Petromagnetic and microprobe studies of rocks in the Sofu underwater volcanic cluster, Izu-Bonin island arc, Pacific Ocean

Abstract

A multidisciplinary study of samples that were dredged during cruises of the R/V Vulkanolog was continued in order to interpret results from a survey of the anomalous magnetic field on the Pacific seafloor. We acquired data on the main ferromagnetic carrier and the nature of magnetization. The naturally occurring magnetic parameters and petromagnetic properties of the rocks that make up the edifices of the Sofu underwater volcanic cluster at the Izu-Bonin island arc have been studied. We provide the results of thermomagnetic, petrologic, and microprobe studies of the dredged samples. It is shown that the main magnetic mineral carriers of natural remanent magnetization are unaltered titanomagnetite and oxidized titanomagnetite. The differentiation of rocks over values of natural remanent magnetization was due to different crystallization conditions for titanomagnetite, while that over magnetic susceptibility was controlled by the magnetic grain size. The structural, mineralogic, and petrographic features of the samples show that they were taken from rocks that were discharged during different phases of eruptions.