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）之间的合作成果，旨在开发工具和策略来“逆向工程”地球大陆地壳的关键矿物——也就是说，了解这些材料是如何以及何时形成的，以及它们起源以来所经历的条件。举例来说，仅仅通过测量一粒沙子的关键化学元素的含量就能“读懂”它的历史，这样做的好处是：它是在一次造山活动中形成的，还是在一次火山喷发中形成的？它从源头传播了多远？它在形成时是否与潜在的有价值的矿床有关？该项目将包括在一定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加入的元素杂质的数量（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为所有研究人员开发“化学工具”，用于破译地球过去的事件和过程——从造山到矿床的形成。RPI进行了十年的实验研究，重点是评估压力(P)和温度(T)对大陆地壳关键矿物中低丰度元素溶解度的影响。这项业务被称为“微量元素热气压测定法”，因为每种应用都是基于一种单一的、边际相容的元素在普通或其他战略矿物相中的浓度（例如，锆石中的Ti）。这些温度计与基于主元素相平衡的“传统”温度计在基本方面有所不同，它们有一个关键优势：如果系统受到适当的约束，单一矿物中单一杂质的浓度可以作为其结晶T和/或p的指标。迄今为止，这种努力已经产生了基于锆石的Ti含量、石英的Ti含量（“TitaniQ”）、金红石的Zr含量和钛矿的Zr含量的热（气压）仪表。通过对感兴趣的相中所有相关杂质的扩散规律进行实验校准，这些温度计的价值得到了显著提高，因此用户可以评估温度计在特定实际应用中的稳健性。迄今为止的努力已经在地球科学领域产生了重大影响（根据文献引用来判断），但是微量元素温度计的开发和“旧”温度计的改进还远远没有完成。提出的工作旨在为地壳系统提供一个完整的温压计工具箱，包括各种P-T指标的交叉检查。具体而言，TitaniQ校准将扩展到更低的P值以应用于火山岩，并且将更彻底地评估锆石中的ti对P的影响。全新的系统和应用也将被追求，包括基于超高压（UHP）岩石中钛矿的P-T指标的开发，石英中的Al以补充TitaniQ，金红石中的Si和Al，以及钾晶石和蓝晶石中的Ti。所有新系统的平衡研究将由有关元素的扩散测量加以补充。此外，实验产品晶体中流体包裹体的压力-体积-温度特性将与微量元素温度计结合使用，以确认我们在相对低P-T应用下校准的准确性。我们提出的研究涉及实验地球化学技术的具体实施，但我们的研究结果的应用扩展到地球科学的大量领域，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.