Journal of Petrology, Volume 39, Issue 4: April 1 1998.

Behavior of Re during Magma Fractionation: an Example from Volcán Alcedo, Galápagos

K. RIGHTER¹, J. T. CHESLEY², D. GEIST³ AND J. RUIZ²

¹LUNAR AND PLANETARY LABORATORY, UNIVERSITY OF ARIZONA, TUCSON, AZ 85721, USA ²DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, TUCSON, AZ 85721, USA ³DEPARTMENT OF GEOLOGY AND GEOLOGICAL ENGINEERING, UNIVERSITY OF IDAHO, MOSCOW, ID 83844-3022, USA

Rhenium concentrations have been measured in eight samples from Volcán Alcedo in the Galápagos 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.

Keywords: fractionation;Galápagos; magnetite; rhenium; osmium

Pages 785-795