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输送到地球深处的高压（P）和高温（T）条件下的过程。在地球深处，粘土、砂岩和石灰岩转变为结晶变质岩，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最初被困在沉积矿物中的流体，如水（H2O）和二氧化碳（CO2）。这些流体对地球过程有重大影响，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，在数百万年的造山运动中逐渐释放出液体。然而，最近的工作表明，变质过程可能会发生相对迅速的短暂爆发，可能会发生在几十万年或更短的时间尺度上，这项研究的目标是测试最近的替代假说快速变质。这项研究可能会将长期存在的变质作用发生在很长时间尺度上的解释转变为更快速的重结晶，变质矿物生长和液体释放的新范式，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属于代表性不足群体的博士后和博士生的培训。这笔赠款还将有助于YouTube频道"每块岩石都有一个故事"上的一系列教育宣传视频，其中包括各种各样的共同主持人（例如，儿童、妇女和来自代表性不足群体的科学家）。“每块岩石都有一个故事”将每块岩石内部存在的非凡故事带入生活。当这些故事由不同的主持人讲述时，孩子们将自己视为科学家，并受到启发，探索自己的地球科学。本提案将采用多管齐下的多学科方法，包括野外工作，实验室分析，地质年代学，研究这种替代假设-岩石转化相对较快，因此液体释放相对较快（测年矿物形成），高温高压实验，以及作为变质重结晶典型例子的矿物石榴石的成核和生长的热力学模拟。野外、实验室和地质年代学研究的重点是对同一露头的岩石进行比较，这些岩石显示出大量的小石榴石和少量的大石榴石。成核理论预测，在类似条件下形成的新晶体（即核）的数量是平衡反应超越量（称为"亲和力"）的函数。石榴石形成的压力-温度条件将由使用拉曼光谱学的包裹体气压测定法和痕量元素温度测定法（例如，石英或石榴石中的石墨; Zr在金红石测温法），看看是否石榴石成核一致的位移从平衡等值线为主体组成，或者他们是否成核在类似的P-T条件下，这将表明显着不同程度的超越在一个共同的点石榴石成核。石榴石形成的Sm-Nd年代学年龄将限制是否从这两种类型的样品在不同的时间成核的平衡等梯度的不同位置相一致，或作为一个单一的造山规模的成核事件的一部分。将使用活塞-气缸装置进行高压和高温下的实验，以确定（a）每种特定岩石成分的平衡石榴石等梯度的位置，以及（B）作为岩石MnO含量的函数，使石榴石成核所需的等梯度的超越量。这将提供现场研究的实验验证，并使细化的热力学性质的锰组分（spessartine）在石榴石。在远离平衡组合的组合演化的热力学建模将提供一个基础，与自然共生的比较，并将完善我们的理解如何在纳米尺度上的变质岩演化。这笔赠款还将为"每块岩石都有一个故事"的新剧集提供剧集内容和共同主持人，这是一个由共同PI Ethan巴克斯特创作的YouTube系列，旨在吸引和激励小学年龄的儿童了解地球科学。在本研究过程中收集或实验创造的岩石将成为一些新剧集的核心内容。Epperly将通过讲故事来展示科学，旨在吸引和激励我们的年轻学习者。讲故事的人将包括共同PI巴克斯特以及其他团队成员，包括妇女和代表性不足的少数民族，以帮助更多的孩子看到自己作为科学家。这个奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.