Refractory trace elements in Ca-Al-rich inclusions in the Allende meteorite

Abstract

Abstract-The condensation temperatures are calculated for a number of refractory trace metals from a gas of solar composition at 10e3 and 1O-4 atm. total pressure. Instrumental neutron activation analysis of C&Al-rich inclusions in the Allende carbonaceous chondrite reveals enrichments of 22.8 f 2.2 in the concentrations of Ir, SC and the rare earths relative to Cl chondrites. Such enrichments cannot be due to magmatic differentiation processes because of the marked differences in chemical behavior between Ir and SC, exhibited by their distributions in terrestrial igneous rocks and meteorites. All of these elements should have condensed from a cooling gas of solar composition above or within the range of condensation temperatures of the major mineral phases of the inclusions, which suggests that these inclusions are hightemperature condensates from the primitive solar nebula. Gas-dust fractionation of these materials may have been responsible for the depletion of refractory elements in the ordinary and enstatite chondrites relative to the carbonaceous chondrites.

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

“…The only property in common to all of these elements is that they have condensation temperatures from a gas of solar composition which are higher than the accretion temperature of the inclusions, 1450°K. Theoretical calculations [6] suggest, although these elements began to condense at widely different temperatures from one another in the range 1500-2000°K, that each one should have been virtually totally condensed at 1450°K. The enrichment factor of 17.7 relative to C1 chondrites indicates that the refractories were efficiently incorporated into 100/17.7 = 5.6 wt.% of the total condensable matter, a value in agreement with condensation calculations [6,7].…”

“…Theoretical calculations [6] suggest, although these elements began to condense at widely different temperatures from one another in the range 1500-2000°K, that each one should have been virtually totally condensed at 1450°K. The enrichment factor of 17.7 relative to C1 chondrites indicates that the refractories were efficiently incorporated into 100/17.7 = 5.6 wt.% of the total condensable matter, a value in agreement with condensation calculations [6,7]. Thus, while individual inclusions are not typical of the mean composition of the high-temperature condensates, a collection of inclusions, being a larger sample of this material, is far more representative.…”

“…The coarse-grained, Ca-Al-rich inclusions in the Allende meteorite are thought to be the earliest, highest-temperature condensates from the cooling vapor of the solar nebula [4,5], representing ~5 wt.% of the total condensable matter [6]. These inclusions are several millimeters to 2 cm in size, subspherical to angular in shape and white in color, sometimes with gray patches.…”

“…Grossman [6] showed that many trace dements are so refractory or form such stable oxides that they should have condensed in the same high-temperature range as the major mineral phases of the coarse-grained inclusions. Many trace element studies [6,[12][13][14][15][16][17][18][19][20][21] have demonstrated enrichment of the inclusions in these refractory elements and their depletion in elements which are more volatile in the solar nebula.…”

“…Many trace element studies [6,[12][13][14][15][16][17][18][19][20][21] have demonstrated enrichment of the inclusions in these refractory elements and their depletion in elements which are more volatile in the solar nebula. The concentration of any particular refractory trace element and, often, the ratio of one refractory to another vary widely from inclusion to inclusion.…”