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

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 delta(53/52) Cr value of -0.124 +/- 0.101 parts per thousand (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 scams of the Bushveld and Great Dyke layered igneous complexes are invariable in their Cr isotope compositions and reproduce within uncertainties the average delta(53/52) Cr-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 (PbCrO4)-O-VI) from various localities yield delta(53/52) Cr-SRM 979 values between 0.640 and 1.037 parts per thousand; 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 Delta(53/52) Cr(crocoite-Cr(VI)aq) of only ca. +0.10 +/- 0.05 parts per thousand at a temperature of 20 degrees C. Thus, the heavy Cr isotope signature of crocoites is most likely the result of repeated redox cycling of chromium in hydrothermal processes. (C) 2008 Elsevier B.V All rights reserved.