Evolution versus cooling of gaseous volcanic emissions from Momotombo Volcano, Nicaragua: Thermochemical model and observations

Details Evolution versus cooling of gaseous volcanic emissions from Momotombo Volcano, Nicaragua: Thermochemical model and observations Evolution versus cooling of gaseous volcanic emissions from Momotombo Volcano, Nicaragua: Thermochemical model and observations

Oct 1989

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989gecoa..53.2591q&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 53, Issue 10, pp.2591-2608

Computer Science

6

Scientific paper

In order to identify the chemical reactions controlling the condensation of a high temperature volcanic gas phase, we have collected and analysed the various gas-related products of Momotombo Volcano, Nicaragua (high temperature gases, water condensates, silica tube sublimates, natural incrustations and aerosols) and the results were then compared with a thermochemical model of the behaviour of 29 major, minor and trace elements (H, B, C, N, O, F, Na, Mg, Al, Si, S, Cl, K, Ca, Ti, Mn, Fe, Cu, Zn, As, Se, Br, Pb, Mo, Sb, Sn, Ag, Au and Cd) under cooling. Theoretical mineral assemblages were then compared to the experimental results. The elemental composition of the starting gas phase (major, minor and trace elements) is in good agreement with previous studies of the Momotombo Volcano. Moreover, this gas was very close to thermodynamic equilibrium under the sampling conditions. Various mineral assemblages are observed in silica tubes sublimates, the distribution of which can be related to the temperature gradient. Most of them (mainly sylvite, halite, magnetite, molybdenite, pyrite, sulfosalts of As, Pb, Se and native sulfur) show good agreement with theoretical results with respect to both their nature and condensation temperature range. On the contrary, the occurrence of some sulfates in the tubes conflicts with the predictions of the model and is attributed to partial contamination by atmospheric oxygen of the gas in the sampling tubes. Conversely, some theoretical compounds were not detected in the tubes. The presence of solid S and H 3 BO 3 in the low temperature fumarolic incrustations is consistent with the model, but not that of gypsum. This discrepancy may be due to their gaseous and metasomatic origin, respectively. Most of the observed crystal morphologies indicate condensation mechanisms occurring under conditions close to thermodynamic equilibrium. However, disequilibria were observed for some species. For instance, the formation of silica results, in part, from the chemical attack on the sampling tube, while the alkali sulfate morphology, dendritic salammoniac and silicon whiskers obviously indicate variations of the growth conditions during sampling. Comparison of the chemistry of both aerosols and water condensates provides reliable information regarding the mechanism of the gas transfer into the atmosphere. Enrichment factors and granulometric distributions of aerosols were also used to corroborate some aspects of the thermochemical model.

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