bcrowell wrote:Half a thread hijack, but I have a question(s) for Teresa, about the Mg concentration remaining constant.
Was the water saturated with respect to dolomite too, or just calcite?
and, although you can't easily test this and it's probably just a guess, was the constant Mg concentration a function of slower precipitation kinetics for dolomite when compared to calcite?
The water came out of the ground undersaturated with respect to aragonite, and only minisculely 0.03 saturated with respect to calcite. Dolomite was saturated to the tune of 0.14.
By the end of the stream run, aragonite saturation had increased to 0.7 (max 0.73), calcite saturation had increased to 0.83 (max 0.88) and dolomite to 2.08 (max 2.12). All the maxes were at the next to last sampling station. I would have expected the saturation indices to continue to decrease, but we ran out of stream (tributary to a river). The max saturations were also where the stream was the steepest and the most tufa was being laid down.
My supposition re Mg followed the findings of Dr. Robert Folk, who has some work to the effect that a higher ratio of Mg to Ca than existed in this stream would have to be present in order for dolomite deposition to occur--hence dolomitization occurring in certain Mg rich ocean basins, and through the infusion of seawater through limestone, which does not typically occur in freshwater basins.
What's interesting is the next valley over has a surface stream with aragonite saturation of 0.39, and calcite saturation of 0.53, but this stream is not depositing tufa.
Posted: Apr 8, 2007 5:57 pm
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