Alteration of chromite during serpentinization of peridotites

Abstract

Trace element (Zn, Co, Mn, Ni, Ga, V, Ti, and Sc) compositions of chromite in mafic-ultramafic rocks are widely used for addressing their petrogenesis and discriminating tectonic environments, but they can be altered by subsolidus re-equilibration and hydrothermal alteration, and the detailed dynamic processes are not clear. The Jinchang ophiolite in SW China is part of the Tethyan ophiolite, and consists mainly of harzburgite with minor lherzolite. These peridotites are strongly serpentinized, and contain variably modified chromite and magnetite. Two types of chromite are identified in harzburgite, including magmatic and altered chromite. The magmatic chromite occurs as granular or amoeboid minerals, or as cores surrounded by the altered chromite, and they have homogeneous major elemental compositions (Mg0.65Fe0.35(Cr0.46Al0.54)2O4), but their trace elements, such as Ni, Ga, V, Zn, Co, and Mn, are variably changed during subsolidus re-equilibration. The altered chromite consists of ferrous chromite (Mg0.15Fe0.85(Cr0.46Al0.11Fe0.43)2O4) and chromian magnetite (Fe(Cr0.15Fe0.85)2O4), which were formed by two stages of hydrothermal alteration. They are depleted in Ga and Sc, and enriched in Zn, Co and Mn during the alteration, but their Ti, Ni and V are increased and decreased in the first and second stage, respectively. Thermodynamic modeling reveals that: (1) Reaction between magmatic chromite and olivine at the first stage of alteration produces ferrous chromite and chlorite under high temperature (610–470 °C) and increasing oxygen fugacity (ΔFMQ increasing from −1 to 3) and water-rich conditions; (2) Chromian magnetite is generated by further alteration of the ferrous chromite in the second stage alteration under low temperature (< 470 °C) and decreasing oxygen fugacity conditions (ΔFMQ decreasing from 3 to −4). In addition, the first stage alteration significantly changed trace-element compositions of the magmatic chromite compared with the second stage alteration that slightly modified the ferrous chromite. This study clarifies trace element migration of chromite in the silicate mineral-hydrothermal fluid system, indicating that chromite microstructures and trace elemental compositions can record the detailed serpentinization processes.