Rare-earth concentrations in 3.5-billion-year-old Onverwacht cherts: An indicator for early Precambrian crustal environments

抄録

3.5-billion-year-old cherts from Onverwacht Group, Swaziland System in the Barberton area, South Africa were analyzed for rare earth elements (REE), Sc, Hf, Fe, Cr, Co and Na by means of instrumental neutron activation analysis (INAA). The Onverwacht cherts showed less fractionated REE abundances compared with those in younger cherts and other sedimentary rocks. The chondrite-normalized La/Yb ratios observed for the samples from the older Theespruit Formation and younger Hooggenoeg Formation of the Onverwacht Group are 1.2∼2.5, and 4.6, respectively. These values are smaller than the La/Yb values observed for the Fig Tree Group (5.5) and Moodies Group (7.8), which overlie the Onverwacht Group in the Barberton area, indicating an increasing trend for La/Yb ratio in the sedimentary rocks with time in the Swaziland System. The results appear to indicate that the relative fractionation between light and heavy REE in the crustal region which had supplied REE to the sedimentary rocks increased with time in the Archaean era from nearly chondritic La/Yb ratio to about the present-day value in a very short time span of a few hundred million years. Absence of Ce anomaly, together with variable Eu anomalies in the REE patterns, possibly shows reducing conditions in the sedimentary environments. It is also suggested that the negative Eu anomalies commonly observed for the post Archaean sedimentary rocks were derived by accumulation of plagioclase in the lower continental crust by sinking of plagioclase in magmas in the upper crust.