Volcanic Unrest by Joachim Gottsmann & Jürgen Neuberg & Bettina Scheu

Author:Joachim Gottsmann & Jürgen Neuberg & Bettina Scheu
Language: eng
Format: epub
ISBN: 9783319584126
Publisher: Springer International Publishing

An alternative way to constrain the composition of gas bubbles is illustrated on Fig. 6. It starts from the observation that bubble nucleation is, from a kinetic point of view, an instantaneous process (e.g., Mourtada-Bonnefoi and Laporte 2002, 2004). Nucleation of a gas bubble draws volatiles from the local melt and the possibility that the initial CO2/H2O of the gas bubble is the same as the local melt should be considered. According to this hypothesis, represented schematically on Fig. 6, the local melt next to the nucleated bubble (Cnew) is volatile-depleted but its CO2/H2O (r, Fig. 6a) is the same than the initial melt (Cinit). Melt and gas bubble compositions are thus both located on a mixing line between Cinit and Cnew which passes through the origin of the H2O–CO2 diagram (Fig. 6b). The net result is the nucleation of individual gas bubbles more CO2-rich than expected from local gas-melt equilibrium (Fig. 5b).

Fig. 6Hypothetical model for the composition of a gas bubble nucleated during decompression of H2O–, CO2-bearing basaltic melts. a detail of the gas-melt interface region in a concentration (Ci) versus distance diagram where Ci refers to the volatile concentration in the melt. The gas bubble is on the left. The two horizontal lines are melt volatile concentrations for H2O (black) and CO2 (grey) drawn as straight lines because bubble nucleation is an instantaneous event. The narrow zone near the interface is the melt region depleted in volatiles drawn to form the bubble. Cnew are volatile concentrations in the depleted melt region. Notice that the H2O/CO2 ratio (r) is identical in both the depleted and non-depleted melt regions because during nucleation volatiles are drawn from the local melt and the initial CO2/H2O of the gas bubble is the same as the local melt. Cinit give volatile concentrations of the pre-decompression melt, Cnew gas/melt interface volatile concentrations left after bubble nucleation at Pfin. Black lettering is used for H2O and grey for CO2. b H2O–CO2 diagram illustrating the evolution during decompression and degassing. The black bold curve gives the schematic location of the theoretical equilibrium degassing trajectory. The two black lines are isobars labelled with initial (Pinit) and final (Pfin) pressures along the decompression path. Melts produced as a result of decompression and bubble nucleation plot on the straight line joining Cinit and Cnew. This line passes through the origin of the diagram because both Cinit and Cnew have the same CO2/H2O ratio. Note that the location of Cnew along this line is arbitrary. The CO2/H2O ratio of the gas bubble (dashed line) is also the same as Cinit and Cnew. It is higher than the gas bubble controlled by local gas-melt equilibrium (Fig. 5)

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