Oxygen isotopic and chemical compositions of cosmic spherules collected from the Antarctic ice sheet: Implications for their precursor materials

Bulk chemical compositions and oxygen isotopic compositions were analyzed for 48 stony cosmic spherules (melted micrometeorites) collected from the Antarctic ice sheet using electron- and ion-microprobes. No clear correlation was found between their isotopic compositions and textures. The oxygen isotopic compositions showed an extremely wide range from −28‰ to +93‰ in δ¹⁸O and from −21‰ to +13‰ in Δ¹⁷O. In δ¹⁸O-δ¹⁷O space, most samples (38 out of 48) plot close to the terrestrial fractionation line, but 7 samples plot along the carbonaceous chondrite anhydrous mineral (CCAM) line. Three samples plot well above the terrestrial fractionation line. One of these has a Δ¹⁷O of +13‰, the largest value ever found in solar system materials. One possible precursor for this spherule could be ¹⁶O-poor planetary material that is still unknown as a meteorite. The majority of the remaining spherules are thought to be related to carbonaceous chondrites.     

Isotopic compositions of oxygen, iron, chromium, and nickel in cosmic spherules: Toward a better comprehension of atmospheric entry heating effects

Large, correlated, mass-dependent enrichments in the heavier isotopes of O, Cr, Fe, and Ni are observed in type-I (metal/metal oxide) cosmic spherules collected from the deep sea. Limited intraparticle variability of oxygen isotope abundances, typically <5‰ in δ¹⁸O, indicates good mixing of the melts and supports the application of the Rayleigh equation for the calculation of fractional evaporative losses during atmospheric entry. Fractional losses for oxygen evaporation from wüstite, assuming a starting isotopic composition equal to that of air (δ¹⁸O = 23.5‰; δ¹⁷O = 11.8‰), are in the range 55%-77%, and are systematically smaller than evaporative losses calculated for Fe (69%-85%), Cr (81%-95%), and especially Ni (45%-99%). However, as δ¹⁸O values increase, fractional losses for oxygen approach those of Fe, Cr, and Ni indicating a shift in the evaporating species from metallic to oxidized forms as the spherules are progressively oxidized during entry heating. The observed unequal fractional losses of O and Fe can be reconciled by allowing for a kinetic isotope mass-dependent fractionation of atmospheric oxygen during the oxidation process and/or that some metallic Fe may have undergone Rayleigh evaporation before oxidation began.In situ measurements of oxygen isotopic abundances were also performed in 14 type-S (silicate) cosmic spherules, 13 from the Antarctic ice and one from the deep sea. Additional bulk Fe and Cr isotopic abundances were determined for two type-S deep-sea spherules. The isotopic fractionation of Cr isotopes suggest appreciable evaporative loss of Cr, perhaps as a sulfide. The oxygen isotopic compositions for the type-S spherules range from δ¹⁸O = −2‰ to + 27‰. The intraspherule isotopic variations are typically small, ∼5% relative, except for the less-heated porphyritic spherules which have preserved large isotopic heterogeneities in at least one case. A plot of δ¹⁷O vs. δ¹⁸O values for these spherules defines a broad parallelogram bounded at higher values of δ¹⁷O by the terrestrial fractionation line, and at lower values of δ¹⁷O by a line parallel to it and anchored near the isotopic composition of δ¹⁸O = −2.5‰ and δ¹⁷O = −5‰. Lack of independent evidence for substantial evaporative losses suggests that much of this variation reflects the starting isotopic composition of the precursor materials, which likely resembled CO, CM, or CI chondrites. However, the enrichments in heavy isotopes indicate that some mixing with atmospheric oxygen was probably involved during atmospheric entry for some of the spherules. Isotopic fractionation due to evaporation of incoming grain is not required to explain most of the oxygen isotopic data for type-S spherules. However spherules with barred olivine textures that are thought to have experienced a more intense heating than the porphyritic ones might have undergone some distillation. Two cosmic spherules, one classified as a radial pyroxene type and the other showing a glassy texture, show unfractionated oxygen isotopic abundances. They are probably chondrule fragments that survived atmospheric entry unmelted.Possible reasons type-I spherules show larger degrees of isotopic fractionation than type-S spherules include: a) the short duration of the heating pulse associated with the high volatile content of the type-S spherule precursors compared to type-I spherules; b) higher evaporation temperatures for at least a refractory portion of the silicates compared to that of iron metal or oxide; c) lower duration of heating of type-S spherules compared to type-I spherules as a consequence of their lower densities.     

Stable carbon and nitrogen isotopic compositions of high molecular weight dissolved organic matter from four U.S. estuaries

High molecular weight dissolved organic matter (HMW-DOM) represents an important component of dissolved organic carbon (DOC) in seawater and fresh-waters. In this paper, we report measurements of stable carbon (δ¹³C) isotopic compositions in total lipid, total hydrolyzable amino acid (THAA), total carbohydrate (TCHO) and acid-insoluble “uncharacterized” organic fractions separated from fourteen HMW-DOM samples collected from four U.S. estuaries. In addition, C/N ratio, δ¹³C and stable nitrogen (δ¹⁵N) isotopic compositions were also measured for the bulk HMW-DOM samples. Our results indicate that TCHO and THAA are the dominant organic compound classes, contributing 33-46% and 13-20% of the organic carbon in HMW-DOM while total lipid accounts for only <2% of the organic carbon in the samples. In all samples, a significant fraction (35-49%) of HMW-DOM was included in the acid-insoluble fraction. Distinct differences in isotopic compositions exist among bulk samples, the compound classes and the acid-insoluble fractions. Values of δ¹³C and δ¹⁵N measured for bulk HMW-DOM varied from −22.1 to −30.1‰ and 2.8 to 8.9‰, respectively and varied among the four estuaries studied as well. Among the compound classes, TCHO was more enriched in ¹³C (δ¹³C = −18.5 to −22.8‰) compared with THAA (δ¹³C = −20.0 to −29.6‰) and total lipid (δ¹³C = −25.7 to −30.7‰). The acid-insoluble organic fractions, in general, had depleted ¹³C values (δ¹³C = −23.0 to −34.4‰). Our results indicate that the observed differences in both δ¹³C and δ¹⁵N were mainly due to the differences in sources of organic matter and nitrogen inputs to these estuaries in addition to the microbial processes responsible for isotopic fractionation among the compound classes. Both terrestrial sources and local sewage inputs contribute significantly to the HMW-DOM pool in the estuaries studied and thus had a strong influence on its isotopic signatures.     

Magnesium and iron isotopes in 2.7 Ga Alexo komatiites: Mantle signatures, no evidence for Soret diffusion, and identification of diffusive transport in zoned olivine

Komatiites from Alexo, Canada, are well preserved and represent high-degree partial mantle melts (∼50%). They are thus well suited for investigating the Mg and Fe isotopic compositions of the Archean mantle and the conditions of magmatic differentiation in komatiitic lavas. High precision Mg and Fe isotopic analyses of 22 samples taken along a 15-m depth profile in a komatiite flow are reported. The δ²⁵Mg and δ²⁶Mg values of the bulk flow are −0.138 ± 0.021‰ and −0.275 ± 0.042‰, respectively. These values are indistinguishable from those measured in mantle peridotites and chondrites, and represent the best estimate of the composition of the silicate Earth from analysis of volcanic rocks. Excluding the samples affected by secondary Fe mobilization, the δ⁵⁶Fe and δ⁵⁷Fe values of the bulk flow are +0.044 ± 0.030‰, and +0.059 ± 0.044‰, respectively. These values are consistent with a near-chondritic Fe isotopic composition of the silicate Earth and minor fractionation during komatiite magma genesis. In order to explain the early crystallization of pigeonite relative to augite in slowly cooled spinifex lavas, it was suggested that magmas trapped in the crystal mush during spinifex growth differentiated by Soret effect, which should be associated with large and coupled variations in the isotopic compositions of Mg and Fe. The lack of variations in Mg and Fe isotopic ratios either rules out the Soret effect in the komatiite flow or the effect is effaced as the solidification front migrates downward through the flow crust. Olivine separated from a cumulate sample has light δ⁵⁶Fe and slightly heavy δ²⁶Mg values relative to the bulk flow, which modeling shows can be explained by kinetic isotope fractionation associated with Fe-Mg inter-diffusion in olivine. Such variations can be used to identify diffusive processes involved in the formation of zoned minerals.     

