The stability of an eclogitic clinopyroxene in the Earth’s mantle: an experimental investigation

The transport of chemical elements down into the Earth’ s transition zone and lower mantle is strongly affected by the stability of minerals as function of pressure and temperature. Whether the slab stagnates or sinks depends on the density and viscosity of the subducted portions that links with the mineralogy and associated phase transitions and decomposition reactions, which have important implications for the origin of deep earthquakes. Clinopyroxene (cpx) is known to dissolve as majoritic component in garnet during subduction. However, the associated slow diffusion can result in metastable cpx at high pressure. To date, experimental studies on the stability of omphacitic cpx as function of pressure are few and sparse in literature. We performed high pressure and temperature experiments to investigate the fate of omphacite during subduction. The experiments were carried out at temperatures between 800 and 1300 °C and pressures of 3 and 6 using the Voggenreiter 840 t, Walker-type multi anvil press available at HPHT Lab at National Institute of Geophysics and Volcanology (INGV, Rome). The starting material employed is a mixture of synthetic omphacitic glass, quartz, dolomite and graphite representative of a carbonated eclogite loaded in a graphite capsule. Tungsten carbide (WC) anvils (F grade) with 17 and 11 mm truncation edge lengths (TEL) were used with chromium-doped MgO octahedra (25 and 18mm edge length) as pressure media. In a second set of experiments, a synthetic omphacitic glass was mixed with Re and ReO2 to act as redox buffer, all loaded in a Re capsule of ~1mm in diameter. WC anvils (F grade) with TEL of 3 mm were used with cobalt-doped MgO octahedra and pyrophillite gaskets as pressure media both optimized to generate high pressure (Tange et al., 2008). The temperature during the experiments was monitored with a W-5%Re/W-26%Re thermocouple. In all experiments, olivine (San Carlos) or quartz powder were used as pressure marker (Ito, 2007). The sample was compressed applying a ram load up to ~5 MN, then heated and kept at a constant temperature within ±10 °C for a period of 20-360 minutes. The recovered samples were mounted in epoxy resin, sectioned and polished for textural observation and chemical analyses with a field-emission scanning electron microscope. Preliminary results from experiments at 3 and 6 GPa show a strong temperature dependence of the content of sodium and aluminum in tetrahedral coordination that is consistent with previous studies. Isothermal runs at higher pressures show the formation of a Na-poor rim in the recovered cpx likely corresponding to the nucleation of a post-clinopyroxene phase (Oguri et al, 2007)