HR: 1340h
AN: T13B-0479    [Abstracts]
TI: Uranium-Thorium Isotopic Analyses of Basalts From Behind the Volcanic Front, Southeastern Guatemala
AU: * Ruscitto, D M
EM: rusc0047@umn.edu
AF: Dept. of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AU: Hirschmann, M M
EM: hirsc022@umn.edu
AF: Dept. of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AU: Edwards, R L
EM: edwar001@umn.edu
AF: Dept. of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AU: Walker, J A
EM: jim@geol.niu.edu
AF: Dept. of Geology and Environmental Geosciences, Northern Illinois University, Davis Hall 312, Normal Rd, DeKalb, IL 60115 United States
AU: Cheng, H
EM: cheng021@umn.edu
AF: Dept. of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AU: Cameron, B I
EM: bcameron@uwm.edu
AF: Dept. of Geosciences, University of Wisconsin- Milwaukee, Lapham Hall 366, Milwaukee, WI 53211 United States
AU: Roggensack, K
EM: kurt.roggensack@asu.edu
AF: Dept. of Geological Sciences, Arizona State University, Box 871404, Tempe, AZ 85287 United States
AB: Abundant, monogenetic cinder cones in SE Guatemala extend over 3000 km² and reach up to ~100 km behind the main volcanic front, providing an ideal location to study across-arc geochemical transects above a continental subduction zone^1,2. Isotopic evidence (e.g. Sr, Nd) for crustal assimilation is not obvious in the basaltic lavas of southeastern Guatemala^1,3, suggesting that the geochemistry of the lavas may record processes in their source. Uranium series nuclides are sensitive to both fluid transfer and melting and may also constrain the timescale of melting, transport, and differentiation^5,6,7,8. Although no historical behind-the-front eruptions are known, geologic evidence strongly suggests that volcanism includes Holocene activity^4,9. Uranium and thorium isotopic ratios were determined on basalt to basaltic andesite, whole-rock lavas from cinder cones of SE Guatemala by inductively coupled plasma and thermal ionization mass spectrometry. Preliminary data trend broadly parallel to the ²³⁰Th-²³⁸U equiline ranging from [²³⁰Th/²³²Th] = 1.10 to 1.35 and [²³⁸U/²³²Th] = 1.26 to 1.43. The data indicate slight ²³⁸U enrichment with an average [²³⁰Th/²³⁸U]= 0.92 ±0.07, [²³⁸U/²³²Th]= 1.32 ±0.03, and U/Th = 0.436 ±0.005. Behind-the-front lavas have [²³⁸U/²³²Th] values slightly greater than lavas of the volcanic front (with the exception of one sample from Tecuamburro)¹⁰. Slab-derived fluid addition and separation of small-degree melts from the wedge result in increased uranium and thorium values respectively^5,6,10. The bulk [²³⁰Th/²³⁸U] value of the erupted lavas will depend upon the relative proportions of fluid-fluxing/melting and the associated timescales involved. Larger U-excesses in monogenetic behind-front lavas suggest that slab-derived fluid addition may dominate farther away from the front while decompression melting plays a larger role in the volcanic front itself, or that lavas behind the main front originate from a more depleted source. Relatively small amounts of disequilibria may indicate: (1) both fluid-addition and melting processes influence the lavas by roughly the same magnitude, (2) assimilation of an older, compositionally similar crust, or (3) lavas are older than inferred. More U-series analyses, in addition to better age constraints from Ar/Ar dating, will allow a detailed understanding of the processes involved behind the volcanic front in southeastern Guatemala. ¹Walker, J.A. et. al. 1995 CMP 120 378-390 ²Walker, J.A. et. al. 2000 JGR 105 18949-18939 ³Patino, L.C. et. al. 2000 CMP 138 265-283 ⁴Cameron, B.I. et. al. 2002 JVG 119 21-50 ⁵Thomas, R.B. et. al. 2002 GCA 66 4287-4309 ⁶Gill and Williams 1990 GCA 54 1427-1442 ⁷Reagan et. al. 1994 GCA 58 4199-4212 ⁸Bourdon et. al. 2003 JGR 108 1-19 ⁹Carr, M.J. et. al. 2005 unpub. Manuscript ¹⁰Mickelson, J.E. 2005 unpub. M.S. thesis from N.I.U.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1040 Radiogenic isotope geochemistry
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Tectonophysics [T]
MN: Fall Meeting 2005