Magnesium isotopic composition of the Earth and chondrites

To constrain further the Mg isotopic composition of the Earth and chondrites, and investigate the behavior of Mg isotopes during planetary formation and magmatic processes, we report high-precision (±0.06‰ on δ²⁵Mg and ±0.07‰ on δ²⁶Mg, 2SD) analyses of Mg isotopes for (1) 47 mid-ocean ridge basalts covering global major ridge segments and spanning a broad range in latitudes, geochemical and radiogenic isotopic compositions; (2) 63 ocean island basalts from Hawaii (Kilauea, Koolau and Loihi) and French Polynesia (Society Island and Cook-Austral chain); (3) 29 peridotite xenoliths from Australia, China, France, Tanzania and USA; and (4) 38 carbonaceous, ordinary and enstatite chondrites including 9 chondrite groups (CI, CM, CO, CV, L, LL, H, EH and EL).Oceanic basalts and peridotite xenoliths have similar Mg isotopic compositions, with average values of δ²⁵Mg = −0.13 ± 0.05 (2SD) and δ²⁶Mg = −0.26 ± 0.07 (2SD) for global oceanic basalts (n = 110) and δ²⁵Mg = −0.13 ± 0.03 (2SD) and δ²⁶Mg = −0.25 ± 0.04 (2SD) for global peridotite xenoliths (n = 29). The identical Mg isotopic compositions in oceanic basalts and peridotites suggest that equilibrium Mg isotope fractionation during partial melting of peridotite mantle and magmatic differentiation of basaltic magma is negligible. Thirty-eight chondrites have indistinguishable Mg isotopic compositions, with δ²⁵Mg = −0.15 ± 0.04 (2SD) and δ²⁶Mg = −0.28 ± 0.06 (2SD). The constancy of Mg isotopic compositions in all major types of chondrites suggest that primary and secondary processes that affected the chemical and oxygen isotopic compositions of chondrites did not significantly fractionate Mg isotopes.Collectively, the Mg isotopic composition of the Earth’s mantle, based on oceanic basalts and peridotites, is estimated to be −0.13 ± 0.04 for δ²⁵Mg and −0.25 ± 0.07 for δ²⁶Mg (2SD, n = 139). The Mg isotopic composition of the Earth, as represented by the mantle, is similar to chondrites. The chondritic composition of the Earth implies that Mg isotopes were well mixed during accretion of the inner solar system.     

Lithium isotopic systematics of peridotite xenoliths from Hannuoba, North China Craton: Implications for melt-rock interaction in the considerably thinned lithospheric mantle

Li concentrations and isotopic compositions of coexisting minerals (ol, opx, and cpx) from peridotite xenoliths entrained in the Hannuoba Tertiary basalts, North China Craton, provide insight into Li isotopic fractionation between mantle minerals during melt-rock interaction in the considerably thinned lithospheric mantle. Bulk analyses of mineral separates show significant enrichment of Li in cpx (2.4-3.6 ppm) relative to olivine (1.2-1.8 ppm), indicating that these peridotites have been affected by mantle metasomatism with mafic silicate melts. Bulk olivine separates (δ⁷Li ∼ +3.3‰ to +6.4‰) are isotopically heavier than coexisting pyroxenes (δ⁷Li ∼ −3.3‰ to −8.2‰ in cpx, and −4.0‰ to −6.7‰ in opx). Such large variation suggests Li elemental and isotopic disequilibrium. This conclusion is supported by results from in situ SIMS analyses of mineral grains where significant Li elemental and isotopic zonations exist. The olivine and opx have lower Li concentrations and heavier Li isotopes in the rims than in the cores. This reverse correlation of δ⁷Li with Li concentrations indicates diffusive fractionation of Li isotopes. However, the zoning patterns in coexisting cpx show isotopically heavier rims with higher Li abundances. This positive correlation between δ⁷Li and Li concentrations suggests a melt mixing trend. We attribute Li concentration and isotope zonation in minerals to the effects of two-stage diffusive fractionation coupled with melt-rock interaction. The earliest melts may have been derived from the subducted oceanic slab with low δ⁷Li values produced by isotopic fractionation during the dehydration of the seawater-altered slab. Melts at later stages were derived from the asthenosphere and interacted with the peridotites, producing the Li elemental and isotopic zoning in mineral grains. These data thus provide evidence for multiple-stage peridotite-melt interaction in the lithospheric mantle beneath the northern North China Craton.     

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.     

Microscale oxygen isotopic exchange and magnetite formation in the Ningqiang anomalous carbonaceous chondrite

We report in situ measurements of O-isotopic compositions of magnetite, olivine and pyroxene in chondrules of the Ningqiang anomalous carbonaceous chondrite. The petrographic setting of Ningqiang magnetite is similar to those in oxidized-CV chondrites such as Allende, where magnetite is found together with Ni-rich metal and sulfide in opaque assemblages in chondrules. Both magnetite and silicate oxygen data fall close to the carbonaceous-chondrite-anhydrous-mineral line with relatively large ranges in δ¹⁸O in magnetite (−4.9 to +4.2‰) and in silicates (−15.2 to −4.5‰). Magnetite and silicates are not in O-isotopic equilibrium: the weighted average Δ¹⁷O (=δ¹⁷O − 0.52 × δ¹⁸O) values of magnetite are 1.7 to 3.6‰ higher than those of the silicates in the same chondrules. The petrological characteristics and O-isotopic disequilibrium between magnetite and silicates suggest the formation of Ningqiang magnetite by the oxidation of preexisting metal grains by an aqueous fluid during parent body alteration. The weighted average Δ¹⁷O of −3.3 ± 0.3‰ is the lowest magnetite value measured in unequilibrated chondrites and there is a positive correlation between Δ¹⁷O values of magnetite and silicates in each chondrule. These observations indicate that, during aqueous alteration in the Ningqiang parent asteroid, the water/rock ratio was relatively low and O-isotopic exchange between the fluid and chondrule silicates occurred on the scale of individual chondrules.     

Lithium isotopic composition and concentration of the upper continental crust

The Li isotopic composition of the upper continental crust is estimated from the analyses of well-characterized shales, loess, granites and upper crustal composites (51 samples in total) from North America, China, Europe, Australia and New Zealand. Correlations between Li, δ⁷Li, and chemical weathering (as measured by the Chemical Index of Alteration (CIA)), and δ⁷Li and the clay content of shales (as measured by Al₂O₃/SiO₂), reflect uptake of heavy Li from the hydrosphere by clays. S-type granites from the Lachlan fold belt (-1.1 to -1.4‰) have δ⁷Li indistinguishable from their associated sedimentary rocks (-0.7 to 1.2‰), and show no variation in δ⁷Li throughout the differentiation sequence, suggesting that isotopic fractionation during crustal anatexis and subsequent differentiation is less than analytical uncertainty (±1‰, 2σ). The isotopically light compositions for both I- and S-type granites from the Lachlan fold belt (-2.5 to + 2.7 ‰) and loess from around the world (-3.1 to + 4.5‰) reflect the influence of weathering in their source regions. Collectively, these lithologies possess a limited range of Li isotopic compositions (δ⁷Li of −5‰ to + 5‰), with an average (δ⁷Li of 0 ± 2‰ at 1σ) that is representative of the average upper continental crust. Thus, the Li isotopic composition of the upper continental crust is lighter than the average upper mantle (δ⁷Li of + 4 ± 2‰), reflecting the influence of weathering on the upper crustal composition. The concentration of Li in the upper continental crust is estimated to be 35 ± 11 ppm (2σ), based on the average loess composition and correlations between insoluble elements (Ti, Nb, Ta, Ga and Al₂O₃, Th and HREE) and Li in shales. This value is somewhat higher than previous estimates (∼20 ppm), but is probably indistinguishable when uncertainties in the latter are accounted for.     

Graphite-bearing CO2-fluid inclusions in granulites: Insights on graphite precipitation and carbon isotope evolution

