Date of Award
Master of Science
The magmatic processes involved in building the plutonic section of the oceanic crust at fast-spreading centers remain debated. At this stage, no intact section of this important lithospheric layer, known as seismic layer 3, has been drilled. Yet, Ocean Drilling Program Hole 1256D, located on the eastern flank of the East Pacific Rise, provides unparalleled opportunities to investigate the mode of emplacement and fabric development of the two uppermost gabbro bodies in this crust. Although inferences made from a drill core remain limited due to the intrinsically linear nature of observations, the samples recently recovered hold potential clues on fabric-forming processes and the magma convection in such small intrusions. Gabbro 1 forms a 52 m-thick body intruded in the texturally granoblastic altered zone of the sheeted dike complex. Gabbro 2, situated below gabbro 1, is only 24 m-thick. Both gabbro bodies lack a macroscopically visible fabric and were, until this study, considered structurally isotropic. We use digital image analysis of petrographic thin-sections and the intercept method (Launeau and Robin, 1996) to determine the shape-preferred orientation of plagioclase phenocrysts in 3-D. Thirty-three sets of three mutually perpendicular thin-sections were prepared and analyzed in this manner. The mode of plagioclase grains ranges from 15 to 20% in the upper interval of gabbro 1, 13 to 36% in the lower interval of gabbro 1, and 9 to 28% in gabbro 2. These values refer to the mode of euhedral to subhedral grains only and do not reflect the total mode of plagioclase in the rock. The two gabbros display a weakly anisotropic shape-preferred orientation characterized by an average shape ratio > 1.050. The aspect ratio indicates plagioclase fabric strength deviation from a perfectly anisotropic distributed population. The aspect ratio in the upper interval of gabbro 1 ranges from 1.068 to 1.153. The aspect ratio in the lower interval of gabbro 1 ranges from 1.074 to 1.183. The aspect ratio in gabbro 2 ranges from 1.056 to 1.220. The fabric of these gabbros also displays broad consistency between nearby specimens. Lineation plunges in the upper interval of Gabbro 1 range from 1° to 36°. Lineations plunge in the lower interval of gabbro 1 range from 0° to 44°. Lineations in gabbro 2 plunge from 6° to 69°. In general, the symmetry of plutonic fabrics provides clues on the nature of magmatic fabric-forming processes. Prolate fabrics support magmatic flow while oblate fabrics rather suggest gravitational settling of crystals. Fabrics in gabbro 1 and gabbro 2 are both prolate and oblate. Fabric in the upper interval of gabbro 1 is more prolate than oblate whereas the lower interval of gabbro 1 and gabbro 2 have even distributions of fabric ellipsoids. Detailed observations of petrographic thin-sections reveal several microstructures suggestive of brittle and plastic deformation in plagioclase grains. Microstructures indicative of plastic deformation include kink-banding, mechanical twinning, and undulose extinction. Microstructures indicative of brittle deformation include submagmatically-fractured laths. These subtle features appear in at least one crystal per thin-section analyzed. With the exception of mechanical twins, shipboard scientists of Expeditions 312 and 335 have not documented these microstructures. Numerical calculations reveal that for both gabbro bodies the Rayleigh Number ranges between 1015 and 1018, depending primarily on the kinematic viscosity of the magmas and temperature change across the intrusions. These values indicate that turbulent flow (Rayleigh number > 106) occurred during emplacement. This type of flow may provide an explanation for the variability of aspect ratio and the variability in linear crystal fabric.
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