2018 Volume 113 Issue 2 Pages 68-81
Triple–layer coronas around corundum occur in the Lützow–Holm Complex at Akarui Point, East Antarctica. The monomineralic layers of green spinel, sapphirine, and plagioclase are arranged in this order from corundum to the matrix hornblende, indicative of the reaction
corundum + hornblende = green spinel + sapphirine + plagioclase + H2O–fluid.
Singular value decomposition analysis in the simplified Na2O–CaO–MgO–Al2O3–SiO2–H2O system implies that either Na2O and CaO, Na2O and MgO, or CaO and SiO2 were open during the corona formation. The position of the initial boundary between corundum and hornblende coincided with the present sapphirine–plagioclase boundary, as judged from the spatial position of the brownish–green spinel in the coronas which is also present in the matrix and hornblende. The flow of Al2O3 decreased outward and completely ceased at the plagioclase–hornblende boundary. This feature together with the monotonic decrease in Al content from corundum to hornblende as well as within sapphirine and plagioclase suggests that diffusion of Al2O3 controlled the corona formation. The net reaction showed an increase in volume, indicating that the products were stable at lower pressures. Therefore, the corona–forming reaction took place during the decompression stage of the Lützow–Holm Complex.