Project Summary: CSEDI Collaborative Research: Electrical conductivity of deformed partially molten rocks: Implications for upper mantle structure and dynamics Intellectual Merit: Geophysical measurements of the upper mantle detect electrical and seismological anomalies at depths located approximately at the depth of the lithosphereasthenosphere boundary. These anomalies are typically attributed to compositional and/or thermal anomalies. The increasingly precise image of the Earths crust and upper mantle obtained by magnetotelluric (MT) surveys highlights the need for more detailed laboratory studies and modeling. Actually, MT measurements are able to distinguish an integrated anomaly signature, as well as spatial anisotropy in behavior that can provide additional constraint on the nature of the anomalous structure. Various models have been suggested to explain these electrical anomalies, such as the presence of a partially molten zone, strain-textured hydrated mantle minerals, or vertical sub-solidus compositional differences. Texturing of the mineral assemblage due to shear deformation can result in preferred alignment of higher conductivity crystallographic orientations or faster seismic velocity directions in major minerals. Melt spatial arrangement in the upper mantle can also result in anisotropic geophysical signatures when it is deformed under high shear strains. Experimental studies of texturing in partially molten samples have been applied to understand mantle seismic signatures, but have received little attention in interpreting mantle magnetotelluric measurements. However, the evolution of melt structure in shear experiments, promoting formation of melt-rich bands, is expected to have a significant effect on both bulk conductivity and electrical anisotropy. We propose a multidisciplinary investigation of the electrical conductivity signature, in terms of both magnitude and anisotropy relative to the shear direction, of partially molten mantle rocks deformed to high shear strain combined with correlation to field electrical measurements. Synthesis of deformed and undeformed partially molten samples (olivine-melt system) and will be performed by Qi and Kohlstedt at the University of Minnesota. Melt compositions will include anhydrous, hydrous and carbon-bearing melts. A few melt-free samples will also be sheared to provide a reference point for analysis of the melt-bearing samples. The effects of temperature, compaction length and total shear strain will be investigated. Samples will be characterized in 2-D with optical and electron microscopy and in 3-D by synchrotron x-ray tomography (in collaboration with Wenlu Zhu, University of Maryland) . Electrical conductivity measurements will be performed on prepared sections of the undeformed and deformed samples, oriented with respect to the maximum applied shear stress, under sub- and super-solidus conditions by Pommier and Tyburczy at Arizona State University. Electrical measurements will be combined with the optical characterization of the samples in order to develop geometry-based conductivity models as a function of physical and chemical parameters (in collaboration with Dr. Stephen Mackwell, LPI). . We will apply our conductivity models to interpret field electrical data, in collaboration with geophysicist Rob Evans at Woods Hole Oceanographic Institute, . to constrain the nature of and processes in the asthenosphere. Broader Impacts: This multi-disciplinary and multi-scale study will strengthen collaborations between the involved laboratories and with colleagues in associated fields and institutions. The research will involve a graduate student and a postdoctoral associate, who will spend time in each of the participating institutions, resulting in significant opportunities for growth. It is especially notable that the NSF has selected ASU as the current host university for the EarthScope National Office, which will promote discussions with geophysicists and their students regarding the application of our laboratory results. Our lab-based models of mantle conductivity are of interest to the geosciences community and will be made available to researchers in the field. The knowledge gained in this research will be incorporated into the undergraduate and graduate classes taught by the PIs and their colleagues.
|Effective start/end date||4/15/13 → 8/13/14|
- National Science Foundation (NSF): $278,441.00