Interactions between deformation and reactive melt percolation in the upper mantle: simple-shear deformation experiment at high pressure and temperature

This study consists of a series of simple shear deformation experiment by using a 6-ram multi-anvil apparatus at 2 GPa and 1150 °C. The samples comprises a dunite aggregate mixed with 10% of Si-rich hydrous melt in chemical disequilibrium with the solid matrix. The samples were deformed at strain rates between 4x10^-5 and 7x10^-4 s^-1. The strain ranged from a γ of 0.3 to 1.5. The reaction between the melt and the olivine induces the precipitation of orthopyroxene and minor amounts of clinopyroxene. The analysis of the melt pockets topology and orthopyroxene shape preferred orientation displays a clear difference between their directions of elongation. The melt pockets are mostly elongated or may form bands orientated at 45 to 75° counterclockwise from the shear plane at senestral shear, i.e. 0 to 30° to the compression axis. The orthopyroxene single crystal and aggregate long axis is parallel to the lineation defined by olivine elongation, which is orientated at 0 to 45° clockwise to the shear plane at senestral shear. In these two direction of melt pockets and orthopyroxene elongation, narrow bands composed by very fine grains (10 times smaller than the average grain size) are developed. Reactive melt percolation may then constitute an efficient mechanism for strain localization in the mantle. Crystal preferred orientation (CPO) measurements on olivine show a maximum concentration of the [010] axes close to the normal of the shear plane. The [100] axes form a girdle close to the shear plane with one or two maxima concentrated normal or normal and parallel to the shear direction, respectively. The [001] axes display a maximum concentration, which becomes closer to the shear direction with the increasing of strain. Such CPO patterns may be explained by different processes: (1) deformation in the presence of high water contents that change the olivine dominant slip system; (2) a partitioning of the deformation between the narrow fine-grained bands elongated with the melt pockets and the orthopyroxenes, and the olivine matrix; and (3) olivine deformation dominated by diffusion creep, which is rate limited by interface reactions. Orthopyroxenes displays stronger CPO than olivines with the maximum concentration of the [001] axes close the shear direction, and a maximum concentration of the [100] and [010] axes close to the normal to the shear plane. This CPO pattern may result from the deformation of the orthopyroxene under relatively high water and Al contents.