Olivine Slip Systems in Isua Ultramafic Rocks Do Not Require Earths Oldest Plate Tectonics

Whether the ~3.83.7 Ga Isua supracrustal belt (SW Greenland) represents Earths oldest plate tectonic subduction remains an open question. Evidence in support of subduction includes type-B olivine crystallographic preferred orientation (CPO) pattern found in some Isua dunites (i.e., the olivine [001] axes maximum parallel to lineation, the olivine [010] axes maximum normal to foliation). This particular CPO pattern is commonly found in mantle wedge of subduction settings, where the water-rich, high-stress condition facilitates activation of the olivine [001](010) slip system via dislocation creep. However, olivine CPO patterns are not only a function of slip systems activated via dislocation creep. Additional controlling factors include olivine shape preferred orientations and other deformation mechanisms (e.g., grain boundary sliding and diffusion creep). Indeed, the type-B olivine CPO pattern has been identified in rocks that experienced various conditions, including dry, shallow crustal intrusions and water-poor mylonites under high shear stress. Accordingly, a type-B olivine CPO pattern alone does not require mantle wedge deformation. To better constrain the deformation conditions of dunites from the Isua suprarustal belt, we examined the dominant slip systems recorded in olivine of three Isua ultramafic samples via electron backscatter diffraction techniques. The subgrain boundaries of analyzed olivine (identified by 210 misorientations) record misorientation axes concentrated along [001], [0kl] or [010] directions. This indicates that dominant dislocation creep in olivine was accomplished by [100](010) (type-A), [001](100) (type-C), and/or [100](001) (type-E) slip systems. In contrast, a [100] maximum of misorientation axes (corresponding to type-B slip) is absent in all three samples. We conclude that the type-B olivine slip system was not favorably activated during deformation of these Isua dunites, and plate tectonic subduction is not required. Alternatively, the observed dominant slip systems for these Isua dunites match those of weakly deformed, shallow-crust ultramafic intrusions. Hence, microstructures of dunites from the Isua supracrustal belt may be formed under crustal conditions, consistent with non-plate tectonic origins for this Eoarchean terrane.