Fluorian garnets from the host rocks of the Skaergaard intrusion: Implications for metamorphic fluid composition

Zoned, silica-deficient, calcic garnets containing up to 5 mol% F substitution for 0 formed during contact metamorphism of basalts by the Skaergaard intrusion in East Greenland. Fluorian calcic garnets occur as a retrograde alteration of prograde wollastonite and clinopyroxene that fills vesicles and vugs in lavas 30-70 m from the intrusion. Paragenetically equivalent phases include fluorite, wollastonite, calcic clinopyroxene, prehnite, quartz, and calcite. Electron microprobe analysis shows that the garnets are :::-:93mol% grossular-andradite (grandite) solid solutions and that the F content does not fully compensate for the silica deficiency, suggesting the presence of a hydrous component in the garnets. The garnets display discontinuous zoning with respect to Al and Fe, and increases in F, calculated OH, and the Si deficiency with increasing Al concentration are observed. The garnets formed at temperatures between 200 and 420°C based on the coexisting mineral assemblage, and stratigraphic reconstructions indicate pressures of ~ 1 kbar. Assessment of isobaric, isothermal phase relations in the system CaO-A1203-Si02-H20HF allows estimation of fluid characteristics in equilibrium with fluorian grandite-bearing assemblages. The presence of fluorite, wollastonite, quartz, and calcite with these garnets combined with fluid inclusion data require that the activity (a) of H20 was ~ 1 and that aH.aF was 10-105 to 10-10.0in the coexisting hydrothermal solutions at 200-420 °C and 1 kbar. If pH was neutral at these pressure and temperature conditions (5.3 ::!:0.1), the value of aF-associated with fluorian garnet formation was 10-4.7 to 10-52. The F content of garnet is extremely sensitive to minor changes in fluid composition. The calculations show that a decrease in pH or an increase in log aFof 0.3 at constant pressure and temperature will decrease the F concentration in garnet from 5 to 0 mol%. The results of this study show that fluorian hydrous grandites provide a mineralogical record of the activities ofF species in coexisting metamorphic and hydrothermal fluids.