Graphite in deep crustal enderbitic (orthopyroxene + garnet + plagioclase + quartz) granulites (740°C, 8.9 kb) of Nilgiri hills, southern India were investigated for their spectroscopic and isotopic characteristics. Four types of graphite crystals were identified. The first type (Gr_I), which is interstitial to other mineral grains, can be grouped into two subtypes, Gr_IA and Gr_IB. Gr_IA is either irregular in shape or deformed, and rough textured with average δ¹³C values of −12.7 ± 0.4‰ (n = 3). A later generation of interstitial graphite (Gr_IB) shows polygonal crystal shapes and highly reflecting smooth surface features. These graphite grains are more common and have δ¹³C values of −11.9 ± 0.3‰ (n = 14). Both subtypes show well-defined Raman shifts suggesting a highly crystalline nature. Cores of interstitial graphite grains have, on average, lower δ¹³C values by ∼0.5‰ compared to that of the rim. The second type of graphite (Gr_II) occurs as solid inclusions in silicate minerals, commonly forming regular hexagonal crystals with a slightly disordered structure. The third type of graphite (Gr_III) is associated with solid inclusions (up to 100 μm) that have decrepitation halos of numerous small (<15 μm) satellite fluid inclusions of pure CO₂ with varying density (1.105 to 0.75 g/cm³). The fourth type of graphite (Gr_IV) is found as daughter crystals within primary type CO₂-fluid inclusions in garnet and quartz. These fluid inclusions have a range of densities (1.05 to 0.90 g/cm³), but in general are significantly less dense than graphite-free primary, pure CO₂ fluid inclusions (1.12 g/cm³). Raman spectral characteristics of graphite inside fluid inclusions suggest graphite crystallization at low temperature (∼ 500°C). The precipitation of graphite probably occurred during the isobaric cooling of CO₂-rich peak metamorphic fluid as a result of oxyexsolution of oxide phases. The oxyexsolution process is evidenced by the magnetite-ilmenite granular exsolution textures and the systematic presence of numerous micron-sized rutile and other oxide inclusions in association with fluid inclusions within garnet, plagioclase, and quartz.The carbon isotope compositions of coexisting CO₂ (in fluid inclusions) and graphite show a fractionation (α₂CO−gr) of ∼6‰ in garnet, consistent with the existing theoretical estimates of α₂CO−gr at 800°C. A subsequent generation of CO₂ inclusions trapped in matrix quartz and quartz segregation have higher δ¹³C values, −4‰ and −2.9‰ respectively. Graphite in quartz segregations also has higher δ¹³C values (−9.8‰) than those in enderbite (−12.7‰). Micro-graphite crystals included in garnet, quartz (enderbite), and quartz (segregation) have average δ¹³C values of −11.1, −10.4, and −8.7‰ respectively, indicating progressive enrichment in ¹³C with a decrease in temperature of recrystallization of respective minerals. This progressive enrichment is also observed in carbon isotope compositions of fluid inclusion CO₂, suggesting isotopic equilibrium during graphite precipitation from CO₂ fluids. Thus, the carbon isotope record preserved in these rocks by the interstitial graphite, CO₂ fluid in enderbite, graphite microcrystals, graphite in quartz segregation, and CO₂ fluid in quartz segregation, suggests a temperature-controlled isotopic evolution. This evolution is in accordance with a closed system Rayleigh-type graphite precipitation process which progressively enriched residual CO₂ in ¹³C.     

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.     

Application of sulphur isotope ratios to examine weaning patterns and freshwater fish consumption in Roman Oxfordshire, UK

This study investigates the application of sulphur isotope ratios (δ³⁴S) in combination with carbon (δ¹³C) and nitrogen (δ¹⁵N) ratios to understand the influence of environmental sulphur on the isotopic composition of archaeological human and faunal remains from Roman era sites in Oxfordshire, UK. Humans (n = 83), terrestrial animals (n = 11), and freshwater fish (n = 5) were analysed for their isotope values from four locations in the Thames River Valley, and a broad range of δ³⁴S values were found. The δ³⁴S values from the terrestrial animals were highly variable (−13.6‰ to +0.5‰), but the δ³⁴S values of the fish were clustered and ³⁴S-depleted (−20.9‰ to −17.3‰). The results of the faunal remains suggest that riverine sulphur influenced the terrestrial sulphur isotopic signatures. Terrestrial animals were possibly raised on the floodplains of the River Thames, where highly ³⁴S-depleted sulphur influenced the soil. The humans show the largest range of δ³⁴S values (−18.8‰ to +9.6‰) from any archaeological context to date. No differences in δ³⁴S values were found between the males (−7.8 ± 6.0‰) and females (−5.3 ± 6.8‰), but the females had a linear correlation (R² = 0.71; p < 0.0001) between their δ¹⁵N and δ³⁴S compositions. These δ³⁴S results suggest a greater dietary variability for the inhabitants of Roman Oxfordshire than previously thought, with some individuals eating solely terrestrial protein resources and others showing a diet almost exclusively based on freshwater protein such as fish. Such large dietary variability was not visible by analysing only the carbon and nitrogen isotope ratios, and this research represents the largest and most detailed application of δ³⁴S analysis to examine dietary practices (including breastfeeding and weaning patterns) during the Romano-British Period.     

