Water exsolution in the magma chamber favors effusive eruptions: Application of Cl-F partitioning behavior at the Nisyros-Yali volcanic area

Abstract

Changes from effusive to explosive behavior are common during single eruptive events. However, in many cases these shifts are recorded between distinct eruptions, separated by periods of volcanic repose. In the quiescent periods, magma chamber processes are likely to play a major role in controlling the style of the next eruption, by modifying the properties of the magmas. Crystallinity, viscosity, dissolved water content and the water saturation state of the melt are among the properties that can easily change during periods of repose, and that can influence the behavior of the magmas during eruption. Here, we focus on the active Nisyros-Yali volcanic center (South Aegean Volcanic Arc) to investigate the water saturation state of the magma reservoir, before effusive and explosive events were triggered. We investigate the evolution of Cl in melt inclusions, and the fractionation of Cl from F in apatite included in other mineral phases. We present an analytical protocol for minimizing F diffusion during EPMA apatite measurements, and to correct for diffusion resulting from variable crystal orientation. The relatively high concentrations of Cl positively correlated with the degree of differentiation in melt inclusions, and the lack of a clear fractionation trend of Cl from F in apatite, indicate that explosive eruptions were generated by magmas that were water-undersaturated during magmatic storage. In effusive eruptions, both melt and apatite inclusions record Cl depletion at magmatic storage conditions. This suggests that effusive events were generated by magmas stored in areas of the reservoir with sufficient volumes of exsolved water to significantly extract Cl from the melt. This is in accordance with mineral-melt hygrometry estimates, which suggest dissolved water contents of ~4.5 wt% for magmas generating explosive events, and in excess of 5.5 wt% for magmas behaving effusively. The presence of exsolved volatiles in the magma reservoirs immediately prior to eruption favors outgassing in the conduit and effusive behavior despite the high water content. This is partly due to increasing the compressibility of the magmatic reservoir upon mafic recharge, and partly due to fostering the development of early gas permeability, at deep levels in the conduit.