconsistent with Mg removal from vacuoles. Our preferred model, however, suggests that planktonic foraminifera synthesize biomolecules that increase the energetic barrier for Mg incorporation. In this model, the need to remove large quantities of Mg from vacuole solutions is avoided. For the high-Mg calcites from echinoids, the precipitation of amorphous calcium carbonate may be responsible for their weaker Mg isotope fractionation.Disregarding superimposed biological effects, it appears that cation light isotope enrichments in CaCO₃ principally result from a chemical kinetic isotope effect, related to the incorporation of cations at kink sites. In this model, the systematics of cation isotope fractionations in CaCO₃ relate to the activation energy required for cation incorporation, which probably reflects the dehydration of the cation and the crystal surface and bond formation at the incorporation site. This kinetic incorporation model predicts (i) no intrinsic dependence on growth rate, unless significant back reaction upon slow growth reduces the isotope fractionation towards that characteristic for equilibrium isotope partitioning (this may be observed for Ca isotopes in calcites), (ii) a small decrease of isotope fractionation with increasing temperature that may be amplified if higher temperatures promote back reaction and (iii) a sensitivity to changes in the activation barrier caused by additives such as anions or biomolecules or by the initial formation of amorphous CaCO₃.     

Oxygen isotope fractionation between calcite and tremolite: an experimental study

Oxygen isotope partitioning between calcite and tremolite was experimentally calibrated in the presence of small amounts of a supercritical CO₂–H₂O fluid at temperatures from 520 to 680° C and pressures from 3 to 10 kbar. The experiments were carried out within the stability field of the calcite-tremolite assemblage based on phase equilibrium relationships in the system CaO–MgO–SiO₂–CO₂–H₂O, so that decomposition of calcite and tremolite was avoided under the experimental conditions. Appropriate proportions of carbon dioxide to water were used to meet this requirement. Large weight ratios of mineral to fluid were employed in order to make the isotopic exchange between calcite and tremolite in the presence of a fluid close to that without fluid. The data processing method for isotopic exchange in a three-phase system has been applied to extrapolate partial equilibrium data to equilibrium values. The determined fractionation factors between calcite (Cc) and tremolite (Tr) are expressed as:10³1n _Cc-Tr=3.80 × 10⁶/T ²-1.67By combining the present data with the experimental calibrations of Clayton et al. (1989) on the calcite-quartz system, we obtain the fractionation for the quartztremolite system: 10³1n _Qz-Tr=4.18 × 10⁶/T ²-1.67Our experimental calibrations are in good agreement with the theoretical calculations of Hoffbauer et al. (1994) and the empirical estimates of Bottinga and Javoy (1975) based on isotopic data from naturall assemblages. At 700 C good agreement also exists between our experimental data and theoretical values calculated by Zheng (1993b). With decreasing temperature, however, an increasing difference between these data appears.Retrograde isotopic reequilibration by oxygen diffusion may be common for amphibole relative to diopside in metamorphic rocks. However, isotopic equilibrium in amphibole can be preserved in cases of rapid cooling.     

Oxygen isotope fractionation during the dolomitization of calcium carbonate

The oxygen isotope fractionation accompanying the hydrothermal dolomitization of CaCO₃ between 252 and 295°C has been investigated. Dolomitization (which occurs via the crystallization of one or more intermediate magnesian calcite phases) is characterised by a progressive lowering in δ⁸O, which smoothly correlates with the change in the Mg/(Mg + Ca) and the $\text{Sr}\text{(Mg + Ca)}$ ratios and with the sequential phase formation. The data support the proposals of Katz and Matthews (1977) that (a) all reaction occurs by solution and reprecipitation, (b) intermediate phases and dolomite form sequentially and (c) the intermediate phases form within limited solution zones surrounding the dissolving precursor. Calculated volumes of the solution zone for the aragonite → low magnesian calcite transformation are within the range 3.7–6.7 × 10^?5 liters (out of 5 × 10^?3 liters, the volume of the bulk solution used in the present study), and agree well with those calculated from strontium and magnesium partitioning data. Dolomite precipitates in apparent isotopic equilibrium with the bulk solution. The temperature dependence of the fractionation is defined by the equation 1000 Inα_D-H2O = 3.06 × 10⁶T^?2 ? 3.24 Dolomite-water fractionations from this equation are significantly lower than those obtained by extrapolation of the Northrop And Clayton (1966) calibration. The reaction zone model can be applied to explain near zero dolomite-calcite oxygen isotope fractionations reported by Epsteinet al. (1964).     

