Banded Iron Formations (BIFs) are archives of Precambrian seawater composition. Presence or absence of negative Ce anomaly (Ce/Ce*) in BIFs has been widely used to understand paleo-redox conditions on the Earth’s surface in the Precambrian. However, whether the extremely negative Ce anomaly associated with the BIFs reflects a primarily depositional signature or not has been questioned and it has been suggested that such signatures could also arise from secondary alterations.1 We report elemental and Nd isotopic data for BIFs and associated clastic rocks from the Sirsi region in southern India. Major and trace element compositions of these BIFs were measured using an Inductively Coupled Plasma Mass Spectrometer (ICP-MS, X series II) while Nd isotope ratio (143Nd/144Nd) measurements were performed using a Thermal Ionization Mass Spectrometer (TIMS, Triton Plus), both at the Centre for Earth Sciences (CEaS), Indian Institute of Science Bangalore, India. The BIF samples are sub-divided into two groups based on their REE+Y (REY) compositions. The group-1 BIFs show seawater-like REY pattern with HREE enrichment over LREEs and super-chondritic Y/Ho (41-52). These BIF samples also lack significant negative Ce anomalies. In contrast, group-2 BIFs show high LREE/HREE enrichment, negative Ce anomaly, and sub-chondritic Y/Ho. Very high values of La/Yb in the group-2 BIFs cannot be explained by simple two-component mixing of basement rock (Dharwar TTG) and pristine Sirsi BIFs. Instead, fluid-rock alteration by LREE enriched, and Ce depleted fluid could explain the observed REY variations. We further utilized Sm-Nd isotope systematics to calculate the timing of this alteration event. These BIFs show lowest RSD (%) in their initial 143Nd/144Nd composition around 0.6 Ga, which we consider as the time of alteration event which affected the Sm/Nd of these rocks. The timing of alteration event coincides with the Pan-African orogeny which had regionally affected the Greater Dharwar Craton. The associated red shales are also characterized by high LREE/HREE ratios and negative Ce anomalies. These shales also show very high Chemical Index of Alteration (CIA) values (83-99) suggesting high degree of chemical weathering. [1] Bonnand et al, (2020) Earth and Planetary Science Letters

The Neoproterozoic Era represents extreme environmental conditions with three major glaciation events. Associated with these glaciation and deglaciation periods are high amplitude positive and negative δ13C excursions which have been observed in the rock records, suggesting perturbations in oceanic biogeochemical cycles. However, whether these isotopic records reflect primary depositional signature and open ocean condition are debated. The Marwar Basin of the Indian Shield preserves one such record of the Ediacaran time period. The magnitude of negative δ13C excursion from the Marwar Basin is comparable with the Shuram excursion.1 We report elemental compositions of sixty-eight carbonate samples collected from five spatially distributed sections from the Bilara carbonates of the Marwar Basin. Selected samples from two of these sections were analyzed for their radiogenic Sr (87Sr/86Sr) and stable Ca (δ44/40Ca) isotopic compositions. Elemental compositions were measured using an Inductively Coupled Plasma Mass Spectrometer (ICP-MS, X series II) while 87Sr/86Sr and δ44/40Ca (reported relative to NIST SRM 915a) values were measured using a Thermal Ionization Mass Spectrometer (TIMS, Triton Plus), both at the CEaS, IISc Bangalore, India. The Bilara Group carbonates are sub-divided into two populations based on non-redox REY anomalies and the Y/Ho ratio. Super-Chondritic Y/Ho (40-52) and positive La anomaly (1.01-2.65) of some samples suggest deposition under open ocean conditions and connectivity to the global ocean. While heterogeneity in δ13C values is evident in samples with low Y/Ho (<40), samples with high Y/Ho (> 40) preserve low δ13C values. No significant correlation has been observed between δ13C and Ce anomaly (Ce/Ce*) suggesting absence of any paleo-redox gradient. The lowest 87Sr/86Sr (~0.7079) observed in the carbonates is comparable with Ediacaran seawater confirming retention of primary depositional signatures in these samples. These carbonates show heavy δ44/40CaSRM915a compositions (1.44‰-2.21‰) typical of Neoproterozoic post-glacial successions2. Our study confirms primary origin and open ocean nature of the late Neoproterozoic δ13C excursions in the Marwar Basin. [1] Ansari et al., (2018) Precambrian Research, [2] Silva-Tamayo et al., (2010) Precambrian Research