MARGINS: Controls on Magma Generation at Incipient Spreading Centers

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Intellectual merit. This study aims to identify the controlling parameters for magmatic diversity in two young rift basins (Salton Sea and Cerro Prieto) within the on-land extension of the Gulf of California. Despite similarities in basin structure and tectonic setting, magmatism in both locations differs strongly. Salton Sea surface and subsurface magmatic rocks are bimodal, whereas intermediate compositions predominate at Cerro Prieto. Because both magmatic systems are associated with economically important geothermal resources, dense three-dimensional sampling exists through geothermal well penetration. Drill cuttings from geothermal wells are invariably contaminated and rocks were exposed to intense alteration at high ambient temperatures. Thus, high spatial resolution uranium series geochronology coupled with oxygen isotopic analysis of zircon by secondary ion mass spectrometry (SIMS) has the strongest prospects for extracting reliable information on the temporal and spatial patterns of magmatic variability. Results from Cerro Prieto will complement existing data for the Salton Sea that indicate episodic rhyolite generation through remelting of hydrothermally altered basaltic crust since at least 400 ka. The integration of these results will provide insights into the changes in lithospheric structure during continental rupture, the composition and thermal state of the lower crust in rift basins, and the maturation of newly accreted crust during incipient ocean spreading.Broader impacts. This research impacts on strategies for renewable energy exploration, and fosters ties between academics and the geothermal industry. Users of the UCLA National Ion Microprobe facility will benefit from SIMS method development tailored to low-yield samples and in-situ analysis. This project provides opportunities for undergraduate research experience.