Micron to sub-micron sized ferromagnetic inclusions in rock forming silicate minerals may give rise to particularly stable remanent magnetizations. When a population of inclusions have a preferred crystallographic or shape orientation in a rock, the recorded paleomagnetic direction and intensity may be biased by magnetic anisotropy. To better understand this effect, we investigated plagioclase grains from oceanic gabbro dredged from the Mid-Atlantic Ridge at 11-17N. The plagioclase grains contain abundant needle and lath shaped magnetite inclusions aligned along specific directions of the plagioclase lattice. Electron back scatter diffraction and anisotropy of magnetic remanence measurements are used to correlate the orientation distribution of the magnetite inclusions in the host plagioclase that contains multiple twin types (Manebach, Carlsbad, Albite and Pericline) with the bulk magnetic anisotropy of the inclusion-host assembly. In unaltered plagioclase, the anisotropy ellipsoid of magnetic remanence has oblate shapes that parallels the plagioclase (010) plane. It is suggested that recrystallization of magnetite inclusions from hydrothermal alteration shifts the relative abundance of the inclusions pertaining to the different orientation classes. We show that the maximum axis of the anisotropy ellipsoid of magnetic remanence parallels the plagioclase [001] direction, which in turn controls the recorded remanent magnetization direction. Our results are relevant for paleointensity and paleodirection determinations and for the interpretation of magnetic fabrics.

We report palaeomagnetic and K-Ar geochronologic results of two volcanic sections from Ethiopia. One section, dated around 29-30 Ma and spanning ~1 km in thickness, is related to the Oligocene Afro-Arabian traps. The second ~700-m-thick section was emplaced during the Miocene in two pulses around 10-11 and 14-15 Ma. We sampled 67 flows (550 cores) of predominantly basaltic rocks at the Oligocene section and 59 rhyolitic to trachybasaltic flows (500 cores) at the Miocene section. The Oligocene section was correlated to subchrons C11r to C11n.1n, with an average emplacement rate of 1m/kyr and instantaneous rates increasing with time from ~0.5 m/kyr near the base to ~1.36 m/kyr towards the top. We combined our results to the available paleomagnetic studies for the Early Oligocene (N = 4; 167 sites), Middle Miocene (N = 2; 125 sites) and Plio-Pleistocene (N = 8; 249 sites) to better understand how geomagnetic secular variation changed through time in the Afro-Arabian region. Recentred directional distributions for all three periods are elongated in the meridian plane (e = 2.7 ± 0.4, 1.8 ± 0.6, 2.3 ± 0.4, respectively), in coherence with field models for a dipole-dominated field. The angular dispersion S of the virtual geomagnetic poles, representative of the vigour of the palaeosecular variation, was higher during the Early Oligocene (S=14.2°|13.2°15.4°) and the Middle Miocene (S=15.0°|13.8°16.5°) than during the Plio-Pleistocene (S=9.7°|9.0°10.5°). As the reversal frequency f during the Early Oligocene is half that for the Plio-Pleistocene, it appears that S and f are uncorrelated in this near-equatorial region.