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.
The main goal of the project was to test the hypothesis that grain size is an important control on gas concentrations in the mantle, using new noble gas measurements in variably deformed mantle rocks. We chose three suites of samples for the study, which included one previously existing suite from St. Paul’s Rocks in the equatorial Atlantic ocean. In this case, the research involved new helium, neon, and argon measurements of variably deformed peridotites. The measurements confirmed the earlier work, in that high noble abundances were found in the most strongly deformed samples, called mylonites, and support the hypothesis that grain size is an important control on grain size. The new measurements on St. Paul's Rocks are important because they include argon. The other two sample sites required field research at Josephine Ophiolite (Oregon, USA) and the Oman Ophiolite (Sultanate of Oman). The field-based strategy was to collect samples across exposures of peridotite shear zones, where the relationship between deformation and noble gas variations could be directly examined, using a combination of field observations and laboratory sample characterization. The Josephine Peridotite and Oman Ophiolites were selected because they have been extensively studied and contain well documented shear zones (where deformation is extreme) along with excellent exposures. In both cases, we attempted to eliminate mineralogy as a variable in the study, and to directly compare deformation (grain size) with gas contents. The Josephine study provided the first detailed results from an ophiolite peridotite and were recently published (Recanati et al., 2012). There were few prior ophiolite noble gas studies, so we did not know if the results would be greatly impacted by post-emplacement alteration and or radiogenic production of important noble gas isotopes such as 4He and 40Ar. The laboratory strategy was to sequentially crush and melt the samples in vacuum, to allow a first look at noble gas residence sites. The results showed that, in all cases, more than 80% of the helium resided within the solid mineral matrix, thus eliminating fluid inclusions (within the minerals) as a factor. The concentration data does show a correlation between grain size and helium content, which supports the hypothesis that deformation adds helium to the peridotite. No relationship could be detected between helium and internal mineral defects (based on decoration of olivine dislocations), so we concluded that grain boundaries, and the defects associated with grain boundaries, are the most likely noble gas residence sites. The helium isotopic data showed that the Josephine peridotites are dominated by mantle helium, despite their great age (~157 Ma). The 3He/4He values found in the Josephine shear zone was remarkably uniform at 6.77 ± 0.2 times the atmospheric value. This surprising result, that mantle helium can be preserved over such long periods, suggests that the peridotites have very low Th and U abundances. Another interesting outcome is that a clinopyroxenite vein, just outside the shear zone, had roughly ten times higher helium abundance than the host rock harzburgite This type of vein is thought to be formed by melt migration in the mantle, and the new data suggests that melt migration plays a key role in noble gas abundances. Tiny amounts of cosmogenic 3He (from cosmic ray produced spallation) were detected in the Josephine peridotite surface samples. The calculated exposure ages are approximately 10 Ka, or just after the retreat of alpine glaciers in the northwestern USA following the last glacial maximum which provides constraints on the landscape evolution. The preliminary results from the Hilti shear zone in Oman do not appear to show a systematic relationship between helium and grain size. In this case, it is unclear if we can exclude the importance of melt migration and mineralogical variations to noble gas storage. Additional data will be required to understand this result in Oman.
