MODELING METAMORPHISM IN THE LABORATORY

The expansion of the goals of metamorphic petrology, from the study of heat and mass transfer in the crust, to the characterization of environments, rates and mechanisms of reaction in a broader range of settings, has been facilitated by a parallel evolution in experimental study. Historically, the only satisfactory method for establishing metamorphic conditions was laboratory experiment, in which model systems were subjected to a known chemical environment. Bowen (1940) helped establish the petrogenetic grid as the framework for linking environmental variables to mineral assemblages. Utilization of petrogenetic grids was limited until the experimental "reversal" provided precise brackets of equilibria involving especially refractory systems, such as Al2SiO5 (Newton, 1966). This spurred an expansion in experimental work, so that mineral stability and activity-composition relations, and fluid equations of state, are now well characterized. The modern petrologist can employ experimentally based, internally consistent data sets to compute petrogenetic grids and mineral compositions for many bulk compositions. To some, this has suggested that the job of the metamorphic petrologist is complete. Instead, it should be seen as a signal that we finally have a firm foundation from which to apply petrologic methods to fundamental problems. As in the past, experimental studies are playing a major role in building on this foundation. For example, recent work integrates minerals datable in situ with phase equilibria to finally realize the goal of quantifying P-T-t paths. Important advances in experimental methods now permit laboratory access to the mysterious environment of high-P fluid-rock interaction. At the other extreme, studies of sub-bar gas-mineral reactions extend the reach of metamorphic methods to meteorite parent bodies. Experimental study of the links between mineral reactions and micro-organism metabolism promise to shed light on the origin and early evolution of life on Earth. Abiotic production of organic compounds in experimental systems is helping to characterize the chemistry of the prebiotic Earth. These new experimental directions exemplify the utility of modeling metamorphic processes in the laboratory, and show how the petrologic framework and methods can be extended to address new problems.