Richard H. Becker

Xenon and krypton isotopes in extraterrestrial regolith soils and in the solar wind

Isotopic distributions of pure solar-wind xenon and krypton are derived from an extensive data base of xenon and krypton compositions evolved from lunar and meteoritic regolith samples by acid-etching or combustion-pyrolysis experiments in several different laboratories. Regolith Xe and Kr are nonuniform mixtures of primary solar-wind components with others arising in situ from cosmic-ray spallation, neutron-capture in iodine and bromine, and, for Xe, from adsorption or shallow implantation of radiogenic isotopes generated by decay of long-extinct radionuclides and still extant in the regolith. A previously unrecognized irradiation effect in Xe has generated variable and often large excesses of 126Xe in both lunar and asteroidal regolith samples. Their most likely production mechanism appears to be solar proton reactions on dispersed surface-correlated iodine, but for most samples this requires substantially more iodine circulating in the topmost regolith than is reflected in their measured contents. We find that the pure solar-wind composition itself can be accurately modeled as a mixture of “UXe” and a heavy-isotope constituent containing primarily only 134Xe and 136Xe components which from other evidence, observed and inferred, appear to have been fundamental constituents of primordial solar-system Xe. Deconvolution of measured heavy-isotope Xe compositions according to this view reveals the presence of an additional rcgolith contribution with an isotopic distribution fully consistent with that of 244Pu fission Xe. A solar-energetic-particle (SEP) Xe component, isotopically distinct from the solar wind composition, is not apparent in this analysis. SEP-like fractionated krypton, however, is clearly a contributor to the regolith Kr system, completely in line with conclusions reached by the Zurich group, along with solar-wind Kr and the expected additions from spallation and neutron capture in dispersed regolithic bromine. A large number of Kr fractions released from a variety of samples of differing antiquities by early acid attack and low-temperature pyrolysiscombustion. and therefore considered to evolve primarily from the shallow, largely SEP- and spallation-free solar-wind implantation zones in grain surfaces, point firmly to an isotopically light and time-invariant composition for pure solar-wind Kr. This composition is essentially identical to that of an ancient Kr component :inferred, from models of atmospheric evolution on the two planets, to have been present in the primordial atmospheres of Earth and Mars.

Long-term changes in solar wind elemental and isotopic ratios: A comparison of two lunar ilmenites of different antiquities

Abstract An ilmenite separate from lunar regolith breccia 79035, a sample presumed to have been exposed to solar wind more than 2 Ga ago, was analyzed for noble gas and nitrogen elemental and isotopic abundances by stepwise oxidation and pyrolysis. The gases appear to be distributed between two distinct reservoirs in the ilmenite, defined by release patterns and isotopic considerations. One of the reservoirs, near grain surfaces, yields elemental ratios that for the most part are “solar” while the other, sited at greater depths within grains, has severely fractionated elemental abundances and generally heavier isotopic ratios as well. Xenon provides an exception to the solar abundance pattern in the near-surface reservoir, being enhanced by about a factor of 2 relative to the expected value. A comparison of the 79035 separate with a previously analyzed ilmenite from soil 71501, which received its solar wind exposure much more recently, indicates that the two-fold xenon enhancement occurs in the fractionated reservoir as well as the “solar” one, and that it may therefore be attributable to a change in the solar wind elemental abundances. Other differences between the two ilmenites occur in helium and neon isotopic ratios and in He Ar elemental ratios. Since mineralogical influences on retentivities of the gases in the two samples should be the same, and possible contributions of non-solar wind components to one ilmenite in preference to the other can generally be eliminated or accounted for, all of these differences may reflect changes in the solar wind over time.

Irradiation records in regolith materials. I: isotopic compositions of solar-wind neon and argon in single lunar mineral grains

Abstract We have applied a stepwise pyrolytic extraction technique to eleven individual lunar regolith grains to investigate the compositions of light noble gases embedded in grain surfaces by solar wind irradiation, with emphasis on the rather poorly known isotopic composition of solar-wind argon. Results are intriguing: average 20 Ne/ 22 Ne ratios observed in early pyrolytic releases from ilmenite grains separated from lunar soils 71501, 79035 and 10084 agree very well with both direct measures of the solar wind neon composition in the Apollo foils and with values obtained in first releases from acid-etched ilmenites by the Zurich laboratory, whereas these same pyrolytic and acid-etch fractions carry argon isotopic signatures that significantly disagree—average 36 Ar/ 38 Ar ratios near 5.8 for thermal extraction compared to 5.4–5.5 for chemical etching at Zurich. Consideration of the isotopic and elemental data from these grains in the context of first-order diffusive modeling calculations points to gas release at low temperatures, without significant isotopic or elemental fractionation, from isolated grain-surface reservoirs of solar wind composition. The physical nature of these reservoirs is presently unknown. In this interpretation the preferred solar wind 20 Ne/ 22 Ne and 21 Ne/ 22 Ne ratios deduced from this study are respectively 13.81 ± 0.08 and 0.0333 ± 0.0003, both within error of the Zurich acid-etch values, and 36 Ar/ 38 Ar = 5.77 ± 0.08. It may be possible to reconcile the discrepancy between the acid-etch and pyrolytic estimates for the solar wind 36 Ar/ 38 Ar ratio in the context of arguments originally advanced by Benkert et al. (1993) to account for their He and Ne isotopic compositions. At the other, high-temperature end of the release profile from one of these grains there are clear isotopic indications of the presence of a Ne constituent with 20 Ne/ 22 Ne close to the 11.2 ratio found at Zurich and attributed by these workers to a deeply-sited component implanted by solar energetic particles.

