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

发生在地壳深处或上地幔中的变质过程无法直接观察到，而且它们跨越的时间尺度比人类的寿命长得多，因此对它们的知识必须来自对它们产生的岩石的研究。要重建地质历史--例如，要了解一条山脉带的形成需要多长时间，它有多古老，它形成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，这些过程编码在深层次形成的矿物中，并被构造过程带到地表。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，石榴石是一种矿物，具有从化学成分中捕捉其历史细节的非凡能力。在生长过程中，石榴石通常会在从晶体核心到边缘的组成元素浓度上产生差异，这些差异会被扩散和原子在固体晶体结构中的运动所改变。扩散修正在不同程度上发生，反映了晶体在其生长后历史中在不同温度和压力下花费的不同时间长度。将这些扩散变质作用转化为有关地质过程的详细信息的关键是定量了解不同元素在石榴石结构中的扩散速率和机制。以前的工作对石榴石中丰富的元素的扩散速度进行了强有力的测量，但如果有非常低丰度的微量元素的可比数据，就有可能对地质历史进行更微妙的解释。通过测量和模拟在高温下部分吸收的石榴石晶体中的扩散效应，朝着这一目标也取得了一些进展。但在更低的温度下需要数据，而且需要比目前研究的更广泛的痕量元素。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，这些方法将把以前的工作扩展到扩散效应跨越较窄距离范围的晶体，因为它们在较低的温度下被吸收。这项研究将调查微量元素被掺入石榴石结构的各种方法，以及不同的掺入方法如何影响扩散速度，无论是单个元素还是可能一起扩散的元素对，以保持晶体中电荷的局部平衡。有关石榴石中痕量元素扩散的新数据应该会增加各种应用的严谨性，如高温热事件和交代事件的高温加热/冷却历史和时间尺度，Sm-ND和Lu-Hf石榴石年代学，对石榴石和副矿物之间变质平衡的理解，以及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.