Geochemical Characterization and Temporal Changes in Parietal Gas Emissions at Mt. Etna (Italy) During the Period July 2000 - July 2003

摘要

Several types of natural gas emissions (soil gas, low temperaturefumaroles, gas bubbling in mud pools) were collected monthly on Mt. Etnavolcano between July 2000 and July 2003 both from its summit and itsflanks. Samples were analysed for the determination of the concentrationsof CO2, CH4, He, H2, CO as well as the isotopic ratios of 13 14 C/ C of CO2(δ13C) and He (R/Ra). The analysed gases were chemically divided into twogroups: air-contaminated (from sites closer to the summit vents of Mt. Etna)and CO2 - rich. Among the latter, samples from the lower SW flank of thevolcano showed high contents of biogenic thermogenic and/or microbialCH4. Isotopic shift in the δ13C values is caused by input of organic CO2and/or by interaction between magmatic CO2 and shallow ground wateras a function of water temperature and CO2 flux from depth. Based on agraphic method applied to δ13TDIC C of some ground water, the inferredisotopic composition of the pristine magmatic gas at Mt. Etna ischaracterised by δ13C values ranging from -2 to -1 000 .During the period July 2000 - July 2003 significant variations wereobserved in many of the investigated parameters almost at all monitoredsites. Seasonal influences were generally found to be negligible, with only alimited effect of air temperature changes on soil CO2 and ground temperaturein only two of the air-contaminated sites. The largest chemical anomalieswere observed in the air-contaminated sites, probably because of thestrong buffering power of local ground water on gases released through themost peripheral areas where the CO2-rich sites are located. The anomalouschanges observed during the study period can be explained in terms of progressive gas release from separate batches of magma that ascend towardsthe surface in a step-wise manner. Data relevant to the period followingthe 2002-03 eruption suggest that magma kept accumulating beneaththe volcano, thus increasing the probability of a new large eruptionat Mt. Etna.