The magnetic anisotropy of mantle peridotites: Example from the Twin Sisters dunite, Washington

The Twin Sisters dunite massif, Washington State, provides unweathered peridotites for magnetic analysis. These rocks have been chosen to test a new magnetic fabric based on measuring the anisotropy of magnetic susceptibility (AMS) in high-field with a vibrating sample magnetometer. Although the dunites are fresh, they host small quantities of secondary magnetite formed along cracks. Under low-field conditions magnetite masks the magnetic fabric of olivine, and thus the magnetic fabric does not reflect mantle flow deformation. Above the magnetic saturation, at a field of 1 T, the ferrimagnetic component is removed and the high-field slope represents the paramagnetic AMS only. This high-field AMS originates from the magnetocrystalline anisotropy of olivine. The high-field AMS are compared with lattice preferred orientation (LPO) fabric data obtained by electron backscatter diffraction. LPO point distributions are formed by high-temperature plastic flow, and match the high-field measurements. In contrast, girdle LPOs present notable departures from the high-field AMS axes.