Magnetic Properties of Single Crystal Clinopyroxenes: Anisotropy of Remanence

Magnetite exsolved from clinopyroxene in subsolidus cooling can have robust remanence characteristics. Exsolved from the host silicate during slow cooling, the magnetite is in the form of lath-shaped needles. With clinopyroxene as a crystal-chemical template, the magnetite needles are arranged in only two directions or arrays (termed X and Z, about 75° apart). The extreme elongation of each magnetite needle produces a dominating shape anisotropy that generates anamolously high coercivities and extremely long relaxation times. The dominance of shape anisotropy also produces a complete restriction in remanence directions. Each needle can have one of the two possible remanence directions, parallel or anti-parallel to its length. Each array can be expected to produce a remanence along its axis (e.g. +X or \m -X), while the presence of two arrays (as in CPX) should produce a remanence that is restricted to a plane (X-Z plane in CPX). Observations of remanence directions in single crystals of CPX show these specific linear and planar restrictions. This type of intracrystalline remanence control is different from the more commonly encountered distribution anisotropy, arising from a bias of preferred orientations in populations of magnetite crystals. The term anisotropy may not be wholly appropriate for the silicate-hosted, shape restricted remanence, which has only two bipolar states within a given CPX crystal. The term polarity would be appropriate, except for its preemptive use in describing the state of the global field. The term anisotropy will be used for now as a general term for the bipolar states of magnetite arrays. Anisotropy of IRM, Mr, Ms, and microscopic coercivity have been measured on single crystals of CPX (ca. 0.1 mg each) from gabbroic cone sheets of early Cretaceous age (Messum Complex, Namibia). IRM values have an abrupt reduction by a factor of 4 through a 30° zone in the X-Z plane. The hysteresis parameter Mr shows a more regular sinusoidal change (also by a factor of 4), reaching a minimum whenever the applied field is most perpendicular to the X and Z arrays. The saturation magnetization (Ms) is less variable, but shows 30% variations when the applied field is perpendicular to the X and Z arrays. Microscopic coercivity has been measured using the detailed hysteresis method of FORC analysis (First Order Reversal Curves). Each array of magnetites can be measured separately by applying the field perpendicular to the other array. Differences in the microcoercivity can be determined for each array, e.g. X=75-100mT while Z=85-125mT. Extreme anisotropy can be seen whenever the applied field is perpendicular to the plane of the arrays (Y direction), such that no microcoercivity population can be detected (up to 700mT) in the Y direction. For IRM, the remanence also appears to be null in the Y direction. The current challenge is to overcome these extreme restrictions in remanence anisotropy, while utilizing the equally extreme remanence stability to decode paleomagnetic field directions and intensities.