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.
