Accessory Minerals and Crustal Processes

大多数影响大陆地壳组成和性质的过程无法直接观测，因为它们发生在地球深处或速度太慢。 因此，需要采取其他战略来研究地壳深部;其中之一是高温高压实验。 在实验室中再现地壳深部条件，可以直接研究矿物生长过程中的杂质吸收和原子在晶体中移动的速率等现象。 反过来，从这些实验室实验中获得的见解可以用来推断地质时期自然环境中发生的过程。 该项目涉及实施实验策略与理论模型相结合，为地质时期地壳演化的“阅读记录”提供工具。该项目具体侧重于“附属”矿物-地壳中丰度不大但分布广泛的矿物，这些矿物往往集中了关键的微量元素和放射性同位素。 其中一种副矿物的稀有晶体（来自澳大利亚的锆石）是已知最古老的陆地物质，可追溯到地球年龄的97%。正在进行的研究包括三个主要的分项目，第一个是校准和开发“地质温度计”，用于估计从地壳岩石中回收的副矿物的结晶温度。 温度历史是地壳样品的整体地质历史的重要组成部分; RPI的实验研究提供了有关杂质掺入锆石、金红石和其他含钛矿物（在锆石中，感兴趣的杂质是钛;在金红石中，它是锆）的系统变化的详细知识。 这些结果使研究人员能够通过分别分析锆石和金红石晶体的钛和锆含量来确定它们的结晶温度。 这项研究已经揭示了世界上最古老的锆石（形成于44亿年前）在现代地质条件下花岗岩形成过程的典型温度下结晶。 这一发现表明，早期地球的地壳过程与今天的地壳过程没有很大的不同。第二个分项目研究了矿物晶体中原子的迁移（扩散）速率，重点是原子通过扩展的晶体缺陷（如亚晶界和位错）快速运动的可能后果。 高压和高温下的实验与理论模型结合使用，以确定同位素年龄信息可能因子（放射性）同位素异常快速原子迁移的意外损失而受到干扰的情况。 这项工作的动机是确定的情况下，天然样品的同位素测量的意义和有用性可能会受到损害的晶体结构中的扩散增强缺陷。第三个子项目的重点是稀有气体氦的原子在辅助矿物的行为。 氦主要是由铀和钍的放射性衰变产生的，它们相对于主要造岩矿物富集在大多数副矿物中。 副矿物的实验室加热导致氦的脱气，脱气模式可以用来约束地质学上的岩石体的温度历史。 反过来，这种热历史提供了关于局部侵蚀速率的信息。 支撑这种“氦热测时”方法的是氦原子在辅助矿物中的迁移速率的知识，最重要的是磷灰石、锆石和钛铁矿。 在过去的十年里，通过对天然样品的热脱气，人们已经了解了很多关于氦扩散特性的知识，但通过对氦原子迁移率的直接实验室研究，可以更明确地回答一些关键问题。 RPI的实验计划包括研究晶体取向和同位素质量（氦-3与氦-4）对氦迁移的影响。

Most of the processes that influence the composition and properties of the continental crust are not accessible to direct observation because they occur too deep in the Earth or too slowly. Alternative strategies are thus needed to investigate the deep crust; one of these is high temperature-pressure experimentation. Laboratory re-creation of deep-crustal conditions allows direct investigation of phenomena such as impurity uptake during mineral growth and the rates at which atoms move around in crystals. Insight gained from these laboratory experiments can, in turn, be used to make deductions about processes occurring in the natural environment over geologic time. This project involves implementation of experimental strategies in combination with theoretical models to provide tools for "reading the record" of crustal evolution over geologic time. The project focuses specifically upon "accessory" minerals -- minerals of only modest abundance in the crust, but which are of widespread occurrence and which tend to concentrate key trace elements and radioactive isotopes. Rare crystals of one of these accessory minerals (zircons from Australia) are the oldest known terrestrial materials, dating back 97% of the age of the Earth.Ongoing research involves three principal sub-projects, the first of which is calibration and development of "geothermometers" for estimating the crystallization temperatures of accessory minerals recovered from crustal rocks. Temperature history is a vital component of the overall geologic history of a crustal sample; experimental studies at RPI are providing detailed knowledge of the systematic variation with temperature of impurity incorporation into zircon, rutile and other titanium-bearing minerals (in zircon, the impurity of interest is Titanium; in rutile, it is Zicronium). The results allow researchers to determine the crystallization temperature of zircon and rutile crystals of unknown origin ("provenance") simply by analyzing them for their titanium and zirconium contents, respectively. This study has already revealed that the world's oldest zircons (formed 4.4 billion years ago) crystallized at temperatures typical of granite-forming processes under present-day geologic conditions. This finding suggests that crustal processes of the very early Earth were not very different from what they are today.The second sub-project addresses the migration (diffusion) rates of atoms in accessory mineral crystals, focusing upon the possible consequences of rapid atomic movements via extended crystal defects such as sub-grain boundaries and dislocations. Experiments at high pressures and temperatures are used in combination with theoretical models to determine the circumstances under which isotopic age information might be perturbed by unexpected loss of daughter (radiogenic) isotopes unusually fast atomic migrations. The motivation of this work is to identify circumstances under which the significance and usefulness of isotopic measurements on natural samples might be compromised by diffusion-enhancing defects in crystal structures.The third sub-project focuses on the behavior of atoms of the noble gas helium in accessory minerals. Helium is produced mainly by radioactive decay of uranium and thorium, which are concentrated in most accessory minerals relative to the major rock-forming minerals. Laboratory heating of accessory minerals causes degassing of helium, and the pattern of degassing can be used to constrain the geologically recent temperature histories of rock bodies. This thermal history provides, in turn, information on local erosion rates. Underpinning this "helium thermochronometry" approach is knowledge of the migration rates of helium atoms in accessory minerals, most importantly apatite, zircon and titanite. Much has been learned about He diffusion properties over the past decade by thermal degassing of natural samples, but a few key questions can be more definitively answered through direct laboratory studies of helium atomic migration rates. The experimental program at RPI includes investigations of the effects of crystal orientation and isotope mass (helium-3 vs. helium-4) on helium migration.