Formation of orthopyroxenite by reaction between peridotite and hydrous basaltic melt: an experimental study

Abstract

The consequences of hydrous basaltic melts and peridotite interaction were examined experimentally in Au–Pd, Pt, and graphite capsules using the reaction couple method. Reactions between a hydrous basaltic andesite (4 wt% H2O) and dunite or lherzolite in an Au–Pd capsule at 1 GPa and 1200 °C produce a melt-bearing orthopyroxenite–dunite sequence. Reactions between a hydrous ferro-basalt and lherzolite in Pt or Au–Pd capsules at 0.8–2 GPa and 1250–1385 °C produce a melt-bearing orthopyroxenite–harzburgite sequence. Reactions between the ferro-basalt and lherzolite in graphite capsules (not designed to retain water) result in a melt-bearing dunite–harzburgite sequence at 1 GPa and a melt-bearing harzburgite–lherzolite sequence at 2 GPa. The orthopyroxenite from the hydrous reaction experiments has a high porosity, and it is separated by a sharp lithological interface from the dunite or harzburgite. Orthopyroxenes in the orthopyroxenite are large in size with resorbed olivine inclusions. Formation of the high-porosity orthopyroxenite in the hydrous melt–rock reaction experiments is determined by the liquidus phase relation of the interface reacting melt and reaction kinetics. Reaction between orthopyroxene-saturated hydrous melt and olivine at melt–rock interface produces orthopyroxenite. Water infiltration induces hydrous melting of the lherzolite, producing a dunite or an orthopyroxene-depleted harzburgite. Efficient diffusive exchange between the partial melt and the hydrous reacting melt promotes orthopyroxene-oversaturation around the melt–rock interfacial region. The simplified experiments reveal end-member processes for understanding the formation of orthopyroxenite in the upper mantle. The presence of orthopyroxenites in mantle samples is a strong indication of hydrous melt and peridotite interaction.