Kinetic isotope effect during reduction of iron from a silicate melt

Iron isotopic compositions measured in chondrules from various chondrites vary between δ⁵⁷Fe/⁵⁴Fe = +0.9‰ and −2.0‰, a larger range than for igneous rocks. Whether these compositions were inherited from chondrule precursors, resulted from the chondrule-forming process itself or were produced by later parent body alteration is as yet unclear. Since iron metal is a common phase in some chondrules, it is important to explore a possible link between the metal formation process and the observed iron isotope mass fractionation. In this experimental study we have heated a fayalite-rich composition under reducing conditions for heating times ranging from 2 min to 6 h. We performed chemical and iron isotope analyses of the product phases, iron metal and silicate glass. We demonstrated a lack of evaporation of Fe from the silicate melt in similar isothermal experiments performed under non-reducing conditions. Therefore, the measured isotopic mass fractionation in the glass, ranging between −0.32‰ and +3.0‰, is attributed to the reduction process. It is explained by the faster transport of lighter iron isotopes to the surface where reduction occurs, and is analogous to kinetic isotope fractionation observed in diffusion couples [Richter, F.M., Davis, A.M., Depaolo, D.J., Watson, E.B., 2003. Isotope fractionation by chemical diffusion between molten basalt and rhyolite. Geochim. Cosmochim. Acta67, 3905-3923]. The metal phase contains 90-99.8% of the Fe in the system and lacks significant isotopic mass fractionation, with values remaining similar to that of the starting material throughout. The maximum iron isotope mass fractionation in the glass was achieved within 1 h and was followed by an isotopic exchange and re-equilibration with the metal phase (incomplete at ∼6 h). This study demonstrates that reduction of silicates at high temperatures can trigger iron isotopic fractionation comparable in its bulk range to that observed in chondrules. Furthermore, if metal in Type I chondrules was formed by reduction of Fe silicate, our observed isotopic fractionations constrain chondrule formation times to approximately 60 min, consistent with previous work.     

Stable Cu and Zn isotope ratios as tracers of sources and transport of Cu and Zn in contaminated soil

Copper and Zn metals are produced in large quantities for different applications. During Cu production, large amounts of Cu and Zn can be released to the environment. Therefore, the surroundings of Cu smelters are frequently metal-polluted. We determined Cu and Zn concentrations and Cu and Zn stable isotope ratios (δ⁶⁵Cu, δ⁶⁶Zn) in three soils at distances of 1.1, 3.8, and 5.3 km from a Slovak Cu smelter and in smelter wastes (slag, sludge, ash) to trace sources and transport of Cu and Zn in soils. Stable isotope ratios were measured by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) in total digests. Soils were heavily contaminated with concentrations up to 8087 μg g⁻¹ Cu and 2084 μg g⁻¹ Zn in the organic horizons. The δ⁶⁵Cu values varied little (−0.12‰ to 0.36‰) in soils and most wastes and therefore no source identification was possible. In soils, Cu became isotopically lighter with increasing depth down to 0.4 m, likely because of equilibrium reactions between dissolved and adsorbed Cu species during transport of smelter-derived Cu through the soil. The δ⁶⁶Zn_IRMM values were isotopically lighter in ash (−0.41‰) and organic horizons (−0.85‰ to −0.47‰) than in bedrock (−0.28‰) and slag (0.18‰) likely mainly because of kinetic fractionation during evaporation and thus allowed for separation of smelter-Zn from native Zn in soil. In particular in the organic horizons large variations in δ⁶⁶Zn values occur, probably caused by biogeochemical fractionation in the soil-plant system. In the mineral horizons, Zn isotopes showed only minor shifts to heavier δ⁶⁶Zn values with depth mainly because of the mixing of smelter-derived Zn and native Zn in the soils. In contrast to Cu, Zn isotope fractionation between dissolved and adsorbed species was probably only a minor driver in producing the observed variations in δ⁶⁶Zn values. Our results demonstrate that metal stable isotope ratios may serve as tracer of sources, vertical dislocation, and biogeochemical behavior in contaminated soil.     

Non-biological fractionation of stable Ca isotopes in soils of the Atacama Desert, Chile

We measured Ca stable isotope ratios (δ^44/40Ca) in an ancient (2 My), hyperarid soil where the primary source of mobile Ca is atmospheric deposition. Most of the Ca in the upper meter of this soil (3.5 kmol m⁻²) is present as sulfates (2.5 kmol m⁻²), and to a lesser extent carbonates (0.4 kmol m⁻²). In aqueous extracts of variably hydrated calcium sulfate minerals, δ^44/40Ca_E values (vs. bulk Earth) increase with depth (1.4 m) from a minimum of −1.91‰ to a maximum of +0.59‰. The trend in carbonate-δ^44/40Ca in the top six horizons resembles that of sulfate-δ^44/40Ca, but with values 0.1-0.6‰ higher. The range of observed Ca isotope values in this soil is about half that of δ^44/40Ca values observed on Earth. Linear correlation among δ^44/40Ca, δ³⁴S and δ¹⁸O values indicates either (a) a simultaneous change in atmospheric input values for all three elements over time, or (b) isotopic fractionation of all three elements during downward transport. We present evidence that the latter is the primary cause of the isotopic variation that we observe. Sulfate-δ³⁴S values are positively correlated with sulfate-δ¹⁸O values (R² = 0.78) and negatively correlated with sulfate δ^44/40Ca_E values (R² = 0.70). If constant fractionation and conservation of mass with downward transport are assumed, these relationships indicate a δ^44/40Ca fractionation factor of −0.4‰ in CaSO₄. The overall depth trend in Ca isotopes is reproduced by a model of isotopic fractionation during downward Ca transport that considers small and infrequent but regularly recurring rainfall events. Near surface low Ca isotope values are reproduced by a Rayleigh model derived from measured Ca concentrations and the Ca fractionation factor predicted by the relationship with S isotopes. This indicates that the primary mechanism of stable isotope fractionation in CaSO₄ is incremental and effectively irreversible removal of an isotopically enriched dissolved phase by downward transport during small rainfall events.     

