Inclusion Barometry and Overstepping Isograd Reactions: How Close is the Approach to Equilibrium During Metamorphism?

本提案中概述的研究目的是检查和量化变质岩在转化过程中接近平衡的程度。对于所有变质作用来说，一个公认的事实是，为了使反应向前进行，必须在一定程度上超越反应边界。也就是说，完全平衡的岩石将不会经历再结晶的驱动力，因此将保持不变。因此，在不同的压力和温度条件下，地壳中存在不同程度的变质再结晶，这就证明了转变确实发生了，因此需要某种程度的超越反应。然而，人们也普遍认为，不平衡（超越）的程度很小，因此在整个岩石中保持接近平衡的状态。年代的历史。这是用平衡相图解释变质条件的传统理由。第二部分。这一建议旨在评估在变质作用期间接近平衡的假设的有效性。提案中提出的初步数据表明，常见变质矿物石榴石的成核与平衡有很大的偏差。从这些初步研究中推断出的超温量在不同的样品中有所不同，从几摄氏度到70多摄氏度和5000巴的压力。更重要的是，推断出的石榴石成核条件与样品达到的最高变质温度和压力的推断条件非常相似。这个结果的含义是，岩石实际上保持不变，直到达到接近最大压力和温度的条件，然后通过一连串的反应转变，而不是沿着地壳压力和温度增加的轨迹经历连续的小变化。这种区别不仅仅是学术上的。一个多世纪以来，变质作用期间接近平衡的假设一直是变质研究的标志，我们对造山事件期间岩石演化的几乎所有理解以及推断出的地壳压力和温度条件都是基于这一假设。本提案中提出的初步结果表明，所有这些关于变质作用条件的先前推论都需要根据这些新发现进行修订。此外，在显著超越后发生的一连串变质转变有可能向地壳释放大量流体（例如H2O），这有可能引发大量变形。甚至有人提出，作为俯冲带特征的深震级大地震可能是由这种机制引发的。这项研究是由一项新技术实现的，通过拉曼光谱测量石榴石中石英、绿帘石或锆石包裹体的残余压力，可以确定石榴石等矿物形成的压力（可能还有温度）。这种新方法所要求的假设是石榴石和包裹体之间达到了机械平衡。将这种新方法的结果与假设化学平衡（传统方法）得到的结果进行比较，可以定量地衡量超越的程度。该研究将通过一系列实验来评估这项新技术的有效性，将该方法应用于研究充分的山带（例如新英格兰中部）中特征明确的样品，并开发新的矿物包裹体-宿主对，将该方法扩展到其他岩石类型。

The aim of the research outlined in this proposal is to examine and quantify the extent to which metamorphic rocks approach equilibrium during transformation. It is an accepted fact of all metamorphic transformations that some degree of overstepping of a reaction boundary is required in order for reactions to proceed in a forward direction. That is, rocks that are perfectly equilibrated will experience no driving force for recrystallization and will thus remain unchanged. Hence the mere existence of different degrees of metamorphic recrystallization under different conditions of pressure and temperature in the crust is evidence that transformations have, indeed, taken place and thus some degree of overstepping of reactions is required. However, it is also generally accepted that the degree of disequilibrium (overstepping) is minor such that a close approach to equilibrium is maintained throughout a rock?s history. This is the traditional justification for employing equilibrium phase diagrams for the interpretation of metamorphic conditions.Part II. This proposal seeks to evaluate the validity of the assumption of a close approach to equilibrium during metamorphism. Preliminary data is presented in the proposal that indicates a substantial deviation from equilibrium for the nucleation of the common metamorphic mineral, garnet. The amount of overstepping inferred from these preliminary studies varies in different samples from a few degrees Celsius to over seventy degrees and 5000 bars of pressure. Even more significant, the inferred conditions of garnet nucleation are very similar to the conditions inferred for the maximum metamorphic temperature and pressure attained by the sample. The implication of this result is that, rather than undergoing a continuum of small changes along a trajectory of increasing pressure and temperature in the crust, the rock remained virtually unchanged until reaching conditions near the maximum pressure and temperature and then transformed through a cascade of reactions. This distinction is not merely academic. The assumption of a close approach to equilibrium during metamorphism has been the hallmark of metamorphic studies for over a century and nearly all of our understanding of the evolution of rocks during mountain building events and the inferred pressure and temperature conditions in the crust are based on this assumption. The preliminary results presented in this proposal imply that all of these previous inferences about conditions of metamorphism will need to be revised in light of these new findings. Furthermore, a cascade of metamorphic transformations occurring after significant overstepping has the potential to release copious amounts of fluids (e.g. H2O) into the crust, which has the potential for triggering substantial deformation. It is even suggested that deep, large magnitude earthquakes that are characteristic of subduction zones might be triggered by such a mechanism. This study is made possible by a new technique by which the pressure (and possibly temperature) at which a mineral such as garnet forms can be determined by measuring the residual pressure of an inclusion of quartz, epidote, or zircon in the garnet by Raman spectroscopy. The assumption required by this new approach is that mechanical equilibrium is attained between the garnet and the inclusion. Comparison of the results from this new method with those that are obtained by assuming chemical equilibrium (the traditional approach) provides a quantitative measure of the degree of overstepping. The proposed study will evaluate the validity of this new technique through a series of experiments, apply the approach to well-characterized samples from well-studied mountain belts (e.g. in central New England), and develop new mineral inclusion-host pairs to extend the method to additional rock types.