Sulfur, carbon and oxygen isotopic compositions of Newania carbonatites of India: implications for the mantle source characteristics

Abstract

This study presents fi rst 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 δ 34 S V – CDT ( − 1.4 to 2 ‰ ) and Δ 33 S ( − 0.001 to − 0.13 ‰ ) values of these carbonatite samples (n = 7) overlap with the S isotope compositions of Earth ’ s mantle. Additionally, the δ 13 C V – PDB and δ 18 O V – 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 fl uid (<250 °C), we suggest that the sul fi de minerals in these carbonatites were formed under a magmatic condition. The mantle like signatures in the δ 34 S, δ 13 C – δ 18 O, and 87 Sr/ 86 Sr values of these carbonatites rule out possible crustal contamination. Coexistence of the sul fi de phase (pyrrhotite) with magnesite in these carbonatites suggests that the sul fi de phase has formed early during the crystallization of carbonatite magmas under reducing conditions. Overall restricted variability in the δ 34 S values of these samples further rules out any isotopic fractionation due to the change in the redox condition of the magma and re fl ect the isotopic composition of the parental melts of the Newania carbonatite complex. A compilation of δ 34 S 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 δ 34 S observed in carbonatites younger than 400 Ma. Such observation could suggest an overall lower oxidation state of carbonatite magmas emplaced prior to 400 Ma.