Oriented inclusions in apatite in a post-UHP fluid-mediated regime (Tromsø Nappe, Norway)
Broska, Igor; Krogh Ravna, Erling J.; Vojtko, Peter; Janák, Marian; Konečný, Patrik; Pentrák, Martin; Bačík, Peter; Luptáková, Jarmila; Kullerud, Kåre
European Journal of Mineralogy Volume 26 Number 5 (2014), p. 623 - 634
published: Oct 1, 2014
ArtNo. ESP147052605002, Price: 29.00 €
We report pyrrhotite, anhydrite and dolomite crystal rods in fluorapatite occurring in silicate-bearing carbonate rocks associated with UHP eclogites in the Tromsø Nappe of the Scandinavian Caledonides in Norway. The apatite-rich rock (up to 10 vol. %) is composed of Mg-rich calcite-dolomite exsolutions, almandine-grossular garnet, low-jadeite clinopyroxene, magnesiohornblende, phlogopite, and accessory minerals represented mainly by zircon, Fe-Ti oxides and allanite. Fluorapatite occurring as euhedral crystals in the carbonate matrix and as inclusions in garnet and clinopyroxene shows up to 45 mol. % of the hydroxylapatite component, traces of CO3 2–, probably CN and small amounts of the britholite and ellestadite components. Pyrrhotite occurs as crystallographically oriented rods parallel to the c axis of the host hydroxyl-bearing fluorapatite either as a dense trellis or in the form of scarce inclusions. Precipitation of pyrrhotite in the fluorapatite was probably facilitated by a volatile sulphur phase (e.g., H2S), which was enclosed within the apatite nano-channels and interacted with Fe in apatite. Anhydrite and dolomite rods have also been identified in the apatite, pointing to the presence of HCO3 in the fluids. The anhydrite is also trapped by exsolved dolomite from calcite in the carbonate matrix. Crystallisation of anhydrite, and probably also the associated pyrrhotite, at about 550–650 °C was deduced from calcite–dolomite thermometry. At these amphibolite-facies, post-UHP conditions rapid pyrrhotite precipitation in the host apatite is presumed. Relaxation of the fluorapatite structure in the a-axis direction during decompression facilitated the formation of the oriented inclusions in apatite.