HR: 10:50h
AN: V52A-03    [Abstracts]
TI: Subduction Contributions in the Trans-Mexican Volcanic Belt: Implications from Lava Chemistry and Hf-Nd-Pb Isotopes
AU: * Cai, Y
EM: cai@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, United States
AU: Goldstein, S L
EM: steveg@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, United States
AU: Langmuir, C H
EM: langmuir@eps.harvard.edu
AF: Harvard University, Dept. of Earth and Planetary Sciences, Cambridge, MA 02138, United States
AU: Gómez-Tuena, A
EM: tuena@geociencias.unam.mx
AF: UNAM, Centro de Geociencias, Querétaro, Mexico
AU: LaGatta, A
EM: alagatta@jorgensenassociates.com
AU: Straub, S M
EM: smstraub@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, United States
AU: Martín Del Pozzo, A
EM: analil@geofisica.unam.mx
AF: UNAM, Instituto de Geofísica, Mexico City, Mexico
AB: Despite thick continental crust, near primitive lavas erupt throughout the Trans-Mexican Volcanic Belt (TMVB). In order to distinguish and better constrain subduction contributions and effects of crustal contamination, we analyzed samples representing subducting sediments from DSDP Site 487, and Quaternary lavas from stratovolcanoes and cinder cones, including alkaline "high-Nb" lavas from the Sierra Chichinautzin Volcanic Field (SCVF) showing negligible subduction signature in its trace element chemistry and representing melts of the mantle wedge. Our primary observations and implications are: (1) The high-Nb SCVF ‘intraplate' lavas define a linear trend along the "Nd-Hf mantle-crust array", defining the composition of the mantle wedge. (2) Popocatepetl and Nevado de Toluca stratovolcanoes show the highest Nd and Hf isotope ratios, higher than the ‘intraplate' lavas, indicating their sources are more "depleted mantle-like" than the regional mantle wedge. (3) The Popo and Toluca chemical and isotopic trends sharply contrast with Pico de Orizaba, which shows classic indications of crustal contamination (e.g. high 207Pb/204Pb, low Nd-Hf isotope ratios), consistent with contamination by local Precambrian crust. (4) Higher Nd-Hf isotopes in Popo and Toluca lavas also correlate with lower Pb isotope ratios, and lower Lu/Hf and Zr/Hf. Together, these data indicate contributions from subducted Pacific oceanic crust and hydrothermal sediment. (5) Popo and Toluca are also enriched in Th/LREE compared with ‘intraplate' lavas, reflecting subducted sediment contributions. (6) Nd-Hf isotope ratios of hydrothermal sediment from DSDP Site 487 lie on the "seawater array", with high Hf isotope ratios compared to the "mantle-crust array". Popo and Toluca Nd-Hf isotopes display a shallower slope than the "intraplate lava Nd-Hf array", reflecting contributions from hydrothermal sediment. Popocatepetl and Toluca lavas therefore avoid substantial crustal contamination of mantle wedge-derived melts, despite traverse through a ca. 35 km thick continental crust. Their compositions can be simply modeled, and reflect components from a composite slab melt (represented by altered Pacific ocean crust and DSDP 487 sediment), plus the regional mantle (represented by high-Nb SCVF lavas). The best estimate of the slab melt is a ca. 5 percent sediment melt plus a low-degree (ca. 0.5 percent) altered oceanic crust melt, with a mixing ratio of about 1:9. Thus, the integrated data allow us to clearly distinguish between mantle and crustal sources and point to substantial subducted slab contributions to TMVB lavas.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 8185 Volcanic arcs
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly