Donald S. Burnett

Quantifying the diffusion kinetics and spatial distributions of radiogenic ^4He in minerals containing proton-induced ^3He

Apatite, titanite and olivine samples were bombarded with a ~ 150 MeV proton beam to produce ~ 10^8 atoms/mg of spallation ^3He. High-precision stepped-heating experiments were then performed in which the artificial ^3He and, for apatite and titanite, the natural radiogenic ^4He were measured to characterize the diffusive behavior of each isotope. Helium-3 diffusion coefficients are in excellent agreement with concurrently and/or previously determined He diffusion coefficients for each mineral. Our results indicate that proton-induced ^3He is uniformly distributed and that radiation damage associated with a proton fluence of ~ 5 x 10^(14) protons/cm^2 does not cause noticeable changes in ^4He diffusion behavior in at least apatite and titanite. Proton-induced ^3He can therefore be used to establish He diffusion coefficients in minerals with insufficient natural helium for analysis or those in which the natural ^4He distribution is inhomogeneous. In addition,step-heating ^4He/^3He analysis of a mineral with a uniform synthetic ^3He concentration provides a means by which a natural ^4He distribution can be determined.

Evidence for the formation of an iron meteorite at 3.8 × 109 years

Rb Sr isotopic analyses were made on silicate inclusions from Kodaikanal, a brecciated fine octahedrite. Twelve analyses of different mineral fractions and separate inclusions were made. The silicates in this iron meteorite are highly enriched in alkalis and show ^(87)Sr/^(86)Sr ratios ranging up to 8.8. The samples lie on a well defined isochron on the Sr-Rb evolution diagram and indicate an age of 3.8 ± 0.1 × 10^9 y and an initial ^(87)Sr/^(86)Sr ratio between 0.69 and 0.73. These data provide unambiguous evidence for the ‘formation’ of younger solid objects in the solar system in a process which demands extensive chemical differentiation. This is evidence for an intermediate history for some meteoritic objects of a different sort than has been previously observed.

Solar Wind Neon from Genesis: Implications for the Lunar Noble Gas Record

Lunar soils have been thought to contain two solar noble gas components with distinct isotopic composition. One has been identified as implanted solar wind, the other as higher-energy solar particles. The latter was puzzling because its relative amounts were much too large compared with present-day fluxes, suggesting periodic, very high solar activity in the past. Here we show that the depth-dependent isotopic composition of neon in a metallic glass exposed on NASAs Genesis mission agrees with the expected depth profile for solar wind neon with uniform isotopic composition. Our results strongly indicate that no extra high-energy component is required and that the solar neon isotope composition of lunar samples can be explained as implantation-fractionated solar wind.

87Rb-87Sr ages of silicate inclusions in iron meteorites

Rb-Sr measurements were made on silicate inclusions extracted from the iron meteorites Toluca, Odessa, Four Corners, Linwood, Pine River, Colomera and Weekeroo Station. Strontium isotopic analyses were made on samples as small as 24 ng. The typical Sr and Rb blanks were 2 × 10^(−9) g and 2 × 10^(−10) g, respectively. In certain cases it was possible to obtain relatively precise isochrons. With the exception of Colomera, all of these samples gave ages compatible with 4.4 to 4.8 × 10^9 y. The Colomera data scatter widely and do not meet the requirements for defining a reliable age. None of the samples are compatible with an age of 6 × 10^9 y. The primary strontium observed or estimated for Toluca and Pine River agrees with that obtained from achondrites. It is evident that extensive Rb-Sr isotopic studies may be made on a large number of rather typical iron meteorites.

40Ar40K ages of silicate inclusions in iron meteorites

Analyses of the argon and potassium in silicate inclusions were made on Weekerro Station, Toluca, Four Corners and Kodaikanal. The 40Ar40K ratios of all samples except Kodaikanal are compatible with an age of 4.5±0.15 × 109yr. These results are in agreement with the SrRb results obtained previously but not with the so-called 40Ar40K ages determined on iron meteorites by other workers. The 40Ar40K age of 3.5±0.1 × 109yr on Kodaikanal is consistent with the 87Sr87Rb age of 3.8±0.1 × 109yr. Estimates of the U + Th content of the silicate inclusions were made based on the 4He contents. Kodaikanal was found to have a very young exposure age for an iron meteorite (⪅40 m.y) based on upper limits to the cosmogenic He, Ne and Ar in the metal phase.

Primary U distribution in scleractinian corals and its implications for U series dating

In this study we use microsampling techniques to explore diagenetic processes in carbonates. These processes are important as they can affect the accuracy of U series chronometry. Fission track maps of deep-sea scleractinian corals show a threefold difference between the minimum and maximum [U] in modern corals, which is reduced to a factor of 2 in fossil corals. We use micromilling and MC-ICP-MS to make detailed analyses of the [U] and δ234Uinitial distributions in corals from 218 ka to modern. Within each fossil coral we observe a large range of δ234Uinitial values, with high δ234Uinitial values typically associated with low [U]. A simple model shows that this observation is best explained by preferential movement of alpha-decay produced 234U atoms (alpha-recoil diffusion). Open-system addition of 234U may occur when alpha-recoil diffusion is coupled with a high [U] surface layer, such as organic material. This process can result in large, whole-coral δ234Uinitial elevations with little effect on the final age. The diagenetic pathways that we model are relevant to both shallow-water and deep-sea scleractinian corals since both exhibit primary [U] heterogeneity and may be subject to U addition.

