COSMOGENIC 3HE SURFACE EXPOSURE DATING OF STRAWBERRY CRATER IN NORTHERN ARIZONA, USA: RESULTS FROM THE SAN FRANCISCO VOLCANIC FIELD REU PROGRAM

The San Francisco Volcanic Field (SFVF) is located in the southern portion of the Colorado Plateau in northern Arizona. Volcanism in the SFVF began about 5 Mya and began to taper off a few hundred thousand years ago, with the most recent eruption occurring around 1080 C.E. at Sunset Crater. This study focuses on Strawberry Crater, a cinder cone and associated lava flow in the SFVF with no previous reliable age constraints. Prior to this study, the only age available for Strawberry was an uncertain K-Ar determination of 46 ± 46 ka. Here, we apply cosmogenic ³He surface exposure dating to the Strawberry Crater cinder cone and lava flow, joining previous ³He studies in the SFVF on the South Sheba, SP Crater, Doney Mountain, and Grand Falls lava flows. Strawberry Crater’s agglutinated, crescent-shaped cone rises 170 m above the surrounding landscape. The associated lava flow has three lobes that cover 6.6 km² and attain a maximum flow length of 3.8 km. Olivine and pyroxene phenocrysts are common in the cone agglutinate but sparse in the lava flow. Five sites were sampled for helium measurements, including three surfaces on the cone rim and two surfaces on the lava flow. Samples on the cone were collected from the outer surfaces of breadcrust bombs welded into the agglutinated rim of the cone. Surfaces sampled on the flow are from squeeze-ups near the margins of the flow with fine surface textures and polygonal cooling cracks. Initial helium data from pyroxenes in the three cone samples yield exposure ages averaging 50±1 ka. One sample yielded a slightly older age, which may be explained by its steeply dipping angle allowing the surface to shed snow or scoria cover that could block cosmic rays and cause a low apparent age. Helium analyses of the two samples from the Strawberry lava flow are in process and will enable comparison of exposure ages from the lava flow surface and the agglutinated cone rim. Our new data provide the first reliable age for Strawberry Crater, and are anticipated to provide insight on potential differences in erosion rates and exposure histories between the lava flow and cone rim.