HR: 0800h
AN: V41D-0806    [Abstracts]
TI: Volatile Contents of Olivine-Hosted Melt Inclusions From the Central Oregon High Cascades
AU: * Ruscitto, D
EM: druscitt@uoregon.edu
AF: Dept. of Geological Sciences, University of Oregon, Eugene, OR 97403,
AU: Johnson, E
EM: ejohns10@uoregon.edu
AF: Dept. of Geological Sciences, University of Oregon, Eugene, OR 97403,
AU: Wallace, P
EM: pwallace@uoregon.edu
AF: Dept. of Geological Sciences, University of Oregon, Eugene, OR 97403,
AU: Kent, A
EM: adam.kent@geo.oregonstate.edu
AF: Dept. of Geosciences, Oregon State University, Corvallis, OR 97330,
AU: Mercer, C
EM: cmercer@uoregon.edu
AF: Dept. of Geological Sciences, University of Oregon, Eugene, OR 97403,
AU: Bindeman, I
EM: bindeman@uoregon.edu
AF: Dept. of Geological Sciences, University of Oregon, Eugene, OR 97403,
AB: The young (~ 16 Ma) and slowly subducting (~ 5 cm/a NNW) Juan de Fuca plate should be mostly dehydrated at typical arc magma generation depths (~ 100 km) beneath the Central Oregon High Cascades, resulting in small amounts of slab-derived fluids infiltrating the overlying mantle wedge (Green and Harry, 1999; Peacock, 2003). To investigate this hypothesis, we analyzed olivine-hosted melt inclusions in mafic tephra deposits from five volcanic centers (Blue Lake Crater, Yapoah Crater, Twin Crater, Palagonite Tuff, and North Sister Volcano) by FTIR, LA-ICPMS, and electron probe to characterize the pre-eruptive volatile contents (H₂O, CO₂, S, F, Cl) and their relationship to major and trace element compositions. Host olivine phenocrysts (Fo_79-84) have δ¹⁸O values of 5.35 ±0.2 ‰. The melt inclusions have compositions of 49 - 54 wt. % SiO₂ and 4.9 - 5.5 wt. % MgO. H₂O contents for the entire dataset vary from ~ 0.2 to 4.0 wt. % and CO₂ contents vary from below detection up to 2000 ppm, implying maximum pressures of entrapment between 1.7 and 3.9 kbar during ascent and degassing (Newman and Lowenstern, 2002). Additionally, within-cone averages for inclusions range from 800 - 1500 ppm F, 900 - 1400 ppm S, and 350 - 1000 ppm Cl. For the set of inclusions from each cone, we assume that the highest H₂O content is least affected by degassing and therefore represents the minimum amount of H₂O in the parent magma. Highly fluid mobile to less fluid mobile element ratios (e.g., Ba/La and Sr/La) correlate positively with maximum H₂O/La ratios for Cascades melt inclusions (r² ~ 0.60; n = 5 volcanic centers). High incompatible element abundances relative to NMORB closely correspond to an EMORB-type source mantle, overprinted with even higher abundances of incompatible and fluid mobile elements (e.g., Ba, U, Pb), resulting in a distinct, negative Nb-Ta anomaly. The volatile data, taken together with olivine δ¹⁸O values, trace element abundances and conservative element ratios (e.g., Nb/Y ~ 0.29 - 0.60), suggest partial melting of an EMORB-type mantle source that has been fluxed by an H₂O-rich slab component even in this hot, dry end-member subduction zone.
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 8430 Volcanic gases
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting