Abstract

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

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