Chemical characteristics of the volcanic gases from Nyiragongo lava lake and the generation of CH 4-rich fluid inclusions in alkaline rocks

Abstract

Methods used previously to remove compositional modifications from volcanic gas analyses for Mount Etna and Erta'Ale lava lake have bean employed to estimate the gas phase composition at Nyiragongo lava lake, based on samples obtained in 1959. H 2O data were not reported in 11 of the 13 original analyses. The restoration methods have been used to estimate the H 2O contents of the samples and to correct the analyses for atmospheric contamination, loss of sulfur and for pre- and pest-collection oxidation of H 2S, S 2, and H 2. The estimated gas compositions are relatively CO 2-rich, low in total sulfur and reduced. They contain approximately 35–50% CO 2 45–55% H 2O, 1–2% SO 2, 1–2% H 2., 2–3% CO, 1.5–2.5% H 2S, 0.5% S 2 and 0.1% COS over,he collection temperature range 102° to 960° C. The oxygen fugacities of the gases are consistently about half an order of magnitude below quartz-magnetite-fayalite. The low total sulfur content and resulting low atomic S/C of the Nyiragongo gases appear to be related to the relatively low f O 2 of the crystallizing lava. At temperatures above 800°C and pressures of 1–1.5 k bar, the Nyiragongo gas compositions resemble those observed in primary fluid inclusions believed to have formed at similar temperatures and pressures in nephelines of intrusive alkaline rocks. Cooling to 300°C, with f O 2 buffered by the rock, results in gas compositions very rich in CH 4 (50–70%) and resembling secondary fluid inclusions formed at 200–500°C in alkaline rocks. Below 600°C the gases become supersaturated in carbon as graphite. These inferences are corroborated by several reports of hydrocarbons in plutonic alkaline rocks, and by the presence of CH 4-rich waters in Lake Kivu — a lake on the flanks of Nyiragongo volcano.