Collaborative Research: Revisiting the water-saturated granite solidus

地球最上层的大陆地壳平均由花岗岩物质组成。花岗岩是火成岩，是由熔融物质冷却和随后结晶形成的岩石的一个子集。含水花岗岩融化的温度（或者在岩浆冷却时，最后一滴熔融物质结晶的温度）被称为花岗岩饱和水固体（G-WSS）。G-WSS是所有地质学中最重要的相界之一。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（如蓝宝石、锂、金和铜）的产生，毁灭性和爆炸性火山爆发的爆发，以及我们的星球在千万年中冷却的速度。G-WSS的位置随深度（压力；P）、温度(T)和体成分的变化而变化。G-WSS类似于含水流体的冰点，组分对岩浆冰点的影响类似于各种盐（如NaCl、KCl、CaCl等）的加入对水冰点下降的影响。60多年前的开创性工作仍然是我们理解G-WSS的基础。然而，来自自然系统的大量观测表明，火成岩在75-100摄氏度的温度下结晶，比广泛接受的G-WSS低。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表明，G-WSS比以前的发现低100摄氏度，这将改变长期以来对花岗岩形成过程、大陆地壳形成、陆相体热结构、板块构造、硬岩岩石学无数方面的看法，并影响具有重要经济意义的矿石的勘探。pi将进行一系列以实验室为基础的实验，系统地重新定义G-WSS，然后将观测结果应用于国家自然历史博物馆收藏的天然岩石。除了为博士生和华盛顿特区的高中生提供研究机会之外，pi还将提供一系列教育推广经验，向国家广场的游客传授古老的岩浆系统是如何产生建筑岩石的，这些岩石构成了国家广场许多最著名的纪念碑和建筑。花岗质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向花岗质大块成分演化，许多岩石的第一次熔体基本上是花岗质。花岗岩饱和水固相（G-WSS）是区分变质界和火成岩界的最低温度相界；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。公认的G-WSS在很大程度上是在60年前通过实验和分析技术确定的，这使得G-WSS可能不准确。在自然系统中，对花岗岩和流纹岩组成岩石的各种热气压测量应用通常返回的温度估计比广泛接受的G-WSS低~ 75-100℃。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，将通过执行本提案中概述的工作来解决：(1)G-WSS的P-T位置是什么？(2)沿G-WSS共存的熔体和晶体的成分是什么？pi将执行一个系统的实验和分析程序，以确定G-WSS的P-T位置以及在跨越大陆地壳的条件下相关的成分变化。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实验运行产品的主要元素组成。傅里叶变换红外光谱和拉曼光谱将用于测量熔体中的水浓度。一种统计学上严格的实验方法，称为实验设计，将被用来确定G-WSS在跨越大陆地壳的一系列压力下的成分。对天然花岗质岩石进行地球化学分析和热气压测定，将揭示岩石记录中低温记录的程度。在0.5 ~ 10 kbar范围内对花岗岩组成岩石进行的初步实验结果表明，G-WSS明显低于普遍接受的估计。更准确地了解G-WSS的位置将有助于调和对花岗岩形成和硅质岩浆系统中储存条件的解释，为了解地球和其他陆地上地壳的热结构提供新的机会，并将影响硬岩岩石学，地球物理学和矿物/矿石勘探的无数方面，这些方面将受益于G-WSS的准确描述。该项目还包括为研究生、来自服务不足社区的华盛顿特区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”，以及一个名为“广场上的岩浆”的外展项目，旨在向公众宣传岩浆作用，以及它是如何创造国家广场上使用的建筑石头的。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。

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.