Carbon isotope fractionation between calcite,graphite and CO2: an experimental study

The partitioning of stable carbon isotopes between calcite, graphite and CO₂ was experimentally determined at temperatures from 500 to 1200 °C and 1 to 15 kbar pressure. Attainment of carbon isotope equilibrium in CO₂-calcite runs was proven by achieving the same fractionation from isotopically opposite directions. The resultant CO₂-calcite fractionation curve for carbon differs from Bottinga's calculation by 1.2 and confirms recent experiments of Chacko et al. and Mattey et al. In CO₂-graphite experiments equilibrium fractions were extrapolated by applying the partial-exchange technique of Northrop and Clayton and by optimizing the contribution of surface reaction in graphite. CO₂-graphite fractionations at temperatures up to 800 °C are in fair agreement with Bottinga's calculation, but yield a surprisingly high fractionation of 5 at upper mantle temperatures. The combination of CO₂-calcite (carbon) and CO₂-graphite fractionation results in a new experimentally determined calcite-grapite fractionation curve, expressed by the equation:

Liquid composition-dependence of calcium isotope fractionation during diffusion in molten silicates

Liquid phase diffusion experiments were carried out to determine whether diffusive isotopic fractionation of a major chemical element (Ca) varies with chemical composition in high-temperature molten silicates. The objective was to determine how differences in silicate liquid structure, such as the ratio of bridging to non-bridging oxygen atoms, as well as bulk transport properties such as viscosity, relate to isotope discrimination during diffusion. This information, in turn, may relate to the lifetimes and sizes of multi-atom structures in the liquid. Diffusion couples consisting of juxtaposed natural mafic and felsic liquids were held at T = 1450 °C and P = 1.0 GPa for durations of 12-24 h in a standard piston-cylinder assembly. Experiments were done using different mafic endmember compositions (two tholeiitic basalts and a ugandite) and a single rhyolite composition. Major-element diffusion profiles and Ca isotope profiles were measured on the recovered quenched glasses. The starting materials were isotopically indistinguishable, but ⁴⁴Ca/⁴⁰Ca variations of ca. 5‰ arose due to a mass dependence of the Ca diffusion coefficients. Results indicate that the mass dependence of Ca diffusion coefficients varies with the magnitude and direction of aluminum gradients and the viscosity of the liquid. Some Ca fractionations result mainly from Al gradients.A simplified multicomponent diffusion model was used to model the experimental results. The model allows for diffusion of Ca in response to gradients in the concentrations of both CaO as well as Al₂O₃, and the model results are consistent with the inferred existence of at least two distinct species of Ca. The magnitude of isotopic discrimination during diffusion also appears to be stronger on the rhyolite versus the basalt/ugandite side of diffusion couples. The results can largely be accounted for by an adaptation of the model of Dingwell (1990), whereby in high silica liquids, Ca diffuses largely by site hopping through a quasi-stationary aluminosilicate matrix, producing strong isotopic effects because the Ca diffusion is not strongly correlated with the movement of the framework atoms. In low-silica liquids, Ca diffusion is correlated with the movement of the other components and there is less mass discrimination. Combining our Ca results with Ca, Mg, and Li data from previous studies, we show that this model can explain most of the cation- and composition-dependence of diffusive isotopic fractionations observed thus far. A key parameter controlling isotopic discrimination is the ratio of the elemental (Ca, Mg, Li) diffusivity to the Eyring (or Si) diffusivity. However, all experiments done so far also exhibit isotopic features that are not yet fully explained; some of these may relate to small temperature gradients in the capsules, or to more complex coupling effects that are not captured in simplified diffusion models.     

