[B-CG07] Decoding the history of Earth: From Hadean to the present

[BCG07-04] The depositional setting of Francevillian Group, Gabonese Republic, revisited from REE geochemistry

★Invited Papers

*Satoshi Yoshida¹, Hisashi Asanuma², Tomohiko Sato³, Yusuke Sawaki¹, Takafumi Hirata⁴ (1.Graduate School of Arts and Sciences, The University of Tokyo, 2.Graduate School of Human and Environmental Studies, Kyoto University, 3.Center for Fundamental Education, Okayama University of Science, 4.Geochemical Research Center, The University of Tokyo,)

Keywords:carbonate rock, in situ analysis, Francevillian Group, Paleoproterozoic, rare earth element, paleo-seawater

The Paleoproterozoic Francevillian Group in the Gabonese Republic is recently attracting increasing attention because it includes distinctive macroscopic structures interpreted as the first eukaryotic fossil. The depositional setting and associated redox condition are, however, rather poorly constrained. Although a shallow marine environment was assumed on the basis of sedimentary structures formed by strong currents, a non-marine environment such as lacustrine is also capable of forming the structures. Thus, a complementary evaluation by geochemical data can assist in understanding its depositional environment. Rare earth element (REE) in carbonate rock is one of the powerful tools for such purposes. Superchondritic Y/Ho value is often used as an index of deposition in a marine setting, and positive/negative Ce anomaly is useful in evaluation of redox condition. Previous studies determined REE abundances of the dolostones of the Francevillian Group by whole rock analysis, and the results exhibited chondritic Y/Ho value and little Ce anomaly. The dolostones include certain amounts of detrital silicate minerals. Furthermore, the carbonate minerals in the rocks are composed of dolomite, Fe- and Mn-rich dolomite, and calcite. This indicates that the dolostones were formed through a complex diagenetic history. Because whole rock analysis cannot separate each phase, we tried to determine REE contents of the dolostones by in situ analysis.
We collected dolostones from the Francevillian Group deposited in the Lastoursville basin. The FB Formation of the Group is mainly composed of dolostones, and its upper part is characterized by high Mn content (> 0.6 wt%). REE contents of the dolostones were analyzed with ICP-MS/MS coupled with a laser ablation sampling technique. The spot analysis demonstrated that the dolostones have superchondritic Y/Ho values (up to 40.6) and show a large variation in Ce anomalies. Based on our recent compilation, modern marine carbonate rocks have Y/Ho values greater than 33, whereas lacustrine carbonate rocks have Y/Ho values lower than 32. Thus, the Y/Ho values obtained by in situ analysis suggest that the Francevillian dolostones were deposited in an ocean setting rather than a lake.
Ce anomaly values show a positive correlation with Fe and Mn contents. This correlation can be explained by the mixing of seawater- and ferromanganese oxide-components. The latter component started from the settled ferromanganese oxide in water column that selectively adsorbed Ce. During the diagenetic reduction of the oxides, this feature was inherited by the Fe- and Mn-rich dolomite. These, in turn, indicate that the seawater had been oxidized enough to produce such a Mn shuttle. On the other hand, Y/Ho values show little correlation with Fe and Mn contents. In the modern ocean, ferromanganese oxides have subchondritic Y/Ho values, which leads to the idea that fractionation by the ferromanganese oxides is responsible for the superchondritic Y/Ho value of seawater. However, in the Paleoproterozoic ocean, because the ferromanganese oxides also have superchondritic Y/Ho values, other factors such as hydrothermal fluids created superchondritic Y/Ho values in seawater.