Melting of the Indarch meteorite (EH4 chondrite) at 1 GPa and variable oxygen fugacity: Implications for early planetary differentiation processes

摘要

In order to derive constraints on planetary differentiation processes, and ultimately the formation of the Earth, it isrequired to study a variety of meteoritic materials and to investigate their melting relations and elemental partitioning at var-iable pressures, temperatures, and oxygen fugacities (f_(O2)). This study reports the first high pressure (HP) and temperature(HT) investigation of an enstatite chondrite (Indarch). Four series of experiments exploring various f_(O2)conditions have beencarried out at 1 GPa in a piston–cylinder apparatus using the EH4 chondrite Indarch. We show that temperature and redoxconditions have important effects on the phase equilibria of the meteorite: the solidus and liquidus temperatures of the silicateportion increase with decreasing f_(O2), and the stability fields of various phases are modified. Olivine and pyroxene are stablearound 1.5 logf_(O2)unit below the iron-wustite buffer (IW-1.5), whereas quartz and pyroxene is the stable assemblage underthe most reducing conditions, between IW-5.0 and IW-4.0, due to reduction of the silicate. While these changes are occur-ring in the silicate, the metal gains Si from the silicate, (Fe, Mg, Mn, Ca, Cr)-bearing sulfides are observed at f_(O2)less thanIW-4, and the partitioning of Ni and Mo are both affected by the presence of Si in Fe–S–C liquids. The f_(O2)has also a sig-nificant effect on the liquid metal–liquid silicate partitioning behavior of Si and S, two possible light elements in planetarycores, and of the slightly siderophile elements Cr and Mn. With decreasing f_(O2), S becomes increasingly lithophile, Si becomesincreasingly siderophile, and Cr and Mn both become strongly siderophile and chalcophile. The partitioning behavior of theseelements places new constraints on models of core segregation for the Earth and other differentiated bodies.