REDOX CONTROL DURING MANTLE MELT INTERACTION

We have investigated redox systematics in the upper mantle under continents and ocean islands, using data on olivine, orthopyroxene, and spinel from both spinel lherzolites and coexisting pyroxenite xenoliths. Calculated f(O2) values at 15 kb for continental xenoliths range from QFM-5.3 to QFM+0.8, with median values (M) of QFM-0.5 for volatile-free lherzolites, QFM-0.3 for volatile-bearing lherzolites, and QFM-0.7 for pyroxenites. Ocean island xenoliths reflect relatively oxidizing conditions; with one exception lherzolites fall within the range QFM-0.7 to QFM+2.1 (M = QFM+1.0), whereas pyroxenites give the range QFM-0.1 to QFM+1.5 (M = QFM+0.8). Fe-Ti-rich pyroxenites show a narrow range in calculated f(O2) values close to the QFM-buffer, suggesting a coupling between redox processes and melt/rock interaction in the upper mantle. We have investigated the ability of mineral/melt partitioning to induce variations in the Fe3+/SIGMA-Fe ratio of mantle-derived melts and the effect of infiltration of Fe3+-rich melts on the redox state of the upper mantle. On the basis of this study we conclude that magmatic processes alone, involving partial melting and melt infiltration, may induce large variations in the redox state of the upper mantle and mantle-derived melts within the stability fields of spinel and plagioclase peridotite. In particular, spinel- and clinopyroxene-poor peridotitic material is prone to oxidation by infiltration of Fe3+-enriched, fractionated melts. This is in accord with the relatively high f(O2), values found in the depleted upper mantle beneath ocean islands.