Cosmogenic 3He production rates revisited from evidences of grain size dependent release of matrix-sited helium
Measurements of the cosmogenic 3 He ( 3 He c ) content of various size aliquots of exposed olivines show that the fine fraction (<140 μm) has 3 He c concentrations between 14 and 100% lower than that of the coarse fractions (0.14–1 mm). Such differences attest to a grain size dependent partial release of 3 He c from the phenocrysts matrix during the preliminary in vacuo crushing. This result might have important implications since most 3 He c measurements have used for ∼20 yr a standard routine based on the fusion of bulk powdered phenocrysts, whatever their grain size. A suite of new data obtained from coarse olivine grains yielded a mean Sea Level High Latitude 3 He c production rate (SLHL P 3 ) of 128±5 and 136±6 at. g −1 yr −1 , depending on the scaling factors used. This new value, which is ∼15% higher than previously published rates, is obtained from 5 ropy flow surfaces of Mt Etna (38°N) and Hawaiian (19°N) volcanoes, at elevations between sea level and 870 m and ranging in age from 1.47±0.05 to 149±23 ka according to independent 14 C or K/Ar dating. 3 He loss during the crushing step might account for the discrepancy between the standard reference value of 110–115 at. g −1 y −1 and the higher SLHL P 3 proposed here. More generally, removal of the powdered fraction before fusion is an important point to consider in further studies in order to avoid any 3 He c systematic underestimates. An altitudinal section has also been sampled on the ropy surface of a ∼1500 yr single flow of Mauna Loa (19°N) which allowed a new empirical atmospheric attenuation length of 149±22 g cm −2 to be documented for 3 He c in olivines between 2400 and 4000 m elevations.