A Detailed Opaque Petrological and Magnetic Investigation of a Single Tertiary Lava Flow from Skye, Scotland—I

Summary The iron‐titanium oxides of 45 samples from a single tertiary lava flow from Skye, Scotland, have been subjected to detailed examination and quantitative measurement. A basically simple assemblage of titanomagnetite (93‐100 per cent) and discrete ilmenite (0‐7 per cent) has, in most samples experienced one or both of two types of alteration, titanomagnetite high‐temperature oxidation and ‘titanomagnetite granulation’, which we define in this paper. Titanomagnetite high‐temperature oxidation, describing the process of ilmenite exsolution and further oxidation, varies rapidly but largely systematically within the flow. Parallel alteration of discrete ilmenite and olivine also occurs. Titanomagnetite granulation, describing the alteration of titanomagnetite to impure rutile granules, only affects samples with limited development of high‐temperature oxidation and varies throughout the lava in a rather unsystematic fashion. While high‐temperature oxidation is almost certainly wholly of deuteric origin titanomagnetite granulation is shown usually to require burial to greater than 900 m for its formation, and thus may not take place until several million years after the extrusion of a lava flow. The mechanisms by which these two types of alteration take place, and their implications for palaeomagnetic interpretation, are discussed.