Deformation of Fe-Ti oxides in gabbroic shear zones from the MARK area

Abstract

The migration of late-stage magmatic fractions along shear zones at seafloor spreading centers will modify bulk composition, temperature, and fluid chemistry, as well as pressure during deformation. These changes will influence the rheological evolution of oceanic shear zones and the time-dependent strength of the ocean crust. Fe-Ti oxides, representing late-stage magmatic fractionates, are volumetrically abundant (>50% in some instances) in many of the core-scale shear zones recovered from Ocean Drilling Program Sites 921, 922, and 923. Yet, little is known about the role of these phases during shear-zone deformation, the mechanisms by which they deform, and the timing of melt migration relative to deformation. Using orientation contrast imaging and selected area diffraction (electron channeling), the petrofabrics and microstructures of Fe-Ti oxides in three texturally contrasting shear zones were determined. Magnetite preserves no strong crystallographic preferred orientation in any of the samples. Ilmenite exhibits a strong preferred orientation in one sample where the [0001] direction is oriented at a high angle to the shear plane. Deformation mechanism maps, grain-size variations, and paleotemperature estimates of 550°-650°C, indicate that the magnetite in two of the samples deformed predominantly by diffusion creep. Ilmenite deformed by dislocation creep in one sample and probably by diffusion creep in a second sample. Melt-assisted grain boundary sliding with post-kinematic grain growth could also account for the lack of crystallographic preferred orientation of recrystallized oxides. In a third sample, primary textures may persist in both magnetite and ilmenite. Through the interaction of magmatic fractionation and deformation, relatively weak phases were concentrated in shear zones during an overall down-temperature deformation path (>900°-550°C). Our results and other investigations of metamorphic ores suggest that Fe-Ti oxides behave as highly ductile phases even at low temperatures (200°-400°C). This property, combined with an ore-concentrating mechanism in shear zones, will promote aseismic creep at relatively shallow levels in the ocean crust (<2 km) and may partly account for the low levels of seismicity along spreading-segment faults.