HR: 1340h
AN: T33B-1369    [Abstracts]
TI: Li Isotopes as Tracers of Fluid-Rock Interactions in Oceanic Hydrothermal Systems: Evidence From the Hess Deep Rift
AU: * Brant, C O
EM: lithiumocean@gmail.com
AF: School of Earth and Ocean Sciences,University of Victoria, PO Box 3055, Victoria, BC V8W 3P6, Canada
AU: Coogan, L A
EM: lacoogan@uvic.ca
AF: School of Earth and Ocean Sciences,University of Victoria, PO Box 3055, Victoria, BC V8W 3P6, Canada
AU: Gillis, K M
EM: kgillis@uvic.ca
AF: School of Earth and Ocean Sciences,University of Victoria, PO Box 3055, Victoria, BC V8W 3P6, Canada
AB: Li isotopes have the potential to be powerful tracers of fluid-rock interactions at mid-ocean ridge hydrothermal systems due to the large isotopic difference between seawater (δ⁷Li = 31‰) and fresh MORB (δ⁷Li = 1.5 to 5.6 ‰). Sampling programs along tectonic escarpments at Hess Deep provide an ideal suite to examine the spatial variability of Li mobility and Li isotopic content within young (1 Ma) lavas and sheeted dikes formed at the fast-spreading East Pacific Rise towards the end of a segment. Previous work has shown that the lavas are relatively fresh, with minor alteration to clay minerals and Fe oxyhydroxides. Sheeted dikes are variably altered to amphibole-dominated assemblages, with localized zones where chlorite dominates. Sr and O isotope data correlate with these regional patterns. Preliminary data, collected by Thermo X-series quadrupole ICP-MS, show that the lavas have similar Li concentrations to fresh MORB (5 to 6 ppm) and are slightly enriched in δ⁷Li (4.1 to 7.7 ‰). There is a positive correlation between Li content and δ⁷Li within the lavas, however, the trend is not a simple mixing line between seawater and fresh MORB, being offset to lower δ⁷Li in the altered component. Similar to previous studies, these relationships support the prediction that ⁶Li is preferentially retained in low temperature clay minerals. Sheeted dykes are depleted in Li (0.8-4.63 ppm) and most samples are enriched in δ⁷Li (6.7-15.4 ‰) relative to fresh MORB. These samples show an inverse relationship between decreasing concentration and increasing isotopic enrichment. This implies that ⁶Li is not simply preferentially retained in the replacement mineral assemblages. The degree of isotopic enrichment appears to increase with increasing clinopyroxene alteration, and is greatest for amphibolite dominated assemblages. A broad positive correlation between Li concentration and δ¹⁸O is observed in the dikes. Thus Li isotopes are sensitive indicators of the nature and extent of hydrothermal interactions for both high and low temperature processes.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 3017 Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424)
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2007 Fall Meeting