Natural fractionation of U/U

The isotopic composition of U in nature is generally assumed to be invariant. Here, we report variations of the ²³⁸U/²³⁵U isotope ratio in natural samples (basalts, granites, seawater, corals, black shales, suboxic sediments, ferromanganese crusts/nodules and BIFs) of ∼1.3‰, exceeding by far the analytical precision of our method (≈0.06‰, 2SD). U isotopes were analyzed with MC-ICP-MS using a mixed ²³⁶U-²³³U isotopic tracer (double spike) to correct for isotope fractionation during sample purification and instrumental mass bias. The largest isotope variations found in our survey are between oxidized and reduced depositional environments, with seawater and suboxic sediments falling in between. Light U isotope compositions (relative to SRM-950a) were observed for manganese crusts from the Atlantic and Pacific oceans, which display δ²³⁸U of −0.54‰ to −0.62‰ and for three of four analyzed Banded Iron Formations, which have δ²³⁸U of −0.89‰, −0.72‰ and −0.70‰, respectively. High δ²³⁸U values are observed for black shales from the Black Sea (unit-I and unit-II) and three Kupferschiefer samples (Germany), which display δ²³⁸U of −0.06‰ to +0.43‰. Also, suboxic sediments have slightly elevated δ²³⁸U (−0.41‰ to −0.16‰) compared to seawater, which has δ²³⁸U of −0.41 ± 0.03‰. Granites define a range of δ²³⁸U between −0.20‰ and −0.46‰, but all analyzed basalts are identical within uncertainties and slightly lighter than seawater (δ²³⁸U = −0.29‰).Our findings imply that U isotope fractionation occurs in both oxic (manganese crusts) and suboxic to euxinic environments with opposite directions. In the first case, we hypothesize that this fractionation results from adsorption of U to ferromanganese oxides, as is the case for Mo and possibly Tl isotopes. In the second case, reduction of soluble U^VI to insoluble U^IV probably results in fractionation toward heavy U isotope compositions relative to seawater. These findings imply that variable ocean redox conditions through geological time should result in variations of the seawater U isotope compositions, which may be recorded in sediments or fossils. Thus, U isotopes might be a promising novel geochemical tracer for paleo-redox conditions and the redox evolution on Earth. The discovery that ²³⁸U/²³⁵U varies in nature also has implications for the precision and accuracy of U-Pb dating. The total observed range in U isotope compositions would produce variations in ²⁰⁷Pb/²⁰⁶Pb ages of young U-bearing minerals of up to 3 Ma, and up to 2 Ma for minerals that are 3 billion years old.     

Boron isotopic composition of atmospheric precipitations and liquid-vapour fractionations

Boron isotope compositions (δ¹¹B) and B concentrations of rains and snows were studied in order to characterize the sources and fractionation processes during the boron atmospheric cycle. The ¹¹B/¹⁰B ratios of instantaneous and cumulative rains and snows from coastal and continental sites show a large range of variations, from −1.5 ± 0.4 to +26.0 ± 0.5‰ and from −10.2 ± 0.5 to +34.4 ± 0.2‰, respectively. Boron concentrations in rains and snows vary between 0.1 and 3.0 ppb. All these precipitation samples are enriched in ¹⁰B compared to the ocean value (δ¹¹B = +39.5‰). An empirical rain-vapour isotopic fractionation of +31‰ is estimated from three largely independent methods. The deduced seawater-vapour fractionation is +25.5‰, with the difference between the rain and seawater fractionations principally reflecting changes in the speciation of boron in the liquid with ∼100% B(OH)₃ present in precipitations. A boron meteoric water line, δD = 2.6δ¹¹B − 133, is proposed which describes the relationship between δD and δ¹¹B in many, but not all, precipitations. Boron isotopic compositions of precipitations can be related to that of the seawater reservoir by the seawater-vapour fractionation and one or more of (1) the rain-vapour isotopic fractionation, (2) evolution of the δ¹¹B value of the atmospheric vapour reservoir via condensation-precipitation processes (Rayleigh distillation process), (3) any contribution of vapour from the evaporation of seawater aerosols, and (4) any contribution from particulate matter, principally sea salt, continental dust and, perhaps more regionally, anthropogenic sources (burning of biomass and fossil fuels). From the δ¹¹B values of continental precipitations, a sea salt contribution cannot be more than a percent or so of the total B in precipitation over these areas.     

