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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Referência Bibliográfica

ZANON, V., PECCERILLO, A., PIMENTEL, A., PACHECO J., (2009) - Explosive activity of the last 16 ka in the island of Faial (Azores - Portugal): insight into the shallow plumbing system of a small central volcano, Geoitalia 2009 (Poster).


The recent (< 16 ka) activity of the Caldeira central volcano in the island of Faial (Azores), North Atlantic Ocean, generated at least 13 explosive eruptions producing mainly pumice fallout deposits. These eruptive events started with the emission of white trachytic pumice, sometimes followed by the extrusion of banded and/or grey benmoreitic pumice. The last three events, which occurred in a time span of about 2 ka showed a preponderance of benmoreite over trachyte, but with a significant reduction of the erupted volumes.
White trachytic pumices are glassy or hypocrystalline, with plagioclase and Na-sanidine as common phenocrystals joined by biotite, clinopyroxene, aegirina, amphibole and magnetite. Apatite occurs as microphenocrystals and in the groundmass, where is also possible to find aenigmatite and eudyalite. The trachytes were generated at shallow depth (2.8 - 7.7 km – i.e. from 0.8 to 2.2 ± 0.65 kbar) with temperature range of 838 - 864 °C and log fO2 of -14.93 and -15.24.

Grey benmoreitic pumices are porphyric (P.I. 14-26%) with plagioclase, Na-sanidine and clinopyroxene as typical phenocrystals, joined by olivine, aegirina, titanomagnetite and amphibole. The last phase occurs in greater amount as microphenocrystals and in the groundmass and indicating a higher pH2O for these magmas. These magmas were generated at a depth of 8-8.7 km (2.3-2.5 kbar ± 0.65 kbar) with temperature of about 1040 °C.
The shift from white to grey pumices is sharp and no intermediate compositions due to mixing have been found, leading to the idea of cyclical intrusive process of benmoreitic magmas.

Pre-16 ka magmatic evolution occurred as a single process of polybaric fractional crystallization from a common parental melt (olivine basalt) and lead to the formation of biotite-and-amphibole poor benmoreite magmas. These benmoreites are mainly expressed as lava flows, which are either almost aphyric or crystal poor, with zoned plagioclase as liquidus phase, followed by olivine, magnetite and rare sanidine. In the groundmass, it is possible to find also augite, aegirina, apatite and aenigmatite. Accumulation of labradoritic-andesinic plagioclase generated anomalous Ba-enriched compositions.

Both the pre-and-post-16 ka benmoreites lie on the same evolutionary path due to polybaric fractional crystallization of plagioclase, clinopyroxene, olivine, and Fe-Ti oxides. Trachytes are located off this trend due to the crystallization/re-absorption of Na-sanidine, amphibole (mostly kaersutite and rarely hastingsite) and biotite. Post-16 ka erupted pumices do not follow a single evolutionary path and do not show mixing with previously erupted benmoreites (> 16 ka).

The low erupted volumes, the monotonous vertical zoning of the deposits and geochemical modeling suggest the existence of a network of vertical feeder dikes, which fed these eruptions from shallow magma reservoirs. The intrusion of the water-rich benmoreites from depth into the shallow plumbing system of the Caldeira volcano determined thermodynamic instability (i.e. feldspar and amphibole re-adsorbing, plagioclase zoning) in the reservoir, resulting in a high explosivity of these eruptions.