ABSTRACT

Recent models of rifting in the Southwest United States have suggested that the mantle beneath the North American Plate has undergone hydration, potentially from shallow subduction of the Farallon Plate beneath the North American Plate from ~80-40 million years ago. The addition of water and other volatile elements (chlorine, sulfur, fluorine) to the continental mantle (lithosphere) may have a profound influence on volcanism and tectonic processes. Water (along with volatile and fluid-mobile trace elements) added to the mantle reduces the melting temperature, allowing magmas to be generated at lower temperatures. Additionally, hydration of the mantle weakens the lithosphere, potentially allowing for a greater extent of rifting. Despite the clear significance of hydration of the mantle, direct constraints on the amount of water in the lithosphere or the source(s) of this water are few. The primary focus of this research therefore is to measure volatile abundances in basaltic lavas to determine the extent to which volatiles have been added to the continental lithosphere. This study will concentrate on the Rio Grande Rift in New Mexico- the extent of rifting varies from N-S making this an ideal location to test for a correlation between hydration of the lithosphere and the extent of rifting and volcanism.

Because magmas degas upon eruption on the Earth's surface, precise measurements of volatile abundances can be problematic. In order to address this dilemma, melt inclusions, small parcels of magma trapped within minerals during crystallization and prior to degassing, will be analyzed to provide constraints on the volatile content of primitive basaltic magmas. Primitive lavas will be collected for this study from N-S and E-W transects along and across the Rio Grande Rift to adequately compare volatile concentrations to the extent of rifting and to look for lateral variations in the degree of mantle hydration. Additionally, lavas of varying ages will be examined to address potential temporal variations in the hydration of the lithosphere and potential transition from lithospheric to asthenospheric mantle melting. Despite prior evidence of enrichment in fluid-mobile trace elements (e.g., high Ba/Nb and Sr/Nd), the source of the enrichment, either in the mantle or the crust, has long been debated. In this new approach, the combination of fluid-mobile trace elements (characteristic of subduction) and volatile concentrations from melt inclusions with whole rock isotope (Sr-Nd-Pb) and trace element analysis will allow us to distinguish potential crustal and mantle components, a significant obstacle to positively identifying mantle hydration. Analytical techniques for melt inclusions have greatly benefited from advancement in microbeam technology. Major elements and volatiles (S, Cl, and F) will be analyzed by electron microprobe at Oregon State University while trace element abundances will be measured by ion microprobe at Arizona State University. Whole isotope analyses will be conducted using the new TIMS facility at University of Texas at Austin resulting in collaborative research at several institutions. Broader impacts of this research include training of undergraduate students in petrologic techniques and the continued development of early career scientists.

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