Earth's habitability is closely tied to its late-stage accretion, during which impactors delivered the majority of life-essential volatiles. However, the nature of these final building blocks remains ...poorly constrained. Nickel (Ni) can be a useful tracer in characterizing this accretion as most Ni in the bulk silicate Earth (BSE) comes from the late-stage impactors. Here, we apply Ni stable isotope analysis to a large number of meteorites and terrestrial rocks, and find that the BSE has a lighter Ni isotopic composition compared to chondrites. Using first-principles calculations based on density functional theory, we show that core-mantle differentiation cannot produce the observed light Ni isotopic composition of the BSE. Rather, the sub-chondritic Ni isotopic signature was established during Earth's late-stage accretion, probably through the Moon-forming giant impact. We propose that a highly reduced sulfide-rich, Mercury-like body, whose mantle is characterized by light Ni isotopic composition, collided with and merged into the proto-Earth during the Moon-forming giant impact, producing the sub-chondritic Ni isotopic signature of the BSE, while delivering sulfur and probably other volatiles to the Earth.
The Mg isotopic compositions of 11 peridotite xenoliths and 21 HIMU-like intraplate basaltic lavas from New Zealand’s South Island and Antipodes Islands, respectively, provide new evidence bearing on ...the origin of continental intraplate alkaline basalts. Olivine and orthopyroxene in the peridotites display very limited Mg isotopic variations (−0.28‰ to −0.17‰ for olivine; −0.24‰ to −0.14‰ for orthopyroxene). The clinopyroxenes have Mg isotopic compositions (−0.37‰ to −0.10‰) slightly lighter than coexisting olivine and orthopyroxene. Calculated whole rock δ26Mg values of the peridotites vary from −0.27‰ to −0.16‰, falling within the normal mantle range (−0.25±0.07‰). By contrast, the δ26Mg values of continental intraplate lavas vary from −0.47‰ to −0.06‰ and negatively correlate with (Gd/Yb)N ratios. The difference in Mg isotopic systematics between peridotites and intraplate basalts reveals that the former cannot be the sole magma source to the latter, and a lithology with a lighter Mg isotopic composition such as carbonated eclogite must also have contributed. Thus, the intraplate lavas with HIMU-like source signatures likely represent the product of mixing of melts derived from peridotite and recycled carbonated eclogite. Magnesium isotopes may therefore be a useful tracer of mantle sources to intraplate volcanism.
The end-Permian mass extinction (EPME) was the most severe extinction event in the past 540 million years, and the Siberian Traps large igneous province (STLIP) is widely hypothesized to have been ...the primary trigger for the environmental catastrophe. The killing mechanisms depend critically on the nature of volatiles ejected during STLIP eruptions, initiating about 300 kyr before the extinction event, because the atmosphere is the primary interface between magmatism and extinction. Here we report Ni isotopes for Permian-Triassic sedimentary rocks from Arctic Canada. The δ
Ni data range from -1.09‰ to 0.35‰, and exhibit the lightest δ
Ni compositions ever reported for sedimentary rocks. Our results provide strong evidence for global dispersion and loading of Ni-rich aerosol particles into the Panthalassic Ocean. Our data demonstrate that environmental degradation had begun well before the extinction event and provide a link between global dispersion of Ni-rich aerosols, ocean chemistry changes, and the EPME.
The Ni isotopic systematics in banded iron formations (BIFs) potentially recorded the Ni isotopic composition of ancient seawater over Precambrian geological history. However, the utility of BIFs as ...proxies requires quantitative knowledge of how Ni isotopes fractionated as dissolved Ni was initially incorporated into iron-rich sediments and how diagenesis may have affected the Ni isotopic systematics. Here we report results of synthesis experiments to investigate the behavior of Ni isotopes during Ni coprecipitation with ferrihydrite and then transformation of ferrihydrite to hematite. Ferrihydrite coprecipitation experiments at neutral pH demonstrated that the dissolved Ni was variably heavier than coprecipitated Ni (likely a mixture of surface-adsorbed and structurally incorporated Ni), with the isotope fractionation becoming larger as the fraction of Ni associated with solid increased (Δ60/58Nisolution-solid=+0.08 to +0.50‰). Further experiments at lower pH (3.7–6.7), in which structurally incorporated Ni likely dominated in solids, documented a decrease in Δ60/58Nisolution-solid from +0.44‰ to −0.18‰ as the pH decreased. The negative value for Δ60/58Nisolution-solid at low pH indicates the enrichment of heavier isotopes in incorporated Ni relative to dissolved and adsorbed Ni, possibly as a result of the presence of a small amount of tetrahedral Ni2+ in addition to octahedral Ni2+ in the ferrihydrite structure. The results of the ferrihydrite experiments thus reflect equilibrium isotope fractionation between three pools of Ni, with δ60/58Ni values in the order of incorporated>dissolved>adsorbed. Hematite was synthesized by transformation of Ni-bearing ferrihydrite in aqueous solution at ∼100°C. A significant amount of Ni (up to 60%) was released (desorbed) from solids into solutions as pH dropped from ∼7 to 4.5–5.5 upon phase transformation. Rinsing of the synthesized hematite in 2M acetic acid released only very small amounts of Ni (<4% of total Ni, presumably surface-adsorbed) that were isotopically heavier (δ60/58Ni=+0.11±0.06‰) than the residues (presumably dominated by incorporated Ni), which had δ60/58Ni of −0.26±0.07‰. The preference of lighter isotopes for the incorporated Ni relative to the surface-adsorbed Ni after phase transformation (most had been released into solution) is probably due to distortion of NiO octahedra in the hematite structure, with weaker NiO bond strengths on average. Hence, the more variable Δ60/58Nisolution-solid values (−0.04 to +0.77‰) observed in hematite experiments most likely reflect thermodynamically driven Rayleigh fractionation, with incorporated Ni unavailable to exchange with dissolved Ni due to continuous reduction in size of the highly reactive surface pool of Ni, through which all solid-solution exchange must occur. Overall, the synthesized hematite was isotopically lighter than the ferrihydrite by ∼0.08‰ in δ60/58Ni, which is however within the current analytical uncertainties (±0.09‰). This implies that earliest diagenesis of BIFs results in very limited change in the isotopic composition of solid-associated Ni. Our experimental results, although conducted in a very simple system that differs from Archean seawater, represent an important step toward reconstruction of the Ni isotopic composition of ancient seawater from Ni isotopic signatures in BIFs.
The distribution of Mg isotopes in minerals is becoming increasingly relevant in Earth science. Usually, they are determined by dissolving mineral concentrates and, after purifying Mg with ion ...exchange resins, analysing the resulting solutions by TIMS or, most often, MC‐ICP‐MS. When applied to individual minerals, these methods are slow and prone to contamination from impurities in the concentrates, inconveniences that may be avoided using spot analysis techniques such as LA‐MC‐ICP‐MS or SIMS, albeit at the price of a large instrumental mass fractionation (IMF) and isobaric interferences, most prominent in the former. Here, we studied the potential of the multi‐collector SHRIMP II ion microprobe for measuring Mg isotopes in Fe‐Mg silicates and oxides. We found that, when corrected for the divergence of the Mg ion paths within the sample chamber caused by the Earth's magnetic field, the SHRIMP's IMF overwhelmingly depends on the mineral species, and the effects of variable chemical composition are negligible. We propose that the IMF is caused by the force constant difference, ∆F, between "hard" and "soft" bonds linking the ions of the studied element to the mineral lattice. Given that ∆F is a constant for each mineral species, we calculated IMF‐correction factors for the most common Mg‐bearing minerals. The thus‐calculated correction factors permit the analysis in the same session, and with reasonable accuracy (within ~ 0.3‰ of the δ26Mg determined by SN‐MC‐ICP‐MS analyses of concentrates), of samples from different mineral species, facilitating the application of Mg isotopes to terrestrial studies.
Key Points
Corrected for the Earth's magnetic field, the SHRIMP's IMF depends on the mineral species, not the chemical composition.
The IMF arises from the force constant difference between "hard" and "soft" bonds, which is characteristic of each mineral.
This permitted the calculation of IMF‐correction factors for most common Mg‐bearing minerals.
