Oxygen–Carbon and Strontium Isotope Composition in Primary Carbonatites of the World: The Data Summary and Linear Trends

DOI: 10.47765/0869-5997-2024-10011

Abstract

A statistical regression analysis was performed of a summary of borrowed literary iso- topic data on 176 carbonatite occurrences worldwide (a total of 1829 paired δ¹⁸О-δ¹³С values in primary carbonatites). For most of the occurrences, distinct linear trends in δ¹⁸О-δ¹³С values were revealed, approximated in diagrams by vectors using the regression formula y = kx + b. Trends in three directions are distinguished. The dominating trend possesses the angular coefficient k in the range of 0.0÷+0.6, averaging at + 0.31 (direct correlation δ¹⁸О-δ¹³С). In the literature, it is associated with the Rayleigh high-temperature fractionation of carbonatite melts or with their sedimentary crustal contamination. The second trend with k from 0.0 to -0.7 (inverse correlation δ¹⁸О-δ1³С) is associated with CO₂ degassing from the melts. The third trend with k from +0.6 to +1.5 is rare. In other carbonatite occurrences, no linear trends in δ¹⁸О-δ¹³С values are detected. This may be a consequence of a combined action of the multidirectional linear trends. In this regard, the initial point of the trend vectors is accepted as a universal isotope indicator for all the carbonatite occurrences. In the δ¹⁸О-δ¹³С diagram, the box PC-90 is outlined in the field of the initial points, including 90 % of the primary carbonatite occurrences. The coordinates of the corner points of the box in δ¹⁸О-δ¹³С  ‰  are  (+5÷-8),  (+5÷-5),  (+5.5÷-2.5),  (+12÷-2.5),  (+12÷-7),  (+9÷-8.5).  The  box  PC-90  is  subdivided into two parts by the carbon isotope composition, using the value δ13C = -5.2 ‰. This suggests that the deep sources differ in carbon isotope composition. No correlation is found in the oxygen–carbon and strontium isotope compositions in the carbonatites, which suggests that the deep sources are also heterogeneous in the strontium isotope composition.