The Transition from Carbonate to Silicate Melts in the CaO--MgO--SiO2--CO2 System

Abstract

The compositions of melts in equilibrium with a lherzolite mineral INTRODUCTION assemblage were determined in the analogue system CaO– The common association of carbonatites with silicaMgO–SiO2–CO2 at 3 GPa. Carbonate liquids coexist with olivine undersaturated alkaline igneous rocks has led to many and two pyroxenes between the solidus for carbonated lherzolite at speculative petrogenetic schemes for their generation. 1250°C and 1450°C. The Ca/(Ca + Mg) ratio of these melts These include limestone assimilation (Shand, 1945) to is 0·64 and the main eVects of rising temperature are increasing produce silica-undersaturated rocks, crystal fractionation SiO2 content (from 1700°C) is approached. The progression from carbonate to from isotopic studies (e.g. Powell et al., 1966; Bell & silicate liquids is, therefore, abrupt and the field of transitional Blenkinsop, 1987a, 1987b) that the primitive magma is compositions (10–30% SiO2) is restricted to very narrow temof mantle origin. An understanding of the genesis of perature intervals at pressures greater than the solidus ledge. All associated carbonate and silicate rocks therefore depends liquids appear to be miscible. In the context of upwelling magma, on determining the compositions of melts that are proour results provide possible insight into the origins of complexes that duced from peridotitic compositions under upper-mantle are considered to contain primary carbonatites. The solidus ‘ledge’ conditions. This study is aimed at the elucidation of between 2·5 and 3 GPa acts as a filter for both carbonatites and melting behaviour in mantle analogue systems. transitional melt compositions. Carbonatites, which have a wide Qualitative interpretation of the behaviour of carstability field at 3 GPa, may rise through the mantle if they are bonated peridotite at solidus and supersolidus conditions isolated from lherzolite by wallrock reaction and production of can be derived from the model system CaO– wehrlite. Transitional carbonate–silicate melts must also, however, MgO–SiO2–CO2 (CMS–CO2) (Fig. 1). Decarbonation react with the mantle at low pressures. This fact, combined with reactions (Wyllie & Huang, 1975a, 1975b, 1976; Eggler, the small range of physical conditions over which they are generated 1978) define the stability fields of carbonate in lherzolitic and their higher (than carbonatite) viscosity, means that they rarely mantle. The following reaction separates regions in the reach crustal levels. Low-CO2 silicate melts, in contrast, are not mantle where CO2 exists as a free fluid phase and where required to react extensively en route to the surface and are abundant. it resides in mineral phases (dolomite): We suggest that the binary nature of some carbonatite complexes