Experimental and field studies of the timescale of igneous processes

火成岩过程时间尺度的实验和现场研究

• 批准号: RGPIN-2019-04424
• 负责人: Shaw, Cliff
• 金额: $ 1.82万
• 依托单位: University of New Brunswick
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2020
• 资助国家: 加拿大
• 起止时间: 2020-01-01 至 2021-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=714877
• 关键词: Experimental; field; studies; timescale; igneous

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.

大陆漂移和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放射性元素（如铀）含量的变化来研究这些过程。这些元素中的大多数的长半衰期限制了它们的使用至少几十万年的时间尺度。我们经常忘记，有地质过程，如火山，发生在几秒钟到几年的时间尺度。为了理解火山活动，我们需要一些方法，我们可以利用熔岩中的矿物质来解决这些短时间尺度的问题。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。被岩浆吸收的橄榄石与之发生反应，产生了不同成分的外壳。当岩浆上升时，化学元素在原始谷物和外皮之间交换，在它们之间形成一个中间成分的区域。这个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。在相当于岩浆房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公里的深度上升到地球表面。 不幸的是，并非所有火山岩中都存在橄榄石时间胶囊，因此我们需要为一系列不同的矿物开发类似的方法。我的建议是和八个学生一起开发矿物单斜辉石，一种火山岩中常见的矿物，作为时间胶囊。单斜辉石是有吸引力的，因为它往往有不同的组成区域安排在一个“树环”一样的模式周围的核心晶体。第一步将是做实验，找出成分和温度如何影响单斜辉石中化学成分的交换速度。我们将把两种不同成分的晶体粘在一起，并将它们加热到岩浆房的温度;高达1200摄氏度，至少100天。两个晶体相遇的地方的成分会在类似于人类头发直径的距离上发生变化。我们将使用电子显微镜来测量成分的变化，然后建立数学模型，解释成分随温度和时间的变化。 一旦我们开发出我们的模型，我们将把它们应用于古代和现代火山熔岩中的单斜辉石，并建立一幅它们在过去是如何工作的画面。当我们在最近爆发的火山上使用这种技术，并在爆发之前，期间和之后进行监测时，我们将能够从我们的时间胶囊中将矿物和岩浆成分的变化，监测信息以及岩浆房中事件的持续时间联系起来。这将使我们能够更好地模拟火山如何工作，并对未来的喷发做出更好的预测。