Nickel isotopic composition of the mantle

This paper presents a detailed high-precision study of Ni isotope variations in mantle peridotites and their minerals, komatiites as well as chondritic and iron meteorites. Ultramafic rocks display a relatively large range in delta Ni-60 (permil deviation in Ni-60/Ni-58 relative to the NIST SRM 986 Ni isotope standard) for this environment, from 0.15 +/- 0.07 parts per thousand to 0.36 +/- 0.08 parts per thousand, with olivine-rich rocks such as dunite and olivine cumulates showing lighter isotope compositions than komatiite, lherzolite and pyroxenite samples. The data for the mineral separates shed light on the origin of these variations. Olivine and orthopyroxene display light delta Ni-60 whereas clinopyroxene and garnet are isotopically heavy. This indicates that peridotite whole-rock delta Ni-60 may be controlled by variations in modal mineralogy, with the prediction that mantle melts will display variable delta Ni-60 values due to variations in residual mantle and cumulate mineralogy. Based on fertile peridotite xenoliths and Phanerozoic komatiite samples it is concluded that the upper mantle has a relatively homogeneous Ni isotope composition, with the best estimate of delta Ni-60(mantle) being 0.23 +/- 0.06 parts per thousand(2 s.d.). Given that >99% of the Ni in the silicate Earth is located in the mantle, this also defines the Ni isotope composition of the Bulk Silicate Earth (BSE). This value is nearly identical to the results obtained for a suite of chondrites and iron meteorites (mean delta Ni-60 0.26 +/- 0.12 parts per thousand and 0.29 +/- 0.10 parts per thousand, respectively) showing that the BSE is chondritic with respect to its Ni isotope composition, with little to no Ni mass-dependent isotope fractionation resulting from core formation. (C) 2016 Elsevier Ltd. All rights reserved.