Volatiles in pantellerite magmas: A case study of the Green Tuff Plinian eruption (Island of Pantelleria, Italy)

The Green Tuff (GT) Plinian eruption, the largest in magnitude at Pantelleria, erupted 3 to 7 km3 DRE of
pantellerite magma and a small volume of trachyte. Fifty-nine anorthoclase-hosted melt inclusions from the
two basal pumice memberswere analyzed by FT-IR spectroscopy in order to assess the pre-eruptiveH2Ocontent
in the pantellerite melt. Microanalytical methods were used to determine major element, Cl, F and S contents.
Melt inclusions and glassy groundmasses have a nearly homogeneous pantelleritic composition (peralkaline
index = 1.9-2.2) and variable water contents ranging from 1.4 to as high as 4.2 wt %, i.e. much higher than
the 1.4 wt % of earlier published studies. The chlorine content is constant at about 1 wt %. Combined Cl and
H2O data were used to estimate a confining pressure of about 50 MPa (depth around 2-3 km) for the GT
magma chamber. The chamber was characterized by a compositional zoning with a dominant pantellerite
overlying a trachyte magma. Soon after the GT eruption, intra-caldera volcanism was dominated by the
eruption of voluminous trachyte lava flows, while pantellerite melt production resumed after about 20 ka
with numerous low-volume, mildly explosive (Strombolian) to effusive eruptions. Comparison with data
from the literature reveals that, despite the different explosivity, the post-caldera Strombolian eruptions
and the GT Plinian eruption were fed by pantelleritic magmas with similar water contents. Chlorine and
CO2 contents suggest that the young magma reservoirs feeding the Strombolian to effusive activity were
deeper (h ≥ 4.5 km) than the much larger (based on erupted volumes) magma chamber which fed the
GT eruption.