B. E. Hagee

Solubility and partitioning of Ne, Ar, Kr, and Xe in minerals and synthetic basaltic melts

We have measured the solubilities of Ne, Ar, Kr and Xe in natural samples of anorthite, diopside, forsterite, spinel and in synthetic basaltic melts. The minerals and melts represent equilibrium pairs in the Fo-An-Di-SiO2 system. Samples were suspended in individual crucibles in a one-bar flowing mixed noble gas atmosphere at 1300 or 1332°C for times ranging from 7–18 days. The solubilities in the minerals increase with increasing noble gas atomic number and typical solubility values are surprisingly high. Samples of a particular mineral (i.e., anorthite) that came from different localities yield distinctly different and reproducible solubility values. This indicates that properties intrinsic to individual samples can influence solubility. Noble gases are likely to be sited in lattice vacancy defects. In contrast, the solubilities in the melts decrease with increasing atomic number. Our data overlap the low end of the range defined previously for natural basalts; however, the dynamic range of the values from Ne to Xe is not as great as for the natural melts. Solubilities correlate well with melt molar volume but poorly with density. The partition coefficients increase with increasing noble gas atomic number for all mineral/melt pairs. Such trends imply that the terrestrial planet atmospheres were not derived from partial melting of chondritic source material. Significant fractionation of Kr relative to Xe is not observed, ruling out an origin for Earths “missing” Xe via magmatic fractionation. Partition coefficient absolute values are frequently near or greater than unity. For example, the ranges for five diopside measurements are the following: DNel, 0.013–0.37; DArl, 0.15–0.84; DKrl, 0.31–2.4; and DXel, 3.2–47. This indicates that magmatic transport is not an efficient mechanism for degassing the terrestrial planets. Our results favor a catastrophic origin for the atmospheres.

Solubility and partitioning of Ar in anorthite, diopside, forsterite, spinel, and synthetic basaltic liquids

We have investigated the solubility and partitioning of Ar in natural anorthite, diopside, forsterite, spinel, and synthetic iron-free basaltic melts. The experiments used a new technique which obviates post-quenching phase separation. Minerals and melts known to be in equilibrium are held in separate crucibles in a one bar flowing noble gas atmosphere at 1300°C or 1332°C. After a specified time the samples are quenched and the gas concentrations measured by mass spectrometry. A reversal and a rate study for Ar in anorthite indicate a close approach to equilibrium solubilities in our experiments. The solubility of Ar in the minerals is surprisingly high. In addition, the solubility of Ar in different samples of a particular mineral run in the same experiment varies more than the solubility in the same sample run in different experiments. This result suggests that noble gases are held in lattice vacancy defects. Other evidence supports defect siting: 1. (i) gases are held in very retentive sites; 2. (ii) solubility trends do not favor interstitial siting. 3. (iii) TEM imaging revealed no anomalous microstructures or dislocation densities. 4. (iv) EXAFS studies of some samples show that Kr has no preferred site in the lattices. Argon solubilities in synthetic silicate melts are lower than those observed experimentally in natural basalts. This difference correlates with the greater molar volume and polymerization of the natural basalts compared to the synthetic melts. The solubility variations greatly affect the absolute values of Ar mineral/ melt partition coefficients. Average anorthite/melt (0.6 ± 0.5) and diopside/melt (0.6 ± 0.5) partition coefficient values suggest that Ar is moderately incompatible. However, given the evidence that Ar solubility in minerals depends on lattice vacancy defect concentrations, it may not be possible to specify the partition coefficient values in a manner analogous to ionic species.

Actinide abundances in ordinary chondrites

Measurements of ^(244)Pu fission Xe, U, Th, and light REE (LREE) abundances, along with modal petrographic determinations of phosphate abundances, were carried out on equilibrated ordinary chondrites in order to define better the solar system Pu abundance and to determine the degree of variation of actinide and LREE abundances. Our data permit comparison of the directly measured Pu/U ratio with that determined indirectly as (Pu/Nd) × (Nd/U) assuming that Pu behaves chemically as a LREE. Except for Guarena, and perhaps H chondrites in general, Pu concentrations are similar to that determined previously for St. Severin, although less precise because of higher trapped Xe contents. Trapped ^(130Xe)/_(136Xe) ratios appear to vary from meteorite to meteorite, but, relative to AVCC, all are similar in the sense of having less of the interstellar heavy Xe found in carbonaceous chondrite acid residues. The Pu/U and Pu/Nd ratios are consistent with previous data for St. Severin, but both tend to be slightly higher than those inferred from previous data on Angra dos Reis. Although significant variations exist, the distribution of our Th/U ratios, along with other precise isotope dilution data for ordinary chondrites, is rather symmetric about the CI chondrite value; however, actinide/(LREE) ratios are systematically lower than the CI value. Variations in actinide or LREE absolute and relative abundances are interpreted as reflecting differences in the proportions and/or compositions of more primitive components (chondrules and CAI materials?) incorporated into different regions of the ordinary chondrite parent bodies. The observed variations of Th/U, Nd/U, or Ce/U suggest that measurements of Pu/U on any single equilibrated ordinary chondrite specimen, such as St. Severin, should statistically be within ±20–30% of the average solar system value, although it is also clear that anomalous samples exist.

Isotopic fractionation of Kr and Xe implanted in solids at very low energies

Abstract We report results on the implantation of Kr and Xe in W under closed system conditions at very low energies (50–500 eV). Investigation of the fraction of gas trapped as a function of time reveals the existence of competing trapping and release mechanisms and analysis of recovered trapped gas and residual gas phases shows that both elemental and isotopic fractionation result from these mechanisms. We determined the mass dependence for the overall implantation process to be at or near m 1 , with heavier isotopes enriched in the implanted gas. This mass dependence is inferred to result from implantation and a combination of diffusive and gas sputtering release mechanisms. Our results reaffirm the conclusion of Bernatowicz and Fahey (1986) that previously observed isotopic fractionation of Kr and Xe in carbonaceous material synthesized in electrical discharges owes its origin to low energy ion implantation and also suggest that this process may be relevant to incorporation of noble gases in early solar system materials. We also discuss the implication of our results for noble gas mass spectrometry.

Response to Comment by W.A. Myers and P.K. Kuroda on “Actinide abundances in ordinary chondrites”

One aspect of our recent paper (HAGEE et al., 1990) on actinide abundances in ordinary chondrites is estimation of the amount of ^(244)Pu present in these meteorites at the time of their formation. This estimate is based on the quantity of ^(244)Pu spontaneous fission Xe, a component whose resolution from other Xe components present must be based on observed isotopic compositions in the meteorite and assumed compositions for the contributing components. In their comments, MYERS and KURODA (1991) advocate a different approach to component analysis of Xe than the one we used and arrive at correspondingly different estimates of fission Xe and ^(244)Pu abundances.