Ni Isotope Fractionation Research Articles

The behavior of nickel isotopes during mantle melting

Mantle-derived mafic rocks show relatively large variations in nickel (Ni) isotopes and mostly isotopically light compared to the bulk silicate Earth (BSE). Whether this signature is due to the source heterogeneity or controlled by melting processes has been a debatable issue. Here, we analyzed Ni isotopic compositions of 18 intraplate basalts from the Sabzevar region, northern Iran and 16 serpentinized peridotites from Cyprus and South China. The Sabzevar basalts were likely sourced from a sulfur-free/barren mantle domain with recycled pyroxenites involved, manifested by their high Cu contents (127–265 ppm), FeT/Mn (58.2–77.4) and Zn/FeT ratios (11.9–16.4). The Ni isotopic compositions of the Sabzevar basalts (average δ60/58Ni = +0.15 ± 0.08 ‰; 2SD) are slightly heavier than the BSE value (average δ60/58Ni = +0.11 ± 0.06 ‰; 2SD), consistent with equilibrium Ni isotope fractionation during mantle silicate melting as predicted by ionic model calculations. Our new data of serpentinized peridotites (average δ60/58Ni = +0.16 ± 0.06 ‰; 2SD), together with previously reported data for oceanic sediments and metabasalts, suggest that recycled lithologies have mantle-like or relatively heavy Ni isotopic compositions. Thus, the isotopically light Ni in mafic rocks is unlikely to be caused by crustal recycling-induced mantle heterogeneity. Rather, the light Ni isotopic signature is caused by the dissolution of recycled sulfides into the mantle melts. We suggest two sulfide dissolution models (“dissolve and go” and “dissolve and equilibrium”) to describe the δ60/58Ni and Cu systematics in terrestrial basalts. In both models, the initial sulfide content (Sinitial) and oxygen fugacity (fO2) exert a major control on Ni isotopic compositions of resulting melts. These two parameters vary among different geological settings. Basalts derived from mantle sources with high Sinitial and fO2 are characterized by light Ni isotopic compositions, whereas basalts sourced from sulfur-free/barren and reduced mantle domains are likely characterized by heavy Ni isotopic compositions, aligning with the characteristic observed in natural samples.

A benthic source of isotopically heavy Ni from continental margins and implications for global ocean Ni isotope mass balance

Nickel (Ni) is a bio-essential element for phytoplankton in the modern oceans, and yet, the global ocean mass balance of Ni has puzzled scientists for decades. Many estimates of total Ni output flux are larger than the total Ni input flux to the ocean. The measurement of Ni stable isotopes (δ60Ni) in earth materials may inform our understanding of ocean biogeochemistry and redox conditions in both the modern ocean and the geological past. An ocean Ni stable isotope imbalance has also concerned scientists, with a global ocean δ60Ni of +1.4 ‰ which is notably heavier than the major source of Ni to the oceans from rivers. Recent efforts to resolve Ni and δ60Ni imbalances have focused on the role which manganese (Mn) oxides play in marine Ni cycling, both in the water column and in marine sediments. Manganese oxide rich marine sediments can supply isotopically heavy dissolved Ni to porewater fluids and seawater, but the source of the isotopically heavy porewater Ni remains unclear. Here we present porewater trace metal concentrations and δ60Ni from two sites from the continental margin off southern California. Porewater Ni concentrations are up to roughly 100-fold higher than deep seawater concentrations (∼1 μM compared to 10 nM, respectively). Porewater δ60Ni is near 0 ‰ in the subsurface region where Ni concentrations are highest, suggesting dissolution of lithogenic material from sediments. Porewater δ60Ni increases dramatically towards the sediment-water interface with values of up to +2.66 ‰, which to our knowledge are the heaviest Ni isotope compositions ever reported for natural materials. Measurements of porewater and sediment Ni and Mn concentrations suggest that Ni is released to porewaters in the Mn-reducing zone, and then removed by newly precipitated Mn oxides as Mn and Ni move towards the oxygenated zone of sediments. A simple Rayleigh model suggests a Ni isotope fractionation of factor of -0.61 ‰ for Ni sorption onto Mn oxides, while a diffusion-reaction model suggests a Ni isotope fractionation of -1.80 to -0.96 ‰, always with preferential adsorption of lighter Ni isotopes. This flux of Ni toward the sediment-water interface, coupled with preferential removal of lighter Ni isotopes in marine sediments, provides evidence supporting an isotopically heavy benthic source of new Ni to the oceans, which we estimate has a magnitude of 0.65 × 108 to 1.66 × 108 moles/year. We find that a benthic flux such as measured here over just 0.27-2.70 % of the ocean seafloor could mix with the river δ60Ni of +0.8 ‰ to achieve the observed deep ocean δ60Ni of +1.4 ‰. This study reveals the similarities in how rivers and continental margin sediments supply heavy Ni isotopes to the ocean. In both continental rivers and margin sediments, terrigenous Ni is released with an δ60Ni near 0.1 ‰, but lighter isotopes are captured by precipitation onto oxides so that the dissolved flux to the ocean is isotopically heavier than the terrigenous material from which Ni was released, and in the case of margin sediments may be heavier than even bulk seawater δ60Ni. We thus recognize continental margin sediments with active Mn cycling as a ‘new’ source of isotopically heavy dissolved Ni to the ocean.