Iron and magnesium isotopic compositions of peridotite xenoliths from Eastern China

We present high-precision measurements of Mg and Fe isotopic compositions of olivine, orthopyroxene (opx), and clinopyroxene (cpx) for 18 lherzolite xenoliths from east central China and provide the first combined Fe and Mg isotopic study of the upper mantle. δ⁵⁶Fe in olivines varies from 0.18‰ to −0.22‰ with an average of −0.01 ± 0.18‰ (2SD, n = 18), opx from 0.24‰ to −0.22‰ with an average of 0.04 ± 0.20‰, and cpx from 0.24‰ to −0.16‰ with an average of 0.10 ± 0.19‰. δ²⁶Mg of olivines varies from −0.25‰ to −0.42‰ with an average of −0.34 ± 0.10‰ (2SD, n = 18), opx from −0.19‰ to −0.34‰ with an average of −0.25 ± 0.10‰, and cpx from −0.09‰ to −0.43‰ with an average of −0.24 ± 0.18‰. Although current precision (∼±0.06‰ for δ⁵⁶Fe; ±0.10‰ for δ²⁶Mg, 2SD) limits the ability to analytically distinguish inter-mineral isotopic fractionations, systematic behavior of inter-mineral fractionation for both Fe and Mg is statistically observed: Δ⁵⁶Fe_ol-cpx = −0.10 ± 0.12‰ (2SD, n = 18); Δ⁵⁶Fe_ol-opx = −0.05 ± 0.11‰; Δ²⁶Mg_ol-opx = −0.09 ± 0.12‰; Δ²⁶Mg_ol-cpx = −0.10 ± 0.15‰. Fe and Mg isotopic composition of bulk rocks were calculated based on the modes of olivine, opx, and cpx. The average δ⁵⁶Fe of peridotites in this study is 0.01 ± 0.17‰ (2SD, n = 18), similar to the values of chondrites but slightly lower than mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB). The average δ²⁶Mg is −0.30 ± 0.09‰, indistinguishable from chondrites, MORB, and OIB. Our data support the conclusion that the bulk silicate Earth (BSE) has chondritic δ⁵⁶Fe and δ²⁶Mg.The origin of inter-mineral fractionations of Fe and Mg isotopic ratios remains debated. δ⁵⁶Fe between the main peridotite minerals shows positive linear correlations with slopes within error of unity, strongly suggesting intra-sample mineral-mineral Fe and Mg isotopic equilibrium. Because inter-mineral isotopic equilibrium should be reached earlier than major element equilibrium via chemical diffusion at mantle temperatures, Fe and Mg isotope ratios of coexisting minerals could be useful tools for justifying mineral thermometry and barometry on the basis of chemical equilibrium between minerals. Although most peridotites in this study exhibit a narrow range in δ⁵⁶Fe, the larger deviations from average δ⁵⁶Fe for three samples likely indicate changes due to metasomatic processes. Two samples show heavy δ⁵⁶Fe relative to the average and they also have high La/Yb and total Fe content, consistent with metasomatic reaction between peridotite and Fe-rich and isotopically heavy melt. The other sample has light δ⁵⁶Fe and slightly heavy δ²⁶Mg, which may reflect Fe-Mg inter-diffusion between peridotite and percolating melt.     

The origin and history of ordinary chondrites: A study by iron isotope measurements of metal grains from ordinary chondrites

Chondrules and chondrites provide unique insights into early solar system origin and history, and iron plays a critical role in defining the properties of these objects. In order to understand the processes that formed chondrules and chondrites, and introduced isotopic fractionation of iron isotopes, we measured stable iron isotope ratios ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe in metal grains separated from 18 ordinary chondrites, of classes H, L and LL, ranging from petrographic types 3-6 using multi-collector inductively coupled plasma mass spectrometry. The δ⁵⁶Fe values range from −0.06 ± 0.01 to +0.30 ± 0.04‰ and δ⁵⁷Fe values are −0.09 ± 0.02 to +0.55 ± 0.05‰ (relative to IRMM-014 iron isotope standard). Where comparisons are possible, these data are in good agreement with published data. We found no systematic difference between falls and finds, suggesting that terrestrial weathering effects are not important in controlling the isotopic fractionations in our samples. We did find a trend in the ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe isotopic ratios along the series H, L and LL, with LL being isotopically heavier than H chondrites by ∼0.3‰ suggesting that redox processes are fractionating the isotopes. The ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe ratios also increase with increasing petrologic type, which again could reflect redox changes during metamorphism and also a temperature dependant fractionation as meteorites cooled. Metal separated from chondrites is isotopically heavier by ∼0.31‰ in δ⁵⁶Fe than chondrules from the same class, while bulk and matrix samples plot between chondrules and metal. Thus, as with so many chondrite properties, the bulk values appear to reflect the proportion of chondrules (more precisely the proportion of certain types of chondrule) to metal, whereas chondrule properties are largely determined by the redox conditions during chondrule formation. The chondrite assemblages we now observe were, therefore, formed as a closed system.     

