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

We have measured cosmogenic 3He 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 Earthʹs dipole moment. Our individual 3He production rate determinations are consistent with previous estimates, and reinforce the feasibility of dating very young (late Holocene) surfaces with the cosmogenic 3He method. Integrated cosmogenic 3He 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 3He from calibration sites in this study is 116±3 atoms g−1 yr−1.