OSL DATING OF QUARTZ XENOCRYSTS FROM THE STRAWBERRY CRATER BASALT FLOW, SAN FRANCISCO VOLCANIC FIELD, ARIZONA
Six samples were collected, two from each of the three lobes of the basalt flow. Under darkroom conditions the outer edges of the rocks were removed and inner portions were crushed and sieved before the quartz xenocrysts (3-7% by volume) were isolated using heavy-liquid separation. To ensure purity, quartz grains were hand-picked under a microscope (four per aliquot) and the response to infrared stimulation was monitored. The single-aliquot regenerative-dose (SAR) technique (Murray and Wintle, 2000) was employed following procedures developed for xenocryst dating of a nearby basalt flow (Rittenour et al., 2012). Representative crushed-rock samples were analyzed using ICP-MS for dose-rate calculation.
Preliminary results produced age estimates of 2-4 ka. These apparent ages are far younger than new well-constrained late Pleistocene OSL ages obtained from the loess overlying the flow. In light of the age of the overlying loess, degree of soil development on the Strawberry Crater flow and poor luminescence characteristics of the xenocrysts, we interpret the xenocryst OSL results as unreliable. Moreover, OSL xenocryst results for the nearby SP Crater flow also returned ages that were much younger than results from other dating techniques (Rittenour et al., 2012). Age underestimation from quartz xenocrysts is likely due to weak luminescence signals that are dominated by unstable medium and slowly decaying components.
While this study did not provide a reliable age for the Strawberry Crater flow, it did identify problems with and the inaccuracy of the application of OSL dating to quartz xenocrysts. Further testing and refinement of the technique will be required to date volcanic xenocrysts using OSL.