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Abstract
Long term time series of volcanic plumes composition constitute valuable indicators of the evolution of the magmatic and volcanic systems. We present here a 4 years long time series of molecular ratios of HF/HCl, HCl/SO2 , SiF4 /SO2 , HF/SiF4 measured in the Popocat epetl 's volcanic plume using ground-based solar absorption FTIR spectroscopy. The instrument based in the NDACC (Network for the Detection of Atmospheric Composition Change) station of Altzomoni site, facing the Popocatepetl volcano, provides an unrivalled precision. The computed mean and standard deviation of the HF/HCl and HCl/SO2 ratios for this period were found to be 0.24±0.03 and 0.11±0.03, respectively. SiF4 was detected in three occasions and the SiF4 /SO2 ratios ranged between (1.9±0.5) x 10e−3 and (9.9±0.4) x 10e−3. The HBr/HCl and HBr/SO2 ratios remained below their detection limit (1.25 x 10e−4 and 1.25 x 10e−5, respectively), given that a part of the HBr has already been converted to other bromine species (e.g.: BrO, Br2 ) a few kilometers downwind of the crater. Combining our time-series with satellite SO2 fluxes and seismic data, we explain the significant long-term HCl/SO2 variations by changes in the conduit and edifice permeabilities, impacting the deep and shallow degassing processes. The high temporal resolution of the data also allows capturing the variation of the volcanic plume composition preceding and induced by a common moderate explosion at Popocatepetl volcano. We interpret the observed variations of the HCl/SO2 ratio during the explosion as a progressive decompression and release of the gas (Taquet et al. 2017) stored in the deep conduit. SiF4 /HCl versus HF/HCl is found to have a linear relation with a slope of -1/4 during the explosions, which implies a conservation of fluorine.
Highlights
Changes in the eruptive dynamics of dome-capped volcanic systems are mainly controlled by the gas content in the magma, directly affecting its viscosity and ascending speed (Martel and Schmidt, 2003; Boudon et al, 2015; Manga et al, 2018)
We present a 4-years long time series of the HCl/SO2, HF/HCl, and SiF4/SO2 ratios obtained from a high spectral resolution (HR) FTIR spectrometer, which forms part of the Network for the Detection of Atmospheric Composition Change (NDACC)1, facing the Popocatépetl volcano
The complete time series of the HCl/SO2, HF/HCl, and SiF4/SO2 ratios and their linear fit weighting-based errors are reported in Figures 2A,B concurrently with the SO2 fluxes (Figure 2C) and seismic activity (Figure 2D)
Summary
Changes in the eruptive dynamics (passive degassing, dome extrusion, or destruction) of dome-capped volcanic systems are mainly controlled by the gas content in the magma, directly affecting its viscosity and ascending speed (Martel and Schmidt, 2003; Boudon et al, 2015; Manga et al, 2018). The short-term variations were rather explained by variations in the permeability of the magma column (Oppenheimer et al, 2002), lava domes growth or change in the lava extrusion rate (Edmonds et al, 2001) Trace gases such as SiF4 in volcanic plumes were studied (Francis et al, 1996; Love et al, 1998; Mori et al, 2002; Stremme et al, 2012; Taquet et al, 2017) for the understanding of the lava dome destruction episodes. The lava dome endures multiple phases of successive inflation, degassing, deflation, and compaction, occasionally punctuated by its partial destruction These processes lead to a progressive weakening of the dome structure, until the final destructive phase (days to weeks in duration), generally evidenced by series of consecutive moderate explosions. A few in-situ measurements were performed since the appearance of the first lava dome in 1996 (Goff et al, 1998, 2001), the composition measurements are principally based on remote sensing techniques
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