Chapter 17
Modeling Dynamic Hydrothermal Processes by Coupling Sulfur Isotope Distributions with Chemical Mass Transfer: Approach
Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: December 7, 1990 | doi: 10.1021/bk-1990-0416.ch017
David R. Janecky Los Alamos National Laboratory, Isotope Geochemistry Group, University of California, Los Alamos, NM 87545
A computational modeling code (EQPS_S) that couples sulfur isotope distri bution and chemical mass transfer reaction calculations has been developed. Isotopic distribution is calculated using standard fractionation factor equa tions. A post processor approach to EQ6 calculations was chosen so that a variety of isotopic pathways could be examined for each reaction pathway. Two types of major bounding conditions were implemented: (1) equilibrium isotopic exchange between sulfate and sulfide species or exchange only accompanying chemical reduction and oxidation events, and (2) existence or lack of isotopic exchange between solution species and precipitated minerals, parallel to the open and closed chemical system formulations of chemical mass transfer model ing codes. All of the chemical data necessary to calculate isotopic distribution pathways is generated by most mass transfer modeling codes and can be input to the EQPS code. Routines are built in to directly handle EQ6 tabular files. Chemical reaction models of seafloor hydrothermal vent processes and accompanying sulfur isotopic distribution pathways illustrate the capabilities of coupling EQPS_S with EQ6 calculations, including the extent of differences that can exist due to the isotopic bounding condition assumptions described above.
Isotopes have long been known to provide unique information about natural hydrothermal systems. Light-stable isotopes of H, O, S, and C, in particular, have received considerable attention due to their measurable fractionation both between minerals and solution and as a function of temperature during hydrothermal processes, and their ubiquitous occurrence and dominant chemical role in such systems (1). While isotopic systematics have been integrated into mineral phase and solution speciation diagrams (2), general computational chemical mass transfer models have largely ignored this source of information until recently (3-5). For investigating reactions involving high temperature, metal transporting hydrother mal solutions, sulfur isotopic processes have been chosen for systematic examination. The stability of multiple sulfur redox states (predominantly S~ and S ) and a relatively large natural range of isotopic compositions (6 *S 20 per mil) provides an excellent probe into important hydrothermal processes of fluid mixing, wall rock interaction, and deep metasomatic reactions. A computer model which examines isotopic processes quantitatively has been implemented. Application to reactions occurring during hydrothermal venting at mid-ocean ridges are presented here to provide examples and an evaluation of the approach. 2
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0097-6156/90/0416-0226$06.00/0 o 1990 American Chemical Society
Melchior and Bassett; Chemical Modeling of Aqueous Systems II ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
17. JANECKY
Modeling Dynamic Hydrothermal Processes
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Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: December 7, 1990 | doi: 10.1021/bk-1990-0416.ch017
APPROACH Chemical reaction pathways for input into the isotopic model have been computed using the EQ3/6 reaction pathway modeling codes (6). Distribution of sulfur isotopes between aqueous species and minerals are calculated using a new computer code (EQPS.S). Isotopic fraction ation factors ( I ) are used by the code to determine the distribution among components as described below. Thus, this approach does not make or apply any assumptions about the chemical mechanism by which isotopic exchange or transfer occurs. The 'descriptive', rather than 'mechanistic' approach, is due in part to the lack of understanding of such mechanisms and inability of chemical reaction codes to handle kinetics of homogeneous solution reactions. Hydrothermal flow and reactions often occur at rates faster than that of sulfur isotopic equilibration (&). Therefore, a variety of isotopic distribution pathways are possible due to competition between three processes: isotopic fractionation (e.g., equilibrium-disequilibrium of exchange), chemical reactions (e.g., sulfur oxidation-reduction reactions), and physical pro cesses (e.g., mixing between solutions, conductive cooling-heating, and/or wall rock reactions). To evaluate the consequences and importance of possible pathways without knowledge of ex plicit mechanisms and kinetics requires examination of the boundary conditions that determine sulfur isotopic distribution and composition attained by the hydrothermal solution and sulfide minerals, rather than only the absolute pathways followed. However, in exploring the effects of some of the boundary conditions and comparing the results to natural products, the relative importance and range of the possible processes involved and the most critical variables can be elucidated. Chemical reaction constraints of interest are redox equilibrium vs. disequilibrium (es pecially whether sulfate reduction progresses appreciably or not) and open vs. closed system processes. These types of constraints are implemented routinely when running chemical reac tion models using EQ3/6 (9,5). Isotopic reaction constraints of interest include parallels to those for chemical reactions: isotopic equilibria or disequilibria between redox species independent of chemical equilibria, and open vs. closed isotopic exchange reactions between aqueous species and precipitated min erals. The latter isotopic process is more complex than that considered for chemical reactions, because precipitated and/or saturated reactant phases may partially redissolve to equilibrate with solution, while not simultaneously exchanging isotopically with the solution. In addition, during isotopic fractionation and chemical reaction it is of interest to examine the extent to which mineral precipitation is coupled with redox processes during incongruent dissolution of a reactant phase. For example, if chemical reduction occurs at a mineral surface, rather than homogeneously in solution, the resulting reduced sulfur may have a tendency to be precipi tated inhomogeneously, thus affecting the resulting mineral isotopic composition and apparent fractionation factors. EQPS Computation Implementation. The EQPS code is composed of of approximately 3500 lines of FORTRAN statements, includ ing comments and local graphics interface. The code does not involve any machine restrictions. CPU time for each of the examples presented below is approximately 35 seconds on a Micro VAX II when chemical mass transfer pathway data is read from EQ6 tabular files. Total moles of all sulfur-bearing components present in the system are required by EQPS from data on tabular files output by EQ6 or input from the user. The input modules allow expansion to integrate with other codes as necessary. The choice of a post-processor mode of operation for EQPS, rather than integration into the EQ6 code as for the oxygen and hydrogen isotopic calculations of Bowers and Taylor (4), was made primarily because of the complexities of sulfur chemistry discussed above. By using the post-processor approach the computationally intensive EQ6 code is only run once for each chemical reaction path, while the simpler and much faster EQPS code can be run multiple times to examine the effects of various isotopic reaction constraints.
Melchior and Bassett; Chemical Modeling of Aqueous Systems II ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
CHEMICAL MODELING OF AQUEOUS SYSTEMS II
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A n isotopic data input file provides E Q P S with coefficients to calculate a fractionation factor (a) for each solution and mineral component (i), relative to a reference solution specie (chosen to be H2S, consistent with previous convention) using: 6
1000 l n a , = ^
· 10 + |
3
· 10 + C
(1)
with A , B , and C being constants for each specie or mineral (7) and temperature (T) in Kelvin. The fractionation factors (α,·) are then used to calculate the composition (
