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 tracing the deep Earth carbon cycle. There are still many open questions, however, regarding proper application this relatively novel proxy to study mantle-derived magmas, including (i) possible effects pressure on mineral-melt fractionation factors, (ii) isotopes be used as tracers recycled marine carbonates in magmas. Kimberlites melts that highly enriched CO2 deepest-sourced magmas (>200 km depth) known erupt at Earth’s surface, providing an excellent opportunity explore these questions. We present data combined with detailed petrographic observations, bulk-carbonate C-O data, bulk-rock major element analyses, a suite 23 well-characterized kimberlite samples from their type-locality (Kimberley, South Africa). These kimberlites have abundant previous evidence surface materials mantle source, low S moderately radiogenic Sr compositions, yet display only limited variations compositions (δ44CaBSE −0.08‰ −0.27‰), average −0.17 ± 0.02‰ (2SE, n = 21). This composition is indistinguishable carbonatites [−0.19 0.03‰ 106)] OIB recent studies [−0.16 0.01‰ 41)], slightly lower than MORB [−0.11 31)]. Although wide range emplacement styles, alteration conditions, extents differentiation, degrees mantle-cargo entrainment (i.e., xenocryst accumulation), we find no correlations between any factors. Instead, low-degree partial melting likely source lithology carbon-bearing garnet lherzolite) yields modelled melt δ44CaBSE values ranging −0.12‰ −0.16‰ (at 1400–1500 °C), agreement measured Kimberley kimberlites. observation, lack heavy signatures examined samples, indicates do not require subducted sources, despite very high contents. several suggested equilibrium factors (e.g., 1000lnαgrt-melt) could significantly different higher pressures due pressure-induced changes CaO bond lengths coordinations), models successfully reproduce using pressure-independent predictions fractionations. It remains possible, differences isotopic peculiar CO2, SiO2) effectively cancelled out by competing effects, future work independently targeting will especially important systematics mantle.