HR: 0800h
AN: V51B-1487    [Abstracts]
TI: U/Th Geochronology of Carbonate Chimneys at the Lost City Hydrothermal Field
AU: * Ludwig, K A
EM: kludwig@u.washington.edu
AF: University of Washington, School of Oceanography, Box 357940, Seattle, WA 98195 United States
AU: Kelley, D S
EM: kelley@ocean.washington.edu
AF: University of Washington, School of Oceanography, Box 357940, Seattle, WA 98195 United States
AU: Shen, C
EM: river@ntu.edu.tw
AF: National Taiwan University, Department of Geosciences, No. 1, Sec. 4, Roosevelt Road, Taipei, 106 ROC Taiwan
AU: Cheng, H
EM: cheng021@umn.edu
AF: University of Minnesota, Department of Geology and Geophysics, 310 Pillsbury Drive SE, Minneapolis, MN 55455-0219 United States
AU: Edwards, R L
EM: edwar001@tc.umn.ed
AF: University of Minnesota, Department of Geology and Geophysics, 310 Pillsbury Drive SE, Minneapolis, MN 55455-0219 United States
AB: U/Th radiochronology is commonly used to date marine carbonates such as corals and sediments that are tens to 600,000 years old. This technique is newly applied to precise dating of hydrothermal carbonate deposits from the Lost City Hydrothermal Field (LCHF). The LCHF, located at 30°N near the Mid-Atlantic Ridge (MAR), is an off-axis, ultramafic-hosted hydrothermal system characterized by up to 60 m tall carbonate chimneys that vent warm (<40-91°C), high pH (9-10) fluids. The field is hosted by 1.5 my old crust on the southern face of the Atlantis Massif, which is composed of variably altered serpentinite with lesser gabbro. Mixing of warm, calcium-rich fluids with surrounding seawater results in the precipitation of variable mixtures of aragonite, calcite, and brucite. Previous work using ¹⁴C dating techniques showed that Lost City has been active for at least 30,000 years. In this study, over thirty carbonate samples from the LCHF have been analyzed using ICP-MS techniques for ²³⁸U - ²³⁴U - ²³⁰Th - ²³²Th dating. Initial results indicate that venting at Lost City may have been on-going for at least 100,000 years. The U/Th systematics of the Lost City carbonates are similar to those of deep sea corals. Concentrations of uranium in Lost City carbonates range from 1-10 ppm, which is similar to corals (2-4 ppm). Endmember vent fluids contain low concentrations of U (less than 0.15 ng/g), suggesting that the U incorporated into the carbonate deposits is seawater-derived (seawater U = 3 ng/g). In addition, the initial ²³⁴U/²³⁸U values of the carbonates are indistinguishable from the value for modern seawater. Ages are corrected to initial ²³⁰Th, which is constrained by analyzing ²³⁰Th/²³²Th values in ambient seawater and young Lost City carbonates. This work confirms that the youngest (modern-~1,000 years) known formations are located along an east-west fault that likely focuses fluid flow within the massif. Initial results also suggest that carbonate vent deposits on the east side of the field are within the same age range (~1,000-~5,000 years), which may have implications for fault activity and fluid flow in this region. The modern to ~100,000 year age range of the structures at Lost City implies sustained faulting, cracking, and fluid flow within the massif that has allowed these structures to remain active for so long.
DE: 0450 Hydrothermal systems (1034, 3017, 3616, 4832, 8135, 8424)
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 1115 Radioisotope geochronology
DE: 3017 Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005