AbstractDespite its increasing application to estimate magma decompression rates for explosive eruptions, the embayment speedometer has long awaited critical experimental evaluation. We present the first experimental results on the fidelity of natural quartzâhosted embayments in rhyolitic systems as recorders of magma decompression. We conducted two high pressureâtemperature isobaric equilibrium experiments and 13 constantârate, continuous isothermal decompression experiments in a coldâseal pressure vessel where we imposed rates from 0.005 to 0.05 MPa sâ1 in both H2Oâsaturated and mixedâvolatile (H2O + CO2)âsaturated systems. In both equilibrium experiments, we successfully reâequilibrated embayment melt to new fluid compositions at 780°C and 150 MPa, confirming the ability of embayments to respond to and record changing environmental conditions. Of the 32 glassy embayments recovered, seven met the criteria previously established for successful geospeedometry and were thus analyzed for their volatile (H2O ± CO2) concentrations, with each producing a good model fit and recovering close to the imposed decompression rate. In one H2Oâsaturated experiment, modeling H2O concentration gradients in embayments from three separate crystals resulted in bestâfit decompression rates ranging from 0.012 to 0.021 MPa sâ1, in close agreement with the imposed rate (0.015 MPa sâ1) and attesting to the reproducibility of the technique. For mixedâvolatile experiments, we found that a slightly variable starting fluid composition (2.4â3.5 wt.% H2O at 150 MPa) resulted in good fits to both H2O + CO2 profiles. Overall our experiments provide confidence that the embayment is a robust recorder of constantârate, continuous decompression, with the model successfully extracting experimental conditions from profiles representing nearly an order of magnitude variation (0.008â0.05 MPa sâ1) in decompression rate.