HR: 0800h
AN: U41A-0701    [Abstracts]
TI: Volatile Origin and Recycling Into the Mantle
AU: * Holland, G
EM: g.holland@man.ac.uk
AF: University of Manchester, Dept. of Earth Sciences University of Manchester, Manchester, M13 9PL United Kingdom
AU: Ballentine, C
EM: Chris.Ballentine@man.ac.uk
AF: University of Manchester, Dept. of Earth Sciences University of Manchester, Manchester, M13 9PL United Kingdom
AB: Noble gas isotopes in magmatic CO2 well gases provide a unique insight into mantle volatile origin and dynamics. Previous work has resolved mantle 20Ne/22Ne ratios and suggests a Solar wind irradiated meteoritic source for mantle He and Ne. This constrains the mechanism of volatile addition to the mantle during accretion and the relationship between volatiles in MORB and OIB (Ballentine et al., 2003). We present here complimentary high precision analyses of Ar and Xe isotopes from three CO2 gas fields from the SW USA; Bravo Dome, Sheep Mountain and McElmo Dome. All fields contain 129Xe excesses from 129I decay. In addition, Sheep Mountain and Bravo Dome contain correlated non-radiogenic 124Xe, 126Xe and 128Xe excesses indicating up to a 10% non-air contribution to these gases. Whilst mass dependent fractionation can theoretically produce correlated excesses in 124Xe-128Xe, it cannot generate the correlated excess of 124Xe-128Xe and radiogenic 129Xe therefore this excess must be due to a resolvable primordial component (Caffee et al., 1999). Preliminary results for the first time show a 38Ar/36Ar ratio in Bravo Dome lower than air implying a 5% contribution from a primordial reservoir with Solar 38/36Ar. The resolved primordial component in Ne, Xe and now Ar produces 132Xe/36Ar and 22Ne/36Ar in the well gases that are similar to those of irradiated protoplanetary material. This is distinct from Solar values and provides crucial corroborative evidence for the interpretation of the mantle 20Ne/22Ne. Furthermore, preservation of the relative amounts of primordial Ar and Xe is most simply explained by recycling and homogenisation of seawater-derived noble gases into the mantle. Mass balance calculations show that seawater recycling would have a negligible impact upon the Ne isotopic ratios. These results suggest that the noble gas subduction barrier (Staudacher et al., 1988) is not effective; that the dominance of air-like non-radiogenic Ar (Kr) and Xe isotopes in the mantle is due to seawater recycling; that this process results in a relatively homogeneous air contribution to the mantle system; and is further consistent with low 129Xe excess observed in OIB being explained by a significant recycled volatile component. Ballentine, C.J. et al., AGU, V52D-06, 2003 Caffee, M.W. et al., Science, 285, 2115-2118, 1999 Staudacher, T. and C.J. Allegre, Earth Planet. Sci. Lett., 89, 173-183, 1988
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1010 Chemical evolution
SC: Union [U]
MN: 2004 AGU Fall Meeting