Experimental determination of magnesium isotope fractionation during higher plant growth

Two higher plant species (rye grass and clover) were cultivated under laboratory conditions on two substrates (solution, phlogopite) in order to constrain the corresponding Mg isotope fractionations during plant growth and Mg uptake. We show that bulk plants are systematically enriched in heavy isotopes relative to their nutrient source. The Δ²⁶Mg_plant-source range from 0.72‰ to 0.26‰ for rye grass and from 1.05‰ to 0.41‰ for clover. Plants grown on phlogopite display Mg isotope signatures (relative to the Mg source) ∼0.3‰ lower than hydroponic plants. For a given substrate, rye grass display lower δ²⁶Mg (by ∼0.3‰) relative to clover. Magnesium desorbed from rye grass roots display a δ²⁶Mg greater than the nutrient solution. Adsorption experiments on dead and living rye grass roots also indicate a significant enrichment in heavy isotopes of the Mg adsorbed on the root surface. Our results indicate that the key processes responsible for heavy isotope enrichment in plants are located at the root level. Both species also exhibit an enrichment in light isotopes from roots to shoots (Δ²⁶Mg_leaf-root = −0.65‰ and −0.34‰ for rye grass and clover grown on phlogopite respectively, and Δ²⁶Mg_leaf-root of −0.06‰ and −0.22‰ for the same species grown hydroponically). This heavy isotope depletion in leaves can be explained by biological processes that affect leaves and roots differently: (1) organo-Mg complex (including chlorophyll) formation, and (2) Mg transport within plant. For both species, a positive correlation between δ²⁶Mg and K/Mg was observed among the various organs. This correlation is consistent with the link between K and Mg internal cycles, as well as with formation of organo-magnesium compounds associated with enrichment in heavy isotopes. Considering our results together with the published range for δ²⁶Mg of natural plants and rivers, we estimate that a significant change in continental vegetation would induce a change of the mean river δ²⁶Mg that is comparable to analytical uncertainties.     

Variations of Li and Mg isotope ratios in bulk chondrites and mantle xenoliths

We present whole rock Li and Mg isotope analyses of 33 ultramafic xenoliths from the terrestrial mantle, which we compare with analyses of 30 (mostly chondritic) meteorites. The accuracy of our new Mg isotope ratio measurement protocol is substantiated by a combination of standard addition experiments, the absence of mass independent effects in terrestrial samples and our obtaining identical values for rock standards using two different separation chemistries and three different mass-spectrometric introduction systems. Carbonaceous, ordinary and enstatite chondrites have irresolvable mean stable Mg isotopic compositions (δ²⁵Mg = −0.14 ± 0.06; δ²⁶Mg = −0.27 ± 0.12‰, 2SD), but our enstatite chondrite samples have lighter δ⁷Li (by up to ∼3‰) than our mean carbonaceous and ordinary chondrites (3.0 ± 1.5‰, 2SD), possibly as a result of spallation in the early solar system. Measurements of equilibrated, fertile peridotites give mean values of δ⁷Li = 3.5 ± 0.5‰, δ²⁵Mg = −0.10 ± 0.03‰ and δ²⁶Mg = −0.21 ± 0.07‰. We believe these values provide a useful estimate of the primitive mantle and they are within error of our average of bulk carbonaceous and ordinary chondrites. A fuller range of fresh, terrestrial, ultramafic samples, covering a variety of geological histories, show a broad positive correlation between bulk δ⁷Li and δ²⁶Mg, which vary from −3.7‰ to +14.5‰, and −0.36‰ to + 0.06‰, respectively. Values of δ⁷Li and δ²⁶Mg lower than our estimate of primitive mantle are strongly linked to kinetic isotope fractionation, occurring during transport of the mantle xenoliths. We suggest Mg and Li diffusion into the xenoliths is coupled to H loss from nominally anhydrous minerals following degassing. Diffusion models suggest that the co-variation of Mg and Li isotopes requires comparable diffusivities of Li and Mg in olivine. The isotopically lightest samples require ∼5-10 years of diffusive ingress, which we interpret as a time since volatile loss in the host magma. Xenoliths erupted in pyroclastic flows appear to have retained their mantle isotope ratios, likely as a result of little prior degassing in these explosive events. High δ⁷Li, coupled with high [Li], in rapidly cooled arc peridotites may indicate that these samples represent fragments of mantle wedge that has been metasomatised by heavy, slab-derived fluids. If such material is typically stirred back into the convecting mantle, it may account for the heavy δ⁷Li seen in some oceanic basalts.     

Controls on dissolved inorganic carbon and δC in cave waters from DeSoto Caverns: Implications for speleothem δC assessments

