Collaborative Research: Overstepping and the Formation of Metamorphic Garnet - Field, Laboratory, Geochronological, Experimental and Modeling Studies

变质作用是沉积岩被运送到地球深处的高压(P)和高温(T)条件的过程。在地球深处，粘土、砂岩和石灰岩转变成结晶变质岩，如片岩、石英岩和大理石。将沉积矿物转化为变质矿物的化学反应也会释放出原本困在沉积矿物中的水（H2O）和二氧化碳（CO2）等流体。这些流体对地球的过程有重大影响，如火山的形成、地震的触发和有价值的矿床的形成。此外，这些液体可能会到达地球表面，影响气候。在过去的50-60年里，大多数科学家都认为变质作用是一个缓慢的过程，在数百万年的造山活动中逐渐释放流体。然而，最近的研究表明，变质过程可能相对迅速地发生在短暂的爆发中，可能发生在数十万年或更短的时间尺度上。这项研究的目的是检验最近关于快速变质作用的另一种假说。这项研究可能会将长期以来关于变质作用发生在很长时间尺度上的解释转变为更快速的再结晶、变质矿物生长和流体释放的新范式，并有可能改变对地球过程的看法。这项工作支持研究机会和培养属于代表性不足群体的博士后和博士生。这笔拨款还将用于YouTube频道“每块石头都有一个故事”上的一系列教育推广视频，这些视频以各种各样的共同主持人为特色（例如，来自代表性不足群体的儿童、妇女和科学家）。“每块石头都有一个故事”将存在于每块石头内部的非凡故事带入生活。当这些故事由不同的主持人共同讲述时，孩子们将自己视为科学家，并受到启发去探索地球的科学。这一建议将通过多管齐下、多学科的方法来研究另一种假设——岩石相对快速地变形，相应地相应地快速释放流体，包括野外工作、实验室分析、地质年代学（确定矿物形成的年代）、高温高压实验和矿物石榴石的成核和生长的热力学模型，石榴石是变质再结晶的一个典型例子。野外、实验室和地质年代学研究将集中在同一露头岩石的比较上，这些岩石显示出大量的小石榴石和很少的大石榴石。成核理论预测，在相似条件下形成的新晶体（即原子核）的数量是超越平衡反应的量（称为“亲和”）的函数。石榴石形成的压力-温度条件将通过使用拉曼光谱的包裹体气压测定法和微量元素测温法(例如，石英或石榴石中的石墨；Zr（金红石测温）来观察石榴石是否在总体组成的平衡等梯度上以一致的位移成核，或者它们是否在相似的P-T条件下成核，这将表明在石榴石成核的共同点上有明显不同程度的超越。利用Sm-Nd地质年代学测定石榴石的形成年龄将限制两种样品的石榴石在不同时间成核是否与平衡等格拉德的不同位置相一致，还是作为单一造山尺度成核事件的一部分。高压和高温下的实验将使用活塞缸装置进行，以确定(a)每种特定岩石成分的平衡石榴石等梯度的位置和(b)石榴石成核所需的等梯度的超越量作为岩石中MnO含量的函数。这将为实地研究提供实验验证，并使石榴石中锰成分（spessartiine）的热力学性质得以改进。在远离平衡的组合中，组合演化的热力学建模将为与天然共生组合的比较提供基础，并将完善我们对变质岩如何在纳米尺度上演化的理解。这笔资金还将为“每块岩石都有一个故事”的新一期节目提供内容和联合主持人，这是一个由伊森·巴克斯特（Ethan Baxter）共同创建的YouTube系列节目，旨在吸引和激励小学生了解地球科学。在这项研究的过程中收集到的岩石——或者通过实验创造出来的——将成为一些新剧集的核心。剧集将通过讲故事的方式呈现科学，旨在吸引和激励我们的年轻学习者。讲故事的人将包括巴克斯特联合主任以及其他团队成员，包括女性和未被充分代表的少数民族，以帮助更多的孩子将自己视为科学家。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Metamorphism is the process by which sedimentary rocks are transported deep into the Earth to high pressures (P) and temperatures (T) conditions. Deep in the Earth, clay, sandstone, and limestone are transformed into crystalline metamorphic rocks like schists, quartzites, and marbles. Chemical reactions that transform sedimentary minerals into metamorphic minerals can also release fluids such as water (H2O) and carbon dioxide (CO2), originally trapped in the sedimentary minerals. These fluids have a major impact on earth processes such as the formation of volcanoes, the triggering of earthquakes, and the formation of valuable ore deposits. In addition, these fluids may make it to the Earth’s surface where they affect the climate. Over the last 50-60 years most scientists worked within the paradigm that metamorphism is a slow process, releasing fluids gradually over millions of years during mountain-building events. However, more recent work suggests that metamorphic processes may occur relatively rapidly in brief bursts that may occur over timescales of hundreds of thousands of years or less. The goal of this study is to test the more recent alternative hypothesis about rapid metamorphism. This study may shift longstanding interpretations that metamorphism occurs over very long timescales to a new paradigm of more rapid recrystallization, metamorphic mineral growth and release of fluids and has the potential to alter views on Earth processes. This work supports research opportunities and the training of postdocs and PhD students belonging to underrepresented groups. This grant will also contribute to a series of educational outreach videos on the YouTube channel “Every Rock Has A Story” that feature