Compared to mid-ocean ridges, the life cycle of back-arc spreading is short, averaging ~15 Ma of extension between arc-ward ridge jumps. Once initiated, back-arc volcanism evolves from relatively arc-like to relatively MORB-like compositions (Taylor and Martinez, 2003; Langmuir et al., 2006). Both of these observations are ascribed to the over-increasing distance to the back-arc spreading center from secondary convective upwelling above a subduction zone (Taylor and Martinez, 2003; Martinez and Taylor, 2006). This hypothesis makes substantial predictions for the geochemistry of any mantle rocks that may be exposed during the end phase of the magmatic system. Melting should become drier, productivity lower and lithospheric melt stagnation more common as a back-arc rift approaches the end of its life cycle. We propose to test this hypothesis using mineral chemistry of spinel and clinopyroxene, and whole rock Os isotopes from a substantial suite of abyssal peridotites recovered from a time line from the Godzilla Mullion in the Parace Vela Rift, the former back arc spreading center behind the Mariana arc (19-20 Ma). A declining magmatic budget would confirm the notion that secondary slab upwelling is the driving force in back-arc spreading in general, and suggests a mechanism for triggering ridge jumps. Preliminary mineral chemical data suggests that a coherent and interpretable melting signal is present in the Godzilla Mullion mantle rocks. Os isotopes will be used to determine whether the depleted signatures found are representative of the recent geodynamic configuration or are inherited ancient signals (e.g., Liu et al., 2008). This work will also provide an important comparison between in-situ back-arc and fore-arc mantle rocks, and serve as a petrologic characterization of mantle rocks supporting IODP drilling proposal 640-Full. This proposal furthers the basic understanding of core-complex style ocean crustal formation in a back-arc. This is an important new and still relatively poorly understood type of crust accretion. For this reason, it belongs to a class of basic geologic problems related to the formation of the ocean floor that can best be investigated via direct ocean floor observations. This project will support the career development of an excellent young female scientist, who will help mentor undergraduate and graduate student participants. She will also extend her current skill set in learning LA-ICP-MS and Os isotope techniques, and participate in research and teaching as appropriate. Grad student support is for an undergrad who began as an REU student on the sampling cruise. This project is part of a collaboration of long standing between the PI, the Japanese Research Institute, JAMSTEC, and the Japanese Hydrographic Office. Separate funding has been acquired for minority undergraduates, including two female undergraduates interested in pursuing work in this area for later graduate school. The UH is the second most diverse among 250 ranked universities (US News and World Report, 2010), serving the surrounding working class Houston community. This project provides an essential point of contact between students from traditionally underserved groups (~60%) at UH and the cutting edge of scientific research. The results of this study also flow directly into the teaching of the PI, who had 105 beginning mineralogy students in Fall 2009, a majority of whom were minorities. The impact on traditionally underserved communities is thus disproportionately large.
