The behavior of Cl, F, and H₂O during the late evolution of alkaline magmas on the Island of Pantelleria

Solovova I.P., Kovalenko V.I., Girnis A.V.

Institute of Ore deposits, petrography, mineralogy and geochemistry RAS (IGEM),

 Moscow, Russia

The Island of Pantelleria is situated in the Straight of Sicily within the Sicily continental rift. The island is made up of a bimodal basalt–pantellerite volcanic association. The formation of the main caldera separates the development of pre- and post-caldera volcanic suites. Ignimbrites with an age of 114 ka mark the boundary between the early (325–124 ka) and late (106–47 ka) sequences. The last eruption of subalkali basalts was documented in 1891. The volcanic rocks are rich in Cl, which was interpreted as an indicator of the “dryness” of pantellerite magmas.

This study is based on the analysis of melt inclusions in minerals. The investigation of inclusions included heating/cooling experiments, electron microprobe analysis, secondary ion mass spectrometry, and Raman spectroscopy.

It was shown that volatile components (CO₂, H₂O, Cl, F) played a significant role during all stages of magma formation of differentiation. Carbon dioxide was the main volatile component in the source region of subalkaline (hawaiite) magmas. The analysis of glasses from melt inclusions in hawaiites showed no more than 0.07 wt % F and Cl. The low content of H₂O in the deep fluids is supported by the investigation of a gas phase in melt inclusions.

The more evolved plumasitic trachytes inherit the enrichment of melts in CO₂ fluids, which were observed in fluid inclusions. In contrast to the hawaiite magmas, inclusions in the trachytes contain up to 1 wt % Cl. The direct analysis of hydrogen (SIMS) in inclusions in anortoclase phenocrysts homogenized at 985œC showed an H₂O content of ~0.3 wt %.

Melt inclusions from the agpaitic trachytes and pantellerites also contain CO₂. Fluid inclusions dominated by CO₂ were found in anorthoclase phenocrysts. Solid CO₂ was obtained during cooling of these inclusions and sublimated at temperatures from –75 to –62œC, which correspond to densities of 0.0037–0.010 g/cm³ and fluid pressures of 9–24 bar at 950œC.

During the examination of some melt inclusions in clinopyroxene from the pre-caldera volcanic rocks, unique evidence was found for the loss of H₂O from inclusions during the process of rock formation. Numerous tiny hematite crystals (identified by EPMA) were observed on the walls of such inclusions (fig 1, a). They were formed owing to FeO oxidation accompanying H₂ diffusion through the host mineral in response to a temperature increase. Based on the composition of anomalous inclusions (~7 wt % FeO in normal inclusions and no more than 1.1 wt % FeO in the inclusions with hematite crystals), the maximum amount of H₂O that escaped from such inclusions is estimated as ~1 wt %. According to SIMS analysis, the quenched glasses contain ~0.9 wt % H₂O, which implies that the primary melts contained approximately 2 wt % H₂O. Similar inclusions were also observed in some anorthoclase phenocrysts.

There are two types of melt inclusions in anorthoclases. The first type comprises completely crystallized inclusions with grains of a Zr-bearing amphibole-like mineral. The inclusions of the second type are dominated by glass. In addition, euhedral and skeletal fluorite crystals were observed in these inclusions. The complete melting of fluorite was observed at 840–890œC The content of F in the melts of the amphibole-bearing inclusions is twice that of the fluorite-bearing inclusions (0.58 and 0.26 wt %, respectively). The daughter amphibole contains up to 1.55 wt % F.

Fluorite in melt inclusions closely associates with salt globules dominated by NaCl and containing minor amounts of Ca and Fe (fig 1, b). The fluid of silicate inclusions also contains NaCl crystals, which rim a gas bubble. The melting temperatures of the salt globules in the glasses of inclusions are 760–740œC in the pre-caldera rocks and 640–600œC in the post-caldera rocks, which correspond in the NaCl–H₂O system to salinities of 93–91 и 71–69 wt %, respectively. Correspondingly, the content of H₂O in the salt globules is 7–9 wt % for the pre-caldera pantellerites and 29–31 wt % in the post-caldera rocks.

Similar salt aggregates were observed in the groundmass glass of agpaitic trachytes and pantellerites, where they coexist with low-density gas vesicles. The salt aggregates are polycrystalline and include approximately 90 vol % NaCl. In addition to Na and Cl, their analysis yielded up to 10 wt % Ca, 0.3 wt % K, 0.4 wt % Fe, and 0.3 wt % F. The salt aggregates contain an opaque phase (less than 1 vol %) and a gas bubble (3–8 vol %), in which liquid water was occasionally observed.

The coexistence low-density fluids and salt melts (brines) in the glasses of melt inclusions and rock groundmass indicates that a fluid phase separated from the melt and unmixed into two phases (brine + vapor) during the late magmatic stage of the evolution of the magmatic system.

Thus, the investigation of melt and fluid inclusions in minerals and groundmass glasses showed that the primary basic (hawaiite) magmas were saturated with respect to CO₂, which remained to be the main fluid component until the formation of agpaitic trachytes in the course of crystallization differentiation. The composition of fluid changed significantly in more evolved silicic melts, which contained considerable amounts of Н₂О, Сl, and, to some extent, F. The formation of such melts could not be related to crystal fractionation alone and required mixing/contamination phenomena.