Abstracts for Annual Meeting of the Mineralogical Society of Japan
2005 Annual Meeting of the Mineralogical Society of Japan
Session ID : K1-P01
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Phase relations in peridotite-CO2 system to 20 GPa
Sujoy Ghosh*Eiji OhtaniKonstantin LitasovAkio SuzukiHidenori Terasaki
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CONFERENCE PROCEEDINGS FREE ACCESS

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Abstract

Carbon dioxide and water are the most important volatile constituents in the Earth and they produce drastic changes in the melting phase relations and partial melt composition of the mantle peridotites. Study of the peridotite-CO2 system is closely related to petrogenesis of kimberlite and diamond. There are a few high pressure mineral inclusions (i.e. majorite garnet and Ca & Mg perovskite) in diamond which suggest that kimberlites may be originated from the transition zone and lower mantle. Several experimental petrologists have studied the kimberlite and basalt-CO2 systems, however the phase relations and melt compositions in the CO2-bearing peridotite at high pressures are poorly constrained. Simplified peridotite-CO2 system (like CMS or CMAS) has been studied at pressures up to 12 GPa (Canil and Scarfe, 1990), whereas complex peridotite-CO2 systems have investigated only at lower pressures (up to 4 GPa, e.g. Wendlandt and Mysen, 1980). In this work we report the results on the phase relations and melt compositions of a model peridotite-CO2 system determined at 10-20 GPa and temperature range from 1200 to 2100oC.Our results show that solidus of carbonated peridotite is consistent with low-pressure data for CMAS-CO2 system. Liquidus phase at 10-20 GPa is majorite garnet. At 10-15 GPa, crystallization sequence with decreasing temperature is garnet, olivine and clinoenstatite. Magnesite is the most important CO2-rich phase stable in peridotite and clinoenstatite is an important phase in carbonated peridotite at 10-15 GPa.The partial melts formed by 10-25% melting at 10-20 GPa has high MgO (26-34 wt.%) and FeO (7.0-10.4 wt.%) and low SiO2 (18-36 wt.%) and Al2O3 (0.5-1.3 wt.%) contents. It contains also 6-12 wt.% CaO, 0.6-2.0 wt.% Na2O and 0.1-0.3 wt.% K2O. The CO2 contents in the melts are 14-32 wt.%. SiO2-poor composition of partial melts is different to the results for melting of anhydrous or water-bearing peridotite. Partial melting of hydrous peridotite produce SiO2-rich melts, which can be related to komatiite magmas (e.g. Litasov and Ohtani, 2002). The composition of low degree partial melts (10%) in present experiments is close to magnesiocarbonatites, whereas higher degree melting (20-25%) produce melts, which is close to kimberlite magmas.

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© 2005 Japan Association of Mineralogical Sciences
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