Iron isotope fractionation in subterranean estuaries

Dissolved Fe concentrations in subterranean estuaries, like their river-seawater counterparts, are strongly controlled by non-conservative behavior during mixing of groundwater and seawater in coastal aquifers. Previous studies at a subterranean estuary of Waquoit Bay on Cape Cod, USA demonstrate extensive precipitation of groundwater-borne dissolved ferrous iron and subsequent accumulation of iron oxides onto subsurface sands. Waquoit Bay is thus an excellent natural laboratory to assess the mechanisms of Fe-isotope fractionation in redox-stratified environments and determine potential Fe-isotope signatures of groundwater sources to coastal seawater. Here, we report Fe isotope compositions of iron-coated sands and porewaters beneath the intertidal zone of Waquoit Bay. The distribution of pore water Fe shows two distinct sources of Fe: one residing in the upward rising plume of Fe-rich groundwater and the second in the salt-wedge zone of pore water. The groundwater source has high Fe(II) concentration consistent with anoxic conditions and yield δ⁵⁶Fe values between 0.3 and −1.3‰. In contrast, sediment porewaters occurring in the mixing zone of the subterranean estuary have very low δ⁵⁶Fe values down to −5‰. These low δ⁵⁶Fe values reflect Fe-redox cycling and result from the preferential retention of heavy Fe-isotopes onto newly formed Fe-oxyhydroxides. Analysis of Fe-oxides precipitated onto subsurface sands in two cores from the subterranean estuary revealed strong δ⁵⁶Fe and Fe concentration gradients over less than 2m, yielding an overall range of δ⁵⁶Fe values between −2 and 1.5‰. The relationship between Fe concentration and δ⁵⁶Fe of Fe-rich sands can be modeled by the progressive precipitation of Fe-oxides along fluid flow through the subterranean estuary. These results demonstrate that large-scale Fe isotope fractionation (up to 5‰) can occur in subterranean estuaries, which could lead to coastal seawater characterized by very low δ⁵⁶Fe values relative to river values.     

Electrochemically controlled iron isotope fractionation

Variations in the stable isotope abundances of transition metals have been observed in the geologic record and trying to understand and reconstruct the physical/environmental conditions that produced these signatures is an area of active research. It is clear that changes in oxidation state lead to large fractionations of the stable isotopes of many transition metals such as iron, suggesting that transition metal stable isotope signatures could be used as a paleo-redox proxy. However, the factors contributing to these observed stable isotope variations are poorly understood. Here we investigate how the kinetics of iron redox electrochemistry generates isotope fractionation. Through a combination of electrodeposition experiments and modeling of electrochemical processes including mass-transport, we show that electron transfer reactions are the cause of a large isotope separation, while mass transport-limited supply of reactant to the electrode attenuates the observed isotopic fractionation. Furthermore, the stable isotope composition of electroplated transition metals can be tuned in the laboratory by controlling parameters such as solution chemistry, reaction overpotential, and solution convection. These methods are potentially useful for generating isotopically-marked metal surfaces for tracking and forensic purposes. In addition, our studies will help interpret stable isotope data in terms of identifying underlying electron transfer processes in laboratory and natural samples.     

