| Autor: |
Kenderes, S. M., Befus, K. S., Bryson, A. N., Whittington, A. G. |
| Předmět: |
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| Zdroj: |
Bulletin of Volcanology; Nov2022, Vol. 84 Issue 11, p1-18, 18p |
| Abstrakt: |
The emplacement behaviors and the associated hazards of silicic lavas remain poorly understood because detailed real-time observations have only been made during a few modern eruptions. Therefore, we rely on older well-preserved glassy rhyolites like Banco Bonito and the VC-1 Rhyolite from the Valles caldera, New Mexico to constrain the emplacement behaviors of silicic lavas. Continuous whole cores, collected from both units during drilling in 1984 by the Continental Scientific Drilling Program, provide the unique opportunity to assess the rheological evolution of the interior of obsidian lavas. Banco Bonito is a large (0.9 km3; up to 150 m thick) obsidian lava coulée. The VC-1 Rhyolite is a relatively thin (~ 20 m) obsidian rhyolite that is not exposed at the surface and has an unclear emplacement history. Evidence for the emplacement dynamics of these two units is preserved as trends in volatile content and thermal history through the thicknesses of the rhyolite obsidians. We acquired 46 glassy samples from the VC-1 drill core through the entire thicknesses of both Banco Bonito and the VC-1 Rhyolite. We measured Archimedean densities, glass water concentrations using Fourier transform infrared spectroscopy (FTIR), and natural cooling rates using relaxation geospeedometry as a function of depth for both rhyolite obsidians. Systematic trends in both datasets provide evidence of the emplacement style of Banco Bonito and the VC-1 Rhyolite in the vicinity of the VC-1 drill hole. Total water contents range from 0.03 ± 0.01 up to 6.93 ± 1.09 wt. % for Banco Bonito, and range from 0.24 ± 0.06 to 1.08 ± 0.05 wt. % for the VC-1 Rhyolite. Both rhyolite obsidians display evidence of having been partially rehydrated. Cooling rates are fastest near the top and bottom of Banco Bonito (up to tens of °C s−1) and are slowest in the center of the flow (< 1 °C year−1). The VC-1 Rhyolite has fast cooling near the bottom of the flow (tens of °C s−1), but top and interior of the flow cooled slowly (< 1 °C year−1). We find that Banco Bonito was likely emplaced in an exogenous or "tank-tread" style because primary magmatic water (OH−) concentrations are at or below predicted solubility values for a given depth and eruption temperature, and measured cooling rates resemble values predicted by a conductive cooling model of a single flow unit. We estimate an emplacement timescale for Banco Bonito of < 3 months by combining a conductive cooling model, estimated glass transition temperatures using dissolved OH− concentrations, and a crustal thickness relationship used on active flows. We conclude that the VC-1 Rhyolite was likely emplaced as a lava that has been partially eroded based on stratigraphic evidence and the absence of fast cooling near the current top of the unit. [ABSTRACT FROM AUTHOR] |
| Databáze: |
Complementary Index |
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