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The Canadian Mineralogist; February 2004; v. 42; no. 1; p. 143-153; DOI: 10.2113/gscanmin.42.1.143
© 2004 Mineralogical Association of Canada
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Articles

KANONAITE FROM THE METAMORPHIC SOLE OF THE OMAN OPHIOLITE: EVIDENCE FOR HIGH-f (O2) CONDITIONS DURING RETROGRADE METAMORPHISM

Yaso Kugimiya1,§, Susumu Umino1, Toshiaki Masuda1 and Yasushi Matsuda2

1 Institute of Geosciences, Shizuoka University, Shizuoka 422-8529, Japan
2 Department of Life Science, Himeji Institute of Technology, Kamigori-cho, Hyogo 678-1297, Japan

§ E-mail address: ykugimiya{at}yahoo.com

Kanonaite has been discovered in a metachert pebble derived from the metamorphic sole of the Oman ophiolite in United Arab Emirates. The kanonaite rims (50 µm in maximum width) manganian andalusite with a distinct chemical discontinuity, and is also found intergrown with muscovite. It contains up to 65 mol.% Mn3+AlSiO5, and the Fe analogue, Fe3+AlSiO5, typically accounts for 3–12 mol.% of the total. Petrographic features suggest that kanonaite formed by reaction between manganian andalusite and quartz, with the addition of K introduced by an aqueous fluid, according to: 6(Al0.62Mn0.30 3+Fe0.08 3+)AlSiO5 + 3SiO2 + 2K+ + 3H2O -> 3(Mn0.60 3+Al0.36Fe0.04 3+)AlSiO5 + K2(Al3.64Fe0.36 3+)(Si6Al2)O20(OH)4 + 2H+. The formation of kanonaite can be attributed to a retrograde hydration event under high-f(O2) buffering according to the solid–solid reactions: 14Mn2O3 + 4SiO2 = 4Mn7SiO12 + O2 and Mn2O3 + SiO2 + Al2SiO5 = 2MnAlSiO5. The formation of kanonaite suggests that the retrograde metamorphism in the sole of the Oman ophiolite involved localized conditions of high f(O2).

Keywords: kanonaite, manganian andalusite, high fugacity of oxygen, metamorphic sole, Oman ophiolite, United Arab Emirates.






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