FLUID INCLUSION EVIDENCE FOR MAGMATIC SILICATE SALINE/CO2 IMMISCIBILITY AND GEOCHEMISTRY OF ALKALINE XENOLITHS FROM VENTOTENE ISLAND, ITALY/

Fluid and melt inclusions and geochemical features of alkali syenite, mafic, and ultramafic cumulate xenoliths in the last ignimbritic event (volcanism up to 300 ky.b.P.) at the island of Ventotene in the Ponti ne archipelago (Gaeta Gulf) were investigated to establish the genesis and evolution of the fluids trapped in the inclusions. Ranges in lead isotopic compositions of the xenoliths as a group are narrow: Pb-206/ Pb-204:18.778-18.864; (207)pb/(204)pb:15.641-15.701; (208)pb/(204)pb:3 8.858-39.090; the values overlap among the groups, implying that the x enoliths are closely related. The xenoliths straddle the best-fit line describing the regional NW-SE variation of values for uranogenic Pb i n volcanic rocks from the Roman alkaline province. The similarity betw een the xenoliths and volcanic rocks permits the interpretation that t he xenoliths are representative of the source region of the volcanic r ocks, residues after partial melting of the source region or fractiona l crystallization of the volcanic rocks, or even that the xenoliths re present assimilants obtained during the evolution of the magmas that p roduced the volcanic rocks. Xenoliths belonging to the ultramafic-mafi c cumulate group contain only silicate melt inclusions +/- vapor bubbl e +/- droplets of an opaque phase and rarely some CO2 trapped in silic ate melt inclusions. Xenoliths in the alkali syenite group have three types of fluid inclusions: (1) single phase vapor and silicate melt in clusions; (2) two-phase silicate melt + salt, silicate melt + CO2 (V), aqueous (L + V), and silicate melt + vapor inclusions, and (3) three- phase and multiphase inclusions: CO2 (L) + CO2 (V) + H2O; silicate mel t + saline melt + H2O +/- birefringent or opaque trapped minerals; H2O + salt + silicate glass +/- birefringent trapped minerals. During hea ting experiments, melting of salt occurs at temperatures from 565 to 8 15 degrees C, depending on the water content of the inclusions. Homoge nization of the vapor bubble occurs from 850-1160 degrees C, and compl ete melting of the silicate glass at about 950 degrees C. The highly v ariable proportions of the individual phases in the silicate melt + sa lt + H2O inclusions and the coexistence of silicate melt + CO2 inclusi ons indicates immiscibility during the crystallization of the magma. P rimary and secondary CO2 inclusions in the alkali syenite suite indica te lower densities (from 0.10 to 0.22 g/cm(3)) than those resulting fr om primary CO2 inclusions in the gabbroic suite (from 0.34 to 0.42 g/c m(3)). The P-T trajectory of the probable fluid evolution path shows t hat the crystallization of gabbro occurred between similar to 1 and 1. 4 kbars, whereas alkali syenite crystallized between similar to 200 an d 400 bars. The secondary H2O inclusions in alkali syenite were trappe d in the later stages of the hydrothermal process and at much lower te mperatures (130-290 degrees C), but at pressures relatively close to t hose of alkali syenite crystallization. The almost isobaric conditions during the final stage of the fluid evolution path are explained by t he very shallow emplacement of the alkali syenite intrusive body.