EAR-PF: A new oxybarometer to quantify spatial and temporal scales of redox variation in subducting slabs

梅根·霍利克罗斯博士获得了美国国家科学基金会EAR博士后奖学金，研究俯冲板块如何记录地球深处的氧气循环。她的研究和公共服务将在史密森学会和耶鲁大学进行。固体地球上氧气的可用性影响着地球的结构、岩石和矿石的化学成分以及大气的组成。在地球的氧化表面和还原内部之间的氧气质量传递发生在俯冲带，但是俯冲板块本身的氧化状态及其对俯冲带的岩石学、地球化学和地球动力学过程的影响仍然知之甚少。霍利克罗斯博士将进行实验，校准榴辉岩中氧气的新代用物，榴辉岩是在俯冲板块上形成的变质岩，它们已经回到了地表。在实验校准之后，她将应用新的代理来确定古代和现代的天然榴辉岩是如何跟踪俯冲带氧化状态的时空变化的。在担任美国国家科学基金会博士后期间，霍利克罗斯博士将在国家自然历史博物馆和耶鲁皮博迪自然历史博物馆主持教育活动，与公众分享地球科学的兴奋。她还将建立一个虚拟的点对点指导和支持小组，将美国8个以上机构的早期职业科学家联系起来。榴辉岩形成于大洋玄武岩高压高温蚀变过程中的俯冲板块上。因此，它们可能保留了玄武岩前体氧化态（或氧逸度，fO2）的重要信息，或者，它们的氧化态可能是整个俯冲系统中氧化物质传质的结果。对榴辉岩的fO2记录进行全面的研究，并了解这些值的地质意义，需要一个实验校准的代理，可以使用常用的分析工具应用于自然样品。榴辉岩矿物组合（这里是金红石、石榴石和辉石）是开发一种新的fO2替代物的绝佳候选者，这种替代物将利用多价微量元素钒(V)对氧化还原环境变化的敏感性。钒在与地球相关的fO2s上转移形态（V2+, V3+, V4+, V5+），因此钒在不同相中的掺入应该随着氧逸度的变化而变化。将进行榴辉石熔融活塞-圆筒实验，以校准金红石、辉石、石榴石和硅酸盐熔体中钒的分配随fO2、压力和温度的变化。新的fO2代理将应用于西非太古代榴辉岩包体和希腊基克拉迪斯俯冲杂岩的始新世地体，以探测俯冲板块在时间（数十亿年的尺度）和空间（俯冲带的尺度）上的氧化还原变化。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。

Dr. Megan Holycross has been awarded an NSF EAR Postdoctoral Fellowship to investigate how subducting slabs record the Earth's deep oxygen cycle. Her research and public service will take place at the Smithsonian Institution and at Yale University. The availability of oxygen in the solid Earth influences the structure of the planet, the chemistry of rocks and ores as well as the composition of the atmosphere. Mass transfer of oxygen between the Earth's oxidized surface and its reduced interior takes place at subduction zones, but the oxidation state of the subducting slab itself, and its consequences for petrological, geochemical and geodynamic processes in subduction zones, is still poorly understood. Dr. Holycross will perform experiments to calibrate a new proxy for oxygen in eclogites, metamorphic rocks formed on subducting slabs that have made their way back to the surface. Following experimental calibration, she will apply the new proxy to determine how ancient and modern suites of natural eclogites track spatial and temporal variations in the oxidation state of subduction zones. During her tenure as an NSF Postdoctoral Fellow, Dr. Holycross will host educational events at both the National Museum of Natural History and the Yale Peabody Museum of Natural History to share the excitement of geoscience with the public. She will also develop a virtual peer-to-peer mentoring and support group to connect early career scientists at 8+ U.S. institutions. Eclogites form on subducting slabs during high-pressure and high-temperature alteration of oceanic basalts. Therefore they may retain important information about the oxidation state (or oxygen fugacity, fO2) of their basalt precursor, or alternately, their oxidation state may be a result of the mass transfer of oxidized materials throughout the subduction system. Comprehensive interrogation of the eclogite fO2 record, and understanding the geological significance of these values, requires an experimentally calibrated proxy that can be applied to natural samples using commonly available analytical tools. Eclogitic mineral assemblages (here, rutile, garnet and pyroxene) are excellent candidates for the development of a new fO2 proxy that will utilize the sensitivity of the multivalent trace element vanadium (V) to changes in redox environment. Vanadium shifts speciation (V2+, V3+, V4+, V5+) over Earth-relevant fO2s, and thus the incorporation of Vanadium in different phases should change as function of oxygen fugacity. Eclogite-melting piston-cylinder experiments will be run to calibrate the partitioning of Vanadium between rutile, pyroxene, garnet and silicate melt as a function of fO2, pressure and temperature. The new fO2 proxy will be applied to Archean eclogite xenoliths from West Africa and an exhumed Eocene terrane from the Cyclades subduction complex in Greece to probe redox changes in subducting slabs over time (scale of billions of years) and space (scale of a subduction zone).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.