HR: 0800h
AN: V51B-0568    [Abstracts]
TI: Mass Dependent Fractionation of Hg Isotopes in Source Rocks, Mineral Deposits and Spring Waters of the California Coast Ranges, USA
AU: * Smith, C N
EM: chris.n.smith@shell.com
AF: Dept. of Geological Sciences, University of Michigan, 1100 N. University Ave., Ann Arbor, MI 48109, United States
AU: Kesler, S E
EM: skesler@umich.edu
AF: Dept. of Geological Sciences, University of Michigan, 1100 N. University Ave., Ann Arbor, MI 48109, United States
AU: Blum, J D
EM: jdblum@umich.edu
AF: Dept. of Geological Sciences, University of Michigan, 1100 N. University Ave., Ann Arbor, MI 48109, United States
AU: Rytuba, J J
EM: jrytuba@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, United States
AB: We present here the first study of the isotopic composition of Hg in rocks, ore deposits, and active hydrothermal systems from the California Coast Ranges, one of Earth's largest Hg-depositing systems. The Franciscan Complex and Great Valley Sequence, which form the bedrock in the California Coast Ranges, are intruded and overlain by Tertiary volcanic rocks including the Clear Lake Volcanic Sequence. These rocks contain two types of Hg deposits, hot-spring deposits that form at shallow depths (<300 m) and silica-carbonate deposits that extend to greater depths (200 to 1000 m), as well as active springs and geothermal systems that release Hg to the present surface. The Franciscan Complex and Great Valley Sequence contain clastic sedimentary rocks with higher concentrations of Hg than volcanic rocks of the Clear Lake Volcanic Field. Mean Hg isotope compositions for all three rock units are similar, although the range of values in Franciscan Complex rocks is greater than in either Great Valley or Clear Lake rocks. Hot spring and silica-carbonate Hg deposits have similar average isotopic compositions that are indistinguishable from averages for the three rock units, although δ²⁰²Hg values for the Hg deposits have a greater variance than the country rocks. Precipitates from dilute spring and saline thermal waters in the area have similarly large variance and a mean δ²⁰²Hg value that is significantly lower than the ore deposits and rocks. These observations indicate there is little or no isotopic fractionation during release of Hg from its source rocks into hydrothermal solutions. Isotopic fractionation does appear to take place during transport and concentration of Hg in deposits, especially in their uppermost parts. Boiling of hydrothermal fluids is likely the most important process causing of the observed Hg isotope fractionation. This should result in the release of Hg with low δ²⁰²Hg values into the atmosphere from the top of these hydrothermal systems and a consequent enrichment in heavy Hg isotopes in the upper crust through time.
DE: 0461 Metals
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 3665 Mineral occurrences and deposits
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting