[OLD] Sulfur Isotopes

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From: T.C. Onscott

Subject: sulfur isotopes

I'm trying to model the sulfur isotopic composition of my "system" and I know the 34S of both the sulfate and the dissolved sulfide. The program doesn't seem to allow me to enter both values, even though in my system we measure the amounts of each.

 

From: Craig Bethke

Subject: Re: sulfur isotopes

That's a good question with only a pretty good answer. As background, React's isotope model assumes that the species in solution are in isotopic equilibrium. This is a pretty good starting point for most isotopes, the most notable exception being sulfate sulfur, which exchanges very slowly due to the strong S-O bonds in the sulfate molecule. React, however, does not require isotopic equilibrium among minerals, and this feature will likely allow you to set up a meaningful model. To set a mineral in isotopic disequilibrium, you “segregate� it (with the “segregate� command, of course). To construct a model, segregate each sulfur-bearing mineral. Then, assign your sulfate measurement to the sulfate mineral(s) and the sulfide value to the sulfide mineral(s). Finally, set your initial fluid 34S to the sulfate value if the fluid is dominated by SO4, or to the sulfide value if it's richer in H2S. This strategy works best, as you might imagine, when the mineral(s) are the dominant reservoir of reduced sulfur reacting with an oxidized fluid, or oxidized sulfur reacting with a reduced fluid. And as always, you should keep the conceptual model in mind as you do your modeling. With care, you should be able to produce useful results.