HR: 11:25h
AN: V42B-05 INVITED    [Abstracts]
TI: Temperature Versus Buoyant Mantle Heterogeneities, Evaluating the Origin of OIB Using the Galapagos Archipelago
AU: * Saal, A E
EM: asaal@brown.edu
AF: Brown University, 324 Brook St., Providence, RI 02192, United States
AU: Bourdon, B
EM: bourdon@erdw.ethz.ch
AF: Dept. Earth Sciences, ETH, Zürich, Zürich, CH- 8092, Switzerland
AU: Kurz, M D
EM: mkurz@whoi.edu
AF: MC&G WHOI, Woods Hole, WH, MA 02540, United States
AU: Hauri, E H
EM: hauri@dtm.ciw.edu
AF: DTM, CIW, 5241 Broad Branch Road, NW Washington, DC 20015, United States
AU: Blusztajn, J
EM: jblusztajn@whoi.edu
AF: MG&G WHOI, Woods Hole, WH, MA 02540,
AU: Blichert-Toft, J
EM: jblicher@ens-lyon.fr
AF: Ecole Normale Supérieure de Lyon, CNRS UMR 5570, Lyon, 69364, France
AU: Hart, S R
EM: shart@whoi.edu
AF: MG&G WHOI, Woods Hole, WH, MA 02540,
AU: Sims, K W
EM: ksims@whoi.edu
AF: MG&G WHOI, Woods Hole, WH, MA 02540,
AB: The existence of hot mantle plumes has recently been questioned; instead tectonics and shallow low-melting heterogeneities, rather than excess temperatures, have been proposed to explain the upwelling of mantle beneath oceanic islands basalts (OIB) [1]. Geophysical, petrological and geochemical arguments have been used to suggest that enriched heterogeneities located shallow within the upper mantle are buoyant due to either melt retention or water lowering its solidus and consequently triggering early melting of the fertile blobs. Thus, the opponents of the hotspot model have suggested that shallow fertile and buoyant blobs could reproduce the effects of temperature, including isotope gradients, as the cause for the generation of OIB [1]. Uranium-series disequilibria measured in oceanic lavas may provide insight into the origin of OIB that is independent of previous arguments. Earlier works have shown that U-series isotopes in basalts are sensitive to variations in the upwelling rates of their mantle source, and those can be used to invoke upward movements that may be associated with mantle plumes [2–4]. Those work also demonstrated that there exist a significant relationship between the extent of U-series disequilibria and the buoyancy flux beneath OIB; where lower buoyancy fluxes are associated with lower rates of melting and hence higher (²³⁰Th/²³⁸U) values [2]. However, most of those works have assumed that the effects of source heterogeneity can be neglected relative to the effects of other key parameters. If the increase in melting rates is truly due to the presence of fertile blobs, then there should be a correlation between clear indices of enrichment such as radiogenic isotopes and extent of U-series disequilibrium in OIB. Furthermore, the fertility of the mantle heterogeneities may or may not be associated with the enrichment of water, which has an important effect on the melting rate and consequently in the U-series isotope of the basalts. To evaluate what factors (temperature versus mantle heterogeneity) are controlling the upwelling mantle beneath oceanic islands we used our data (including volatile and U-series isotopes) for lavas from across the Galapagos archipelago combined with previously published results from the adjacent Galapagos Spreading Ridge lavas [5]. These results suggest that the most important factor controlling the variation in mantle upwelling velocities beneath the Galapagos archipelago and adjacent ridge is the difference in mantle temperature. References 1. Plates, Plumes, and Paradigms (eds Foulger, G.R., Natland, J.H., Presnall, D.C. & Anderson, D.L.) (GSA Special Paper 388, Geological Society of America, Boulder, 2005). 2. Bourdon, B. et al. Nature 444, 713-717, 10.1038 (2006). 3. Bourdon, B. & Sims, K. W. W. in U-series Geochemistry (eds Bourdon, B., Lundstrom, C., Henderson, G. & Turner, S. P.) 215–253 (Mineralogical Society of America, 2003).
DE: 1033 Intra-plate processes (3615, 8415)
DE: 1038 Mantle processes (3621)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting