Chemistry and mineralogy of high-temperature gas discharges from Colima volcano, Mexico. Implications for magmatic gas-atmosphere interaction

Gases, condensates and silica tube precipitates were collected from 400 deg reesC (Z2) and 800 degreesC (Z3) fumaroles at Colima volcano, Mexico, in 19 96-1998. Volcanic gases at Colima were very oxidized and contain up to 98% air-due to mixing with air inside the dome interior, close to the hot magma tic body. An alkaline trap method was used to collect gas samples, therefor e only acidic species were analysed. Colima volcanic gases are water-rich ( 95-98 mol%) and have typical S/C/Cl/F ratios for a subduction type volcano. deltaD-values for the high-temperature Z3 fumarolic vapour vary from -26 t o -57 parts per thousand. A negative deltaD-Cl correlation for the Z3 high- temperature fumarole may result from magma degassing: enrichment in D and d ecrease in the Cl concentration in condensates are likely a consequence of input of "fresh" batches of magma and an increasing of volcanic activity, r espectively. The trace element composition of Colima condensates generally does not diff er from that of other volcanoes (e.g. Merapi, Kudryavy) except for some enr ichment in V, Cu and Zn. Variations in chemical composition of precipitates along the silica tube from the high-temperature fumarole (Colima 1, fumaro le Z3), in contrast to other volcanoes, are characterized by high concentra tions of Ca and V, low concentration of Mo and a lack of Cd. Mineralogy of precipitates differs significantly from that described for silica tube expe riments at other volcanoes with reduced volcanic gas. Thermochemical modell ing was used to explain why very oxidized gas at Colima does not precipitat e halite, sylvite, and Mo- and Cd-minerals, but does precipitate V-minerals and native gold, which have not been observed before in mineral precipitat es from reduced volcanic gases. (C) 2001 Elsevier Science B.V. All rights r eserved.