Compositional variations of coexisting phases with degree of melting of peridotite in the upper mantle

Abstract

A new technique using radioactive tracers was developed for accurate determination of the degree of melting of rocks. Trace amounts of a compound (CO/sub 2/ is used here) or an element that preferentially enters the melt phase are determined. A glass of the starting material is used as standard. The proportion of partial melt equals the ratio of the amount of tracer in the standard and the unknown. Using this technique, to garnet peridotite nodules, samples 1611 (from a Lesotho kimberlite) and 66SAL-1 (from Hawaiian nephelinite tuff), were subjected to temperatures from their solidi to 1750/sup 0/C at 20 and 35 kbar pressure. Both periodite nodules show similar melting behavior. The degree of melting was plotted against temperature, and such melting curves show a distinct change of slope wherever one or more phases disappear.