Eur. J. Mineral. This paper has been presented at the ECROFI XIII 1996, 8, 1401-1420 Symposium in Sitges, Spain (June 1995) Magmatic (silicates/saline/sulfur-rich/C02) immiscibility and zirconium and rare-earth element enrichment from alkaline magma chamber margins : evidence from Ponza Island, Pontine Archipelago, Italy HARVEY E. BELKIN ! '*, BENEDETTO DE VIVO 2 , ANNAMARIA LIMA 2 and KÁLMÁN TOROK 3 1 - U. S. Geological Survey, MS 959, Reston, 22092 VA, USA - Dipartimento di Geofisica e Vulcanologia, Università di Napoli Federico II, Via Mezzocannone 8, 1-80134 Napoli, Italy - Department of Petrography and Geochemistry, Eôtvos University, Muzeum Krt/4, H-1088 Budapest, Hungary Abstract: Fluid inclusions were measured from a feldspathoid-bearing syenite xenolith entrained in trachyte from Ponza, one of the islands of the Pontine Archipelago, located in the Gulf of Gaeta, Italy. The feldspathoid-bearing syenite consists mainly of potassium feldspar, clinopyroxene, amphibole, biotite, titanite, manganoan magnetite, apatite with minor nosean, Na-rich feldspar, pyrrhotite, and rare cheralite. Baddeleyite and zirkelite occur associated with manganoan magnetite. Detailed electron-microprobe analysis reveals enrichments in REE, Y, Nb, U, Th as well as Cl and F in appropriate phases. Fluid inclusions observed in potassium feldspar are either silicate-melt or aqueous inclusions. The aqueous inclusions can be further classified as. (1) one-phase vapor, (2) two-phase (V + L) inclusions, vapor-rich inclusions with a small amount ofCÜ2 in most cases; homogenization of the inclusions always occurred in the vapor phase between 359 and 424°C, salinities vary from 2.9 to 8.5 wt. % NaCl equivalent; and. (3) three-phase and multiphase inclusions (hypersaline/sulfur-rich aqueous inclu- sions sometimes with up to 8 or more solid phases). Daughter minerals dissolve on heating before vapor/liquid homogenization. Standardless quantitative scanning electron microscope X-ray fluorescence analysis has tentatively identified the following chloride and sulfate daughter crystals; halite, sylvite, glauberite, arcanite, anhydrite, and thenardite. Melting of the daughter crystals occurs between 459 and 536°C (54 to 65 wt. % NaCl equivalent) whereas total homogenization is between 640 and 755 °C. The occurrence of silicate-melt inclusions and high- temperature, solute-rich aqueous inclusions suggests that the druse or miarolitic texture of the xenolith is late-stage magmatic. The xenolith from Ponza represents a portion of the peripheral magma chamber wall that has recorded the magmatic/hydrothermal transition and the passage of high solute fluids enriched in chlorides, sulfur, and incom- patible elements. Key-words: fluid inclusions; melt inclusions; immiscibility; baddeleyite, zirkelite, chloride and sulfate daughter crystals, volcanology; Ponza. * Email: hbelkin@usgs.gov 0935-1221/96/0008-1401 $5.00 © 1996 E. Schweizerbart'sche Verlagsbuchhandlung. D-70176 Stuttgart