Redox Reactions in Antigorite Serpantinites

Abstract

We investigated serpentinites that contain a significant amount of magnetite, originating from ophiolites and tectonic melanges. Variations in the amount of magnetite were observed in serpentinites dominated by antigorite. While some samples exhibited a prevalence of iron oxides, others contained almost none. The Mg# [100*Mg/(Mg+Fe) atomic] values of the studied samples were found to range from 79 to 88. However, for antigorite, the Mg# value can reach up to 92 in magnetite-rich rocks. In samples devoid of iron oxides, this value decreases, indicating the preservation of relic protolith textures. Serpentinites, despite having different formation mechanisms, typically result from the hydration of peridotite. Hydration involves redox reactions, which yield brucite and magnetite. The formation of magnetite occurs primarily due to the preferential partitioning of Mg over Fe2+ in serpentine compared to olivine or orthopyroxene. Consequently, the excess iron, mainly derived from olivine, predominantly generates magnetite through redox reactions involving iron-rich brucite (Fe2+(OH)2) or reacting with available oxygen or product H2 (or CH4). The presence of magnesite and dolomite has been identified in the samples. Based on the obtained findings, it can be inferred that rocks devoid of magnetite indeed contain magnetite produced by serpentinization, but ongoing reactions with fluid allowing iron back-reaction serve to re-equilibrate fO2 to preserve the entire rock's Mg#. Although this process may be challenged by the extended storage of antigorites at high P - T conditions, it appears to be a relatively closed system for oxides.