U-Th systematics and 230Th ages of carbonate chimneys at the Lost City Hydrothermal Field

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 Th-230 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 Th-230/Th-232 atomic ratios of the vent fluids range from 1 (+/- 10) x 10(-6) to 11 (+/- 5) x 10(-6), are less than those of seawater, and indicate that the vent fluids may contribute a minor amount of non-radiogenic Th-230 to the LCHF carbonate chimney deposits. Chimney U-238 concentrations range from 1 to 10 mu g/g and the average chimney corrected initial delta U-234 is 147.2 +/- 0.8, which is not significantly different from the ambient seawater value of 146.5 +/- 0.6. Carbonate Th-232 concentrations range broadly from 0.0038 +/- 0.0003 to 125 +/- 16 ng/g and Th-230/Th-232 atomic ratios vary from near seawater values of 43 (+/- 8) x 10(-6) up to 530 (+/- 25) x 10(-3). Chimney ages, corrected for initial Th-230, 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. (C) 2011 Elsevier Ltd. All rights reserved.