TAILIEUCHUNG - .New Frontiers in Integrated Solid Earth Sciences Phần 10

Công trình tiên phong của họ đã gây ra những khám phá hơn nữa xenoliths ultradeep chứa garnet majoritic mắc-ma có chứa kali siêu mafic ở Java Ontong cao nguyên Malaita, phía tây nam Thái Bình Dương (Collerson et al, 2000). | Ultradeep Rocks and Diamonds in the Light of Advanced Scientific Technologies 377 eclogite took place. Their pioneering work has triggered further discoveries of ultradeep xenoliths containing majoritic garnet from potassic ultramafic magmas at the Ontong Java Plateau of Malaita southwest Pacific Collerson et al. 2000 . Within one of these xenoliths Collerson et al. 2000 have described majoritic garnet in association with Ca- and Mg-perovskite Al-silicate phase with undetermined structure and microdiamond. The conventional geobarometry based on the chemistry of majoritic garnet suggested pressure of 22 GPa whereas the Al-silicate phase was assumed to be crystallized at 27 GPa according to experiments. Taking in account the calculations and assumption above Collerson et al. 2000 have suggested the depth of the Malaita xenolith formation at 600-670 km. However such a deep origin was later questioned by Neal et al. 2001 . Because evaluation of depths from which mantle peridotites originate is always a subject of strong discussions the majoritic garnet - or its product of decompression presented by pyrope with exsolution lamella of pyroxenes - remains one of the best indicators of the very high-pressure environments. It was verified by many experiments conducted in different laboratories that the majoritc garnet is stable at P 5 GPa . Akaogi and Akimoto 1977 Irifune 1987 . The composition of majorite is represented by the complex solid solution M3VI11 Al2-2nMnSin VI Si3Oi2 where M Mg2 Fe2 and Ca2 0 n 1 and superscripts indicate cations oxygen coordination. When pressure rises above 5 GPa the garnetprecursor is transformed into majorite supercilisic garnet with Si SiIV SiVI 3 cations per formula unit the silica content also increases because the Al3 and Cr3 are replaced by M and Si4 cations . Smith and Mason 1970 . Therefore because the Si content in the octahedral site of majoritic garnet increases with increasing pressures Akaogi and Akimoto 1977 Irifune 1997 .

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