Pre-BIF, Negative Ce-Anomaly in Mid-Archean Shales From the Eastern Indian Craton.
Abstract
We report preliminary geochemical data for the Iron Ore Group (IOG) of rocks from the Eastern Indian Craton. These rocks have been intruded by 3.0-3.1 Ga old phases of the Singhbhum and Bonai granites and thus have been assigned a Mid-Archean age (~ 3.2Ga). The IOG rocks represent a sequence of volcano-sedimentary litho-units starting with a ~1km thick basal lava flow (lower lava) followed successively upward by a shale (lower shale), Banded Iron Formation (BIF), a banded shale (upper shale) with associated lava flow (upper lava). The occurrence of pillow lava in the lower part of the IOG indicates that the lavas erupted under submarine conditions. The Chondrite-normalized Rare Earth Element (REE) patterns of both the Lower and Upper lava samples show a near-uniform fractionation pattern. However, the lower lavas show a smaller degree of fractionation (LaN/LuN = 4-6) than the upper lavas (LaN/LuN = 8-10), suggesting an evolved and already fractionated source for the Upper Lava rocks. The negative Eu-anomaly in all the lavas is due to separation of plagioclase in the magma chamber beneath the sea floor. The REE patterns of the Lower Shale samples are very similar to those of the underlying lavas indicating their affinities with those lavas. The lower shale samples exhibit a large negative Ce-anomaly (Ce/Ce* = 0.3-0.45). A smaller degree of negative Ce-anomaly (Ce/Ce* = 0.8-0.9) is also seen in the Lower Lava samples. Oxidative scavenging of cerium by Fe-Mn oxyhydroxides in the marine environment is believed to result in a negative Ce-anomaly in the seawater and in the phases precipitating in equilibrium with seawater. Hydrous minerals in basalts forming in equilibrium with seawater can impart a mild negative Ce-anomaly as observed in the Lower Lava samples. A more pronounced Ce-anomaly in the shales is a result of further subaqueous, hydrothermal disintegration of the basalts. Ce-anomaly in such ancient rocks, suggest oxygenation of the deeper waters as early as 3.1 Ga, much earlier than the precipitation of the overlying Banded Iron Formation. This has important implications regarding the oxygen budget in the early Earth and the rise of oxygen. Positive Ce-anomaly in an Upper Shale sample is due to the associated occurrence of Manganese deposits in these shales.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2003
- Bibcode:
- 2003AGUFM.V42D0381C
- Keywords:
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- 1010 Chemical evolution;
- 1020 Composition of the crust;
- 1045 Low-temperature geochemistry;
- 1050 Marine geochemistry (4835;
- 4850);
- 1065 Trace elements (3670)