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。它的化学活动(即逸度)是决定：(1)地幔熔融形成的岩浆的组成，(2)岩石中存在并相互平衡的矿物，(3)伴生岩浆流体中化学物种的分布，以及(4)元素在矿物形成和蚀变过程中的分配的控制因素。这项研究涉及该国不同地区的三个机构的合作，利用高温/大气压实验来实现研究目标，并对西南印度洋海脊的海底火成岩进行了地球化学比较。岩石类型包括大洋中脊玄武岩，也被称为MORB，以及深海橄榄岩，它代表了一旦MORB被熔融过程提取出来，地球上地幔中剩下的东西。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。该项目将对一名女博士生进行交叉培训，她将在三个实验室之间穿梭，接受最先进的地球化学分析技术培训，为她提供出色的职业发展和联网机会。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早期的，另一人来自EPSCoR(刺激竞争性研究的实验计划)州(罗德岛)的一个机构。研究人员将使用Smithsonian公共推广引擎来帮助实施和促进与海底岩石有关的实践学习活动，在那里科学家可以直接与公众互动。史密森学会还将主持和传播相关的在线迷你课程。这项研究涉及岩石学、岩石学和实验工作，以考察上地幔的氧逸度及其变异性。岩石样本来自海底的一个区域，该区域非常接近玄武岩和橄榄岩物质的丰度。大多数样品已经对主要和微量元素以及关键的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专注于氧逸度。这项研究包括对上地幔研究中两个最常用的氧气压计进行交叉校准：(1)氧逸度由XANES测定的大洋中脊玄武岩的Fe3+/总Fe比值确定，X射线吸收近边缘结构，以及(2)根据电子探针测量橄榄岩的氧逸度，采用穆斯堡尔校准的尖晶石标准。由于蚀变能显著影响氧气压计的特征，因此它的影响将通过比较新鲜的和经过不同程度的蚀变的橄榄岩的氧逸度结果来检验。实验将在1大气压下进行，在立式气体混合炉中进行，氧逸度范围为QFM(石英、镁橄榄岩、磁铁矿)缓冲液，温度范围至1225摄氏度。项目目标包括：(1)确定上地幔的氧逸度，(2)检查控制MORB和深海橄榄岩中氧逸度的过程，以及(3)试图了解地幔中氧逸度在多长尺度上的变化。

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.