Investigation of Metamorphic Timescales

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The theory of Plate Tectonics provides an explanation for the formation of mountain chains as the crumpling of the Earth's crust where two plates collide. Based on the apparent longevity of mountain belts, the slow progress of plate motions, and calculations of the time scales required to heat the crust to the temperatures recorded by metamorphic rocks in mountain belts, it has generally been assumed that mountain building processes take many tens to hundreds of millions of years. Preliminary studies of the principal investigator, however, have revealed that the entire cycle of heating and cooling for at least some metamorphic rocks occurred over no more than a few million years and perhaps in as little as a few hundred thousand years. These time scales are one to two orders of magnitude faster than previously believed and are on the same order as human evolutionary timescales (a few million years at most). That is not to imply that mountains appear out of flat lands in a few generations of humans, but it does present the radically new proposition that mountain belt evolution may have affected human migratory pathways and associated climate conditions.This proposal seeks to test this new hypothesis by conducting a comprehensive study of the timescales of metamorphic cycling in the classic metamorphic terrane of central New England, USA. The study will involve extensive sample collection along strike from northern Vermont to southern Connecticut, analysis of the samples by standard petrographic techniques (optical microscopy, microstructural analysis, elemental mapping using the electron microprobe, cathodoluminescence imaging, trace element analysis, reaction history analysis, thermobarometry, and pressure-temperature path calculation. The novel aspect of this research is to apply newly developed transport models to samples that display clear evidence for diffusional modification of chemical zoning. The theoretical justification for this approach is the fact that diffusion is thermally activated, so most diffusion occurs within 25-50 degrees of the maximum temperature experienced by a sample. Indeed, the largest uncertainty stems from uncertainty in this maximum temperature and new thermodynamic models are being developed to better constrain this value. The initial application of this new method by the PI revealed the short time scale described above and provided the impetus for this proposal. Cursory examination of samples already in the PIs collection indicate that samples appropriate for this type of analysis are readily available, and several extensions of the method to other minerals and diffusion couples will be explored. The goal of this specific proposal is to constrain the temperature-time history of the entire metamorphic belt of central New England. The insights gained from this study will provide justification for its application in other terranes which will ultimately provide greater understanding and appreciation for the true dynamic nature of the Earth.