A- to B-Type Olivine Fabric Transitions Associated with Hydration, Dehydration and Shear Above the Farallon Flat Slab

Abstract:

We investigate mantle rocks associated with hydration, dehydration and shear above the Farallon flat-slab at its contact with the base of North America. The rocks we focus on are ultramafic inclusions hosted within serpentinized ultramafic microbreccia diatremes of the Navajo Volcanic Field (New Mexico) that erupted to the surface at the waning stages of the Laramide orogeny. A large number of petrological and geochronological studies have pinpointed the origin of these rocks to the hydrated mantle wedge above the Farallon slab as well as tectonically eroded and entrained fragments of the plate interface. We combine petrological observations and EBSD measurements of olivine grainsize and LPO to examine the effects of hydration on olivine fabric development in different parts of the supra-subduction zone mantle. The rocks examined include weakly deformed to strongly foliated tectonites we interpret to represent partially hydrated fragments of the upper plate mantle; and mylonites and ultramylonites we interpret to represent deformed fragments of the plate interface. The rocks deformed at temperatures ranging from 500-900°C based on thermometry, and olivine compositions in some record heating just before incorporation in the diatreme mix. We observe the following:

1. Tectonites exhibit A-type bulk olivine LPOs, but show transitions to B-type LPO in local, fine-grained, dynamically recrystallized regions associated with hydrous minerals.
2. Mylonites and ultramylonites with stable chlorite and/or antigorite and recrystallized grainsizes of less than 10µm show strong B-type olivine LPOs. 
3. A single mylonite with recrystallized grainsizes of ~35µm shows evidence for prograde metamorphism and dehydration through the chlorite breakdown reaction at temperatures above ~770°C. It contains no hydrous minerals and shows a strong A-type olivine LPO.

Together these rocks demonstrate a strong correlation between hydration under high stress conditions, and B-type olivine LPO, consistent with experimental studies that indicate this fabric forms at high water contents and high stress. If interpreted in terms of supra-subduction zone seismic anisotropy, these rocks are consistent with a transition from trench-parallel fast directions close to the trench to trench-perpendicular fast directions away from the trench.