A Detailed Opaque Petrological and Magnetic Investigation of a Single Tertiary Lava Flow from Skye, Scotland?II Spatial Variations of Magnetic Properties and Selected Relationships between Magnetic and Opaque Petrological Properties

Abstract

Summary Natural remanence, initial susceptibility, saturation magnetization and strong field thermal properties have been measured on 45 basalt cores representing two vertical sections through the flow separated horizontally by 16 m. The form and magnitude of the profiles for every magnetic property differ in the two sections, which means that lateral variations are quite comparable to vertical variations in this lava. Stability of rernanence, as determined by a.c. partial demagnetization, is found to be related to two opaque petrological properties. High stability is associated either with titanomagnetite high-temperature oxidation (ilmenite exsolution etc.) or with titanomagnetite granulation. Granulation consists of areas of titanomagnetite replaced by fine aggregates of rutile and is described in detail in Part I of this paper. A relationship exists between strong field heating curves and natural titanomagnetite high-temperature oxidation. The Curie point spectrum has two isolated peaks. As natural oxidation increases the relative intensity of the high Curie point component increases. The possibility of titanomagnetite granulation, believed to be formed several million years after lava eruption, causing remagnetization is discussed and rejected. 1. Magnetic properties measured Natural remanence direction and intensity, weak field susceptibility, saturation magnetization and strong field Curie points have been measured for all the samples described in Part I of this paper. With the exception of natural remanence directions, these properties together with derived parameters and opaque petrological properties are listed in Table 1. Mean natural remanence directions for 32 oriented samples at different a.c. fields are listed in Table 2. Natural remanence values were measured for each specimen at several a.c. fields using an astatic magnetometer. From the changes in intensity and direction of natural remanence during a.c. partial demagnetization to 200 peak oersteds a cumulative stability factor, S2,,,,, has been obtained. Definition of S has been given by Wilson, Haggerty & Watkins (1968). Briefly, a sample suffering change in neither