Igneous petrology through the lens of non-traditional stable isotopes

人们很早就认识到，地质作用会导致元素的同位素比率发生变化。然而，对于C、H、O、N、S以外的元素，在高温下同位素变化被认为是微不足道的。这种看法在大约20年前发生了戏剧性的变化，当时质谱学的进步表明，较重元素(如镁、铁)的同位素实际上在其同位素组成中显示出可分辨的变化。这些同位素变化(或分馏)为化学分析提供了补充，在化学分析提供模棱两可的解释的情况下特别有用。大多数研究涉及平衡同位素分馏，即所研究的熔体和晶体的化学和同位素组成不随时间变化。然而，我的研究计划侧重于动力学同位素分馏。这是当一个系统走向其平衡状态时发生的同位素分馏。与平衡分馏相比，动力学分馏往往较大。我的研究计划将使用记录在单个晶体中的这些独特的动力学特征来确定火山下面的岩浆如何随着时间的推移而演化。晶体内的化学变化记录了岩浆条件(温度、压力和成分)的变化，在某些情况下，可以提供有关导致喷发的事件的时间线的关键信息。使用激光烧蚀多收集器电感耦合等离子体质谱(LA-MC-ICPMS)等原位技术测量晶体内的同位素分布，可以为晶体如何以及在什么时间尺度上获得其化学分布提供基本限制。稳定同位素也可以解释在这些晶体中发现的包裹体的历史。使用稳定的同位素，火山的管道结构-地表以下岩浆运动的几何和时间尺度-可以放心地重建。为了实现这一目标，我的团队将在同位素地球化学和实验岩石学的交叉点上工作，通过在实验室模拟岩浆条件并将我们所学到的应用于自然样品。我的研究计划将有助于从根本上了解活火山的内部运作。

It has long been recognized that geological processes can cause variations in the isotopic ratios of an element. For elements other than C, H, O, N, S, however, isotopic variations were thought to be insignificant at high temperatures. This perception changed dramatically about two decades ago, when advances in mass spectrometry revealed that isotopes of heavier elements (e.g., Mg, Fe) do, in fact, show resolvable variations in their isotopic compositions. These isotopic variations (or fractionations) provide a complement to chemical analyses and are especially useful in instances where chemical analyses provide ambiguous interpretations. Most studies are concerned with equilibrium isotope fractionations, in which the chemical and isotopic composition of the melts and crystals being studied do not change with time. My research program, however, focuses on kinetic isotope fractionation. This is the fractionation of isotopes that occurs as a system moves towards its equilibrium state. Kinetic fractionations are often large compared to equilibrium fractionations. My research program will use these unique kinetic signatures recorded within a single crystal, to determine how magmas beneath volcanoes evolve through time. Chemical variations within a crystal records changes in magmatic conditions (temperature, pressure, and composition) and, in some cases, can provide key information regarding the timeline of events leading up to an eruption. Measuring isotope profiles within a crystal using in-situ techniques, such as laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS), can provide fundamental constraints on how, and on what timescale, a crystal obtained its chemical profile. Stable isotopes can also illuminate the history of inclusions found in these crystals. Using stable isotopes, the plumbing structure of a volcano - the geometry and timescales of magma movement below the surface - can be reconstructed with confidence. To achieve this goal, my group will work at the intersection of isotope geochemistry and experimental petrology, by simulating magmatic conditions in the laboratory and applying what we learn to natural samples. My research program will contribute to a fundamental understanding of the inner workings of active volcanoes.