Journal of Petrology Advance Access originally published online on December 10, 2007
Journal of Petrology 2008 49(2):225-238; doi:10.1093/petrology/egm078
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Experimental Investigation of the Upper Thermal Stability of Mg-rich Actinolite; Implications for Kiruna-Type Iron Deposits
Department of Geological Sciences and Environmental Studies, Binghamton University, Binghamton, NY 13902-6000, USA
RECEIVED JANUARY 9, 2007; ACCEPTED NOVEMBER 14, 2007
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Abstract |
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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°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°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 compositional re-equilibration but strong nucleation of Fe-rich cummingtonite. 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 carried out on the thermal decomposition of a natural actinolite (Fe-number = 0·22) from Pleito Melón, Chile, indicating the breakdown of actinolite to clinopyroxene, orthopyroxene, quartz, and water at 780°C and 1 kbar, and 850°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°C/Fe-number at 1 kbar and –546°C/Fe-number at 4 kbar. The high thermal stabilities (750–900°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.
KEY WORDS: actinolite; mineral stability; Kiruna deposits, thermodynamic values; cummingonite
*Corresponding author. Present address: Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, NV 89154-4010, USA. E-mail: haroldo.lledo{at}unlv.edu
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