[H-RE13] Earth Resource Science

[HRE13-P06] Evidence of oxidative shallow ocean recorded in 3.2 Ga banded iron formation by chromium concentrations, Moodies Group, Barberton Greenstone Belt, South Africa

*Yoko Ohtomo¹, Haruki Matsukawa², Yasuhiro Nagashima², Yoshiaki Kon³, Tsubasa Otake¹, Takeshi Kakegawa⁴, Tsutomu Sato¹ (1.Faculty of Engineering, Hokkaido University, 2.Graduate School of Engineering, Hokkaido University, 3.National Institute of Advanced Industrial Science and Technology, 4.Graduate School of Science, Tohoku University, 6-3, Aramaki Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan)

Keywords:Great Oxidation Event, Banded Iron Formation, Chromium, Barberton Greenstone Belt, Moodies Group

Redox condition of earth’s surface environments before 2.45 Ga Great oxidation event (GOE) has an important role for understanding Archaean metallic element cycles and the ore formations. Fluctuation of microbial activities prior to prosperity of cyanobacteria and oxidation state might be expected in shallow ocean before GOE. In such a transitional environment, behavior of redox-sensitive elements can be important factors to evaluate oxidation state. 3.2 Ga Moodies Group, Barberton Greenstone belt, South Africa, is focused as a significant archive of shallow ocean environment before GOE which consists of detrital sedimentary rocks including Banded Iron Formations (BIFs), chemical deposits associated with microbial activities. Here, we performed geological, petrological, and geochemical investigation with detailed observations of minerals containing chromium (Cr), redox-sensitive element, to understand fluctuation of redox conditions on 3.2 Ga shallow BIFs of Moodies Group.
Geological survey was conducted in Moodies Hills and Gate of Paradise, Barberton Greenstone Belt. MdS2 in Sheba Hills indicated the most significant Fe-Cr concentrations and was subjected to detailed observation. These samples were classified into Sandy-siltstone (6.1–16.3 wt% Fe₂O₃), Carbonate rich (Carb-) siltstone (21.6–32.8 wt% Fe₂O₃) and Magnetite rich (Mgt-) siltstone (18.9–50.2 wt% Fe₂O₃) by petrographic observations. Contents and stable isotope ratios of organic carbon in these samples indicate 0.03–0.29 wt% and ~−27 per mil, suggesting the biogenic origin. Carbon stable isotope ratio of carbonate ranges −12.5 to −3.4 per mil, which indicates isotopic contribution of organic carbon to carbonate.
Mgt-siltstone is composed of alternating magnetite rich-/clastic quartz rich-/carbonate-rich layers. The main host minerals for Cr are chromite with ~50 μm diameter surrounded by magnetite and single euhedral magnetite grains with 5–150 μm diameter. Trace element concentrations in magnetite were measured by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Magnetite in Mgt-siltstone showed significantly higher Cr concentration than those in previous reports, probably due to the shallow oxic depositional environments of the Moodies Hills. The results of the LA-ICP-MS analysis also indicated variations of Cr concentrations in magnetite depending on microlayers, suggesting that Cr concentrations at the primary precipitation were preserved in Moodies Hills BIF. On the other hand, in Sandy- and Carb-siltstone, chromite is rare, although bulk Cr concentrations are almost identical. Main host minerals for Cr in Sandy- and Carb-siltstone are biotite and chlorite, which contain 1.22±0.66wt% and 1.54±0.58wt% Cr₂O₃ respectively, whereas biotite and chlorite in Mgt-siltstone contain 0.26±0.40wt% Cr₂O₃.
The positive correlation of Fe-Cr concentrations, variations of the main host minerals for Cr associated with rock facies, significantly high Cr concentrations in magnetite and the microscopic variations suggest that oxic environments allowing dissolution of Cr⁶⁺ were spread in 3.2 Ga shallow ocean. After the primary precipitation, Cr might be distributed to silicate (chromite, biotite) or concentrated in magnetite during early diagenesis, however, the spatial range of the migrations was under the microlayer level.