HR: 08:45h
AN: V11C-04    [Abstracts]
TI: Lithium Isotopic Composition of Guatemalan Jadeitites
AU: * Simons, K K
EM: ksimons@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964
AU: Langmuir, C H
EM: langmuir@eps.harvard.edu
AF: Harvard University, 20 Oxford St, Cambridge, MA 02138
AU: Harlow, G E
EM: ghalow@amnh.org
AF: American Museum of Natural History, Central Park West at 79th St, New York, NY 10024
AB: Lithium (Li) isotopes have been measured on Guatemalan jadeitites using MC-ICP-MS, to better understand the evolution of fluids and fluid exchange during subduction. These jadeitites are high-pressure ($>$6 Kbar), low-temperature (200 to 400C) rocks in serpentinite m‚lange that have metasomatic signatures (including high Li concentrations), and are interpreted as having crystallized from slab-derived fluids. They occur on both sides of the Motagua Fault Zone of Guatemala, which is where the North American and Caribbean plates meet. We have measured the Li isotopic compositions of both jadeite and phengitic muscovite from jadeitites occurring both north and south of the fault. The range in Li isotopes is 0 to -4 \permil, relative to the standard LSVEC, and the precision is 0.5 \permil. There is no large systematic offset in the isotopic ratio north and south of the fault, nor is there a fractionation in isotopic ratio between jadeite and phengite within a given rock. These initial values may suggest that the fluids being sampled by these HPLT phases are isotopically light, compared to the Li isotopic ratio of the mantle (+4 \permil), that of seawater (+30 \permil) or that of altered ocean crust or sediments (\sm 0 to +25 \permil). Further analyses are needed to constrain the source and nature of these fluids as well as the process that generates such a large isotopic fractionation in precipitates of a fluid that equilibrated with an isotopically heavy protolith.
DE: 3665 Mineral occurrences and deposits
DE: 3670 Minor and trace element composition
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting