BEYOND EQUILIBRIUM THERMOBAROMETRY: SHIFTING THE PARADIGM TOWARDS KINETICS (Invited Presentation)

A mainstay of metamorphic petrology is the use of pseudosections and composition isopleths (i.e. garnet) to infer PT conditions and paths. Unfortunately, these methods give the wrong answer. Pseudosection analysis requires knowledge of the effective bulk composition (EBC) and the length scale of local equilibrium, neither of which is known with sufficient accuracy to provide robust results. Furthermore, these methods require the system be close to equilibrium throughout, an assumption that is not supported by recent advances.

Our recent work utilizing quartz-in-garnet (QuiG) barometry combined with chemical zoning in the cores of garnets from central Vermont revels that the EBC during initial garnet growth is depleted in MnO and CaO relative to the whole rock bulk composition. The major source of Al in garnet is chlorite and chlorite contains insufficient Mn/unit of chlorite to supply the needed Mn for garnet. Therefore, Mn must come from the interior of the chlorite. Ca in garnet comes primarily from plagioclase and diffusion of Ca is slow due to the coupling of CaAl=NaSi. Supply of Ca to garnet requires recrystallization of plagioclase. This implies that the EBC is controlled, in part, by both the lattice diffusion of MnO in chlorite (as well as probably FeO and MgO) and the recrystallization kinetics of plagioclase. The ability to infer the EBC for any specific rock from first principles is beyond our present capabilities thus rendering any methods that depend on the EBC to be problematic.

QuiG barometry also provides an estimate of the PT conditions of garnet nucleation that is independent of assumptions of equilibrium. Our data from several terranes reveal considerable overstepping of the garnet isograd reaction (i.e. out of equilibrium). Published studies that present PT paths from garnet zoning are also entirely consistent with garnet nucleating and growing nearly at constant PT, indicating that the published PT paths are spurious. Methods that rely on element partitioning (e.g. exchange thermometry and trace element partitioning) appear relatively robust in the face of considerable overstepping. Net transfer barometers such as GASP, however, provide incorrect results if equilibrium is assumed when applied to overstepped assemblages and new theoretical treatments are required to yield accurate results.