HR: 0800h
AN: V31C-0633    [Abstracts]
TI: Lithium Isotope Systematics in the western Trans-Mexican Volcanic Belt: Initial Results
AU: * Ryan, J G
EM: ryan@shell.cas.usf.edu
AF: Department of Geology University of South Florida, 4202 East Fowler Ave. SCA 528, Tampa, FL 33620
AU: Fuller, T
EM: tkfuller@mail.usf.edu
AF: Department of Geology University of South Florida, 4202 East Fowler Ave. SCA 528, Tampa, FL 33620
AU: Johnston, R
EM: rjohnston@mail.usf.edu
AF: Department of Geology University of South Florida, 4202 East Fowler Ave. SCA 528, Tampa, FL 33620
AU: Mui, E
EM: emui@mail.usf.edu
AF: Department of Geology University of South Florida, 4202 East Fowler Ave. SCA 528, Tampa, FL 33620
AB: The westernmost segment of the Trans-Mexican Volcanic Belt includes both calc-alkaline stratocones and smaller alkaline volcanic centers. Toward trying to resolve the contributions of slab-derived components, the sub-arc mantle, and assimilated arc crust to the compositions of TMVB, we have been examining the B-Be-Li and Li isotopic signatures of lavas erupted in and around Volcan de Colima and the Michoacan-Guanouato volcanic field. Colima Graben lavas (high K calc-alkaline andesites from the active stratocone, minettes from monogenetic cones surrounding Colima, and older lamprophyric lava flows in the rift basin) all show δ⁷Li of +4 ± 1 ‰. Calc-alkaline lavas verge toward the lower values in this range (+3.3 ‰), while the alkaline lavas range up to +5 ‰. B/Be ratios show marked differences between calc-alkaline and alkaline Colima Graben lavas (see Hochstaedter et al 1996); our early results indicate a similar, but subtler distinction may be preserved in Li isotope systematics. Interestingly, the evolved lamprophyric lavas analyzed show no clear Li isotopic differences from the more primitive minettes, although other parameters suggest crustal assimilation played a role in their petrogenesis. Lavas from Paricutin, in the Michoacan-Guanouato volcanic field, have distinctly lower δ⁷Li than the Colima lavas, as +2 ± 0.5 ‰. Paricutin lavas show strong evidence for assimilation of older granitic arc crust with B/Be ratios different from those currently being generated via slab-mantle chemical exchanges in the modern TMVB arc. δ⁷Li shows little variation in the lavas analyzed and no systematics with SiO₂ or other useful parameters for assimilation. Thus, the lower δ⁷Li in these lavas may be an indication of an isotopically distinct source mantle beneath Paricutin.
DE: 1025 Composition of the mantle
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005