Significant variability in the δ44/40Ca of global carbonatites: implications for carbonate recycling, magma differentiation and source-mantle mineralogy

Stable Ca isotopic composition (δ^44/40Ca) of crustal carbonates are typically lighter than that of the bulk silicate Earth value ( 1.05 ‰). Hence, δ^44/40Ca of mantle-derived rocks can potentially trace recycled crustal carbonates into the mantle. We report the Ca isotopic compositions of globally distributed carbonatites (n = 46), which are unique igneous rocks with more than 50% modal carbonate minerals, with eruption ages ranging from Precambrian until recent. The δ^44/40Ca (w.r.t. SRM915a) of these carbonatites show a large range (0.35 ‰ to 1.26 ‰), which is significantly higher than the analytical uncertainty (0.08‰) of the measurements performed using TIMS at CEaS, IISc. These samples are well-characterized in terms of their major and trace element geochemistry as well as Nd, Sr, B, C, and O isotopic compositions for selected samples. No systematic trend is observed between δ^44/40Ca of the carbonatites and their eruption ages. Significant variability is observed in δ^44/40Cavalues in samples from individual provinces including those from the Oka complex in Canada (0.44 ‰ - 1.26 ‰, n= 8), Ambadongar (0.53 ‰ - 1.1 ‰, n= 8) and the Newania complexes (0.44 ‰ - 0.83 ‰, n= 4) in north-west India, the South Indian carbonatites (0.65 ‰ - 0.91 ‰, n= 3) and carbonatites from the Palabora complex in South Africa (0.35 ‰ - 0.84‰, n= 3). The δ^44/40Caof carbonatites from Oka, Newania and the Ambadongar show strong correlations with Ca/Mg, Ca/Fe as well as CaO and MgO contents. The δ^44/40Caof the Oka and Ambadongar carbonatites show correlated variations with their Mg# and K/Rb ratios, respectively. The large variability in δ^44/40Caof global carbonatites is explained in terms of: (1) presence of isotopically lighter ancient subducted carbonates in the mantle-source regions and carbonate metasomatism of the mantle, (2) partial melting and differentiation of the carbonatite magma and (3) heterogeneity in the source-mantle mineralogy of carbonatites.