Maria Luce Frezzotti

Melt-mineral-fluid interactions in ultramafic nodules from alkaline lavas of Mount Etna (Sicily, Italy): Melt and fluid inclusion evidence

Abstract Clinopyroxene phenocrysts and ultramafic nodules in the “Ancient Alkaline Lavas” of Mount Etna contain melt and fluid inclusions which represent immiscible trapping of basaltic melt and CO2. Inclusions consist of glass + vapor (CO2) hosted by clinopyroxene with kaersutite, magnetite, spinel and apatite daughter minerals (type I), and liquid (CO2) + vapor (CO2) + glass secondary inclusions (type II) with highly variable fluid/glass ratios. Electron-microprobe investigations on type I melt inclusions indicate a glass of trachitic composition, with extremely high chlorine contents (0.5–1 wt.%), which we believe evolved from an initial melt composition of an alkali-basalt. Homogenization temperatures (Th) (n=128) of type II CO2 + glass inclusions display a bimodal fluid density distribution, with high-density (0.61–0.75 g/cm3) CO2 fluids in the nodules, and low-density (0.23–0.61 g/cm3) CO2 fluids in the phenocrysts. A two-step degassing process is proposed with high-density immiscible CO2 fluids, formed at pressures of about 4.3 kbar. The density distribution suggests interaction events between nodules, ascending lavas, and fluid phases, with a peak around 3.5 kbar. A second generation of lower-density CO2 is recorded around 2.8 kbar, most likely prior to eruption.

Evidence of magmatic CO2-rich fluids in peraluminous graphite-bearing leucogranites from Deep Freeze Range (northern Victoria Land, Antarctica)

Fine-grained peraluminous synkinematic leuco-monzogranites (SKG), of Cambro-Ordovician age, occur as veins and sills (up to 20–30 m thick) in the Deep Freeze Range, within the medium to high-grade metamorphics of the Wilson Terrane. Secondary fibrolite + graphite intergrowths occur in feldspars and subordinately in quartz. Four main solid and fluid inclusion populations are observed: primary mixed CO2+H2O inclusions + Al2SiO5 ± brines in garnet (type 1); early CO2-rich inclusions (± brines) in quartz (type 2); early CO2+CH4 (up to 4 mol%)±H2O inclusions + graphite + fibrolite in quartz (type 3); late CH4+CO2+N2 inclusions and H2O inclusions in quartz (type 4). Densities of type 1 inclusions are consistent with the crystallization conditions of SKG (≈750°C and 3 kbar). The other types are post-magmatic: densities of type 2 and 3 inclusions suggest isobaric cooling at high temperature (≈700–550°C). Type 4 inclusions were trapped below 500°C. The SKG crystallized from a magma that was at some stage vapour-saturated; fluids were CO2-rich, possibly with immiscible brines. CO2-rich fluids (±brines) characterize the transition from magmatic to post-magmatic stages; progressive isobaric cooling (T<670°C) led to a continuous decrease offO2 can entering in the graphite stability field; at the same time, the feldspars reacted with CO2-rich fluids to give secondary fibrolite + graphite. Decrease ofT andfO2 can explain the progressive variation in the fluid composition from CO2-rich to CH4 and water dominated in a closed system (in situ evolution). The presence of N2 the late stages indicates interaction with external metamorphic fluids.

Fluid inclusions in xenoliths yield evidence for fluid evolution in peralkaline granitic bodies at Pantelleria (Italy)

Abstract Fluid inclusions are rare in peralkaline granitoid xenoliths entrained in volcanic units at Pantelleria, Italy, suggesting that the shallow magma bodies in this peralkaline center do not develop extensive hydrothermal systems. Only one of the samples studied, an alkali-feldspar granite xenolith, contains abundant fluid inclusions in quartz. Four types of aqueous fluid inclusions are distinguished: Type I brines, including early, highly saline, NaCl-bearing inclusions (Th L = 508–560°C; salinity = 60–68 eq. wt.% NaCl) that occur in random groups (Type Ia) and late, NaCl-bearing inclusions (Th L = 386–450°C; salinity = 37–45 eq. wt.% NaCl) along secondary healed fractures (Type Ib); Type II vapor-rich aqueous inclusions (Th V > 600°C) associated with Type Ia inclusions; Type III biphase (L + V) aqueous inclusions (Th L = 360–390°C) along secondary healed fractures; and minor Type IV high-density biphase (L + S) brines. The earliest fluids to be trapped (T > 600°C) are represented by vapor-rich inclusions (Type II), which suggest boiling of the fluid phase. No coexisting brines were trapped, however, as all brines were trapped at temperatures below 560°C. High-salinity fluid phases circulated in the system down to temperatures of about 380°C, suggesting prolonged endogeneous fluid circulation in the shallow intrusive body that was the source for the xenolith. In contrast to the cooling history of many shallow-level granitic bodies, there is no evidence of pervasive circulation of low-temperature (