Nickel isotope fractionation during intense weathering of basalt: Implications for Ni output from continental weathering

Nickel isotope fractionation during continental weathering is not well constrained due to the limited research on Ni isotopes during weathering of mafic rocks such as basalt. This study investigated two weathering profiles (strongly and extremely weathered) derived on basalts in tropical China. The strongly weathered profile was dominated by kaolinite and divided into three layers with depth (soil, saprolite and rock). The extremely weathered profile was mainly composed of Fe- and Al-(oxyhydr)oxides and kaolinite, and included a special ferricrust layer, iron-rich duricrust with ferruginous nodules, between the soil and saprolite. We analysed the Ni speciation (e.g. Fe-oxide phases and residual phase) and isotopic composition of soil, ferricrust, saprolite, rock and water samples in the weathering profiles. Pore water and groundwater were collected underlying the extremely weathered profile, and the size-distribution of particulate Ni (1 kDa–450 nm) in water samples was analysed.Nickel in both profiles was mainly hosted in the residual phase (>71% of total Ni) which included primarily secondary silicates and Al-(oxyhydr)oxides. Weathering intensity controlled the mineral compositions and physico-chemical properties of the weathering profiles as well as Ni isotopic variations. Nickel redistribution (∼30%) and isotope fractionation (Δ60Niregolith-rock = −0.08‰ to 0.07‰) were slight in the strongly weathered profile due to the dominance of secondary silicates in Ni speciation. For the extremely weathered profile, significant Ni depletion (∼67%) and isotope fractionation (Δ60Niregolith-rock = −0.25‰ to 0.32‰) were related to the transformation of secondary silicates into Fe- and Al-(oxyhydr)oxides during lateritic weathering. The positive correlation of δ60Ni and Ni concentrations indicated that Ni isotope fractionation was controlled by Ni adsorption and incorporation of secondary minerals. In the lower saprolite, the calculated isotope fractionation between Ni remaining in the profile and leaching to the solution (Δ60Nisolid-solution = −0.47‰) indicated that Ni isotope fractionation was mainly controlled by the preferential adsorption of isotopically light Ni on Fe- and Al-(oxyhydr)oxides. However, in the soil and ferricrust, acidic pH (∼5.0) might reduce Δ60Nisolid-solution to −0.28‰ by releasing the isotopically light adsorbed Ni, leading to the predominance of incorporated Ni in Fe- and Al-(oxyhydr)oxides. Specifically, the highest δ60Ni value (0.27 ± 0.05‰) and Ni concentration in the upper saprolite indicated that the isotopically heavy Ni leached from the soil and ferricrust was scavenged by precipitating secondary silicates. Finally, the estimated isotopic composition of Ni leached from the extremely weathered profile (δ60Nioutput = 0.02 ± 0.15‰) was close to that of pore water (0.06 ± 0.01‰) where Ni was mainly present in the particulate phase. Compared with groundwater (δ60Ni = 0.33 ± 0.04‰) dominated by dissolved Ni, Ni output from the profile likely consisted of isotopically heavy dissolved Ni and light particulate Ni. Overall, this study provides a comprehensive understanding of Ni redistribution and speciation in basalt weathering profiles, highlighting the significance of weathering intensity in controlling Ni isotope fractionation and its implications for Ni outputs from continental weathering.

Nickel isotope fractionation factors between silicate minerals and melt

Quantitative knowledge of inter-mineral and mineral–melt isotopic fractionation factors is of crucial importance to interpret variability of mass-dependent isotope ratios in terrestrial rocks and meteorites. The isotopic composition of Ni shows great promise as a tracer of planetary differentiation, but the lack of solid constraints on the sign and magnitude of equilibrium Ni isotope fractionation factors hampers its further development. We address this issue by studying Ni isotope fractionation between silicate minerals and melt using a combination of carefully selected natural samples and olivine crystallisation experiments.Mineral separates from xenoliths are used to determine the Ni isotope fractionation behaviour of pyroxenes, spinel, and garnet relative to olivine. Nickel isotope fractionation between olivine and orthopyroxene is negligible, whereas clinopyroxene shows large variability that does not reflect equilibrium. Spinel has a clear affinity for the heavier isotopes of Ni relative to olivine; garnet has the lightest Ni isotope composition at equilibrium. The key parameter that dictates the sign and magnitude of Ni isotope fractionation during partial melting and crystallisation, however, is the olivine–melt fractionation factor. We present data for a natural basalt that contains olivine phenocrysts in chemical equilibrium with the host glass, and furthermore employ olivine crystallisation experiments in a 1-atmopshere furnace. Although the experiments invariably suffer from Ni loss to the Pt wire used to suspend the experiments in the furnace, these effects were carefully considered and quantified. The experiments limit the olivine–melt Ni isotope fractionation factor to lie between zero and slightly negative, which agrees very well with the natural datum, and we derive a best estimate value for Δ60/58NiOl–melt of −0.142 ± 0.031 ‰ at 1000 K. Hence, olivine has a subtle but clearly resolvable affinity for the lighter isotopes of Ni relative to basaltic melt. The implication is that equilibrium partial melts of mantle peridotite should have a slightly heavier Ni isotope composition than their source. Available Ni isotope data for mid-ocean ridge basalts show some overlap with modelled partial melts but extend to notably lighter Ni isotope compositions, potentially reflecting source heterogeneity or diffusive fractionation during melt transport.

