The relationship of hydrothermal fluid composition and crustal residence time to maturity of vent fields on the Juan de Fuca Ridge
The 228Ra/ 226Ra ratio of hydrothermal fluids reflect the residence time of the fluids within the crust as well as the age-since-melting of the basalt with which the fluids interact. We have taken into account the variable 228Ra/ 226Ra ratio in basalt source rocks to improve the calculation of residence time in four hydrothermal systems on the Juan de Fuca Ridge (JDFR). Apparent large differences in residence time (based on the incorrect assumption that the Ra activity ratios are equal to the parent Th activity ratios in the rock) disappear when the correct rock activity ratios are used. Fluid residence times from the onset of high-temperature (200°C) reaction are ≤3 years which is short compared to the mean life of 228Ra. It thus appears that on the JDFR, variability of the fluid 228Ra/ 226Ra ratio reflects the basalt age and not significant differences in fluid residence time. Within the present data set, there is an apparent correlation of increasing water/rock ratio with increasing system age, which may reflect the increased importance of faulting, higher permeability, and flow rate with age. However, within the resolution of the method there is no apparent correlation of water/rock ratio with residence time. Finally, the residence times are short relative to hypothesized storage periods for brines within the ocean crust. A potential implication of this result is that postulated, long-lived, high-chlorinity brine reservoirs must be extensively diluted by young, Ra-bearing fluids that have reacted significantly with hot rock.