Fe isotopes as a key to understanding fluid-rock processes during hydration of oceanic crust

University Of Wyoming, Laramie WY

Investigators

Abstract

Serpentinization, the reaction of olivine and pyroxene, the two most prevalent minerals in the mantle in the oceanic lithosphere, with aqueous fluids is the primary alteration reaction in the ocean crust. Through this reaction, H2O is taken up into the rock and bound in the structure of minerals that replace olivine and pyroxene as they react with magmatic fluids and/or seawater. Vast volumes of ocean crust have undergone serpentinization; and this reaction results in a myriad of interesting and poorly known processes, such as the development of ultra-mafic hosted hydrothermal vents that are home to unusual microbes living both on and below the seafloor in the deep sea. In subduction zones, serpentinized oceanic lithosphere dives down into Earth's mantle where increasing temperature causes serpentine to react and release its bound H2O, which then lowers the melting point of overlying rocks and causes magmatism and island arc volcanism, such as that which occurs in Japan and the Alaskan Aleutian Islands, and cycles H2O from surface reservoirs back into the mantle. Thus, understanding serpentinization processes is essential for our knowledge of how the Earth works, how ore deposits associated with convergent tectonic margins are formed, and how life can exist deep in the ocean crust far from light and the input from organic matter settling down from the sea surface. This research provides a new means to understand the early part of the serpentinization process, using the isotopes of Iron (Fe) in minerals that form and form from serpentine. Samples from cores from holes drilled into the ocean crust will be analyzed as will samples from the Josephine Ophiolite in California and from the Oman and New Caledonian ophiolites, all of which represent seafloor that has been thrust onto the continentals. Analyses of mineral phases will be performed by electron microprobe. Electron Energy-Loss Spectroscopy will be used to determine the ferric iron content of the serpentines; and Fe isotopes will be measured on a thermal ionization mass spectrometer. Analysis of the resulting data will be assisted by thermodynamic modeling and determinations of the various oxidation states of Fe. The main goals of this research are to investigate the processes by which Fe in serpentinizing crust moves, determine how magnetite, a major Fe bearing mineral, forms during serpentinization, and explore how non-traditional stable isotopes of Fe can be used to track the oxidation and mobility of Fe irrespective of the formation of magnetite. Specific hypotheses are that the Fe isotopic signature of chlorite and temolite rims around olivine will be close to zero per mil because little to no magnetite is produced in the reactions. If the signature is found to be light, then it is likely to indicate Fe fractionated as it moved through the solution phase to create magnetite. It is further predicted that the Fe isotopic signature of magnetite in serpentine veins is heavy compared to that in the bulk rock. Additional mineral specific fractionation questions will be examined and addressed. Broader impacts of the work include support of faculty at the University of Wyoming, which is an institution in an EPSCoR state (i.e., state that does not receive significant federal funding). It also involves graduate student training in cutting-edge technology and international collaboration with Australian scientists. To increase public awareness of the research and the science of marine geology, the awardees will work closely with the University of Wyoming Geology Museum to create a new series of exhibits on the seafloor and ocean science. The project has additional broader impact in that it informs the economic geology and formation of ore deposits fields.