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Rapid fractionation of copper with magmatic volatiles in volcanic systems

Lucy McGee 1,2, John Foden1, Justin Payne2,3

  • Affiliations:  1Department of Earth Sciences, University of Adelaide, Adelaide, Australia; 2Mineral Exploration Cooperative Research Centre, Australia; 3University of South Australia STEM, Adelaide, Australia

  • Presentation type: Poster

  • Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 23

  • Programme No: 4.1.14

  • Theme 4 > Session 1

Abstract

With evidence for hydrous volatile-rich evolution, saturation and degassing, active and well-sampled arc volcanoes like Soufriere Hills Volcano (SHV) on the island of Montserrat are targets where volatile partitioning and transport of metals out of the silicate melt can be assessed. SHV andesites entrain enclaves of more mafic composition formed at the interface between rising magma and stalled fractionated magma bodies. Previous (210Pb/226Ra)0 isotope analyses (which track recent (<100 years) volatile loss or gain in the magmas) showed volatile capture by the enclave-forming mafic magma at the start of SHV's most recent eruptive episode, by contrast with the andesites with evidence of continual degassing. The samples were analysed for δ65Cu as a monitor of Cu fractionation during transport between separate phases. The andesite lavas have uniform δ65Cu values close to 0‰. The earliest enclaves however show quite negative δ65Cu values (down to -2.5‰) and there is positive linear covariation of d65Cu with age. Correlation between 210Pb-excess and negative δ65Cu in the early enclaves suggests a fractionation mechanism for Cu relating to magmatic volatile phases operating at the beginning of the eruptive episode and waning with time. This also puts a time constraint of <100 years on this mechanism. Transport of Cu via exsolved fluid is unlikely as heavy δ65Cu would be expected, rather than the negative δ65Cu (and high elemental Cu) observed. A source of light δ65Cu could involve stripping of Cu from magmatic sulphides in the deeper crust during periods of mafic magma recharge and fluid saturation during decompression.