CHARACTERIZATION OF MESOZOIC DIKES FROM MAURITANIA, WESTERN AFRICA AND IMPLICATIONS FOR THE COHERENCY OF THE AFRICAN CONTINENT
Detailed stepwise alternating field (AF) and thermal demagnetizations performed at the University of Rochester show linear decay to the origin after the removal of a low coercivity, low unblocking temperature overprint. Magnetic susceptibility measured using a KLY-3S Kappabridge reveal Curie points of 570-580°C, suggesting the presence of magnetite (Fe3O4). This is confirmed by the presence of a low temperature inflection point at ~ -153°C, which is characteristic of the cubic to monoclinic transition in magnetite known as the Verwey transition. Magnetic hysteresis loops measured with an Alternating Gradient Force Magnetometer (AGFM) show a range of domain states, with Mr/Ms values ranging from 0.05 to 0.33. The presence of nearly pure magnetite suggests high temperature oxidation upon initial cooling; we will test this hypothesis with SEM studies. Overall, we believe that the magnetic minerals in these rocks are high-resolution recorders of the Triassic-Jurassic magnetic field and thus can be used to study the coherency of the African continent during the rifting that formed the Atlantic Ocean.