Thermodynamics of pressure release melting of a veined plum pudding mantle

This study explores the thermodynamics of adiabatic decompression melting o f peridotitic mantle containing pyroxenite veins that have lower solidi tha n the peridotite. When a vein of lower solidus-temperature material melts a djacent to more refractory material, additional heat will flow into the mel ting region to increase its melting productivity. If pyroxenite veins have a solidus-depletion gradient ((partial derivativeT(m)/partial derivativeF)( P)) like that of olivine or peridotite, then the melting of the veins is en hanced by up to a factor of 4 by this heat. However, the solidus-depletion gradient of pyroxenites is apparently lower than that of peridotites; thus pyroxenite melting would be even more enhanced. If pyroxenite veins have a gentler solidus-pressure (T-P) dependence (i.e., lower (partial derivativeT (m)/partial derivativeP)(F)) than that of peridotite solidi, than although these veins will experience enhanced melting while they are the only meltin g assemblage, they will stop melting soon after their peridotite matrix beg ins to melt. During large-scale peridotite melting the material ascends alo ng a T-P path close to that of the periodotite solidus, so that the mixture 's temperature remains lower than the solidus of the residual pyroxenite, a nd pyroxenite melting ceases throughout the shallower sections of the melti ng column. If pyroxenitic material makes up a large fraction of the mantle mixture (similar to 20%), then the heat consumed by deep pyroxenite melting cools the ascending mantle mixture enough so that peridotite melting begin s similar to5-10 km shallower than it would in the absence of precurser pyr oxenite melting. After recycling into the mantle, the melt extraction resid ue will again melt if it is reheated to ambient mantle temperatures and ris es again to shallow depths.