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Master of Science (M.S.)

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Iceland, Rhyolite, Partial Melt, Fractionation, Magma Mixing


The 8-9 million year old volcanic complex of Oxnadalur is host to large-volume basalt flows, small and large volume rhyolite ash and lava flows, and a gabbroic intrusion. Both the plagioclase and pyroxene phenocrysts of the basalt are larger in size in the younger flows. The rhyolite ashes contain no primary crystals, but numerous basalt xenoliths and pumice fragments. The rhyolite lava flows are banded, with only the oldest containing phenocrysts of sanidine and plagioclase. One rhyolite flow is a mingled hybrid of two glasses, each containing plagioclase, pyroxene, and hornblende. Whole rock major and trace element analyses indicate a mixing trend among all of the units in the complex; yet abundant xenoliths in the ashes make this less data less dependable. In situ major and trace element analyses were performed via electron microprobe show two distinct populations in the variation diagrams, with the basalts and rhyolites separated by a compositional gap. Electron microprobe analyses also show that the plagioclase of the basalts and the gabbro are normally zoned with distinct calcic cores and sodic rims; this is also true for the mingled hybrid flow. Rare earth element analyses done via laser ablation inductively coupled plasma mass spectrometry, show that the phenocrysts are enriched in the light and depleted in the heavy rare earth elements. Rare earth element abundances in the glasses have a trend similar to that of ocean island basalt rather than that of mid ocean ridge basalt. Plagioclase geothermometry and amphibole geobarometry indicate that the magma chambers were replenished by new batches of melt and may have existed at a shallow level in the crust just prior to being erupted. Oxygen isotope ratios are depleted compared to those of typical mid ocean ridge basalts, typically indicating that the source melt was partially melted from a hydrothermally altered layer in the crust. As the δ18O values are whole rock, the depletion may be the result of any sub solidus interaction with low δ18O water. The data indicate that multiple shallow reservoirs evolved separately, with limited communication while being intruded by new magma throughout the lifespan of the complex.

First Advisor

Sheila J Seaman