A study of redox environment of the primitive earth by using rare earth element abundances.

点击翻译按钮获取中文摘要

(1) A precise instrumental neutron activation method for determining rare earth elements including Ce and Eu by use of a LEPS-detector for low energy gamma-ray measurement was established. The method offers good accuracy comparable with the massspectrometric isotope dilution technique under desirable conditions. This method is particularly suitable for small scale samples. (2) Antarctic achondrites showed Ce anomalies in their rare earth patterns. However, these Ce anomalies were found to have bee produced by alteration in the Antarctic ice and thus have no relation to the redox conditions in the primitive planets. Three of the Antarctic achondrites were found to have come from the lunar highlands, possibly from the far side of the moon. (3) Rare earth patterns of the sedimentary rocks from Africa suggest rare earth abundances in the earth's crust have not changed much during these 3 b.y., but have change rapidly from chondritic to present-day crustal abundances about 3.0 to 3.5 b.y. ago. 3.5-b.y.-old Onverwacht cherts showed no Ce anomaly but showed variable Eu anomalies. This indicates that the sedimentary environment of cherts was considerably more reducing than those for younger cherts. (4) Ca, Ti-rich and Fe, Cr, Ni-rich new types of spherules were found in the Antarctic ice. Both types show similar rare earth patterns with anomalies in Nd and Sm, indicating both types have a common origi, possibly a meteoritic impact. (5) Minerals separated from a coarse-grained Ca,Al-rich inclusions in the Allende meteorite showed Yb anomalies. These Yb anomalies must have originated either from contamination of a component produced by condensation in the solar nebula or partitioning under an extremely reducing conditions. The latter possibility suggests that Yb anomalies may be used as a redox indicator for very reducing environments.