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Journal of Petrology | Volume 39 | Number 11-12 | Pages 1943-1951 | 1998
© Oxford University Press 1998

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

K. R. Moore* and B. J. Wood

Department of Earth Sciences, Wills Memorial Building, University of Bristol Queen's Road,Bristol BS8 1RJ, UK

Received September 30, 1997; Revised typescript accepted June 23, 1998


  Abstract

The compositions of melts in equilibrium with a lherzolite mineral assemblage were determined in the analogue system CaO–MgO–SiO2–CO2 at 3 GPa. Carbonate liquids coexist with olivine and two pyroxenes between the solidus fo carbonated lherzolite at 1250°C and 1450°C. The Ca/(Ca + Mg) ratio of these melts is 0.64 and the main effects of rising temperature are increasing SiO2 content (from <4.3 to 7.5 wt %) and decreasing CO2 content. Between 1475°C and ~1525°C the SiO2 conten of th liquid increases dramatically from 10 to 30% and, thereafter, the CO2 content decreases rapidly as the CO2-absent invariant point (at >1700°C is approached. The progression from carbonate to silicate liquids is, therefore abrupt and the field of transitional compositions (10–30% SiO2) is restricted to very narrow temperature intervals at pressures greater than the solidu ledge. All liquids appear to be miscible. In the context of upwelling magma, our result provide possible insight into the origins of complexes that are considered to contain primary carbonatites. The solidus ‘ledge’ between 2.5 and 3 GPa acts a a filter for both carbonatites and transitional melt compositions. Carbonatites, which have a wide stability field at 3 GPa, may rise through the mantle if they are isolated from lherzolite by wallrock reaction and production of wehrlite. Transitional carbonate–silicate melts must also, however, react with the mantle at low pressures. This fact, combined with the small range of physical conditions over which they are generated and higher (than carbonatite) viscosity, means that they rarely reach crustal levels. Low-CO2 silicate melts, in contrast, are not required to react extensively en route to the surface and are abundant. We suggest that the binary nature of some carbonatite complexes may be controlled by the compositions of primary mantle melts produced at pressures greater tha the solidus ledge.

KEY WORDS: carbonatites; primary liquids; CO2 saturation; metasomatism

* Corresponding author. Telephone: 0117 928 9000, ext. 4780, Fax: 0117 925 3385. e-mail: K.R.Moore{at}bris.ac.uk


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