Opacitization conditions of hornblende in Bezymyannyi volcano andesites (March 30, 1956 eruption)

• Published in: Petrology 2008, Vol.16 (1), p.19-35
• Main Authors: Plechov, P. Yu, Tsai, A. E., Shcherbakov, V. D., Dirksen, O. V.
• Format: Article
• Language: English
• Subjects: Earth and Environmental Science; Earth Sciences; Geochemistry; Mineralogy
• ISSN: 0869-5911; 1556-2085
• DOI: 10.1134/S0869591108010025

The paper is devoted to the conditions under which opacite rims developed around hornblende grains in andesite of the catastrophic eruption (March 30, 1956) of Bezymyannyi volcano, Kamchatka. The opacite rims were produced by a bimetasomatic reaction between hornblende and melt with the development of the following zoning: hornblende → Px + Pl + Ti- Mag → Px + Pl → Px → melt. Biometasomatic reaction was accompanied by the active removal of CaO from the rim, addition of SiO 2 , and more complicated behavior of other components. The hornblende also shows reactions of its volumetric decomposition under near-isochemical conditions. The opacite rims developed under isobaric conditions, at a pressure of approximately 6 kbar. The main reason for the instability of the hornblende was the heating of the magma chamber from 890 to 1005°C due to new hot magma portion injection. The time interval between the injection and the start of eruption was estimated from the thickness of the opacite rims and did not exceed 37 days. Hence, the March 30, 1956, eruption was not related to the volcanic activity in November of 1955 but to the injection of a fresh magma portion in February–March of 1956.