Collaborative Research: Origin of Magnetite and Magnetic Remanence in Submarine Basaltic Glass and Implications for Glass Paleointensities

点击翻译按钮获取中文摘要

Submarine basaltic glass is increasingly being used in studies of geomagnetic field intensity variation, yet the origin of the magnetic remanence in glass remains poorly understood, leading some to question the validity of paleointensity estimates from glass. To recover valid paleointensity estimates, the magnetic remanence in the glass must be a primary thermoremanence - i.e. magnetization must be locked in at high temperatures when the glass initially cools. If suggestions that the magnetization is instead a low-temperature chemical remanence are true, then paleofield estimates from basaltic glass might be invalid. This study is designed to elucidate the process by which (titano)magnetite forms in basaltic glass, as well as the conditions under which it forms and its fidelity as a recorder of geomagnetic field variations. Glass is synthesized under a range of conditions (oxygen fugacity, cooling rate, composition) and in a known magnetic field. The starting materials are either powdered natural tholeiitic basalt or simplified synthetic compositions prepared by mixing reagent-quality component oxides. Reheating oxidation diffusion experiments, combined with Rutherford backscattering spectrometry and analytical transmission electron microscopy, allow an exploration of the field of conditions under which and the process by which magnetite precipitates and grows in glass. The results have direction implications not only for the origin of magnetite in glass, but also for its stability during reheating paleointensity experiments. The microanalytical techniques also allow characterization of grain size and composition, which are complimented by magnetic characterization using a suite of rock magnetic techniques on both the synthetic glass samples, as well as natural glass samples of varying ages. In this way the resulting magnetic mineralogy and magnetic properties of the synthesized glasses are readily compared to those of natural glasses. Finally, paleointensity experiments provide a direct test of the ability of the synthetic glass samples to accurately record the ambient field intensity.