Investigation of Metamorphic Timescales

板块构造理论为山脉的形成提供了一种解释，即两个板块碰撞时地壳的褶皱。根据造山带的明显寿命，板块运动的缓慢进展，以及对地壳加热到造山带中变质岩记录的温度所需的时间尺度的计算，人们普遍认为造山过程需要数千万至数亿年。然而，首席研究员的初步研究表明，至少有一些变质岩的整个加热和冷却循环发生在不超过几百万年的时间里，也许只有几十万年。这些时间尺度比以前认为的要快一到两个数量级，并且与人类进化的时间尺度（最多几百万年）处于同一个数量级。这并不意味着山脉出现在几代人的平地上，但它确实提出了一个全新的命题，即山带演化可能影响了人类的迁移路径和相关的气候条件。这项提议旨在通过对美国新英格兰中部经典变质带的变质循环时间尺度进行全面研究来验证这一新假设。这项研究将涉及沿着佛蒙特州北方到康涅狄格州南部的走向广泛收集样品，通过标准岩相学技术（光学显微镜、显微结构分析、使用电子探针的元素绘图、阴极发光成像、微量元素分析、反应历史分析、热压法和压力-温度路径计算）分析样品。这项研究的新颖之处在于将新开发的运输模型应用于显示出化学分区扩散改性的明确证据的样品。这种方法的理论依据是扩散是热激活的，因此大多数扩散发生在样品经历的最高温度的25-50度内。事实上，最大的不确定性源于这一最高温度的不确定性，新的热力学模型正在开发中，以更好地约束这一值。PI对这种新方法的初步应用揭示了上述的短时间尺度，并为这一提议提供了动力。粗略检查已经在PI收集的样品表明，适合这种类型的分析样品是现成的，和其他矿物和扩散对的方法的几个扩展将进行探索。这一具体建议的目标是限制整个新英格兰中部变质带的温度-时间历史。从这项研究中获得的见解将为它在其他领域的应用提供理由，最终将使人们更好地理解和欣赏地球的真正动态性质。

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.