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

¹ Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
² Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA
³ Department of Geological Sciences, Central Washington University, Ellensburg, Washington 98926, USA
⁴ Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
⁵ Nuclear and Radiochemistry, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
⁶ Department of Earth and Planetary Science, MC4767, University of California, Berkeley, California 94720-4767, USA

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; 2). 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; 2) and the ³⁶Cl concentration (41.2 ± 8.8 ka; 2) 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.

Key Words: dating Quaternary basalts • U-Th-Ra disequilibria • ³He and ³⁶Cl cosmogenic ages • erosion rates

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