HR: 11:35h
AN: V52A-06    [Abstracts]
TI: Subduction in high fluid fluxing environment and the origin of high-δ ¹⁸O and high- δ ⁷Li lavas in Mt. Shasta, Cascade arc, California.
AU: * Martin, E
EM: ermartin@uoregon.edu
AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403, United States
AU: Bindeman, I
EM: bindeman@uoregon.edu
AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403, United States
AU: Grove, T L
EM: tlgrove@MIT.EDU
AF: Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, Cambridge, MA 02139, United States
AB: This study presents analyses of O-isotopes in olivine and orthopyroxene phenocrysts from representative samples of Mt. Shasta's lavas and correlates them with Li isotopes and trace elemental ratios. The measured δ ¹⁸O_Ol-values range from 5.31‰ to 6.08‰, is up to 1‰ higher than the mantle value. Added to the presence of primitive magnesian andesite and high primary water content (up to 12wt%), the high-δ ¹⁸O measured makes the Mt. Shasta exceptional. The composition of the analyzed olivine phenocrysts shows that these crystals are in equilibrium with their host whole rocks, and were derived by melting of a peridotitic source based on trace element ratios in olivine. Minor (<5%) contamination makes these rocks a good assessment to mantle-derived magma. The origin of the high-δ ¹⁸O mantle-derived signature results from the interaction with unusually high fluid fluxes from high-δ ¹⁸O slab and subducted sediments. A model based on the flux-melting process is proposed in order to explain why in a general case the subduction fluid signature is lost during the magma genesis and why in exceptional cases, such as Mt Shasta, it is partially preserved. Unusual features such as a fracture zone or volcanic chain subduction lead to a fluid flux that is high enough to maintain a high-δ ¹⁸O after reacts the mantle wedge and after dilution by mantle-melt when crossing the hydrated mantle solidus. Consequently, by melt re-equilibration with the fluid, high-δ ¹⁸O magmas are generated. The comparison between the Li- and O-isotopes compositions of Mt. Shasta and neighboring volcanoes from the rear-arc (i.e., Medicine Lake), show that both stable isotope values decrease from front to the rear arc. This decrease is explained by the slab dehydration process, due to the Rayleigh loss of heavy isotope with subduction progress and the fact that deeper and deeper portions of the slab are dehydrated. The "adakitic" signature (Sr/Y up to 160) of the Mt. Shasta's lavas appears to result from high fluid fluxing. According to geochemical and petrological characteristics of the Mt. Shasta's high-Mg andesites, it appears that these siliceous, hydrous, and high-δ ¹⁸O magmas worldwide are generated in specific, high-fluid-flux environments and are not ubiquitous as general primary magma types in arc petrogenesis.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting