Analysis of a carbonate (calcite vein) sample yielded d18O of 6.09 (V-SMOW) and d13C of -4.23 (V-PDB), and only 47 ppm Sr. Stable isotopic data indicate 'mantle signature', while Sr abundance is very low (Sr isotopic ratio is not available yet). Can the sample be of a carbonatite vein? If carbonatite, what could be the reason of its extremely low Sr content? Please suggest me some relevant literature on petrogenesis of such type of carbonatite.
Article Deducing the source and composition of rare earth mineralisi...
Abstract
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 87Sr/86Sr 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% REE2O3, comprise assemblages of monazite-(Ce), strontianite and baryte forming hexagonal pseudomorphs after probable burbankite. The 87Sr/86Sr 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 (δ18O + 3 to + 5‰VSMOW and δ13C − 3.5 to − 3.2‰VPDB). In contrast, dolomite in the same samples has similar δ13C values but much higher δ18O, corresponding to increasing degrees of exchange with low-temperature fluids (
Dear Himadri Basu ,
sometimes several secondary processes such as low-temperatures alteration, high -temperatures fractionation or crustal assimilation, causing post-emplacement changes in δ18O and δ13C, make the interpretation of primary mantle fingerprint particularly difficult.
As regards the Sr content, it is very low if compared with other carbonatite complexes (see primary oceanic carbonatite in Carnevale et al., 2021, Minerals).
You should be evaluate also the other petrological/geochemical parameters (mineral paragenesis, trace elements, Sr-Nd-Pb systematics, etc...).
Best regards,
Gabriele Carnevale
Thanks Dr. Gabriele for your suggestions and paper. Is it possible that the calcite precipitated as a result of interaction of mantle derived CO2 initially with crustal hydrous fluid and subsequently that fluid interacted with other crustal rocks (Ca-rich)?
Regards,
Himadri Basu
Carbonatites are rarely only calcite. Is there something else in the vein? What is the host rock? Are there any reaction rims or selvages between the vein and the host rock? Do you have the full composition of calcite?
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