第15回ジオダイナミクスセミナー
　　　Geodynamics　Seminar

　　“Phase relations in the MORB+H2O system
　　　from 10 to 20 GPa”
　　　
　　　　講　師：岡本　和明(ミラノ大学研究員）
　　　　　
　　　　　主催　：　愛媛大学地球深部ダイナミクス研究センター
　　　　　日時　：　２００１年９月７日　（金）　午後５時〜
　　　　　場所　：　愛媛大学理学部講義棟　２０１教室

Phase relations in the MORB+H2O system from 10 to 20 GPa

　　　　　　　　　Kazuaki Okamoto (Univ. Milan)

I conducted ultrahigh P experiments in the MORB+ H2O system at T lower than 1200 °C to decipher the fate of oceanic crust in the coldest slab at the mantle transition zone. The result is as follows. The jadeite eclogite facies (Grt+ Jd) is bounded at the lower P side by the omphacite eclogite facies (Grt+omp). The boundary runs between 13 and 14 GPa, at 700 °C, and between 14 and 15 GPa, at 900 and 1100 °C. The jadeite eclogite facies is bounded at high P side by CaPv garnetite facies (Grt+CaPv). The boundary runs between 900 and 1000 °C, at P = 19 GPa, between 16 and 17 GPa, at 800 °C, and between 18 and 19 GPa, at 900 °C.

It is enlarged omphacite dissolution in garnet within 1 GPa of pressure range, between 14 and 15 GPa. Over 17 GPa, decrease of dissolution of pyroxene component into garnet is caused by enlargement of Na dissolution of hollandite. At 18 GPa, Ca-perovskite is exsolved from garnet reached its maximum Ca content (Gasparik, 1996).

Estimated H2O content in subducting oldest oceanic crust indicates a sudden decrease of water content at 15 GPa. This is simply caused by decrease of CaEs component in clinopyroxene. At further higher pressures than 19 GPa, stishovite, and new hydrous phase identified by Otani et al. (2001) would be water container.