HR: 1340h
AN: V23B-1427    [Abstracts]
TI: Evidence for diffusive loss of cosmogenic ³He during vacuum crushing of mafic phenocrysts
AU: Puchol, N
EM: nicolas.puchol@ens-lyon.fr
AF: Ecole Normale Superieure de Lyon, 46 allee d Italie, LYON, 69364, France
AU: * Blard, P
EM: blard@gps.caltech.edu
AF: California Institute of Technology, MS 170-25 1200 E. California Blvd., PASADENA, CA 91125, United States
AU: Farley, K A
EM: farley@gps.caltech.edu
AF: California Institute of Technology, MS 170-25 1200 E. California Blvd., PASADENA, CA 91125, United States
AB: In vacuum crushing is an efficient technique to selectively release the primordial helium component trapped within olivine and pyroxene phenocrysts. However, contrary to previous assumptions, recent studies have shown that this method may cause significant release of matrix sited cosmogenic ³He (³Hec). Because this loss may bias both the determination of magmatic ³He/⁴He ratios (Yokochi et al., 2005) and the accuracy of ³Hec measurements (Blard et al., 2006), it is essential to understand what mechanism is responsible and under what conditions ³Hec loss is manifest. For this reason, olivines and pyroxenes with various amounts of matrix-sited ³He (from 10⁷ to 10¹¹ at.g^-1) were crushed in air or in vacuum using different crushing devices. The sample temperature was carefully controlled during each crushing experiment, and ranged from 25 to 325 ° C. The resulting powders were then sieved to obtain several homogeneous grain fractions ranging between <10 microns and >300 microns. The ³Hec concentrations measured in each fraction clearly show that significant ³Hec loss (>20%) affects only the finest fraction (<10 microns) and, importantly, only under hot conditions (T>300 ° C). Even the smallest fractions (<10 microns) quantitatively retain matrix-sited ³Hec when crushed under cold conditions (T<25 ° C), regardless of the duration and energy of crushing. These results invalidate the previously proposed mechanism that involved spallation tracks and implied a purely grain size control of the magnitude of loss (Yokochi et al., 2005). Moreover, new diffusion experiments were carried out to constrain the diffusivity of matrix-sited helium in crushed olivines. When used to model diffusive ³Hec loss as a function of grain size during crushing, these new data predict the observed release fairly well. Therefore, we conclude that temperature-enhanced volume diffusion is the main mechanism controlling the release of ³Hec during crushing. For future applications, special attention should thus be paid to control both the grain size and the temperature of the sample during crushing. References Blard, P.-H. et al. (2006) EPSL 247, 222-234. Yokochi, R. et al. (2005) G-cubed 6, doi:10.1029/2004GC000836.
DE: 1105 Quaternary geochronology
DE: 1130 Geomorphological geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting