Volatiles contents, degassing and crystallisation of intermediate magmas at Volcan de Colima, Mexico, inferred from melt inclusions.

In volatile-saturated magmas, degassing and crystallisation are interrelated processes which influence the eruption style. Melt inclusions provide critical information on volatile and melt evolution, but this information can be compromised significantly by post-entrapment modification of the inclusions. We assess the reliability and significance of pyroxene-hosted melt inclusion analyses to document the volatile contents (particularly HO) and evolution of intermediate arc magmas at Volcán de Colima, Mexico. The melt inclusions have maximal HO contents (≤4 wt%) consistent with petrological estimates and the constraint that the magmas crystallised outside the amphibole stability field, demonstrating that pyroxene-hosted melt inclusions can preserve HO contents close to their entrapment values even in effusive eruptions with low effusion rates (0.6 m s). The absence of noticeable HO loss in some of the inclusions requires post-entrapment diffusion coefficients (≤1 × 10 m s) at least several order of magnitude smaller than experimentally determined H diffusion coefficient in pyroxenes. The HO content distribution is, however, not uniform, and several peaks in the data, interpreted to result from diffusive HO reequilibration, are observed around 1 and 0.2 wt%. HO diffusive loss is also consistent with the manifest lack of correlations between HO and CO or S contents. The absence of textural evidence supporting post-entrapment HO loss suggests that diffusion most likely occurred via melt channels prior to sealing of the inclusions, rather than through the host crystals. Good correlation between the melt inclusion sealing and volcano-tectonic seismic swarm depths further indicate that, taken as a whole, the melt inclusion population accurately records the pre-eruptive conditions of the magmatic system. Our data demonstrate that HO diffusive loss is a second-order process and that pyroxene-hosted melt inclusions can effectively record the volatile contents and decompression-induced crystallisation paths of vapour-saturated magmas. [ABSTRACT FROM AUTHOR]