Iron-titanium oxide minerals in block-and-ash-flow deposits: implications for lava dome oxidation processes

摘要

The Ikeshiro pyroclastic-flow deposit at Yufu volcano, Kyushu in Japan, consists of typical block-and-ash flows generated by collapse of the Ikeshiro lava dome erupted ca. 2000 years ago. Lava clasts in the Ikeshiro pyroclastic-flow deposit were previously found to consist to two types of rock sample with different magnetic mineral assemblages established by rock magnetic experiments [Saito et al., 2003, J. Volcanol. Geotherm. Res. 126, 127-142]. We examined iron-titanium oxides in samples from the Ikeshiro pyroclastic-flow deposit with an optical microscope and with an electron microprobe analyzer. As a result, samples were classified into two types with different iron-titanium oxide mineral assemblages. Type A oxides are characterized by homogeneous titanomagnetite and titanohematite. Type B oxides are exsolved and composed of two or three phases: Ti-poor titanomagnetite, titanohematite, pseudobrookite and rutile. The reconstituted compositions of type B oxides show the same Fe/Ti ratio as type A oxides. This indicates that type B oxides are produced by oxidation of type A oxides. Type A oxides yield an equilibrium temperature of about 800-850℃ at an oxygen fugacity of NNO+2 using a two-oxide geothermobarometer. This indicates that deuteric oxidation in the lava dome separated samples of each type. Type A rocks originated in oxygen-poor parts of the lava dome with temperatures of about 800-850℃, while type B rocks originated in oxygen-rich parts of the lava dome. Type A oxides remained unoxidized, while type B oxides oxidized and were transformed into complex grains by intense deuteric oxidation. The collapse of the lava dome generated the Ikeshiro pyroclastic flow. All oxides were quenched from about 800-850℃ and preserved their compositions by rapid cooling.