Melting and Volatile Interactions in the 50-70 Kbar Mantle

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Small quantities of the volatile molecules H2O and CO2 are found magmas and rocks from the Earth's upper mantle. These volatiles can have important effects on such problems as the origin of magmas, the melting temperature of the mantle as a function of depth, the existence of melts along geotherms in the primitive mantle, and the chemical differentiation of the mantle. Experimental research completed in the past 30 years has focused on answering these questions at pressures up to 35 kbar, reaching only into the uppermost mantle. The upper pressure was largely determined by apparatus limitations. In the last three years, a burst of activity focusing on the 70-250 kbar pressure range has occurred due to the advent of the multiple anvil high pressure device. Fragments of upper mantle rocks found in kimberlite breccias record fossil geotherms (temperature profiles) with an inflection (change) in their pressure-temperature trajectories at pressure of about 50 kbar. The deepest samples record pressures of 60 kbar. Diamonds found in these garnet peridotite fragments and in the kimberlites suggest somewhat higher pressures. The inflection in the geotherm suggests that major processes occur in this pressure range. The proposed problems that will investigate the interaction of volatiles with minerals and magmas at a 50-70 kbar pressure range are: *Which H2O reservoir minerals are stable, and do they have a required constituent such as potassium? *What is the effect of CO2 on magma compositions? *What is the effect of pressure on the ferric/ferrous iron ratio in magmas? *What is the relationship between H2O-CO2-oxygen and primary magma composition? A newly developed apparatus, an active girdle piston-cylinder with a routine operating range to 70 kbar, will be used to explore these questions.