TY - JOUR
T1 - Iron - Titanium oxide minerals in block-and-ash-flow deposits
T2 - Implications for lava dome oxidation processes
AU - Saito, Takeshi
AU - Ishikawa, Naoto
AU - Kamata, Hiroki
N1 - Funding Information:
We are grateful to D.J. Dunlop, Ö. Özdemir and B. Scaillet for helpful reviews that improved the manuscript. The electron microprobe analyses were done at Beppu Geothermal Research Laboratory of Kyoto University, at Institute for Frontier Research on Earth Evolution of Japan Marine Science and Technology Center, and at Venture Business Laboratory of Kobe University. We would like to thank Y. Tatsumi and N. Tomioka for permission to use the microprobe in their laboratory. We also thank T. Kawamoto and H. Shukuno for instructing and helping with electron microprobe analyses. M. Funaki and M. Ozima are acknowledged for helping with identification of iron–titanium oxide minerals using the optical microscope. I. Iizawa helped with digital image processing. We also acknowledge comments and suggestions by M. Torii. This research was supported in part by a Grant-in-Aid of the Fukada Geological Institute.
PY - 2004/12/15
Y1 - 2004/12/15
N2 - 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 °C 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 °C, 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 °C and preserved their compositions by rapid cooling.
AB - 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 °C 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 °C, 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 °C and preserved their compositions by rapid cooling.
KW - Block-and-ash flow
KW - Deuteric oxidation
KW - Lava dome
KW - Pseudobrookite
KW - Titanohematite
KW - Titanomagnetite
UR - http://www.scopus.com/inward/record.url?scp=11044230651&partnerID=8YFLogxK
U2 - 10.1016/j.jvolgeores.2004.07.006
DO - 10.1016/j.jvolgeores.2004.07.006
M3 - 学術論文
AN - SCOPUS:11044230651
SN - 0377-0273
VL - 138
SP - 283
EP - 294
JO - Journal of Volcanology and Geothermal Research
JF - Journal of Volcanology and Geothermal Research
IS - 3-4
ER -