Determining eruption ages and erosion rates of Quaternary basaltic volcanism from combined U-series disequilibria and cosmogenic exposure ages

We present U-238-Th-230 -Ra-226 disequilibria and cosmogenic He-3 and (CI)-C-36 data for the Bluewater flow of the Zuni-Bandera volcanic field in western New Mexico. The U-238-Th-230 disequilibria measured on separated groundmass phases yield an internal isochron age of 68 ka (+24/-20 ka; 2 sigma). This value cannot be directly compared with surface exposure ages unless erosion rates are known. The apparent (zero erosion) ages determined from both the 3 He concentration (47.5 +/- 5 ka; 2 sigma) and the (CI)-C-36 concentration (41.2 +/- 8.8 ka; 2 sigma) are significantly younger than the U-Th isochron age. When minimum estimates of surface erosion based on flow morphology are considered, the 3 He concentrations indicate a minimum exposure age of 60 ka, in good agreement with the U-Th isochron age, with a minimum erosion rate of 1.7 mm/k.y. and an erosion rate as high as 5 mm/k.y. in other locations. Correcting for erosion has little effect on the model (CI)-C-31 age and, as a result, the (CI)-C-36 age is significantly younger than the U-Th isochron age and erosion-corrected He-3 ages; this discordance is attributed to a lack of closed-system behavior in the (CI)-C-36 system. These new ages have local significance for the geochronology of the Zuni-Bandera volcanic field; however, their larger significance is in their applicability to dating Quaternary basalts and quantifying erosion rates.