AN ANALYSIS OF VARIATIONS IN ISENTROPIC MELT PRODUCTIVITY

The amount of melt generated per unit pressure drop during adiabatic u pwelling, the isentropic melt productivity, cannot be determined direc tly from experiments and is commonly assumed to be constant or to decr ease as melting progresses. From analysis of one- and two-component sy stems and from calculations based on a thermodynamic model of peridoti te partial melting, we show that productivity for reversible adiabatic (i.e. isentropic) depressurization melting is never constant; rather, productivity tends to increase as melting proceeds. Even in a one-com ponent system with a univariant solid-liquid boundary, the 1/T depende nce of (partial derivative S/partial derivative T)(P) and the downward curvature of the solidus (due to greater compressibility of liquids r elative to minerals) lead to increased productivity with increasing me lt fraction during batch fusion (and even for fractional fusion in som e cases). Similarly, for multicomponent systems, downward curvature of contours of equal melt fraction between the solidus and the liquidus contributes to an increase in productivity as melting proceeds. In mul ticomponent systems, there is also a lever-rule relationship between p roductivity and the compositions of coexisting liquid and residue such that productivity is inversely related to the compositional distance between coexisting bulk solid and liquid. For most geologically releva nt cases, this quantity decreases during progressive melting, again co ntributing to an increase in productivity with increasing melting. The se results all suggest that the increases in productivity with increas ing melt fraction (punctuated by drops in productivity upon exhaustion of each phase from the residue) predicted by thermodynamic modelling of melting of typical mantle peridotites using MELTS are neither artif acts nor unique properties of the model, but rather general consequenc es of adiabatic melting of upwelling mantle.