Calibration of cosmogenic 3He production rates from Holocene lava flows in Oregon, USA, and effects of the Earth's magnetic field

We have measured cosmogenic He-3 production rates in olivine phenocrysts from four radiocarbon-dated Holocene lava flows in Oregon. The flows span the period between 2 and 7 ka when there were significant fluctuations in the intensity of the Earth's dipole moment. Our individual He-3 production rate determinations are consistent with previous estimates, and reinforce the feasibility of dating very young (late Holocene) surfaces with the cosmogenic He-3 method. integrated cosmogenic He-3 production rates exhibit small temporal Variations during the Holocene, supporting predictions that production rates at mid-latitudes are weakly affected by geomagnetic modulation of cosmic ray flux. However, the time-varying difference between geographic and geomagnetic latitude caused by secular variation of dipole axis position may represent an important source of error (as much as 5%) in Holocene surface exposure ages and production rate calibrations. The best value for the integrated Holocene production rate of cosmogenic He-3 from calibration sites in this study is 116 +/- 3 atoms g(-1) yr(-1). (C) 1999 Elsevier Science B.V. All rights reserved.