228Th/ 228Ra ages of a barite-rich chimney from the Endeavour Segment of the Juan de Fuca Ridge

Abstract

Ages in the range 1–15 yr for chimneys deposited from hydrothermal vents from the Endeavour Segment of the Juan de Fuca Ridge are estimated based on the ingrowth of 228Th from 228Ra. Cleaning samples by digestion in an acid bath confirms that the mineral barite is the phase being dated. A detailed examination of sample ALV 2463 R7 collected from the Clam Bed Site yielded a vertical growth rate of 5.8 cm/yr and a radial growth rate of 2.6 cm/yr. This investigation revealed that this chimney did not grow in a regular layer by layer accretion. Therefore, an average of ages obtained from several barite crystals a few centimeters apart (<10 cm) was used to define the age of each segment. Ages obtained from the sulfide-rich Main Field Site sample ALV 2466 R5 suggest that the barite in this sample is from late-stage precipitation, in contrast to the Clam Bed sample where barite formed the primary matrix. These samples conform to different models of barite incorporation into hydrothermal chimneys.