Nickel mass balance and isotopic records in a serpentinic weathering profile: Implications on the continental Ni budget

During serpentinite rock weathering, Ni is concentrated in the regolith owing to residual and secondary enrichment, forming a Ni-rich deposit under tropical conditions. This study presents geochemical, isotopic, and mineralogical data for Ni from the Serra do Puma Complex (SPC) weathering profile in Carajas (Brazil). The Ni fluxes, redistribution, and Ni isotopic fractionation magnitudes were quantified along the entire weathering profile: rock, saprock, lower and upper saprolite, and limonite. The results show that chlorite and serpentine are the primary Ni-scavenging phases in saprolite, whereas Fe oxyhydroxides are the main Ni-hosting minerals in the limonite unit. The mass balance model confirmed a global Ni gain at the weathered profile scale, with larger Ni enrichment in the upper saprolite. The isotopic dataset obtained in this study contributes greatly to the knowledge of the current Ni cycle on regional and global scales. The detailed insights into Ni isotopes in the SPC, coupled with chemical and mineralogical composition, allow for the first time a Ni mass balance and Ni isotopic values for an entire weathering profile in an Amazonian context. The Ni isotopic profile agrees with the preferential retention of light Ni isotopes in the residual material relative to the parent rock during weathering processes, with the Δ60Ni limonite-saprock of −0.72‰. In the SPC, major isotopic fractionation was notably recorded during saprolitization, while limonitization was accompanied by an overall Ni loss without significant Ni isotopic fractionation. As already observed in previous studies, heavier Ni isotopes, preferentially leached during weathering processes, can be further mobilized downward and lost from the profile or incorporated in secondary minerals in the saprolite and limonite sections. The Ni isotopic mass balance model indicated that the Ni loss from the weathered serpentinite profile was isotopically heavy, in agreement with the isotopically heavier composition of the dissolved load of Amazonian rivers. The isotopically light Ni pool, associated with the significant Ni gain encountered in the upper saprolite, is notable in the SPC weathering profile and confirms the existence of a light Ni isotope reservoir in the continent.

Nickel Isotope Fractionation During Magmatic Differentiation

AbstractThe behavior of nickel (Ni) isotopes during magmatic differentiation is not adequately explored. Here, we find that tholeiitic rocks in the Kīlauea Iki (KI) lava lake, Hawai'i, show progressively lighter Ni isotopic compositions with increasing magmatic differentiation, whereas calc‐alkaline rocks from the thick Kamchatka arc (30–45 km), located at the convergent boundary of the Eurasian and Pacific plates show increasing Ni isotope values as MgO and Ni decrease. Forty‐three global intermediate‐felsic continental igneous rocks analyzed in this study display large Ni isotopic variations, with the Eoarchean samples having light Ni isotopic compositions that fall in the trend defined by the KI lavas, and the post‐Eoarchean samples showing systematically heavier Ni isotopic compositions overlapping those of Kamchatka arc rocks. The isotopic dichotomy results from the crystallization of isotopically heavy magnetite during low‐pressure differentiation of KI lavas, whereas the participation of sulfide separation that removes isotopically light Ni during high‐pressure differentiation of magmas traversing thick continental crust. Combined with Rhyolite‐MELTS and sulfur concentration at sulfide saturation simulations, we demonstrate that the Ni isotope fractionation during magmatic differentiation is mainly controlled by the tempo of magnetite crystallization and sulfide formation, which is a function of pressure, oxygen fugacity, and water activity. High‐pressure calc‐alkaline differentiation usually suppresses magnetite crystallization while stabilizing sulfide, leading to heavy Ni isotopic compositions for the evolved magmas, significantly deviating from the low‐pressure fractionation trend seen in the KI lavas. Ni isotopes can be used in the future as a tracer of magmatic differentiation and processes of continent formation and differentiation.

Nickel isotope ratios trace the process of sulfide-silicate liquid immiscibility during magmatic differentiation

Sulfide-silicate liquid immiscibility (i.e., the coexistence of two liquid phases in equilibrium) plays a significant role in the processes of planetary-scale differentiation, the formation of Earth’s crust, and the genesis of magmatic sulfide ore deposits. The stable isotope geochemistry of nickel can potentially take advantage of fractionation signals produced in magmatic systems where immiscible sulfide and silicate liquid have equilibrated. Variations in Ni isotope ratio beyond those found in sulfide-undersaturated basaltic rocks can provide a hallmark of fractionation processes that control chalcophile and siderophile element abundances. We report high-precision Ni isotope ratio data for a stratigraphically-controlled sequence of Siberian Trap basalts in a volcanic edifice centered over the world’s largest concentration of magmatic sulfide ore deposits at Noril’sk-Talnakh, Russia. Nickel isotope ratios (δ60Ni (‰) = [(60Ni/58Ni)sample/(60Ni/58Ni)SRM986-1] × 1000) in the basaltic rocks range from +0.07 ± 0.02‰ to +1.00 ± 0.04‰ and correlate negatively with Ni and precious metal abundance levels, indicating extensive Ni isotope fractionation due to sulfide saturation of the silicate magma. A Rayleigh fractionation model fits the observed results and enables a precise estimate of the fractionation factor (α) between sulfide liquid and silicate melt to be 0.99965 (i.e., 103*lnα = −0.35). Our study illustrates the application of Ni isotope ratio data in understanding sulfide-silicate liquid immiscibility and the broader implications with respect to the formation of the largest known magmatic sulfide ore deposits in association with the Siberian Trap.

Mass-dependent nickel isotopic variations in achondrites and lunar rocks

We present high-precision mass-dependent nickel isotopic data for a comprehensive suite of achondrites and lunar rocks, providing key insights into the early planetary differentiation and Earth-Moon system formation. The primitive achondrites display high Ni contents and invariant Ni isotopic compositions. Incomplete core-mantle differentiation in primitive achondrite parent bodies resulted in the retention of metal in the mantle, which dominated the Ni budget and accounted for the bulk chondritic Ni isotopic values. The highly reduced differentiated achondrites, aubrites and an ungrouped achondrite (NWA 8409), have variable, and extremely light Ni isotopic compositions. Acid leaching experiments demonstrate that the sulfides are a significant host of light Ni isotopes in aubrites. The most extreme Ni isotope values of aubrites may be due to large Ni isotope fractionation accompanied by silicate-sulfide-metal separation during differentiation of the parent bodies, and subsequent global disruptive collision and reassembly with variably high proportions of sulfides enriched in the mantle. The howardite-eucrite-diogenite (HED) meteorites show Ni isotopic variations that are positively correlated with Ni/Co ratios, a feature that cannot be produced by igneous differentiation. Late accretion of high-Ni and high-Ni/Co chondritic materials after core formation of their likely parent body, Vesta, could have accounted for this correlation. Thus, the primitive silicate mantle of Vesta may have sub-chondritic Ni isotopic compositions, implying possible Ni isotope fractionation during core-mantle differentiation of small planetary bodies. The lunar breccia meteorites have homogenously chondritic Ni isotope values, together with their high Ni/Co of bulk rock and metals therein, suggesting impact contamination. Lunar basalt meteorites have low Ni/Co ratios and are systematically isotopically lighter than the breccias, displaying a positive correlation between Ni isotope value and Ni/Co ratio, as that seen in the HEDs. Therefore, the Ni isotopic systematics in lunar rocks also indicates the effect of late accretion, with the primitive lunar mantle having sub-chondritic Ni isotope values. This implies that the Moon-forming impactor, Theia, was likely an aubrite-like differentiated planetary body whose mantle was enriched in light Ni isotopes. We suggest that there was significant Ni isotope fractionation between core and mantle during differentiation of early formed small planetary bodies, but this signature can be obscured by late accretion in the bulk achondrite records.

Sulfate-driven anaerobic oxidation of methane inferred from trace-element chemistry and nickel isotopes of pyrite

In marine sediments, formation of pyrite (FeS2) is promoted by both organoclastic sulfate reduction (OSR) and sulfate-driven anaerobic oxidation of methane (SD-AOM), and these two microbial pathways might yield differing patterns of sulfur and nickel isotopic fractionation and trace-element enrichment. To better understand these pathways, we analyzed the geochemistry of authigenic pyrite aggregates from the Upper Quaternary of the western Andaman Sea (International Ocean Discovery Program, Expedition 353, Site U1447A). 34S-enriched pyrites (to ∼+41‰) in Unit Ib are interpreted as products of SD-AOM, and their enrichments in Co and Ni and low δ60Ni values (−0.63‰ to −0.09‰) may represent diagnostic signatures of a reverse-methanogenesis microbial pathway. In contrast, 34S-depleted pyrites (∼–46‰ to –38‰) in both Units I and II are consistent with sulfide formation through early diagenetic remineralization of organic matter, and their relative enrichments in Cu and V and heavier δ60Ni values (to +0.41‰) may be diagnostic of this alternative microbial pathway. This study reports for the first time a fractionation of Ni isotopes linked to preferential uptake of isotopically light Ni by the enzyme Mcr (M reductase enzyme) in reverse methanogenesis and demonstrates that OSR- and SD-AOM-associated pyrites are potentially distinguishable on the basis of their δ60Ni compositions. Such geochemical signatures may prove useful in determining processes of pyrite formation in paleo-depositional systems and in identifying ancient methane emission events.

The effect of alkalinity on Ni–O bond length in silicate glasses: Implications for Ni isotope geochemistry

Equilibrium mass-dependent (“stable”) isotopic fractionation of an element during magmatic processes is driven by a contrast in bonding environment between minerals and silicate melt, which is expressed as an isotopic fractionation factor. A quantitative understanding of such isotopic fractionation factors is vital to interpret observed isotopic variations in magmatic rocks. It is well known that the local environment and the bond strength of an element dictate the sign and magnitude of isotopic fractionation between minerals, but it is uncertain how the structure and chemical composition of a silicate melt can affect mineral-melt isotopic fractionation factors. To explore this, we studied the coordination environment of nickel (Ni) in different silicate glasses using extended X-ray absorption fine structure (EXAFS) measurements at the German synchrotron X-ray source (DESY).We determined Ni–O bond lengths in a suite of synthetic but near-natural silicate glasses using EXAFS and found that the former vary systematically with melt alkalinity, which is best described by the parameter ln[1 + (Na + K)/Ca]. With increasing melt alkalinity, Ni occupies more IV-fold coordinated sites, which are associated with a shorter Ni–O bond length. Next, we use the ionic model, which allows to predict isotopic fractionation factors based on the difference in bond length between two phases. We find that more alkaline melts have a stronger preference for the heavier isotopes of Ni than less alkaline melts. This implies that the magnitude of mineral-melt Ni isotope fractionation factors, for instance between olivine and melt, will depend on the alkalinity of the melt. At magmatic temperatures, however, the variation in fractionation factors caused by melt alkalinity will rarely exceed 0.05 ‰ and is thus mostly negligible, in particular in the realm of basaltic melt compositions. Nevertheless, the relationship between melt alkalinity and fractionation factor reported here can be used to extrapolate empirical data for mineral-melt Ni isotope fractionation factors, once such data become available, to the full range of magma compositions on Earth and other Solar System bodies.

Nickel isotopic composition of the upper continental crust

Establishing the nickel (Ni) isotopic composition of the upper continental crust (UCC) is crucial for using the Ni isotope system to trace biogeochemical processes and understand crust-mantle interactions. This study reports the Ni isotopic composition of eighty-four well-characterized upper crustal samples, including granites, granodiorites, tonalite-trondhjemite-granodiorite (TTG), loess, river sediments and glacial diamictites, to constrain the Ni isotopic composition of the UCC. Significant variations in δ60Ni are revealed for I-type (0.02–0.26‰), A-type (−0.05–0.08‰) and S-type (0.08–0.36‰) granites for the first time. These Ni isotopic variations are attributed to magmatic differentiation for I- and A- type granites and source heterogeneity for S-type granites. The δ60Ni values of fine-grained clastic sediments (including loess, river sediments and glacial diamictites) range from −0.01‰ to 0.23‰. Such δ60Ni variations cannot be explained by Ni isotopic fractionation during chemical weathering because there are no clear correlations between δ60Ni and Ni/Al2O3, or the chemical index of alteration (CIA). Instead, the δ60Ni variations in fine-grained clastic sediments are likely inherited from source rocks. The δ60Ni values of our samples for 3.2–3.5 Ga TTGs (0.00–0.13‰), 2.4–2.5 Ga TTGs (0.04–0.13‰) and < 0.4 Ga granites (excluding S-type granites) are statistically indistinguishable (P < 0.05, student's t-test), implying limited variation of δ60Ni in the felsic igneous UCC since 3.5 Ga. Similarly, the δ60Ni values of glacial diamictites suggest insignificant temporal variation in the weathered UCC since 2.4 Ga. The data gathered in this study combined with literature data yields an arithmetic mean δ60Ni value of 0.12 ± 0.15‰ (2SD) for the UCC (ranging from −0.07‰ to 0.36‰). And the weighted average δ60Ni is estimated to be 0.07 ± 0.10‰ (2SD) or 0.11 ± 0.09‰ (2SD) depending on the assumed δ60Ni of the metamorphic rocks. Thus, a lithology-weighted average δ60Ni needs to be further determined by future studies when the δ60Ni values of metamorphosed sedimentary rocks in the UCC are constrained.

Nickel Isotope Fractionation As an Indicator of Ni Sulfide Precipitation Associated with Microbially Mediated Sulfate Reduction.

Microbially mediated sulfate reduction is a promising cost-effective and sustainable process utilized in permeable reactive barriers (PRB) and constructed wetlands to treat mine wastewater. Laboratory batch experiments were performed to evaluate nickel (Ni) isotope fractionation associated with precipitation of Ni-sulfides in the presence of the sulfate-reducing bacterium (SRB) Desulfovibrio desulfuricansT (DSM-642). Precipitates were collected anaerobically and characterized by synchrotron powder X-ray diffraction (PXRD), scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM-EDS), and transmission electron microscopy (TEM). Solid-phase analyses showed that the precipitates associated with bacteria attached to the serum bottle walls were characterized by enhanced size and crystallinity. Lighter Ni isotopes were preferentially concentrated in the solid phase, whereas the solution was enriched in heavier Ni isotopes compared to the input solution. This fractionation pattern was consistent with closed-system equilibrium isotope fractionation, yielding a fractionation factor of Δ60Nisolid-aq = -1.99‰. The Ni isotope fractionation measured in this study indicates multiple Ni reaction mechanisms occurring in the complex SRB-Ni system. The results from this study offer insights into Ni isotope fractionation during interaction with SRB and provide a foundation for the characterization and development of Ni stable isotopes as tracers in environmental applications.

Nickel isotope fractionation in komatiites and associated sulfides in the hart deposit, Late Archean Abitibi Greenstone Belt, Canada

Extremely light and highly variable δ60Ni values have been observed in komatiite-associated magmatic sulfides in recent studies. In this study, we examine the mechanisms of Ni isotope fractionation between silicate and sulfide liquids in the Hart komatiite-associated Fe-Ni-Cu-sulfide system. We assess the petrogenetic significance of these mechanisms using Ni isotope and concentration data. The concentration of Ni in bulk rock varies from 774 to 2690 ppm in komatiite samples with no sulfide minerals to 8380–39,300 ppm in samples almost entirely consisting of sulfide minerals. The δ60Ni values vary from +0.14‰ in komatiite samples with no sulfide minerals to −1.06‰ in samples dominantly consisting of sulfide minerals. A theoretical model of fractionation between the komatiitic lava and sulfide xenomelt with nickel isotope exchange followed by fractional crystallization during crystallization of the sulfide melt can produce a range of δ60Ni values from +0.17‰ to −1.02‰ in sulfide-rich rocks depending on the extent of fractional crystallization and the amount of trapped melt between the sulfide mineral grains, which corresponds well with the range of values observed in these rocks. This proposed model requires fractionation of Ni isotopes between sulfide liquid and the earliest formed sulfide crystals during crystallization. Effects of later crystallization during peritectic reactions and subsolidus exsolution could be tested by in situ measurements of Ni isotopes in different textural varieties of pentlandite that formed over a large range of temperatures during cooling.

The essential bioactive role of nickel in the oceans: Evidence from nickel isotopes

The role of nickel (Ni) in ocean biogeochemical cycles is both under-studied and controversial. Strong correlations between Ni and organic carbon in modern and ancient marine sediments suggest a prominent biogeochemical role over a substantial portion of Earth history. Addition of Ni to culturing and seawater incubation experiments produces strong responses in terms of cell growth, particularly of nitrogen-fixing organisms. But the implied limiting role for phytoplankton growth is inconsistent with observations in the real ocean, specifically that photic zone Ni concentrations never descend to the very low values that characterise other bioactive, and often bio-limiting, metals like iron. These two observations can be reconciled if a large portion of the total dissolved Ni present in open-ocean surface waters is not bio-available on short timescales. Here we present new Ni concentration and stable isotope data from the GEOVIDE transect in the North Atlantic. We interpret these new data in the light of the growing database for Ni stable isotopes in the modern ocean, with implications for the biogeochemical importance of Ni.In the new North Atlantic dataset, the lowest Ni concentrations (1.8-2.6 nmol/L) and highest δ60Ni (up to +1.67‰) are associated with low nitrate, south of the subarctic front (SAF). By contrast, stations at latitudes north of the SAF, with higher surface nitrate, show very subdued variation in Ni concentrations throughout the entire depth of the water column (3.6±0.3 nmol/L, mean and 2SD), and no variation in δ60Ni beyond the narrow global deep-ocean range (+1.33±0.13 ‰). These North Atlantic Ni isotope data also show relationships with nitrogen isotope effects, observed in the same samples, that are suggestive of a link between Ni utilisation, isotope fractionation and nitrogen fixation.The global dataset, including the new data presented here, reveals a biogeochemical divide with Ni isotope fractionation only occurring in low latitude surface waters. A simple observationally constrained three-dimensional model of Ni cycling suggests that the creation of this isotopically heavy, Ni-poor, end-member, together with the physical circulation and remineralisation at depth, can explain the global Ni-δ60Ni systematics. Taken together, these findings hint at Ni-N co-limitation in the modern ocean. We advocate for more extensive and detailed culturing/incubation studies of this neglected metal in order to elucidate its potentially crucial biogeochemical role.

Nickel isotope fractionation during precipitation of Ni secondary minerals and synchrotron-based analysis of the precipitates

The determination of fractionation factors associated with important biogeochemical processes controlling Ni availability in the environment is necessary to confidently use Ni isotope signatures as tracers in environmental studies. In this paper we present experimental results on Ni isotope fractionation during the precipitation of Ni secondary minerals (Ni hydroxide, Ni hydroxycarbonate and Ni sulfide minerals) that can control Ni fate and mobility in settings where high dissolved Ni(II) concentrations may pose a serious threat to the environment. Results show a preferential partition of lighter isotopes into the solid phase with associated fractionation factors ε of −0.40‰ ± 0.04‰, −0.50‰ ± 0.02‰, and −0.73‰ ± 0.08‰ relative to the hydroxide, carbonate and sulfide systems, respectively. Early kinetically induced isotope fractionation, followed by possible re-equilibration with the reacting solutions was suggested for the three investigated systems. Distortions of Ni-O bonds in the octahedra constituting the structures of Ni hydroxide and hydroxycarbonate minerals could have also contributed to the isotopic fractionation measured in these systems. In contrast, for Ni sulfide system, the magnitude of Ni isotope fractionation seemed to be determined by water and sulfide ligands exchange in solution prior to mineral nucleation. Although there is insufficient evidence to determine whether complete equilibrium occurred in the three studied systems, the fractionation factors reported in this study can provide useful indicators of Ni isotope fractionation associated with fast precipitation of secondary minerals involved in the sequestration of Ni from contaminated environments. These findings also contribute to the characterization of Ni isotope systematics which is still in the early stages of development.

δ60Ni and δ13C Composition of Serpentinites and Carbonates of the Tekirova Ophiolite, Turkey, and Meatiq Ophiolite, Egypt

Nickel isotope fractionation patterns in continental ultramafic environments generally show a depletion of δ60Ni in weathered rocks and an enrichment in bedrock samples. The present study focuses on stable Ni isotope fractionation patterns in carbonate-rich, ultramafic ophiolite samples with concomitant fluids at an active serpentinization site in southwestern Turkey, with a comparison to results from an inactive serpentinization site in the Eastern Desert of Egypt with carbonate-rich samples. All solid phase data from the inactive serpentinization area are consistent with previously reported values from serpentinites, whereas the solid precipitates in the active area (SW Turkey) give values slightly heavier than previously reported data. However, the Ni isotopic signatures in the active serpentinization system likely reflect the scavenging of light Ni by iron oxide and carbonate precipitation, as has been previously demonstrated in laboratory coprecipitation experiments. It is also possible that the active system results resemble previous laboratory experimental results that show a relatively strong initial fractionation between fluids and solids, which then diminishes with time due to aging of the precipitates.

Iron Isotope Compositions of Coexisting Sulfide and Silicate Minerals in Sudbury-Type Ores from the Jinchuan Ni-Cu Sulfide Deposit: A Perspective on Possible Core-Mantle Iron Isotope Fractionation

Many studies have shown that the average iron (Fe) isotope compositions of mantle-derived rocks, mantle peridotite and model mantle are close to those of chondrites. Therefore, it is considered that chondrite values represent the bulk Earth Fe isotope composition. However, this is a brave assumption because nearly 90% of Fe of the Earth is in the core, where its Fe isotope composition is unknown, but it is required to construct bulk Earth Fe isotope composition. We approach the problem by assuming that the Earth’s core separation can be approximated in terms of the Sudbury-type Ni-Cu sulfide mineralization, where sulfide-saturated mafic magmas segregate into immiscible sulfide liquid and silicate liquid. Their density/buoyancy controlled stratification and solidification produced net-textured ores above massive ores and below disseminated ores. The coexisting sulfide minerals (pyrrhotite (Po) &gt; pentlandite (Pn) &gt; chalcopyrite (Cp)) and silicate minerals (olivine (Ol) &gt; orthopyroxene (Opx) &gt; clinopyroxene (Cpx)) are expected to hold messages on Fe isotope fractionation between the two liquids before their solidification. We studied the net-textured ores of the Sudbury-type Jinchuan Ni-Cu sulfide deposit. The sulfide minerals show varying δ56Fe values (−1.37–−0.74‰ (Po) &lt; 0.09–0.56‰ (Cp) &lt; 0.53–1.05‰ (Pn)), but silicate minerals (Ol, Opx, and Cpx) have δ56Fe values close to chondrites (δ56Fe = −0.01 ± 0.01‰). The heavy δ56Fe value (0.52–0.60‰) of serpentines may reflect Fe isotopes exchange with the coexisting pyrrhotite with light δ56Fe. We obtained an equilibrium fractionation factor of Δ56Fesilicate-sulfide ≈ 0.51‰ between reconstructed silicate liquid (δ56Fe ≈ 0.21‰) and sulfide liquid (δ56Fe ≈ −0.30‰), or Δ56Fesilicate-sulfide ≈ 0.36‰ between the weighted mean bulk-silicate minerals (δ56Fe[0.70ol,0.25opx,0.05cpx] = 0.06‰) with weighted mean bulk-sulfide minerals (δ56Fe ≈ −0.30‰). Our study indicates that significant Fe isotope fractionation does take place between silicate and sulfide liquids during the Sudbury-type sulfide mineralization. We hypothesize that significant iron isotope fractionation must have taken place during core–mantle segregation, and the bulk Earth may have lighter Fe isotope composition than chondrites although Fe isotope analysis on experimental sulfide-silicate liquids produced under the varying mantle depth conditions is needed to test our results. We advocate the importance of further research on the subject. Given the close Fe-Ni association in the magmatic mineralization and the majority of the Earth’s Ni is also in the core, we infer that Ni isotope fractionation must also have taken place during the core separation that needs attention.

Nickel isotope fractionation as a function of carbonate growth rate during Ni coprecipitation with calcite

The fractionation of Ni isotopes during Ni coprecipitation with calcite was measured at pH = 6.2 and pCO2 = 1 atm as a function of calcite growth rate. Light Ni isotopes are preferentially incorporated into calcite during its coprecipitation, which is likely due to a longer NiO bond length in calcite compared to that of the Ni aquo complex. The extent of Ni isotope fractionation between Ni in the solid and the aqueous fluid phase increases from −0.3 to −0.9‰ as the calcite growth rate slows from 10−7.3 to 10−8..3 mol m−2 s−1. This behaviour can be attributed to the strong hydration of the Ni2+ aqueous ion. As mineral growth rates depend strongly on the degree of supersaturation of the fluid relative to the mineral, the results of this study suggest that the Ni isotopic composition of natural calcite can potentially provide insight into the saturation state of seawater with respect to calcite at the time that this mineral formed. In addition, calculations based on our results suggest that the incorporation of Ni into calcite could be a significant sink of light Ni in the ocean.

