Magnetic versus crystallographic fabrics in a basaltic lava flow

Abstract

Combined anisotropy of magnetic susceptibility (AMS) and crystallographic studies were performed within a quaternary lava flow, for which we have a thorough knowledge of the flow direction. Microscope examinations, microprobe analyses and studies of rock magnetic properties show that AMS is carried by homogeneous multidomain Ti-rich titanomagnetite ( x≈0.6). This mineral, forming either subhedral isolate individuals or grouped in various elongated aggregates, represents a late crystallized interstitial phase. AMS determined from samples collected in the lower, the middle and the upper part of the flow is characterized by a well-defined magnetic foliation whereas magnetic lineation is generally poorly grouped and therefore meaningless. The foliation plane of the lower and upper part of the flow show an obliquity, which is interpreted as an opposite imbrication indicating a westward flow in agreements with the known flow direction. In the lower part of the lava flow, the tight correlation between AMS and plagioclase lattice preferred orientation suggests that a silicate framework control the titanomagnetite crystallization and thus the orientation of the AMS ellipsoid.