Diffusion of Trace Elements in Garnet: Rates, Mechanisms, and Theory

发生在地壳深处或上地幔的变质过程无法直接观察到，它们跨越的时间尺度比人类的寿命长得多，因此对它们的了解必须来自对它们产生的岩石的研究。为了重建地质历史-例如，为了了解一个山脉带形成需要多长时间，它有多古老，它产生或被侵蚀的速度有多快-我们必须能够阅读这些过程的记录，这些过程被编码在矿物中，这些矿物形成于很深的地方，并通过构造过程被带到地表。这项研究的中心目标是更好地了解如何阅读石榴石中的此类记录，石榴石是一种具有非凡能力的矿物，可以在其化学成分中捕获其历史的细节。石榴石在生长过程中，通常会从晶体的核心到边缘产生组成元素浓度的差异，并且这些变化会通过扩散进行修改，即原子在固体晶体结构中的运动。扩散的修改发生在不同程度上，反映了不同的时间长度的晶体在不同的温度和压力在其生长后的历史。将这些扩散修改转化为地质过程的详细信息的关键是不同元素通过石榴石结构扩散的速率和机制的定量知识。以前的工作已经产生了强大的测量扩散速率的元素丰富的石榴石，但更细致的解释地质历史将是可能的，如果可比数据可用于微量元素，那些具有非常低的丰度。通过测量和模拟在高温下部分再吸收的石榴石晶体中的扩散效应，也取得了一些进展。 但是需要在较低温度下获得数据，并且需要比迄今为止研究的更广泛的微量元素。 该项目将通过开发和部署高空间分辨率的化学分析方法来获得这些数据，这些方法将把先前的工作扩展到晶体中，因为晶体在较低温度下被再吸收，因此扩散的影响范围较窄。该研究将调查微量元素被纳入石榴石结构的各种手段，以及不同的纳入方式如何影响扩散速率，无论是单个元素还是可能扩散在一起的成对元素，以保持晶体中电荷的局部平衡。石榴石中微量元素扩散的新数据应增加严格的应用，如高温加热/冷却的历史和时间尺度的高温热和交代事件，钐-钕和Lu-Hf石榴石年代学，了解石榴石和辅助矿物之间的变质平衡，地幔矿物和熔体的稀土模式的解释。

Metamorphic processes that take place deep in the earth's crust or in its upper mantle cannot be observed directly, and they span time scales much longer than human lifetimes, so knowledge of them must come from study of the rocks they produce. To reconstruct geologic histories -- to learn, for example, how long it takes for a mountain belt to form, how ancient it is, how rapidly it arose or was eroded -- we must be able to read the record of those processes that is encoded in minerals that formed at great depth and have been brought to the surface by tectonic processes. The central goal of this research is to learn better how to read such records in garnet, a mineral with a remarkable ability to capture details of its history in its chemical composition. Garnet, during growth, commonly develops differences in the concentrations of constituent elements from the cores of crystals out to their rims, and these variations are modified by diffusion, the movement of atoms through the solid crystal structure. Diffusional modifications occur to varying degrees, reflecting the various lengths of time that the crystals spent at different temperatures and pressures during their post-growth histories. The key to transforming these diffusional modifications into detailed information on geologic processes is quantitative knowledge of the rates and mechanisms of diffusion of different elements through the garnet structure. Prior work has produced robust measurements of diffusion rates for elements that are abundant in garnet, but a much more nuanced interpretation of geologic history would be possible if comparable data were available for trace elements, those with very low abundance. Some progress toward this goal has also been made, by measuring and modeling the effects of diffusion in garnet crystals that have been partially resorbed at very high temperatures. But data are needed at lower temperatures, and for a broader suite of trace elements than have so far been investigated. This project will obtain those data by developing and deploying high-spatial-resolution methods of chemical analysis that will extend prior work to crystals in which the effects of diffusion span a narrower range of distance because they have been resorbed at lower temperatures. The research will investigate various means by which trace elements are incorporated into the garnet structure, and how different means of incorporation may affect diffusion rates, both for individual elements and for pairs of elements that may diffuse together in order to maintain local balance of electrical charges in the crystal. New data on trace-element diffusion in garnet should add rigor to applications as diverse as high-temperature heating/cooling histories and timescales of high-temperature thermal and metasomatic events, Sm-Nd and Lu-Hf garnet geochronology, understanding of metamorphic equilibration between garnet and accessory minerals, and interpretation of rare-earth patterns of mantle minerals and melts.