Mass spectrometric analyses of He, Ne and Ar were performed on the mesosiderites, Clover Springs, Crab Orchard, Patwar, and Hainholtz with comparative analyses also conducted on the achondrite Binda. Gases were released in incremental temperature steps so that variation of spallogenic compositions could be studied. All samples were found to be rich in cosmogenic Ne and Ar. The 21Ne/22Ne ratio showed a variation between 0.83 and 1.00 indicating a definite dependence on the combined effects of target element abundances and cosmic ray shielding. The shielding effect seems to be especially pronounced in the mesosiderites. The 36Ar/38Ar ratio for spallation was found to be ∼0.65 in agreement with previous work. Cosmic ray exposure ages determined from cosmogenic Ar abundances fall in the range 40 to 87myr for mesosiderites. Crab Orchard gave a K-Ar age of 3.2 × 109yr. The Binda achondrite gave 44myr and 3.2 × 109yr respectively for cosmic ray exposure and K-Ar ages.
The cosmogenic xenon isotope patterns were calculated independently for each temperature fraction from Patwar and also for the Estherville mesosiderite data of KAISER and RAJAN. A distinct correlation with temperature was observed which is comparable to other results on achondrites and lunar materials. These spallation product compositional variations appear to depend on target and energy variations. Cosmogenic yields of 129Xe are also distinctly correlated, suggesting an insignificant contribution of radiogenic 129Xe, i.e., from extinct 129I decay. The 129Xe/126Xe spallation ratio varied from 0.7 in the high temperature fractions from Estherville to about 1.8 in the low temperature fractions from Patwar.
Rb-Sr whole-rock ages have been determined on 10 small-sized granitic masses in Japan having simple thermal history. The Rb-Sr ages are 4∼9m.y. older than the corresponding K-Ar biotite ages, except for two Neogene granites which show concordant ages. The Rb-Sr whole-rock ages for these masses probably represent the time of emplacement, whereas the K-Ar biotite ages indicate the time of uplift and cooling, thus the slight discordance in the two age systems may be explained as indicating the cooling period of the granitic masses. The comparison between the published Rb-Sr whole-rock and K-Ar mineral ages for Japanese granitic rocks reveals that the rocks are grouped into three on the basis of the concordance-discordance mode: nearly concordant, discordant with the age difference of about 50m.y., and grossly discordant. Discordant ages for granitic rocks generally have rather large errors in Rb-Sr whole-rock ages, and careful evaluation is needed for these ages.
Partition coefficients for rare earth elements have been determined experimentally for diopside-liquid pairs by using two systems, Di90-Abs-An5 and Di80-Fo10-Qz10 (wt%). The partition coefficient patterns obtained by partial melting of silicate system maintained beneath solidus have three remarkable features: (1) There is a peak in the vicinity of Gd. (2) The pattern for heavy REE span is warped up with increasing atomic number. (3) The small negative Ce anomaly is seen. These patterns can be interpreted as reflecting effect of overlapping two peaks which correspond to M1 and M2 site in the pyroxene structure. The effects of temperature and pressure on the partition coefficient pattern are qualitatively discussed.
Isotopic anomalies observed in meteorites for Ca, Ba, Nd and Xe are explained as due to the alteration of the isotopic ratios by a combined effect of mass fractionation, neutron-capture and cosmic-ray irradiation processes, which took place prior to and during the period of formation of the solar system. The neutron-capture process appears to have occurred at a temperature higher than that of the earth's surface.
An intense irradiation of the solar system material by the cosmic-rays originating from a nearby supernova may have been sufficient to produce in certain meteorites an excess of 107Ag, which is the decay product of 6.5 × 106-year 107Pd from the 106Pd (n, γ)107Pd reaction
The difference in the isotopic compositions of barium from some meteorites and from the terrestrial barium is explained as due to the alteration of the isotopic ratios by a combined effect of mass-dependent fractionation, neutron-capture and cosmic-ray irradiation processes, which took place prior to and during the period of formation of the solar system. The abundances of 134Ba and 135Ba in the meteorite Bruderheim seem to be slightly enhanced due to the decays of 2.06-year 134Cs and 2.3 × 106-year 135Ba. A small difference in the isotopic compositions of cerium in the Bruderheim meteorite and the terrestiral sample can also be attributed to neutron-capture processes, which occurred during an early irradiation period. It is shown that the barium and xenon isotopic anomalies observed in meteorites are closely related to each other.