Phase equilibria constraints on the chemistry of hot spring fluids at mid-ocean ridges

Details Phase equilibria constraints on the chemistry of hot spring fluids at mid-ocean ridges Phase equilibria constraints on the chemistry of hot spring fluids at mid-ocean ridges

Dec 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55.3559s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 55, Issue 12, pp.3559-3580

Mathematics

Logic

12

Scientific paper

Recent advances in experimental and theoretical geochemistry have made it possible to assess both homogeneous and heterogeneous equilibria involving a wide range of aqueous species at temperatures and pressures appropriate to model hydrothermal alteration processes at mid-ocean ridges. We have combined selected aspects of the chemistry of hot spring fluids with constraints imposed by a geologically reasonable assemblage of minerals in the system Na 2 O-K 2 O-CaO-MgO-FeO-Fe 2 OrAl 2 O 3 -SiO 2 -H 2 O-HCl-H 2 S to assess the effect of temperature on the composition of the aqueous phase and the activities of mineral components in plagioclase and epidote solid solutions. Assuming f O 2( g ) and f S 2( g ) controlled by pyrite-pyrrhotite-magnetite equilibria, a constant dissolved Ca concentration, and a dissolved Cl concentration equivalent to that of seawater, increasing temperature from 250 to 400°C at 500 bars results in systematic changes in the composition of mineral phases, which in turn constrain pH and the distribution of aqueous species. The model predicts that dissolved concentrations of Fe, SiO 2 , K, H 2 S, and H 2 increase, while Na and pH (25°c) decrease with increasing temperature. pH ( in-situ) decreases slightly with increasing temperature, and has a value of 5.16 at 400°C. Dissolved Mg concentrations do not exceed 1 mmolal at any temperature investigated. Allowing for differences in pressure and total dissolved Cl, the predicted effect of temperature on fluid chemistry is in good agreement with results from basalt alteration and plagioclase + epidote + quartz recrystallization experiments, and in some cases, with results from hot spring fluids at mid-ocean ridges. Some vent fluids, in particular NGS (EPR, 21°N) and vent-4 (EPR, 11°N), however, reveal pH and/or dissolved Fe, H 2 S, H 2 , and SiO 2 concentrations, which are difficult to reconcile with measured temperatures in comparison with results of temperature dependent mineral solubility calculations, and suggest that these fluids have lost heat by conduction on ascent to the seafloor. That many hot spring vent fluids are characterized by variable degrees of conductive heat loss renders measured temperatures unreliable as indicators of the maximum temperature of subseafloor hydrothermal alteration processes. The implications of this are significant for hot spring fluids which reveal large Cl variations relative to seawater, since likely mechanisms to account for such variability typically require temperatures in excess of those inferred for subseafloor reaction zones by simply correcting measured temperatures for the effects of adiabatic cooling.

Affiliated with

Also associated with

No associations

Rating

Phase equilibria constraints on the chemistry of hot spring fluids at mid-ocean ridges does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.

If you have personal experience with Phase equilibria constraints on the chemistry of hot spring fluids at mid-ocean ridges, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Phase equilibria constraints on the chemistry of hot spring fluids at mid-ocean ridges will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFWR-SCP-O-1538230