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

合作研究：超越和变质石榴石的形成 - 现场、实验室、地质年代学、实验和建模研究

• 批准号: 2147527
• 负责人: Ethan Baxter
• 金额: $ 10.4万
• 依托单位: Boston College
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2022
• 资助国家: 美国
• 起止时间: 2022-02-01 至 2025-01-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2147527&HistoricalAwards=false
• 关键词: Collaborative; Research; Overstepping; Formation; Metamorphic

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.

变质作用是沉积岩进入地球深处达到高压 (P) 和高温 (T) 条件的过程。在地球深处，粘土、砂岩和石灰岩转化为片岩、石英岩等结晶变质岩。将沉积矿物转化为变质矿物的化学反应也会释放出水（H2O）和二氧化碳（CO2）等流体，这些流体最初被困在沉积矿物中。对地球过程的影响，例如火山的形成、地震的引发以及有价值的矿床的形成。此外，这些流体可能会在过去 50-60 年中到达地球表面，对气候产生最大的影响。科学家们认为，变质作用是一个缓慢的过程，在造山事件期间逐渐释放流体，然而，最近的研究表明，变质过程可能会在数百年的时间尺度内发生相对较快的短暂爆发。这项研究的目的是检验关于快速变质作用的最新替代假设。这项研究可能会将长期的解释，即变质作用发生在很长的时间尺度上，转变为更快速的重结晶、变质矿物生长和释放的新范式。这项工作有可能改变对地球过程的看法，并为属于代表性不足群体的博士后和博士生提供培训。这笔赠款还将有助于 YouTube 频道上的一系列教育宣传视频。 “每块岩石都有一个故事”由不同的共同主持人（例如，儿童、妇女和来自弱势群体的科学家）组成，“每块岩石都有一个故事”将每块岩石中存在的非凡故事带入生活。这些故事由不同的共同主持人讲述，孩子们将自己视为科学家，并受到启发去探索地球的科学。该提案将研究另一种假设——岩石相对较快地转变，从而导致液体相对较快地释放与一个多管齐下、多学科的方法，包括现场工作、实验室分析、地质年代学（矿物形成年代学）、高温压力实验以及矿物石榴石成核和生长的热力学建模（作为变质重结晶实验室和地质年代学的典型例子）。研究将集中于对来自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，成核理论预测了新晶体的数量。在相似条件下形成的核（即核）是超出平衡反应量（称为“亲和力”）的函数。石榴石形成的压力-温度条件将通过使用拉曼光谱和夹杂物气压测定的组合来确定。微量元素测温（例如，石榴石中的石英或石墨；金红石测温中的 Zr），以查看石榴石是否以与平衡等梯度一致的位移成核整体成分或它们是否在相似的 P-T 条件下成核，这表明在石榴石成核的共同点上的超越程度显着不同，使用 Sm-Nd 地质年代学的石榴石形成年龄将限制两种类型样品的石榴石是否在同一时间成核。与平衡等梯度的不同位置一致的不同时间，或作为单个造山尺度成核事件的一部分，将使用高压和高温进行实验。活塞缸装置，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出量，作为岩石中 MnO 含量的函数。提供现场研究的实验验证，并能够细化石榴石中锰组分（锰铝榴石）的热力学模型。远离平衡的组合将为与天然共生体进行比较提供基础，并将完善我们对变质岩如何在纳米尺度上演化的理解。这笔赠款还将为“每块岩石都有”的新剧集提供剧集内容和联合主持人。 “一个故事”是由联合 PI Ethan Baxter 创建的 YouTube 系列节目，旨在吸引和启发小学生了解通过这项研究过程收集或实验创建的岩石，这将是其中一些项目的核心内容。新剧集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 Baxter 以及包括女性和代表性不足的少数族裔在内的其他团队成员，以帮助更多的孩子将自己视为科学家。法定使命，并通过使用基金会的智力优点和更广泛的影响审查标准进行评估，被认为值得支持。