HR: 1330h
AN: V32B-1014    [PDF]
TI: Oxygen isotope geochemistry of the May 10th Anahatan eruption.
AU: * OLeary, J A
EM: oleary@gps.caltech.edu
AF: Caltech, MC 100-23, 1200 E. California Blvd, Pasadena, CA 91125 United States
AU: Eiler, J M
EM: oleary@gps.calltech.edu
AF: Caltech, MC 100-23, 1200 E. California Blvd, Pasadena, CA 91125 United States
AU: Fischer, T
EM: fischer@unm.edu
AF: University of New Mexico University of New Mexico, Northrop Hall, Albuquerque, NM 87131 United States
AU: Hilton, D
EM: drhilton@ucsd.edu
AF: Scripps Institute of Oceanography, UCSD, Mail Code 0244, La Jolla, CA 92093 United States
AU: Jaffee, L
EM: ljaffe@ucsd.edu
AF: Scripps Institute of Oceanography, UCSD, Mail Code 0244, La Jolla, CA 92093 United States
AB: The Anahatan Eruption provides an opportunity to examine, in detail, the oxygen isotope chemistry of a zero age pyroclastic eruption. The petrology and trace element geochemistry of oceanic arc magmas is highly influenced by the introduction of subducted material to the mantle wedge. Oxygen isotope ratios, though sensitive to the addition of slab materials to the mantle, are also modified by fractional crystallization and contamination of magmas and sub-solidus alteration. In addition, trace element geochemical variations are found within single lava flows [1]. Zero age samples allow analysis of groundmass free of alteration minerals and the intensity of sampling provides coverage of geochemical variation within products of an individual flow. We report oxygen isotope ratios from 12 Anahatan samples that include lava, tephra, and a volcanic bomb. Our purpose is to compare Anahatan samples to each other and to other arc volcanoes and to assess oxygen isotope ratio heterogeneity within this single eruption. We determined oxygen isotope ratios for groundmass from all twelve samples. Lava sample $\delta^{18}$O values vary from 5.5 to 6.1$\permil$, with an average value of 5.9$\pm$.3$\permil$. Tephra samples were separated into two vesicular, glassy fractions based on color and inclusions. The dark, oxide-bearing fraction has an average $\delta^{18}$O value of 5.63 $\pm$ .07$\permil$ (n = 8) while the light, oxide-free fraction has an average $\delta^{18}$O value of 5.86 $\pm$ .08 $\permil$ (n = 7). These fractions may not represent the bulk chemistry of tephra samples. Electron probe analyses are needed to characterize the major element chemistry of selected grains. The average oxygen isotope ratio of all Anahatan groundmass samples, 5.70$\pm$.3 $\permil$, is typical for an oceanic arc and is similar to previously observed $\delta^{18}$O values in Mariana arc volcanoes [2]. Lava oxygen isotope compositions are consistent with oxygen isotope fractionation by crystallization with high $\delta^{18}$O values in silica-rich samples ($>$60 wt % SiO$\_{2}$) and low $\delta^{18}$O in a silica-poor sample ($\sim$~50 wt %). The uniform oxygen isotope composition within each tephra groundmass fraction, over a range in silica content that is comparable to the range in lava silica content, is unexpected and is likely due to our selection of visually similar grains that do not reflect the heterogeneity of the bulk sample. The .26$\permil$ fractionation between light and dark fractions of tephra groundmass may be a due to bulk composition, fractional crystallization, or variable degassing history. [1] Maclennan et al. (2003) JGRb 108, art. Number 2007 [2] Ito and Stern (1985/86) EPSL 76, 312-320
DE: 1020 Composition of the crust
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting