Abstract:
Plate tectonics is a process that is unique and fundamental to our planet, and midocean ridges are the surface expression of the constructional end of this cycle, where new tectonic plates are formed. As mantle material upwells to fill in the gap created by the spreading apart of tectonic plates, a number of processes can occur, including melting, melt-rock interaction, melt crystallization, and/or deformation Several of these processes can occur simultaneously, and their interaction is as yet poorly understood, although they may have a profound effect on the behavior of the uppermost mantle beneath mid-ocean ridges. I use petrographic, microstructural/textural, and electron backscatter diffraction (EBSD) lattice-preferred orientation (LPO) analyses to explore igneous and deformational processes occurring in the mantle beneath four mid-ocean ridges. LPO analyses show that abyssal peridotites record some homogeneous deformation in the mantle during upwelling. Paleomagnetic reorientation of olivine LPOs in peridotites from Ocean Drilling Program Leg 209 Hole 1274A preserve a small angle (<40 degrees ) between the paleomagnetic vector and the mantle flow direction, and thus show that the mantle beneath the Mid-Atlantic Ridge (MAR) experienced complex/3D mantle flow, complex/3D tectonics, or both. Textural observations show that peridotites from the MAR, the Southwest Indian Ridge (SWIR), the Gakkel Ridge, and the East Pacific Rise (EPR) experienced diffuse melt percolation and melt-rock interaction along most grain boundaries late in their upwelling history after large-scale deformation ceased. This suggests that deformation localized in the presence of melt while the rocks were at high temperature (>1100 degrees C). LPO and textural variations within a suite of peridotites from the Southwest Indian Ridge show that late-stage melt-rock interaction and deformation were heterogeneous on the kilometer scale. Predictions of seismic anisotropy based on LPO measurements in the SWIR peridotites range from 5.8-8.6%, and show that seismic anisotropy may be affected by the late-stage interaction between melt and deformation. Furthermore, a comparison of olivine LPOs and the associated seismic anisotropy predictions from the MAR, the Gakkel, the SWIR, and the EPR show that the presence of melt may affect olivine slip systems, and thus deformation in the presence of varying amounts of melt may result in variations in seismic anisotropy. This document includes six chapters and five appendices. In the electronic version of this document, the chapters are compiled in a single main file; appendices are available as supplemental files.