Sulfur isotope systematics of the 1982 El Chichón trachyandesite: an ion microprobe study

Sulfur isotopic compositions were determined by ion microprobe for 36 spots on anhydrite crystals in trachyandesitic pumices erupted from El Chichón Volcano in 1982. Individual anhydrite crystals are homogeneous in δ³⁴S, within the ±1‰ (2σ) uncertainty of the method, but crystal-to-crystal variations are large (+2.5 to +10.9‰). The mean δ³⁴S for anhydrite (+6.4 ± 2.1‰, 1σ) is significantly lower than earlier results for bulk anhydrite separates (+9.0 to +9.2‰). The difference between the mean δ³⁴S values in these two populations may reflect a grain-size effect, with heavier sulfur concentrated in smaller anhydrite crystals, few of which were analyzed by ion microprobe. Variations in δ³⁴S show no correlation with complex textures in anhydrite revealed by cathodoluminescence color. Ion-microprobe analyses of δ³⁴S were also obtained on six ovoid-shaped inclusions of pyrrhotite, chalcopyrite, and/or intermediate sulfide solid solution hosted by silicate or oxide crystals, interpreted to be magmatic (δ³⁴S = −0.1 to +2.7‰; mean +0.7‰), and on four irregularly shaped multiphase sulfide fragments in the matrix, interpreted as xenocrystic, which range widely in δ³⁴S (−3.7 to +5.5‰). We evaluate four different mixing scenarios involving (1) magmatic anhydrite and sedimentary sulfate, (2) magmatic anhydrite and hydrothermal anhydrite, and anhydrite and coexisting sulfide crystals precipitated in different domains of a common magma reservoir that were affected by (3) different degrees of degassing or (4) different degrees of crustal sulfur contamination. The model involving physical contamination of sedimentary sulfate is considered untenable. The other three models are considered to be viable, but none of them can explain all observations. The results of this study and other recent investigations prompt a re-evaluation of the sulfur budget for the 1982 El Chichón eruption. We estimate that 2.2 × 10¹³ g of S was emitted, and that 58 wt.% of the sulfur was present as anhydrite prior to eruption, with the remainder in a vapor phase, with H₂S/SO₂ ≈ 9. The bulk magmatic δ³⁴S value for the 1982 El Chichón trachyandesite is estimated as +4.1 to +5.8‰, typical of the relatively heavy sulfur isotopic compositions that characterize subduction-related magmas.     

Ion microprobe analysis of graphite from ca. 3.8 Ga metasediments, Isua supracrustal belt, West Greenland: Relationship between metamorphism and carbon isotopic composition

In-situ ion microprobe measurements of carbon isotopic compositions of graphite were made in seven metasediments and two carbonate rocks from the ca. 3.8 Ga Isua supracrustal belt, West Greenland. The δ¹³C values of micron-scale graphite globules in the metasediments and the carbonate rocks vary from -18 to +2‰ and from -7 to -3‰, respectively. The maximum δ¹³C value of graphite globules in the metasediment rises from -14 to -5‰, as the metamorphic grade increases from epidote-amphibolite to upper amphibolite facies. In a single hand specimen, the δ¹³C values of graphite inclusions in garnet are ∼7‰ lower on average than those outside garnet. Similarly, graphite armored by quartz apparently shows a few permil lower δ¹³C values than those on grain boundaries between noncarbonate minerals. The fact that early crystallized minerals include relatively ¹³C-depleted graphite indicates that the regional metamorphism increased the δ¹³C values of the Isua graphite. This is consistent with the regional trend of ¹³C-enrichment accompanied by the increase of metamorphic grade. The minimum fractionation between graphite and carbonate is consistent with the equilibrium fractionation at about 400 to 550 °C. These observations indicate that isotopic exchange with isotopically heavy carbonate caused ¹³C-enrichment of Isua graphite. The δ¹³C values of graphite reported here (δ¹³C > -18‰) were produced either as a metamorphic modification of organic carbon with initially much lower δ¹³C values, or as an abiological reaction such as decomposition of carbonate. If the isotopic exchange between carbonate and graphite during regional metamorphism controlled the ¹³C-enrichment of Isua graphite, previously reported large ¹³C-depletion of graphite, especially armored by apatite (Mojzsis et al., 1996) was probably premetamorphic in origin. This supports the existence of life at Isua time (ca. 3.8 Ga).     

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.     

