HR: 0800h
AN: V31C-0632    [Abstracts]
TI: Hf Isotope Evidence for Subducted Basalt and Sediment Contributions to the Eastern Trans-Mexican Volcanic Belt
AU: * Cai, Y
EM: cai@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Tuena, A G
EM: tuena@geociencias.unam.mx
AF: Centro de Geociencias Universidad Nacional Aut˘noma de M‚xico, Campus Juriquilla, Quer‚taro, 76230 Mexico
AU: Capra, L
EM: lcapra@geologia.unam.mx
AF: Instituto de Geologˇa Universidad Nacional Aut˘noma de M‚xico, Ciudad Universitaria, M‚xico D. F., 04510 Mexico
AU: Straub, S M
EM: smstraub@fas.harvard.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Straub, S M
EM: smstraub@fas.harvard.edu
AF: Department of Earth and Planetary Sciences Harvard University, 20 Oxford St, Cambridge, MA 02138 United States
AU: Goldstein, S L
EM: steveg@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Langmuir, C H
EM: langmuir@eps.harvard.edu
AF: Department of Earth and Planetary Sciences Harvard University, 20 Oxford St, Cambridge, MA 02138 United States
AB: Magmas generated at thick crust continental arcs often have enriched continental crust-like trace element patterns and Pb-Sr-Nd isotope ratios that are intermediate to both upper mantle and crustal compositions. Thus it is difficult to distinguish between contributions from (a) the subducted basalt and the upper mantle wedge, and (b) subducted sediment and the continental crust. These issues have been the focus of major controversy. Here we show evidence for subduction contributions to lavas in a classic thick crust environment. In Eastern Trans-Mexican Volcanic Belt, the upper continental crust is 30 km to 45 km thick. However, primitive mafic lavas erupt on many sites across the arc. We have analyzed the subducting sediments as represented by DSDP 487, located seaward of the trench, where the lower third of the sediment column has strongly hydrothermal pelagic features and the upper two-thirds is composed of terrigenous sediments. The pelagic sediments have distinctive features that could be used to identify a subduction component in the volcanics, including high REE/Hf, negative Ce anomalies, and Nd-Hf isotopes that lie on the "seawater array" and offset from the "mantle-crust" array. We have focused on a unique series of lavas from volcano Nevado de Toluca, located southwest of Mexico City. These lavas show negative Ce anomalies coupled with low REE/Hf and Zr/Nd ratios. Hf-Nd isotope ratios show a shallow trend compared to the mantle-crust array, consistent with a pelagic component. In addition, Hf isotopes show a striking positive correlation with Ce anomalies that trend toward the pelagic sediment compositions. These and other observations provide clear evidence for a component from subducted sediment in the lavas. In addition, there is a negative correlation of Lu/Hf and Hf isotopes that requires a mixing endmember with MORB-like Hf isotope ratios but with lower than MORB Lu/Hf. This indicates a melt from eclogitic subducted basalt. Compared to other volcanoes from the Eastern Trans-Mexican Volcanic Belt, Nevado de Toluca so far shows the most significant pelagic sediment melt contribution.
DE: 1040 Radiogenic isotope geochemistry
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005