Vapor-bubble growth in olivine-hosted melt inclusions

Melt inclusions record the depth of magmatic processes, magma degassing paths, and volatile budgets of magmas. Extracting this information is a major challenge. It requires determining melt volatile contents at the time of entrapment when working with melt inclusions that have suffered post-entrapment modifications. Several processes decrease internal melt inclusion pressure, resulting in nucleation and growth of a vapor bubble and, time permitting, diffusion of volatiles (especially CO (sub 2) ) into the vapor bubble. Previous studies have shown how this process may lead to most of the CO (sub 2) in the bulk melt inclusion being lost to the bubble. Without reconstruction, most of the melt inclusion data in the literature vastly underestimate the CO (sub 2) concentrations of magmas by measuring the glass phase only. Methods exist that attempt to reconstruct the entrapped CO (sub 2) contents, but they can be difficult to apply and do not always yield consistent results. Here, we explore bubble growth, evaluate CO (sub 2) reconstruction approaches, and develop improved experimental and computational approaches. Piston-cylinder experiments were conducted on olivine-hosted melt inclusions from Seguam (Alaska, U.S.A.) and Fuego (Guatemala) Volcanoes at the following conditions: 500-800 MPa, 1140-1200 degrees C for Seguam and 1110-1140 degrees C for Fuego, 4-8 wt% H (sub 2) O in the KBr brine filling the experimental capsules, and run durations of 10-120 min. Heated melt inclusions form well-defined S-CO (sub 2) trends consistent with degassing models. CO (sub 2) contents are enriched by a factor of approximately 2.5, on average, relative to those of the glasses in unheated melt inclusions, whereas S contents of heated and unheated melt inclusion glasses overlap, indicating that insignificant amounts of S partition into the vapor bubble. For naturally quenched melt inclusions, relatively low closure temperatures for CO (sub 2) diffusion enables some CO (sub 2) to enter vapor bubbles during quench, whereas higher closure temperatures for S diffusion limits its loss to vapor bubbles. We evaluate the timescales of post-entrapment processes and use the results to develop a new computational model to restore entrapped CO (sub 2) contents: melt inclusion modification corrections (MIMiC). Heated melt inclusion data are used as a benchmark to evaluate the results from MIMiC and other published methods of CO (sub 2) reconstruction. The methods perform variably well. Key advantages to our experimental technique are accurate measurements of CO (sub 2) contents and efficient rehomogenization of large quantities of melt inclusions. Our new computational model produces more accurate results than other computational methods, has similar accuracy to the Raman method of CO (sub 2) reconstruction in cases where Raman can be applied (i.e., no C-bearing phases in the bubble), and can be applied to the vast body of published melt inclusion data. To obtain the most robust data on bubble-bearing melt inclusions, we recommend taking both experimental- and MIMiC-based approaches.