Abstract
One model for formation of obsidian pyroclasts suggests that they form
through sintering of ash particles on volcanic conduit walls, which are
subsequently torn out and entrained in the gas-particle dispersion out
of the erupting vent. Here, we investigate microlite abundances and
textures in obsidian pyroclasts in order to determine the time required
to produce adequate numbers of microlites, and hence the pyroclasts
themselves. We measured microlite number densities (MNDs) and microlite
and vesicle orientations in obsidian pyroclasts in tephra deposits from
the 1340 A.D. North Mono eruption. MNDs increase with decreasing
dissolved H2O concentrations. Also, microlite spatial orientations
become less aligned and differ more from vesicle orientations with
decreasing dissolved H2O concentrations. MNDs increase from the second
layer (P2) through the final layer (P10). To investigate timescales
required to replicate MNDs in the North Mono obsidian, we performed
time, temperature and pressure-controlled experiments with rhyolitic
glass from the same eruption. MNDs in our experiments initially increase
with decreasing pressure (50-35 MPa), then decrease as pressure
decreases further(35-10 MPa). MNDs in obsidian from layers P2-P10 were
replicated in ~7 hours or less. Based on these
observations we propose a model where during the initial phase of the
North Mono eruption most obsidian formed close to the magmatic
fragmentation depth, equilibrated for short time periods (< 7
hours) and were then erupted out of the volcanic vent. These obsidian
clasts have lower MNDs than subsequent phases, and microlites are well
aligned with each other and with vesicles, reflecting their short
residence time in the conduit, higher dissolved H2O contents, and lower
viscosities. During later phases of the North Mono eruption obsidian
formed at various depths in the conduit, equilibrating for longer
periods of time (≤ ~7 hours) before being erupted out of
the vent or sintering together with other clasts and equilibrating at
shallower depths before being erupted. These obsidian clasts have higher
MNDs than earlier phases of the eruption, and microlites are not well
aligned with each other or with vesicles, reflecting their variable
residence times in the volcanic vent, lower dissolved H2O contents, and
higher viscosities.