Fuerteventura - Assessment of a calibration site for cosmogenic 3He exposure dating with the 40Ar/39Ar incremental heating method

We measured cosmogenic 3He concentrations in olivine and clinopyroxene phenocrysts from flow top samples on a MAP 215-50 sector mass spectrometer with a crushing device and a diode laser extraction system. Exposure age calculations yielded ages in the range 38.9 ± 4.0 ka to 62.3 ± 6.7 ka for the youngest lava flow and the data series is in broad agreement with the argon data up to 250 ka and reveals a more continuous time line of volcanism during the late Pleistocene on the island. However, the dataset was not sufficient for calculation of production rates for in situ Terrestrial Cosmogenic 3He as many samples showed signs of erosion. Calculated erosion rates range from none to as high as 7.3 mm/kyr assuming a rock density of 2.9 g/cm2. This finding puts a constraint on the use of Fuerteventura as a calibration site for exposure histories older than 50-100 ka. A comparison with cosmogenic 36Cl data supports these findings and indicates substantial weathering.