Collaborative Research: Revisiting the water-saturated granite solidus

合作研究：重新审视水饱和花岗岩固相线

• 批准号: 2120599
• 负责人: Michael Ackerson
• 金额: $ 5.9万
• 依托单位: Smithsonian Institution
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-09-01 至 2024-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2120599&HistoricalAwards=false
• 关键词: Collaborative; Research; Revisiting; water; saturated

Earth’s uppermost continental crust is composed on average of granitic material. Granites are igneous rocks, a subset of rocks that form by the cooling and subsequent crystallization of molten materials. The temperature at which a water-bearing granite melts (or, upon cooling of a magma, the temperature at which the last drop of molten material crystallizes) is known as the granitic water-saturated solidus (G-WSS). The G-WSS is one of the most important phase boundaries in all of geology. Its location in pressure-temperature space controls the formation of our continents, the generation of economically important gem and ore deposits (e.g., sapphire, lithium, gold, and copper), the eruption of devastating and explosive volcanic eruptions, and how rapidly our planet has cooled over eons. The position of the G-WSS changes with depth (pressure; P), temperature (T) and bulk composition. The G-WSS is analogous to the freezing point of aqueous fluids, and the compositional effect on magmatic freezing points is analogous to changes to the freezing point depression of water caused by addition of various salts (e.g., NaCl, KCl, CaCl, etc.). Pioneering work performed over 60 years ago remains the basis for our understanding of the G-WSS. However, numerous observations from natural systems suggests igneous rocks crystallize at temperatures ~75–100 degrees C lower than the widely accept¬¬ed G-WSS. These observations combined with advances in experimental and analytical techniques provide the motivation and opportunity to re-investigate the location of the G-WSS. The PI’s preliminary work surprisingly demonstrated that the G-WSS is 100 degrees C lower than previous findings, which will transform long-standing views on granite formation processes, continental crust formation, thermal structure in terrestrial bodies, plate tectonics, innumerable aspects in hard-rock petrology and affect explorations of economically important ores. The PIs will conduct a series of laboratory-based experiments to systematically re-define the G-WSS, and then apply observations to the natural rocks contained in the National Museum of Natural History collections. Beyond providing research opportunities to PhD students and Washington DC high school students from under-served communities, the PIs will also produce a series of educational outreach experiences to teach National Mall visitors how ancient magmatic systems generated building stone rocks that compose many of the National Mall’s most famous monuments and buildings.Granitic and rhyolitic rocks are the end-product of continental crust differentiation. Most magmatic systems evolve towards granitic bulk compositions during crystallization, and the first melts of many rocks are broadly granitic. The granitic water-saturated solidus (G-WSS) is the lowest temperature phase boundary fundamentally separating metamorphic and igneous realms; thus, understanding its location in -pressure-temperature-composition space is critical for interpreting the rock record. The accepted G-WSS was largely determined 60 years ago using experimental and analytical techniques that leave open the possibility that the G-WSS may be inaccurate. In natural systems, various thermobarometric applications to granitic and rhyolitic composition rocks commonly return temperature estimates ~75–100 degrees C lower than the widely accept¬¬ed G-WSS. The availability of modern experimental and analytical approaches and the low temperature estimates for mineral crystallization in granitic rocks raise two overarching questions that will be resolved by performing work outlined in this proposal: (1) What is the P–T position of the G-WSS?, and (2) What are the compositions of melts and crystals that coexist along the G-WSS? The PIs will perform a systematic experimental and analytical program to determine the P–T position of the G-WSS and related compositional variations over conditions that span the continental crust. Experiments will be conducted in cold-seal pressure vessels (P5 kbar) and piston-cylinder devices (P5 kbar). The PIs will use electron probe microanalysis to measure major element compositions of experimental run products. Fourier transform infrared and Raman spectroscopy will be used to measure water concentrations in the melt. A statistically rigorous experimental approach, called a design of experiments, will be employed to determine compositions along the G-WSS over a range of pressures spanning the continental crust. Geochemical analyses and thermobarometry of natural granitic rocks will reveal the extent to which low temperatures are recorded in the rock record. Preliminary results from experiments performed from 0.5 to 10 kbar on granitic composition rocks demonstrate that the G-WSS is significantly lower than unanimously accepted estimates. A more accurate understanding of the position of the G-WSS will help to reconcile interpretations of granite formation and storage conditions within silicic magmatic systems, provide new opportunities to understand the thermal structure of the crust on Earth and other terrestrial bodies, and will influence myriad aspects of hard-rock petrology, geophysics, and mineral/ore exploration that will benefit from an accurate description of the G-WSS. This program also includes research opportunities for graduate students, DC-local high school students from underserved communities, development/implementation of Next Generation Science Standards for 5-8 grade students across the country, and an outreach program called “Magmas on the Mall” aimed at educating the broad public on magmatism and how it created the building stones used across the National Mall.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.

