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

变质作用是沉积岩在高压(P)和温度(T)条件下深入地球的过程。在地球深处，粘土、砂岩和石灰岩转化为晶质变质岩，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可以释放出水(H2O)和二氧化碳(CO2)等流体，这些流体最初被困在沉积矿物中。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藏的形成。此外，这些流体可能会到达地球表面，影响气候。在过去的50-60年里，大多数科学家都在这样的范式下工作：变质作用是一个缓慢的过程，在数百万年的造山过程中逐渐释放出流体。然而，最近的研究表明，变质过程可能相对快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。这项研究可能会改变长期以来的解释，即变质作用发生在非常长的时间尺度上，转向更快速的重结晶、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对属于代表性不足群体的博士后和博士后的培训。这笔赠款还将用于YouTube频道“每个岩石都有一个故事”频道上的一系列教育宣传视频，其中包括不同的联合主持人(例如，来自代表性不足群体的儿童、妇女和科学家)。《每个岩石都有一个故事》将存在于每个岩石中的不同寻常的故事赋予了生命。当这些故事由不同的联合主持人讲述时，孩子们将自己视为科学家，并受到启发来探索地球科学本身。这项提议将调查这个替代假设-岩石相对快速的转变和相应的相对快速的流体释放，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(矿物形成年代测定)、高温-压力实验，以及作为变质重结晶的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将重点放在对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，在类似条件下形成的新晶体(即原子核)的数量是平衡反应(称为“亲和力”)越越量的函数。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量和微量元素测温(例如，石英石或石榴石中的石墨石；金红石测温法中的Zr)相结合来确定，以确定石榴石是在与块体成分的平衡等温线一致的位移处成核，还是在类似的P-T条件下成核，这表明在石榴石的共同成核点上存在显著不同的超越程度。利用Sm-ND年代学测定石榴石的形成年龄将制约两种样品的石榴石是在不同的时间成核，与平衡等值线的不同位置一致，还是作为单一造山尺度成核事件的一部分。将使用活塞筒装置在高压和高温下进行实验，以确定(A)每种特定岩石成分的平衡石榴石等值线的位置，以及(B)成核石榴石所需的等值线超出量与岩石中的MnO含量的函数关系。这将为野外研究提供实验验证，并使石榴石中锰组分(石榴石)的热力学性质得以改进。对远离平衡组合的组合演化进行热力学模拟，将为与天然共生岩石进行对比提供基础，并将加深我们对变质岩如何在纳米尺度上演化的理解。这笔赠款还将为YouTube上由少年派·伊桑·巴克斯特创作的连续剧《每个岩石都有一个故事》提供新的剧集内容和联合主持人，以吸引和启发小学生了解地球科学。在这项研究过程中收集到的--或实验创造的--岩石将成为一些新剧集的核心。剧集将通过讲故事的方式展示科学，旨在吸引和激励我们的年轻学习者。故事讲述者将包括共同的Pi Baxter以及其他团队成员，包括妇女和未被充分代表的少数族裔，以帮助更多的儿童将自己视为科学家。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.