Evaluating Hydration and Artificial Aging Effects on the Paleointensity in Natural Glass

Abstract

Young natural volcanic glasses have been successfully used to recover Earth’s geomagnetic field intensity (paleointensity). However, the magnetic stability and reliability of volcanic glass as a paleomagnetic recorder over geologic time is unclear. Paleointensity estimates may be influenced by natural processes that alter magnetic mineralogy. Previous results from paleointensity and rock magnetic experiments suggest that post-emplacement hydrothermal alteration can alter the magnetic remanence and can possibly cause paleointensity experiments to fail. Low-temperature hydration and natural relaxation of the glass structure over time may also adversely impact paleointensity results. In this study, rhyolitic and basaltic glass specimens underwent artificial aging and artificial hydration treatments to observe how the magnetic mineralogy and resulting magnetic properties are affected. The fresh rhyolitic glass contained pseudo-single-domain to multidomain low-Ti titanomagnetite, and basaltic glass contained single-domain and superparamagnetic grains of medium to low-Ti titanomagnetite. Artificial aging took place by heating in air at 200-400°C under anhydrous conditions for up to 240 days. Hydration was induced at 200 MPa pressure with elevated temperatures of 300℃ and 450℃ at different time intervals. Before and after aging or hydration, samples underwent experimental procedures to assess the impact of the aging or hydration treatments on magnetic mineralogy and behavior during paleointensity experiments. Aged samples were subject to a modified Thellier-Thellier paleointensity experiment, isothermal remanent magnetization (IRM) acquisition experiments, hysteresis and first order reversal curve (FORC) experiments, and thermal demagnetization of a three-component IRM. Hydrated samples were subject to hysteresis and FORC experiments, and IRM acquisition experiments. IRM acquisition experiments on artificially aged samples showed increases in saturation IRM and a decrease in coercivity in both rhyolitic and basaltic glass specimens. These trends in magnetic properties are believed to have arisen from a growth of existing grains within the basaltic and rhyolitic glasses. Paleointensity experiments showed that with increased aging temperature, basaltic glasses experience more alteration during paleointensity laboratory reheating experiments. This is not seen in rhyolitic glasses. Hydration experiments resulted in inconsistent changes in coercivity and magnetization over treatment. Changes in coercivity and magnetization in basaltic glasses were much greater than rhyolitic glasses. These changes may be explained by magnetic grain growth, loss of material, select dissolution of the finest magnetic grains, and possible oxidation in basaltic glass samples based on IRM experiments. Hydration rims appeared prominently in nearly all hydrated samples, with some rhyolitic glasses experiencing a hydrated interior while only one basaltic sample showed hydration within the interior. While young volcanic glass could be used as a good paleomagnetic recorder, results of this study suggest that older material might pose several problems. Older material could be hydrated, rehydrated, or have a change in the glass structure that results in a change in the magnetic mineral assemblage and therefore incorrect paleointensity and paleomagnetic data. It is recommended that the glass properties and hydration states of older glasses should be further studied before carrying out paleomagnetic studies.