Calcium Isotopes in Natural and Experimental Carbonated Silicate Melts

The calcium stable isotopic compositions of mantle-sourced rocks and minerals were investigated to better understand the carbon cycle in the Earth’s mantle. Bulk carbonatites and kimberlites were analyzed to identify a geochemical signature of carbonatite magmatism, while inter-mineral fractionation was measured in co-existing Ca-bearing carbonate and silicate minerals. Bulk samples show a range of composition deviating from the bulk silicate Earth δ44/40Ca composition indicating signatures of magmatic processes or marine carbonate addition to source materials. Δ44/40Cacarbonate-silicate values range from -0.55‰ to +1.82‰ and positively correlate with Ca/Mg ratios in pyroxenes. A series of experiments designed to equilibrate clinopyroxene with carbonated melts of varying compositions were conducted to constrain the systematics of Ca isotopic fractionation in these systems. Results show significant fractionation between phases, with δ44/40Ca in Cpx ranging from 0.37 to 2.75‰ heavier than quenched melts. However, the influence of thermal diffusion on these results is not fully understood.