Geochemical Investigation of Xenoliths From the Central Rio Grande Rift and Colorado Plateau: Constraints on Lithosphere Evolution and Possible Delamination

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"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."Intellectual Merit: This proposal seeks funding to study the evolution of the lithospheric mantle beneath the central Rio Grande Rift and Colorado Plateau margin. The project will provide insights into a variety of questions related to the tectonic and volcanic evolution of this province, including the role of Farallon plate subduction and dehydration in generation of "enriched" mantle sources of Rio Grande Rift lavas. However, this proposal seeks primarily to answer one simple question: Has the original Proterozoic-age lithospheric mantle beneath the central Rio Grande Rift and Colorado Plateau margin been actively removed by delamination or convective erosion, as suggested by several recent geophysical studies? If correct, these studies suggest that lithosphere delamination, previously recorded beneath the Sierra Nevada, may be more prevalent than currently recognized and may be triggered by extensional as well as compressional tectonic regimes. Correlation of Hf- and Os-isotope variations in peridotite xenoliths with mineralogic constraints on degree of melt depletion will allow constraints on the timing of melt depletion recorded in xenoliths from the Rio Grande Rift and Colorado Plateau. Also, peridotite clinopyroxene trace element abundances combined with Sm-Nd isotope compositions will constrain the age of metasomatic overprints observed in portions of the Colorado Plateau and Rio Grande Rift. Together, these data will allow us to distinguish between Proterozoic, melt-depleted and possibly metasomatized lithospheric mantle and young, fertile asthenosphere. This study will thus provide a direct test of the lithosphere delamination model proposed by Gao et al. (2004) and Song & Helmberger (2007) to explain the presence of anomalously low seismic velocities in the shallow mantle beneath the central rift and plateau margin and complementary, anomalously high velocities within a slab-like structure dipping beneath the Central Plains lithosphere. Identification of asthenosphere-derived peridotite at the relatively shallow depths (~40-60 km) represented by peridotite xenolith suites throughout the region would support the proposed delamination models, and would be inconsistent with simple passive thinning of the lithosphere in response to extension. In contrast, identification of ancient lithosphere at these depths would suggest significantly less lithosphere removal and would require reevaluation of current interpretations of the available seismic tomographic data. Insights gained from this study on the chemical and physical evolution of lithospheric mantle prior to and during continental rifting will expand our understanding of the triggers and mechanisms involved in continental rifting and continental break-up. This remains a fundamental question in plate tectonics.Broader Impacts: Students at the graduate and undergraduate level will participate in all aspects of this project, from data acquisition to interpretation and dissemination of results at national conferences and through peer-reviewed publications. Undergraduate student involvement in fieldwork and analytical aspects of the proposed research will allow the selected student to discover the excitement of scientific research outside the classroom. One undergraduate has already completed an honors thesis examining the petrology of xenoliths from our study area. We plan to extend the success of this initial pilot project by incorporating additional undergraduates into all aspects of the research program. This proposal will foster cross-discipline collaboration and partnerships through organization of interdisciplinary workshops focused on the evolution of the Rio Grande Rift and Colorado Plateau.