Nickel isotopes and rare earth elements systematics in marine hydrogenetic and hydrothermal ferromanganese deposits

Attention is now being given to Ni isotope systematics in hydrogenetic marine ferromanganese (Fe-Mn) crusts as paleoceanographic proxies. Previous work focused on identifying both mineralogy (post-depositional) and source effects (Gall et al. 2013; Gueguen et al. 2016), in particular regarding hydrothermal inputs in the oceans and the response of Ni isotope biogeochemical cycling through time. The most important sink for Ni in the oceans is the Fe-Mn oxides sink, but estimation of its Ni isotope composition is only based on hydrogenetic Fe-Mn crusts. In this study, we investigated a range of Fe-Mn deposits including Fe-Mn deposits variably affected by hydrothermal inputs, including hydrothermal deposits from the Lau back-arc basin (South West Pacific) and Lo'ihi seamount (Hawaii), hydrogenetic crust and nodules from the Bauer Basin (Pacific Ocean). Nickel isotope ratios were measured by multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) using a double-spike (61Ni and 62Ni) correction method. The combination of Ni isotopes and rare earth element (REE) geochemistry show that Ni isotope fractionation in Fe-Mn deposits is essentially controlled by formation processes of the deposits (such as the rate of formation, the initial Mn-phase and sorption processes) which are also related to the depositional environment. Consistent with previous studies, pure hydrogenetic crusts are characterized by isotopically heavy Ni isotope signatures (δ60/58Ni values range from ‰ 0.9 and 2.5‰) and well-developed positive Ce anomalies. In contrast, mixed hydrothermal‑hydrogenetic crust and nodules from the Bauer Basin (East Pacific) display negative Ce anomaly and lighter δ60/58Ni values (0.3‰ to 0.4‰), which are interpreted as the result of far-field hydrothermal inputs of Fe-Mn precipitates from the East Pacific Rise.Nickel in hydrothermal deposits from the Lau Basin (0.5 and 1.1‰) and Lo'ihi seamount (−0.8 to −1.5‰) is isotopically lighter than in hydrogenetic Fe-Mn crusts. Light δ60/58Ni values in Lo'ihi deposits is due to the removal of Ni during Ni adsorption from seawater and from the hydrothermal fluid (between 0 and 1.4‰) on Fe-oxides followed by isotope fractionation between the fluid and the mineral phase. Results suggest that Ni isotopes in hydrothermal Fe-rich deposits are strongly fractionated relative to the seawater/fluid source due to partial removal of Ni on Fe-phases. Hydrothermal Mn-oxides deposits from the Lau Basin acquired their Ni isotope signature through Ni adsorption and continuous exchange of Ni with seawater. We propose that the systematic difference in Ni isotope signatures between hydrogeneous and hydrothermal Fe-Mn deposits is related to the mechanisms of Ni uptake into oxide minerals (e.g., birnessite vs. todorokite; Fe-oxides vs. Mn-oxides) which depend on the rate of formation and the source of Mn and Fe to marine ferromanganese deposits (i.e., depositional environment) rather than Ni sources.

Ni isotope fractionation during coprecipitation of Fe(III)(oxyhydr)oxides in Si solutions

The dramatic decline in aqueous Ni concentrations in the Archean oceans during the Great Oxygenation Event is evident in declining solid phase Ni concentrations in Banded Iron Formations (BIFs) at the time. Several experiments have been performed to identify the main removal mechanisms of Ni from seawater into BIFs, whereby adsorption of Ni onto ferrihydrites has shown to be an efficient process. Ni isotopic measurements have shown limited isotopic fraction during this process, however, most experiments have been conducted in simple solutions containing varying proportions of dissolved Fe and Ni as NO3 salts, as opposed to Cl salts which are dominant in seawater. Further, Archean oceans were, before the advent of siliceous eukaryotes, likely saturated with amorphous Si as seen in the interlayered chert layers within BIFs. Despite Si being shown to greatly affect the Ni elemental partitioning onto ferrihydrite solids, no studies have been made on the effects of Si on the Ni isotope fractionation. Here we report results of multiple coprecipitation experiments where ferrihydrite precipitated in mixed solutions with Ni and Si. Ni concentrations in the experiments ranged between 200 and 4000 nM for fixed concentrations of Si at either 0, 0.67 or 2.2 mM. The results show that Si at these concentrations has a limited effect on the Ni isotope fractionation during coprecipitation of ferrihydrite. At 0.67 mM, the saturation concentration of cristobalite, the isotopic fractionation factors between the precipitating solid and experimental fluid are identical to experiments not containing Si (0.34 ± 0.17‰). At 2.2 mM Si, and the saturation concentration of amorphous silica, however, the Ni isotopic composition of the ferrihydrite solids deviate to more negative values and show a larger variation than at low or no Si, and some samples show fractionation of up to 0.5‰. Despite this seemingly more unstable fractionation behaviour, the combined results indicate that even at as high concentrations of Si as 2.2 mM, the δ60Ni values of the forming ferrihydrites does not change much. The results of our study implicate that Si may not be a major factor in fractionating stable Ni isotopes, which would make it easier to interpret future BIF record and reconstruct Archean ocean chemistry.