Experimental and field studies of the timescale of igneous processes

大陆漂移，山脉生长的时间尺度比人类的寿命长得多。地质学家通过测量矿物中铀等放射性元素的数量变化来研究这些过程。大多数这些元素的长半衰期将它们的使用限制在至少几十万年的时间尺度上。我们经常忘记，有一些地质过程，例如火山的地质过程，发生在几秒钟到几年的时间尺度上。为了了解火山过程，我们需要一些方法来利用熔岩中的矿物来解决这些短暂的时间尺度。在过去的六年里，我和我的学生利用岩浆和矿物橄榄石之间的反应作为时间胶囊。被岩浆捕获的橄榄石与之发生反应，形成了不同成分的外壳。随着岩浆上升，化学元素在原始颗粒和外壳之间交换，形成了它们之间的中间成分区域。这一区域的组成和宽度的变化取决于许多因素，其中最重要的是时间。在与岩浆室相同的温度下对橄榄石进行的实验，导致了描述成分如何随时间变化的数学模型。我们使用这些模型和熔岩中的橄榄石时间胶囊来证明，岩浆可以在不到两个小时的时间内从50公里深的地方上升到地球表面。*不幸的是，并非所有的火山岩都存在橄榄石时间胶囊，所以我们需要为一系列不同的矿物开发类似的方法。我的建议是与八名学生一起开发单斜辉石这种矿物，这是火山岩中常见的一种矿物，作为时间胶囊。单斜辉石在这方面很有吸引力，因为它通常在晶体核心周围有不同组成的区域，排列成树环状的图案。第一步将进行实验，以找出组成和温度如何影响单斜辉石中化学成分的交换速度。我们将把两个不同成分的晶体粘在一起，并将它们加热到岩浆室的温度；高达1200摄氏度，持续至少100天。两个晶体交汇处的成分将在一段类似于人类头发直径的距离内发生变化。我们将使用电子显微镜测量成分的变化，然后建立数学模型，解释成分随温度和时间的变化。*一旦我们开发了我们的模型，我们将把它们应用于古代和现代火山熔岩中的单斜辉石，并建立它们过去是如何工作的图景。当我们将这项技术用于最近喷发的火山，并在喷发前、喷发期间和喷发后进行监测时，我们将能够从我们的时间胶囊中联系矿物和岩浆成分的变化、监测信息以及岩浆室中事件的持续时间。这将使我们能够更好地建立火山如何工作的模型，并对未来的喷发做出更好的预测。

Continents drift and mountains grow on timescales much longer than a human lifespan. Geologists study these processes by measuring changes in the amount of radioactive elements, like uranium, in minerals. The long half-life of most of these elements restricts their use to timescales of at least a few hundred thousand years. We often forget that there are geologic processes, such as those at volcanoes, which occur over timescales of seconds to years. To understand volcanic processes we need ways that we that can use minerals in lava to resolve these short timescales. In the last six years, my students and I have used reactions between magma and the mineral olivine as a time capsule. Olivine picked up by magma reacts with it and creates a rind of different composition. As the magma rises, chemical elements are exchanged between the original grain and the rind making a region of intermediate composition between them. The change in composition and width of this region depend on many factors, the most important of which is time. Experiments with olivine, at temperatures equivalent to those in magma chambers, led to mathematical models that describe how composition changes with time. We have used these models and olivine time capsules in lava to show that magma can rise to the earth's surface from a depth of 50 km in less than two hours.***Unfortunately, olivine time capsules are not present in all volcanic rocks, so we need to develop a similar method for a range of different minerals. My proposal is to work with eight students to develop the mineral clinopyroxene, a common mineral in volcanic rocks, as a time capsule. Clinopyroxene is attractive for this because it often has zones of different composition arranged in a “tree-ring” like pattern around the core of the crystal. The first step will be to do experiments to find out how composition and temperature affect the speed of exchange of the chemical components in clinopyroxene. We will do this by sticking together two crystals of different composition and heating them to the temperature of a magma chamber; up to 1200 Celsius for, at least 100 days. The composition where the two crystals meet will change over a distance similar to the diameter of a human hair. We will use an electron microscope to measure the changes in composition and then make mathematical models that will account for changes in composition with temperature and time.***Once we develop our models, we will apply them to clinopyroxene in lava from ancient and modern volcanoes and build up a picture of how they have worked in the past. When we use this technique at volcanoes that have erupted recently and have been monitored before, during and after an eruption, we will be able to relate changes in mineral and magma composition, monitoring information, and the duration of events in the magma chamber from our time capsules. This will allow us to make better models of how volcanoes work and make better predictions of future eruptions.