Subducted carbonates not required: Deep mantle melting explains stable Ca isotopes in kimberlite magmas

Ca isotope geochemistry has great potential for improving our understanding of magmatic systems and for tracing the deep Earth carbon cycle. There are still many open questions, however, regarding the proper application of this relatively novel proxy to the study of mantle-derived magmas, including (i) the possible effects of pressure on mineral-melt Ca isotope fractionation factors, and (ii) the potential for Ca isotopes to be used as tracers of recycled marine carbonates in mantle-derived magmas. Kimberlites are mantle-derived melts that are highly enriched in CO2 and are the deepest-sourced mag-mas (>200 km depth) known to erupt at Earth's surface, providing an excellent opportunity to explore these questions. We present Ca isotope data combined with detailed petrographic observations, bulk -carbonate C-O isotope data, and bulk-rock major element analyses, for a suite of 23 well-characterized kimberlite samples from their type-locality (Kimberley, South Africa). These kimberlites have abundant previous evidence for recycled surface materials in their mantle source, including low S isotope and mod-erately radiogenic Sr isotope compositions, yet display only limited variations in their Ca isotope compo-sitions (844CaBSE of-0.08%degrees to-0.27%degrees), with an average of-0.17 +/- 0.02%degrees (2SE, n = 21). This composition is indistinguishable from average carbonatites [-0.19 +/- 0.03%degrees (2SE, n = 106)] and OIB from recent studies [-0.16 +/- 0.01%degrees (2SE, n = 41)], and slightly lower than average MORB [-0.11 +/- 0.02%degrees (2SE, n = 31)]. Although our samples display a wide range of emplacement styles, alteration conditions, extents of mag-matic differentiation, and degrees of mantle-cargo entrainment (i.e., xenocryst accumulation), we find no correlations between Ca isotopes and any of these factors. Instead, we find that low-degree partial melt-ing of the likely kimberlite source lithology (i.e., carbon-bearing garnet lherzolite) yields modelled melt 844CaBSE values ranging between-0.12%degrees and-0.16%degrees (at 1400-1500 degrees C), in agreement with the mea-sured Ca isotope compositions of the Kimberley kimberlites. This observation, and the lack of heavy car-bon isotope signatures in the examined samples, indicates that kimberlites do not require subducted carbonates in their mantle sources, despite their very high CO2 contents. Although several recent studies have suggested that equilibrium mineral-melt Ca isotope fractionation factors (e.g., 1000lnagrt-melt) could be significantly different at higher pressures (i.e., due to pressure-induced changes in CaAO bond lengths and coordinations), our models successfully reproduce the kimberlite data using pressure-independent predictions for mineral-melt fractionations. It remains possible, however, that differences in isotopic frac-tionation due to the peculiar composition of kimberlite melts (e.g., high CO2, low SiO2) are effectively can-celled out by competing pressure effects, and future work independently targeting these factors will be especially important for our understanding of Ca isotope fractionation in mantle-derived melts and the Ca isotope systematics of Earth's mantle.(c) 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

Automated summary

The study investigates the Ca isotope compositions of kimberlites and their relation to mantle-derived magmas and the deep Earth carbon cycle. The results show that kimberlites have similar Ca isotope compositions as carbonatites and OIB, and slightly lower than MORB. The findings suggest that kimberlites do not require subducted carbonates in their mantle sources, despite their high CO2 contents. The study also suggests that the pressure effects on Ca isotope fractionation may not be significant for kimberlite melts.