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

Abstract

A statistical regression analysis was performed of a summary of borrowed literary isotopic 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.