Extreme variability of deformation mechanisms at tiny scale in natural peridotite: the effect of phase distribution and mineral reactions

Abstract:

What is the role of mineral distribution / proportion and fluid during mantle exhumation in an extensional context such as an ocean-continent transition? A microstructural study is undertaken on inherited sub-continental mantle at some distance to the continent. We sampled peridotite containing discrete mylonitic shear-zones away (~400 m) from the exhumation surface and mylonitic peridotite found as clasts in a cataclasite at the top of the exhumed mantle exposed at the paleo-seafloor.

The mylonitic blocks in the cataclasite are formed by unmixed areas of olivine (ol) - orthopyroxene (opx) or clinopyroxene (cpx) - amphibole (amph). The amph has all crystallographic axes parallel to cpx axes with [001] axes perpendicular to the foliation plane suggesting epitaxial growth without stress. However, in the ol – opx areas, olivine has [001] axes parallel to the lineation and [010] axes perpendicular to the foliation plane suggesting [001] slip. This mylonite is probably the record of a deep and early deformation before fluid percolation.

The peridotite is amph bearing spinel lherzolite and displays a pervasive high temperature foliation. The peridotite contains ultra-mylonitic zones formed by elongated cpx and ol grains (1:10 aspect ratio) forming ribbons and sliding planes. Olivine [100] axes are parallel to the lineation and [010] axes perpendicular to the foliation plane suggesting [100] slip. However, [100]_cpx axes are parallel to the lineation and [100]_ol axes, while [010]_cpxaxes are perpendicular to the foliation plane. The shape, size and crystallographic preferred orientations of ol and cpx suggest the interplay between diffusion creep, grain boundary sliding and mineral precipitation. The discrete mylonitic shear zones cutting the peridotite are 1cm to 10cm-thick and represent an extreme localization of the deformation. They contain a mixed matrix of ol, pyroxenes, spinel and amph with grains up to 15 µm in size with round shapes. The ol shows a strong concentration of [010] axes perpendicular to foliation plane with other axes forming girdles within the foliation plane suggesting axial [010] deformation.

During mantle exhumation the mineral distribution and proportion, the fluid percolation and the mineralogical reaction lead to the activation of variable deformation mechanisms at mm or cm scale.