HR: 14:00h
AN: T23D-02    [Abstracts]
TI: Geochemical Tracing of Mantle Flow above Subduction Zones
AU: * Pearce, J A
EM: PearceJA@cf.ac.uk
AF: School of Earth and Planetary Sciences, Cardiff University, Cardiff, CF103YE United Kingdom
AU: Barry, T L
EM: tbarry@bgs.ac.uk
AF: Isotope Geosciences Laboratory, British Geological Survey, Keyworth, NG12 5GG United Kingdom
AU: Millar, I L
EM: ilm@bas.ac.uk
AF: Isotope Geosciences Laboratory, British Geological Survey, Keyworth, NG12 5GG United Kingdom
AU: Leat, P T
EM: PTLE@bas.ac.uk
AF: British Antarctic Survey, Madingley Road, Cambridge, CB3 0ET United Kingdom
AU: Stern, R J
EM: rjstern@utdallas.edu
AF: Geosciences Department, University of Texas at Dallas, Richardson, TX 75083-0688 United States
AB: Geochemical tracing may be used to track mantle flow above and behind subduction zones and so provide an independent test of the applicability of seismic anisotropy measurements. The theory is that, if mantle flow is accompanied by decompression, then extraction of small degree melts from multi-component mantle leads to compositional gradients in the mantle in both isotope and trace element space. These gradients may be obtained by inverting geochemical data from the products of mantle melting. If mantle flow is accompanied by addition of a subduction fluid, then simultaneous melting and subduction component addition may also produce compositional gradients. Numerical experiments enable compositional gradients to be quantified in terms of the extent of melt extraction, mantle temperature and other variables. In addition, isotope and trace element systematics provide evidence for the provenance of the mantle entering the subduction system, with Hf isotopes and immobile trace elements providing a means of establishing provenance even from magmas generated directly above the dehydrating subducted plate. This work focuses on a series of geochemical maps which enable mantle flow to be traced for range of oceanic arc basin systems (Izu-Bonin-Mariana, Tonga-Vanuatu, Scotia, Manus) and, provisionally, some continental systems (Japan, Cascades). Using maps based on geochemical proxies for melt extraction (such as Ta/Yb), subduction-addition (such as Th/Ta) and mantle provenance (such as epsilon-Hf v epsilon Nd), it is possible to demonstrate the existence of a wide range of mantle flow regimes. Thus, the Izu and Japan systems appear to be characterised by simple trench-orthogonal flow, the Mariana system by dispersion away from several separate centers of mantle upwelling, the Tonga-Vanuatu system by unidirectional flow from beneath the Pacific plate in the north, and the Scotia system by bi-directional flow from both north and south. In a number of these cases, isotopic fingerprinting using immobile isotope ratios is critical for establishing the ultimate source of the mantle: for example, the mantle entering the Scotia system may be seen to originate from the Atlantic Bouvet domain rather than the Atlantic Tristan or the Pacific domains. These results are broadly in keeping with seismic anisotropy measurements to date while providing greater coverage but, of course, geochemical tracing is restricted to areas of magmatic activity. Integration of geophysical and geochemical methods may therefore be necessary to provide the maximum information on mantle flow.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8150 Plate boundary--general (3040)
DE: 3670 Minor and trace element composition
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting