Radiochemical constraints on the crustal residence time of submarine hydrothermal fluids: Endeavour Ridge
The 210Pb/Pb and 228Ra/ 226Ra ratios measured in fluids and particles venting from the Endeavour Ridge are used to constrain the crustal residence time of the converting hydrothermal fluid from the initiation of basalt alteration where Mg +2 loss from seawatcr results in rapidly falling pH conditions, to termination at seafloor venting. The 210Pb/Pb ratios of hot, low Mg fluids are close to that of the basalts, suggesting a residence time of no greater than ten years. Particles associated with these vents have slightly higher ratios which may in part be due to scavenging of seawater 210Pb. The 228Ra/ 226Ra ratios of the fluids and an associated Ba-rich particle sample were also close to the basalt ratio, futher constraining the residence time to 3 years or less. These estimates indicate that the mass of fluid interacting with newly formed crust at any one time is less than 9 × 10 13kg. if the axial heat flux is to be no greater than 30% of the total advective heat loss from the oceanic crust.