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

The Payún Matru volcanic field encompasses more than 12,000 km2 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 3He exposure age dating in the Los Volcanes area. Olivines from the sampled basalts have low concentrations of U and Th and a minimal radiogenic 4He correction. We used the recently established R correction factor to correct our measured 3He concentrations for magmatic 3He and 4He and radiogenic 4He. Exposure ages for primary lava flow features covering a large area just east of where Route 40 crosses the Río 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 Río 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 Río Grande bridge yielded two ages of 0 and 2 ka, and is presumed to be historical.

► We determined exposure ages of basalts in the Payún Matru volcanic field, Argentina.
► 3He exposure ages from older basalts in the NW of the field range from 41 to 43 ± 3 ka.
► Our exposure ages are older than a recent K→Ar age on the same flow of 26 ± 5 ka.
► The youngest basalt flows in the field have exposure ages of 2 ± 2 ka.