Studies of the Geochemical Similarity of Plutonium and Samarium and Their Implications for the Abundance of ²⁴⁴Pu in the Early Solar System

Geochemistry

Jones, John Hume ; Patterson, Clair C.

University:California Institute of Technology

Department:Geological and Planetary Sciences

关键词: Geochemistry;   

Others  :  https://thesis.library.caltech.edu/3930/3/jones_jh_1981.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

^PuD and ^SmD have been measured for diopsidic pyroxene and whitlockite in the system Di-An-Ab (± Ca₃(PO₄)₂). ^PuD was determined by fission track radiography and ^SmD was determined on the same samples by beta radiography. The analytical difficulties encountered in using photographic emulsions to determine beta fluences have been studied in detail. These studies have resulted in a technique which should be generally applicable to the radiography of low energy beta emitters (Appendix I).

^PuD and ^SmD for clinopyroxene are 0.17 and 0.31, respectively, at about 1250°C and ^SmD/^PuD = 1.9. The addition of 1.4% P₂O₅ to the DiAnAb system lowers ^SmD and ^PuD by factors of 1.5 and 2.0, respectively; this P₂O₅ addition raises the ^SmD/^PuD ratio to 2.6. ^PuD and ^SmD for whitlockite are 3.8 and 6.4, respectively.

The problem of approach to interfacial equilibrium is discussed in detail, and D values of crystals grown using various thermal histories are compared. Whitlockite D values appear to be independent of thermal history, implying a close approach to equilibrium at the crystal-liquid interface. Differences are seen between pyroxene D values for different thermal histories, and these variations are compared to the results of a simple kinetic disequilibrium model. The result of this comparison suggests that interfacial equilibrium was closely approached and that variations in pyroxene D values are mainly due to the temperature dependence of partitioning. This appears to be true for both Sm and Pu, even though the partitioning of other elements (e.g. Al) may be influenced by kinetic disequilibrium.

The magnitudes of ^SmD and ^PuD and (≤ 0.3) for clinopyroxene imply that Pu and the light REE's are difficult to fractionate during ordinary igneous processes. ²⁴⁴Pu/²³⁸U ratio measurements of meteorites are reviewed in light of the results of this study and a ²⁴⁴Pu/²³⁸U ratio of 0.005 ± 0.002 for the early solar system is favored. This is supportive of the ²⁴⁴Pu/²³⁸U ratio calculated by Marti et al. (1977) and suggests that the Podosek (1970a) value of 0.015 is invalid.