The²¹⁰PbPb 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 of²¹⁰Pb, by in-situ growth from²²²Rn (Krishnaswami and Turekian, 1982). Stable lead is derived only from the alteration of basalt.

The²¹⁰Pb/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 the²³⁸UP/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 of²¹⁰Pb, 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 the²¹⁰Pb/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 in²¹⁰Pb/Pb across the second sample. This is consistent with alteration of the original²¹⁰Pb/Pb distribution by extensive remobilization reactions which are inferred from the mineralogic and textural relationships of the sample.