Experimental Calibration of the Olivine-melt Ni Thermometer Under Hydrous Conditions: Applications to Hygrometry, Oxybarometry and Olivine Phenocryst Growth Rates

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

Water plays a unique role in the planetary evolution of Earth, particularly at subduction zones. The recycling of oceanic water into the mantle at subduction zones, the tectonic setting where continental crust forms, triggers partial melting and the formation of hydrous basalts, which differentiate to form continental crust, unique to Earth within our solar system. One of the many challenges associated with the study of hydrous basalts is that current thermometers cannot be accurately applied to them. The goal of this project is to calibrate a Ni-based olivine-melt thermometer, which is independent of melt water contents and can thus be applied to hydrous basalts to obtain accurate temperatures of formation. It will also allow the concentration of water in basaltic liquids to be determined at the onset of crystallization. This project will engage two Ph.D. graduate students and at least three undergraduate students, who will be trained in the operation of various experimental apparatus and analytical techniques, as well as thermodynamic modeling; all of these tools and skills have important applications in material science. The project will support students from underrepresented groups in a research team that has demonstrated a commitment to expanding the diversity of those in the sciences. The specific goals of this proposal are to perform hydrous phase-equilibrium experiments in a cold-seal apparatus under controlled fO2 and fH2O conditions on a variety of natural basalt and basaltic andesite liquids over a range of temperature and pressure, which will enable calibration of how these parameters, together with melt composition, affect the partitioning of Ni between olivine and liquid. In addition, the depression of the olivine liquidus as a function of melt water content (up to ¡Ü 8 wt%) will be experimentally mapped out for several melt compositions. Once a well-calibrated thermometer and hygrometer are in hand, it will be applied to >100 basalts from the Mexican and Cascades volcanic arcs and compared to results from olivine-hosted melt inclusions. Two additional experimental goals are to measure crystal growth rates of olivine phenocrysts under hydrous melt conditions, especially for natural samples that contain olivine with diffusion-limited growth textures, and to test whether Fe-Mg KD values obtained in hydrous olivine-melt experiments can be used to place constraints on the oxidation state of natural basalts.