Oxygen isotope fractionation between synthetic aragonite and water: Influence of temperature and Mg concentration

Aragonite was precipitated in the laboratory at 0, 5, 10, 25, and 40 °C to determine the temperature dependence of the equilibrium oxygen isotope fractionation between aragonite and water. Forced CO₂ degassing, passive CO₂ degassing, and constant addition methods were employed to precipitate aragonite from supersaturated solutions, but the resulting aragonite-water oxygen isotope fractionation was independent of the precipitation method. In addition, under the experimental conditions of this study, the effect of precipitation rate on the oxygen isotope fractionation between aragonite and water was almost within the analytical error of ±∼0.13‰ and thus insignificant. Because the presence of Mg²⁺ ions is required to nucleate and precipitate aragonite from Na-Ca-Cl-HCO₃ solutions under these experimental conditions, the influence of the total Mg²⁺ concentration (up to ∼0.9 molal) on the aragonite-water oxygen isotope fractionation was examined at 25 °C. No significant Mg²⁺ ion effect, or oxygen isotope salt effect, was detected up to 100 mmolal total Mg²⁺ but a noticeable isotope salt effect was observed at ∼0.9 molal total Mg²⁺.On the basis of results of the laboratory synthesis experiments, a new expression for the aragonite-water fractionation is proposed over the temperature range of 0-40 °C:

1000lnα_{aragonite-water}=17.88±0.13(10³/T)-31.14±0.46     

Oxygen isotope fractionation factors involving cassiterite (SnO2): II. determination by direct isotope exchange between cassiterite and calcite

Direct oxygen isotope fractionation between cassiterite and calcite has been investigated experimentally at 15 kbar with temperature ranging from 800 to 1000°C. Combined with the quartz-calcite fractionation measured with the same technique (Clayton et al., 1989), the calcite-cassiterite and quartz-cassiterite oxygen isotope fractionations can be expressed as:

     

Calculation of sulfur isotope fractionation in sulfides

The increment method has been successfully applied to calculate thermodynamic isotope fractionation factors of oxygen in silicates, oxides, carbonates, and sulfates. In this paper, we modified the increment method to calculate thermodynamic isotope fractionation factors of sulfur in sulfides, based on chemical features of sulfur-metal bonds and crystal features of sulfide minerals. To approximate the bond strength of sulfides, a new constant, known as the Madelung constant, was introduced. The increment method was then extended to calculate the reduced partition function ratios of sphalerite, chalcopyrite, galena, pyrrhotite, greenockite, bornite, cubanite, sulvanite, and violarite, as well as the isotope fractionation factors between them over the temperature range from 0 to 1000 °C. The order of ³⁴S enrichment in these nine minerals is pyrrhotite > greenockite > sphalerite > chalcopyrite > cubanite > sulvanite > bornite > violarite > galena. Our improved method constitutes another model for calculating the thermodynamic isotope fractionation factors of sulfur in sulfides of geochemical interest.     

Calcium isotope fractionation in modern scleractinian corals

The ⁴⁴Ca/⁴⁰Ca ratios of cultured (Acropora sp.) and open ocean (Pavona clavus, Porites sp.) tropical reef corals are positively correlated with growth temperature. The slope of the temperature-fractionation relation is similar to inorganic aragonite precipitates. However, δ^44/40Ca of the coral aragonite is offset from inorganic and sclerosponge aragonite by about +0.5‰. This offset can neither be explained by the very fast, biologically controlled calcification of scleractinian corals, nor as a consequence of calcification from a partly closed volume of fluid. As corals actively transport calcium through several cell layers to the site of calcification, the most likely explanation for the offset is a biologically induced fractionation. Our results indicate a limited use of Ca isotopes in scleractinian corals as temperature proxy.     

