Mantle pyroxenites: deformation and seismic properties

The physical properties of peridotite are approximated by those of olivine for most mantle models. However, significant volumes of pyroxene-rich lithologies suggest a rheologically heterogeneous upper mantle, and thus interpretations of upper mantle properties that provide constraints to model large-scale processes, may be inaccurate. This study endeavours to better constrain the physical properties of pyroxenites by characterising their microstructure and by assessing different textural states at different stages of microstructural evolution in the "subduction factory".For that purpose and for consistency of methodology, I developed an automated script to process quantified microstructural data. I then applied this to two contrasting peridotite and ophiolite massifs bearing large volumes of pyroxenites.In the Cabo Ortegal complex, Spain, dunites and layered pyroxenites crop out in what is interpreted as a preserved paleo-arc root that experienced deformation in the deep parts of a subducting slab. Investigation of pyroxenite microstructures reveals that they all reacted to deformation in a similar way, independent of composition. Petrofabrics suggest that layered pyroxenites can preserve structures and textures inherited from high-temperature deformation, could help to localize shearing, and may represent preferred pathways for later fluid percolation.In the Trinity ophiolite, California, USA, well-preserved outcrops of mantle and crustal lithologies were studied. Microstructures from the mantle domains of the ophiolite reveal that peridotites and pyroxenites recorded high temperature plastic deformation involving melt percolation. In the crustal section, cumulate pyroxenites occur in the Bear Creek magma chamber. The microstructures of layered cumulate pyroxenites from the crustal section have a strong planar fabric with a weak linear component and likely formed by compaction and crystal-settling. Microstructural data from the Cabo Ortegal complex and the Trinity ophiolite are then used to compute models of seismic properties for a layered pyroxenite-rich domain. The effect of compositional layering on seismic properties was incorporated into the models to produce the most geologically realistic results. The models suggest that layered pyroxeniterich domains should induce distinctive signal deviations relative to peridotite domains devoid of pyroxenites. Using these data from Cabo Ortegal and Trinity, I investigated the relative influence of the textural state and the modal abundance on the seismic properties. The models suggest that plastically deformed and poorly-oriented pyroxenites have comparable seismic properties whereas pyroxenites affected by compaction would yield the most anisotropic seismic signal.This quantification of pyroxenite microstructures with implications for the rheology and seismicity, is a novel step towards a better understanding of the role of heterogeneities in the geodynamic behaviour of the upper mantle.