a b s t r a c tModeling magmatic degassing, or how the volatile distribution between gas and melt changes at pressure varies, is a complex task that involves a large number of thermodynamical relationships and that requires dedicated software. This article presents the software D-Compress, which computes the gas and melt volatile composition of five element sets in magmatic systemsIt has been calibrated so as to simulate the volatiles coexisting with three common types of silicate melts (basalt, phonolite, and rhyolite). Operational temperatures depend on melt composition and range from 790 to 1400°C. A specificity of D-Compress is the calculation of volatile composition as pressure varies along a (de)compression path between atmospheric and 3000 bars. This software was prepared so as to maximize versatility by proposing different sets of input parameters. In particular, whenever new solubility laws on specific melt compositions are available, the model parameters can be easily tuned to run the code on that composition. Parameter gaps were minimized by including sets of chemical species for which calibration data were available over a wide range of pressure, temperature, and melt composition. A brief description of the model rationale is followed by the presentation of the software capabilities. Examples of use are then presented with outputs comparisons between D-Compress and other currently available thermodynamical models. The compiled software and the source code are available as electronic supplementary materials.

[1] Erebus volcano in Antarctica offers an exceptional opportunity to probe the dynamics of degassing -its behavior is characterized by an active lava lake through which sporadic Strombolian eruptions occur. Here, we develop a framework for interpreting contrasting degassing signatures measured at high temporal resolution, which integrates physical scenarios of gas/melt separation into a thermodynamic model that includes ©2012. American Geophysical Union. All Rights Reserved.1 of 24 new volatile solubility data for Erebus phonolite. In this widely applicable framework, the measured gas compositions are backtracked from surface to depth according to physical templates involving various degrees of separation of gas and melt during ascent. Overall, explosive signatures can be explained by large bubbles (gas slugs) rising slowly in equilibrium from at least 20 bars but at most a few hundred bars in a magmatic column closer to the stagnant end-member than the convecting end-member. The span of explosive signatures can be due to various departure depths and/or slug acceleration below a few tens of bars.Results also reveal that explosive gases last equilibrated at temperatures up to 300 C colder than the lake due to rapid gas expansion just prior to bursting. This picture (individual rise of gas and melt batches from a single, potentially very shallow phonolitic source) offers an alternative to the conclusions of previous work based on a similar data set at Erebus, according to which differences between quiescent and explosive gas signatures are due to the decompression of two deep, volatile-saturated sources that mixed to various degrees (phonolite at 1-3 kbar and basanite at 5-8 kbar).

Partitioning experiments between a basaltic melt from Mt. Etna and a low-density hydrous fluid or vapor containing H 2 O or H 2 O-CO 2 were performed at 1200-1260 °C, at pressures between 1 and 200 MPa, either near the nickel-nickel oxide (NNO) buffer or at two log units above it (NNO+2), and with different chloride concentrations. Most of the experiments were done at chloride-brineundersaturated conditions, although at the highest Cl concentrations explored brine saturation might have been reached. The average partition coefficients (D Cl fluid/melt) over the range of Cl concentrations were derived on a weight basis by plotting the calculated concentrations of Cl in the fluid phase versus the measured ones in the melt. For H 2 O-Cl experiments in which the Cl concentration in the melt was ≤ 0.4 wt.%, a negative dependence between D Cl f/m and pressure is observed. D Cl fluid/melt in H2O+Cl-bearing experiments ranges between 11-14 at 1 and 25 MPa to 6 at 200 MPa at NNO; and between 4 at 50 MPa and 13 at 100 MPa at ΔNNO≥2. Addition of CO 2 at NNO yields lower partition coefficients than in CO 2-free conditions over the pressure range investigated. The negative pressure dependence observed for H 2 O-Cl experiments disappears when CO 2 is present in the system. Overall, once CO 2 is introduced in the system, Cl fugacity in the silicate melt tends to increase, thus resulting in a decrease of D Cl f/m. Application to Mt. Etna shows that the composition of the volcanic plume in terms of Cl records very shallow pressures of magma equilibration with its exsolved fluid.