Copper isotope fractionation in acid mine drainage

We measured the Cu isotopic composition of primary minerals and stream water affected by acid mine drainage in a mineralized watershed (Colorado, USA). The δ⁶⁵Cu values (based on ⁶⁵Cu/⁶³Cu) of enargite (δ⁶⁵Cu = −0.01 ± 0.10‰; 2σ) and chalcopyrite (δ⁶⁵Cu = 0.16 ± 0.10‰) are within the range of reported values for terrestrial primary Cu sulfides (−1‰ < δ⁶⁵Cu < 1‰). These mineral samples show lower δ⁶⁵Cu values than stream waters (1.38‰ ? δ⁶⁵Cu ? 1.69‰). The average isotopic fractionation (Δ_aq-min = δ⁶⁵Cu_aq − δ⁶⁵Cu_min, where the latter is measured on mineral samples from the field system), equals 1.43 ± 0.14‰ and 1.60 ± 0.14‰ for chalcopyrite and enargite, respectively. To interpret this field survey, we leached chalcopyrite and enargite in batch experiments and found that, as in the field, the leachate is enriched in ⁶⁵Cu relative to chalcopyrite (1.37 ± 0.14‰) and enargite (0.98 ± 0.14‰) when microorganisms are absent. Leaching of minerals in the presence of Acidithiobacillus ferrooxidans results in smaller average fractionation in the opposite direction for chalcopyrite (Δ_aq-min^o=-0.57±0.14‰, where min^o refers to the starting mineral) and no apparent fractionation for enargite (Δ_aq-min^o=0.14±0.14‰). Abiotic fractionation is attributed to preferential oxidation of ⁶⁵Cu⁺ at the interface of the isotopically homogeneous mineral and the surface oxidized layer, followed by solubilization. When microorganisms are present, the abiotic fractionation is most likely not seen due to preferential association of ⁶⁵Cu_aq with A. ferrooxidans cells and related precipitates. In the biotic experiments, Cu was observed under TEM to occur in precipitates around bacteria and in intracellular polyphosphate granules. Thus, the values of δ⁶⁵Cu in the field and laboratory systems are presumably determined by the balance of Cu released abiotically and Cu that interacts with cells and related precipitates. Such isotopic signatures resulting from Cu sulfide dissolution should be useful for acid mine drainage remediation and ore prospecting purposes.     

石英一水一盐体系氧同位素分馏作用

温度为180—550℃,盐度(wt.％)分别为0、5、25和40条件下,在高压釜内完成了由硅胶合成石英的氧同位素分馏作用实验研究,目的是了解:①盐同位素效应;②△t值对同位素分馏的影响;③温度与同位素分馏系数的关系。研究资料表明:低温条件下矿物和纯水之间同位素平衡作用不可能发生;影响含氧矿物(初)之间氧同位素平衡速率的因素包括盐度、△t值大小和温度等;我们的研究还表明,盐度对同位素分馏作用同系数无影响,即不存在所谓的“同位素盐效应”。在180—550℃温度范围内,不同盐度条件下获得的石英-水氧同位素分馏实验方程为:10001nα_(石英-水)=3.306×10~5T~(-2)—2.71。     

Carbon isotope fractionation in wood during carbonization

A significant uncertainty exists as to whether δ¹³C values in charcoal meaningfully represent the stable isotopic content of the original material, with studies suggesting variable responses to both natural and laboratory heating. An extensive study was undertaken using fully homogenised samples of wood taken from Eucalyptus spp., Quercus robur and Pinus radiata. The results demonstrate that the duration of heating had no tangible effect on the final composition of the charred material, with the δ¹³C and carbon content of wood fixed after 30 min of heating. Furthermore, all three wood types become progressively depleted in ¹³C with increasing temperature. The results demonstrate that even at temperatures commonly reached in natural fires (<450 °C) isotopic fractionation of up to 1.3‰ can take place indicating that the absolute values obtained from charcoal extracted for paleoenvironmental reconstruction must be interpreted with caution.     

Pressure effect on magnesian calcite coating calcite and synthetic magnesian calcite seeds added to seawater in a closed system

