Genesis of Harrat Basalts in Western Saudi Arabia

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This project seeks to understand the formation of large-volume basalt lava fields that erupt onto continental crust, where they provide important nutrients for terrestrial and aquatic life as well as geothermal and mineral resources for human use. The research is pursued in Western Saudi Arabia because the thermal and material structure of the crust itself is a barrier to basalt volcanism, and yet the study areas have been volcanically active for up to 30 million years. The work uses the geochemistry of basalt lavas as a lens through which to explore the long-term evolution of the lowermost crust, i.e., the “underlayer” of the continent that cannot be observed directly. The research project will determine how plate tectonic processes acting in the region have affected crustal thickness and integrity, potentially thinning and weakening the continent substantially and thereby encouraging volcanic activity. The work is conducted in partnership with Saudi colleagues who are interested in diversifying their economic portfolio beyond petroleum and natural gas resources. Funds from this project support a doctoral candidate and several undergraduate students from backgrounds that are underrepresented in the geosciences; help develop a middle-school learning unit in cooperation with a local teacher and a museum display focused on the artistic, cultural, religious and ecological significance of the Saudi lava fields. This work will create new knowledge about the long-term evolution of our planet that helps us prepare for a sustainable and diverse future. Continental basalts are the most common subaerial volcanic features on Earth but melting beneath thick continental crust is difficult to achieve. Geochemical and geochronologic data, integrated with geophysical evidence, will be used to determine the mechanism(s) of mantle melting and the mineralogic, isotopic and thermo-barometric parameters of mantle source domains that contribute to alkaline mafic volcanism in two long-lived western Arabian Harrats (Uwayrid and Ash Shaam). The geochemical and isotopic investigation will document evolution of source domains and mantle melting mechanisms in a region of complex tectonics and will test whether rejuvenated mafic alkaline volcanism in two Arabian harrats results from lithospheric drip magmatism. Geochemical evidence for this process has focused on documenting individual small-volume volatile-rich melt batches, contributions from a pyroxenitic source, small-scale temporal trends of increasing melting depth and temperature, and a positive correlation between depth and degree of mantle melting. The research will pursue this evidence through robust major and trace element analysis of lavas and individual crystals in lavas and xenoliths from Harrats Uwayrid and Ash Shaam. High precision 40Ar/39Ar dating will establish temporal trends in magma chemistry, and Sr-Nd-Pb-Hf-He isotopic study of lavas and xenoliths will establish source characteristics. The research explores contributions from the Afar plume, ambient upper mantle and metasomatized lithosphere, and documents spatial and temporal variations to these contributions across the Arabian plate. The findings have implications for transport of plume material, for the development of gravitationally unstable zones in metasomatized lithospheric mantle, and for evolution of the Dead Sea Fault and northern Red Sea.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.