Jeff and all,
Introduction of oxygenated and carbonated ground water can easily
transform basalt to your assemblage. It is difficult to identify a single
reaction when dealing with complex solid solutions, and they are probably
irreversible anyway (metastable basalt minerals transforming to hydrous
silicates and carbonate).
Simple equilibria that illustrate the types of reactions involved are:
diopside + H2O + CO2 = tremolite + calcite + quartz
anorthite + calcite + H2O = clinozoisite + CO2
anorthite + calcite + magnetite + O2 = epidote + CO2
Eric
>Friends,
>
> I have spent a fair amount of time trying to find a solution to one
>of our research problems. There are metamorphosed
>Paleoproterozoic sills (basaltic) associated with turbidites, along
>an assumed rifted margin in what is now the Black Hills region of
>South Dakota. Many of the sills contain the assemblage:
>actinolite+quartz+/-chlorite+/-epidote+/-clinozoisite+variable
>amounts of calcite. Note that plagioclase is quite rare and that the
>amount of calcite is greater near contacts with the
>metasedimentary country rocks. Quartzites and phyllites
>constitute the vast majority of metasedimentary material.
>Carbonate rocks are almost nonexistent. The phyllites contain
>white micas, chlorite and only rarely, biotite porphyroblasts.
>
> Our problem is two fold. A) What reaction removed calcic
>plagioclase (it is quite common in some other sills in the region)?
>and B) Where did all the calcite come from? We have done a
>moderately good job of searching literature on lower-grade
>metamorphism of metabasites. I would like to account for the two
>issues with one reaction or set of reactions, but nothing
>satisfactory has yet been found. Unfortunately, we do not have
>mineral chemistry on the assembled phases. Any insights on the
>situation will be greatly appreciated. Thanks for your help.
>Jeff Greenberg
>
>Jeffrey K. Greenberg, Professor and Chair
>Dept. of Geology and Environmental Science
>Wheaton College, Wheaton, IL 60187
>(630)752-5866
> Quote 1, "I never would have seen
> it, if I hadn't believed
> it."
> Quote 2, "Nothing is impossible for
> the man who doesn't have
> to do it himself."
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