FEEDBACK BETWEEN FIELD AND EXPERIMENTAL METHODS IN PETROLOGY, TOWARD ADVANCED UNDERSTANDING OF METAMORPHIC AND IGNEOUS PROCESSES

Experimental petrology seeks to answer questions posed by natural mineral assemblages, with the advantage of constraining pressure-temperature (P-T) conditions and initial composition. Early interest in petrologic phenomena may tend naturally toward field observations, as the products of Earth-generated experiments are more widespread and accessible than experimental laboratories. However, conditions of the natural processes must be inferred. As geology became a recognized science, histories deduced from field-petrologic observations indicated relative differences in mode of origin, with Hutton's plutonists prevailing over Werner's neptunists. Field-based advances over the next century remained based primarily on inference, though Hall employed sealed gun barrels for the first experiment in metamorphic petrology, following this up by contrasting the textures of variably cooled silicate melts. Sorby opened up the micro-petrologic realm, from which previously cryptic textural details of interaction between and within phases could be deduced.

The advent of systematic experimental work was advanced greatly by founding of the Carnegie Institution of Washington (CIW), including the Geophysical Laboratory, early last century. Advanced field-based inferences had begun to capitalize on the formalization of physical chemistry, including the Gibbs thermodynamic phase rule. Prior to laboratory experimentation at CIW, Bowen described details of differentiation in diabase sills, and later confirmed by experiment gravity settling of liquidus crystals. Likewise, prior to experimental work, Eskola established the concept of metamorphic facies based on relative differences in P-T conditions, though absolute constraints still were absent. Access to crustal P-T conditions in the laboratory began constraining natural processes, even though geological time scales are inaccessible.

Bowen established the experimental-petrology laboratory at the University of Chicago, in which most seminal experiments of Julian R. Goldsmith and Robert C. Newton grew. These numerous contributions all advanced the interpretation of natural petrogenesis, including fluid-involved processes and field testing in Indian granulites, in the true spirit of the discipline.