MSA ROEBLING MEDAL LECTURE: THE THREE PARTNERS OF METAMORPHIC PETROLOGY

In addition to supplying the initial motivation for experimental and theoretical petrology, field observations in metamorphic terranes have generated a remarkable feed-back mechanism with which to test the less-direct, and sometimes ambiguous, conclusions of the laboratory-based approaches. A simple example is the Al₂SiO₅ minerals. “Geo-experimental” phase diagrams, in which equilibrium pressure-temperature relations of the three common polymorphs were constructed entirely from geologic criteria, challenged the validity of the results of experimental petrology in the 1960’s. This audacious geo-experimental approach was upheld by the results of later experiments. Vindication of the quantitative field-based approach in the Al₂SiO₅ system served notice to the metamorphism community that “field-testing” of conclusions from the indoor methods can provide essential evidence with which to critique and solidify the dataset of petrology. The geo-experimental approach has been applied successfully to more complex mineral systems, including those with solid solutions, such as zoisite-clinozoisite and the sodic plagioclase “peristerite” feldspars.

Minerals containing volatile components provide an important but difficult avenue for on-going investigation, because they can yield information about complex metamorphic fluids, about which little definitive data yet exist. The minerals biotite, apatite, cordierite, scapolite, carbonates and Fe-Ti oxides may contain, in their chemical make-up, records of the activities of H₂O, CO₂, SO₂, O₂, F, Cl, and S in mineralizing fluids which have acted to modify the deeper crust and the upper mantle. Experimental and theoretical investigations on these minerals in the presence of multi-component fluids, together with appropriately selected “field-testing”, can serve to interpret such important earth processes as recycling of surface materials through the mantle and the generation of volatile-fluxed magmatism.