Testing Models of Mantle Upwelling: Microstructure, Crystallography, and Seismic Anisotropy of Peridotites From 15 Degrees N, Mid-Atlantic Ridge

Owing to the scarcity of oriented mantle peridotite samples, few microstructural and crystallographic data are currently available to constrain models of mantle upwelling and deformation beneath slow-spreading ridges. Here we present quantitative data regarding the shape- and lattice-preferred orientations (SPO and LPO) of olivine and orthopyroxene in peridotites from Hole 1274A, drilled to a depth of 156 mbsf, on the western flank of the rift valley wall at 15° 39' N on the Mid-Atlantic Ridge during ODP Leg 209. With as little as 50% alteration in places, our samples are the freshest peridotites recovered during Leg 209. The rocks are protogranular harzburgites with 75-82% olivine, 17-22% orthopyroxene, 0.8-1.4% clinopyroxene, and 0.9-2.4% spinel, and one dunite with 96.3% olivine, 0.1% opx, 1.7% clinopyroxene, and 1.9% spinel . The samples preserve evidence of melt-rock interaction and weak lithospheric deformation, but the primary asthenospheric SPO is preserved. Olivine and orthopyroxene exhibit a weak but consistently measurable foliation; no spinel lineation is readily apparent. Crystallographic data, collected using Electron Backscatter Diffraction (EBSD), are based on measurements of as many as 1100 olivine crystals and 530 orthopyroxene crystals per sample in rocks from ten sections of core (recovered from depths ranging from 20 - 150 mbsf). Olivine crystals show a strong [010] maximum subperpendicular to foliation, with a less strongly developed [100] maximum and diffuse clusters of [001] axes roughly parallel to the foliation plane. This fabric is consistent with dislocation creep operating at ~1200° C predominantly on the olivine (010)[100] slip system. The relatively weakly developed [100] and [001] maxima may be attributed to a low strain rate, or to a component of slip in another slip system. Orthopyroxene crystals show a [001] maximum parallel to the foliation plane, with a [100] maximum subperpendicular to the foliation plane, indicative of the (100)[001] slip system active at ~1200° C. Rotation of the SPO and LPO to a common magnetic declination, using shipboard paleomagnetic data, shows a consistent down-hole olivine foliation dipping at ~80°, with sub-horizontal olivine [100] and [010] axes. The orthopyroxene foliation is less well constrained, but, dipping at ~50°, is discordant to the olivine foliation. Removing the effects of tectonic rotation, we find that at the time of the onset of magnetization in these rocks, olivine [100] axes were subparallel to the ridge axis, and olivine [010] axes were subvertical (resulting in a subhorizontal foliation). Although the fabrics are relatively weak and there is large uncertainty in the amount and direction of tectonic rotation, this sample reorientation suggests that the direction of asthenospheric flow in this portion of the mantle beneath the slow-spreading Mid-Atlantic Ridge was most likely parallel to the ridge axis. Careful mapping of grain boundaries using both petrographic and EBSD data allows for the complete reconstruction of the original (pre-alteration) fabric of these rocks. Using this reconstruction, we have calculated the original P-wave anisotropy of the core to be 5.7%. This anisotropy is similar to the distance-averaged seismic anisotropy measured by seismic experiments along strike to the west of this locality.