HR: 0800h
AN: V41A-1423    [Abstracts]
TI: Magmatic Volatile Variations Along a Trench-Perpendicular Transect in the Central Trans-Mexican Volcanic Belt
AU: * Johnson, E R
EM: ejohns10@uoregon.edu
AF: Department of Geological Sciences University of Oregon, 1272 University of Oregon, Eugene, OR 97403
AU: Wallace, P J
EM: pwallace@uoregon.edu
AF: Department of Geological Sciences University of Oregon, 1272 University of Oregon, Eugene, OR 97403
AU: Delgado Granados, H
EM: hugo@tonatiuh.igeofcu.unam.mx
AF: Universidad Autonoma de Mexico, Departamento de Vulcanologia, Ciudad Universitaria, DF 04510 Mexico
AB: To investigate volatiles (H₂O, CO₂, S, Cl) in subduction-related basaltic magmas, we have analyzed olivine-hosted melt inclusions from five basaltic centers located at varying distances from the trench in the Michoacan-Guanajuato Volcanic Field (MGVF), a part of the Trans-Mexican Volcanic Belt. Two of the cinder cones, Volcan Jorullo and Cerro El Astillero, are located near the volcanic front, about 100 km above the subducting Cocos plate (Pardo and Suarez, 1995). These cones erupted primitive lavas and tephra (Jorullo, Fo_88-92 olivine phenocrysts in early erupted tephra; Astillero, Fo₈₈ olivine). Paricutin, located about 20 km behind the front, erupted in 1943 with olivine compositions of Fo₈₄. Cerro San Miguel lies 68 km from the volcanic front, and has olivine of Fo₈₇ in early tephra. Hoya de Alvarez, a basaltic tuff ring, is located farthest from the trench, 160 km behind the volcanic front. This tuff ring is part of the Valle de Santiago region in the northern MGVF, which is related to subduction and influenced by extension from the nearby Chapala-Tula fault zone. The Hoya de Alvarez deposits are alkalic and contain abundant megacryst fragments of lower-Mg olivine (Fo₇₇). Olivine-hosted melt inclusions from all volcanic centers show a range of H₂O and CO₂ contents reflecting crystallization during ascent and degassing. The range in CO₂ contents is broadly similar for Jorullo (44-962 ppm), Astillero (914-1306 ppm), and San Miguel (595-1176 ppm), indicating crystallization pressures of 3.4 to 4.2 kbars. Melt inclusions from Hoya de Alvarez have similar to higher CO₂ (665-5523 ppm) and crystallization pressures from 1 to >5 kbars, suggesting relatively deep crystallization of some olivine megacrysts in the Valle de Santiago. Maximum H₂O contents at each center, which should most closely represent primary magmatic values, are highest at the volcanic front (Jorullo 5.2 wt% H₂O; Astillero 4.2 wt% H₂O). Similar to slightly lower maximum H₂O values are found behind the front (Paricutin 4.0 wt% [Luhr, 2001]; San Miguel 3.7 wt% H₂O) whereas Hoya de Alvarez, far behind the front, has very low water (max. value 1.5 wt% H₂O). Melt inclusion S and Cl contents are also variable at each center as a result of degassing. As with H₂O, maximum S and Cl contents are highest at the volcanic front (Jorullo, 3000ppm S, 1435ppm Cl; Astillero 2500ppm S, 1235ppm Cl), and decrease behind the front (Paricutin 1200ppm S, 1100ppm Cl [Luhr, 2001]; San Miguel 2120ppm S, 1020ppm Cl; Hoya de Alvarez 1250ppm S, 830ppm Cl). Our results document a decrease in the amount of subduction-recycled volatiles (H₂O, S, Cl) transferred to the mantle wedge with increasing slab depth beneath central Mexico.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1043 Fluid and melt inclusion geochemistry
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3640 Igneous petrology
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005