The non-chondritic Ni isotope composition of Earth's mantle

摘要

Nickel is a major element in the Earth. Due to its siderophile nature, 93% of Ni is hosted in the core and the Ni isotope composition of the bulk silicate Earth might inform on the conditions of terrestrial core formation. Whether Earth's mantle is fractionated relative to the chondritic reservoir, and by inference to the core, is a matter of debate that largely arises from the uncertain Ni isotope composition of the mantle. We address this issue through high-precision Ni isotope measurements of fertile- to melt-depleted peridotites and compare these data to chondritic meteorites. Terrestrial peridotites that are free from metasomatic overprint display a limited range in delta Ni-60/58 (deviation of Ni-60/Ni-58 relative to NIST SRM 986) and no systematic variation with degree of melt depletion. The latter is consistent with olivine and orthopyroxene buffering the Ni budget and isotope composition of the refractory peridotites. As such, the average Ni isotope composition of these peridotites (delta Ni-60/58 = 0.115 +/- 0.011 parts per thousand) provides a robust estimate of the delta Ni-60(/)58 of the bulk silicate Earth. Peridotites with evidence for melt metasomatism range to heavier Ni isotope compositions where the introduction of clinopyroxene appears to drive an increase in delta Ni-60(/)58. This requires a process where melts do not reach isotopic equilibrium with buffering olivine and orthopyroxene, but its exact nature remains obscure. Chondritic meteorites have variability in delta Ni-60(/)58 due to heterogeneity at the sampling scale. In particular, CD chondrites are displaced to isotopically lighter values due to sorption of Ni onto ferrihydrite during parent body alteration. Chondrites less extensively altered than the CII chondrites show no systematic differences in delta Ni-60(/)58 between classes and yield average delta Ni-60(/)58 = 0.212 +/- 0.013 parts per thousand, which is isotopically heavier than our estimate of the bulk silicate Earth. The notable isotopic difference between the bulk silicate Earth and chondrites likely results from the segregation of the terrestrial core. Our observations potentially provide a novel constraint on the conditions of terrestrial core formation but requires further experimental calibration. (C) 2019 Elsevier Ltd. All rights reserved.