HR: 17:45h
AN: V34B-08    [Abstracts]
TI: Lithium Isotopic Composition of Intra-Oceanic Arc Hydrothermal Fluids - Initial Results From the Tonga Kermadec Arc
AU: Massoth, G J
EM: gary.massoth@gmail.com
AF: GNS Science, P.O. Box 30-368, Lower Hutt, 5010, New Zealand
AU: * Chan, L
EM: glchan@lsu.edu
AF: Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803-4101, United States
AU: Butterfield, D A
EM: david.a.butterfield@noaa.gov
AF: NOAA Pacific Marine Environmental Laboratory, 7600 Sand Point Way, NE, Seattle, WA 98115-6349, United States
AB: Active hydrothermal venting has recently been discovered on submarine volcanoes along the Tonga-Kermadec Arc. Li isotopic compositions of high temperature fluids from mid-ocean ridge systems have been well studied as an indicator of seawater-basalt exchange. we present here initial Li isotope data for hydrothermal fluids collected from the volcanoes along the arc: Volcano 1 and Volcano 19 on the Tonga arc and Monowai, Giggenbach, Macauley, Brothers, and Healy along the Kermadec arc. The system features shallow venting depths, phase separation, magmatic degassing, and pervasive diffuse flows. Black smoker venting occurs on Volcano 19 (maximum fluid temperature 245°C) and Brothers NW vents (290°C). δ⁷Li (relative to L-SVEC) of high temperature end-member fluids at these two locations are 8.2 and 7.2‰ respectively. These isotopic values are in the typical range for mid-ocean ridge crest hot springs, which average 7± 1‰ globally (Bray, 2001). The similarity in fluid composition may reflect comparable Li isotopic compositions of arc volcanic substrates and mid-ocean ridge basalts. Most low temperature fluids (<70°C, e.g. at Brothers cones, Healy, and Giggenbach) are variably enriched in Li and have δ⁷Li between 15‰ and the seawater value (32‰). Only Monowai shows slight Li depletion and higher δ⁷Li (33.5‰) than seawater. The trends of Li versus Mg vary widely between sites, reflecting very different water/rock ratios. However, high and low temperature fluids converge to a general linear relationship between δ⁷Li and Mg/Li and Cl/Li suggesting that the diffuse flows are essentially the product of subsurface mixing of ambient seawater and high temperature vent fluids. The compositions of diffuse flows differ from that of Baby Bare, a 60°C warm spring water from a ridge flank hydrothermal system in that the latter has lost both Mg and Li as a result of low temperature reaction (Wheat and Mottl, 2000). High δ⁷Li (33‰) relative to Mg/Li and Cl/Li has been observed at Macauley and, together with high Mg and Si, is consistent with acid leaching of chemically weathered rocks. Several sites (Volcanoes 1 and 19, Monowai, Giggenbach and Brothers) provide evidence of phase separation, but Li isotopic fractionation associated with this process appears to be insignificant. Available data also do not indicate anomalous composition in samples containing magmatic volatiles. Our initial results thus suggest that high temperature vent fluids from the Tonga-Kermadec main arc have similar Li isotope compositions as those from mid-ocean ridge systems. Further work is in progress to investigate the nature of diffuse flows. References:
Bray, A.M., 2001, PhD thesis, University of New Hampshire.
Wheat, C.G., Mottl, M.J., 2000. Geochim. Cosmochim. Acta 64, 629-642.
DE: 4832 Hydrothermal systems (0450, 1034, 3017, 3616, 8135, 8424)
DE: 4870 Stable isotopes (0454, 1041)
DE: 8185 Volcanic arcs
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting