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

(1)建立了用LEPS探测器测量低能伽马射线的精密仪器中子活化法测定稀土元素Ce和Eu的方法。在理想的条件下，该方法具有与质谱学同位素稀释法相当的准确度。该方法特别适用于小规模样品的测定。(2)南极非球粒陨石的稀土配分模式显示出Ce异常。然而，这些Ce异常是由南极冰蚀变产生的，因此与原始行星的氧化还原条件无关。其中三块南极无球粒陨石被发现来自月球高地，可能来自月球的另一边。(3)来自非洲的沉积岩的稀土配分模式表明，地壳中的稀土丰度在这3年内变化不大，但从球粒陨石到现代地壳的丰度变化迅速，约为3.0~3.5b.y。那是以前的事了。3.5b.y-老的Onverwacht硅质岩没有显示Ce异常，但显示出可变的Eu异常。这表明硅质岩的沉积环境比年轻硅质岩的沉积环境要强得多。(4)在南极冰层中发现了富钙、富钛和富铁、铬、镍的新型球体。这两种类型都显示出相似的稀土配分模式，具有Nd和Sm的异常，表明这两种类型具有共同的成因，可能是陨石撞击。(5)从阿连德陨石中粗粒富钙、富铝包裹体中分离出的矿物显示出Yb异常。这些Yb异常可能源于太阳星云中凝聚产生的成分的污染，或者是在极端还原的条件下发生的分配。后一种可能性表明，Yb异常可能被用作非常还原的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.