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Cosmogenic 3He exposure ages of basalt flows in the northwestern Payún Matru volcanic field, Mendoza Province, Argentina

2012; Elsevier BV; Volume: 19; Linguagem: Inglês

10.1016/j.quageo.2012.10.004

1878-0350

David W. Marchetti, Scott A. Hynek, Thure E. Cerling,

Geological and Geochemical Analysis

Abstract The Payun Matru volcanic field encompasses more than 12,000 km 2 in the west-central Argentine province of Mendoza. The northwestern part of the field, called Los Volcanes, includes multiple basaltic lava flows and numerous cinder cones and scoria fields. We sampled primary lava flow features (tumuli, pahoehoe and aa flow surfaces) from two different basaltic lava flows and one fluvial strath for cosmogenic 3 He exposure age dating in the Los Volcanes area. Olivines from the sampled basalts have low concentrations of U and Th and a minimal radiogenic 4 He correction. We used the recently established R correction factor to correct our measured 3 He concentrations for magmatic 3 He and 4 He and radiogenic 4 He. Exposure ages for primary lava flow features covering a large area just east of where Route 40 crosses the Rio Grande range from 41 to 43 ka with external uncertainties on the order of 7%. A recently published K→Ar age of 26 ± 5 ka was sampled within a few hundred meters of three of our cosmogenic exposure age samples and appears to be from the same basalt flow. Given the large uncertainties associated with dating young lava flows, the K→Ar age and our cosmogenic ages are in reasonable agreement; however, they do not overlap within uncertainties and suggest possible systematic discrepancies between the two methods of up to 20% in this instance. A sample from a bedrock strath carved by the Rio Grande into basalt yielded a surface exposure age of 17 ± 1 ka. The toe of a much younger-looking aa flow about 5 km west of the Rt. 40 Rio Grande bridge yielded two ages of 0 and 2 ka, and is presumed to be historical.