Helium distribution in a mantle shear zone from the Josephine Peridotite
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A previous study of oceanic mylonites suggested that peridotite helium concentrations are correlated with the degree of high-temperature ductile deformation in the mantle. In order to test this result, this study combines helium measurements with characterization of the deformation state of harzburgite samples in a small (6 m width) ductile mantle shear zone from the Josephine Peridotite, Oregon, USA. All measurements were made by coupled in vacuo crushing and melting, demonstrating that most of the helium (> 80%) resides within the solid phases rather than fluid or melt inclusions. The present study confirms the influence of deformation on helium contents, but only at the highest shear strain (gamma > 20) are helium contents significantly higher. The highest helium concentration, by roughly a factor of two, is found in the center-most sample, which also has grain size reduction by a factor of similar to 4. Dislocations and sub-grain boundaries are present in all samples and do not correlate with helium concentrations. Mineralogy also appears to have a negligible influence in this shear zone, as modal mineralogy is relatively homogeneous, with all samples being harzburgites. These observations suggest that the increase in helium concentration is related to grain size reduction, with grain boundaries proposed as an additional storage site for helium in the mantle. The present data also characterize the isotopic composition of the Josephine Peridotite: He-3/He-4 = 6.7 +/- 0.2 Ra (n = 33, between 6.3 and 7.1 Ra). The presence of cosmogenic He-3 in the matrix is indicated by the helium isotopic composition released by melting: He-3/He-4 = 8.5 +/- 0.3 Ra (n = 10; from 7.9 to 10.9). This corresponds to an exposure age of 10 Kyr, which is approximately concordant with the end of the last glacial maximum. Very little radiogenic helium is present in the samples, suggesting extremely low uranium and thorium contents ([U] < 0.3 ppb). Helium isotope measurements in four samples outside the shear zone suggest that mineralogy and melt infiltration are also important factors for understanding helium storage in the mantle. (C) 2012 Elsevier B.V. All rights reserved.