Serpentine decomposition in the presence of graphite and metallic iron at P-T conditions of the Earth’s upper mantle

Abstract

Abstract Serpentinized peridotite is an important part of subducting oceanic crust. Despite its small amount in the oceanic lithosphere (about 10%), serpentine is believed to be the main source of water containing about 13 wt.% OH-groups. There has been recently developing a hypothesis that subduction of crustal material is linked with formation of diamond including its super-deep variety named CLIPPIR (Cullinan-like large, inclusion-poor, relatively pure, irregularly shaped and resorbed). The present paper is focused on experimental crystallization of silicate and oxide minerals during serpentine decomposition in the presence of elemental carbon and metallic iron at high pressure and high temperature. The experiments were carried out using a multiple-anvil split-sphere type high pressure apparatus (BARS) at pressures 4–5.5 GPa and temperatures of 1350–1450°C. The following main phases were identified after experiments in the serpentine–graphite–iron system: olivine, orthopyroxene, garnet, chromite, metallic iron, iron carbide, graphite. Olivine is a predominant silicate mineral while metallic iron and solid solution of carbon in iron (γ–phase) are abundant in the samples as well. The processes of mineral transformation in the experiment occurred in the presence of fluid of complex composition. In contrast to the oxidizing systems which mainly contain CO2, CO and H2O, the present experiment also contained a hydrocarbon-rich fluid. Our experimental modelling of interaction between the oxidizing and reducing components of the fluid demonstrated a nonequilibrium state with significant heterogeneity in fO2: in short-time runs iron content of olivine varied widely, but longer experiments showed less variations (14.1–18.3 wt.% FeO). It is suggested that iron content of olivine can be considered as an indicator of redox changes in the medium. The excess of metallic iron induces equilibrium which is characterized by formation of minerals corresponding to the reducing conditions. Our results confirmed that Mg-rich olivine and Mg-rich orthopyroxene can form in silicate rock if metallic iron is involved. In such conditions crustal carbon captured by subducting plate can recrystallize in the form of diamond with inclusions of metal–sulfide–silicate composition.