Phase Equilibria of Pantelleria Trachytes (Italy): Constraints on Pre-eruptive Conditions and on the Metaluminous to Peralkaline Transition in Silicic Magmas

The relationships between trachytes and peralkaline rhyolites (i.e. pantellerites and comendites),
which occur in many continental rift systems, oceanic islands and continental intraplate settings, is
unclear. To fill this gap, we have performed phase equilibrium experiments on two representative
metaluminous trachytes from Pantelleria to determine both their pre-eruptive equilibration conditions
(pressure, temperature, H2O content and redox state) and liquid lines of descent. Experiments
were performed in the temperature range 750–950 C, pressure 0 5–1 5 kbar and fluid saturation
conditions with XH2O [¼H2O/(H2OþCO2)] ranging between zero and unity. Redox conditions were
fixed below the nickel–nickel oxide buffer (NNO). The results show that at 950 C and melt water
contents (H2Omelt) close to saturation, trachytes are at liquidus conditions at all pressures.
Clinopyroxene is the liquidus phase, being followed by iron-rich olivine and alkali feldspar.
Comparison of experimental and natural phases (abundances and compositions) yields the following
pre-eruptive conditions: P¼160 5 kbar, T¼925625 C, H2Omelt¼261wt %, and fO2 between
NNO– 0 5 and NNO– 2. A decrease in temperature from 950 C to 750 C, as well as of H2Omelt,
promotes a massive crystallization of alkali feldspar to over 80 wt %. Iron-bearing minerals show
gradual iron enrichment when T and fO2 decrease, trending towards the compositions of the
phenocrysts of natural pantellerites. Despite the metaluminous character of the bulk-rock compositions,
residual glasses obtained after 80 wt % crystallization evolve toward comenditic compositions,
owing to profuse alkali feldspar crystallization, which decreases the Al2O3 of the melt, leading
to a consequent increase in the peralkalinity index [PI¼molar (Na2OþK2O)/Al2O3]. This is the first
experimental demonstration that peralkaline felsic derivatives can be produced by low-pressure
fractional crystallization of metaluminous mafic magmas. Our results show that the pantelleritic
magmas of basalt–trachyte–rhyolite igneous suites require at least 95 wt % of parental basalt crystallization,
consistent with trace element evidence. Redox conditions, through their effect on Fe–Ti
oxide stabilities, control the final iron content of the evolving melt.