Iron isotope compositions (δ56Fe) of ferromanganese (Fe-Mn) crusts and nodules are reliable proxies for understanding the biogeochemical cycling of Fe in the ocean. Fe-Mn nodules/crusts are characterized by low δ56Fe values (–0.8 to –0.05 ‰ IRMM 014), while the dissolved Fe in the deep ocean is 56Fe enriched (d56Fe range from +0.2 to +0.8 ‰). Here, we report Fe isotope compositions of top scrapings of sixteen Mn nodules and three Fe-Mn crusts and their geochemical compositions from the Central Indian Basin (CIB) to understand Fe isotope compositions. Based on their morphology and elemental compositions, the CIB nodules are divided into three groups: ‘hyrogenetic,’ ‘diagenetic,’ and ‘mixed’ types. The range of δ56Fe values (-0.63 to -0.06 ‰) for CIB nodules and crusts is similar to those from different parts of the world ocean. The δ56Fe values of the hydrogenetic group of CIB nodules are consistent with a fractionation model involving selective adsorption 56Fe onto organic ligands (siderophore complexes). Using a fractionation factor of -0.77 ‰ between seawater and nodules, we estimate that δ56Fe of the CIB deep seawater dissolved Fe range between +0.28 and +0.63 ‰ similar to the Atlantic deep seawater. The δ56Fe values of mixed nodules correlate positively with Mn/Fe and concentrations of Mn, Cu, Zn, Mo, Cd, Sb, and Tl, and negatively with Fe, Be, Sc, Co, Zr, Nb, and rare earth elements (REE). These mixing lines attest to variable proportions of metals from diagenetic and hydrogenetic sources.

Primary melt compositions and mantle potential temperature (Tp) was calculated for c.a. 3.4 Ga -2.21 Ga volcanic rocks e.g., Basalts, Dikes, Komatiites, Komatiitic-basalts from different Indian cratons, using the data sourced from previously published literature. The results are then compared with the previously published values of Tp from different cratons of the world. In addition, we have also calculated the Poisson’s ratio value for the Primary melts from Indian cratons and other cratons of the world as well. A volume normative mineralogy was used for the input to calculate the Poisson’s ratio. For temperature, we have used the Tp value of the corresponding Primary melts, and the pressure was kept constant at 1 bar. The measured values of Poisson’s ratios are then plotted against their respective ages. Poisson’s ratio estimates from different cratons range between 0.2764-0.2840 for 3.5 Ga to 2.6 Ga time window, and records a rise at 2.5 Ga, reaching 0.288 at 2 Ga. The average value of Poisson’s ratio from 3.5 Ga to 2.6 Ga is 0.2791± 0.0021(SD), whereas it is 0.282923±0.00283 from 2.5 Ga to 1.3 Ga. However, the number of data points from 1.3 Ga to 500 Ma is very less, and the modern world (i.e., 500 Ma- Present) records an average Poisson’s ratio of 0.285 ± 0.004. Nevertheless, the scenario flips around for a few cratons e.g., Superior craton of Canada, the Baltic shield of eastern Europe, and Bundelkhand craton of India; as these cratons are showing an extraordinarily high Poisson’s ratio between 2.8 Ga-2.7 Ga. The average Poisson’s ratio of 0.287±0.0021 with a high Tp value of 1550°C-1630°C is recorded from Superior craton and the Baltic shield. Tp value from the Bundelkhand craton, on the other hand, is low (1388°C). Similar to the present day, subduction dominated Poisson’s ratio from Superior craton and Baltic shield with high Tp value at 2.7 Ga suggests a possibility of manifestation of subduction even at higher temperature conditions. These phenomena, however, appear to be craton-specific rather than a general approach, as no such values of Poisson’s ratio have been documented from any other craton in the world between 3.5 Ga-2.7 Ga. The other way around for the above condition might be explained by a proto-tectonic activity such as drip-tectonics or sagduction. This is because, although the Poisson’s ratio is high for these cratons during 2.8 Ga- 2.7 Ga, rigid plate subduction at such a high temperature seems to be a difficult task. On the other hand, multiple peaks in Tp values with a certain interval between 3.5 Ga-2.5 Ga indicate cyclic mantle overturn events, which might have terminated the possibility of subduction. However, a lower Tp with a high Poisson’s ratio from Bundelkhand might suggest a very similar rigid plate motion during the end of Archaean. Key Word- Mantle Potential Temperature, Poisson’s Ratio, Primary Melt