We propose to investigate the formation of discordant dunite in the Trinity and Josephine peridotites, in northern California and southern Oregon. Quantification of the degree and nature of wall rock reaction in natural examples is crucial. If wall rock reaction commonly has a significant effect, then this process may be as important as melting and crystal fractionation in the production of magmas in the mantle. We propose to test our hypothesis by measuring major and trace element compositions of pyroxene, olivine and spinel in carefully selected samples of dunite and host peridotite. We will analyze individual crystals in thin section using the election and ion microprobes at MIT and Woods Hole. Use of major and trace element data on cores of individual mineral grains to estimate parental liquid compositions, and solution of a series of mass balance equations for these elements, should provide a quantitative estimate of the relative importance of crystal fractionation and wall rock reaction in the genesis of discordant dunite.
