EXPERIMENTALLY TESTING EMBAYMENTS AS RECORD KEEPERS OF MAGMATIC ASCENT

The style and explosivity of volcanic eruptions are largely controlled by magmatic ascent rate, the timescale of magma transport from the source reservoir. Ascending melt decompresses, which lowers volatile solubility and forces rapid gas exsolution. This rate of decompression is challenging to quantify. Here, we experimentally test the validity of melt embayment diffusion geospeedometry, a new method to constrain ascent rate. Embayments are tube-shaped pockets of glass trapped within a crystal host. Unlike melt inclusions, embayments are never enclosed by the crystal and remain connected to the exterior melt, leaking volatiles via diffusion in response to decreasing solubility. The resulting concentration gradients are thought to preserve a direct, diffusion-limited record of ascent rate, but the fidelity of embayments as diffusion geospeedometers remains unverified. In our experimental program, we reproduce diffusional reequilibration during decompression using synthetic embayments. We drilled holes, ~100 µm in diameter and 100-320 µm long, into 3 mm diameter quartz cores for a controlled, simple geometry. The embayed cores were loaded into platinum capsules with finely powdered Bishop Tuff rhyolite and 1-5 mg of H₂O and welded shut. Sealed capsules were then loaded in cold-seal pressure vessels and heated in furnaces to equilibrate at magmatic conditions of 200 MPa and 800 ^oC for 24 hours. Multi-step decompressions at 20 MPa hr^-1 were then performed before rapidly quenching the pressure vessels. Quartz cores were ground into thin wafers to doubly expose glass within the embayments. These were analyzed for water contents using Fourier Transform Infrared Spectroscopy. Two embayments do not capture diffusive loss and instead maintain ~3 wt. % H₂O from interior to exterior. One embayment preserves an H₂O concentration gradient with a diffusional timescale identical to the experimental decompression rate. The experimental embayments are filled with glass, bubbles, and microlites. Average microlite number density of 10^4.9 mm^-3 provides a secondary ascent speedometer consistent with decompression at 20 MPa hr^-1. Our preliminary experiments are equivocal and suggest that further testing is needed to address the utility of embayments as record keepers of magmatic ascent rate.