JpGU-AGU Joint Meeting 2020

Presentation information

[E] Oral

S (Solid Earth Sciences ) » S-MP Mineralogy & Petrology

[S-MP37] Supercontinents and crustal evolution

convener:Madhusoodhan Satish-Kumar(Department of Geology, Faculty of Science, Niigata University), Krishnan Sajeev(Centre for Earth Sciences, Indian Institute of Science), Tomokazu Hokada(National Institute of Polar Research), Yasuhito Osanai(Division of Evolution of Earth Environments, Faculty of Social and Cultural Studies, Kyushu University)

[SMP37-06] Bulk rock Nd-Sr and multiple-sulfur isotope compositions of carbonatites from the Phalaborwa complex, South Africa

★Invited Papers

*Anupam Banerjee1, Madhusoodhan Satish-Kumar1 (1.Niigata University)

Keywords:Recycling, Carbonatites and alkaline silicate rocks, Mass independant fractionation of sulfur isotopes

Carbonatites are unique mantle-derived rocks with more than 50% modal abundance of igneous carbonates and they mostly occur in association with alkaline silicate rocks. Since carbonatites and alkaline silicate rocks are highly enriched in Sr, Nd and S concentrations than any other terrestrial igneous rocks, they are less susceptible to crustal contamination. Therefore, such igneous rock suites provide an ideal opportunity to trace crust-mantle interaction through recycling and mantle heterogeneity. While 143Nd/144Nd and 87Sr/86Sr of any magmatic rocks are suitable tracers for distinguishing depleted versus enriched mantle source regions, sulfur isotopic compositions of these rocks are also useful in delineating crustal recycling owing to the higher δ34S of crustal sediments than primitive and/or depleted mantle1. Additionally, Δ33S of these rock samples would indicate the relative timing of crustal sources since only Archean sediments could possess a non-zero Δ33S value2. A recent study, based on the combined S and Pb isotopic compositions of bulk rock carbonatites and in-situ measurements of sulfide minerals, from the ~2.06 Ga Phalaborwa complex of South Africa suggests a possible role of Archean sediments in their mantle source regions3. However, this study is limited to only three carbonatites and their constituent sulfur-bearing minerals. Thorough multiple isotopic investigations are required on multiple samples from this peculiar carbonatite occurrence to characterize the mantle source region of this igneous suite and its interaction with recycled components.

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.