Alicudi

Schematic Map of Alicudi Island. da Lucchi et al., (2008).

The Island of Alicudi is located at the western end of the Aeolian archipelago. It rises about 2000 m above the sea floor and 675 m above sea level. Tthe emergent part of the island was constructed during three main stages of activity, each separated by a summit collapse. Calc-alkaline basalt and basaltic andesite lavas plus minor pyroclastics were erupted during the first two stages; andesite lava flows and domes were emplaced during the third phase in the summit crater and along the south-eastern flank of the cone. K/Ar measurements give ages younger than 60 ka for the lowest exposed rocks, and about 28 ka for the latest andesitic lava flows and domes.

All rocks display a holocrystalline to hypocrystalline porphyritic texture, and contain phenocrysts of plagioclase (An₈₂-₆₇), olivine (Fo₇₉-₆₈), and diopside to salite clinopyroxene that are set in a groundmass consisting of the same phases plus Ti-magnetite and glass. Orthopyroxene and brown hornblende are present in basaltic andesites and andesites, where olivine is scarce and strongly resorbed; accessory phases include Fe-Ti oxides and apatite.

Alicudi rocks, like other Aeolian volcanics, contain a variety of metamorphic and magmatic xenoliths. Metamorphic xenoliths are represented by predominant quartz-rich rocks displaying evidence of partial melting along grain boundaries, and by a few biotite gneiss and granulite lithologies. Metamorphic xenoliths are more abundant in the mafic rocks than in the andesites. Igneous xenoliths include gabbros, diorites, and a few ultramafic inclusions made up of clinopyroxene and olivine. Some of the ultramafic xenoliths are cumulate in origin, whereas others display a typical granoblastic texture with kinked olivine, Cr-diopside, and accessory Cr-spinel, which probably denote an upper mantle origin.

Alicudi rocks range in composition from calc-alkaline basalts to high-K andesites. Basalts exhibit the most primitive compositions over the Aeolian arc. Variations of several major and trace elements point to a derivation of the whole Alicudi series from a single magma by fractional crystallisation. However, variable isotopic signatures call for the involvement of some additional process in magma evolution. It has been suggested the effect of a combined early fractional crystallisation process, followed by bulk assimilation of wall rocks. Fractional crystallisation of olivine, pyroxene and plagioclase produced a suite of basalt to andesite magmas, which underwent assimilation of wall rocks during their rise to the surface. The more fluid and hotter mafic magmas dissolved higher amounts of crustal rocks than the andesitic liquids, due to their higher temperature and to turbulent flow through volcanic conduits.

Bibliography

• Peccerillo. A. Plio-Quaternary Volcanism in Italy. (2005)
• Rachel Hunt (2012): DOCUMENTING THE ORIGIN OF COMPOSITIONAL DIVERSITY OF SUBDUCTION ZONE MAGMATISM, ALICUDI, AEOLIAN ARC (SOUTHERN TYRRHENIAN SEA, ITALY) USING IN SITU PLAGIOCLASE DATA (A Thesis Presented to The Graduate Faculty Central Washington University)

Basaltic Andesite
Clinopyroxene and plagioclase (with Sieve texture) embedded in a a glassy groundmass. PPL image, 2x (Field of view = 7mm)	Clinopyroxene and plagioclase (with Sieve texture) embedded in a a glassy groundmass. PPL image, 2x (Field of view = 7mm)	Clinopyroxene and plagioclase (with Sieve texture) embedded in a a glassy groundmass. PPL image, 2x (Field of view = 7mm)
Clinopyroxene and plagioclase (with Sieve texture) embedded in a a glassy groundmass. XPL image, 2x (Field of view = 7mm)	Clinopyroxene and plagioclase (with Sieve texture) embedded in a a glassy groundmass. PPL image, 2x (Field of view = 7mm)	Clinopyroxene and plagioclase (with Sieve texture) embedded in a a glassy groundmass. PPL image, 2x (Field of view = 7mm)
Partially resorbed Olivine, Clinopyroxene and plagioclase (with Sieve texture) embedded in a a glassy groundmass. PPL image, 2x (Field of view = 7mm)	Partially resorbed Olivine, Clinopyroxene and plagioclase (with Sieve texture) embedded in a a glassy groundmass. XPL image, 2x (Field of view = 7mm)	Partially resorbed Olivine, Clinopyroxene and plagioclase (with Sieve texture) embedded in a a glassy groundmass. PPL image, 2x (Field of view = 7mm)
Clinopyroxene and plagioclase (with Sieve texture) embedded in a a glassy groundmass. PPL image, 2x (Field of view = 7mm)	Clinopyroxene and plagioclase (with Sieve texture) embedded in a a glassy groundmass. XPL image, 2x (Field of view = 7mm)

Basalt
Olivine, Plagioclase and Pyroxene crystals. XPL image, 2x (Field of view = 7mm)	Olivine, Plagioclase and Pyroxene crystals. XPL image, 2x (Field of view = 7mm)	Olivine inclusions in a Pyroxene crystal. XPL image, 10x (Field of view = 2mm)
Plagioclase and Pyroxene crystals. XPL image, 2x (Field of view = 7mm)	Plagioclase and Pyroxene crystals. XPL image, 2x (Field of view = 7mm)	Olivine, Plagioclase and Pyroxene crystals. XPL image, 2x (Field of view = 7mm)
Olivine, Plagioclase and Pyroxene glomerocryst. XPL image, 2x (Field of view = 7mm)	Plagioclase and Pyroxene crystals. XPL image, 2x (Field of view = 7mm)	Olivine, Plagioclase and Pyroxene crystals. XPL image, 2x (Field of view = 7mm)
Olivine, Plagioclase and Pyroxene crystals. PPL image, 2x (Field of view = 7mm)	Olivine, Plagioclase and Pyroxene crystals. XPL image, 2x (Field of view = 7mm)	Olivine, Plagioclase and Pyroxene crystals. XPL image, 2x (Field of view = 7mm)
Olivine, Plagioclase and Pyroxene crystals. PPL image, 2x (Field of view = 7mm)	Olivine, Plagioclase and Pyroxene crystals. XPL image, 2x (Field of view = 7mm)	Olivine, Plagioclase and Pyroxene crystals. PPL image, 2x (Field of view = 7mm)
Olivine, Plagioclase and Pyroxene crystals. XPL image, 2x (Field of view = 7mm)	Olivine, Plagioclase and Pyroxene crystals. PPL image, 2x (Field of view = 7mm)	Olivine, Plagioclase and Pyroxene crystals. XPL image, 2x (Field of view = 7mm)
Olivine, Plagioclase and Pyroxene crystals. XPL image, 2x (Field of view = 7mm)	Olivine, Plagioclase and Pyroxene crystals. PPL image, 2x (Field of view = 7mm)	Olivine, Plagioclase and Pyroxene crystals. XPL image, 2x (Field of view = 7mm)