ACTINOLITE IN KIRUNA-TYPE IRON DEPOSITS I: UPPER-THERMAL STABILITY OF MG-RICH ACTINOLITE

The occurrence of actinolite in magnetite deposits of possible magmatic origin has prompted an experimental investigation of the upper-thermal stability of Mg-rich actinolite to determine how the stability of actinolite decreases with increasing Fe content. Experiments were done on the compositional re-equilibration of natural tremolite (molar Fe/(Fe+Mg)=Fe#=0.014) in the presence of synthetic clinopyroxene (Ca_0.88Fe_0.68Mg_0.40Si_2.02O₆), synthetic pigeonite/orthopyroxene (Ca_0.14Fe_1.15 Mg_0.67Si_2.02O₆), quartz, and water to a more Fe-rich actinolite over the range of 900-600°C, 1 and 4 kbar, Ni-NiO oxygen buffer, for durations of 1-2 weeks. Bulk composition of the mineral mixture is close to actinolite with Fe#=0.5. These experiments constitute a half-reversal of the amphibole composition which, when approached from a Mg-rich starting composition, provide information on the minimum Fe content of actinolite at a given temperature. Compositional changes were monitored by electron microprobe analysis of amphibole rim compositions and/or overgrowths on the original tremolite. At 4 kbar and 900-800°C, tremolite shows strong re-equilibration with overgrowths of an Fe-rich but low-Ca (1.7>Ca>1.4) actinolite; Fe-rich cummingtonite (Ca<0.7) begins to nucleate at 860°C. At 800-700°C, tremolite shows weak re-equilibration but nucleation of Fe-rich cummingtonite is strong. Similar results were observed at 1 kbar, with tremolite showing strong re-equilibration to low-Ca actinolite at 790-600°C with cummingtonite nucleation at 800°C and below. The wide variation in Ca contents of the re-equilibrated amphiboles was unexpected.

Additional univariant reversal experiments were done on the thermal decomposition of a natural actinolite (Fe#=0.22) from Pleito Melon, Chile, indicating the breakdown boundary to be at 780°C/1kbar and 860°C/4 kbar. Considering only amphiboles with Ca > 1.7 apfu, the thermal stability of actinolite is observed to decrease in a linear manner over the P-T range investigated with a dT/dFe# slope of –372 °C/Fe# at 1 kbar and –546 °C/Fe# at 4 kbar. These results were modeled thermodynamically using ternary regular-solution theory for amphibole and pyroxenes and show that actinolites with Fe# as high as 0.3 could be stable in the presence of Fe-P-rich magmas to at least 750°C at 1 kbar.