a diverse slate of co-hosts (e.g., kids, women, and scientists from underrepresented groups). “Every Rock Has A Story” brings to life the remarkable stories that exist inside every rock. When these stories are told by a diverse cast of co-hosts, kids see themselves as scientists and are inspired to explore the science of the Earth themselves.This proposal will investigate this alternative hypothesis — that rocks transform relatively rapidly with consequential relatively rapid release of fluids with a multi-pronged, multidisciplinary approach that includes field work, laboratory analysis, geochronology (dating mineral formation), high temperature-pressure experiments, and thermodynamic modeling of the nucleation and growth of the mineral garnet as a type example of metamorphic recrystallization. The field, laboratory and geochronological studies will focus on a comparison of rocks from the same outcrop that display numerous small garnets and few large garnets. Nucleation theory predicts that the number of new crystals (i.e. nuclei) that form under similar conditions is a function of the amount of overstepping of the equilibrium reaction (called the “affinity”). The pressure-temperature conditions of garnet formation will be determined from a combination of inclusion barometry using Raman spectroscopy and trace element thermometry (e.g., quartz or graphite-in-garnet; Zr in rutile thermometry) to see if garnets nucleated at consistent displacements from the equilibrium isograd for the bulk composition or whether they nucleated at similar P–T conditions, which would suggest significantly different degrees of overstepping at a common point of garnet nucleation. Ages of garnet formation using Sm-Nd geochronology will constrain whether garnets from both types of samples nucleated at different times consistent with the different locations of the equilibrium isograd, or as part of a single orogenic-scale nucleation event. Experiments at high pressures and temperatures will be conducted using piston-cylinder apparatus in order to determine (a) the location of the equilibrium garnet isograd for each specific rock composition and (b) the amount of overstepping of the isograd required to nucleate garnet as a function of the MnO content of the rock. This will provide experimental verification of the field studies and enable refinement of the thermodynamic properties of the Mn-component (spessartine) in garnet. Thermodynamic modeling of assemblage evolution in far-from-equilibrium assemblages will provide a basis for comparison with the natural parageneses and will refine our understanding of how metamorphic rocks evolve on a nano scale. The grant will also provide episode content and co-hosts for new episodes of “Every Rock Has A Story”, a YouTube series created by co-PI Ethan Baxter to engage and inspire elementary school age children about the geosciences. Rocks collected — or experimentally created — through the course of this research will be the centerpiece of some of the new episodes. Episodes will present the science through storytelling, designed to engage and inspire our young learners. Storytellers will include co-PI Baxter as well as other team members including women and underrepresented minorities to help more children see themselves as scientists.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.