Collaborative Research: Oxygen Fugacity (i.e., Chemical Activity) in the Upper Mantle: Intercalibration of Upper-Mantle Oxybarometers with State-of-the-Art Analytical Techniques

氧是地幔中的一种重要元素，构成了大多数岩石的主体。它的化学活性（即逸度）是决定以下因素的控制因素：（1）地幔熔融形成的岩浆的成分，（2）岩石中存在的矿物及其相互平衡，（3）相关岩浆流体中化学物质的分布，以及（4）矿物形成和蚀变过程中元素在矿物之间的分配。这项研究涉及该国不同地区的三个机构的合作，采用高温/大气压实验来实现研究目标，并对西南印度洋脊海底火成岩的地球化学进行比较。 岩石类型包括洋中脊玄武岩，也称为MORB，以及深海橄榄岩，它们代表了MORB在熔融过程中被提取后在地球上地幔中留下的东西。 这项工作的更广泛影响包括来自大学和史密森自然历史博物馆的科学家之间的互动，推进科学基础设施的方法开发，以及研究和教育的紧密结合。该项目将涉及一名女博士生的交叉培训，她将在三个实验室之间旅行，接受最先进的地球化学分析技术培训，为她提供出色的专业发展和建立联系的机会。还将对本科生和博士后进行培训。该项目支持三名女性研究人员，其中一人是职业生涯初期，另一人来自EPSCoR（刺激竞争性研究的实验方案）州（罗得岛）的一个机构。 研究人员将使用史密森尼公共外展引擎来帮助实施和促进与海底岩石的实践学习活动，在那里科学家直接与公众互动。 史密森学会还将主办和传播相关的在线迷你课程。 这项研究涉及岩石学、岩相学和实验工作，以检查上地幔的氧逸度及其变化。 岩石样本来自一个区域的海底，有一个特殊的丰富的玄武岩和橄榄岩材料在附近。 大多数样品已经对主要和微量元素以及关键放射性同位素进行了很好的分析表征，使研究能够以更有效的方式关注氧逸度。 这项研究包括对上地幔研究中最常用的两种氧气压计进行交叉校准：（1）根据XANES、X射线吸收近边结构、光谱学确定的洋中脊玄武岩的Fe 3 +/总Fe比率确定氧逸度;（2）使用穆斯堡尔校准尖晶石标准品通过橄榄岩电子探针测量得出氧逸度。由于蚀变可以显着影响氧气压计签名，其影响将通过比较新鲜和蚀变橄榄岩，经历了不同程度的蚀变的氧逸度结果进行检查。实验将在垂直气体混合炉中在1 atm下在一定范围的氧逸度下进行，所述氧逸度范围包括在1225 ℃的温度范围内的QFM（石英、镁橄榄石、磁铁矿）缓冲液。 项目目标包括：（1）确定上地幔的氧逸度，（2）研究控制MORB和深海橄榄岩中氧逸度的过程，（3）试图了解地幔中氧逸度在什么长度尺度上变化。

Oxygen is an important element in Earth's mantle and makes up the bulk of most rocks. Its chemical activity (i.e. fugacity) is a controlling factor in determining: (1) the composition of magmas that form from mantle melting, (2) the minerals that are present and in equilibrium with one another in a rock, (3) the distribution of chemical species in associated magmatic fluids, and (4) the partitioning of elements between minerals during their formation and alteration. This research involves a collaboration of three institutions in different parts of the country, employs high temperature/atmospheric-pressure experiments to achieve research goals, and carries out a comparison of the geochemistry of seafloor igneous rocks from the Southwest Indian Ridge. Rock types include mid-ocean ridge basalts, also known as MORB, and abyssal peridotites which represent what is left over in Earth's upper mantle once MORB has been extracted by melting processes. Broader impacts of the work include interaction between scientists from universities and the Smithsonian Museum of Natural History, methods development that advances the infrastructure for science, and a strong integration of research and education. The project will involve the cross-training of a female PhD student who will travel between three laboratories and be trained in state-of-the-art geochemical analytical techniques, providing her with outstanding professional development and networking opportunities. Training of undergraduate students and a postdoc will also take place. The project supports three female researchers, one of whom is early career, and another who is from an institution in an EPSCoR (Experimental Program to Stimulate Competitive Research) state (Rhode Island). The researchers will use the Smithsonian public outreach engine to help implement and promote hands-on learning activities with seafloor rocks where the scientists interact directly with the public. The Smithsonian will also host and disseminate associated online mini-lessons. This research involves petrologic, petrographic, and experimental work to examine the oxygen fugacity of the upper mantle and its variability. Rock samples come from an area of the seafloor that has an exceptional abundance of both basaltic and peridotitic material in close proximity. Most samples have already been well characterized analytically for major and trace elements and key radiogenic isotopes, allowing the research to focus on oxygen fugacity in a more efficient way. The research includes the cross-calibration of the two most frequently used oxybarometers in upper mantle studies: (1) oxygen fugacity determined from Fe3+/Total Fe ratios of mid-ocean ridge basalts determined by XANES, X-ray Absorption Near Edge Structure, spectroscopy and (2) oxygen fugacity derived from electron microprobe measurements of peridotite using Mossbauer-calibrated spinel standards. Because alteration can significantly impact oxybarometer signatures, its effects will be examined by comparing oxygen fugacity results of fresh and altered peridotites that have undergone various degrees of alteration. Experiments will be performed at 1 atm in a vertical gas mixing furnace at a range of oxygen fugacities that bracket the QFM (Quartz, Forsterite, Magnetite) buffer at a range of temperatures to 1225 C. Project goals include: (1) determining the oxygen fugacity of the upper mantle, (2) examining the processes that control the fugacity of oxygen in MORB and abyssal peridotite, and (3) trying to understand over what length scale oxygen fugacity in the mantle varies.