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

The article contains the results of statistical processing of a large summary of δ 18 О-δ 13 С isotope values in the primary carbonatites of the world. From literary sources, 1593 paired values δ 18 О-δ 13 С from 173 carbonatite occurrences of the world were collected. This report exceeds all previously published reports on С-О isotopes in carbonatites by quantity of the used values and carbonatite occurrences. Statistical data analysis is performed on diagrams in the coordinates δ 18 О (‰, V-SMOW) - δ 13 С (‰, V-PDV). For each carbonatite occurrence, not only the arithmetic mean values are calculated, but also the regression line. Distinct linear trend of δ 18 О-δ 13 С values is found in half of the carbonatite occurrences. The starting, middle, and ending points of the trend line are determined. The slope of the trend line (angular coefficient) varies over a wide range. The trend is dominated by an average angular coefficient of 0.30 (positive correlation δ 18 О-δ 13 С). In the literature, it is associated with the Rayleigh high-temperature fractionation of carbonatite melts or with their sedimentary contamination. Half of the carbonatite occurrences do not show a linear trend of δ 18 О-δ 13 С values, probably due to the combined action of multidirectional trends. The initial ratio 87 Sr/ 86 Sr in the used carbonatite occurrences varies from 0.701 to 0.708. Statistics show no correlation of 87 Sr/ 86 Sr with the δ 18 О-δ 13 С system.


Introduction
Oxygen and carbon isotope composition of carbonatites were summarized in a How to cite this paper: Bolonin number of previous works. The largest number of δ 18 О-δ 13 С values (about 440) was collected and used to construct histograms in [1]. In the work [2], 56 values from 8 carbonatite occurrences of Kola Alkaline Province were used, linear trends were identified. In the work [3], 70 analyzes from 20 carbonatite occurrences of Siberia and Mongolia were used; a diagram was proposed for determining the type of mantle using the ratio of O-C isotopes. In the work [4], the fields of point values are outlined in the δ 18 О-δ 13 С diagram for the Greenland, Europe, and North and South America regions without a division for individual carbonatite occurrences. The fields of primary igneous carbonatites on the δ 18 О-δ 13 С diagram are outlined in the works [1] [5] [6] [7]. These fields are used in the analysis of local isotope data in carbonatite studies. This paper uses 1593 pairs of conjugate values of δ 18 О-δ 13 С out of 173 carbonatite occurrences of the world. In addition, a linear regression analysis of the values is performed for most occurrences. This paper exceeds all previously published reports on С-О isotopes in carbonatites by quantity of the used values and carbonatite occurrences. Data on the ratio 87 Sr/ 86 Sr in 92 carbonatite occurrences are taken additionally from the sources used. The limited size of the article does not allow to provide a complete database and a list of used references (about 100 titles).
Carbonatite occurrences are represented by bodies of various shapes and sizes (complexes, massifs, dikes, facies zones). Isotope analysis is applied to carbonatite rocks (sovite, alvikite, beforsite, etc.), monofractions of calcite, dolomite, ankerite, siderite. Authors of literature classify the analyzed material as primary carbonatites (PC). This is mainly done on the basis of petrographic studies, in which secondary endo-and exogenous minerals are not detected. Single anomalous values are excluded from the primary category by the author of the article. Numerous δ 18 О-δ 13 С values refer to secondary carbonatites in the used literature: carbonate tuffs and lavas, hydrothermal veins, hydrothermally altered and recrystallized carbonatites, weathered and oxidized carbonatites, secondary calcite. Data on secondary carbonatites is not used in this article.
All isotope diagrams have a horizontal x-axis δ 18 О (‰, V-SMOW) and a vertical y-axis δ 13 С (‰, V-PDV). The equal scale of both axes, a multiple of 1‰, allows to visually comparing the shape of the point sets (point fields) and the slope of the trend lines. The names of carbonatite occurrences and their identification number (ID) are coordinated with the database [8] and are given in English transcription.

Summary Data
The diagram in Figure 1 contains 1593 points of the δ 18 О-δ 13 С from 173 carbonatite occurrences, including various carbonatite facies in one occurrence. The number of points in the individual occurrences varies from 2 to 54. The points fill a very wide field. Analysis of the field is complex and incorrect due to the variable number and large scatter of points that characterize individual carbonatite occurrences. The contours PC-98% and PC-90% presented in the diagram are proved in Figure 2 and Figure 3.    The averaged shape of the field of points for three groups of occurrences with a linear trend and for one group without a trend is modeled in Figure 6. The    Table. Occurrences without a linear trend also imply the presence of a starting point. This follows from the previously made assumption that the nonlinear point field 6d in Figure 6 is the result of the cumulative influence of trends 6a, 6b and 6c. All trends are directed upwards δ 18 О, but in different directions along δ 13 С. Therefore, the starting point of field 6d must have δ 18 О equal to the minimum of the statistical sample. The δ 13 С value in some approximation can be taken equal to the average of the sample (Figure 6(d)). The δ 18 О-δ 13 С of the starting point in the occurrences without the identified linear trend is also given in the Table. The diagram shows two groups of points ( Figure 3): 1) the starting point of the trend line in the occurrences with a linear trend (n = 70); 2) the starting point of nonlinear fields in other occurrences (n = 103). The second group includes occurrences without a linear trend (n = 70), and also occurrences with only two points δ 18 О-δ 13 С (n = 24) and with a difference δ 18 Оmax -δ 18 Оmin < 0.5 (n = 9) that were excluded from the trend analysis. Visual analysis of the diagram allows to delineate the internal contour PC-90% in addition to the PC-98% contour justified in Figure 2. This contour includes a compact group of 90% starting points. The vertical line δ 18 О = 7.9‰ divides the contour PC-90% into two parts, each of which is 45% of the total number of starting points.    The PC-90% contour can be considered acceptable for primary carbonatites.

Conclusions
In-depth petrographic and other argumentation of the primary nature of carbonates is required for occurrences outside this contour. The PC-45% (δ 18 О < 7.75‰) contour with a high probability includes only primary carbonatites with a mantle source of a carbonate substance and with minimal effect of isotope fractionation or contamination of melts. A greater influence of these factors is expected for occurrences in the PC-45% (δ 18 О > 7.75‰) contour. Strontium in carbonatite occurrences has a wide variation of the initial 87 Sr/ 86 Sr ratio from 0.701 to 0.708. This variation and the absence of correlation between 87 Sr/ 86 Sr and the δ 18 О-δ 13 С allow both mantle and crustal contamination of carbonatite magmas.
The stated statistical data on the O, C and Sr isotope composition in primary carbonatites leave room for additional and alternative judgments.

Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this paper.