Collaborative Research: Testing Source vs. Crustal Processing in High-Mg# Arc Magmas by Os-O-He-Olivine Systematics

“会聚板块边缘”是固体地球地球化学循环的主要界面，是形成和维持宜居地球的基础。在收敛的板块边缘，一个由地壳和最上层地幔组成的岩石圈板块被拉入或俯冲到另一个岩石圈板块之下。在大约80-140公里的深度，俯冲板块释放出富含气候活性挥发性元素（H2O，卤素）的物质，并通过排成弧形的火山活动将其“循环”回地球表面。这样的弧形成了“环太平洋火山带”，以富含挥发物、毁灭性的硅岩浆喷发而闻名，这些火山喷发可能导致致命的气候扰动（如坦博拉火山1815年）和海啸（如喀拉喀托火山1883年、2018年），对人口稠密的地球上的人类居住构成永久性的重大威胁。因此，了解物质循环如何导致硅，爆炸性弧火山活动仍然是地球科学研究的核心主题。在研究俯冲再循环时，一个特别的挑战是它发生在地球内部深处（在几公里到200公里之间），因此无法直接观察到。因此，科学家们依靠从岩浆成分（如橄榄石和尖晶石晶体）中获得的间接观测，这些观测携带着地球内部许多公里深处发生的过程的信息。两名美国妇女（其中一名残疾妇女）计划与来自美国、墨西哥、苏格兰和法国的国内和国际同事合作和交流，从墨西哥年轻火山喷发的火山岩中收集这类信息。该项目将以促进多样性为目标培养研究生和本科生。为了将新研究成果应用到初高中地球科学课程中，将动员K-12教师，并在每年定期举行的开放日（Open day）上向普通大众进行宣传。该项目主要研究跨墨西哥火山带（TMVB）的镁铁质至硅质岩浆中的橄榄石及其尖晶石包裹体，该火山带是一个主要的俯冲带，以其灾难性火山爆发的高潜力而闻名。许多研究认为，在构造TMVB的45公里厚大陆地壳基底中，硅质岩浆通过熔融作用获得了致命的爆发力。然而，一些研究表明，这些岩浆中含有的橄榄石晶体的组成与这一假设不一致。值得注意的是，来自TMVB中部的橄榄石具有δ 18o和3He/4He同位素信号，这表明它们的寄主岩浆主要形成于地壳基底以下的地幔，在那里它们通过大量俯冲的板块物质结合而变成硅质。在本项目中，我们将通过从TMVB中部（Popocatepetl和Sierra Chichinautzin）和东部（Serdan盆地）以及岩浆不受俯冲影响的弧后区域的关键位置精心挑选的橄榄石和块岩中获得额外的187Os/188Os同位素数据，进一步验证这两种截然不同的假设。将Os-He-O同位素比值与橄榄石和含铬尖晶石的组成数据相结合，推断橄榄石结晶的温度和氧逸度，从而建立硅熔体形成模式的整体模型。总的来说，该项目将为板块俯冲和弧火山作用之间的连通性及其在调节地球长期演化和维持宜居地球中的作用提供新的限制。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

'Convergent plate margins' are major interfaces of the solid Earth geochemical cycle that are fundamental to the formation and maintenance of habitable Earth. At convergent plate margins, a lithospheric plate consisting of the Earth's crust and uppermost mantle is drawn - or subducted - beneath another lithospheric plate. At around 80-140 kilometers depth, the subducted plate releases material rich in climate-active volatile elements (H2O, halogens) and 'recycles' it back to the Earth's surface via the activity of volcanoes aligned in arcs. Such arcs form the 'Pacific Ring of Fire', which is renowned for volatile-rich, devastating eruptions of silicic magmas that can cause deadly climate perturbations (e.g. Tambora 1815) and tsunamis (e.g. Krakatoa 1883, 2018), and that are a permanent major threat to human habitation on the densely populated Earth. Thus, understanding how material cycling leads to silicic, explosive arc volcanism remains a theme at the core of geoscience research. A particular challenge when studying subduction recycling is that it takes place deep in the Earth's interior (between a few and 200 km) and thus cannot be observed directly. Scientists therefore rely on indirect observations obtained from components in magmas (e.g. olivine and spinel crystals) that carry information on processes occurring at many kilometers depth within the Earth. Two US-based women (one with a disability) plan to collect such information from volcanic rocks erupted from young Mexican volcanoes, in collaboration and exchange with national and international colleagues from the US, Mexico, Scotland and France. The project will train graduate and undergraduate students with the goal to promote diversity. K-12 teachers will be engaged in order to transfer the new research results into the Earth Science curriculum for middle and high school students, and the general public will be informed at regular annual Open Days.This project focuses on olivine and its inclusions of Cr-spinel from mafic to silicic magmas in the Trans-Mexican Volcanic Belt (TMVB), a major subduction zone that is well-known for its high potential for catastrophic volcanic eruptions. Many studies propose that the silicic magmas gain their deadly explosive power through melt processing in the 45 km thick continental crustal basement on the which the TMVB is constructed. However, some studies suggest that the composition of olivine crystals contained in these magmas is inconsistent with this hypothesis. Remarkably, olivines from the central TMVB have delta18O and 3He/4He isotope signals that suggest that their host magmas were primarily formed in the mantle below the crustal basement, where they became silicic through incorporation a large amount of subducted slab material. In this project we will further test these two contrasting hypotheses by obtaining additional 187Os/188Os isotope data from olivine and bulk rocks that have been carefully selected from key locations in the central (Popocatepetl and Sierra Chichinautzin) and eastern (Serdan basin) TMVB, as well as from the rear-arc region where magmas are not significantly affected by subduction. The Os-He-O isotope ratios will be combined with data on the composition of olivine and included Cr-spinels to infer the temperatures and oxygen fugacity of olivine crystallization, enabling a holistic model on the mode of silicic melt formation. Overall, the project will provide new constraints on the connectivity between slab subduction and arc volcanism and its role in regulating the long-term evolution and maintenance of habitable Earth.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.