FeTi oxide mineralogy and the origin of normal and reverse remanent magnetization in dacitic pumice blocks from Mt. Shasta, California

Abstract

Detailed mineralogical analyses and rock magnetic experiments have made it possible to directly identify the FeTi oxide phases responsible for the normal and reverse magnetic components of two dacitic pumice blocks from Mt. Shasta, California. Both samples contain a normal component carried by 100 μm size multi-domain (MD) titanomagnetite (Usp 11–24). One sample also contains a second normal component carried by < 10 μm size pseudo-single domain (PSD) or single domain (SD) Ti-free magnetite (Usp 1) found in the dacitic glass. The MD titanomagnetite and PSD or SD magnetite dominate the strong field magnetic signal, but only the PSD or SD magnetite has any influence on the remanence signal. Unlike the strong field signal, the remanence signal of both samples is dominated by a reverse NRM component. This reverse component is carried by 100 μm size ferrian ilmenite (Ilm 53–65). The compositions of the ilmenites in both samples are within the range of compositions (Ilm 50–75) known to have the ability to acquire self-reversing thermoremanent magnetizations (TRM). The results of the Lowric-Fuller test indicate that the remanence signal is dominated by PSD or SD carriers. Because one sample contains only large MD titanomagnetite and no SD Ti-free magnetite (in addition to ferrian ilmenite), the ferrian ilmenite must be a PSD or SD carrier. Oxide and pyroxene geothermometry indicate the FeTi oxides in the pumice crystallized at temperatures between 880 and 945°C. This temperature range is within the disordered region of the ilmenite-hematite phase diagram for Ilm 53–65. Previous work on synthetic Ilm 70 and Ilm 80 has shown that cooling through the order-disorder transition into the ordered region develops a transformation-induced microstructure consisting of cation-ordered domains with disordered domain boundaries. An Ilm 58–59 grain from one of the Mt. Shasta samples was examined in the transmission electron microscope and was found to contain 100–200 Å diameter cation-ordered domains. These domains arose during cooling through the transition temperature, which is estimated at 800°C for Ilm 58–59. The presence of the disordered domain boundaries provides an explanation for the magnetic behavior of the ferrian ilmenite. (1) The disordered boundaries are the higher Curie point phase necessary for the operation of the self-reversal mechanism. (2) The disordered domain boundaries either inhibit the formation of magnetic domain walls or restrict magnetic domain wall movement accounting for the PSD or SD behavior of the ferrian ilmenite.