HR: 1340h
AN: U33A-0010    [Abstracts]
TI: Evidences of cosmogenic 3He loss during the phenocrysts step-crushing: implications for production rates calibration
AU: * Blard, P
EM: blard@cerege.fr
AF: CRPG CNRS, Rue Notre Dame des Pauvres, Vandoeuvre les Nancy, 545001 France
AU: * Blard, P
EM: blard@cerege.fr
AF: CEREGE CNRS Univ Paul Cezanne, Europole de l Arbois, Aix en Provence, 13545 France
AU: Pik, R
EM: rpik@crpg.cnrs-nancy.fr
AF: CRPG CNRS, Rue Notre Dame des Pauvres, Vandoeuvre les Nancy, 545001 France
AU: Lave, J
EM: jerome.lave@ujf-grenoble.fr
AF: LGCA CNRS Univ Joseph Fourier, Maison des Geosciences, Grenoble, 38041 France
AU: Bourles, D
EM: bourles@cerege.fr
AF: CEREGE CNRS Univ Paul Cezanne, Europole de l Arbois, Aix en Provence, 13545 France
AU: Yokochi, R
EM: yokochi@crpg.cnrs-nancy.fr
AF: CRPG CNRS, Rue Notre Dame des Pauvres, Vandoeuvre les Nancy, 545001 France
AU: Burnard, P
EM: peteb@crpg.cnrs-nancy.fr
AF: CRPG CNRS, Rue Notre Dame des Pauvres, Vandoeuvre les Nancy, 545001 France
AU: Trusdell, F
EM: trusdell@usgs.gov
AF: HVO USGS, Hawaii Volcanoes National Park, Hilo, HI 96718 United States
AB: Measurement of cosmogenic 3He (3Hec) in mafic phenocrysts (olivines and clinopyroxenes) is based since ca. 20 years on a two-step routine. The first step is supposed to selectively release the magmatic helium component (contained in the fluid inclusions) through in vacuo crushing, while the second step extracts all the remaining helium components, including the cosmogenic one, through fusion of the obtained powder. One of the ways to refine the precision and the accuracy of 3Hec production rates, which is one of the main goals of the CRONUS EU project, is to check the robustness of this measurement technique. In the present study, specific tests have been conducted in order to evaluate a possible release of 3Hec during the preliminary crushing, as proposed by Yokochi et al. [2005]. After the in vacuo crushing step of the phenocrysts, samples have been sieved to separate the fine (less than 0.14 mm) from the coarse (from 0.14 to 1 mm) fraction. Their subsequent melting shows that the 3Hec content of the fine fraction is from 20 to 80 % lower than that of the coarse fraction. Such differences most likely result from partial release of in situ produced 3Hec via the cosmic ray induced spallation tracks (ca. 10 microns long) present in the phenocryts matrix. Since most of the 3Hec calibration studies have been performed by fusing the crushed bulk phenocrysts, the main implication of these results is that the 3Hec Sea Level High Latitude (SLHL) production rates reference value ranging from 110 to 115 at.g-1.yr-1 might be underestimated. Indeed, a suite of new data performed on uncrushed olivines and clinopyroxenes phenocrysts from flows of Etna (38°N) and Hawaiian (20°N) volcanoes yield a mean SLHL rate from 10 to 15 % higher than the currently used production rates. This revisited value is obtained from 7 ropy flow surfaces located from sea level to 850 m, ranging in age from 1.4 to 150 ka according to independent 14C or K/Ar dating. It is corrected for sampling depth, spatial scaling, past magnetic fluctuations, island subsidence and mask topography. More generally, as the preliminary crushing step often remains necessary to measure the primordial helium isotopic composition, removing the powdered fraction before fusion is thus an important point to consider in order to avoid any 3Hec systematic underestimate. Yokochi et al., G3, 6 (1), 2005
DE: 1094 Instruments and techniques
DE: 1130 Geomorphological geochronology
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 4918 Cosmogenic isotopes (1150)
SC: Union [U]
MN: Fall Meeting 2005