Behavior of Re during Magma Fractionation: an Example from Volcan Alcedo, Galapagos

Abstract

Rhenium concentrations have been measured in eight samples from Volcan Alcedo in the Galapagos Archipelago—four basalts, an icelandite, a dacite and a rhyolite (all part of a fractionation trend), and a magnetite phenocryst-rich separate from a rhyolite. This study shows that Re concentrations increase (from 234 to 850 ppt) during the FeO and TiO2 enrichment trend, and indicates that Re is incompatible in olivine, augite and plagioclase. Once magnetite stabilizes and fractionates, the iron and titanium enrichment trend in the melt reverses, and Re concentration drops from 610 ppt in the icelandite to 36 ppt in the rhyolite. The magnetite-rich separate has an extremely high Re content of 40 ppb–further evidence for compatibility of Re in magnetite. The paucity of sulfides in this suite suggests that magnetite is a significant host phase for Re. Re–SiO2 trends in the Alcedo suite can be modeled with a magnetite–melt partition coefficient (D) of 20–50 for a sulfide-free system and D of 20–26 for a sulfide-bearing system. Compatibility of Re in magnetite suggests a valence of 3+ or 4+ for Re in terrestrial magmatic processes. The distribution of Re and Os in cubic oxides may be controlled by crystal chemistry.