Experimental constraints on the textures and origin of obsidian pyroclasts

Abstract

Obsidian pyroclasts are commonly preserved in the fall deposits of explosive silicic eruptions. Recent work has suggested that they form by sintering of ash particles on the conduit walls above the fragmentation depth, and are subsequently torn out and transported in the gas-particle dispersion. Although the sintering hypothesis is consistent with the general vesicle textures and dissolved volatiles in obsidian pyroclasts, previous sintering experiments do not capture all of the textural complexities observed in the natural pyroclasts. Here, we design experiments in which unimodal and bimodal distributions of rhyolitic ash are sintered at temperatures and H2O pressures relevant to shallow volcanic conduits, and under variable cooling rates. The experiments produce dense, welded obsidian that have a range of textures similar to those observed in natural pyroclasts. We find that using a unimodal distribution of particles produces obsidian with evenly distributed trapped vesicles, while a bimodal initial particle distribution produces obsidian with domains of poorly vesicular glass among domains of more vesicle-rich glass. We also find that slow cooling leads to resorption of trapped vesicles, producing fully dense obsidian. These broad features match those found in obsidian pyroclasts from the North Mono (California, USA) rhyolite eruption, providing strong support to the hypothesis that obsidian can be produced by ash sintering above the fragmentation depth during explosive eruptions.

A general model for welding of ash particles in volcanic systems validated using in situ X-ray tomography

“…We note here that our assumption of spherical particles undergoing diffusion of volatiles may be invalid at large polydispersivity for cases where the smallest particles are in equilibrium while the largest particles are far from equilibrium. This is discussed elsewhere (Gardner et al 2019).…”

“…Similarly, the solubility of water increases as the particles cool, which may cause them to re-hydrate (McIntosh et al 2014;Ryan et al 2015). Mass diffusion of water in or out of particles may occur on timescales similar to the timescale of welding (Sparks et al 1999;Gardner et al 2017Gardner et al , 2018Gardner et al , 2019; consequently, we anticipate that diffusion of water can affect the rate of welding through its impact on melt viscosity.…”

“…Disequilibrium of volatile speciesparticularly of wateris important because the solubility changes as the pressure and temperature environment of the particles changes, driving diffusion in or out of the particles during welding (Sparks et al 1999;Gardner et al 2018). In the case of water, this has a strong impact on the viscosity of the particle (Hess and Dingwell 1996) affecting welding rate (Grunder et al 2005;Gardner et al 2018Gardner et al , 2019. Confining (or lithostatic) pressure resulting, for example, from the weight of aggrading particles at the base of a pyroclastic density current, is important because it provides a stress that pushes the droplets together, accelerating welding.…”

Vesicle shrinkage in hydrous phonolitic melt during cooling

“…Cooling rates in decompression experiments are usually much faster (up to 150 K s −1 ), but T and c H2O are significantly higher and melt viscosities are lower compared to the sintering experiments of Gardner et al (2019). Consequently, vesicle shrinkage is still expected during cooling of vesiculated melts (McIntosh et al 2014;Marxer et al 2015;Allabar and Nowak 2018).…”

“…Measured Φ glass and c H2Oglass therefore may not represent the molten state of the sample prior to cooling, especially when cooling rates are low. Slow cooling (~ 10 K•min −1 ) in sintering experiments using rhyolitic glass powder (T of 1023-823 K, P of 22 MPa and H 2 O concentrations up to ~ 2.2 wt%) in the presence of fluid leads to resorption of H 2 O vesicles resulting in fully dense obsidian (Gardner et al 2019).…”