HR: 0800h
AN: V41D-0793    [Abstracts]
TI: Lithium in melt inclusions records crustal assimilation at Volcan Jorullo, Mexico
AU: * Feineman, M D
EM: mdf12@psu.edu
AF: Pennsylvania State University, Dept. of Geoscieces, University Park, PA 16802, United States
AU: Johnson, E
EM: ejohns10@darkwing.uoregon.edu
AF: University of Oregon, Dept. of Geological Sciences, Eugene, OR 97403, United States
AU: Wallace, P
EM: pwallace@darkwing.uoregon.edu
AF: University of Oregon, Dept. of Geological Sciences, Eugene, OR 97403, United States
AU: Kobayashi, K
EM: katsura@pheasant.misasa.okayama-u.ac.jp
AF: Okayama University at Misasa, Institute for Study of the Earth's Interior, Misasa, Tottori, 682- 0913, Japan
AU: Moriguti, T
EM: moriguti@misasa.okayama-u.ac.jp
AF: Okayama University at Misasa, Institute for Study of the Earth's Interior, Misasa, Tottori, 682- 0913, Japan
AU: Nakamura, E
EM: eizonak@misasa.okayama-u.ac.jp
AF: Okayama University at Misasa, Institute for Study of the Earth's Interior, Misasa, Tottori, 682- 0913, Japan
AU: Rubin, K
EM: krubin@hawaii.edu
AF: University of Hawaii / SOEST, Department of Geology and Geophysics, Honolulu, HI 96822, United States
AB: Trace element concentrations and Li isotope ratios have been analyzed in olivine-hosted melt inclusions found in tephras from Volcan Jorullo, a monogenetic cinder cone of the Michoacan-Guanajuato Volcanic Field, Mexico. The melt inclusions range from basalt to basaltic andesite in composition, with K₂O and incompatible trace element concentrations increasing with SiO₂ content. Lithium concentrations range from 2 ppm in the most primitive inclusions to 12 ppm in the most evolved. Some of the lava flows from Jorullo contain abundant granitic xenoliths, and most contain plagioclase xenocrysts, suggesting that the lavas have assimilated some amount of crustal material. Lithium concentrations in the lava flows are 10-12 ppm, while the Li concentration in a granitic xenolith from one of the flows is 29 ppm. We interpret the basaltic melt inclusions containing 2 ppm Li to represent the primitive melt, while the higher Li in the more evolved melt inclusions and the lava flows results from assimilation of granitic country rock. The positive correlation between Li/Y and Eu/Eu* in the melt inclusions supports this theory of Li enrichment by crustal assimilation. The δ⁷Li in the melt inclusions ranges from typical arc lava values of 1.1±1.1‰ in the most primitive (low-Li) melt inclusions to extremely light values down to -9.2±0.9‰ in the more evolved (high-Li) melt inclusions. The significance of these data lies in determining whether the light Li in the evolved melt inclusions represents the initial composition of the contaminated melt itself, or if the Li isotope ratios in these inclusions have been altered by post-entrapment processes such as diffusive exchange with olivine or with the evolving melt through the olivine host. Lithium is known to diffuse quite quickly, and furthermore ⁶Li has been shown to diffuse more quickly than ⁷Li, which could result in kinetic fractionation of Li in the melt inclusions post-entrapment. However, it is puzzling that the most evolved melt inclusions, which were presumably the last melt inclusions to be trapped prior to or during eruption, have the most fractionated δ⁷Li. Lithium isotopic analysis of granitic xenoliths collected from the lava flows will determine if the country rock can provide a direct source of light Li. We propose that the high mobility of Li combined with high concentrations of Li in crustal rocks relative to basalts may combine to make Li an ideal tracer of crustal interaction even in relatively primitive lavas.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1036 Magma chamber processes (3618)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1043 Fluid and melt inclusion geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting