Micromagnetic, rockmagnetic and mineralogical studies on Dacitic Pumice from the Pinatubo Eruption (1991, Phillipines) Showing self‐reversed TRM

Abstract

Detailed micromagnetic, rockmagnetic and mineralogical investigations on dacitic pumice from the eruption of Mt. Pinatubo (1991, Phillipines) showing reversed NRM/TRM are presented. Two intergrown hemoilmenite‐phases in chemically zoned particles were detected as being responsible: a rim‐(or covering) phase consisting of a weak‐ferromagnetic (disordered) hemoilmenite phase (low saturation magnetization but magnetically hard like hematite, FeTiO3 content 53–57 mol percent) and a core‐phase consisting of a ferrimagnetic (ordered) hemoilmenite phase (high saturation magnetization but magnetically softer, constant FeTiO3 content ≈ 58 mol percent). Preliminary studies were carried out in order to elucidate the magnetic interactions (possibly exchange coupling) responsible for the self‐reversal of the NRM/TRM. A preliminary and schematic model is presented which summarizes the main points of acquisition of a self‐reversed TRM according to our results.