Abstract
We present new H 2 O and CO 2 solubility data in mafic to intermediate alkaline magmas from Fasnia and Garachico volcanoes, Tenerife. H 2 O-and CO 2-saturated experiments were conducted at ∼50-400 MPa, 1200°C, and f O 2 from 2 log units below the NiNiO solid buffer to 3.2 log units above it. Although existing solubility models for alkali-rich mafic magmas broadly describe H 2 O and CO 2 behaviour, associated errors are worthy of consideration since they usually exceed 15-20%. For this reason, we have determined the specific solubility laws of basanitic and phonotephritic melts from the Canary Islands. Results show similar H 2 O solubilities for both compositions, whereas the basanite can dissolve an average of ∼45% more CO 2 than the phonotephrite. By combining these data, we have established a simple empirical model that allows us to calculate melt inclusion entrapment pressures accurately and, therefore, better understand the inner workings of volcanic oceanic islands. Application to El Hierro 2011-2012 and young (<20 ka) basanites from this location shows that previous barometric estimates were, on average, overestimated by 15-28%. Our results suggest that magmas rising from depth experienced a first but short episode of equilibration at 8-10 km, whereas the bulk of the crystallization occurred during the subsequent dyke injection, ascent, and degassing at P ⇐ 200 MPa (6-1.5 km).
Highlights
Understanding magmatic volatiles behaviour in terms of the maximum amount that can be dissolved in a silicate melt under a given set of conditions is crucial for improving the knowledge of magmatic processes and degassing mechanisms involved in volcanic eruptions [Moore, 2008, Moore and Carmichael, 1998, Oppenheimer et al, 2014]
In CO2 and H2O-saturated experiments, approximately 80 to 100% of the iron was kept in the melt, except for FC4b, GC05, GC3b, and GC4b, in which the loss reaches 22–25%
Despite that the SEM inspection of these four charges did not reveal the presence of mineral phases within the glasses, we cannot rule out the possibility of an occasional crystallization of Fe–Ti oxides non-detectable by SEM (
Summary
Understanding magmatic volatiles behaviour in terms of the maximum amount that can be dissolved in a silicate melt under a given set of conditions (pressure, temperature, melt composition, and redox state) is crucial for improving the knowledge of magmatic processes and degassing mechanisms involved in volcanic eruptions [Moore, 2008, Moore and Carmichael, 1998, Oppenheimer et al, 2014]. Understanding the mechanisms that control the solubility of the different volatile species dissolved in magmas is crucial to decipher surface gas signals, changes in the eruptive dynamic, and correctly interpretate melt inclusion (MI) data. The increase of NBO/T species in the melt enhances CO2 solubility [e.g., Brooker et al, 2001a,b, Iacono-Marziano et al, 2012] This parameter alone cannot fully capture the details of CO2 solubility behaviour observed in magmas of broadly similar compositions. CaO and K2O contents play an important role in enhancing CO2 solubility in magmas [e.g., Behrens et al, 2009, Dixon, 1997, Lesne et al, 2011b, Moore, 2008, Scaillet and Pichavant, 2005, Vetere et al, 2014]
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