GEOCHEMICAL ASPECTS OF PRIMARY MAGMA ACCUMULATION FROM EXTENDED SOURCE REGIONS

Abstract

Primary basaltic magmas are accumulated after they have migrated through extensive regions of residual mantle. The migrating magma may react to various degrees with the residuum, so that the composition of the primary magma may change during migration and accumulation. Two extreme accumulation models, with and without equilibration with the residua, are considered. The trace element concentrations of melts generated by both models are similar to those of batch melting, when their average degree of melting is used for calculations of melt concentrations of batch melting. This result is independent of the size of the distribution coefficients within the range 10-10-4. The convenient equation for batch melting may therefore be retained if it is noted that the determined degree of melting approximates the average degree of accumulative melting. It appears difficult, if at all possible, to distinguish between different accumulation models on the basis of trace element concentrations of accumulated magmas.