The influence of hydrothermal fluid composition and advection rates on black smoker chimney mineralogy: Insights from modeling transport and reaction

A detailed study of black smoker chimneys was carried out by drawing together mineralogical observations and measured fluid compositions. The modeling technique, which establishes a link between vent fluid chemistry and vent deposit mineralogy, is used to examine the influence of the physical environment on transport across short length scales (centimeters) which are subject to steep thermal ( ∼350°C) and chemical gradients, and to predict saturation states of minerals as functions of position within chimney walls. Owing to short length scales, steep gradients, and constancy of bounding fluid compositions, concentration profiles are controlled by transport; the effect of reaction rates on profiles is assumed to be negligible. Precipitation at points within the chimney wall is thus kinetically controlled; local equilibrium does not apply, and precipitation must be suppressed when calculating distributions of species and saturation states of minerals. Model calculations are performed in a series of four steps. The first is speciation of measured fluid compositions at in situ temperature and pressure conditions. Results of calculations are plotted on stability diagrams of log fO 2 - log f S 2 and log fO 2 - log [ (a Fe 2+ ) (a cu + ) 2 ] to examine how closely the predicted saturation states fit with observations of minerals coexisting with the hydrothermal fluid at the vent sites. The second step is calculation of heat and mass transport across the chimney wall, the third is speciation of the pore fluid compositions calculated to exist in the wall subsequent to transport, and the fourth is comparison of calculated mineral saturation states to observations of naturally occurring chimney samples. Application of the modeling technique to five vent sites which exhibit distinct fluid compositions allows reproduction of observed mineralogy. Model results demonstrate the sensitivity of geochemical processes to the physical environment (porosity, tortuosity, permeability, mineral composition), to rates of advection of seawater inward or hydrothermal fluid outward across chimney walls, and to endmember fluid composition.