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，对宜居地球的形成和维持至关重要。在会聚的板块边缘，一个由地壳和最上面的地幔组成的岩石圈板块被拉到或俯冲到另一个岩石圈板块之下。在大约80-140公里深的地方，俯冲板块释放出富含气候活跃的挥发性元素(水、卤素)的物质，并通过弧形排列的火山活动将其回收回地球表面。这些弧形形成“太平洋火环”，以富含挥发性物质的毁灭性硅质岩浆喷发而闻名，这些硅质岩浆会造成致命的气候扰动(例如坦博拉1815年)和海啸(例如喀拉喀托1883年，2018年)，对人口稠密的地球上的人类居住构成永久的重大威胁。因此，了解物质循环是如何导致硅质火山爆发的，仍然是地球科学研究的核心主题。研究俯冲再循环时的一个特别挑战是，它发生在地球内部深处(几公里到200公里之间)，因此无法直接观察到。因此，科学家们依赖于从岩浆成分(例如橄榄石和尖晶石晶体)获得的间接观测，这些成分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(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，从墨西哥年轻火山喷发的火山岩中收集此类信息。该项目将培养研究生和本科生，目标是促进多样性。K-12教师将致力于将新的研究成果转化为初中生和高中生的地球科学课程，并将在每年的开放日定期向公众通报。该项目重点关注跨墨西哥火山带(TMVB)的橄榄石及其从镁铁质到硅质岩浆中的铬尖晶石包裹体，TMVB是一条主要的俯冲带，以其巨大的灾难性火山喷发潜力而闻名。许多研究认为，硅质岩浆是通过熔融作用在建造TMVB的45公里厚的大陆地壳基底中获得致命的爆炸力的。然而，一些研究表明，这些岩浆中含有的橄榄石晶体的成分与这一假设不一致。值得注意的是，来自TMVB中部的橄榄石具有Delta18O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下方的地幔中，在那里它们通过合并大量俯冲的板片物质而变得硅化。在这个项目中，我们将通过从橄榄石和块状岩石中获得额外的187Os/188Os同位素数据来进一步检验这两个相反的假设，这些岩石是从中部(Popocatepetl和Sierra Chichinautzin)和东部(Serdan盆地)的关键位置以及从岩浆不受俯冲显著影响的弧后地区精心挑选的。Os-He-O同位素比值将与橄榄石和包括铬尖晶石在内的橄榄石组成的数据相结合，以推断橄榄石结晶的温度和氧逸度，从而建立一个关于硅质熔体形成模式的整体模型。总体而言，该项目将对板块俯冲和弧火山作用之间的连通性及其在调节宜居地球的长期演化和维护方面的作用提供新的限制。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.