HR: 09:15h
AN: V11B-05    [Abstracts]
TI: Subduction of Seawater Explains MORB and OIB Compositions
AU: * Holland, G
EM: g.holland@manchester.ac.uk
AF: University of Manchester, Oxford Road, Manchester, M13 9PL United Kingdom
AU: Ballentine, C
EM: chris.ballentine@manchester.ac.uk
AF: University of Manchester, Oxford Road, 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 on CO2 well gas from Bravo Dome, New Mexico has for the first time enabled the mantle He/Ne/Ar/Kr isotopic and elemental abundance to be determined without the ambiguity associated with air contamination in typical mantle samples. This has allowed: i) resolution of the mantle 20Ne/22Ne ratios, confirming a Solar wind irradiated meteoritic source for mantle He and Ne; ii) determination of the mantle 40Ar/36Ar ratio and; iii) determination of the relative elemental abundance of He/Ne/Ar/Kr in the well gas mantle source (Ballentine et al., 2005a). We have re-sampled this gas field with more focus on the mantle rich portion of the field and extended the analyses to include Xe abundance and isotopic composition. He and Ne, in the Earth's mantle preserve a Solar-like isotopic composition. In contrast, Ar and Kr preserve an isotopic composition indistinguishable from that of the modern atmosphere, after correction for radiogenic additions. In addition to the air-like and radiogenic components, Xe shows correlated non-radiogenic 124Xe, 126Xe and 128Xe excesses. These show up to a 10 percent primordial/Solar contribution to these isotopes and confirm earlier work on a single sample (Caffee et al., 1999). With multiple samples we can now correct for air contamination and show the proportion of air-like Xe to Primordial/Solar Xe in the convecting mantle to be ~0.8. After correction for the primordial Xe component, we further show that the elemental composition of the non-radiogenic heavy noble gases in the convecting mantle is indistinguishable from that of seawater. We discuss the consequences of seawater recycling for other volatiles (water, D/H, nitrogen) and argue that this is indeed the most likely source of heavy noble gases in the mantle. This requires that the noble gas subduction barrier (Staudacher and Allegre, 1988) is not effective. Enhanced concentrations of subducted recycled volatiles into the plume source are suggested by numerical models of mantle convection (Ballentine et al., 2005b) and also account for the lower ratio of radiogenic to non-radiogenic heavy noble gas isotopes, higher water content and variable deuterium abundances in ocean island basalts. Thus, a seawater recycling model is proposed which provides a geochemically consistent explanation for MORB and OIB elemental and isotopic compositions and enables us to provide a quantitative estimate of the seawater flux over geological time to the whole mantle reservoir. Ballentine, C.J et al., Nature, 433, 33-38 (2005a). Ballentine, C.J. et al., AGU abstract, this meeting (2005b). Caffee, M.W., et al., Science, 285, 2115-2118, (1999). Staudacher, T. and Allegre C.J., Earth Planet. Sci. Lett., 89, 173-183, (1988).
DE: 1025 Composition of the mantle
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1038 Mantle processes (3621)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005