The volatile component of some pumice-forming alkaline magmas from the Azores and Canary Islands

✍ Scribed by J. A. Wolff; M. Storey

• Publisher: Springer
• Year: 1983
• Tongue: English
• Weight: 952 KB
• Volume: 82
• Category: Article
• ISSN: 0010-7999
• DOI: 10.1007/bf00371176

✦ Synopsis

The abundances of pre-eruptive magmatic volatile species in the system H-O-S may be determined by application of thermodynamic methods to phenocryst assemblages commonly found in volcanic rocks, as demonstrated by Rutherford and Heming (1978). These methods are applied to alkaline pumice deposits, of airfall and ignimbrite type, from Tenerife (Canary Islands), Sao Miguel and Faial (Azores). It is argued that reliable temperature and fOz estimates can be obtained from titanomagnetite-ilmenite pairs containing appreciable quantities of minor elements. fO 2 buffering mechanisms found in rhyolitic magmas appear not to operate in more alkaline liquids, fH20 is estimated using biotite; the high values found are shown to be compatible with the violently explosive nature of the magmas concerned, fS 2 is estimated from pyrrhotite composition, fH 2, fHzS, fSO2, fSO 3 are calculated from gas equilibria. Water fugacity may be very roughly estimated for non-biotite bearing samples from data on the sulphur species. Abundances of these species are similar in alkaline and calc-alkaline salic magmas. Volcanological implications, relating to the release of volatiles during explosive eruptions, are considered.