Abstract and subjects
We present (super 238) U- (super 230) Th- (super 226) Ra disequilibria and cosmogenic (super 3) He and (super 36) Cl data for the Bluewater flow of the Zuni-Bandera volcanic field in western New Mexico. The (super 238) U- (super 230) Th disequilibria measured on separated groundmass phases yield an internal isochron age of 68 ka (+24/-20 ka; 2sigma ). This value cannot be directly compared with surface exposure ages unless erosion rates are known. The apparent (zero erosion) ages determined from both the (super 3) He concentration (47.5+ or -5 ka; 2sigma ) and the (super 36) Cl concentration (41.2+ or -8.8 ka; 2sigma ) are significantly younger than the U-Th isochron age. When minimum estimates of surface erosion based on flow morphology are considered, the (super 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 (super 36) Cl age and, as a result, the (super 36) Cl age is significantly younger than the U-Th isochron age and erosion-corrected (super 3) He ages; this discordance is attributed to a lack of closed-system behavior in the (super 36) Cl 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.