Global Sr-Nd-Os isotope compositions and highly siderophile elements of carbonatites

The examination of more than 500 samples of worldwide carbonatites on the basis of Sr-Nd isotope systematics allows dividing them into three groups which do not follow conventional grouping. Carbonatites with a mantle signature and variable amounts of incorporated sedimentary material form the first two groups. The carbonatites in the third group have resolvedly lower ¹⁴³Nd/¹⁴⁴Nd signatures at a given e_Sr(i) value that cannot easily be explained by crustal contamination but rather by derivation of carbonatite parental melts from the mantle source metasomatized by 2-6% of slab-derived melts with low Sm/Nd during the late Archean (~2.7 Ga).

                As a consequence of high concentrations of Sr and Nd, this system is more resilient to overprint compared to Re-Os elemental and isotope systematics. Overall, 44 carbonatite samples were investigated for Re–Os compositions; of these, 15 samples were excluded due to modification by late-stage hydrothermal alteration. The remaining carbonatites still yield a wide range of gOs from ~4 to 4000. Whether this wide range is a result of the presence of recycled crust or originates from previous carbonatite metasomatism remains unclear.

                Extremely low concentrations of highly siderophile elements (HSE) in carbonatites and common nugget effects observed are the result of the behavior of sulfur-bearing phases as the main HSE carrier and oxygen fugacity. Carbonated silicate melts are capable to host significant amounts of PGE but during ascent, S-rich magma will separate from carbonatite melts leading to the sequestration of PGE due to higher distribution coefficients, leaving carbonatite melt depleted in PGE.