Inversion of partial melting through residual peridotites or clinopyroxenes

Abstract

This paper presents two partial melting inversion methods that permit calculation of the degree of partial melting and source composition for cogenetic melting residues (residual peridotites or clinopyroxenes) : the concentration ratio (CR) method and the slope-intercept (SI) method. Neither method requires assumptions regarding source incompatible element concentrations or ratios. The CR method uses variations of between-residue concentration ratios for two incompatible trace elements that have different bulk distribution coefficients to obtain the degrees of partial melting. Source concentrations can be calculated after obtaining the partial melting degree. The SI method employs the slope and the intercept from the C ra C rb vs. C ra or 1 C rb vs. 1 C ra diagram to obtain their source concentrations. The degrees of partial melting can be calculated after obtaining the source concentrations. Theoretically, the CR method for residual peridotites or clinopyroxenes may be used for modal and nonmodal batch and nonmodal fractional melting. By comparison, the SI method for residual peridotites can only be applied for modal batch melting while this method for residual clinopyroxenes may be used for both modal and nonmodal batch melting. The CR method and the SI method for residual peridotites are illustrated using the orogenic lherzolites from Lanzo as an example. Results show that both the CR method and the SI method for batch melting can be used for the inversion of partial melting through residues after low degrees of partial melting.