The stable Cr isotope inventory of solid Earth reservoirs determined by double spike MC-ICP-MS

Abstract

We present the first comprehensive set of stable Cr isotope data for the major igneous silicate Earth reservoirs, Cr(III)-rich ores and minerals, and hydrothermal chromates. These were determined by MC-ICP-MS using a double spike technique. Mantle xenoliths, ultramafic rocks, cumulates, as well as oceanic and continental basalts share a common Cr isotope composition with an average δ 53/52Cr value of − 0.124 ± 0.101‰ (2 SD) relative to the isotopically certified chromium standard NIST SRM 979. An isotopic difference between mantle xenoliths and basalts, as was reported for iron, was not observed for chromium. Thus, the change in oxidation state that is observed when solid mantle rocks, containing only trivalent chromium, partially melt to form basaltic melts, which predominantly contain bivalent chromium, does not cause any measurable Cr isotope fractionation. Chromite separates from major chromitite seams of the Bushveld and Great Dyke layered igneous complexes are invariable in their Cr isotope compositions and reproduce within uncertainties the average δ 53/52Cr SRM 979 value of igneous silicate Earth reservoirs. This is important for environmental stable Cr isotope studies, because it labels the approximate isotope composition of industrial Cr(VI) pollutants. The Cr isotope compositions of hydrothermal lead chromates (crocoites PbCr VIO 4) from various localities yield δ 53/52Cr SRM 979 values between 0.640 and 1.037‰; these Cr isotope compositions are substantially heavier than those of igneous silicate rocks from which the chromium was leached. Precipitation experiments revealed an isotope fractionation of Δ 53/52Cr (crocoite–Cr(VI)aq) of only ca. + 0.10 ±0.05‰ at a temperature of 20 °C. Thus, the heavy Cr isotope signature of crocoites is most likely the result of repeated redox cycling of chromium in hydrothermal processes.