Long-term cosmogenic 3He production rates from 40Ar/39Ar and K–Ar dated Patagonian lava flows at 47°S

Exceptionally well-preserved basaltic lava flows on the cool, arid, eastern side of the southern Andes near Lago Buenos Aires (LBA) at Lat. 47°S provide ideal sites for calibration of in situ produced cosmogenic 3He. We report new 3He measurements and independent age determinations using unspiked K–Ar and 40Ar/39Ar incremental heating measurements from two basalt flows. Both age techniques give concordant ages, with analytical uncertainties of <5% demonstrating their utility for dating surfaces older than ∼35 ka (the limit of 14C dating). The weighted mean age of the Cerro Volcán flow is 109±3 ka and of the Rio Pinturas flow is 68±3 ka. Scaled to sea level and high latitude [D. Lal, Earth Planet. Sci. Lett. 104 (1991) 424–439] and standard atmospheric pressure [J.O. Stone, J. Geophys. Res. 105 (2000) 23 753–23 759], 3He production rates on the Cerro Volcán flow are 130±4 atoms g−1 a−1 in clinopyroxene and 139±4 atoms g−1 a−1 in olivine (±2σ), consistent with a small compositional dependence of the production rate. The 3He production rate in olivine from the younger Rio Pinturas flow is 129±6 atoms g−1 a−1, which is indistinguishable from that obtained on Cerro Volcán. The 3He production rate is at least 11% higher than previous determinations from low-mid latitudes in the Northern Hemisphere. Corrections for magnetic field strength variability lower the scaled LBA production rates by only ∼1%, and do not account for the discrepancy. We infer that the enhanced production rates are caused by lower atmospheric pressure in Patagonia during glacial periods. The Southern Westerlies, which are associated with the steep circumpolar atmospheric pressure gradient, are presently centered on Lat. 50°S, but based on independent marine and terrestrial evidence are thought to have migrated north at least 5° during glacial periods. The inferred paleobarometry provides independent evidence for northward shifts in the patterns of Southern Hemisphere atmospheric circulation.