Some Calculations Pertaining to an Integrated Chemical and Stable-Isotope Model of the Origin of Mid-Ocean Ridge Hydrothermal Systems

Abstract

Chemical and isotopic changes accompanying seawater-basalt interaction in axial mid-ocean 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 model involves 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 \( {\delta ^{{\rm{18}}}}{{\rm{O}}_{{{\rm{H}}_{\rm{2}}}{\rm{O}}}} \) at 350°C of +2.0 per mil (H. Craig, pers, comm., 1984). This is a realistic model, because in the oceanic crust: (1) the grade of hydrothermal metamorphism increases sharply downward; (2) the water-rock ratio is high (>50) at low temperatures and low (<0.5) at high temperatures; and (3) the model allows for back-reaction of earlier-formed minerals during the course of reaction progress. The results are compared to the major-element chemistry (Von Damm et al., 1985) and isotopic compositions (Craig et al., 1980) of the hydrothermal solutions presently emanating from vents at 21°N on the East Pacific Rise. The calculated solution chemistry correctly predicts complete loss of Mg and SO4 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 350°C calculated \( \delta {{\rm{D}}_{{{\rm{H}}_{\rm{2}}}{\rm{O}}}} \) is +2.6 per mil, in excellent agreement with analytical determinations on 21°N fluids. The calculated chemical, mineralogic, and isotopic changes in the rocks are also in accord with observations on altered basalts dredged from mid-ocean ridges (Humphris and Thompson, 1978; Stakes and O’Neil, 1982), as well as with data from ophiolites (Gregory and Taylor, 1981). The model demands that the major portion of the water-rock interaction occur at temperatures of 300° – 350°C, because interaction at temperatures below approximately 250°C results in negative \( {\delta ^{{\rm{18}}}}{{\rm{O}}_{{{\rm{H}}_{\rm{2}}}{\rm{O}}}} \) shifts, contrary to the observed positive δ18O values of the fluids exiting at mid-ocean ridge vents.