Effect of pyroxene and spinel on the kinetics of serpentinization

Serpentinization, a hydrothermal alteration of ultramafic rocks (typically komatiites and peridotites) at relatively low temperatures (e.g., ≤500 °C), greatly modifies the physical and chemical properties of the oceanic lithosphere, and it may act as a source of energy for the origin of life at the early history of Earth. However, the kinetics of serpentinization, especially the influence of pyroxene and spinel on the rates of serpentinization, still remain poorly constrained. Here we performed hydrothermal experiments at 300 °C and 3.0 kbar with natural olivine and peridotite as starting materials. The purposes are to (1) compare the rates of olivine and peridotite serpentinization, and (2) study the influence of pyroxene and spinel on the rates of serpentinization. The results have revealed that peridotite is serpentinized at rates much faster than olivine, which is attributed to the influence of pyroxene and spinel. Hydrothermal experiments were performed at 300 °C and 3.0 kbar with mechanical mixtures of olivine and spinel or pyroxene as starting materials. A great increase in the rates of serpentinization was observed compared to serpentinization kinetics in olivine-only experiments, which suggests that pyroxene and spinel increase the rates of serpentinization. Chemical compositions of olivine/pyroxene-derived serpentine show that pyroxene lost around 50% of its Al during peridotite serpentinization. Previous experimental results have revealed that Al accelerates the rates of serpentinization by 1-2 orders of magnitude. These experimental results provide a new mechanism for serpentinization processes. Pyroxene and spinel also influence the production of iron oxide during serpentinization.