Magnesium isotopic homogeneity of San Carlos olivine: a potential standard for Mg isotopic analysis by multi-collector inductively coupled plasma mass spectrometry.

Abstract

Previous analyses on San Carlos olivine from Arizona (USA) have shown inter-laboratory δ(26) Mg differences of up to 0.67‰, while mantle olivine samples worldwide are homogeneous at a current analytical uncertainty of ~0.1‰. The differing measurements on San Carlos olivine may be attributable to analytical artifacts or sample heterogeneity. The latter must be ruled out before using it as a standard for Mg isotopic analysis. To examine sample homogeneity, two different batches of San Carlos olivine from a lherzolite and four batches from a harzburgite have been analyzed together with coexisting harzburgitic pyroxene. In addition, the effect of acid purity on resin performance and the reusability of AG50W-X8 resin for Mg separation have been evaluated by processing another batch of lherzolitic San Carlos olivine and Hawaiian seawater through both new and used resins cleaned with different acids. Six different batches of olivine grains from two San Carlos peridotite xenoliths show homogeneous δ(26) Mg values to within 0.03‰, and all the mineral phases in the harzburgite are in Mg isotope equilibrium. Furthermore, there is no resolvable δ(26) Mg shift in either lherzolitic San Carlos olivine or Hawaiian seawater by using either new or used resins that were cleaned with single-distilled or double-distilled acids. The new data are consistent with the narrow δ(26) Mg range of mantle olivine worldwide, while they stand in contrast to the wide range measured on the same San Carlos olivine powder in different laboratories. Therefore, previous inter-laboratory discrepancies reflect analytical artifacts instead of sample heterogeneity, and San Carlos olivine is a suitable standard for Mg isotopic analysis. Copyright © 2016 John Wiley & Sons, Ltd.