HR: 08:15h
AN: V31H-02    [Abstracts]
TI: Using H2O and trace element ratios to produce a spatial map of magmatic H2O contents throughout the Trans-Mexican Volcanic Belt
AU: * Johnson, E R
EM: ejohns10@uoregon.edu
AF: University of Oregon, 1272 University of Oregon, Eugene, OR 97403,
AU: Wallace, P J
EM: pwallace@uoregon.edu
AF: University of Oregon, 1272 University of Oregon, Eugene, OR 97403,
AU: Manea, V C
EM: vlad@geociencias.unam.mx
AF: UNAM Campus Juriquilla, Apdo. postal 1-742, Queretaro, 76001, Mexico
AB: Along with fluids, trace elements are released during dehydration of subducting sediment and altered oceanic crust. Large ion lithophile elements are typically fluid mobile, and thus may be used as tracers for fluid fluxing. We used melt inclusion H₂O and trace element data from nine cinder cones across the subduction-related Michoacan-Guanajuato Volcanic Field (MGVF) of central Mexico to assess the fluid mobility of trace element species. We found correlations between H₂O and Sr/La, Ba/Nb, Ba/Y, Pb/Y, Sr/Ti, suggesting that Sr, Ba, and Pb are present in fluids released from the downgoing slab. Additionally, we used regression lines for these correlations to estimate magmatic H₂O for cinder cones across Mexico. We have applied the Sr/La and Sr/Ti relationships to the extensive dataset of cinder cone lava and scoria analyses from the MGVF by Hasenaka and Carmichael (1985). In order to see 2-D spatial patterns in H₂O across the MGVF, we plotted the localities and the calculated H₂O contents on a digital elevation model of Mexico. Initial results from this modeling show that, like our melt inclusion data, magmatic H₂O contents are generally high (>3 wt%) across a broad region from the volcanic front to ~100 km behind the front. High H₂O concentrations (4-6 wt%) are most abundant along the volcanic front, whereas much lower values (1-2 wt%) occur in an extensional region far behind the front. The relationship between H₂O and trace element ratios can also be extended to other regions of Mexico, as the correlation between H₂O/La and Sr/La is consistent to the east in the Chichinautzin Volcanic Field (Cervantes and Wallace, 2003) and to the west in the Colima Graben. Using analyses from these and other regions, we have created a spatial map of H₂O contents across the Trans- Mexican Volcanic Belt, enabling us to see trends both along and across the arc. We can then use these spatial maps to relate patterns in H₂O content to subduction processes such as arc migration over time and slab devolatilization. Using 2-D geodynamic models we can correlate the breakdown of hydrous minerals in the slab and mantle wedge to spatial variations in magmatic H₂O.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1043 Fluid and melt inclusion geochemistry
DE: 1065 Major and trace element geochemistry
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting