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

Abstract:

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.