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.
This project compared magmatic rocks in two active rift basins: the Salton Sea (southern California, USA) and Cerro Prieto (northern Baja California, Mexico). These basins are locations where continental crust breaks apart, giving way to incipient oceanic crust formation. Results from this study indicate that magmatism in this region is ongoing: the newly determined eruption age for Red Hill, a small rhyolite volcano at the southern end of the Salton Sea, is only approximately 2,500 years. Zircon crystals in magmatic rocks from the Salton Trough and Cerro Prieto also indicate that magmatic intrusions occurred in this location since many tens of thousands of years. Oxygen isotopic compositions in zircon together with Nd isotopic compositions in whole-rocks reveal that magmas are dominantly generated by re-melting of oceanic crust similar in composition to that of the East Pacific Rise, but overprinted by circulation of surface-derived waters. This process has previously not been recognized for the northern Gulf of California rift, and has implications for the deep structure and thermal balance at rifted continental margins. The analytical techniques that were developed and refined during this project benefit other researchers that use the UCLA National Ion Microprobe facility. This project also trained undergraduates in operating high-end analytical equipment. The outcome of this research has impacts on volcanic hazard assessment for the region, strategies for geothermal energy exploration, and also fostered ties between academic institutions and the geothermal industry.
