A 3-COMPONENT MIXING MODEL FOR RIDGE-CREST HYDROTHERMAL FLUIDS

The hydrothermal fluid chemistry data set for sediment starved ridge axes is used to estimate endmember compositions and to constrain a three-component mixing model based on that of Von Damm, using numerical methods to estimate the endmember compositions. The data set includes Fe, Cl, H2S, Na, K, Ca, and Sr, at 11-13 degrees N and 21 degrees N East Pacific Rise, North and South Cleft Segment (Southern Juan de Fuca Ridge), TAG, and MARK. Q-mode factor analysis shows that the data can be explained (99.9%) by 3 factors, A constrained optimization of a weighted least-squares algorithm, assuming conservative mixing and with the constraints that all endmember concentrations and contributions are non-negative and that fractional contributions sum to one, is used to partition the data set into endmembers. Mixing of the calculated endmembers -consistently found to be phase-separated brine, vapor, and hydrothermally altered seawater -reproduces the composition of the vent fluids. The model is consistent with the presence of a convecting brine layer in the primary reaction zone at depth, as suggested by Bischoff and Rosenbauer, that transfers properties to the overlying circulating seawater by release of a ''vapor'' and by diffusion, We suggest that future experimental work examine solubility controls in conjunction with and subsequent to phase separation, and work with a wider range of fluid salinities.