White Paper Title: 
Fluid crustal residence time

There is general agreement that the primary characteristics (temperature, pH, redox state, major element composition) of many high-temperature (black smoker) MOR hydrothermal fluids are produced by reaction with mid-ocean ridge basalt (MORB) at a water/rock ratio (W) near 1 under conditions approaching equilibrium with a secondary mineral assemblage near 400°C at 300–500 bars. Phase separation is responsible for the large changes in chloride and other major elements that have been observed in such fluids. Rock composition can be affected by the degree of prior alteration and can also cause variations in fluid chemistry in some circumstances. Fluid chemistry, as well as biochemical characteristics, should also be affected by the reaction pathway and residence time of fluids within the oceanic crust. For many fluid chemical properties, the high temperature reaction zone overprints the prior history of low-temperature reaction in the recharge zone, so the most important temporal measurement is the time between the onset of high-temperature water-rock reaction and venting at the seafloor. The residence time of hydrothermal fluid (defined here as the time elapsed between being heated to 200°C and exiting the seafloor) should depend directly on the depth of the heat source and the permeability along the path of fluid convection. Measurement of naturally occurring radioisotopes in vent fluids and associated sulfide deposits have been used to constrain the crustal residence time of the hydrothermal fluid from the initiation of high temperature rock alteration. The activity of two isotopes in vent fluids, 210Pb (t1/2 = 22.3 y) and 228Ra (t1/2 = 5.77 y) are used as chronometers for the time that the fluid has resided under high temperature conditions of the crust. Specific to this meeting, I would like to incorporate new radium isotopic data (fluid and rock) from Lau, EPR and Endeavour into collaborative manuscripts with other vent geochemists and biologists.