HR: 0800h
AN: V51B-0524    [Abstracts]
TI: A Global Dataset of Noble gas Concentrations and Their Isotopic Ratios in Volcanic Areas
AU: * Abedini, A A
EM: aabedini@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Hurwitz, S
EM: shaulh@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: The extent to which ocean islands are derived from the deep mantle (mantle plumes) or from chemical heterogeneities embedded within the mantle convective flow has long been debated. Noble gases have unique properties that provide significant information regarding this debate and make them important geodynamic tracers. The study of noble gas isotopic compositions in active tectonic and volcanic areas is central to understanding the origins of major volcanic anomalies. For example, helium isotope composition is considered to be the most unambiguous geochemical indicator of a lower mantle plume component in surface rocks, and its variability is often taken as the strongest evidence for a layered mantle. We have compiled all the published global noble gas data from MOR, ocean islands, seamounts, and volcanic arcs, extending existing datasets which are limited to samples from oceanic rocks (Farley and Neroda, 1998; Graham, 2002). Our data set contains information on helium, neon, argon, and xenon concentrations, as well as their isotopic ratios. Where available we also included the isotopic ratios of lead, strontium, neodymium, and carbon. Overall, there are more than 5,000 entries in the database, which is sub-divided both by material sampled (e.g., volcanic glass, different minerals, fumarole, spring) and by tectonic setting (MOR, ocean islands, volcanic arcs). Our extended dataset is consistent with earlier studies and shows that helium isotope ratios in MORB glass in the Pacific, Indian and Southern Atlantic Oceans have a sharp peak between 7 and 9 RA. The large peak in MORB samples correlates with the maxima in samples from volcanic arcs, probably implying that the upper mantle has a uniform helium isotope ratio. In the North Atlantic there is a broader distribution with a maximum at 11 Ra. In contrast, helium isotope ratios in ocean-island basalts (OIB) are highly variable. As noted by previous workers, the helium isotope ratios in OIB have a large peak at 8 RA and a second peak at 13 RA, separated by a pronounced minimum at about 10 RA. In most cases, higher-than-MORB He-isotope ratios coincide with deep mantle plumes revealed by seismic tomography (Montelli et al., 2004). The database will be available through the World Wide Web and will allow examination of some unresolved scientific problems. Farley K.A., Neroda E., Ann. Rev. Earth Planet. Sci., 1998. Graham, D. W., Rev. Mineral. Geochem., 2002. Montelli, R. et al., Science. 2004.
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8125 Evolution of the Earth
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting