WHY ARE OBSIDIANS CRYSTAL-FREE?

Obsidians erupt and cool as crystal-poor or crystal-free lavas on Earth’s surface. The existence of obsidians presents several paradoxes that challenge our knowledge of eruptive (i.e., decompression, ascent, emplacement, and cooling) and crystallization (i.e., nucleation and growth kinetics) processes. Effusive eruption of an obsidian lava, rather than explosive eruption of pumice and ash, requires sufficiently slow decompression to allow degassing and outgassing of the ascending magma. But slow decompression should result in nucleation and growth of crystals (“microlites”) and although some obsidians contain crystals, others are crystal-free. The glassy texture of obsidians is often explained to be the result of “rapid cooling,” but some obsidian flows are 10s or 100s of meters thick and would have cooled over timescales of weeks to years – timescales that are hardly “rapid.” We use suites of single-step decompression experiments performed on rhyolite obsidians from Valles Caldera (New Mexico, U.S.A.) and South Sister (Oregon, U.S.A.) to examine crystallization rates in decompressing silicic magmas. We calculate instantaneous nucleation and growth rates for plagioclase and sanidine as functions of supersaturation (the difference between equilibrium and observed crystallization) using the SNGPlag model of Andrews and Befus (2020). Our results show that as a magma decompresses from >100 MPa (P_TOTAL=P_H2O) and ascends towards the surface, feldspar nucleation and growth rates increase as supersaturation increases. But pressures fall to <30 MPa, nucleation and growth rates decrease, even though supersaturation continues to increase. Feldspar crystallization effectively ceases as magma degassing drives the melt viscosity above ~10⁷ Pas. Our work shows that obsidians record transit from magma storage depths to ~1 km at ~0.03 to ~0.3 m/s (speeds sufficiently slow to allow degassing (Burgisser and Gardner, 2004), but fast enough to prevent or retard crystallization), but their cargo of feldspar microlites provides virtually no information about the final km of ascent or emplacement on the surface, and the lack of microlites does not indicate “rapid cooling.”