Origin of olivine melilitites — chemical and experimental constraints

摘要: A comparison of the chemistry olivine melilitites (all possible primary magmas) from Hawaiian Islands (Oahu), Balcones Province (Texas), northern Hessen (Germany), Schwabische Alb (Germany) and Cape (South Africa) shows systematic changes between these provinces in order listed. Al2O3/TiO2 Na2O/K2O decrease, CaO/Na2O CaO/Al2O3 CaO + MgO/SiO2 increase, CaO/MgO remains roughly constant. These chemical features constrain models origin fairly tightly. Constant indicates partial melting under P, T conditions where dolomite can be a solidus phase peridotite composition. Increasing suggests increasing amounts CO2 source region, decreasing degrees (K2O P2O5 increase) depth origin. Conditions were determined on an melilitite (no. 2927) Tasmania by Brey Green (1977) who concluded that it derived as 5% melt pyrolite composition at about 27 kbars 1160°C with 7–8% H2O 6–7% dissolved melt. More undersaturated should greater xCO2 region. To test this model CaCO3 MgCO3 was added to 2927 such bulk matched most South Africa. The influence varying tested 30 35 kbars. Olivine clinopyroxene are liquidus phases higher modified than 2927, but separated garnet orthopyroxene wide field alone. stability increases rapidly lower pressure four saturation (ol, opx, cpx, ga) is approached. Thus inferred like those Africa originate contents region for less compositions. Kimberlites (from Russia Lesotho) form chemically continuous spectrum similar CaO/Al2O3, Al2 O3/TiO2 MgO/SiO2. However, MgO/CaO ratio 3–4 times melilitites. It suggested therefore, kimberlite magmas products very low peridotitic magnesite stable more refractory because carbonation reactions.