COSMOGENIC 3HE SURFACE EXPOSURE DATING OF SP CINDER CONE AND LAVA FLOW IN NORTHERN ARIZONA, USA: RESULTS FROM THE SAN FRANCISCO VOLCANIC FIELD REU PROGRAM

The San Francisco Volcanic Field (SFVF) in northern Arizona contains over 600 volcanoes, many of which remain undated or lack reliable ages. SP Crater is a basaltic cinder cone in the northern SFVF with steeply sloping sides composed of loose scoria. An associated lava flow, which emerged from the north side of the cone, is characterized by blocky features, lava levees, and squeeze-ups, with many surfaces that retain original flow textures. Previously reported ages on the flow range from ~70 ka using K/Ar, Ar/Ar, and cosmogenic ²¹Ne and ³He dating techniques to ~6 ka using optically stimulated luminescence on quartz xenocrysts in the flow. The large discrepancy between previous age determinations in combination with the geomorphically young appearance of the SP Crater cone and flow have led to controversy over the true age of these features. To further resolve the age of the SP Crater eruption, we measured cosmogenic ³He concentrations in pyroxene from thirteen sites; including five bombs embedded in agglutinate on the cone rim and eight squeeze-ups on the flow with original flow surfaces. Initial results place a mean age of 59 ±1 ka on the flow and an age of 50 ±1 ka on the cone. These new data broadly agree with previously reported K/Ar, Ar/Ar, ²¹Ne and ³He age estimates, but lead to more precise exposure-age determinations than in prior work. Pyroxene compositions were obtained by electron microprobe analyses of each sample and range narrowly from En_48.3 to En_49.6, showing that compositional influences on the ³He production rate are not responsible for the apparent age difference between the cone rim and the flow. We hypothesize that prior cinder cover on the cone rim, while no longer present at the sample sites, may have blocked incoming cosmic radiation in the past, accounting for the younger apparent age of the cone surfaces relative to the lava flow exposure ages. Additional samples from both the cone and the flow are being processed for ³He exposure ages in order to further evaluate the apparent age discrepancy between the SP flow and cone.