Open volcanic systems: evidence for deep gas loss

Abstract

<p>Previous studies of Vulcanian eruptive products have shown that the respective volcanic conduits were filled for the<br>most part with low-porosity magma prior to eruption. Comparison with the theoretical porosity distribution<br>expected from closed-system degassing suggests that gas loss must have taken place at great depth within the<br>magmatic column. At such high pressures, however, porosities are low enough to rule out traditional gas loss<br>mechanisms. We tested if channelling, an outgassing mechanism based on bubble connection due to high crystal<br>content proposed to occur in mushy magma reservoirs, could also happen in volcanic conduits. We reanalysed<br>phenocryst, microlite, and porosity data from recent eruptions of Merapi volcano, Indonesia, Soufrière Hills<br>volcano, Montserrat, and Tungurahua volcano, Ecuador. Overall, these magmas had crystal contents high enough<br>for outgassing to occur by channelling. Gases could be channelled out of the magma columns at various levels<br>during ascent to yield mostly gas-depleted magma columns prior to explosive behaviour. Such outgassing by<br>channelling has thus the capacity to influence eruptive style. Depending on the phenocryst content, microlite<br>growth during ascent can either foster, or impede gas escape by channelling. Considering the pervasive occurrence<br>of microlites and ensuing high crystal contents in volcanic conduits, the high likelihood of channelling implies that<br>other outgassing mechanisms might not be as dominant as previously envisioned.</p>