Abstract
Active volcanoes exhibit diffuse gas emanations through the ground, the most abundant species of which is CO2. However, the relationship between diffuse degassing and volcanic activity is not often clear and some volcanoes may have low diffuse degassing levels despite having strong volcanic activity. The main goals of this study are to quantify diffuse CO2 degassing and determine whether patterns exist in relation to volcanic activity through the study of Turrialba, Poas and Irazu, three active volcanoes in Costa Rica which are at different stages of activity. Structural controls of spatial distribution of diffuse degassing were also investigated. Measurement campaigns were conducted using the accumulation chamber method coupled with 10 cm depth ground temperature sampling with the aim of estimating the total diffuse CO2 degassing budget. The total amount of CO2 emitted diffusely by each volcano is approximately 113 ± 46 t/d over ~ 0.705 km2 for Turrialba, 0.9 ± 0.5 t/d for Poas over ~ 0.734 km2, 3.8 ± 0.9 t/d over ~ 0.049 km2 for Irazu's main crater, and 15 ± 12 t/d over 0.0059 km2 for Irazu's north flank. Turrialba and Poas volcano diffuse degassing budget represent about 10\% of the whole gas output. Both volcanoes were in a transitional stage and the opening of new conduits may cause a loss in diffuse degassing and an increase of active degassing. Numerous diffuse degassing structures were also identified. One of which at Turrialba was closely associated with the collapse of a crater wall in 2014 during the initiation of a new period of heightened eruptive activity. Similar structures were also observed on the outer slopes of the west crater, suggesting strong alteration and perhaps destabilization of the upper outer cone. Irazu's north flank is highly permeable and has experienced intense hydrothermal alteration.
Highlights
Carbon dioxide is a relatively inert and abundant magmatic gas (Gerlach and Graeber, 1985) that is relatively insoluble in melt and exsolves at depth (Pan et al, 1991; Papale et al, 2006)
In order to check the accuracy of our mathematic model, a Monte Carlo simulation was performed according to the inferred probability law
In both of these hypotheses, the presence of diffuse degassing on the north flank implies an ongoing alteration process and a modification of the cohesion of the rocks in this steep area that increases the possibility of gravitational instability (López and Williams, 1993)
Summary
Carbon dioxide is a relatively inert and abundant magmatic gas (Gerlach and Graeber, 1985) that is relatively insoluble in melt and exsolves at depth (Pan et al, 1991; Papale et al, 2006). A study of the Usu-san volcano (Japan), which had an eruption from March 2000 until September 2001, has shown that CO2 diffuse degassing increased substantially to 340 t/d 6 months before the eruption and decreased to 39 t/d 3 months after the start of the eruption (Hernández et al, 2001). Based on these studies there does not seem to be a clear relationship between diffuse degassing and volcanic activity
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