# Experimental determination of phase relationships of a chemically-zoned peralkaline silicic reservoir: the example of Green Tuff eruption at Pantelleria (Italy)

Pierangelo Romano, Joan Andujar, Bruno Scaillet, Silvio Rotolo

► To cite this version:

Pierangelo Romano, Joan Andujar, Bruno Scaillet, Silvio Rotolo. Experimental determination of phase relationships of a chemically-zoned peralkaline silicic reservoir: the example of Green Tuff eruption at Pantelleria (Italy). 19th EGU General Assembly, 2017, Vienne, Austria. pp.18744. hal-03552729

HAL Id: hal-03552729

https://hal.science/hal-03552729

Submitted on 3 Feb 2022

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# Experimental determination of phase relationships of a chemically-zoned peralkaline silicic reservoir: the example of Green Tuff eruption at Pantelleria (Italy)

Pierangelo Romano (1), Joan Andujar (2), Bruno Scaillet (2), and Silvio Rotolo (1)

(1) Dipartimento di Scienze della Terra e del Mare, Università di Palermo, Palermo, Italy, (2) Institut des Sciences de la Terre d'Orléans, CNRS-Université d'Orléans, Orlean, France

Phase equilibrium experiments are recognized as an excellent method to determine the pre-eruptive conditions of magmas inasmuch they are extremely sensitive to small variation in major elements composition in the studied rocks. Trachytes and peralkaline rhyolites (i.e pantellerite and comendite) usually represent the felsic end-member in continental rift systems and oceanic island settings. Pantelleria island, almost entirely made up of trachyte and pantellerite, is well known in the petrological literature as being the type locality of pantellerites. In this study we present the results of phase equilibrium experiments performed on representative peralkaline rhyolite (pantellerite) and metaluminous trachytes of the Green Tuff eruptions of Pantelleria, the sole known compositionally zoned ignimbrite at this volcanic location, which varies from a crystal-poor pantellerite at the base towards a crystal-rich trachyte at the top of the eruptive sequence. Crystallization experiments were performed in the temperature range 750-950°C, pressure 1-1.5 kbar, fluid saturation conditions with XH₂O (=H₂O/H₂O+CO₂) between 0 and 1 and redox conditions fixed around the FMQ (Fayalite-Magnetite-Quartz) buffer.

Results show that at 900 °C pantelleritic starting compositions are well above their liquidus, regardless their water content. At T < 800°C clinopyroxene is the liquidus phase followed by amphibole and alkali feldspar. Aenigmatite and quartz crystallize at 750°C and XH₂O lower than 0.8. In contrast, the trachytic composition at 800°C is highly crystallized regardless its water content. The liquidus phase is clinopyroxene crystallizing at 950°C and XH₂O < 0.8 followed by iron-rich olivine and alkali feldspar. Iron-bearing minerals record the effect of both H₂O and fO₂, showing progressive iron enrichment when XH₂O decreases. Alkali feldspar becomes the most abundant mineral phase for XH₂O < 0.8 at 900°C or XH₂O < 1 at 850°C both at 1 and 1.5 kbar. Experiments reproduced the mineral assemblages of the natural rocks, i.e. the pre-eruptive conditions were constrained at P ~ 1 kbar, and is found that the compositional zoning in magma chamber (pantellerite to trachyte) is related to a temperature gradient (750°C-900°) within the reservoir. Moreover, our results allow demonstrating that a peralkaline liquid derivate can be produced from a metaluminous trachyte at T< 850°C after extensive alkali feldspar crystallization (~ 80 wt%).