Dr. Eric Goergen is awarded an NSF Earth Sciences Postdoctoral Fellowship to develop an integrated program of research and education at Brown University. The goal of this project is focused on exploring the kinetic driving forces and mechanistic interactions fundamental to the development of reaction-induced strain localization in rocks. This project aims to investigate simultaneous reaction and deformation processes through a series of deformation experiments carried out on rocks of lherzolite bulk composition. These experiments are designed to explore strain localization through three approaches: 1) investigating the known positive correlation between reaction rate and strain rate; 2) characterizing the time dependent evolution of microstructures associated with reaction-induced strain localization through a series of experiments carried out to increasing amounts of finite strain; and 3) investigating the effect of bulk composition on the progress of reaction and localization by varying modal amounts of phases present in starting materials. Experiments will be analyzed by focusing on the microstructural evolution through chemical and crystallographic analysis to develop models of the kinetic and mechanic history within and between experiments. In addition, the choice of the lherzolite chemical system and pressure-temperatures for these experiments will also provide fundamental rheologic data on the upper mantle in the context of evolving bulk composition and phase assemblages.
This project also aims to incorporate a strong educational component through the integration of undergraduate research. Dr. Goergen has developed two independent projects that are appropriate for the undergraduate level and will also contribute strongly to the research objectives stated above. These projects will both further the intellectual development of young geoscientists by laying a strong foundation in sound research as well as, further the professional development of Dr. Goergen by gaining necessary experience in the integration research and teaching through the advising of undergraduate theses.
This primary goal of this project was to quantify—through experimentation—the processes controlling the interaction between metamorphic and deformation processes. The interaction of these processes is observed in natural rocks to cause weakening of the affected lithology due to the localization of deformation into shear zones (ductile faults). This project focused on two studies: i) quantifying the evolution of rock strength in a system experiencing simultaneous deformation and metamorphic reaction. Specifically, we were interested in the spinel to plagioclase lherzolite transformation, a metamorphic reaction that has been shown to localize deformation into natural shear zones in exhumed upper mantle rocks. ii) investigating the role of water in controlling the strength of plagioclase-rich mafic rheologies. In this study, we focused on deformation of natural diabase (a fine-grained gabbro consisting of ~50-50 mix of plagioclase and clinopyroxene) with varying initial and final water contents. While the former study was the primary study related to this project, the experimental protocol was very complicated and our results are therefore only now reaching a mature stage. However the latter project has led to some surprising and interesting conclusions of how water affects the strength of the lower crust with implications for Earth as well as Venus and the moon. Results from our deformation experiments on natural diabase illustrate that the addition of small amounts of water to plagioclase-rich lithologies yield big returns in terms of the resulting strength of the material. Our microstructural data suggest that, at the P-T conditions of our experiments (900?C and 1 GPa), plagioclase is controlling the strength of diabase. Although the weakening effect of water on silicate minerals has been known to exist for decades, the mechanisms and specific mineral water contents associated with weakening is not well known, particularly for plagioclase. Our data illustrate that water-related weakening in plagioclase is not gradational—meaning weakening occurs rapidly over a discrete range of the amount of water contain in plagioclase (~0.01 wt % H2O dissolved in plagioclase crystals). The data suggest that this weakening is equivalent to a 1000 times reduction in the strength of the material. The results of our diabase experiments have profound implications for the strength of the lower crust. The strength of the lower crust controls a wide range of processes that are important to long-term stability of mountain ranges, the geochemical evolution of the upper mantle (through delamination of the lower curst), and the interpretation of seismic hazards. The contribution of the lower crust to determining seismic risk arises from the contribution of the lower crust to post-seismic relaxation and subsequent fault loading after an earthquake; both of these are dependent on the mechanical characteristics of the lower crust. Our data also have important implications for the strength evolution of the crust of Venus. Venus is generally interpreted to contain very little water due to the lack of evidence for plate tectonics and, therefore, the presumed high strength of the Venusian crust. However, our data illustrate that the lower crust of Venus could still contain appreciable water and retain its high mechanical strength. In addition to contributing to the training of an early career scientist, three undergraduate students at Brown University were trained in the design and interpretation of experimental research. This project has also led to the development of new experimental protocols that will be of use to the broader experimental research community.
