U/Th systematics and 230Th ages of carbonate chimneys at the Lost City hydrothermal field

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doi: 10.1016/j.gca.2011.01.008
Author(s): Ludwig, Kristin A.; Shen, Chuan-Chou; Kelley, Deborah S.; Cheng, Hai; Edwards, R. Lawrence
Author Affiliation(s): Primary:
University of Washington, School of Oceanography, Seattle, WA, United States
Other:
National Taiwan University, Taiwan
University of Minnesota, United States
Volume Title: Geochimica et Cosmochimica Acta
Source: Geochimica et Cosmochimica Acta, 75(7), p.1869-1888. Publisher: Elsevier, New York, NY, International. ISSN: 0016-7037 CODEN: GCACAK
Note: In English. Includes appendices. 94 refs.; illus., incl. 3 tables
Summary: The Lost City Hydrothermal Field (LCHF) is a serpentinite-hosted vent field located 15 km west of the spreading axis of the Mid-Atlantic Ridge. In this study, uranium-thorium (U-Th) geochronological techniques have been used to examine the U-Th systematics of hydrothermal fluids and the 230Th ages of hydrothermally-precipitated carbonate chimneys at the LCHF. Fluid sample analyses indicate that endmember fluids likely contain only 0.0073 ng/g U or less compared to 3.28±0.03 ng/g of U in ambient seawater. For fluid samples containing only 2-21% ambient seawater (1.1-11 mmol/kg Mg), Th concentration is 0.11-0.13 pg/g and surrounding seawater concentrations average 0.133±0.016 pg/g. The 230Th/232Th atomic ratios of the vent fluids range from 1(±10)×10-6 to 11(±5)×10-6, are less than those of seawater, and indicate that the vent fluids may contribute a minor amount of non-radiogenic 230Th to the LCHF carbonate chimney deposits. Chimney 238U concentrations range from 1 to 10 µg/g and the average chimney corrected initial δ234U is 147.2±0.8, which is not significantly different from the ambient seawater value of 146.5±0.6. Carbonate 232Th concentrations range broadly from 0.0038±0.0003 to 125±16 ng/g and 230Th/232Th atomic ratios vary from near seawater values of 43(±8)×10-6 up to 530(±25)×10-3. Chimney ages, corrected for initial 230Th, range from 17±6 yrs to 120±13 kyrs. The youngest chimneys are at the intersection of two active, steeply-dipping normal faults that cut the Atlantis Massif; the oldest chimneys are located in the southwest portion of the field. Vent deposits on a steep, fault-bounded wall on the east side of the field are all <4 kyrs old, indicating that mass wasting in this region is relatively recent. Comparison of results to prior age-dating investigations of submarine hydrothermal systems shows that the LCHF is the most long-lived hydrothermal system known to date. It is likely that seismic activity and active faulting within the Atlantis Massif and the Atlantis Fracture Zone, coupled with volumetric expansion of the underlying serpentinized host rocks play major roles in sustaining hydrothermal activity at this site. The longevity of venting at the LCHF may have implications for ecological succession of microorganisms within serpentinite-hosted vent environments.
Year of Publication: 2011
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 03 Geochronology; Absolute age; Actinides; Atlantic Ocean; Atlantis Massif; Atlantis fracture zone; Cenozoic; Chimneys; Cretaceous; Dates; Faults; Foraminifera; Fracture zones; Hydrothermal vents; Invertebrata; Isotope ratios; Isotopes; Leg 158; Lost City hydrothermal field; Mesozoic; Metaigneous rocks; Metals; Metamorphic rocks; Metasomatic rocks; Microfossils; Mid-Atlantic Ridge; Mid-ocean ridges; Normal faults; North Atlantic; Ocean Drilling Program; Ocean floors; Protista; Radioactive isotopes; Sea water; Serpentinite; TAG hydrothermal field; Tertiary; Th-230; Th/U; Thorium; U-238/U-234; Uranium; Vents; Water-rock interaction
Coordinates: N260812 N260814 W0444933 W0444935
N300000 N302000 W0420000 W0421500
Record ID: 2011051985
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands