An integrated chemical and stable‐isotope model of the origin of Midocean Ridge Hot Spring Systems
- 10 December 1985
- journal article
- Published by American Geophysical Union (AGU) in Journal of Geophysical Research
- Vol. 90 (B14) , 12583-12606
- https://doi.org/10.1029/jb090ib14p12583
Abstract
Chemical and isotopic changes accompanying seawater‐basalt interaction in axial midocean ridge hydrothermal systems are modeled with the aid of chemical equilibria and mass transfer computer programs, incorporating provision for addition and subtraction of a wide‐range of reactant and product minerals, as well as cation and oxygen and hydrogen isotopic exchange equilibria. The models involve stepwise introduction of fresh basalt into progressively modified seawater at discrete temperature intervals from 100° to 350°C, with an overall water‐rock ratio of about 0.5 being constrained by an assumed δ18OH2O at 350°C of +2.0 per mil (H. Craig, personal communication, 1984). This is a realistic model because: (1) the grade of hydrothermal metamorphism increases sharply downward in the oceanic crust; (2) the water‐rock ratio is high (>50) at low temperatures and low (4 and substantial increases in Si and Fe; however, discrepancies exist in the predicted pH (5.5 versus 3.5 measured) and state of saturation of the solution with respect to greenschist‐facies minerals. The calculated δDH2O is +2.6 per mil, in excellent agreement with analytical determinations. The calculated chemical, mineralogic, and isotopic changes in the rocks are also in good accord with observations on altered basalts dredged from midocean ridges (Humphris and Thompson, 1978; Stakes and O'Neil, 1982), as well as with data from ophiolites (Gregory and Taylor, 1981). Predicted alteration products include anhydrite and clay minerals at low temperatures and a typical albite‐epidote‐chlorite‐tremolite (greenschist) assemblage at 350°C. The models demand that the major portion of the water‐rock interaction occur at temperatures of 300°–350°C. Interaction at temperatures below approximately 250°C results in negative δ18OH2O shifts, contrary to the observed positive δ18O values of the fluids exiting at midocean ridge vents. Hydrogen isotope fractionation curves by Suzuoki and Epstein (1976), Lambert and Epstein (1980), and Liu and Epstein (1984), among others, are compatible with the model, and require δDH2O to increase at all temperatures as a result of seawater‐basalt interaction.Keywords
This publication has 93 references indexed in Scilit:
- Mineralogy and stable isotope geochemistry of hydrothermally altered oceanic rocksPublished by Elsevier ,2003
- The critical point and two-phase boundary of seawater, 200–500°CPublished by Elsevier ,2002
- Oxygen and hydrogen isotope studies of serpentinization in the Troodos ophiolite complex, CyprusPublished by Elsevier ,2002
- Growth history of hydrothermal black smoker chimneysNature, 1983
- Crustal Processes of the Mid-Ocean RidgeScience, 1981
- The mineralogy and the isotopic composition of sulfur in hydrothermal sulfide/sulfate deposits on the East Pacific Rise, 21°N latitudeEarth and Planetary Science Letters, 1981
- Oxygen isotope fractionation factors between anhydrite and water from 100 to 550°CEarth and Planetary Science Letters, 1981
- Methane and hydrogen in East Pacific Rise hydrothermal fluidsGeophysical Research Letters, 1979
- Hydrogen and oxygen isotope evidence for sea-water-hydrothermal alteration and ore deposition, Troodos complex, CyprusGeological Society, London, Special Publications, 1977
- Thermodynamics of hydrothermal systems at elevated temperatures and pressuresAmerican Journal of Science, 1969