A METHOD FOR QUANTITATIVELY ESTIMATING DIFFUSE AND DISCRETE HYDROTHERMAL DISCHARGE

Submarine hydrothermal fluids discharge as undiluted, high-temperature jets and as diffuse, highly diluted, low-temperature percolation. Est imates of the relative contribution of each discharge type, which are important for the accurate determination of local and global hydrother mal budgets, are difficult to obtain directly. In this paper we descri be a new method of using measurements of hydrothermal tracers such as Fe/Mn, Fe/heat, and Mn/heat in high-temperature fluids, low-temperatur e fluids, and the neutrally buoyant plume to deduce the relative contr ibution of each discharge type. We sampled vent fluids from the north Cleft vent field on the Juan de Fuca Ridge in 1988, 1989 and 1991, and plume samples every year from 1986 to 1991. The tracers were, on aver age, 3 to 90 times greater in high-temperature than in low-temperature fluids, with plume values intermediate. A mixing model calculates tha t high-temperature fluids contribute only approximately 3% of the flui d mass flux but > 90% of the hydrothermal Fe and > 60% of the hydrothe rmal Mn to the overlying plume. Three years of extensive camera-CTD sl ed tows through the vent field show that diffuse venting is restricted to a narrow fissure zone extending for 18 km along the axial strike. Linear plume theory applied to the temperature plumes detected when th e sled crossed this zone yields a maximum likelihood estimate for the diffuse heat flux of 8.9 X 10(4) W/m, for a total flux of 534 MW, cons idering that diffuse venting is active along only one-third of the fis sure system. For mean low- and high-temperature discharge of 25-degree s-C and 319-degrees-C, respectively, the discrete heat flux must be 26 6 MW to satisfy the mass flux partitioning. If the north Cleft vent fi eld is globally representative, the assumption that high-temperature d ischarge dominates the mass flux in axial vent fields leads to an over estimation of the flux of many non-conservative hydrothermal species b y about an order of magnitude.