Unraveling the factors controlling the carbon chemistry and transport of carbon within extant karst systems has important implications concerning the assessment of time-series δ¹³C records of speleothems. Here we report the results of a 3-year study of total dissolved inorganic carbon [DIC] and δ¹³C_DIC from cave waters at DeSoto Caverns (Southeastern USA) that offer valuable insight on carbon transport and the accompanied isotope fractionations from end-member sources to speleothems.[DIC] and δ¹³C_DIC values of cave waters range from 0.2 to 6.0 mM and 2.7 to −12.9 (‰ VPDB), respectively. [DIC] and δ¹³C_DIC of “seasonal drips” show seasonal, albeit noisy, variability and are inversely related (δ¹³C_DIC = −2.49[DIC] + 0.64, r² = 0.84). A shallow pool fed by multiple drips shows a bimodal δ¹³C_DIC distribution with an isotopically heavier mode during winter (−4‰ to −5‰ VPDB) relative to summer months (−9‰ to −10‰ VPDB). A multi-year trend of decreasing water availability during the study period is not reflected in a response of cave water carbon chemistry suggesting that rainfall amount may not be a significant controlling factor of the carbon chemistry. Coupled cave air winter ventilation/summer stagnation and varying CO₂ fluxes through the soil horizon and epikarst exert the strongest influence on seasonal [DIC] and δ¹³C_DIC variability. Measured values of high [DIC] and low δ¹³C_DIC from cave waters collected during the summer/early fall closely approximate isotopic equilibrium conditions. Conversely, low [DIC] and high δ¹³C_DIC values during winter/early months indicate kinetically enhanced isotopic fractionations within the cave waters. The kinetically enhanced isotopic fractionation of partitioned between degassed CO₂ and precipitated CaCO₃(1000lnα_{[(CO2-HCO3)+(CaCO3(AR)-HCO3)]/2}) is greater by about a factor of two (−6.7 ± 0.3‰) relative to the same isotopic fractionation under equilibrium conditions (−3.1‰).On the basis of ¹⁴C mass balance and paired ¹⁴C-U/Th measurements we estimate that on average about ∼23% of C delivered annually by the drips to the aragonite stalagmites is derived from ¹⁴C-dead dolomite cap while the remainder of ∼77% is derived from ¹⁴C-live biomass. δ¹³C measurements of aragonite (n = 12) sampled from the tips of active speleothems during the summer months are consistent with theoretical aragonite δ¹³C values calculated using the shallow pool summer/early fall data thus confirming the δ¹³C seasonality in both drips and coeval aragonite. δ¹³C values of an active stalagmite section spanning the last 200 years show a normal distribution with a mean of −7.1 ± 1.2‰ (n = 81) and a mode of −7‰ to −8‰ that are statistically indistinguishable from the annual mean and mode of all dripwaters. Thus secular time-series δ¹³C records of stalagmites at DeSoto Caverns with resolving power >10⁻¹ year will likely carry the imprints of drip annual means that record climate-driven δ¹³C seasonal biases.     

Li isotope fractionation in peridotites and mafic melts

We have measured the Li isotope ratios of a range of co-existing phases from peridotites and mafic magmas to investigate high-temperature fractionations of ⁷Li/⁶Li. The Li isotopic compositions of seven mantle peridotites, reconstructed from analyses of mineral separates, show little variation (δ⁷Li 3.2-4.9‰) despite a wide range in fertility and radiogenic isotopic compositions. The most fertile samples yield a best estimate of δ⁷Li ∼ 3.5‰ for the upper mantle. Bulk analyses of olivine separates from the xenoliths are typically ∼1.5‰ isotopically lighter than co-existing orthopyroxenes, suggestive of a small, high-temperature equilibrium isotope fractionation. On the other hand, bulk analyses of olivine phenocrysts and their host melts are isotopically indistinguishable. Given these observations, equilibrium mantle melting should generate melts with δ⁷Li little different from their sources (<0.5‰ lighter). In contrast to olivine and orthopyroxene, that dominate peridotite Li budgets, bulk clinopyroxene analyses are highly variable (δ⁷Li = 6.6‰ to −8.1‰). Phlogopite separated from a modally metasomatised xenolith yielded an extreme δ⁷Li of −18.9‰. Such large Li isotope variability is indicative of isotopic disequilibrium. This inference is strongly reinforced by in situ, secondary ion mass-spectrometry analyses which show Li isotope zonation in peridotite minerals. The simplest zoning patterns show isotopically light rims. This style of zoning is also observed in the phenocrysts of holocrystalline Hawaiian lavas. More dramatically, a single orthopyroxene crystal from a San Carlos xenolith shows a W-shaped Li isotope profile with a 40‰ range in δ⁷Li, close to the isotope variability seen in all terrestrial whole rock analyses. We attribute Li isotope zonation in mineral phases to diffusive fractionation of Li isotopes, within mineral phases and along melt pathways that pervade xenoliths. Given the high diffusivity of Li, the Li isotope profiles we observe can persist, at most, only a few years at magmatic temperatures. Our results thus highlight the potential of Li isotopes as a high-resolution geospeedometer of the final phases of magmatic activity and cooling.     

Copper isotopic zonation in the Northparkes porphyry Cu-Au deposit, SE Australia

Significant, systematic Cu isotopic variations have been found in the Northparkes porphyry Cu-Au deposit, NSW, Australia, which is an orthomagmatic porphyry Cu deposit. Copper isotope ratios have been measured in sulfide minerals (chalcopyrite and bornite) by both solution and laser ablation multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). The results from both methods show a variation in δ⁶⁵Cu of hypogene sulfide minerals of greater than 1‰ (relative to NIST976). Significantly, the results from four drill holes through two separate ore bodies show strikingly similar patterns of Cu isotope variation. The patterns are characterized by a sharp down-hole decrease from up to 0.8‰ (0.29 ± 0.56‰, 1σ, n = 20) in the low-grade peripheral alteration zones (phyllic-propylitic alteration zone) to a low of ∼−0.4‰ (−0.25 ± 0.36‰, 1σ, n = 30) at the margins of the most mineralized zones (Cu grade >1 wt%). In the high-grade cores of the systems, the compositions are more consistent at around 0.2‰ (0.19 ± 0.14‰, 1σ, n = 40). The Cu isotopic zonation may be explained by isotope fractionation of Cu between vapor, solution and sulfides at high temperature, during boiling and sulfide precipitation processes. Sulfur isotopes also show an isotopically light shell at the margins of the high-grade ore zones, but these are displaced from the low δ⁶⁵Cu shells, such that there is no correlation between the Cu and S isotope signatures. Fe isotope data do not show any discernable variation along the drill core. This work demonstrates that Cu isotopes show a large response to high-temperature porphyry mineralizing processes, and that they may act as a vector to buried mineralization.     

Oxygen isotope fractionation between apatite-bound carbonate and water determined from controlled experiments with synthetic apatites precipitated at 10-37 °C

The oxygen isotope fractionation between the structural carbonate of inorganically precipitated hydroxyapatite (HAP) and water was determined in the range 10-37 °C. Values of 1000 ln α() are linearly correlated with inverse temperature (K) according to the following equation: 1000 ln α() = 25.19 (±0.53)·T⁻¹ − 56.47 (±1.81) (R² = 0.998). This fractionation equation has a slightly steeper slope than those already established between calcite and water ( [O’Neil et al., 1969] and [Kim and O’Neil, 1997]) even though measured fractionations are of comparable amplitude in the temperature range of these experimental studies. It is consequently observed that the oxygen isotope fractionation between apatite carbonate and phosphate increases from about 7.5‰ up to 9.1‰ with decreasing temperature from 37 °C to 10 °C. A compilation of δ¹⁸O values of both phosphate and carbonate from modern mammal teeth and bones confirms that both variables are linearly correlated, despite a significant scattering up to 3.5‰, with a slope close to 1 and an intercept corresponding to a 1000 ln α() value of 8.1‰. This apparent fractionation factor is slightly higher or close to the fractionation factor expected to be in the range 7-8‰ at the body temperature of mammals.     

Redox-driven stable isotope fractionation in transition metals: Application to Zn electroplating

Redox processes are ubiquitous in Earth science and are often associated with large isotope fractionations. In a previous study, voltage-dependent amplification of stable isotope fractionation was observed for an Fe reduction process. Here, we describe experiments showing a similar effect for a second transition metal, zinc. After electrochemical reduction, the composition of plated Zn metal is enriched in the light isotope (⁶⁴Zn) with respect to the Zn²⁺ leftover in solution, with a voltage-dependent fractionation factor. Results from voltage-dependent electroplating experiments are in good agreement with a second data set following equilibrium fractional isotope evolution of Zn isotopes during an electroplating process which stepwise removes most of the Zn from the aqueous reservoir. Taken together, the results indicate a voltage-dependent isotope fractionation (in permil) of ⁶⁶Zn with respect to ⁶⁴Zn to be equal to −3.45 to 1.71 V. The negative slope trend is in contrast with previously published results on iron isotope fractionation during electroplating which shows a positive slope. These results are interpreted using an extension of Marcus theory, which predicts isotope fractionations as a function of driving force in an electrochemical system. Taken together with observations of natural fractionation of redox-sensitive and non redox-active elements, our modified Marcus theory provides a framework for quantitatively predicting transition metal isotope geochemical signatures during environmentally relevant redox processes in terms of simple energetic parameters.     

Calcium isotope (δCa) fractionation along hydrothermal pathways, Logatchev field (Mid-Atlantic Ridge, 14°45′N)

We investigate the Logatchev Hydrothermal Field at the Mid-Atlantic Ridge, 14°45′N to constrain the calcium isotope hydrothermal flux into the ocean. During the transformation of seawater to a hydrothermal solution, the Ca concentration of pristine seawater ([Ca]_SW) increases from about 10 mM to about 32 mM in the hydrothermal fluid endmember ([Ca]_HydEnd) and thereby adopts a δ^44/40Ca_HydEnd of −0.95 ± 0.07‰ relative to seawater (SW) and a ⁸⁷Sr/⁸⁶Sr isotope ratio of 0.7034(4). We demonstrate that δ^44/40Ca_HydEnd is higher than that of the bedrock at the Logatchev field. From mass balance calculations, we deduce a δ^44/40Ca of −1.17 ± 0.04‰ (SW) for the host-rocks in the reaction zone and −1.45 ± 0.05‰ (SW) for the isotopic composition of the entire hydrothermal cell of the Logatchev field. The values are isotopically lighter than the currently assumed δ^44/40Ca for Bulk Earth of −0.92 ± 0.18‰ (SW) [Skulan J., DePaolo D. J. and Owens T. L. (1997) Biological control of calcium isotopic abundances in the global calcium cycle. Geochim. Cosmochim. Acta61,(12) 2505-2510] and challenge previous assumptions of no Ca isotope fractionation between hydrothermal fluid and the oceanic crust [Zhu P. and Macdougall J. D. (1998) Calcium isotopes in the marine environment and the oceanic calcium cycle. Geochim. Cosmochim. Acta62,(10) 1691-1698; Schmitt A. -D., Chabeaux F. and Stille P. (2003) The calcium riverine and hydrothermal isotopic fluxes and the oceanic calcium mass balance. Earth Planet. Sci. Lett. 6731, 1-16]. Here we propose that Ca isotope fractionation along the fluid flow pathway of the Logatchev field occurs during the precipitation of anhydrite. Two anhydrite samples from the Logatchev Hydrothermal Field show an average fractionation of about Δ^44/40Ca = −0.5‰ relative to their assumed parental solutions. Ca isotope ratios in aragonites from carbonate veins from ODP drill cores indicate aragonite precipitation directly from seawater at low temperatures with an average δ^44/40Ca of −1.54 ± 0.08‰ (SW). The relatively large fractionation between the aragonite precipitates and seawater in combination with their frequent abundance in weathered mafic and ultramafic rocks suggest a reconsideration of the marine Ca isotope budget, in particular with regard to ocean crust alteration.     

