Temperature-HF fugacity trends during crystallization of calcite carbonatite magma in the Fen complex, Norway

Abstract

AbstractCalcite carbonatite (søvite), associated apatite cumulates and contact metasomatic phlogopite-rocks in the Fen complex (S. Norway) contain coexisting apatite and phlogopite. Two distinct compositional groups are recognized: fluorapatite (XF = 0.5–0.8) coexists with hydroxy-phlogopite with XF = 0.11–0.26, whereas hydroxyapatite (XF = 0.34–0.44) coexists with low-F hydroxy-phlogopite (XF ≈ 0.03). Apatite-biotite geothermometry suggests that the minerals equilibrated at igneous temperatures (less than 1200 to ca. 625 °C), and were not significantly disturbed by late low-temperature re-equilibration. This, combined with HF-barometry based on apatite-fluid and phlogopite-fluid F-OH exchange equilibria, makes it possible to recognize two crystallization trends at different levels of hydrogen fluoride fugacity. The existence of such trends reflects internal buffering of the HF fugacity by apatite and/or phlogopite. The recorded differences in HF fugacity suggest that two or more independent or semi-independent lines of magmatic descent gave rise to calcite carbonatite magma in the Fen complex. Combined apatite-phlogopite geothermometry and hydrogen fluoride barometry is a useful tool enabling us to see through post-magmatic alteration of carbonatites, and to establish primary magmatic controls even in cases where carbonate and iron-titanium oxide minerals have re-equilibrated at much lower temperatures.