Subduction of carbonates and carbonated eclogites into the mantle plays an important role in transporting carbon into deep Earth. However, to what degree isotopic exchanges occur between carbonate ...and silicate during subduction remains unclear. Here we report Mg and O isotopic compositions for ultrahigh pressure metamorphic marbles and enclosed carbonated eclogites from China. These marbles include both calcite- and dolomite-rich examples and display similar O but distinct Mg isotopic signatures to their protoliths. Their δ(26)Mg values vary from -2.508 to -0.531‰, and negatively correlate with MgO/CaO ratios, unforeseen in sedimentary carbonates. Carbonated eclogites have extremely heavy δ(18)O (up to +21.1‰) and light δ(26)Mg values (down to -1.928‰ in garnet and -0.980‰ in pyroxene) compared with their protoliths. These unique Mg-O isotopic characteristics reflect differential isotopic exchange between eclogites and carbonates during subduction, making coupled Mg and O isotopic studies potential tools for tracing deep carbon recycling.
Magnesium isotopic compositions are reported for twenty‐four international geological reference materials including igneous, metamorphic and sedimentary rocks, as well as phlogopite and serpentine ...minerals. The long‐term reproducibility of Mg isotopic determination, based on 4‐year analyses of olivine and seawater samples, was ≤ 0.07‰ (2s) for δ26Mg and ≤ 0.05‰ (2s) for δ25Mg. Accuracy was tested by analysis of synthetic reference materials down to the quoted long‐term reproducibility. This comprehensive dataset, plus seawater data produced in the same laboratory, serves as a reference for quality assurance and inter‐laboratory comparison of high‐precision Mg isotopic data.
Les compositions isotopiques du magnésium sont fournies pour vingt‐quatre matériaux géologiques de référence internationaux, comprenant des roches ignées, métamorphiques et sédimentaires, ainsi qu'une phlogopite et des serpentines. La reproductibilité à long terme de la détermination isotopique du Mg, basée des analyses sur quatre ans d’échantillons d'olivine et d'eau de mer, était ≤ 0.07% (2s) pour δ26Mg et ≤ 0.05% (2s) pour δ25Mg. La précision a été testée par l'analyse de matériaux de référence synthétiques jusqu’à la reproductibilité à long terme indiquée. Cette base de données complète, ainsi que des données d'eau de mer produites dans le même laboratoire, servent de référence pour l'assurance qualité et la comparaison inter‐laboratoires de haute précision des données isotopiques du Mg.
Magnesium isotopic systematics has been increasingly used to trace the biogeochemical cycle of Mg in soil systems, and Fe oxides are the critical soil components that affect the geochemical ...behaviours of elements in soils. The role of Fe oxides in fractionating Mg isotopes, however, remains unclear. Here, Mg isotopic compositions are reported for typical Fe-Mn nodules (FMNs), surrounding soils, soil waters, and soil surface waters for a paddy soil profile, and stream waters, and rainwaters in southwestern China to improve our understanding of the processes that control the Mg isotopic compositions in soil systems. Further sequential extraction experiments are conducted to separate two pools of Mg in the FMNs and soils: structural Mg and exchange Mg. The FMNs (−1.39 to −1.58‰) are isotopically lighter than surrounding soils (−0.59 to −0.85‰) but heavier than soil waters (−1.59‰), and surrounding soils are isotopically lighter than parent granite (−0.25‰). The difference in Mg isotopic compositions between FMNs and surrounding soils reflects different sources of Mg in the mineral crystal structures. Structural Mg in surrounding soils is mainly from the chemical weathering of parent granite. By contrast, structural Mg in FMNs is from soil waters because of the frequently repeated dissolution and precipitation of Fe oxides under alternating redox conditions. Enrichment of heavy Mg isotopes in the FMNs relative to soil waters results from preferential incorporation of 26Mg via Mg2+ substitution for Fe3+ in goethite. Given that the exchangeable Mg (−1.62 to −1.91‰) is significantly enriched in light Mg isotopes, the lighter Mg isotopic compositions in surrounding soils relative to their parent granite can be explained by the retention of light Mg isotopes in exchangeable sites of Mg-depleted minerals (kaolinite). Exchangeable Mg in FMNs (−1.79 to −2.15‰) is also shown to be enriched in light Mg isotopes. These light isotopic compositions of exchangeable Mg can be explained by a combination of carbonate contribution and isotope fractionation processes on the soil exchange fractions. Ion-exchange processes preferentially remove heavy Mg isotopes from soil minerals, leaving soil exchange fractions hosting light Mg isotopes. Additionally, river waters draining carbonates contributes light Mg isotopes to soil exchangeable fractions. Our study demonstrates that the development of Mg-depleted clay minerals and Fe oxides can considerably lower the soil δ26Mg values, highlighting the major roles of these two soil minerals in controlling soil Mg isotopic compositions.
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