Cl,N, C isotopic analysis of common agro-chemicals for identifying non-point source agricultural contaminants

The isotopic compositions of commercially available herbicides were analyzed to determine their respective ¹⁵N, ¹³C and ³⁷Cl signatures for the purposes of developing a discrete tool for tracing and identifying non-point source contaminants in agricultural watersheds. Findings demonstrate that of the agrochemicals evaluated, chlorine stable isotopes signatures range between δ³⁷Cl = −4.55‰ and +3.40‰, whereas most naturally occurring chlorine stable isotopes signatures, including those of road salt, sewage sludge and fertilizers, vary in a narrow range about the Standard Mean Ocean Chloride (SMOC) between −2.00‰ and +1.00‰. Nitrogen stable isotope values varied widely from δ¹⁵N = −10.86‰ to +1.44‰ and carbon stable isotope analysis gave an observed range between δ¹³C = −37.13‰ and −21.35‰ for the entire suite of agro-chemicals analyzed. When nitrogen, carbon and chlorine stable isotope analyses were compared in a cross-correlation analysis, statistically independent isotopic signatures exist suggesting a new potential tracer tool for identifying herbicides in the environment.     

Sources and biological fractionation of Silicon isotopes in the Eastern Equatorial Pacific

Silicon isotopes in dissolved silicic acid were measured in the upper four kilometers between 4°N and 3°S latitude at 110°W longitude in the eastern Equatorial Pacific. Silicon isotopes became progressively heavier with silicic acid depletion of surface water as expected from biological fractionation. The value of ε estimated by applying a steady-state isotope fractionation model to data from all stations between 4°N and 3°S was −0.77 ± 0.12‰ (std. err.). When the analysis was restricted to those stations whose temperature and salinity profiles indicated that they were directly influenced by upwelling of the Equatorial Undercurrent (EUC), the resulting value of ε was −1.08 ± 0.27‰ (std. err.) similar to the value established in culture studies (−1.1‰). When the non steady state Rayleigh model was applied to the same restricted data set the resulting value of ε was significantly more positive, −0.61 ± 0.16‰ (std. err.). To the extent that the equatorial system approximates a steady state these results support a value of −1.1‰ for the fractionation factor for isotopes of Si in the sea. Without the assumption of steady state the value of ε can only be constrained to be between −0.6 and −1.1‰. Silicic acid in Equatorial Pacific Deep Water below 2000 m had a near constant δ³⁰Si of +1.32 ± 0.05‰. That value is significantly more positive than obtained for North Pacific Deep Water at similar depths at stations to the northwest of our study area (0.9-1.0‰) and it is slightly less positive than new measures of the δ³⁰Si of silicic acid from the silicic acid plume centered over the Cascadia basin in the Northeast Pacific (Si(OH)₄ > 180  μM, δ³⁰Si = +1.46 ± 0.12‰ (SD, n = 4). We show that the data from the equator and Cascadia basin fit a general trend of increasing δ³⁰Si(OH)₄ with increasing silicic acid concentration in the deep sea, but that the isotope values from the Northeast Pacific are anomalously light. The observed level of variation in the silicon isotope composition of deep waters from this single ocean basin is considerably larger than that predicted by current models based on fractionation during opal formation with no isotope effect during dissolution. Confirmation of such high variability in deep water δ³⁰Si(OH)₄ within individual ocean basins will require reassessment of the mechanisms controlling the distribution of isotopes of silicon in the sea.     

Lithium isotope systematics in a forested granitic catchment (Strengbach, Vosges Mountains, France)

Over the last decade it has become apparent that Li isotopes may be a good proxy to trace silicate weathering. However, the exact mechanisms which drive the behaviour of Li isotopes in surface environments are not totally understood and there is a need to better calibrate and characterize this proxy. In this study, we analysed the Li concentrations and isotopic compositions in the various surface reservoirs (soils, rocks, waters and plants) of a small forested granitic catchment located in the Vosges Mountains (Strengbach catchment, France, OHGE http://ohge.u-strasbg.fr). Li fluxes were calculated in both soil profiles and at the basin scale and it was found that even in this forested basin, atmospheric inputs and litter fall represented a minor flux compared to input derived from the weathering of rocks and soil minerals (which together represent a minimum of 70% of dissolved Li). Li isotope ratios in soil pore waters show large depth dependent variations. Average dissolved δ⁷Li decreases from −1.1‰ to −14.4‰ between 0 and −30 cm, but is +30.7‰ at −60 cm. This range of Li isotopic compositions is very large and it encompasses almost the entire range of terrestrial Li isotope compositions that have been previously reported. We interpret these variations to result from both the dissolution and precipitation of secondary phases. Large isotopic variations were also measured in the springs and stream waters, with δ⁷Li varying from +5.3‰ to +19.6‰. δ⁷Li increases from the top to the bottom of the basin and also covaries with discharge at the outlet. These variations are interpreted to reflect isotopic fractionations occurring during secondary phase precipitation along the water pathway through the rocks. We suggest that the dissolved δ⁷Li increases with increasing residence time of waters through the rocks, and so with increasing time of interaction between waters and solids. A dissolution precipitation model was used to fit the dissolved Li isotopic compositions. It was found that the isotopic compositions of springs and stream waters are explicable by an isotopic fractionation of −5‰ to −14‰ (best fit −10.8‰), in agreement with Li incorporation into clay. In soil solutions, it was found that isotopic fractionation during secondary precipitation is larger (at least −23‰), suggesting a major role for different secondary phases, such as iron oxides that maybe incorporate Li with a higher isotope fractionation.     

