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Abstract Mantle xenoliths from the Southern Alps, New Zealand, provide insight into the origin of mantle seismic anisotropy related to the Australian‐Pacific plate boundary. Most xenoliths from within 100 km lateral distance of the Alpine Fault are coarse grained, but a small number are finer grained protomylonites. The protomylonites contain connected networks of fine grains with a different crystallographic preferred orientation (CPO) to coarse porphyroclasts in the same xenolith, suggesting that protomylonites and coarse‐grained samples record different deformation kinematics. The CPOs of fine grains in protomylonites have monoclinic symmetry, with the 2‐fold rotation axis normal to a plane that contains olivine [010] and orthopyroxene [100] maxima, suggesting that the protomylonite deformation involved significant simple shear. Some coarse‐grained samples contain unconnected lenses and layers of fine grains with the same CPO as the coarse grains. Microstructures suggest that these fine grains formed by subgrain rotation recrystallization and that protomylonites may represent an up‐strain progression of this microstructure, where the connectivity of fine grains has allowed them to localize shear and develop a new Alpine Fault CPO. The samples tell us about the state of the mantle at 25 Ma, in the early history of the plate boundary. If this suite of samples is representative of the mantle beneath the Alpine Fault in the present day, then we can interpret the complex seismic anisotropy patterns in the lithospheric mantle as representative of blocks containing variably rotated older CPOs juxtaposed by narrow shear zones associated with Alpine Fault deformation.
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Microstructural Shift due to Post‐Deformation Annealing in the Upper Mantle
Abstract Syntectonic microstructural evolution is a well‐known phenomenon in the mantle and lower crust associated with two main processes: grain size reduction through dynamic recrystallization and development of crystallographic preferred orientation (CPO). However, the effects of annealing via static recrystallization on grain size and CPO have been largely overlooked. We investigated mantle annealing by analyzing a suite of kimberlite‐hosted garnet peridotite xenoliths from the Wyoming Craton. We focus on five xenoliths that show microstructures reflecting different degrees of recrystallization, with annealed grains characterized by distinctive faceted boundaries crosscutting surrounding, nonfaceted matrix grains. These textures are indicative of discontinuous static recrystallization (DiSRX). Electron backscatter diffraction analysis further demonstrates a ∼10°–20° misorientation between DiSRXed grains and the matrix grains, resulting in an overall weaker CPO. These characteristics are remarkably similar to microstructures observed in samples that were annealed after deformation in the laboratory. Measurements of the thermal conditions and water contents associated with the last equilibration of the xenoliths suggests that high homologous temperatures (T/Tm > 0.9) are necessary to induce DiSRX. We postulate that annealing through DiSRX occurs under high temperatures after a short episode of intense deformation (years to hundreds of years) with timescales for annealing estimated as weeks to years, significantly slower than the timescale of hours expected for a kimberlitic magma ascent. We conclude that microstructural transformation due to DiSRX will occur during transient heating events associated with mantle upwelling, plumes, and lithospheric thinning.
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- Award ID(s):
- 1719208
- PAR ID:
- 10453233
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 22
- Issue:
- 3
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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