Collaborative Research: Tracking Novel Metal Isotope Signatures during Subduction Metamorphism

俯冲作用是控制地球化学演化的关键地质过程。通过这一过程，带有与水圈和大气低温相互作用的化学特征的沉积物以及因与海水相互作用而改变的海洋结壳被输送到地球深处。随着深度和压力的增加，俯冲板块矿物学的变质变化导致流体的释放，在某些情况下，熔融，其中一些元素比其他元素更丰富。这些物质可能会通过弧形火山作用返回地球表面，留下一块化学变化的板条，继续深入地幔。这项研究的主要目的是利用钼(Mo)和铊(Tl)同位素示踪系统更好地了解俯冲变质过程中板块与上覆地幔之间发生的化学交换。Mo和Tl的同位素在海洋中经历了强烈的分馏，导致沉积物和蚀变洋壳中的不同同位素特征，最终转移到俯冲带。最近对火山弧岩中Mo和Tl同位素体系的研究表明，Tl同位素与俯冲带的输入有很好的匹配，但大多数弧体的Mo同位素组成与俯冲带的输入之间存在很大的失配。这项工作将集中在Mo和Tl同位素系统发散行为的起源上，并提供对这些元素的地球深部循环的新理解。这项工作使用了两套特征良好的高压-低温(HPLT)变质岩，以模拟俯冲过程中沉积岩中发生的化学过程和矿物学变化。来自西部阿尔卑斯山的片岩和相关岩石表现出一种变质梯度，最终达到接近3 Gpa的压力，但之前的研究表明，只有有限的可移动元素损失。来自加利福尼亚的Catalina Sist代表了较温暖的俯冲变质条件，并显示了相当大的活动元素损失。将测定两个样品组的块状岩石Mo和Tl同位素组成，以确定这些同位素体系在HPLT变质过程中是否经历了分馏。通过LA-ICPMS的现场分析将确定哪些矿物含有这些元素，以及它们在渐变变质过程中如何重新分布。对提纯矿物分离的同位素分析将确定矿物相之间是否发生同位素分馏。这一裁决反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Subduction is a key geologic process governing the Earth’s chemical evolution. Through this process, sediments bearing the chemical signatures of low-temperature interaction with the hydrosphere and atmosphere are delivered, along with ocean crust altered by interaction with seawater, to the Earth’s deep interior. With increasing depth and pressure, metamorphic changes to the subducting slab’s mineralogy lead to the release of fluids and, in some cases, melts, enriched in some elements over others. These materials may return to the Earth’s surface via the route of arc volcanism, leaving behind a chemically altered slab that continues to sink deeper into the mantle. The research broadly aims to better understand the chemical exchange that occurs between the slab and overlying mantle during subduction metamorphism using the molybdenum (Mo) and thallium (Tl) isotope tracer systems. Isotopes of Mo and Tl experience strong fractionation in the oceans, leading to distinct isotope signatures in sediments and altered ocean crust that are ultimately transferred to subduction zones. While recent research on the Mo and Tl isotope systems in volcanic arc rocks indicate that Tl isotopes match subduction zone inputs well, there is a large mismatch between the Mo isotope composition of most arcs and subduction zone inputs. The work will focus on the origins of the divergent behaviors of the Mo and Tl isotope systems and provide new understanding of the deep Earth cycling of these elements. The work uses two well-characterized suites of high pressure-low temperature (HPLT) metamorphic rocks as analogs for the chemical processes and mineralogical changes that occur in sedimentary rocks during subduction. The Schistes Lustres and related rocks from the western Alps exhibit a metamorphic gradient that ultimately reaches pressures approaching 3 GPa, but prior work has shown only limited mobile-element loss. The Catalina Schist from California represents warmer subduction metamorphic conditions and shows considerable mobile element loss. The bulk-rock Mo and Tl isotope compositions of the two sample suites will be determined in order to ascertain whether these isotope systems experience fractionation during HPLT metamorphism. In situ analysis via LA-ICP-MS will determine which minerals host these elements and how they are redistributed during progressive metamorphism. Isotope analysis of purified mineral splits will determine whether isotopic fractionation occurs between mineral phases.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.