Accessory Minerals and Crustal Processes

副矿物和地壳过程

• 批准号: 0440228
• 负责人: E. Bruce Watson
• 金额: $ 57.72万
• 依托单位: Rensselaer Polytechnic Institute
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-01-01 至 2010-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0440228&HistoricalAwards=false
• 关键词: Accessory; Minerals; Crustal; Processes

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.

影响大陆地壳组成和性质的大多数过程不能直接观察到，因为它们发生在地球太深或太慢。因此，需要其他策略来研究地壳深处；其中之一就是高温高压实验。通过实验室重建深部地壳条件，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度等现象。从这些实验室实验中获得的洞察力反过来可以用来推断自然环境中随着地质时间的推移而发生的过程。该项目涉及结合理论模型实施实验战略，以提供“读取”地质时期地壳演化记录的工具。该项目特别侧重于“辅助”矿物--在地壳中只有中等丰度的矿物，但广泛存在，而且往往集中关键的微量元素和放射性同位素。这些辅助矿物之一的稀有晶体(来自澳大利亚的锆石)是已知的最古老的陆地物质，可以追溯到地球年龄的97%。正在进行的研究涉及三个主要子项目，第一个是校准和开发用于估计从地壳岩石中回收的辅助矿物结晶温度的“地质温度计”。温度历史是地壳样品总地质史的重要组成部分；RPI的实验研究提供了有关锆石、金红石和其他含钛矿物中杂质随温度的系统变化的详细知识(在锆石中，感兴趣的杂质是钛；在金红石中，杂质是锆)。这一结果使研究人员能够简单地通过分析未知来源的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)在当今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并没有太大的不同。第二个子项目研究了辅助矿物晶体中原子的迁移(扩散)速率，重点是通过扩展的晶体缺陷，如亚晶界和位错，快速原子运动的可能后果。高压和高温下的实验与理论模型相结合，以确定在什么情况下，同位素年龄信息可能会受到子体(放射性)同位素异常快速迁移的意外损失的干扰。这项工作的动机是确定在哪些情况下，天然样品的同位素测量的重要性和有用性可能会因晶体结构中的扩散增强缺陷而受到影响。第三个分项目侧重于辅助矿物中惰性气体氦原子的行为。氦主要是由铀和钍的放射性衰变产生的，相对于主要的造岩矿物，它们集中在大多数副矿物中。副矿物的实验室加热会导致氦的脱气，而脱气的模式可以用来约束岩体的地质近代温度历史。这一热历史反过来又提供了有关局部侵蚀速率的信息。这种“氦热计时”方法的基础是对辅助矿物中氦原子迁移率的了解，其中最重要的是磷灰石、锆石和钛矿。在过去的十年里，通过对天然样品的热脱气，人们对氦的扩散特性有了很多了解，但通过对氦原子迁移率的直接实验室研究，可以更明确地回答几个关键问题。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。