R. Pik J. Lave D. Bourles P.G. Burnard R. Yokochi B. Marty F. Trusdell P.-H. Blard 2006 <div class="abstract svAbstract " data-etype="ab"><p id="">Measurements of the cosmogenic ³He (³He_c) content of various size aliquots of exposed olivines show that the fine fraction (&lt;140&nbsp;μm) has ³He_c concentrations between 14 and 100% lower than that of the coarse fractions (0.14–1&nbsp;mm). Such differences attest to a grain size dependent partial release of ³He_c from the phenocrysts matrix during the preliminary in vacuo crushing. This result might have important implications since most&nbsp;³He_c measurements have used for ∼20&nbsp;yr a standard routine based on the fusion of bulk&nbsp;<i>powdered</i> phenocrysts, whatever their grain size. A suite of new data obtained from coarse olivine grains yielded a mean Sea Level High Latitude ³He_c production rate (SLHL P₃) of 128±5 and 136±6&nbsp;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&nbsp;m and ranging in age from 1.47±0.05 to 149±23&nbsp;ka according to independent ¹⁴C or K/Ar dating. ³He loss during the crushing step might account for the discrepancy between the standard reference value of 110–115&nbsp;at. g⁻¹ y⁻¹ and the higher SLHL P₃ 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 ³He_c&nbsp;systematic underestimates.</p><p id="">An altitudinal section has also been sampled on the ropy surface of a ∼1500&nbsp;yr single flow of Mauna Loa (19°N) which allowed a new empirical atmospheric attenuation length of 149±22&nbsp;g cm⁻² to be documented for ³He_c in olivines between 2400 and 4000&nbsp;m elevations.</p></div> application/pdf 10.1016/j.epsl.2006.05.012 en Elsevier Cosmogenic 3He production rates revisited from evidences of grain size dependent release of matrix-sited helium article