A three-component mixing model for ridge-crest hydrothermal fluids

Abstract

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, H 2S, Na, K, Ca, and Sr, at 11–13°N and 21°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.