What do microlites tell us about obsidian pyroclasts?

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Introduction
Understanding magmatic degassing is critical to understanding volcanic eruptions.Obsidian pyroclasts record degassing, but how the clasts form must be understood in order to correctly interpret the information they contain.To be able to understand what is recorded in obsidian, we constrain timescales for the formation of obsidian pyroclasts from the ~1340 A.D. North Mono eruption, and we investigated their physical evolutions.

Microlite and vesicle alignments
Microlites and vesicles become progressively aligned with increasing strain.We compared standard deviations of trend (σφ) and plunge (σθ) of vesicles and microlites in individual obsidian clasts in an effort to better understand the phyical conditions of the environment that produced them; low σφ and σθ = better alignment.

Eruptive Model
Early in the eruptive sequence, obsidian pyroclasts sintered quicker and were mostly produced at greater depths.Higher dissolved H 2 O contents / lower viscosities allowed relaxation and efficient alignment of microlites and vesicles.In later phases of the eruptive sequence, obsidian pyroclasts took longer to sinter and were produced mostly at shallower depths.Particles were more viscous and they retained distorted vesicle shapes.Higher viscosities prevented microlites from aligning as efficiently.
Most obsidian pyroclasts in the North Mono eruption were produced in 7 hours or less.

Microlite Experiments
We investigated ranges of microlite number densities (MND) in obsidian pyroclasts from the North Mono eruption, and then conducted pressure, temperature and time-controlled microlite growth experiments in order to calibrate microlite growth and constrain obsidian sintering timescales.Pressure was hydrostatic (equal in all directions).Microlites in the experiments are randomly oriented.
Blue diamonds = layer P2, green triangles = layer P10.Yellow and red figures outline MND of layers P4 and P6, respectively.Pressures for obsidian pyroclasts calculated from coexisting H 2 O and CO 2 contents after Liu et al. (2005).Errors for pyroxene and feldspars are ~10%.Errors were calculated by counting microlites multiple times in multiple samples.

Most obsidian pyroclasts in the
of 3 bits of ash deformed vesicles into single alignment.Sample was erupted before microlites could fully align.σφ 104.9 σθ 18.2 σφ 114.3 σθ 21.9 Microlites (and vesicles) demonstrate sintering process Stratigraphic trends in obsidian clasts • MND increase with stratigraphic height.Therefore sintering times of obsidian pyroclasts are progressively longer from P2, through P4 and P6, to P10. • Percentage of obsidian pyroclasts with distorted-type vesicles (D, StrD) increases markedly with stratigraphic height.• Every layer contains obsidian pyroclasts with multiple variations of microlite and vesicle textures and alignments.Therefore there is no easily definable stratigraphic trend in microlite (or vesicle) alignments.Fragmentation front Higher MND = more sintering time Lower dissolved volatile content = more viscous Distorted type vesicles dominate shear Lower MND = less sintering time Higher dissolved volatile content = less viscous Non-distorted type vesicles dominate shear Obsidian productio n moves upward with time North Mono eruptive sequence were produced in 7 hours or less.Contour lines with numbers on top are isochrons that show experimental MND as a function of durations (in hours) of experiments at different pressures.the North Mono eruptive sequence tephra deposit at location marked on inset map.Schematic section is not to scale.
volatile contents decreases with stratigraphic height.Blue diamonds = layer P2, green triangles = layer P10.Yellow and red figures outline volatiles contents of layers P4 and P6, respectively.The average error of analyses is shown.(Modified fromGardner et al., 2017.)relative percentage of distorted type vesicles

In samples with mainly S-type vesicles, microlites are poorly aligned (high σφ) with each other and with vesicles. In samples with mainly RE or STE-type vesicles, microlites are generally well aligned (low σφ) with each other and with vesicles. In samples with mainly D or StrD-type vesicles, microlites are variably aligned with each other and with vesicles.
Vesicles MicrolitesSample has three distinct textural regions.Vesicles (mostly STE) in each region are all strongly aligned with each other.Microlites are either not aligned or are aligned roughly planar with vesicles.