The oxygen-isotope composition of chondrules and isolated forsterite and olivine grains from the Tagish Lake carbonaceous chondrite

The oxygen-isotope compositions (obtained by laser fluorination) of hand-picked separates of isolated forsterite, isolated olivine and chondrules from the Tagish Lake carbonaceous chondrite describe a line (δ¹⁷O = 0.95 * δ¹⁸O − 3.24; R² = 0.99) similar to the trend known for chondrules from other carbonaceous chondrites. The isolated forsterite grains (Fo_99.6-99.8; δ¹⁸O = −7.2‰ to −5.5‰; δ¹⁷O = −9.6‰ to −8.2‰) are more ¹⁶O-rich than the isolated olivine grains (Fo_39.6-86.8; δ¹⁸O = 3.1‰ to 5.1‰; δ¹⁷O = −0.3‰ to 2.2‰), and have chemical and isotopic characteristics typical of refractory forsterite. Chondrules contain olivine (Fo_97.2-99.8) with oxygen-isotope compositions (δ¹⁸O = −5.2‰ to 5.9‰; δ¹⁷O = −8.1‰ to 1.2‰) that overlap those of isolated forsterite and isolated olivine. An inverse relationship exists between the Δ¹⁷O values and Fo contents of Tagish Lake isolated forsterite and chondrules; the chondrules likely underwent greater exchange with ¹⁶O-poor nebular gases than the forsterite. The oxygen-isotope compositions of the isolated olivine grains describe a trend with a steeper slope (1.1 ± 0.1, R² = 0.94) than the carbonaceous chondrite anhydrous mineral line (CCAM_slope = 0.95). The isolated olivine may have crystallized from an evolving melt that exchanged with ¹⁶O-poor gases of somewhat different composition than those which affected the chondrules and isolated forsterite. The primordial components of the Tagish Lake meteorite formed under conditions similar to other carbonaceous chondrite meteorite groups, especially CMs. Its alteration history has its closest affinities to CI carbonaceous chondrites.     

Graves Nunataks 95209: A snapshot of metal segregation and core formation

GRA 95209 may provide our best opportunity to date to understand the earliest stages of core formation in asteroidal bodies. This lodranite preserves a physically, chemically, and mineralogically complex set of metal-sulfide veins. High-resolution X-ray computed tomography revealed three distinct lithologies. The dominant mixed metal-silicate-sulfide matrix is cut by metal-rich, graphite-bearing veins exceeding 1 cm in width and grades into a volumetrically minor metal-poor region. Silicate compositions and modal abundances are typical for lodranites, while the mineralogy of the metal-sulfide component is complex and differs among the three lithologies. Kamacite and troilite occur with chromite, tetrataenite, schreibersite, graphite, and a range of phosphates. An ³⁹Ar-⁴⁰Ar age of 4.521 ± 0.006 Ga measures the time of closure of the K-Ar system. Carbon rosettes within the metal-rich vein are nitrogen-poor, well crystallized, include kamacite sub-grains of composition comparable to the host metal, and are essentially isotopically homogeneous (δ¹³C ∼ −33‰). In contrast, carbon rosettes within metal of the metal-poor lithology are N-poor, poorly crystallized, include kamacite grains that are Ni-poor compared to their host metal, and are isotopically heterogeneous (δ¹³C ranging from −50 to +80‰) even within a single metal grain. The silicate portion of GRA 95209 is similar to the lodranite EET 84302, sharing a common texture, silicate mineral compositions, and Ar-Ar age. GRA 95209 and EET 84302 are intermediate between acapulcoites and lodranites. Both experienced Fe,Ni-FeS melting with extensive melt migration, but record only the onset of silicate partial melting with limited migration of silicate melt. The complex metal-sulfide veins in GRA 95209 resulted from low-degree partial melting and melt migration and intruded the matrix lithology. Reactions between solid minerals and melt, including oxidation-reduction reactions, produced the array of phosphates, schreibersite, and tetrataenite. Extensive reduction in the metal-rich vein resulted from its origin in a hotter portion of the asteroid. This difference in thermal history is supported by the graphite structures and isotopic compositions. The graphite rosettes in the metal-rich vein are consistent with high-temperature igneous processing. In contrast, the carbon in the metal-poor lithology appears to preserve a record of formation in the nebula prior to parent-body formation. Carbon incorporated from the solar nebula into a differentiating asteroid is preferentially incorporated in metal-sulfide melts that form a core, but does not achieve isotopic homogeneity until extensive thermal processing occurs.     

