Olivine-hosted melt inclusions as an archive of redox heterogeneity in magmatic systems

Friday, 19 December 2014
Margaret E Hartley1, Oliver Shorttle1, John Maclennan1, Yves Moussallam2 and Marie Edmonds1, (1)University of Cambridge, Cambridge, United Kingdom, (2)CNRS, Paris Cedex 16, France
Crystal-hosted melt inclusions are able to preserve information about the geochemical diversity of melts present within magmatic systems, including information about both the oxygen fugacity (fO2) of their mantle source and the redox evolution of their carrier melt. However, the ferric iron proportions (Fe3+/ΣFe) measured in olivine-hosted melt inclusions are partially controlled by post-entrapment processes, such that inclusions may no longer preserve a record of the fO2 at which they were trapped. Post-entrapment crystallisation (PEC) of olivine onto the inclusion walls during cooling sequesters Fe2+ into olivine. Olivine-hosted melt inclusions may also maintain H2O and fO2equilibrium with their external environment via coupled proton and metal vacancy diffusion through the olivine crystal lattice.

In this study we present a combination of XANES, major, trace and volatile element (C, H, S, F, Cl) analyses from a suite of 100 olivine-hosted melt inclusions from the AD 1783 Laki eruption, Iceland. The inclusions are hosted in Fo86-Fo68 olivines, and have experienced up to a maximum of 7% PEC. They preserve a diverse range of melt compositions similar to that seen in global mid-ocean ridge basalts. Composition-dependent CO2-H2O solubility models have been used to determine the pressures of inclusion trapping. Many of the melt inclusions have experienced diffusive H+ re-equilibration with their external environment: trace element depleted inclusions with low initial H2O concentrations have gained H+ via diffusive exchange with a more H2O-rich carrier melt, which is a consequence of concurrent mixing and crystallisation of diverse primary melt compositions in the Laki magmatic system. This sample set therefore presents a unique opportunity to deconvolve the post-entrapment crystallisation and diffusion processes that modify Fe3+/ΣFe in olivine-hosted melt inclusions, permitting the recovery of the true extent of magmatic redox variability present at the time of inclusion trapping.