Crystal chemical constraints on inter-mineral Fe isotope fractionation and implications for Fe isotope disequilibrium in San Carlos mantle xenoliths

Abstract

The origin of variations in iron isotope compositions of mantle minerals is uncertain, and predictions of equilibrium inter-mineral iron isotope fractionation conflict. This hinders interpretation of the petrologic and geochemical implications of Fe isotope data from mantle lithologies. To address this, we present a revised ionic model for predicting equilibrium iron isotope fractionation between mantle minerals and use it to interpret measured inter-mineral iron isotopic fractionation from five distinct mantle xenolith lithologies from San Carlos, Arizona. The samples represent a broad range of modal abundances and include lherzolite, harzburgite, dunite, clinopyroxenite, and websterite. The xenoliths exhibit Fe-isotopic variation between minerals in a single sample, and between samples. In all cases where spinel and olivine coexist, the 57Fe/54Fe of spinel is greater than that of the corresponding olivine, agreeing with expectations for equilibrium fractionation from theory (ionic model), but disagreeing with predictions based on Mössbauer data. The 57Fe/54Fe values of clinopyroxenes from the xenoliths show no clear systematic differences. We interpret this to be a result of varying degrees of metasomatism, perhaps involving interaction with a melt. The spinel peridotite samples (lherzolite, harzburgite, and dunite) are partially melted residual mantle that exhibit a decrease in whole-rock 57Fe/54Fe with increasing olivine abundance. This is consistent with progressive extraction of a 57Fe-rich partial melt. The clinopyroxenite has the highest whole-rock 57Fe/54Fe, consistent with its origin as a cumulate from an unrelated magma possessing elevated 57Fe/54Fe. The websterite sample is transitional to Group II type xenoliths, has the lowest whole-rock 57Fe/54Fe of the investigated samples, and likely experienced a more complex metasomatic history. This study demonstrates that the Fe isotope compositions of San Carlos xenoliths and their component minerals record the complex petrologic history and local heterogeneity of the subcontinental mantle lithosphere.