Cosmogenic 3He production rates revisited from evidences of grain size dependent release of matrix-sited helium

Measurements of the cosmogenic He-3 (He-3(c)) content of various size aliquots of exposed olivines show that the fine fraction (< 140 lurr) has He-3(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 He-3(c) from the phenocrysts matrix during the preliminary in vacuo crushing. This result might have important implications since most He-3(c) measurements have used for similar to 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 He-3(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 similar to 15% higher than previously published rates, is obtained from 5 ropy flow surfaces of Mt Etna (38 degrees N) and Hawaiian (19 degrees N) volcanoes, at elevations between sea level and 870 in and ranging in age from 1.47 +/- 0.05 to 149 +/- 23 ka according to independent C-14 or K/Ar dating. He-3 loss during the crushing step might account for the discrepancy between the standard reference value of 110-115 at. g(-1) yr(-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 He-3(c) systematic underestimates. An altitudinal section has also been sampled on the ropy surface of a similar to 1500 yr single flow of Mauna Loa (19 degrees N) which allowed a new empirical atmospheric attenuation length of 149 +/- 22 g cm(-1) to be documented for 3 He, in olivines between 2400 and 4000 m elevations. (c) 2006 Elsevier B.V. All rights reserved.