EVIDENCE FOR HYDROUS HIGH-MGO MELTS IN THE PRECAMBRIAN

Prevailing petrogenetic models for Precambrian high-MgO melts such as komatiites invoke crystallization from nearly anhydrous melts (much le ss than 0.5% H2O) generated by partial melting of mantle peridotite at temperatures of less than or equal to 1900 degrees C and pressures of less than or equal to 18 GPa. However, ultramafic cumulate and gabbro zones of komatiitic and other high-MgO units in Precambrian greenston e belts contain vesicles and minor to major amounts (less than or equa l to 25%) of igneous amphibole. The textures (oikocrysts, rims on inte rcumulate pyroxene, and mineral inclusions within orthocumulate olivin e) and the water-rich compositions (1.00%-2.50% H2O) of igneous amphib oles from the Archean Abitibi belt indicate crystallization in situ fr om significantly hydrous melts while the melt fraction was still as hi gh as 40%-50%. Comparisons to experimental phase equilibria suggest th at the residual melts from which the amphiboles crystallized contained 3%-4% H2O, and adjustments for fractional crystallization suggest tha t the initial melts may have contained as much as 2% H2O.H2O contents of this magnitude would require substantial revision of the nearly anh ydrous models for Precambrian high-MgO melts, possibly permitting gene ration at lower temperatures and pressures, lowering their densities a nd viscosities, increasing their eruptibility, and enhancing the forma tion of spinifex textures.