Collaborative Research: Equilibrium and Kinetic Studies of New Trace Element Thermobarometers

该项目是伦斯勒理工学院(RPI)和锡拉丘兹大学(SU)合作开展的，旨在开发工具和策略，对地球大陆地壳的关键矿物进行“逆向工程”--即了解这些物质是如何以及何时形成的，以及它们自产生以来所经历的条件。例如，想一想，仅仅通过测量一粒沙子中关键化学元素的含量就能“读懂”它的历史的好处：它是在造山过程中形成的，还是在火山喷发期间形成的？它从源头走了多远？它在形成时是否与一个潜在的有价值的矿藏有关？该项目将涉及在一系列温度和压力条件下对选定矿物进行实验室合成，然后测量生长过程中元素杂质的含量(例如，石英中铝和钛的浓度在很大程度上取决于石英形成的温度和压力)。该项目更广泛的目的是开发“化学工具”，供所有研究人员用来破译地球过去的事件和过程--从造山到矿藏形成。在RPI进行了十年的实验研究，重点是评估压力(P)和温度(T)对陆壳关键矿物中低丰度元素溶解度的影响。这项技术被称为“痕量元素温压计”，因为每一种应用都是基于一种常见或战略性矿物相(例如，锆石中的钛)中单一的、边缘相容的元素的浓度。这些温压计在根本上不同于基于主要元素相平衡的“传统”温压计，它们有一个关键的优势：如果系统受到适当的限制，单一矿物中单一杂质的浓度可以作为其结晶T和/或P的指示器。到目前为止，这项工作已经产生了基于锆石的钛含量、石英的钛含量(“钛iQ”)、金红石的锆含量和钛铁矿的锆含量的温度(巴罗)计。通过对感兴趣阶段中所有相关杂质的扩散定律进行实验校准，这些温度气压计的价值已显著提高，因此用户可以评估针对特定现实应用的温度气压计的稳定性。迄今为止的努力在地球科学界产生了重大影响(从文献引文判断)，但微量元素温度计的发展和对“旧”温度计的改进还远未完成。拟议的工作旨在为地壳系统提供一个完整的温度气压计工具箱，其中包括对各种P-T指标的交叉检查。具体地说，TianiQ校准将扩展到更低的P，以应用于火山岩，并且将更彻底地评估钛锆石对P的影响。还将寻求全新的系统和应用，包括开发超高压岩石柯石英中的钛、石英中的铝以补充钛的iQ、金红石中的硅和铝以及钾石和蓝晶石中的钛的P-T指示剂。所有新体系的平衡研究将得到有关元素的扩散测量的补充。此外，来自实验运行产品的晶体中流体包裹体的压力-体积-温度特性将与痕量元素温压计一起使用，以确认我们在相对较低的P-T应用中校准的准确性。这项拟议的研究涉及实施专门用于实验地球化学的技术，但我们的成果的应用范围广泛，不仅包括火成岩和变质岩石学，还包括矿床研究、构造地质学、构造学和沉积学。

This project is a collaborative effort between Rensselaer Polytechnic Institute (RPI) and Syracuse University (SU) to develop tools and strategies to "reverse engineer" key minerals of Earth's continental crust - that is, to learn how and when these materials formed and the conditions experienced since their origin. Consider, for example, the benefits of being able to "read" the history of a single grain of sand simply by measuring its content of key chemical elements: Was it formed during an episode of mountain building or during a volcanic eruption? How far did it travel from its source? Was it associated with a potentially valuable ore deposit at the time of formation? This project will involve laboratory synthesis of selected minerals over a range of temperature and pressure conditions, followed by measurement of the amounts of elemental impurities incorporated during growth (the concentrations of aluminum and titanium in quartz, for example, depend strongly on the temperature and pressure of quartz formation). The broader purpose of the project is to develop "chemical tools" for all researchers to use in deciphering events and processes of our planet's past - from mountain building to formation of ore deposits.A decade of experimental research at RPI has focused on evaluating the effects of pressure (P) and temperature (T) on the solubilities of low-abundance elements in key minerals of the continental crust. This enterprise has been called "trace-element thermobarometry" because each application is based on the concentration of a single, marginally compatible element in a common or otherwise strategic mineral phase (e.g., Ti in zircon). These thermobarometers differ in fundamental ways from "conventional" thermobarometers based on major-element phase equilibria, and they have a key advantage: if the system is properly constrained, the concentration of a single impurity in a single mineral can be used as an indicator of its crystallization T and/or P. To date, this effort has produced thermo(baro)meters based on the Ti content of zircon, the Ti content of quartz ("TitaniQ"), the Zr content of rutile, and the Zr content of titanite. The value of these thermobarometers has been enhanced significantly through experimental calibration of the diffusion laws for all relevant impurities in the phases of interest, so users can assess the robustness of the thermobarometers for specific real-world applications. Efforts to date have had significant impact in the geoscience community (as judged by literature citations), but the development of trace-element thermobarometers and the improvement of "old" ones is far from complete. The proposed work is aimed at providing a full toolbox of thermobarometers for crustal systems that includes cross-checks of the various P-T indicators. Specifically, the TitaniQ calibration will be extended to lower P for application to volcanic rocks, and Ti-in-zircon will be more thoroughly assessed for P effects. Entirely new systems and applications will also be pursued, including the development of P-T indicators based on Ti in coesite for ultra high-pressure (UHP) rocks, Al in quartz to complement TitaniQ, Si and Al in rutile, and Ti in both K-spar and in kyanite. Equilibrium studies of all new systems will be complemented by diffusion measurements of the relevant elements. Further, the pressure-volume-temperature properties of fluid inclusions in crystals from experimental run products will be used in conjunction with trace-element thermobarometers to confirm accuracy of our calibrations at relatively low P-T applications. The proposed study involves implementation of techniques specifically in experimental geochemistry, but the applications of our results extend across a substantial expanse of geoscience, including not only igneous and metamorphic petrology but also ore-deposits research, structural geology, tectonics, and sedimentology.