Meltingphase relations of model carbonatedperidotite from2 to 3GPa in the systemCaO-MgO-Al_2O_3-SiO_2-CO_2 and further indication of possible unmixing between carbonatite and silicate liquids

摘要

Melting phase relations of model carbonated peridotite in the system CaO-MgO-Al_2O_3-SiO_2-CO_2 from 2 to 3 GPa are reported. Experimentally produced melts, which are model carbonatites, with approximately 36–40 wt % CaO, 12–17 wt % MgO, 0.2–1.5wt % Al_2O_3, 1–4wt % SiO_2, and 40–42 wt % CO_2 (carbon dioxide) are present at all pressures investigated. At 2.8 and 3GPa, carbonatitic melts are seen experimentally at temperatures that are very close to the vapor-free (CO_2) peridotite solidus and are found in equilibrium with forsterite, orthopyroxene, clinopyroxene, and garnet. Solidus phase relations with isobaric and pressure-temperature invariant points, defining the so-called carbonated peridotite solidus ledge, are also reported from 2.1 to 3GPa. A divariant region exists from 2 to 2.6GPa wherein two, compositionally different melts are present. In this region, these two melts, carbonatitic and silicate in composition, coexist with crystalline phase assemblage and free vapor. The silicate liquid has approximately 30–48 wt % SiO_2 and approximately 6 to 20 wt % of dissolved CO_2. The presence of carbonatitic and silicate liquids is interpreted to be due to liquid immiscibility. On the basis of melting phase relations reported here, we conclude that (a) the ledge is a feature along which model carbonatitic liquids are produced by reaction of silicates and CO_2 vapor and (b) alkali-free carbonatites and silicate melts can form through melt unmixing at depths of ~60–80km in the Earth’s mantle.