GP004-03
Effects of Artificial Aging and Hydration on Magnetic Mineralogy in Volcanic Glasses

Monday, 14 December 2020: 08:45
Virtual
Julie A Bowles, University of Wisconsin Milwaukee, Geosciences, Milwaukee, WI, United States, Sebastian Simon Fearn, University of Wisconsin Milwaukee, Department of Geosciences, Milwaukee, WI, United States, Fatimah Abdulghafur, Macquarie University, Department of Geosciences, Sydney, Australia; University of Wisconsin Milwaukee, Geosciences, Milwaukee, United States and Julia E Hammer, University of Hawaii at Manoa, Honolulu, HI, United States
Recorded Presentation
Abstract:
Young volcanic glass can be been seen as an attractive target for paleointensity studies because it contains very fine-grained (titano)magnetite (TM) that is shielded from alteration by the glass. How this meta-stable material behaves over geologic time is not well understood, however. To explore the effects of aging and hydration on magnetic mineralogy, we undertook a series of artificial aging and hydration experiments on fresh, unhydrated glass of basaltic and rhyolitic composition. The fresh basaltic glass has a mixture of single-domain and superparamagnetic TM with variable Ti content, while the rhyolitic glass pseudo-single-domain low-Ti TM.

The aging experiments were intended to simulate the effects of structural relaxation and possible devitrification under anhydrous conditions over millions of years. Samples were annealed in air between 200°C and 400°C for up to 240 days. Results demonstrate that with increasing anneal time and temperature, a saturation isothermal remanence (sIRM) increases by up to ~40% and the coercivity of remanence (Bcr) decreases by ~15% (rhyolitic) and ~30% (basaltic). The effects are greater in basaltic glass, which we interpret to arise from the lower surface area:volume ratio of the starting TM crystals.

The hydration experiments were conducted at elevated pressure (200 MPa) and temperature (300 – 450 °C) for up to 15 days. Compared to the aging results, patterns with respect to temperature and time are less clear. Basaltic glass shows large but non-systematic changes in both sIRM (-70% to +180%) and Bcr (-40% to +125%), while the rhyolitic glass shows significantly smaller changes (+/- 20%). This may be partially explained by greater heterogeneity in the starting materials compared to the aging experiments. Visual inspection of the hydrated samples also shows intra- and inter-sample heterogeneity in the degree of alteration.

Overall, aging results are consistent with the coarsening of pre-existing titanomagnetite particles, while the hydration results reflect a more complicated alteration process that may include both destruction and growth of pre-existing particles and/or the nucleation of new particles. In-progress electron microscopy, Fourier-transform infrared spectroscopy, and calorimetry work will aid in interpretation and in making ties to natural samples.

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