Superimposing Magma Chamber and Melting Regime Processes: How Easy is it to (Mis)Interpret the Liquid Products?

Abstract

Choice of partial melting model. Less-thanperfect fractional melting processes (O'Hara 1993) yield liquid products which quite closely simulate products of equilibrium partial melting (EPM); a useful first approximation is that the partial melting event supplies a liquid which is an EPM product of the true source region composition. Choice of magma chamber process. Less-thanperfect fractional crystallisation processes (O'Hara 1993, O'Hara and Mathews 1981) in magma chambers yield liquid products which quite closely simulate those of equilibrium partial crystallisation (EPC) of the gross input to the magma chamber, including any materials assimilated from the roof and walls; a useful first approximation is that the partial crystallisation event supplies a liquid which is an EPC product (the equations are identical to EPM) of the average magma chamber input composition. Simplifying assumptions. In this formulation the space problem is ignored and contamination by assimilation of country rock or previously erupted lavas assumed to be non-existent. It is further assumed that the distribution coefficient, d, for each element remains constant throughout both processes and is the same in each process. A simple two-stage model The simplest twostage model of a 'real' system in which erupted