Gabbro microstructure and crystallography from Pito Deep: Evidence for Gabbro Glacier Flow
Abstract
The processes of gabbroic lower ocean crust accretion at fast spreading ridges remain enigmatic; a key constraint, however, is the orientation and fabric of the gabbros themselves. Here we present a study of the microstructure and crystallographic preferred orientation (CPO) of 23 gabbros from the upper 900m of fast-spread lower crust formed at the East Pacific Rise and exposed at the Pito Deep Rift. Samples include gabbros, olivine gabbros, troctolites, and one anorthosite; all with magmatic fabrics with varying strengths of mineral layering/foliation. Thin sections were cut parallel and perpendicular to observable mineral foliations in order to obtain accurate mineral shape preferred orientations (SPO) via hand tracing using an optical microscope. CPO was determined by Electron Backscatter Diffraction (EBSD). Eight oriented samples (3 using GeoCompass, and 5 from video footage) reveal a sub-vertical mineral foliation sub-parallel to the sheeted dikes/ridge axis from 335-863 meters below the gabbro-sheeted dike transition (mbsdt). Two GeoCompass-oriented samples were rotated to their original orientations using magnetic remanence vectors, and give CPO lineations dipping at 30° & 60° within the foliation plane. Petrographic inspection shows that crystal-plastic deformation in the form of deformation twins in plagioclase and undulose extinction /sub-grain development in olivine, is rare in the upper 72m, becomes noticeable at 177mbsdt, and is very common below 662mbsdt. Texturally equilibrated plagioclase with 120° triple junctions is common in the deepest samples. Mean 2D plagioclase axial ratios decrease with depth, indicating that plagioclase crystals become more equant, and likely equilibrated, deeper in the section. One sample at 72mbsdt shows relatively strong plagioclase alignment (Alignment Factor [AF] = 44), however, the deeper samples show a systematic increase in AF from a near random value of 6 at 248mbsdt, to an AF of 35 at 863mbsdt. The plagioclase CPO in most samples is a typical magmatic fabric with maxima for {010}, single girdles for <100> (sometimes with weak point maxima/crystallographic lineation), and weak to poorly defined single girdles for {001}. Most thin sections do not contain sufficient clinopyroxene or olivine to generate recognizable CPOs. The few samples with a mafic mineral CPO, however, are consistent with those for plagioclase. CPO appears to be correlated with SPO as shown by the linear correlation of AF and the crystallographic foliation (F = ratio of 1st and 2nd eigenvalues for {010}). Plagioclase F is relatively strong for the shallow sample at 72mbsdt, but much like the AF, it becomes weak at 248mbsdt, and increases in strength to the bottom of the section. We conclude that the presence of a vertical mineral/CPO foliation, together with the increase in AF, strength of CPO, and proportion of grains showing crystal-plastic deformation with depth, is best interpreted in terms of a gabbro-glacier flow model for the accretion of the uppermost 900m of lower crust at Pito Deep.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMOS13D1765B
- Keywords:
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- 3035 MARINE GEOLOGY AND GEOPHYSICS / Midocean ridge processes;
- 3625 MINERALOGY AND PETROLOGY / Petrography;
- microstructures;
- and textures