A Correlation Between Remanent Magnetism and Petrological and Chemical Properties of Tertiary Basalt Lavas from Mull, Scotland.

Summary. A description is given of a petrological examination of a set of normally and reversely magnetized samples from tertiary basalt lava flows on the Isle of Mull. The collected evidence suggests that the normally magnetized samples are most likely to retain part of their original thermo‐remanence. The reversely magnetized samples probably acquired their magnetization during a diffusion process in the opaque minerals which took place at an undetermined time after the consolidation of the lavas. The distribution of four features is found to correlate with the two senses of natural remanent magnetization. These features are the alteration products of decomposed olivine, the grain size and crystal form and the internal nature of the primary opaque minerals‐especially exsolution of ilmenite from titanomagnetite. Normally magnetized samples are found to contain olivine replaced only by serpentinous minerals together with relatively small well‐crystallized, optically‐homogeneous (unexsolved) primary magnetite grains. On the other hand, reversely magnetized samples are found to contain olivine replaced by serpentine, secondary iron oxides and a red hydrated iron oxide (iddingsite). In addition, the primary magnetite grains in this group are relatively large, show poor crystal form and are typified by varying amounts of ilmenite exsolved as lamellae in the octahedral planes of the magnetite.