Other
Scientific paper
Jul 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59.2941d&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 14, pp.2941-2953
Other
2
Scientific paper
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 Pontine 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: 206 Pb/ 204 Pb:18.778-18.864; 207 Pb/ 204 Pb:15.641-15.701; 208 Pb/ 204 Pb:38.858-39.090; the values overlap among the groups, implying that the xenoliths are closely related. The xenoliths straddle the best-fit line describing the regional NW-SE variation of values for uranogenic Pb in volcanic rocks from the Roman alkaline province. The similarity between the xenoliths and volcanic rocks permits the interpretation that the xenoliths are representative of the source region of the volcanic rocks, residues after partial melting of the source region or fractional crystallization of the volcanic rocks, or even that the xenoliths represent assimilants obtained during the evolution of the magmas that produced the volcanic rocks. Xenoliths belonging to the ultramafic-mafic cumulate group contain only silicate melt inclusions ± vapor bubble ± droplets of an opaque phase and rarely some CO 2 trapped in silicate melt inclusions. Xenoliths in the alkali syenite group have three types of fluid inclusions: (1) single phase vapor and silicate melt inclusions; (2) two-phase silicate melt + salt, silicate melt + CO 2 (V), aqueous (L + V), and silicate melt + vapor inclusions, and (3) three-phase and multiphase inclusions: CO 2 (L) + CO 2 (V) + H 2 O; silicate melt + saline melt + H 2 O ± birefringent or opaque trapped minerals; H 2 O + salt + silicate glass ± birefringent trapped minerals. During heating experiments, melting of salt occurs at temperatures from 565 to 815°C, depending on the water content of the inclusions. Homogenization of the vapor bubble occurs from 850-1160°C, and complete melting of the silicate glass at about 950°C. The highly variable proportions of the individual phases in the silicate melt + salt + H 2 O inclusions and the coexistence of silicate melt + CO 2 inclusions indicates immiscibility during the crystallization of the magma. Primary and secondary CO 2 inclusions in the alkali syenite suite indicate lower densities (from 0.10 to 0.22 g/cm 3 ) than those resulting from primary CO 2 inclusions in the gabbroic suite (from 0.34 to 0.42 g/cm 3 ). The P-T trajectory of the probable fluid evolution path shows that the crystallization of gabbro occurred between ~1 and 1.4 kbars, whereas alkali syenite crystallized between ~200 and 400 bars. The secondary H 2 O inclusions in alkali syenite were trapped in the later stages of the hydrothermal process and at much lower temperatures (130-290°C), but at pressures relatively close to those of alkali syenite crystallization. The almost isobaric conditions during the final stage of the fluid evolution path are explained by the very shallow emplacement of the alkali syenite intrusive body.
Ayuso R. A.
de Vivo Benedetto
Lima Arlindo
Lirer L.
Török K.
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