HR: 11:05h
AN: T32A-04 INVITED     [Abstracts]
TI: Across-arc Volatile Variations in the Michoacan-Guanajuato Volcanic Field, Mexico: Investigating Slab Devolatilization and the Mantle Source for Basaltic arc Volcanoes
AU: * Johnson, E R
EM: ejohns10@uoregon.edu
AF: Department of Geological Sciences, University of Oregon, United States
AU: Wallace, P
EM: pwallace@uoregon.edu
AF: Department of Geological Sciences, University of Oregon, United States
AU: Delgado Granados, H
EM: hugo@tonatiuh.igeofcu.unam.mx
AF: Departamento de Vulcanologia, Instituto de Geofisica, UNAM, Mexico
AB: Volatile abundances in mafic arc magmas provide information on slab devolatilization processes, and thus give insight into fluid recycling in subduction zones. In order to investigate both magmatic and mantle volatile (H₂O, CO₂, S, Cl) variations in a subduction zone, we have analyzed olivine-hosted melt inclusions from nine basaltic centers located at varying distances from the trench in the Michoacan-Guanajuato Volcanic Field (MGVF), Mexico. Additionally, we have used trace element data to assess the variability in mantle source compositions across the arc. By sampling mainly primitive basaltic centers (many contain Fo_88-91 olivine), we can compare magmatic volatile contents of near-primary magmas across the arc. Our data show that magmatic H₂O is high (3.0- 5.0 wt%) from the volcanic front to 160 km behind the front. Other volatiles (CO₂, S, and Cl) also show high concentrations across the arc. We used these data to estimate mantle volatile contents based on trace element partial melting models (e.g., Kelley et al., 2006). The degree of mantle partial melting beneath the MGVF varies from 13-24% for our analyzed cones. Calculated mantle volatile contents (0.5-1.0 wt% H₂O, 100-200 ppm Cl, and 100-650 ppm CO₂) are highly elevated compared to depleted MORB mantle values. Additionally, trace element data for each of the cones show that the mantle beneath the MGVF is heterogeneous. Most cones show the relative depletions in Nb and Ta typical of subduction zone melts. However, two samples show enriched Nb-Ta values, suggesting these melts originated from a more OIB-like source. Consistently high mantle H₂O across the arc, combined with high Cl and CO₂, suggest that flux of water and other volatiles is not limited to the region beneath the volcanic front. Instead, these data support several dehydration models, including continuous dehydration of the slab to great depths, down-dragging of hydrated mantle above the slab, and trenchward migration of the volcanic arc over time. However, better geophysical modeling of the thermal structure of the downgoing slab coupled with age constraints on volcanism in the region are necessary to further assess models for fluid fluxing beneath the MGVF.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1043 Fluid and melt inclusion geochemistry
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3621 Mantle processes (1038)
SC: Tectonophysics [T]
MN: 2007 Joint Assembly