Origin Of Olivine Melilitites - Chemical And Experimental Constraints

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Journal Article: Origin Of Olivine Melilitites - Chemical And Experimental Constraints

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

G. Brey

Published Journal 
Journal of Volcanology and Geothermal Research, 1978

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G. Brey. 1978. Origin Of Olivine Melilitites - Chemical And Experimental Constraints. Journal of Volcanology and Geothermal Research. (!) .