Collaborative Research: An Experimental and Analytical Investigation of the Parameters That Influence Measured CO2 in Plagioclase-Hosted Melt Inclusions in MORB

火山是地球的主要特征，当它们喷发时，会对生命、基础设施和生态系统造成重大破坏。当它们发生在海底时，它们还可能引发海啸，有可能对沿海社区造成毁灭性破坏。因此，我们对岩浆过程的了解对于了解地球如何运作以及帮助我们了解岩浆事件和火山喷发可能造成的危险都是重要的。这项研究的重点是关于地球深处发生的岩浆过程的故事，这些过程是通过捕获在斜长石晶体中的熔体的微小包裹体来讲述的，斜长石晶体是地壳中最普遍的矿物之一。到目前为止，由于扩散和圈闭后的泄漏，斜长石中的熔体包裹体还不能被认为是水、二氧化碳或S等挥发性地球化学物种的可靠原始信息档案。熔融包裹体的起源以及它们在斜长石历史上何时何地形成也一直存在疑问。这项研究结合了创新的实验室实验和全面的受控校准研究，旨在重建斜长石中熔体包裹体的原始挥发性成分，这些熔体来自于太平洋胡安德富卡海脊(Juan De Fuca Ridge)上喷发的超叶状熔岩，位于华盛顿和加拿大边界以西。实验将对同一套样品中的斜长石和共存的橄榄石进行相互比较，并评估捕获后结晶和重新加热过程对测量的包裹体挥发性含量的影响程度。时间序列实验将在不同温度(从1200摄氏度到1270摄氏度)和升温速率的加热阶段进行。包裹体的主要元素成分将通过电子探针进行分析；痕量元素成分将使用激光电感耦合质谱仪进行测定；傅立叶变换红外光谱将用于验证大型包裹体的CO_2和H_2O成分；离子探针和显微拉曼光谱将用于分析CO_2、H_2O、S和镁。拟议进行的实验将确定挥发物在地壳中的传输过程中或在实验加热过程中通过扩散、气泡形成和通过晶体裂缝泄漏等过程从包裹体中损失的机制。这项研究的目标是更好地测量大洋中脊环境中的岩浆中橄榄石和斜长石的矿物结晶深度。需要解决的问题包括：包裹体中的后捕获结晶和气泡的形成如何影响熔融包裹体的测量挥发分含量；不同的均一方法和冷却速度如何影响测量的挥发分含量；不同挥发性物质的测量值表明了结晶货物的形成深度和洋壳的二氧化碳收支；斜长石熔融包裹体提供的信息与橄榄石熔融包裹体提供的信息相比如何。这项工作的更广泛影响包括开发对其他领域具有潜在影响的新实验技术，更好地了解火山过程和了解岩浆生成的矿藏方面的社会影响。它还将涉及两名研究生和两名本科生的培训。

Volcanoes are major Earth features and, when they erupt, can cause significant damage to life, infrastructure, and ecosystems. When they occur on the seafloor, they can also trigger tsunamis that have the potential to cause devastating damage to coastal communities. Thus, our understanding of magmatic processes is important for both understanding how the Earth works as well as for helping us understand the dangers magmatic events and volcanic eruptions can cause. This research focuses on the story about magmatic processes that occur deep in the Earth as told by tiny inclusions of melt trapped in crystals of plagioclase, one of the most ubiquitous minerals in the Earth's crust. Up until now, melt inclusions in plagioclase have not been considered reliable archives of primary information for volatile geochemical species like H2O, CO2, or S due to diffusion and post entrapment leakage. There have also been questions about the origin of melt inclusions and when and where they form in the history of the plagioclase in which they occur. This research combines innovative laboratory experiments and comprehensive controlled calibration studies to reconstruct the original volatile composition of melt inclusions in plagioclase from ultra-phyric lavas erupted on the seafloor in the Pacific Ocean on the Juan de Fuca Ridge, just west of the Washington and Canadian border. The experiments will be done on both plagioclase and coexisting olivines from the same suite of samples to compare against one another and assess the degree to which post entrapment crystallization and the process of reheating impact the measured volatile content of the inclusions. Time-series experiments will be conducted on a heating stage at various temperatures (from 1200 C to 1270 C) and heating rates. Analyses of the major element composition of the inclusions will be carried out via electron microprobe; trace element compositions will be determined using laser inductively coupled mass spectrometry; Fourier transform infrared spectroscopy will be used to verify CO2 and H2O compositions of large inclusions; and the ion microprobe and micro-Raman spectroscopy will be used to analyze CO2, H2O, S, and Mg. The proposed experiments that will be carried out will determine the mechanisms by which volatiles are lost from inclusions during transport in the crust or during experimental heating via process such as diffusion, formation of bubbles, and leakage through cracks in the crystal. Goals of the research are to produce better measurements of the depth of mineral crystallization from magmas in mid-ocean ridge environments for both olivine and plagioclase. Questions to be addressed include how post entrapment crystallization in the inclusion and the formation of bubbles affect the measured volatile contents of the melt inclusions; how different homogenization methodologies and quench rates influence measured volatile contents; what the measured values of different volatile species indicate about the depth of formation of the crystal cargo and the CO2 budget of the ocean crust; and how does the information from plagioclase melt inclusions compare to that coming from olivine melt inclusions. Broader impacts of the work include development of new experimental techniques that have potential implications to other fields, societal impacts in terms of better understanding volcanic processes and understanding magmatically generated ore deposits. It will also involve the training of two graduate students and two undergraduates.