Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v11a2006h&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #V11A-2006
Physics
1025 Composition Of The Mantle, 8124 Earth'S Interior: Composition And State (1212, 7207, 7208, 8105), 8430 Volcanic Gases
Scientific paper
Noble gas isotopes in magmatic CO2 well gases provide a unique insight into mantle volatile origin and dynamics [1-3]. Previous work has resolved mantle 20Ne/22Ne ratios consistent with a solar wind irradiated meteoritic source for mantle He and Ne [1]. This is distinct from Solar Wind values that might be expected if the primary mechanism of terrestrial mantle volatile acquisition was through the gravitational capture of solar nebula gases [see 4]. Within the CO2 well gases a primordial component has also been resolved in the non- radiogenic Xe isotopic composition [2,3]. Using multicollector mass spectrometry we have observed a 124Xe/130Xe excess of 1.85 percent over air plus/minus 0.17 percent for the least air contaminated samples. At this level of precision we are for the first time able to differentiate between a trapped meteoritic origin (average carbonaceous chondrite or Q Xe) rather than Solar Wind origin as the primordial Xe component. The well gases also contain Kr which, in the least air contaminated sample, have a correlated 86Kr/82Kr excess of 0.55 percent over air plus/minus 0.04 percent. Whilst mass dependent fractionation can theoretically produce correlated excesses in 124Xe-128Xe and 82Kr-86Kr isotopes, no fractionation from air is observed in 38Ar/36Ar [3] and the Kr excesses are in the opposite sense to that of Xe. From 136Xe excesses, Kr fission yield from Pu and U can be calculated and subtracted from the Kr isotopic signature. This fission-corrected signature is most reasonably explained as a primordial component. This is the first time that primordial Kr has ever been resolved in a terrestrial sample. The primordial Kr isotopic signature is distinct from Solar Wind Kr and is consistent with the primordial Kr also originating as a trapped component within meteorites. We are now able to demonstrate that both the light (He and Ne) and Heavy (Kr and Xe) noble gas origin in the terrestrial mantle is consistent with a trapped component during the accretionary process and can discount gravitational capture as the principle mechanism of interior planetary volatile acquisition. References: [1] Ballentine, C.J. et al., Nature, 433. 33-38, (2005) [2] Caffee, M.W. et al., Science, 285, 2115- 2118, (1999) [3] Holland G. and Ballentine C. J., Nature, 441, 186-191 (2006) [4] Pepin R.O. and Porcelli D. Rev. Min. Geochem. 47, 191-246 (2002)
Ballentine Chris
Cassidy Mary
Holland Glenn
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