2021 年 116 巻 3 号 p. 121-128
This study presents first report of the sulfur isotopic compositions of carbonatites from the Mesoproterozoic Newania complex of India along with their stable C and O isotope ratios. The δ34SV–CDT (−1.4 to 2‰) and Δ33S (−0.001 to −0.13‰) values of these carbonatite samples (n = 7) overlap with the S isotope compositions of Earth’s mantle. Additionally, the δ13CV–PDB and δ18OV–SMOW values of these carbonatites also show overlapping compositions to that of Earth’s mantle. Based on these mantle–like stable isotopic compositions of carbonatites along with their higher crystallization temperature (~ 600 °C) compared to a hydrothermal fluid (<250 °C), we suggest that the sulfide minerals in these carbonatites were formed under a magmatic condition. The mantle like signatures in the δ34S, δ13C– δ18O, and 87Sr/86Sr values of these carbonatites rule out possible crustal contamination. Coexistence of the sulfide phase (pyrrhotite) with magnesite in these carbonatites suggests that the sulfide phase has formed early during the crystallization of carbonatite magmas under reducing conditions. Overall restricted variability in the δ34S values of these samples further rules out any isotopic fractionation due to the change in the redox condition of the magma and reflect the isotopic composition of the parental melts of the Newania carbonatite complex. A compilation of δ34S of carbonatites from Newania and other complexes worldwide indicates limited variability in the isotopic composition for carbonatites older than 400 Ma, which broadly overlaps with Earth’s asthenospheric mantle composition. This contrasts with the larger variability in δ34S observed in carbonatites younger than 400 Ma. Such observation could suggest an overall lower oxidation state of carbonatite magmas emplaced prior to 400 Ma.