87Rb87Sr isochron and 40K40Ar ages of the norton county achondrite

Analysis of seven different portions of Norton County yielded Rb/Sr ratios ranging from 0.15 to 2.3 and permitted the determination of an isochron with high precision. The Rb and Sr concentrations were found to be as low as ∼0.16 ppm. All of the data lie to within their experimental errors on a line in the Sr evolution diagram of slope 0.0654 ± 0.0014, and intercept 0.700 ± 0.002. This determines an age of 4.7 ± 0.1 × 10^9 yr for λ = 1.39 × 10^(−11) yr^(−1). ^(40)K^(40)Ar ages on samples containing 42, 53 and 107 ppm K gave ages of 4.2 – 4.5 × 10^9 yr which are compatible with the Rb-Sr age. The K appears to be concentrated in alkali feldspar grains of about 5–50 microns. The ^(87)Rb^(87)Sr age and primary ^(87)Sr/^(86)Sr value is of sufficient precision to permit the age resolution of meteoritic objects which were formed 0.3 × 10^9 yr apart. No evidence of element redistribution was found for Norton Country as indicated by the consistency of our Rb-Sr and K-Ar results.

Nucleosynthesis in the early history of the solar system.

Abstract This paper revises the model of Fowler, Greenstein and Hoyle (FGH) for the nucleosynthesis of D, Li, Be and B by high energy particles from the Sun during the early history of the solar system. In this model these nuclei are produced by spallation reactions, mainly on O16, in metric-sized planetesimals. Large numbers of neutrons are also produced. A fraction of these are thermalized and react byB10(n, α)Li7, Li(n, α)H3, to produce the terrestrial Li and B isotopic ratios. Additional D is produced by H1(n, γ) on hydrogen retained in the planetesimals as H2O. The total energy required in high energy particles is about 2 × 1044 ergs. The nuclear calculations have been generalized in an approximate manner to include a dependence on the duration of the irradiation caused by the long lifetime of Be10. The first stage of the calculation yields the required spallation yields and time-integrated neutron flux to produce the terrestrial Li, Be and B abundances and isotopic ratios. The required flux is 4 × 1021 n/cm2, identical with that obtained by FGH, and does not depend significantly on the choice of irradiation time. The predicted spallation yields depend more strongly on the irradiation time. Those are compared with cloud chamber data for O16 + 300 MeV neutrons. The predicted low spallation yield for Be9, which merely reflects its low relative abundance, is consistent with the cloud chamber data. Good agreement is not obtained for the B Li spallation ratio; however, for large values of the irradiation time, this could be due to uncertainties in the parameters. This does not seem possible for short irradiation times, however. Tho nuclear processes are less reasonable if C is present in the planetesimals or if higher energy particles are assumed in order to get appreciable amounts of LiBeB from spallation of SiFe as well as from O16. The second stage of the calculation yields the required hydrogen concentration, H Si ≈ 1 , and “dilution factor,” Fd ≈ 20 (approximately the ratio of unirradiated to irradiated material) to give the terrestrial D H ratio. FGH calculated Fd = 10 and H Si = 8 . The amount of water is considerably reduced in the present calculations. Although a loss of O during the formation of the Earth still must be postulated, the amount to be lost is much less than in tho FGH case. We point out that the FGH model is compatible with suggestions that the Moon has a high water content and that biotic material has formed in the carbonaceous chondrites and on the lunar surface. The nucleosynthesis of C13 in its present terrestrial abundance appears quite feasible; however, in this case the solar C 13 C 12 ratio will be much less than that observed terrestrially. Conflicting experimental results exist on this point at the present time. Observed C 13 C 12 variations in meteorites appear to be due to chemical fractionation. The fact that the isotopic composition of Li, Gd, and K in stone meteorites is identical with that found terrestrially requires that both terrestrial and meteoritic material were subjected to the same particle flux and had the same fraction of material irradiated. This implies that the Earth and the meteorites had a common initial history if the basic features of this model are to be retained. A lunar origin for stone meteorites could very well provide the required astrophysical situation to meet this requirement, whereas an asteroidal origin presents many more difficulties.

Experimental geochemistry of Pu and Sm and the thermodynamics of trace element partitioning

The partitioning of Pu and Sm between diopside/liquid and whitlockite/liquid has been investigated experimentally to evaluate the geochemical coherence of Pu and the light REEs. ^(Pu)D/^(Sm)D is 2 ~ for both diopsidic pyroxene and whitlockite. This small amount of fractionation would be decreased further if Pu were compared to Ce or Nd. Our experimental results thus validate the suggestion that Pu behaves as a LREE during igneous processes in reducing environments. Our data and the data of Ray et al. (1983) indicate that temperature rather than melt composition is the most important control on elemental partitioning. This is true even though we demonstrate that additions of only 1–2 wt.% of P2_O_5 to the diopside-anorthite-albite system change ^(Pu)D_(cpx) by a factor of two. Our data suggest that P_2O_5 in aluminosilicate melts serves as a complexing agent for the actinides and lanthanides.

Cosmogenic rare gases and the 40K-40 Ar age of the kodaikanal iron meteorite*

Measurements have been made of the cosmogenic He, Ne, and Ar in the metal phase and of the K-Ar ages of two glass inclusions in the Kodaikanal iron meteorite. The small amounts of cosmogenic rare gas are best interpreted as due to a low cosmic ray exposure age. From empirical production rates, ages of 12, 15 and 15 m.y. were obtained from the 3 He, 21 Ne and 38 Ar concentrations, respectively. The 3 He/ 21 Ne, 3 He/ 4 He and 4 He/ 38 Ar ratios are all consistent with low shielding during the exposure. The feldspar and glass have approximately equal K-Ar ages of 3.5 ± 0.1 and 3.3 ± 0.1 × 10 9 yr, respectively. These results place constraints on possible origins for the glass and feldspar.