Fabric transition of olivine as the cause for an anisotropic seismic discontinuity in the mantle of the northern Slave craton, Canada

Abstract

The olivine fabric is commonly used to infer mantle flow directions from seismic anisotropy. Despite this, the response of olivine fabric to variations in high pressure, stress, olivine water content—and its subsequent impact on seismic anisotropy of the upper mantle — remains ambiguous. Here we studied 27 peridotite xenoliths from the adjacent Jericho and Muskox kimberlite pipes in the northern Slave craton (Canada) to correlate the olivine fabric with the seismic architecture of the lithospheric mantle. The xenoliths from depths of 70–190 km were analyzed for water content of olivine, and a subset of these samples were examined for the crystallographic preferred orientation of olivine and pyroxenes, major element compositions of minerals, and trace element compositions of garnet and clinopyroxene. Water content of olivine increases with depth, correlating with a decrease in the olivine Mg# (molar Mg/(Mg + Fe)) and increasing clinopyroxene modes. The dominant axial [010] olivine fabric coexists with two distinct fabrics above and below 120 km. These auxiliary fabrics transition from the A-type to the B-type, a change likely driven by increased pressure and the metasomatically-controlled high water content. This offers an explanation for an anisotropic seismic discontinuity observed at 120 km in the Northern Slave. The upper layer is defined by the preferred alignment of the fast [100] olivine axes along the N-S direction of the subhorizontal foliation plane, whereas in the lower layer the olivine [100] axes align parallel to the NE-SW direction. The discontinuity reflects the long, incremental formation process of the cratonic lithosphere that juxtaposes segments with distinct fossil flow directions, as evidenced by distinct fabric orientations. The upper layer may represent the older, depleted peridotites formed during the Mesoarchean craton stabilization, concurrently with the overlying crust. The deeper Proterozoic layer may have been added through accretion, subduction, plume-related melting, and modified by metasomatic events.