HR: 11:05h
AN: V52A-04    [Abstracts]
TI: Implications for Melt Differentiation Processes in the Central Mexican Volcanic Belt from 'Zoned' Monogenetic Volcanoes
AU: * Straub, S M
EM: smstraub@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory of Columbia University, 61 Route 9W , Palisades, NY 10964, United States
AU: Gomez-Tuena, A
EM: tuena@geociencias.unam.mx
AF: Centro de Geociencias UNAM , Campus Juriquilla, Querétaro, Qro 76230, Mexico
AU: Goldstein, S L
EM: steveg@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory of Columbia University, 61 Route 9W , Palisades, NY 10964, United States
AU: Cai, Y M
EM: cai@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory of Columbia University, 61 Route 9W , Palisades, NY 10964, United States
AU: Martin-Del Pozzo, A
EM: analil@geofisica.unam.mx
AF: Instituto de Geofisica UNAM, Ciudad Universitaria, Circuito Institutos, Mexico, D.F 04510, Mexico
AB: Across-arc mass balances require the knowledge of composition and the origins of the mass of melt. This "melt mass" may derive from slab, mantle, or crustal sources, whereby these possible sources will be reflected in magma chemistry. In order to identify the arc melt sources, we are studying a broad range of alkaline ('intraplate- type') to calc-alkaline ('arc type') Holocene volcanic rocks in the Central Mexican Volcanic Belt (CMVB) constructed on thick continental crust (~40-47 km). Sr-Nd-Pb isotope ratios similar to those from subarc mantle xenoliths in all erupted magmas, together with trace elements, point towards a dominant mantle origin of melts along with some slab-derived component, but negligible crustal additions. In order to understand how isotope and trace elements relate with major element diversity, we focus on monogenetic volcanoes with a significant zonation in major elements (e.g. Chichinautzin, Guespalapa, Suchiooc). Monogenetic volcanoes build in a single event and hence their magmas must derive from similar sources. Within zoned monogenetic centers, two different trends are recognized: (1) a 'source trend'; and (2) 'differentiation trends'. The 'source trend' describes the trend of decreasing FeO* and TiO₂ with increasing Mg#, which precedes high-Mg# (>60-75) calc- alkaline and alkaline magmas. The 'source trend' parallels the tholeiitic trend of Fe-enrichment with decreasing Mg# observed in oceanic basalts, but culminates at lower overall maxima of FeO* (~9 wt%) and TiO₂(~2 wt%) (FeO* ~16 wt% and TiO₂ ~3.5 wt%). High-Ni olivines contained in alkaline and calc-alkaline 'source trend' magmas suggest that the 'source trend' is generated through repeated mantle melting induced by fluid addition from slab (Straub et al., G³, submitted). This supports models of mantle origin of primary high-Mg# andesite melts. However, the majority of CMVB magma have lower Mg#- numbers and plot on trajectories that emanate from any point of the 'source trend' towards lower FeO* and TiO₂ with decreasing Mg#. These 'differentiation trends' are preserved fully or partially within individual monogenetic centers. Our preliminary trace element and isotope data suggest that these 'differentiation trends' can neither by explained simply by fractional crystallization nor by crustal assimiliation but require more complex petrogenesis that may involve serial addition of slab components to mantle sources. These competing models are testable by comprehensive trace elements and isotope studies in combination with mineral and melt inclusions work on zoned monogenetic centers.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly