Behavior of Re during magma fractionation: an example from Volcán Alcedo

Rhenium concentrations have been measured in eight samples from strongly compatible (e.g. Allègre & Luck, 1980). Whereas Volcán Alcedo in the Galápagos Archipelago—four basalts, an there is some evidence for the compatibility of Os in a icelandite, a dacite and a rhyolite (all part of a fractionation trend), variety of common igneous minerals such as olivine and and a magnetite phenocryst-rich separate from a rhyolite. This study chromite (e.g. Brügmann et al., 1987; Hart & Ravizza, shows that Re concentrations increase (from 234 to 850 ppt) 1995), there is very little quantitative information avail-during the FeO and TiO2 enrichment trend, and indicates that Re able about the partitioning behavior of Re; such data is incompatible in olivine, augite and plagioclase. Once magnetite are essential if we are to be able to model the Re–Os stabilizes and fractionates, the iron and titanium enrichment trend system in magma chamber processes such as crystal in the melt reverses, and Re concentration drops from 610 ppt in fractionation and assimilation. the icelandite to 36 ppt in the rhyolite. The magnetite-rich separate A classic method for evaluating compatible and in-has an extremely high Re content of 40 ppb—further evidence for compatible behavior of elements in magmatic systems is compatibility of Re in magnetite. The paucity of sulfides in this through the use of chemical variation diagrams (e.g. suite suggests that magnetite is a significant host phase for Re. Harker, 1909; Bowen, 1928). For most tholeiitic suites, Re–SiO2 trends in the Alcedo suite can be modeled with a the variation of a given element can be linked to the magnetite–melt partition coefficient (D) of 20–50 for a sulfide-free appearance (or disappearance) of certain minerals in th