OSL DATING OF XENOLITHS IN THE SP FLOW, SAN FRANCISCO VOLCANIC FIELD, NORTHERN ARIZONA

SP Crater is a basaltic cinder cone in the San Francisco Volcanic Field of northern Arizona. Previous studies, using ⁴⁰Ar/³⁹Ar and K-Ar dating of the lava flow emanating from the base of the cone, suggest that the eruption occurred 70±4 ka. This age, however, is inconsistent with the unweathered appearance of the cone and flow. The SP lava flow contains ~5% phenocrysts of 1-2 mm olivine and ~2-3% of 1mm plagioclase. The flow also contains ~2-3% of quartz xenoliths (up to 4 mm in diameter), likely derived from the Proterozoic basement in the region.

Optically Stimulated Luminescence (OSL) dating is commonly used to date Quaternary sediments. Although the older thermoluminescence (TL) dating technique has been has been applied to lava flows, direct OSL dating of xenoliths within a flow has yet to be accomplished. Traditionally, the OSL technique dates the most recent exposure of sediments to sunlight, which resets the luminescence signal. Heat is also capable of resetting this luminescence signal. Because the xenoliths in the basalt are assumed to have been heated far beyond the luminescence annealing temperature for quartz of 500ºC, the date obtained reflects the time of eruption. Quartz xenoliths were isolated from three bulk samples under dim amber laboratory light conditions and mounted for analysis at 3-4 grains per aliquot. Samples were analyzed using the single-aliquot regenerative-dose (SAR) procedure (Murray and Wintle, 2000).

Preliminary OSL results suggest that the eruption of SP Crater and flow occurred at ~ 2-4 ka. These new results fit with local geomorphic relations and have significant implications for eruption frequencies in the San Francisco Volcanic Field. This new application of OSL dating has great potential for dating young basalt flows that contain quartz xenoliths and that have proven difficult to date by other techniques.