PSEUDOTACHYLITE AS A MAGNETIC RECORDER OF EARTH'S AMBIENT FIELD

Pseudotachylites are frictionally melt-quenched products during high speed co-seismic faulting, during frictional heating along oblique impact surfaces inside meteorites, and due to impact cratering on the Earth. Experimental frictional melting of magnetite-free granites produces dispersed submicron-sized inclusions of magnetite by the oxidation of Fe in melt-susceptible mafic minerals, in artificial pseudotachylites (Nakamura et al., 2002). This result documented that they acquire a stable thermal remanence in submicron magnetites during the rapid cooling of the melt. Therefore, it seems that they hold great potential for paleointensity determinations of contemporaneous magnetic fields during seismic faulting or impact cratering events. However there has been few attempt to determine the paleointensity from pseudotachylites. Here, we present Thellier paleointensity results of post-impact pseudotachylites from the North-range of the 1.85Ga Sudbury impact structure, Canada. They formed in large displacement fault systems during gravitational collapse of the impact-generated transient cavity (Thompson & Spray, 1996). These are 1-8m wide veins in thickness, so that we estimated from an analytical heat conduction calculation that they might have cooled in a several months. Curie temperature determination and scanning electron microscope studies of the pseudotachylites indicate a low-Ti magnetite as the carrier of the remanence. Preliminary determinations from three pseudotachylites meet reliability criteria and yield a virtual dipole moment of 3.48*10²²(am²) which is in half of the present field. This value is supported by a low paleointensity derived from impact melt sheet in the Sudbury Igneous Complex (Schwarz & Symons, 1970). Therefore, pseudotachylite with the cooling rate could acquire an Earth's ambient field at 1.85Ga impact event, although the cooling rate is too slow to record a transient magnetic field, such as a current-induced magnetic field during seismic faulting or impact cratering events.