Geochemical Modelling of Melting and Cumulus Processes: A Theoretical Approach

Abstract

Mantle melting and fractional crystallization are two fundamental processes that control the compositional variations in the erupted basaltic melts. Most of the geochemical variations in the primary mantle-derived melts are generally attributed to mantle heterogeneity. With simple forward modelling techniques, it is illustrated that different melting types (batch, fractional and continuous) are capable of producing large variations in elemental abundances in the primary magmas derived from a homogeneous mantle source. There is no reason to invoke mantle heterogeneity. Similarly, variations in the cumulate and corresponding residual liquid geochemistry are demonstrated alluding to rare earth element abundances. Variable partition coefficients for cumulus phases and dif- ferent amounts of intercumulus liquid depict pseudo liquid-lines-of-descent in cumulate rocks on X–Y type plots. It is documented that assimilation fractional crystallization (AFC) produces greater variations in trace element concentrations in both cumulates and residual liquids. It is found that infinitesimally small solid and liquid compositions, amended by AFC, have similar trace element abun- dances. Two Indian examples are cited to support the theoretical modelling of mantle melting and cumulus processes presented here.