Updated COMAGMAT-5: Modeling the Effects of Sulfide Precipitation in Parallel to the Crystallization of Alumino-Chromian Spinel

Abstract An updated version of the COMAGMAT-5.3 program is presented, which enables simulations of the silicate-sulfide immiscibility in parallel to the crystallization of Al–Cr spinel and other rock-forming minerals. The principal changes include a completed recalibration of the previous Fe–Ni sulfide solubility model (Ariskin et al., 2013) and incorporation of equations describing spinel–melt equilibria in a wide range of magmatic systems (Nikolaev et al., 2018a, 2018b). This allowed us to more accurately specify the link between the compositions of immiscible sulfides and magma crystallization temperatures and to correct alumina partitioning between the model spinel and crystallizing melt. The updated COMAGMAT-5.3 can be used for calculations of the crystallization of basaltic to komatiitic magmas, as well as the history of solidification of mafic to ultramafic cumulates, including the relative proportions of Al–Cr spinel and immiscible sulfides. The applications are exemplified by the solidification of sulfide-bearing primitive olivine cumulate from the inner-contact zone of the Yoko-Dovyren intrusion in Northern Transbaikalia, Russia. It is established that maximum crystallization proportions of Al–Cr spinel as high as 3.5 wt% are observed at the Ol–Sp cotectic, which is followed by an abrupt decrease to slightly negative values (because of spinel dissolution) during the crystallization of plagioclase-bearing assemblages. This results in an inflection point on the trend of spinel composition evolution, which changes from a decrease in the Cr/Al ratio in the field of olivine to its increase when plagioclase starts to crystallize. The first estimates are presented for the effect of Cr in pyroxenes on the crystallization proportion and composition of the Al–Cr spinel. A model calculation shows that this factor results in longer lasting spinel dissolution when pyroxenes start to crystallize, and the composition of this spinel is shifted toward Cr-poorer derivatives.