HR: 09:00h
AN: V31H-05    [Abstracts]
TI: Comparative light element â€" Li isotope systematics of arc volcanic rocks from the TMVB to Guatemala: Roles for the slab, crust, and mantle in magma genesis
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, United States
AU: Walker, J
EM: jim@geol.niu.edu
AF: Department of Geology and Environmental Geosciences, Northern Illinois University Davis Hall 312, Normal Rd., DeKalb, IL 60115, United States
AU: Teipel, A
EM: ateipel@geol.niu.edu
AF: Department of Geology and Environmental Geosciences, Northern Illinois University Davis Hall 312, Normal Rd., DeKalb, IL 60115, United States
AU: Grose, C
EM: kdwormy@yahoo.com
AF: Department of Geology, University of South Florida, 4202 East Fowler Ave, SCA 528, Tampa, FL 33620, United States
AU: Lindsey, M
EM: lindsey3@mail.usf.edu
AF: Department of Geology, University of South Florida, 4202 East Fowler Ave, SCA 528, Tampa, FL 33620, United States
AU: Wagner, R
EM: seestar@bigfoot.com
AF: Department of Geology, University of South Florida, 4202 East Fowler Ave, SCA 528, Tampa, FL 33620, United States
AU: Fuller, T
EM: tfuller@mail.usf.edu
AF: Department of Geology, University of South Florida, 4202 East Fowler Ave, SCA 528, Tampa, FL 33620, United States
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, United States
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, United States
AB: Active arc volcanism occurs under dramatically variable conditions of slab age, dip, temperature and subduction rate; and crustal thickness and composition along the southern margin of Mexico (the TMVB) into the Central American Arc in Guatemala (Ipala Graben region). This study has engaged both undergraduate and graduate researchers over four years in an examination of B-Be-Li and δ⁷Li systematics of young lavas from the TMVB and the Ipala Graben. Dr. Jim Luhr was generous in his support of this effort, hosting undergraduate researchers at NMNH to examine and select samples for analysis, and providing numerous insights into the volcanic petrology and history of the Trans-Mexican Belt. Lavas of the TMVB differ from those in Guatemala in overall lower B enrichments and B/Be ratios (avg. B/Be <15 in the TMVB vs. >20 in Guatemala), though as one moves eastward along the TMVB arc front B/Be increases toward values observed at the Guatemalan volcanic front. Behind-the-front arc lavas show lower B/Be in both arcs, though in the Ipala Graben regular cross-arc declines are not observed, and young lavas from the far eastern end of the graben may have elevated B contents and B/Be. Li/Yb ratios of the Ipala Graben suite are typical for arcs (2-6, at Dy/Yb of ~2), while TMVB lavas range to Li/Yb =13 and Dy/Yb = 4, indicative of mineralogically anomalous mantle sources. However, δ⁷Li signatures of the TMVB and Ipala Graben samples examined span nearly identical ranges of +2‰ to +5.5‰. In both the Ipala Graben and TMVB suites, δ⁷Li values indicate a role for assimilated, isotopically distinct crustal rocks (TMVB: +1.5‰ to +3.4‰; Ipala: +0.9‰ to -2.6‰). Markedly higher B/Be in the Ipala suite points to a greater role for low-T slab-derived fluid inputs. Elevated LILE abundances in some western TMVB suites have been interpreted as the result of earlier, integrated slab additions (Hochstaedter et al, 1996), but the similar and MORB-like δ⁷Li in both systems point to the importance of an upper mantle reservoir uniform in its Li isotopic signature in the genesis of arc lavas along these margins.
DE: 1030 Geochemical cycles (0330)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting