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Carbonatites host some of the largest and highest grade rare earth element (REE) deposits but the composition and source of their REE-mineralising fluids remains enigmatic. Using C, O and  87 Sr/ 86 Sr isotope data together with major and trace element compositions for the REE-rich Kangankunde carbonatite (Malawi), we show that the commonly observed, dark brown, Fe-rich carbonatite that hosts REE minerals in many carbonatites is decoupled from the REE mineral assemblage. REE-rich ferroan dolomite carbonatites, containing 8–15 wt% REE 2 O 3 , comprise assemblages of monazite-(Ce), strontianite and baryte forming hexagonal pseudomorphs after probable burbankite. The  87 Sr/ 86 Sr values (0.70302–0.70307) affirm a carbonatitic origin for these pseudomorph-forming fluids. Carbon and oxygen isotope ratios of strontianite, representing the REE mineral assemblage, indicate equilibrium between these assemblages and a carbonatite-derived, deuteric fluid between 250 and 400 °C (δ 18 O + 3 to + 5‰ VSMOW  and δ 13 C − 3.5 to − 3.2‰ VPDB ). In contrast, dolomite in the same samples has similar δ 13 C values but much higher δ 18 O, corresponding to increasing degrees of exchange with low-temperature fluids (< 125 °C), causing exsolution of Fe oxides resulting in the dark colour of these rocks. REE-rich quartz rocks, which occur outside of the intrusion, have similar δ 18 O and  87 Sr/ 86 Sr to those of the main complex, indicating both are carbonatite-derived and, locally, REE mineralisation can extend up to 1.5 km away from the intrusion. Early, REE-poor apatite-bearing dolomite carbonatite (beforsite: δ 18 O + 7.7 to + 10.3‰ and δ 13 C −5.2 to −6.0‰;  87 Sr/ 86 Sr 0.70296–0.70298) is not directly linked with the REE mineralisation.   Electronic supplementary material  The online version of this article (10.1007/s00410-017-1412-7) contains supplementary material, which is available to authorized users.

contributions to mineralogy and petrology

Deducing the source and composition of rare earth mineralising fluids in carbonatites: insights from isotopic (C, O, 87Sr/86Sr) data from Kangankunde, Malawi