POLY-MINERAL CRYSTAL SIZE DISTRIBUTIONS: IMPORTANT CLUES TO METAMORPHIC RATES AND PROCESSES ALONG SEGMENTS OF PRESSURE-TEMPERATURE-TIME PATHS

Metamorphic rocks have crystal size distributions (CSD) that provide insight into intensive parameters (P, T, X, dT/dt, dP/dt, dX/dt) that were important in their formation. For a metapelite that has crossed the garnet, staurolite and Al-silicate isograds, each isograd will produce minerals with a distinctive CSD that is related to the reaction kinetics operative at that particular stage of the rock’s history. Consider two end member cases for the same whole rock composition: (1) an isograd can produce a few big crystals (b) or (2) many small crystals (s). There are eight combinations of these end member cases (b or s) for rocks that have crossed the three isograds: sG, sSt, sAS; sG, bSt, sAS; sG sSt, bAS; sG, bSt, bAS; bG, sSt, sAS; bG, bSt, sAS; bG, sSt, bAS; bG, bSt, bAS. Each of these combinations is the result of different variations of intensive parameters that are imposed by the metamorphic environment.

Parameters that interact with P,T and t to determine CSD in metamorphic rocks are the reaction affinity (A), the effective diffusion coefficient (D), and the distribution of previously formed phases in the rock. A controls the nucleation rate, D governs the size of equilibration volumes that form around minerals to inhibit nucleation, and the distribution of previously formed minerals determines how A is locally buffered by the metastable assemblage for a given D.

Both A and D are sensitive to the tectonic environment that controls dT/dt and dP/dt. Over stepping a reaction changes A, which depends on dT/dt & dP/dt imposed by tectonics, and the dT/dP of the equilibrium reaction. For constant D, larger A produces a higher nucleation rate and smaller crystal sizes. The major control on D depends on the amount of fluid along the grain boundaries: D in dry rocks is several orders of magnitude smaller than D in wet rocks. Thermally dominated regional PT path segments (large dT/dP) in metapelites form big porphyroblasts or segregations because they cross dehydration reactions that wet grain boundaries, greatly increasing D. Conversely, decompression dominated prograde PT path segments (small positive dT/large negative dP) in metapelites can produce many small crystals because of rehydration reactions that dry out grain boundaries, lowering D so that domains of equilibrium have small volumes when the new minerals nucleate and begin to grow.