PHASE-RELATIONS OF A NATURAL MARID COMPOSITION AND IMPLICATIONS FOR MARID GENESIS, LITHOSPHERIC MELTING AND MANTLE METASOMATISM

Melting experiments were performed on a natural mica-amphibole-rutile- ilmenite-clinopyroxene (MARID) sample from the Kaapvaal mantle lithosp here (AJE137) at 20 to 35 kbar and 800 to 1450-degrees-C. A solidus wa s determined at 1260-degrees-C and 30 kbar above which phlogopite, cli nopyroxene and olivine were stable with an alkali-rich silicate melt. Olivine is the only crystallizing phase just below the liquidus of the AJE137 bulk composition and K-richterite was only stable in the sub-s olidus region (less-than-or-equal-to 1100-degrees-C at 30 kbar). These results are consistent with previous studies in more simple systems. In experiments with 10 wt% added water the solidus was depressed by ca . 300-degrees-C and K-richterite was stabilized above this solidus. MA RIDs represent a potential low-temperature component in the lithospher ic mantle beneath the Kaapvaal Craton of southern Africa. The addition of > 10 wt% water (with less than a 120-degrees-C rise of temperature above the geotherm) to this mantle region would create conditions for the melting of this component. This may then be incorporated in any c ontinental flood basalt parent magma that traverse this lithospheric m antle. The derivation of MARIDs from a silicate melt of their bulk com position, even if water saturated, is considered unlikely as such smal l degree melts could not sustain the elevated liquidus temperatures re quired (> 1200-degrees-C at 30 kbar) in a cold (< 800-degrees-C at 30 kbar) mantle lithosphere. MARID xenoliths may be produced by the inter action of an alkali-rich fluid with a peridotite or as the residue to a group II kimberlitic parent magma that has undergone fractionation o f olivine and the exsolution of a carbonatite component.