Experimental Investigation of the Link Between Water Loss and Oxygen Fugacity in Olivine-Hosted Melt Inclusions

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

Intellectual Merit. Water plays a central role in the generation and evolution of magmas in every tectonic setting. Difficulties in determining pre-eruptive water concentrations in magmas have hindered quantification of its influence on magmatic processes. The solubility of H2O in silicate melts drops substantially with decreasing pressure, so that a magma containing several weight percent dissolved H2O in the shallow crust is left with only a few thousand parts-per-million H2O following eruption. Olivine-hosted melt inclusions provide a direct source of information on the pre-eruptive H2O contents of degassed magmas because the strength of the host crystal protects the melt inclusion from the decompression experienced by the entraining magma. The main uncertainty involved with deriving pre-eruptive H2O concentrations from olivine-hosted melt inclusions is the potential for loss or gain of H+ (protons) by diffusion through the host olivine. Loss or gain of H2O from olivine-hosted melt inclusions via lattice diffusion requires leaving behind an O2- for every 2 protons lost, and scavenging an O2- for every 2 protons gained in order to maintain charge neutrality. This produces an increase or decrease, respectively, of the fugacity of oxygen within the inclusion. It has, therefore, been proposed that Fe redox reactions associated with ingress or egress of H+ severely limit the amount of H2O that can move into or out of an olivine-hosted melt inclusion. However, recent experiments indicate that iron redox reactions do not limit either the amount or rate of water loss/gain by the inclusion, suggesting that oxygen fugacity within the inclusion is moderated by a previously overlooked mechanism. Conceptually, H2O loss/gain in melt inclusions may be linked to point defect-mediated oxygen fugacity re-equilibration. This model suggests that oxygen fugacity fluctuations in melt inclusions are efficiently and effectively moderated by re-equilibration with the host olivine. An experimental and theoretical study is proposed to rigorously test this model and to provide a more thorough understanding of the process of H2O loss/gain for olivine-hosted melt inclusions. Goals are to quantify the rate-limiting process for H2O loss/gain in olivine-hosted melt inclusions through a series of (1) dehydration experiments, carried out on naturally occurring H2O-rich inclusions, and (2) hydration experiments carried out on low-H2O inclusions using H2O that is isotopically enriched in both 18O and D (2H). Results from these experiments, combined with existing partitioning and diffusion data from the literature, will be used to develop a numerical model to evaluate timescales for diffusive re-equilibration of H2O in olivine-hosted melt inclusions over a range of geologic scenarios. The model will be made broadly available to the scientific community to be used as a tool for interpreting natural melt inclusions. The combined results from the experimental and theoretical portions of the proposed study will provide the insights necessary to more accurately assess the reliability of olivine-hosted melt inclusions as indicators of pre-eruptive H2O contents of degassed lavas. Broader Impacts. The proposed research (1) advances discovery and understanding while promoting teaching, training and learning, (2) broadens participation of underrepresented groups, and (3) enhances infrastructure for research and education through the involvement of graduate students in the MIT/WHOI Joint Program, undergraduate students participating in the WHOI Summer and Minority Student Fellow programs, and incorporation of results from this study into graduate courses taught by the PIs and by development of a numerical model that will be made broadly available to the scientific community to be used as a tool for interpreting natural melt inclusions.