Abstract
At mantle pressures, partial melts of peridotite + CO2 are carbonate-rich, but contain insufficient sodium to form most carbonatites and associated silicate magmas. A possible source rock is eclogite, and extrapolations from experiments suggest that eclogite + CO2 will coexist with melt along geotherms of low heat flow. Thermochemical calculations yield high carbonate activities, and imply that the melt is carbonatitic. The melt can be modelled in the system jadeite–calcite–CO2 and will, on emplacement, form mineral assemblages characteristic of a wide range of alkalic rocks. The tectonic settings of carbonatites are consistent with an origin as partial melts of eclogite + carbonate + CO2 under areas of low surface heat flow, and suggest that carbonatite emplacement is not related to mantle hotspots.
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