地球最上层的大陆地壳平均由花岗岩材料组成，是由熔融材料冷却和随后结晶形成的岩石的一个子集。岩浆（最后一滴熔融物质结晶的温度）被称为花岗岩水饱和固相线（G-WSS），G-WSS 是最重要的固相线之一。它在压力-温度空间中的位置控制着我们大陆的形成、经济上重要的宝石和矿床（例如蓝宝石、锂、金和铜）的生成、毁灭性和爆炸性火山的喷发。火山喷发，以及我们的星球在亿万年里冷却的速度有多快。 G-WSS 的位置随着深度（压力；P）、温度（T）和体积成分的变化而变化。类似于水性流体的冰点，而成分对岩浆冰点的影响类似于添加各种盐（例如 NaCl、KCl、CaCl 等）引起的水冰点降低的变化。 60 多年前的研究仍然是我们理解 G-WSS 的基础，然而，来自自然系统的大量观测表明火成岩的结晶温度比广泛的温度低约 75-100 摄氏度。这些观察结果与实验和分析技术的进步相结合，为重新研究 G-WSS 的位置提供了动力和机会。 PI 的初步工作令人惊讶地证明，G-WSS 的温度为 100 摄氏度。低于以前的发现，这将改变长期以来对花岗岩形成过程、大陆地壳形成、陆地热结构、板块构造、硬岩岩石学的无数方面的看法，并影响对PI 将进行一系列基于实验室的实验，系统地重新定义 G-WSS，然后将观察结果应用于国家自然历史博物馆收藏的天然岩石，并为博士生提供研究机会。 PI 还将制作一系列教育推广体验，向国家广场游客介绍古代岩浆系统如何生成建筑石块，这些岩石构成了国家广场许多最著名的纪念碑和建筑。和流纹岩是大陆地壳分异的最终产物，大多数岩浆系统在结晶过程中演化为花岗岩体成分，许多岩石的第一熔体大致是花岗岩质的水饱和固相线（G-WSS）是最低温度的阶段。边界从根本上区分了变质岩和火成岩区域；因此，了解其在压力-温度-成分空间中的位置对于解释岩石记录至关重要 60。几年前，使用实验和分析技术，G-WSS 可能不准确。在自然系统中，对花岗岩和流纹岩成分岩石的各种温压测量通常返回的温度估计值比广泛接受的温度低约 75-100 摄氏度。现代实验和分析方法的可用性以及花岗岩中矿物结晶的低温估计提出了两个首要问题，这些问题将通过执行本提案中概述的工作来解决：（1）什么是 P-T G-WSS 的位置？（2）​​沿着 G-WSS 共存的熔体和晶体的成分是什么？ PI 将执行系统的实验和分析程序来确定 G-WSS 的 P-T 位置。以及跨大陆地壳条件下的相关成分变化，实验将在冷密封压力容器（P5 kbar）和活塞缸装置（P5 kbar）中进行。PI 将使用电子探针微量分析来测量主要成分。实验运行产品的元素成分将用于测量熔体中的水浓度，将采用称为实验设计的严格实验方法来确定沿 G-WSS 在一定范围内的成分。跨越大陆地壳的压力的地球化学分析和天然花岗岩的热压测量将揭示岩石记录中记录的低温程度，在 0.5 至 10 kbar 的压力下进行的实验。花岗岩成分岩石表明，G-WSS 明显低于一致接受的估计值，更准确地了解 G-WSS 位置将有助于协调对硅质岩浆系统内花岗岩形成和储存条件的解释，为理解提供新的机会。地球和其他陆地物体的地壳热结构，并将影响硬岩岩石学、地球物理学和矿物/矿石勘探的各个方面，这些方面将受益于 G-WSS 该计划的准确描述。还包括为研究生、来自服务欠缺社区的华盛顿本地高中生提供研究机会、为全国 5-8 年级学生制定/实施下一代科学标准，以及旨在教育该奖项反映了 NSF 的法定使命，并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，被认为值得支持。