Modeling of trace-element composition of sulfide liquid in a crystallizing basalt magma: Development of the R-factor concept

This paper presents principles of the method of modeling metal concentrations in sulfide at the occurrence of silicate–sulfide liquid immiscibility during middle and late stages of the crystallization of mafic–ultramafic magmas. The proposed approach combines the preliminary modeling of the crystallization of heterogeneous (melt + crystals) and initially sulfide-undersaturated systems with the calculation of the trace-element composition of sulfide liquid after its appearance in the system. The first part of the problem is solved using the sulfide version of the COMAGMAT-5 program, and the second part employs the modified Campbell–Naldrett equation. It was shown that a delay in silicate–sulfide liquid immiscibility and the low proportion of separated sulfide to major minerals are important factors of the geochemical evolution of sulfide liquid. By the example of the behavior of Cu, Pd, and Re, we discussed the reasons for differences between calculations for crystallizing multiphase cumulates and simplified (sulfide–silicate melt) systems.