MELTING EXPERIMENTS IN THE SYSTEMS CAO-MGO-AL2O3-SIO2 AND MGO-SIO2 AT3 TO 15 GPA

The results of multi-anvil melting experiments are reported for a rang e of compositions in the system CaO-MgO-Al2O3-SiO2. The liquidus cryst allization fields for forsterite, orthopyroxene, clinopyroxene, and ga rnet have been mapped out at 10 GPa, as have their intersections at va rious cotectics. The composition of the liquid that is multiply satura ted in forsterite, orthopyroxene, clinopyroxene, and garnet has been a lso determined to within +/-0.5-1.0 wt% (2 sigma), and the result is i n excellent agreement with a previous estimate (Herzberg 1992). These experiments confirm that the effect of pressure is to reduce Al2O3 and increase MgO and SiO2 in magmas formed by the melting of garnet Iherz olite with increasing pressure (Herzberg 1992). Melting experiments in the system MgO-SiO2 also have been performed to constrain how pressur e affects the compositions of liquids that are saturated in harzburgit e [L + Ol + Opx]. Experiments in both CaO-MgO-Al2O3-SiO2 and MgO-SiO, demonstrate that there is a maximum normative olivine content to liqui ds formed by initial or advanced melting of peridotite in the upper ma ntle, and this occurs at 7 to 8 GPa. For most peridotites that undergo decompression melting in a plume, clinopyroxene and garnet are the fi rst crystalline phases to melt out and, with a few important but rare exceptions, the experimentally constrained liquids are unlike most vol canic rocks. Advanced anhydrous melting will yield liquids with a resi dual harzburgite mineralogy [L + Ol + Opx], and these liquids are simi lar in composition to most komatiites with Cretaceous and 2700 Myr Arc haean ages.