Metasomatism: Marble Hosted Talc Deposits

David Mogk, Department of Earth Sciences, Montana State University

Topic: Metamorphic petrology, metamorphic processes, mass balance, metasomatism
Course type: Upper level undergraduate course or graduate level course

Description

Talc formation in southwest Montana occurs as a metasomatic replacement of dolomitic marbles. We have demonstrated that this occurs at temperatures <300 C and at pressures < 2 Kbar in what is probably a constant volume process. The formation of talc requires a huge amount of mass transfer, and a time integrated water rock (w/r) ration of >800! This is also associated with net transport of Si and Mg into the system, and Ca out of the system.

In this modeling exercise, we will demonstrate the mass fluxes required to metasomatise 1 meter of marble into a pure talc rock. But, we will construct the model so that different assumptions may be tested: 1) varying the initial bulk composition, 2) varying the physical conditions, and thus the solubilities of mobile components 3) conserving different parameters such as volume, or fix a specific component (e.g. Mg-constant) to show how the chemical and physical properties of the system must change under these different conditions.

Learning Goals or Outcomes

1. Comparison with talc deposits as observed in the field, hand sample and thin section with model outputs;
2. Reinforcement of principles of metamorphic petrology and geochemistry: phase equilibria, mass balance
3. Demonstrating to students the power of quantitative modeling approaches.

How would you assess whether those goals have been met?

Successful completion of modeling activities in which a) students have formulated hypotheses that are tested with the models, and b) written interpretion of the results.

References

Anderson, D.L., Mogk, D.W., and Childs, J.F. (1990) Petrogenesis and timing of talc formation in the Ruby Range, southwestern Montana. Economic Geology, 85, 585-600.