Evaluation of results from the fourth and fifth IAVCEI field workshops on volcanic gases, Vulcano island, Italy and Java, Indonesia

The major purpose of field work-shops on volcanic gases, organized by the I AVCEI Commission on the Chemistry of Volcanic Gases, is the collection and analysis of volcanic gas discharges with the aim to develop and improve tec hniques for the geochemical surveillance of active volcanoes. The fourth an d fifth workshops were held at Vulcano island, Italy, in 1991 and on Java i sland, Indonesia, in 1994, respectively. Gas samples were collected from fo ur gas vents by nine groups at Vulcano and from eight gas vents by eight gr oups on Java. The quality (e.g. scatter of the data) of most of the results , reported from these two workshops, is sufficient to permit a broad chemic al classification of the discharge and meaningful thermodynamic interpretat ion. In most cases, the majority of the data for individual gas vents clust er closely around the median values, suggesting that the median values are the best estimates of chemical composition. There is, however, also a consi derable scatter of the analytical data, and this scatter warns us to not re ly too heavily on a single analytical value, in particular on a value for C H4 and CO, because analytical data for these species often show a wide scat ter. This warning is particularly relevant for chemical monitoring of volca nic activity. Further improvement of the sampling and analytical techniques as well as more detailed comparison of the techniques is required to reduc e such uncertainty in order to interpret the volcanic activity and hydrothe rmal conditions. (C) 2001 Elsevier Science B.V. All rights reserved.