Formation of orthopyroxenite by reaction between peridotite and hydrous basaltic melt: an experimental study
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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.
KeywordsMelt–peridotite interaction Hydrous melt Orthopyroxenite Dunite Harzburgite Composite xenolith
We thank Joseph Boesenberg for assistance in electron microprobe analysis, Colin Jackson for help with interferometer measurement, and Henry Dick for useful comments and suggestions to an earlier version of the manuscript. Constructive reviews by Véronique Le Roux, an anonymous reviewer, and editor Othmar Müntener helped to improve this manuscript and are greatly appreciated. CW is grateful to Chenguang Sun and Lijing Yao for their hospitality and help during his visits to Brown University. This study was supported in part by Key Basic Research Program of China (2015CB856101) and Natural Science Foundation of China (91014004) to WX, NSF grants EAR-0911501 and OCE-1156706 to YL, and a fellowship provided by the China Scholarship Council (201306170092) to CW.
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