Fluorphlogopite and fluortremolite in Adirondack marbles and calculated C–O–H–F fluid compositions

Fluorphlogopites (up to XF = 0.96) and fluortremolites (up to XF = 0.82) have been found in Grenville marbles near Balmat, New York. In these minerals high fluorine contents correlate inversely with iron solid solution, but do not appear to affect the stable isotope fractionation of hydrogen.

The substitution of fluorine for hydroxyl in micas and amphiboles is partly responsible for stabilizing hydrous minerals in the granulite facies marbles of the Adirondacks. The importance of this effect can be calculated from thermochemical data and a knowledge of F/OH distribution in natural assemblages. Phlogopite concentrates fluorine relative to tremolite. The F/OH distribution coefficient does not vary with metamorphic grade but differs significantly in the most F-rich sample from the other samples. Application of volatilization equilibria in marbles without consideration of possible fluorine substitution can lead to large errors in estimated pressures, temperatures and fluid compositions.

The fugacities of eight C–O–H–F fluid components are restricted by the assemblage fluorphlogopite + calcite + quartz + graphite. At 650°C, 6 kbar, the estimated range in these values is: log fH₂0 = 2.47 to 3.37, log fCO₂ = 4.52 to 4.22, log fCH₄ = 0.25 to 2.35, log fCO = 2.07 to 1.92, log fH₂ = 0.34 to 1.39, log fO₂ = −18.01 to −18.31, log fHF = −1.27 to −0.37 and log fF₂ = −34.19 to −33.43. H₂O and CO₂ were the dominant fluid components in these graphitic marbles and CH₄ was minor. Calculations of many oxidation-fluoridation equilibria (fF₂vs. fO₂) for common calcsilicate minerals support these values of fF₂ and fO₂. The inferred fO₂ is within 0.5 log unit of the QFM buffer, near that commonly inferred for other Adirondacks marbles. Minimum values of log fF₂ are −35.0 for end-member fluorphlogopite, −34.4 for end-member fluortremolite and −34.1 for norbergite plus fluortremolite in calcite marbles at these P and T. Such values of fF₂ may not be unusual suggesting that fluorphlogopite and fluortremolite could be relatively common rock-forming minerals in iron-poor marbles.

The mineral assemblage fluortremolite + fluorphlogopite + diopside + calcite + quartz + graphite and fluortremolite + calcite + graphite + norbergite restrict $ΔGf923∘$ (fluortremolite) to −10,378± 11 kjoules relative to available free energy data for the other phases.