Indices of multidomain magnetic behavior in basic igneous rocks: Alternating-field demagnetization, hysteresis, and oxide petrology

Abstract

Multidomain type alternating-field (AF) demagnetization characteristics like those reported by Lowrie and Fuller for multidomain (MD) magnetite have been discovered in two rock types: (1) coarse-grained dioritic intrusives, and (2) continental basalts of low oxidation state. These are the first reported examples of MD type whole-rock AF characteristics. Mixed single-domain (SD) and MD type characteristics have been measured for two other rocks, a gabbro from the same area as the diorites and a basalt of oxidation index 3. Hysteresis properties of the diorites and gabbro (remanence ratio Jrs/Js 5) are consistent with predominantly MD remanence carriers, and examination of the oxide petrology suggests that these carriers are homogeneous titanomagnetite grains 200–600, μm in size. However, where a stable component of natural remanence can be isolated, the AF characteristics of this component are of SD type. Hysteresis properties of the basalts are less definitive than those of the intrusives but show a clear trend from low (0.127) to high (0.259) Jrs/Js values with increasing oxidation index, in agreement with the corresponding change from MD type to SD type AF characteristics. The MD remanence carriers appear to be 80–100 μm titanomagnetite grains. The Lambertville diabase and Michikamau anorthosite have an anomalous combination of MD hysteresis and SD type AF characteristics. This behavior is not understood but could conceivably prove to be a characteristic signature of rocks containing sub-microscopic magnetite within silicate minerals. Although the results reported here do not constitute a definitive test, the excellent correlation among grain size of the oxide fraction, overall domain state as indicated by hysteresis, and AF demagnetization results for most of the intrusive and volcanic rocks examined favors the view that the Lowrie-Fuller criterion can detect MD carriers of remanence in nature.