Tracking the behaviour of persistently degassing volcanoes using noble gas analysis of Pele's hairs and tears: A case study of the Masaya volcano (Nicaragua)

We provide evidence that noble gasses (NGs) trapped in Pele's hairs and tears can be used as a tool to monitor the degassing behaviour of persistently active volcanic systems. We investigated 4He, 36Ar, 40Ar abundances and 4He/40Ar* and 40Ar/36Ar isotopic ratios in samples collected in March 2015 and March 2016 from Masaya volcano (Nicaragua). The resurgence of a lava lake in December 2015 provided the opportunity to monitor NG variations during a specific volcanic event. The variations in the NG abundances of individual glass particles are here attributed to solubility-controlled NG fractionation between melt and bubbles, and consequently correlated with sample vesicularity and vesicle size. Inhomogeneous incorporation of 36Ar, along with kinetic mass fractionation during magma degassing, vesiculation and fast-quenching of the samples after eruption, are the mechanisms that control the variability of the 40Ar/36Ar ratios observed in the samples. From March 2015 to March 2016, our data indicate a general increase of mean 40Ar* abundances from 1.9E−8 ± 1.4E−8 cc/g to 9.3E−8 ± 8.9E−8 cc/g and decrease in mean 4He/40Ar* ratios from 0.37 ± 0.15 to 0.18 ± 0.13. We ascribe this to the upward migration of a gas-rich magma from depth that recharged the shallow magmatic reservoir of Masaya at some point between the two collection periods. Since this model agrees well with the physical volcanology of Masaya and with gas plume measurements spanning our collection period, we suggest that NG in Pele's hairs and tears can be further used as a valuable tool to monitor degassing processes at Masaya volcano and, when samples are available, at other persistently degassing systems (e.g. Etna, Hawaii).