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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) - Fissural volcanism and centralized systems: geochemical processes in multiple plumbing systems. The case study of Faial island (Azores - Portugal), Geoitalia 2009 (Poster).​

Resumo


Faial (Azores archipelago), located in the north Atlantic Ocean, is a small island of young age constituted by the remnant of an old shield volcano (>580 ka), almost completely buried by younger deposits from the central volcano of Caldeira (410 ka – 0.98 ka) and two fissural zones characterized by extensional tectonics, namely the Capelo Fracture Zone (6 ka - 1957/58) and the Horta Fracture Zone (< 11 ka). These central and fissural volcanic systems were active during overlapping periods. This island represents a suitable case study to understand how a plumbing system develops in an oceanic geodynamic setting and how and when contemporarily active magmatic systems worked.

To this aim, more than 148 samples of lavas and pyroclastic rock were collected for preliminary investigations and 93 samples have been used for geochemical and petrographical studies. All sampled rocks belong to an alkaline series with Na/K >1 and compositional range from olivine basalts to trachytes.

The magmas erupted by the two fissural zones are poorly evolved and range in composition from olivine-basalts to hawaiites. These magmas originated at pressure of about 12.6-13.6 kbar (e.g. >40km) and temperature of 1227-1117 ±30°C with oxidizing conditions (log fO2 = -5.7/-4.6). Alkalibasaltic-hawaiitic compositions crystallized clinopyroxene and olivine under a pressure of 8.7-9.9 kbar, corresponding to a depth range of 28.5-32.2 km, with an oxygen fugacity of -6.6 ±0.59 and a temperature range of 1044-1068 °C.

Mantle source is quite homogeneous and the majority of the products are nefeline-normative. The basalts from the Horta Fracture Zone tend to be more HREE rich than the equivalent rocks from the Capelo Fracture Zone, which can be addressed to a minor difference in the amount of garnet participating to the process of mantle melting.

Whole rock compositions underwent some degree of evolution and cannot be considered as primitive. However, geochemical modeling excluded fertile peridotites as possible source rocks. Sampled ultramafic xenoliths are wherlites, pyroxenites and dunites. These are probably restites left by the melting process of an already partly depleted composition. Geochemical modeling reproduced these compositions with a 5-8% partial melting of a pyroxenite with garnet as residual phase.

The presence of ultramafic xenoliths and skeletal olivines and the poorly vesicular texture of these magmas are suggestive for a general fast ascent through the crust in sub-vertical feeder dykes.
The evidences of late-stage syn-eruptive mixing between glassy and crystallized melts of the same composition indicate heterogeneous conditions of magma ascent due to the existence of a v through the shallow feeder conduit.

The volcanic series produced by the Caldeira central volcano starts with nefeline-normative olivine basalts, which re-equilibrated in the mantle at a depth of about 25 km (7.6 kbar) and a temperature range of 1156-1056 °C. Evolved compositions, from hyperstene-normative basalts and hawaiites up to trachytes, occurred at pressures from 3.2 to 0.8 ± 0.65 kbar (2.8-11.2 km) with temperature range of 838-864 °C and log fO2 from -14.93 to -15.24. The process responsible for the evolution was a polybaric fractional crystallization of clinopyroxene, olivine, plagioclase, magnetite, ilmenite, apatite ±sanidine ±biotite ±amphibole starting from a common parental melt.

Geochemical modeling revealed the absence of any interaction between the two basaltic fissural systems and the central volcano, in fact no mixing patterns have been found so far.

The magmas of Caldeira are quite degassed and evidence thermodynamic disequilibria (i.e. partly resorbed olivines and feldspars), suggesting dynamic instability in the reservoir leading to geochemical re-homogenization of the magmas.

Observações


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