Geothermal waters from the Taupo Volcanic Zone,New Zealand: Li,B and Sr isotopes characterization

Chemical and isotopic data for 23 geothermal water samples collected in New Zealand within the Taupo Volcanic Zone (TVZ) are reported. Major and trace elements including Li, B and Sr and their isotopic compositions (δ⁷Li, δ¹¹B, ⁸⁷Sr/⁸⁶Sr) were determined in high temperature geothermal waters collected from deep boreholes in different geothermal fields (Ohaaki, Wairakei, Mokai, Kawerau and Rotokawa geothermal systems). Lithium concentrations are high (from 4.5 to 19.9 mg/L) and Li isotopic compositions (δ⁷Li) are homogeneous, ranging between −0.5‰ and +1.4‰. In particular, it is noteworthy that, except for the samples from the Kawerau geothermal field having slightly higher δ⁷Li values (+1.4%), the other geothermal waters have a near constant δ⁷Li signature around a mean value of 0‰ ± 0.6 (2σ, n = 21). Boron concentrations are also high and relatively homogeneous for the geothermal samples, falling between 17.5 and 82.1 mg/L. Boron isotopic compositions (δ¹¹B) are all negative, and display a range between −6.7‰ and −1.9‰. These B isotope compositions are in agreement with those of the Ngawha geothermal field in New Zealand. Lithium and B isotope signatures are in a good agreement with a fluid signature mainly derived from water/rock interaction involving magmatic rocks with no evidence of seawater input. On the other hand, Sr concentrations are lower and more heterogeneous and fall between 2 and 165 μg/L. The ⁸⁷Sr/⁸⁶Sr ratios range from 0.70549 to 0.70961. These Sr isotope compositions overlap those of the Rotorua geothermal field in New Zealand, confirming that some geothermal waters (with more radiogenic Sr) have interacted with bedrocks from the metasedimentary basement. Each of these isotope systems on their own reveals important information about particular aspects of either water source or water/rock interaction processes, but, considered together, provide a more integrated understanding of the geothermal systems from the TVZ in New Zealand.     

Silicon isotopes in meteorites and planetary core formation

The silicon (Si) isotope compositions of 42 meteorite and terrestrial samples have been determined using MC-ICPMS with the aim of resolving the current debate over their compositions and the implications for core formation. No systematic δ³⁰Si differences are resolved between chondrites (δ³⁰Si = −0.49 ± 0.15‰, 2σ_SD) and achondrites (δ³⁰Si = −0.47 ± 0.11‰, 2σ_SD), although enstatite chondrites are consistently lighter (δ³⁰Si = −0.63 ± 0.07‰, 2σ_SD) in comparison to other meteorite groups. The data reported here for meteorites and terrestrial samples display an average difference Δ³⁰Si_{BSE−meteorite∗} = 0.15 ± 0.10‰, which is consistent within uncertainty with previous studies. No effect from sample heterogeneity, preparation, chemistry or mass spectrometry can be identified as responsible for the reported differences between current datasets. The heavier composition of the bulk silicate Earth is consistent with previous conclusions that Si partitioned into the metal phase during metal-silicate equilibration at the time of core formation. Fixing the temperature of core formation to the peridotite liquidus and using an appropriate metal silicate fractionation factor (ε ∼0.89), the Δ³⁰Si_{BSE−meteorite∗} value from this study indicates that the Earth core contains at least 2.5 and possibly up to 16.8 wt% Si.     

Br/Cl ratios and O, H, C, and B isotopic constraints on the origin of saline waters from eastern Canada

Saline groundwaters were recovered from undisturbed (Restigouche deposit) and active (Brunswick #12 mine) Zn-Pb volcanogenic massive sulfide deposits in the Bathurst Mining Camp (BMC), northern New Brunswick, Canada. These groundwaters, along with fresh to brackish meteoric ground and surface waters from the BMC, have been analyzed to determine their major, trace element and stable isotopic (O, H, C, and B) compositions. Saline groundwaters (total dissolved solids = 22-45 g/L) are characterized by relatively high Na/Ca ratios compared to brines from the Canadian Shield and low Na/Cl_molar and δ¹¹B isotopic compositions (−2.5‰ to 11.1‰) compared to seawater. Although saline waters from the Canadian Shield commonly have oxygen and hydrogen isotopic compositions that plot to the left of the global meteoric water line, those from the BMC fall close to the water line. Fracture and vein carbonate minerals at the Restigouche deposit have restricted carbon isotopic compositions of around −5‰ to −6‰. The carbon isotopic compositions of the saline waters at the Restigouche deposit (+12‰ δ¹³C_DIC) are the result of fractionation of dissolved inorganic carbon by methanogenesis. We suggest that, unlike previous models for shield brines, the composition of saline waters in the BMC is best explained by prolonged water-rock reaction, with no requirement of precursor seawater. We suggest that elevated Br/Cl ratios of saline waters compared to seawater may be explained by differential uptake of Br and Cl during groundwater evolution through water-rock reaction.