The case for an unfractionated244Pu/238U ratio in high-temperature condensates

Abstract

The coarse-g~ained, Ca-rich inclusions in the AUende meteorite are the highest-temperature condensates from the cooling solar nebula and, as such, the oldest solid objects in the solar system. All refractory elements with condensalion points above the accretion temperature of the inclusions whose eoneentrarions in them have been measured are seen to be present in the inclusions in unfractionated proportion to one another relative to C1 ehondrites when data ate averaged for a large number of inclusions. Observational data for U and theoretical data for both U and Pu'suggest that these elements exhibited refractory behavior in the solar nebula. An experiment is proposed in which fissiogenic Xe and U contents are measured in a suite of these inclusions to obtain the 244pu/238U ratio of the solar system at the time of initial condensation with an uncertainty of + 15%.

Core formation, evolution, and convection: a geophysical model

“…-The concentra tions of U and Th are listed in Table 4. The value of U is the mean concen tration from nine coarse-grained Allende inclusions (Ganapathy and Grossman, 1976) and is consistent with other results (Wanke et al, 1974). The concentration of Th is obtained by assuming the cosmic abundance ratio of Th/U n 0 4.…”

“…The enrichment factor for refractories between Allende inclusions and Cl chondrites is about 20 (Wanke et al, 1974;Martin, 1975, Ganapathy andGrossman, 1976). The only elements of these groups that we consider in detail are U and Th due to their significance as long-lived heat sources.…”

General constraints on the age and chemical evolution of the Galaxy

“…The aar~~/Zs%u abundance ratio is found from decay pr0duct.s of 241Pu. Ganapathy and Grossman (1976) proposed that the solar system *4nPu/*JsU a,bundance ra,tio be determined from coarse-grained Ca-Al-rich inclusions from Allende since they found that in ten such coarsegrained inclusions the average relative abundance of 21 refractory elements was unfractionated with respect to Cl chrondrite abundances. The advantage of these inclusions is that absolute concentra,tions of these elements are some 18 times greater than t,he Cl abundances.…”

Theories of the origin of the solar system 1956- 1985

“…This interpretation was supported by Lawrence Grossman (1973) and his colleagues (Grossman and Clark, 1973;Grossman and Larimer, 1974;Grossman and Olsen, 1974;Olsen and Grossman, 1974;Grossman, 1975;Ganapathy and Grossman, 1976;Grossman and Steele, 1976).…”

“…The major opposition to lunar-terrestrial similarity came from the Anders group, which favored coaccretion after fractionation in the solar nebula; they argued that the moon was formed in a circumterrestrial orbit from material that had condensed at higher temperatures than the Earth (Ganapathy and Anders, 1974). Another version of the coaccretion theory, developed by Ruskol (1971aRuskol ( , 1971b attributed compositional differences to processing of incoming planetesimals by collisions with the circumterrestrial swarm over a long period of time (10 8 yrs); volatiles would be removed from the outer edge by the solar wind, silicates would be broken up and remain in the swarm, while iron would pass through and be accreted by the growing Earth.…”