Noble Gas Behavior During Upper Mantle Deformation

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

Noble Gas Behavior During Upper Mantle Deformation Intellectual merit.  This proposal seeks funds to test the hypothesis that deformation is an important control on noble gas contents in mantle minerals. This hypothesis is based on a systematic study highly sheared rocks (mylonites) from St. Paul's Rocks (equatorial Atlantic ocean) and from the ocean floor. Preliminary study shows that He content varies with degree of deformation, with the highest contents in the most deformed ultramylonites. Coupled vacuum crushing and melting experiments show that most of the helium and neon within the mylonites is contained in the mineral matrices rather than fluid or melt inclusions: only 5 to 18% of the total helium is released by crushing. He/Ne ratios in St. Paul's Rocks vary widely (~ 20x) with deformation and mineralogy, with the highest He/Ne ratios (and helium concentration) found in the finest grained ultramylonite peridotite. It is suggested that very high neon and helium contents in deformed rocks is related to diffusive trapping within defects at mantle pressures. However, the present data set is limited to a very small number of samples, and the influences of mineralogy and alteration need to be assessed. It is proposed to test the gas-deformation hypothesis with further measurements at St. Paul's Rocks and to explore the influence of mineralogy on the isotopes and elemental abundances of helium, neon and argon. Helium concentrations are so high in the St. Paul's Rocks samples that new laser fusion experiments are proposed to directly determine the helium residence sites with in situ measurements. In addition, a systematic sampling across several shear zones at two ophiolites: Josephine (Oregon, USA) and Oman is proposed. The shear zones in these two ophiolites are ideal because deformation characteristics are well documented and field exposures are sufficient to allow sampling of complete transects, from undeformed to deformed peridotite. Broader impacts.  Noble gas isotope measurements in rocks and minerals are of broad interest to the earth science community. Studies of noble gases in mantle derived rocks are important to models of the earth's deep interior because unradiogenic noble gas isotopic compositions, found in some oceanic islands, provide some of the few observations that support the existence of deep undegassed reservoirs in the earth. This research crosses the traditional disciplinary boundaries between isotope geochemistry and mineral physics, which may yield important advances in our understanding of both fields. Validation of the deformation hypothesis would require reevaluation of models for noble gas behavior during mantle melting as well as gas migration along faults in the crust. This will support a graduate student in the MIT/WHOI Joint Program in Oceanography and a Postdoctoral Investigator. The work on St. Paul's Rocks will also involve collaboration with two Brazilian geoscientists (S. Sichel and T. Campos), and will foste international exchanges between students and scientists from Brazil and WHOI.