HR: 0800h
AN: V41B-0610    [Abstracts]
TI: Chlorine Isotope Ratios of Volcanic Glasses from Arc, Back-Arc and Mid-Ocean Ridges Settings - First Results of an Ion Microprobe Study
AU: * Straub, S M
EM: smstraub@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, United States
AU: Layne, G
AF: Memorial University of Newfoundland, St. John's, NL P.O. Box 4200, St. John's, A1C 5S7, Canada
AB: It has recently been demonstrated that chlorine isotopes ratios have a limited range in δ³⁷Cl (~ -2 to +0.5‰) in Earth's crustal and mantle rocks which implies little or no isotopic fractionation by the solid- rock geochemical cycle during the differentiation of Earth (Sharp et al. Nature 2007; Bonifacie et al. Chem Geol 2007; Barnes et al. 2007 Goldschmidt). Here we report preliminary results of a study of Cl isotope by secondary ion mass spectrometry (SIMS) of volcanic glasses from arc, back-arc and mid-ocean ridge settings that conform to these results. The samples investigated comprise 7 samples of basaltic to rhyolitic glass shards and melt inclusions from the Izu Bonin volcanic front that comprise the last 13 million years of arc history. In addition, four basaltic and basaltic andesitic glasses from the Shikoku and Mariana backarc basins, and three basalts from mid ocean ridges (Juan de Fuca Ridge, East Pacific Rise) were analyzed with an Cameca IMS 1280 at WHOI during two sessions in August 2006 and May 2007, respectively. After correction for matrix-related and session-dependent instrumental mass fractionation, the samples investigated fall within an overall range of 3‰ in δ³⁷Cl at an error of ±1.5‰ per sample (one standard deviation). Sample averages from the two sessions are comparable within 0.5‰ absolute. Within this error, the δ³⁷Cl values of arc, backarc and MORB glasses are indistinguishable. Assuming that the MORB samples analyzed are within the range of δ³⁷Cl of MORB determined by gas source isotope ratio mass spectrometry (IRMS; Sharp et al. Nature 2007; Bonifacie et al. Chem Geol 2007), the range of the glasses investigated completely overlaps with the range of the recently published high-precision data of MORB and arc volcanic rocks. While these first results agree with studies of bulk analyses, it is obvious that the large error associated with the SIMS method (~ ±1-1.5‰ versus ~ ± 0.12-0.26‰ obtained by IRMS) does not allow for using the δ³⁷Cl obtained by SIMS as geochemical tracer within a single tectonic setting. However, from the data thus far collected, there appears to be potential for further reducing the systematic error and thus to improve the potential of the SIMS method as geoanalytical tool.
DE: 1030 Geochemical cycles (0330)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting