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 changes with increasing Fe content. Experiments were carried out primarily on the compositional re-equilibration of natural tremolite [molar Fe/(Fe + Mg) = Fe-number 0.014] in the presence of synthetic clinopyroxene (Ca0.80Fe0.67Mg0.54Si2.00O6), synthetic pigeonite/orthopyroxene (Ca0.08Fe1.19Mg0.70Si2.02O6), quartz, and water to a more Fe-rich actinolite over the range of 600-880 degrees C, 1 and 4 kbar, at the Ni-NiO oxygen buffer, for durations of 1-2 weeks. The bulk composition of the mineral mixture is close to actinolite with Fe-number 0.5. These experiments constitute a half-reversal of the amphibole composition, which, when approached from a Mg-rich starting composition, provides 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 880-800 degrees 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 degrees C. At 800-700 degrees C, tremolite shows weak compositional re-equilibration but strong nucleation of Fe-rich cummingtonite. Similar results were observed at 1kbar, with tremolite showing strong re-equilibration to low-Ca actinolite at 790-600 degrees C with cummingtonite nucleation at 800 degrees C and below. The wide variation in Ca contents of the re-equilibrated amphiboles was unexpected. Additional univariant reversal experiments were carried out on the thermal decomposition of a natural actinolite (Fe-number 0.22) from Pleito Melon, Chile, indicating the breakdown of actinolite to clinopyroxene, orthopyroxene, quartz, and water at 780 degrees C and 1kbar, and 850 degrees C and 4 kbar. Considering only amphiboles with Ca > 1.7 a.p.f.u., the thermal stability of actinolite is observed to decrease in a linear manner over the P-T range investigated with a dT/dFe-number slope of -372 degrees C/Fe-number at 1kbar and -546 degrees C/Fe-number at 4 kbar. The high thermal stabilities (750-900 degrees C) of actinolites with Fe-numbers in the range of 0-0.4 overlap with the range of water-saturated melting for a typical andesite or tonalite. These conditions also overlap the field of experimental Fe-P-rich melt formation, suggesting that actinolite may have an igneous origin in Kiruna-type ore deposits.
