The C and O isotopes in calcites from aillikites and carbonatites of the Beloziminsky and Tomtor massifs (Siberia, Russia)

The BelayaZima (645-622 Ma) (Ashchepkov et al., 2020) and Tomtor 700 and ~400 Ma) (Vladykin et al., 2016) ultrabasic alkaline rocks (AR) and carbonatites  (CA)  massifs (UMARC) are located on the SW and NW borders of the Siberian platform, respectively.

Mass-spectrometer FINNIGAN MAT-253 with Gas-Bench II flow (pure He) were used to determine δ13С and δ18O of carbonates from the AR and CA in  BelayaZima and Tomtor massifs (Fig. 1,). Values of δ18O of calcite and δ13С AR (27 samples) is 7.1 to 10.7 ‰ and 6,0 up to-4.0‰, respectively, and the calcite of the CA (116 samples) from 7.0 to 12‰ and from -6,5 to -4.1‰, with most values located in the field of primary magmatic carbonates (Fig. 3). Comparison of δ18O and δ13C in AR and CA from Aillik Bay, Labrador (Tappe et al., 2006), and BelayaZima shows nearly coincidence. In AR, average (av) δ18O_av = 9.11±0.14 (σ) and δ13C_av = -5.13±0.14(σ). In CA, δ18O_av = 8.19±0.086 (σ) and δ13C_av = -5.73±0.012 (σ). Given the greater stability of carbon isotopes compared to oxygen isotopes under the influence of post-magmatic processes (Santos and Claeton, 1995). The δ13C difference between AR and CA , ~0.6%, is an attribute of the initial sources.

On the δ18O -, δ13C-diagram (Fig.1) the population of the CA is partially aligned with the field PIC " Taylor Box" and almost completely with the field PIC by Demeny (1998). δ13С and δ18O values for AR different – the " Taylor Box" is a single value, while the bulk of the points were in the field of primary magmatic carbonates (PIC) (Demeni 1998) points on the δ18O-δ13С diagram for AR, and, especially, CA located along the δ18O axis, marking the horizontal trend is typical for many CA of UMARC  in Africa, Brazil, Canada etc. The prevalence of this trend indicates the global nature of the factors leading to its appearance, primarily the impact on carbonates of post-magmatic carbonless fluids.

The prevalence of this trend indicates the global nature of the factors leading to its appearance, mainly the post-magmatic carbonless fluids influence. A striking example is the δ18O of calcites from Tomtor deposit, unchanged by supergenic processes, along which a thick (100-150 m) weathering crust is developed. It is possible that AR and CA of the BelayaZima massif were formed under higher PT conditions. Due to their longer cooling time, the fractionation of oxygen isotopes possibly was controlled by the Soret effect, when heavy isotopes are concentrated in low-temperature areas under thermogradient conditions (Bindeman et al., 2013; Li, Liu, 2015). Possibly, the same effect makes an additional contribution to the isotopic heterogenization of carbon and oxygen in magmatic melts.

Support: RFBR 19-05-00788, RSF 18-17-00120, Russian Ministry of Education and Science

Fig.1 Aillikites:

Carbonatites:

Fig.2