Origin of olivine melilitites — chemical and experimental constraints

Abstract

A comparison of the chemistry of olivine melilitites (all possible primary magmas) from the Hawaiian Islands (Oahu), the Balcones Province (Texas), northern Hessen (Germany), Schwäbische Alb (Germany) and Cape Province (South Africa) shows systematic changes between these provinces in the order listed. Al 2O 3/TiO 2 and Na 2O/K 2O decrease, CaO/Na 2O CaO/Al 2O 3 and CaO + MgO/SiO 2 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/SiO 2 suggests increasing amounts of CO 2 in the source region, decreasing degrees of partial melting (K 2O and P 2O 5 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% H 2O and 6–7% CO 2 dissolved in the melt. More undersaturated olivine melilitites should be derived from greater depth and greater x CO 2 in the source region. To test this model CaCO 3 + MgCO 3 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 x CO 2 was tested at 30 and 35 kbars. Olivine + clinopyroxene are liquidus phases to higher x CO 2 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 x CO 2 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 CO 2 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/Al 2O 3, lower Al 2 O 3/TiO 2 and higher CaO + MgO/SiO 2. 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.