Date of Award

1-1-2011

Document Type

Campus Access Thesis

Department

Earth and Ocean Sciences

Sub-Department

Geology

First Advisor

Michael Bizimis

Abstract

Mantle peridotite xenoliths found within the rejuvenated stage lavas on the islands of O`ahu and Kaua`i, Hawaii provide a unique window into the oceanic mantle. These peridotites are thought to represent variably metasomatized fragments of the Pacific lithosphere. If these peridotites are undisturbed MORB residues, there should be a strong correlation with depth and melt depletion, with the most fertile peridotites being the deepest. I will present high precision Ca concentration data on high purity olivine separates from Hawaiian peridotites obtained by standard addition HR-ICP-MS to determine the depth at which these peridotites last equilibrated, and test the depth vs. depletion hypothesis. Together with new olivine major and trace element data, combined with preexisting clinopyroxene (cpx) and orthropyroxene major and trace element data, I have reconstructed the P-T range of the Hawaiian spinel peridotites using the Ca-in-olivine barometer and the cpx-opx thermometer (TBKN). To correctly apply the thermobarometric equations, I first use the trace element concentrations in olivine and the apparent olivine/ cpx partition coefficients to screen the samples for disequilibrium processes. After this correction, the DCaol/cpx shows no correlation with other highly incompatible elements (e.g. Sr, Ba, Ce), while the DHREEol/cpx overlap both experimental and theoretical values suggesting high temperature equilibrium. The calculated equilibrium pressures for the Salt Lake Crater peridotites show a deeper origin, (up to 37kb, i.e. the base of the lithosphere) than the Pali vent peridotites (11 to 15kb) and Kauai peridotites (up to 20kb). This is consistent with the presence of deeper garnet pyroxenites found at the Salt Lake Crater vent and their absence in the other locations. These data is the first direct thermobarometric evidence that the Salt Lake Crater peridotites indeed represent the deeper parts of the oceanic lithosphere than other xenolith-bearing localities in Hawaii. When looking at depletion indicators, such as Cr in spinel, there is no clear correlation between pressure and melt depletion, which would be expected for undisturbed MORB residual lithosphere. The deeper origin of the Salt Lake Crater peridotites together with previous data that shows highly unradiogenic 187Os/188Os values and ancient (~2Ga old) Re-depletion ages for many of the same samples, suggests that the base of the oceanic lithosphere under Hawaii may contain fragments of ancient recycled oceanic lithosphere that is brought to shallow levels by the Hawaiian plume.

Rights

© 2011, Shawn William Wallace

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