Date of Award

Spring 2019

Document Type

Open Access Thesis

Department

Earth and Ocean Sciences

First Advisor

Michael Bizimis

Abstract

Water influences geodynamic processes such as melting, deformation and rheology. Yet its distribution in the oceanic upper mantle is primarily known indirectly from melt inclusions and glasses of erupted mantle melts (i.e. MORB and OIB). To constrain the mechanisms influencing the distribution of water in the mantle, we analyzed 15 peridotite xenoliths from Savaiʻi and 2 dunite xenoliths from Taʻū (Samoa) for water concentrations (by polarized FTIR) and major/trace elements. Savaiʻi peridotites are among the most melt depleted oceanic xenoliths recovered (degrees of melt depletion ≥15%). They show strong evidence of transient metasomatism by both carbonatite and silicate melts, with highly variable Ti and Zr depletions and LREE enrichments down to the grain scale, but which are up to an order of magnitude more depleted than expected for metasomatism. LA-ICP-MS analyses of inclusion-rich and - poor domains within single mineral grains reveal significant heterogeneity and patterns with strong carbonatite and silicate characteristics. Despite this, water concentrations of olivines (up to 4 ppm H2O) and orthopyroxene (17 to 89 ppm H2O) are among the lowest reported in oceanic xenoliths. Orthopyroxene H2O correlate with indicators of melt depletion (i.e. Al, Mg#) and not metasomatism (i.e. LREE, Ti). Yet, they are higher than expected for melt depletion, but also up to two orders of magnitude lower than expected for metasomatism by presumably water-rich melts. FTIR transects across mineral grains show generally flat water profiles, indicating no significant water loss during ascent, and show no significant difference between inclusion-rich and -poor domains. Also, Raman Spectroscopy on single inclusions shows the presence of CO2 but lack H2O. If the melt-inclusion trails had several weight % water, as estimated for carbonatites, diffusion of water from the inclusions into the host phase would lower their water contents and simultaneously enrich water across the grain. This mechanism, but on a larger scale, likely also affected the Taʻū dunites. Their clinopyroxenes show trace element systematics consistent with their host lavas, but with water contents several times lower than expected. Thus, melt-rock interactions in the oceanic lithosphere are complex processes that result in variable hydration of the lithosphere.

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© 2019, Aaron Wolfgang Ashley

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