Trace element and isotope Geochemistry of Neoar-chean carbonate rocks from the Dharwar craton, southern India: Implications for depositional environments and mantle influence on ocean chemistry

摘要

Neoarchean carbonate rocks of the Vanivilas Formation (2.7 Ga) occur extensively in greenstone belts of western Dharwar craton, are associated with banded Fe-Mn formations and clastic sediments including diamictites, preserve well-developed stro-matolitic structures. We intend to understand the depositional environments of these carbonates, extent of oxygenation in shallow oceans and to examine the mantle and continent inputs to Neoarchean oceans. Depleted LREEsn and slightly enriched HREE_(SN) patterns, Y/Ho ＞ 28, La, Y and Eu spiking and varying Ce anomalies shown by most of the samples, are consistent with their marine origin. However, 4 samples exhibit negative Ce anomaly indicating presence of oxygen supporting the hypotheses of localised, protected, shallow marine oxygenation in redox stratified late Meso- to Neoarchean oceans. δ~(13)C values are well preserved (mean = -0.27‰ (PDB)) similar to other Archean marine carbonates from other cratons and show no correlation with δ~(18)O and Mn/Sr ratios. Whereas, the δ~(18)O values (mean= -10.78‰) show considerable depletion and can be attributed to later interactions with diagenetic/meteoric fluids. The mantle dominated, yet continental affected Neoarchean ocean chemistry is apparent from Sr-Nd isotope systematics. The lowest ~(87)Sr/~(86)Sri of carbonates, 0.7024 reflects ocean water Sr isotopic composition at 2.7 Ga, is higher than the model seawater evolution curve of Shields and Veizer, (2002). Similarly, other recent studies on Archean carbonates and barites reported higher ~(87)Sr/~(86)Sri values. This is due to the existence of Paleoarchean continental crust subjected to chemical weathering resulting in increased continental flux than previously thought of. Range of Nd values (-8.39 to 5.83) suggest that the carbonates had variable contributions of Nd from a long term depleted mantle through hydrothermal fluids, as well as, from 3.5 Ga old continental crust in the Dharwar craton. The carbonates were deposited in the Neoarchean ocean at variable depths within the shelf and those deposited at shallower and deeper environments are characterized respectively, by inputs derived from chemical weathering of ＞3.5 Ga old continental crust and mantle through hydrothermal fluids associated with submarine volcanic activity.