Emplacement dynamics of Banco Bonito lava at Valles Caldera, NM inferred from volatile content and thermal history

Abstract:

Unexpected emplacement behaviors for silicic lavas were first observed during the most recent effusive eruption of Puyehue-Cordón Caulle (2011-2013). The rhyolitic lava was seen inflating up to 40 m and advancing largely via break-out lobes, however, it is unclear if these behaviors are unique to Cordón Caulle or common to all silicic lavas. Lava flow emplacement dynamics are controlled by the rheology of the lava. Apparent viscosities of silicate melts are a function of temperature (T), composition (X), and texture in the form of bubble and crystal content. To constrain the rheology requires understanding the complex evolution of these variables as a function of time (t) and space. Here, we constrain the emplacement of Banco Bonito, a ~150-m-thick, rhyolitic obsidian lava with a volume of ~4 km³ that erupted from Valles Caldera, NM ~67 ka. We characterized volatile content (X), thermal history (T-t), and glass texture for a suite of samples collected from the Banco Bonito drill core, which provide a high-resolution one-dimensional view of the entire thickness of the lava. We used relaxational geospeedometry and Fourier-transform infrared spectroscopy (FTIR) to constrain the natural cooling rates and volatile content as a function of depth through Banco Bonito. H₂O_tot values (3570 cm⁻¹) are below anticipated solubility limits at relevant pressures and temperatures, and increase as a function of depth. Cooling rates of the lava are fastest at the top and bottom of the flow (~10⁻¹ K s⁻¹) and are slowest just above and below the rhyolite’s crystalline core (~10⁻⁹ K s⁻¹). When compared to a one-dimensional conductive cooling model using temperature-dependent thermal properties, the experimentally constrained natural cooling rates are 2 to 5 orders of magnitude faster than expected at the top and bottom, and 3 to 4 orders of magnitude slower than expected in the flow interior. Therefore, the thermal history cannot be described by conduction alone, but with the release of latent heat of crystallization in the flow interior slowing cooling and convective heat loss at the surface accelerating cooling. In contrast to Cordón Caulle, our results suggest that Banco Bonito advanced as a single unit in a tank-tread fashion where blocks tumble down the flow front to be overridden as the flow advanced.