[SMP37-06] Bulk rock Nd-Sr and multiple-sulfur isotope compositions of carbonatites from the Phalaborwa complex, South Africa
Keywords:Recycling, Carbonatites and alkaline silicate rocks, Mass independant fractionation of sulfur isotopes
Here we report multiple-sulfur isotopic compositions of whole-rock carbonatites and associated alkaline silicate rocks along with their 87Sr/86Sr and 143Nd/144Nd from the Phalaborwa carbonatite complex of South Africa. Whole-rock sulfur isotopic compositions of these rocks have been measured with SF6 method using Thermo Scientific MAT 253 gas source Ion Ratio Mass Spectrometer at Niigata University. The analytical uncertainty of δ34S, based on long-term measurements of sulfur isotope standards S-1 and S-2, is better than 0.5‰. Radiogenic Nd and Sr isotopic compositions of the carbonatites and the associated silicate rocks have been measured using MAT 251 Thermal Ionization Mass Spectrometer at Niigata University. Preliminary results obtained from these samples show a wide range in δ34S for carbonatites and alkaline silicate rocks (-0.8 to 4.2‰ and -4.0 to 12‰, respectively). Mass independent fractionation (MIF) of sulfur isotopes is observed for most of the samples, as revealed by their positive Δ33S values. On the contrary, two silicate rocks show no significant MIF S isotopes. The initial (calculated at 2.06 Ga) 87Sr/86Sr(t) and εNd(t) of carbonatites and some silicate rocks show overlapping values, ranging from 0.703665 to 0.707879 and -6.6 to -8.3, respectively. Two silicate rocks, those showing mass-dependent fractionation of S isotopes, have extremely non-radiogenic 87Sr/86Sr(t) (0.693161 and 0.698217) and CHUR like εNd(t) (-0.3 and 0.4) isotopic compositions. Based on the radiogenic Sr and Nd isotopic compositions of these rocks, we suggest that parent magma for this carbonatite complex is derived from an enriched mantle reservoir. Such enrichment of the mantle source region is perhaps the result of recycling of crustal sediments. This result is also consistent with the δ44/40Ca of a different set of carbonatite samples from this location suggesting a possible role of crustal recycling process4. Since most of the rock samples used in this study show a large range in δ34S and MIF Δ33S isotopic composition, we suggest that such enrichment of mantle source must have happened prior to the global oxidation event at ~2.45 Ga.
References: 1Labidi et al. (2013) Nature, 501(7466), 208-211; 2Farquhar et al. (2000) Science, 289, 756–758; 3Bolhar et al. (2020) Earth Planet. Sc. Lett., 530, 115939; 4Chakrabarti & Banerjee (2019), AGU Fall Meeting, Abstract ID: V51E-0093.