HR: 1340h
AN: V13A-1445    [Abstracts]
TI: Evidence for Dehydrated Slab Melt Components in the Sources of the Izu Bonin Arc and the Mexican Volcanic Belt
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: Harvard University, 20 Oxford Street, Cambridge, MA 02138 United States
AU: Gomez Tuena, A
EM: tuena@geosciencias.unam.mx
AF: Centro de Geosciencias, Campus Juriquilla, Queretaro, 76230 Mexico
AU: Langmuir, C H
EM: langmuir@eps.harvard.edu
AF: Harvard University, 20 Oxford Street, Cambridge, MA 02138 United States
AB: Understanding the pathways of fluids and melts in subduction zones relies on deciphering the processes that generate the unique trace elements patterns of arc magmas. While it is common consensus that these patterns reflect interaction of slab-derived fluid and/or melt components with the subarc mantle, the details of these processes remain highly controversial. One approach towards solving the problem is searching comprehensive high quality data sets that are now becoming available from the global spectrum of arcs. Should common signatures emerge through the veil of the structural and compositional diversity, those must have general meaning for arc magma genesis. Similar slab melt components appear to exist in the rear-arc region of the intra-oceanic Izu Bonin Volcanic Arc and the arc front region near Zitacuaro of the continental Mexican Volcanic Belt. These arcs are opposites in the global spectrum of volcanic arcs: they differ widely in a range of tectonic and compositional parameters, among them the thickness and composition of the crust, as well as the composition of the subarc mantle and the age and composition of the subducting slab. In either arc, a subordinate group of K-rich mafic lavas exist that - despite displaying certain key characteristics of oceanic and continental arc magmas - differ systematically from the dominant calc-alkaline and tholeiitic arc series. In both arcs, the K-rich subgroups show typical oceanic vs. continental arc differences in the abundances of CaO, Na$_{2}$O, FeO and highly incompatible trace elements. However, relative to the calc-alkaline and tholeitic series, the K-rich subgroups have always lower SiO$_{2}$, higher MgO and mg\#, as well as lower ratios of highly fluid mobile elements to rare earth elements (e.g. lower Pb/Nd, Li/Yb), higher LREE abundances, steeper REE patterns and often higher MREE/HREE ratios. However, their prominent negative Nb-Ta and Hf-Zr anomalies clearly attest to a subduction influence. In both arcs, the Pb-Sr-Nd isotope systematics are consistent with all arc magmas being mixtures of slab (sediment, oceanic crust) and mantle components. However, in the Nd/Pb vs Pb isotope diagram, the high-K subgroups are systematically displaced to higher Nd/Pb values at comparable Pb isotopes ratios relative to the calc-alkaline series. We suggest that the high-K subgroups originate from mantle source regions that were infiltrated by composite slab melts (sediment and oceanic crust) derived from a previously dehydrated slab. Since the contribution of the sediment to the composite slab melt is comparatively minor ($<$8% in Izu Arc, and $<$20% in the Mexican Volcanic Belt), the negative Nb-Ta and Hf-Zr anomalies cannot be inherited from the sediment melt component alone. Therefore, additional fractionation is required in order to generate the distinct trace element patterns of the high-K subgroups that may occur either during slab melting or during melt/rock interaction in the mantle. In contrast, the calc-alkaline and tholeiitic series show a stronger influence of slab fluid and sediment melt components. Owing to their end member character in either arc, the presence of the high-K subgroups support hence models of progressive slab and wedge evolution for arcs.
DE: 3640 Igneous petrology
DE: 1025 Composition of the mantle
DE: 1065 Trace elements (3670)
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting