HR: 0800h
AN: V41D-0800    [Abstracts]
TI: Evidence for Slab Melt Contributions to the Mexican Volcanic Belt and Other Young Hot Slab Arcs from Lu-Hf Isotopes
AU: * Goldstein, S L
EM: steveg@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Rt. 9W, Palisades, NY 10964, United States
AU: Cai, Y M
EM: cai@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Rt. 9W, Palisades, NY 10964, United States
AU: Langmuir, C H
EM: langmuir@eps.harvard.edu
AF: Harvard University, 20 Oxford St, Cambridge, MA 02138,
AU: LaGatta, A
EM: alagatta@jorgensenassociates.com
AF: Lamont-Doherty Earth Observatory, 61 Rt. 9W, Palisades, NY 10964, United States
AU: Straub, S M
EM: smstraub@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Rt. 9W, Palisades, NY 10964, United States
AU: Gomez-Tuena, A
EM: tuena@servidor.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Campus Juriquilla, Queretaro, 76230, Mexico
AU: Martin del Pozzo, A
EM: analil@igeofcu.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Instituto de Geofisica, Mexico, DF 04510, Mexico
AB: Despite major advances in delineating the processes that govern magma generation at convergent margins, the problem persists of distinguishing slab, mantle wedge, and crustal contributions. A corrollary question is whether there is significant melting of subducted ocean crust. Especially in thick crust regions, the importance of crustal versus mantle contributions to lavas represents a long-standing fundamental issue in arc magma geochemistry. We show that frontal arc magmas from the Central Mexican Volcanic Belt (CMVB), including the large andesitic stratovolcanoes Popocatepetl and Nevado de Toluca, display negligible crustal contamination, and contain substantial contributions from melting of subducted Pacific ocean crust. Despite ca. 50 km thick continental crust, the CMVB erupts near primitive lavas including "high-Nb" alkaline basalts that show negligible "subduction signatures" in their trace element patterns. These "high-Nb" basalts define the regional mantle wedge composition in isotope-trace element space. The "normal" calcalkaline lavas form a negative correlation between Hf isotopes and Lu/Hf. One endmember is like the high Nb basalts representing the regional mantle wedge. The other endmember has higher Hf isotopes (approaching values of Pacific MORB) and very low Lu/Hf of less than 0.04 (e.g. compared to typical values of ca. 0.2 in Pacific MORB). The low Lu/Hf values require low degree partial melting of a source rich in garnet. The high Hf isotopes require a depleted mantle source with isotopes like Pacific MORB. Together the Lu-Hf data indicate a substantial component derived from melting of eclogitic Pacific ocean crust. A key feature of the data is that the stratovolcano lavas showing the largest slab melt signature also show the highest Hf isotope ratios and thus are more "depleted mantle-like" than the regional mantle wedge. Thus, the integrated data allow us to clearly distinguish between mantle and crustal sources in the CMVB and point to substantial subducted slab melt contributions to these lavas. Preliminary data from the Colima region in western Mexico also indicate subducted ocean crust melt in stratovolcano lavas. The oceanic crust at the trench near Western and Central Mexico ranges in age from Pliocene to Miocene, thus represents subduction of a young hot slab. The southern Cascades are another case where a young hot slab is being subducted beneath a thick continental crust, and where both calcalkaline and high-Nb alkaline lavas are erupted. These suites show analogous trace element and isotopic relationships to Mexico that delineate contributions from melting of the subducted oceanic crust to the calcalkaline lavas. In contrast, the subducted ocean crust melt signature is absent from western Pacific arcs, where old cold slabs are being subducted, such as the Marianas, New Britain, and Kermedec. Thus the global data thus suggest that Lu- Hf isotopes represent an effective tracer for slab melting, and that young hot slabs melt and old cold slabs don't. We note that our Colima samples are from Jim Luhr, whose interest, enthusiastic support for MVB studies, and generosity are hereby acknowledged and greatly appreciated, and whose scientific counsel is greatly missed.
DE: 1025 Composition of the mantle
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1040 Radiogenic isotope geochemistry
DE: 3619 Magma genesis and partial melting (1037)
DE: 3621 Mantle processes (1038)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting