An estimate of hydrothermal fluid residence times and vent chimney growth rates based on 210Pb/Pb ratios and mineralogic studies of sulfides dredged from the Juan de Fuca Ridge

Abstract

The 210PbPb ratios across two sulfide samples dredged from the Juan de Fuca Ridge are used to estimate the growth rate of the sulfide material and the residence time of the hydrothermal fluid within the oceanic crust from the onset of basalt alteration. 210Pb is added to the hydrothermal fluid by two processes: (1) high-temperature alteration of basalt and (2) if the residence time of the fluid is on the order of the 22.3-year half-life of 210Pb, by in-situ growth from 222Rn ( Krishnaswami and Turekian, 1982). Stable lead is derived only from the alteration of basalt. The 210Pb/Pb ratio across one sample was ∼ 0.5 dpm/10 −6 g Pb, and across the other it was ∼ 0.4 dpm/10 −6 g Pb. These values are quite close to the 238UP/b ratios of basalts from the area, suggesting that the residence time of the hydrothermal fluid from the onset of basalt alteration is appreciably less than the mean life of 210Pb, i.e., the time required for ingrowth from the radon. An apparent growth rate of 1.2 cm/yr is derived from the slope of the 210Pb/Pb curve for one of the samples. This is consistent with its mineralogy and texture which suggest an accretionary pattern of development. There is no obvious sequential growth pattern, and virtually no gradient in 210Pb/Pb across the second sample. This is consistent with alteration of the original 210Pb/Pb distribution by extensive remobilization reactions which are inferred from the mineralogic and textural relationships of the sample.