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

(1)建立了一种用于低能伽马射线测量的低质量质子阱探测器精确测定稀土元素Ce和Eu的仪器中子活化方法。在理想的条件下，该方法具有与质谱同位素稀释技术相当的准确性。这种方法特别适用于小尺度的样品。(2)南极无球粒陨石稀土模式呈现Ce异常。然而，这些Ce异常被发现是由南极冰的蚀变产生的，因此与原始行星的氧化还原条件无关。其中三颗南极无球粒陨石被发现来自月球高地，可能来自月球的远端。(3)非洲沉积岩的稀土模式表明，在这3亿年期间，地壳中的稀土丰度变化不大，但在3.0 ~ 3.5亿年以前，地壳中的稀土丰度从球粒陨石到现今的变化很快。3.5 -b.y。古Onverwacht岩未见Ce异常，但有Eu异常。这表明燧石的沉积环境比年轻燧石的沉积环境要低得多。(4)在南极冰中发现了富Ca、ti和富Fe、Cr、ni的新型球粒。两种类型显示出相似的稀土模式，Nd和Sm异常，表明两种类型具有共同的起源，可能是陨石撞击。(5) Allende陨石中富含Ca、al的粗粒包裹体中分离出的矿物呈Yb异常。这些Yb异常要么是由于太阳星云中凝结产生的成分受到污染，要么是在极端还原条件下的分裂。后一种可能性表明，Yb异常可以用作非常还原环境的氧化还原指标。

(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.