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. 47degreesS provide ideal sites for calibration of in situ produced cosmogenic He-3. 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 similar to 35 ka (the limit of C-14 dating). The weighted mean age of the Cerro Volcan flow is 109 +/- 3 ka and of the Rio Pinturas flow is 68 +/- 3 ka. Scaled to sea level and high latitude [D. La1, Earth Planet. Sci. Lett. 104 (1991) 424-439] and standard atmospheric pressure [J.O. Stone, J. Geophys. Res. 105 (2000) 23 753-23 759], He-3 production rates on the Cerro Volcan flow are 130 +/- 4 atoms g(-1) a(-1) in clinopyroxene and 139 +/- 4 atoms g(-1) a(-1) in olivine (+/-2sigma), consistent with a small compositional dependence of the production rate. The He-3 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 Volcan. 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 similar to 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. 50degreesS, but based on independent marine and terrestrial evidence are thought to have migrated north at least 5degrees during glacial periods. The inferred paleobarometry provides independent evidence for northward shifts in the patterns of Southern Hemisphere atmospheric circulation. (C) 2003 Elsevier Science B.V. All rights reserved.