When pure crystalline calcite seeds are added to supersaturated seawater, precipitation results in a coating which with time equilibrates at atmospheric pressure with seawater and corresponds to a calcite containing probably only 2 or 3% of MgCO₃ (mole fraction).If synthetic crystalline magnesian calcite is added, the surface layer equilibrates not only with respect to seawater but also in relation with the crystalline sites initiating precipitation. Adding Mg_0.03Ca_0.97CO₃ results in a coating with a solubility close to that of calcite. This confirms that the surface coating on pure calcite seeds contains about 2 or 3% MgCO₃ (K'_sp = 10?6.30).The surface layer precipitated on a synthetic Mg_0.08Ca_0.92CO₃ equilibrates finally with a carbonate more soluble than calcite (K'_sp = 10?5.94) corresponding to the seeds composition.Experiments at 1000 kg cm t-2 imply that when magnesian calcites are precipitated at the surface of calcite or magnesian calcite seeds, the precipitate must be hydrated, otherwise pressure accelerated recrystallization or rearrangement with loss of Mg would thermodynamically be impossible.By changing the pressure of a seawater sample originally saturated with a solid carbonate phase, changes in pH result from the effect of pressure on the dissociation constants of carbonic acid and boric acid causing either undersaturation or supersaturation with respect to the solid. By changing pressure we can show whether precipitation, dissolution and recrystallization are reversible processes if pH is taken as criteria of reversibility.     

Silicon isotope fractionation during magmatic differentiation

The Si isotopic composition of Earth’s mantle is thought to be homogeneous (δ³⁰Si = −0.29 ± 0.08‰, 2 s.d.) and not greatly affected by partial melting and recycling. Previous analyses of evolved igneous material indicate that such rocks are isotopically heavy relative to the mantle. To understand this variation, it is necessary to investigate the degree of Si isotopic fractionation that takes place during magmatic differentiation. Here we report Si isotopic compositions of lavas from Hekla volcano, Iceland, which has formed in a region devoid of old, geochemically diverse crust. We show that Si isotopic composition varies linearly as a function of silica content, with more differentiated rocks possessing heavier isotopic compositions. Data for samples from the Afar Rift Zone, as well as various igneous USGS standards are collinear with the Hekla trend, providing evidence of a fundamental relationship between magmatic differentiation and Si isotopes. The effect of fractionation has been tested by studying cumulates from the Skaergaard Complex, which show that olivine and pyroxene are isotopically light, and plagioclase heavy, relative to the Si isotopic composition of the Earth’s mantle. Therefore, Si isotopes can be utilised to model the competing effects of mafic and felsic mineral fractionation in evolving silicate liquids and cumulates.At an average SiO₂ content of ∼60 wt.%, the predicted δ³⁰Si value of the continental crust that should result from magmatic fractionation alone is −0.23 ± 0.05‰ (2 s.e.), barely heavier than the mantle. This is, at most, a maximum estimate, as this does not take into account weathered material whose formation drives the products toward lighter δ³⁰Si values. Mass balance calculations suggest that removal of continental crust of this composition from the upper mantle will not affect the Si isotopic composition of the mantle.     

Copper isotope fractionation by desert shrubs

Copper has two naturally occurring stable isotopes of masses 63 and 65 which can undergo mass dependent fractionation during various biotic and abiotic chemical reactions. These interactions and their resulting Cu isotope fractionations can be used to determine the mechanisms involved in the cycling of Cu in natural systems. In this study, Cu isotope changes were investigated at the organismal level in the metal-accumulating desert plant, Prosopis pubescens. Initial results suggest that the lighter Cu isotope was preferentially incorporated into the leaves of the plant, which may suggest that Cu was actively transported via intracellular proteins. The roots and stems show a smaller degree of Cu isotope fractionation and the direction and magnitude of the fractionations was dependent upon the levels of Cu exposure. Based on this and previous work with bacteria and yeast, a trend is emerging that suggests the lighter Cu isotope is preferentially incorporated into biological components, while the heavier Cu isotope tends to become enriched in aqueous solutions. In bacteria, plants and animals, intracellular Cu concentrations are strictly regulated via dozens of enzymes that can bind, transport, and store Cu. Many of these enzymes reduce Cu(II) to Cu(I). These initial results seem to fit into a broader picture of Cu isotope cycling in natural systems where oxidation/reduction reactions are fundamental in controlling the distributions of Cu isotopes.