[S-IT41_28PM1] 海洋プレートの一生：誕生から解体，そして復活

[SIT41-15] クロミタイト温度圧力履歴の指示者としての高Ni・Mgかんらん石

*三浦 真¹、荒井 章司¹、山本 伸次²、シュメレフ ブラディミール³ (1.金沢大学大学院 自然科学研究科、2.東京大学大学院 総合文化研究科、3.ウラル ロシア科学アカデミー 地質・地球化学研究科)

キーワード：かんらん石, Ni・Mg拡散, ポディフォーム・クロミタイト, 低圧クロミタイト, 超高圧クロミタイト, 温度圧力履歴

High Ni and Mg olivines were found in dunites enveloping podiform chromitites from Oman, Ray-Iz and Luobusa ophiolites. These high Ni and Mg olivines occur only in dunite adjacent to chromitite. This characteristic suggests subsolidus Ni and Mg diffusion from the chromites of the chromitite. In the case of dunite enveloping concordant chromitite from Oman ophiolite, olivines show high NiO (up to 0.5 wt %) and Fo (around 92 mol %) contents. This is not the case, however, for the dunite envelope around the discordant chromitite in the Oman ophiolite. On the other hand, olivines in dunite enveloping UHP chromitites from Ray-Iz and Luobusa ophiolites are extraordinarily high in Fo value (94 - 96) and NiO (around 0.5 wt %). Silicate exsolution lamellae in spinel from UHP chromitites and concordant chromitite suggest that these chromitites have experienced substantial cooling, and probably decompression, for a longer priod than the discordant chromitite from Oman. According to the well-known Ni and Mg diffusion coefficients in olivine, the high-Ni and -Mg olivine in the dunite envelope may constrain the cooling duration of the chromitite and the history of ophiolite. Podiform chromitites are enigmatic in origin, and their origins should be systematically classified to understand concerning mantle processes. Their temporal relationship is a clue to solve this problem.