Collaborative Research: Non-peridotite Melting in Plume-influenced Extensional Environments: Lithological Heterogeneity of the African Superplume and African Lithosphere

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Intellectual Merit. This project seeks to determine the source lithology (e.g., peridotite, pyroxenite) of mantle melts that contributed to Oligocene Ethiopian continental flood basalt volcanism that preceded development of the East African Rift. Infiltration of mantle melts can play a central role in the initiation and evolution of lithospheric thinning by weakening and focusing strain. Thus, understanding the composition, and in particular the mineralogy, of the sources of mantle melts is critical to constrain the conditions attending melt generation. Non-peridotite lithologies such as pyroxenite in an upwelling plume or metasomes within the lithospheric mantle are potentially fertile mantle sources capable of producing significant volumes of magmas. In contrast typical peridotite lithologies may not produce significant melt under these conditions (i.e., smaller degrees of mantle decompression, or lower values of mantle potential temperature). Moreover, mantle underlying the study area is characterized by significant seismic attenuation, which has been attributed to the presence of an African superplume upwelling of hot mantle material. The proposed study aims to investigate the relative contributions of lithospheric vs. plume-like mantle as primary magma sources. Specifically, mantle source lithologies (i.e. peridotite versus pyroxenite) will be evaluated via geochemical characterization of a well-preserved 2000 m thick section of flood basalts from the western Ethiopian Plateau that has never before been studied. The possible contribution of enriched lithospheric mantle to melt generation will be assessed through a parallel study of unusual Si-undersaturated rocks erupted in a Miocene shield volcano, for which preliminary data imply magma derivation from ancient African lithospheric mantle. The analytical program will involve measurement of major and selected trace elements, Sr-Nd-Pb-Hf isotopes, and minor elements in olivine, and will be interpreted within a temporal framework to be established by new Ar/Ar geochronology and paleomagnetic studies designed to constrain the timing, duration, and eruptive rates of these deposits. Together these data will provide new insights into processes of basalt generation. Broader Impacts. The data resulting from this project will offer the first petrologic insights into the origin of the geophysically-deduced compositional component of the African superplume and will be directly relevant to ongoing geophysical projects ongoing in the region. This project will support the early career research of Tyrone Rooney (Ph.D., 2006), and fosters international collaboration through interactions with researchers at Addis Ababa University. The project represents a new collaboration between scientists at five US institutions (Rooney-MSU, Herzberg-Rutgers, Kappleman/Holt-UT Austin, Spell-UNLV, and Konter-UTEP), all of whom will contribute to the analytical measurements, interpretations, and modeling. The project utilizes and extends the applications of state-of-the-art analytical facilities at Michigan State University (laser ablation ICP-MS), Rutgers University (electron microprobe), UT Austin (paleomagnetic lab), UNLV (Noble gas MS), and UTEP (MC-ICPMS). The project also will support mentoring of a Ph.D. student from MSU through the wide range of collaborative interactions at the five US institutions and researchers in Ethiopia.