CHLORINE, FLUID IMMISCIBILITY, AND DEGASSING IN PERALKALINE MAGMAS FROM PANTELLERIA, ITALY

This paper documents immiscibility among vapor, highly saline liquid, and silicate melt during the crystallization of peralkaline rhyolites from Pantelleria, Italy, prior to their eruption. Experiments conducte d in a muffle furnace and with a high-temperature heating stage reveal ed three major types of silicate melt inclusions trapped in quartz phe nocrysts. After entrapment in the host phenocryst, type I inclusions c ontained silicate melt. Type II inclusions contained silicate melt + h ydrosaline melt (approximately 60-80 wt% NaCl equivalent), and type II I inclusions contained silicate melt + H2O-CO2 vapor. Two inclusions c ontained all three immiscible fluids: vapor, hydrosaline melt, and sil icate melt. Fluid inclusions within outgassed matrix glass, viewed at room temperature, are interpreted as the crystallized equivalents of t he hydrosaline melts within type II inclusions. These inclusions, 2-10 mum in size, consist of a bubble typically surrounded by a spherical shell of halite. The presence of both vapor and hydrosaline melt in th e magma indicates that the pantellerite was saturated with subcritical NaCl-H2O fluids. At a given temperature and pressure, the fixed activ ity of Cl in these two fluids determines the activity and concentratio n of Cl in the silicate melt. The high concentrations of Cl in these p antellerites (approximately 9000 ppm) are thus a function of the low a ctivity coefficient for NaCl in pantellerite relative to metaluminous silicate liquids. The Cl contents of Pantellerian rhyolites indicate e quilibration at pressures between 50 and 100 MPa. The high Cl contents of outgassed pantellerites may be due to minimal loss of HCI (not NaC l) during eruption, as compared with metaluminous rhyolites, which exs olve more HCl-rich vapors. Discrepancies between the results of heatin g-stage experiments and longer muffle-furnace experiments indicate tha t measurements of melting and homogenization temperatures of melt incl usions may not be accurate unless sufficient time (> 1 h) is allowed f or equilibration at magmatic temperatures.