Effects of Aging on Magnetic Mineralogy of Natural Volcanic Glass: Implications for Geomagnetic Paleointensity Recorders

Abstract Natural volcanic glasses are well represented in the geologic record, and typically contain near‐ideal single‐domain particles required for standard Thellier‐type absolute paleointensity experiments. Young (<∼50–100 ka) glasses have been demonstrated to reliably record Earth's magnetic field. However, it is unclear how the magnetic mineralogy and magnetization might change with age as the metastable glass structure relaxes. Here, we attempt to systematically address issues surrounding glass relaxation and devitrification. We subjected a set of natural basaltic and rhyolitic glasses to controlled annealing experiments at temperatures between 200°C and 400°C and assessed how the magnetic properties and glass structure (as assessed by the glass transition temperature, T g ) change over time. We compare the results to bulk magnetic properties and T g for a suite of volcanic glasses spanning over seven orders of magnitude in age. Annealed samples show an increase in isothermal remanent magnetization acquisition, a decrease in coercivity, and basaltic samples show an increase in unblocking temperatures. The results are consistent with a coarsening of pre‐existing magnetic particles rather than precipitation of new oxides. The natural data are more difficult to interpret, but trends in average parameters are consistent with a coarsening of magnetic particles in some—but not all—samples with age, and this appears to be accompanied by a reduction in T g . While the annealing experiments take place under many different thermodynamic conditions compared to naturally aged samples, we suggest caution when using geologically older glasses for paleointensity analyses.