The effect of early diagenesis on the Fe isotope compositions of porewaters and authigenic minerals in continental margin sediments

Iron isotope compositions in marine pore fluids and sedimentary solid phases were measured at two sites along the California continental margin, where isotope compositions range from δ⁵⁶Fe = −3.0‰ to +0.4‰. At one site near Monterey Canyon off central California, organic matter oxidation likely proceeds through a number of diagenetic pathways that include significant dissimilatory iron reduction (DIR) and bacterial sulfate reduction, whereas at our other site in the Santa Barbara basin DIR appears to be comparatively small, and production of sulfides (FeS and pyrite) was extensive. The largest range in Fe isotope compositions is observed for Fe(II)_aq in porewaters, which generally have the lowest δ⁵⁶Fe values (minimum: −3.0‰) near the sediment surface, and increase with burial depth. δ⁵⁶Fe values for FeS inferred from HCl extractions vary between ∼−0.4‰ and +0.4‰, but pyrite is similar at both stations, where an average δ⁵⁶Fe value of −0.8 ± 0.2‰ was measured. We interpret variations in dissolved Fe isotope compositions to be best explained by open-system behavior that involves extensive recycling of Feflux. This study is the first to examine Fe isotope variations in modern marine sediments, and the results show that Fe isotopes in the various reactive Fe pools undergo isotopic fractionation during early diagenesis. Importantly, processes dominated by sulfide formation produce high-δ⁵⁶Fe values for porewaters, whereas the opposite occurs when Fe(III)-oxides are present and DIR is a major pathway of organic carbon respiration. Because shelf pore fluids may carry a negative δ⁵⁶Fe signature it is possible that the Fe isotope composition of ocean water reflects a significant contribution of shelf-derived iron to the open ocean. Such a signature would be an important means for tracing iron sources to the ocean and water mass circulation.     

Isotopic fractionations associated with phosphoric acid digestion of carbonate minerals: Insights from first-principles theoretical modeling and clumped isotope measurements

Phosphoric acid digestion has been used for oxygen- and carbon-isotope analysis of carbonate minerals since 1950, and was recently established as a method for carbonate ‘clumped isotope’ analysis. The CO₂ recovered from this reaction has an oxygen isotope composition substantially different from reactant carbonate, by an amount that varies with temperature of reaction and carbonate chemistry. Here, we present a theoretical model of the kinetic isotope effects associated with phosphoric acid digestion of carbonates, based on structural arguments that the key step in the reaction is disproportionation of H₂CO₃ reaction intermediary. We test that model against previous experimental constraints on the magnitudes and temperature dependences of these oxygen isotope fractionations, and against new experimental determinations of the fractionation of ¹³C-¹⁸O-containing isotopologues (‘clumped’ isotopic species). Our model predicts that the isotope fractionations associated with phosphoric acid digestion of carbonates at 25 °C are 10.72‰, 0.220‰, 0.137‰, 0.593‰ for, respectively, ¹⁸O/¹⁶O ratios (1000 lnα^∗) and three indices that measure proportions of multiply-substituted isotopologues . We also predict that oxygen isotope fractionations follow the mass dependence exponent, λ of 0.5281 (where ). These predictions compare favorably to independent experimental constraints for phosphoric acid digestion of calcite, including our new data for fractionations of ¹³C-¹⁸O bonds (the measured change in Δ₄₇ = 0.23‰) during phosphoric acid digestion of calcite at 25 °C.We have also attempted to evaluate the effect of carbonate cation compositions on phosphoric acid digestion fractionations using cluster models in which disproportionating H₂CO₃ interacts with adjacent cations. These models underestimate the magnitude of isotope fractionations and so must be regarded as unsucsessful, but do reproduce the general trend of variations and temperature dependences of oxygen isotope acid digestion fractionations among different carbonate minerals. We suggest these results present a useful starting point for future, more sophisticated models of the reacting carbonate/acid interface. Examinations of these theoretical predictions and available experimental data suggest cation radius is the most important factor governing the variations of isotope fractionation among different carbonate minerals. We predict a negative correlation between acid digestion fractionation of oxygen isotopes and of ¹³C-¹⁸O doubly-substituted isotopologues, and use this relationship to estimate the acid digestion fractionation of for different carbonate minerals. Combined with previous theoretical evaluations of ¹³C-¹⁸O clumping effects in carbonate minerals, this enables us to predict the temperature calibration relationship for different carbonate clumped isotope thermometers (witherite, calcite, aragonite, dolomite and magnesite), and to compare these predictions with available experimental determinations. The success of our models in capturing several of the features of isotope fractionation during acid digestion supports our hypothesis that phosphoric acid digestion of carbonate minerals involves disproportionation of transition state structures containing H₂CO₃.