Irradiation records in regolith materials, II: Solar wind and solar energetic particle components in helium, neon, and argon extracted from single lunar mineral grains and from the Kapoeta howardite by stepwise pulse heating

Abstract High-resolution stepped heating has been used to extract light noble gases implanted in a suite of 13 individual lunar ilmenite and iron grains and in the Kapoeta howardite by solar wind (SW) and solar energetic particle (SEP) irradiation. Isotopic analyses of gases evolved at low temperatures from the lunar grains confirm the neon and argon compositions obtained by Pepin et al. (Pepin R. O., Becker R. H., and Schlutter D. J., “Irradiation records in regolith materials, I: Isotopic compositions of solar-wind neon and argon in single lunar regolith grains”, Geochim. Cosmochim. Acta63, 2145–2162, 1999) in an initial study of 11 regolith grains, primarily ilmenites. Combination of the data sets from both investigations yields 20Ne/22Ne = 13.85 ± 0.04, 21Ne/22Ne = 0.0334 ± 0.0003, and 36Ar/38Ar = 5.80 ± 0.06 for the lunar samples; the corresponding 36Ar/38Ar ratio in Kapoeta is 5.74 ± 0.06. The neon ratios agree well with those measured by Benkert et al. (Benkert J.-P., Baur H., Signer P., and Wieler R., “He, Ne, and Ar from the solar wind and solar energetic particles in lunar ilmenites and pyroxenes”, J. Geophys. Res. (Planets) 98, 13147–13162, 1993) in gases extracted from bulk lunar ilmenite samples by stepped acid etching and attributed by them to the SW. The 36Ar/38Ar ratios, however, are significantly above both Benkert et al.’s (1993) proposed SW value of 5.48 ± 0.05 and a later estimate of 5.58 ± 0.03 from an acid-etch analysis of Kapoeta (Becker R. H., Schlutter D. J., Rider P. E., and Pepin R. O., “An acid-etch study of the Kapoeta achondrite: Implications for the argon-36/argon-38 ratio in the solar wind”, Meteorit. Planet. Sci. 33, 109–113, 1998). We believe, for reasons discussed here and in our earlier report, that 5.80 ± 0.06 ratio most nearly represents the wind composition. The 3He/4He ratio in low-temperature gas releases, not measured in the first particle suite, is found in several grains to be indistinguishable from Benkert et al.’s (1993) SW estimate. Elemental ratios of He, Ne, and Ar initially released from grain-surface SW implantation zones are solar-like, as found earlier by Pepin et al. (1999). Gases evolved from these reservoirs at higher temperatures show evidence for perturbations from solar elemental compositions by prior He loss, thermal mobilization of excess Ne from fractionated SW components, or both. Attention in this second investigation was focused on estimating the isotopic compositions of both the SW and the more deeply sited SEP components in regolith grains. Several high-temperature “isotopic plateaus”—approximately constant isotopic ratios in gas fractions released over a number of consecutive heating steps—were observed in the close vicinities of the SEP ratios for He, Ne, and Ar reported by Benkert et al. (1993). Arguments presented in the text suggest that these plateaus are relatively free of interferences from multicomponent mixing artifacts that can mimic pure component signatures. Average SEP compositions derived from the stepped-heating plateau measurements are in remarkable agreement with the Zurich acid-etch values for all three gases.

Helium and Neon Abundances and Compositions in Cometary Matter

Materials trapped and preserved in comets date from the earliest history of the solar system. Particles captured by the Stardust spacecraft from comet 81P/Wild 2 are indisputable cometary matter available for laboratory study. Here we report measurements of noble gases in Stardust material. Neon isotope ratios are within the range observed in “phase Q,” a ubiquitous, primitive organic carrier of noble gases in meteorites. Helium displays 3He/4He ratios twice those in phase Q and in Jupiters atmosphere. Abundances per gram are surprisingly large, suggesting implantation by ion irradiation. The gases are probably carried in high-temperature igneous grains similar to particles found in other Stardust studies. Collectively, the evidence points to gas acquisition in a hot, high ion-flux nebular environment close to the young Sun.

Nitrogen and light noble gases in Shergotty

Abstract Two samples of Shergotty and one of EETA 79001 lithology A have been analyzed for nitrogen and light noble gases. Nitrogen yields are of the order of one ppm, with isotopic ratios within a few per mil of the terrestrial atmospheric composition after correction for spallogenic nitrogen. The nitrogen data do not indicate the presence of any significant amounts of the high-δ 15 N component seen in EETA 79001 glass, nor do they show the isotopically light component, with δ 15 N of about −35% o , reported previously for Shergotty. The possibility that these components are present but masked by variations in the Ar N ratio of the light component cannot be excluded. The spallogenic noble gas data are consistent with cosmic ray irradiation of both Shergotty and EETA 79001 at shallow shielding depths. Cosmic-ray exposure ages are in the range 0.5 to 0.8 My for EETA 79001 and 2.0 to 5.2 My for Shergotty, depending on the choice of object size and shielding. Interpretation of the spallation data is complicated by uncertainties in production rates, unknown contributions from solar cosmic rays, and the possibility that Shergotty has suffered some He loss. In the absence of He loss, the exposure age of Shergotty would lie in the range ~2.0 to ~3.5 My. Radiogenic argon appears to be very heterogeneously distributed on a small scale in Shergotty, due either to small scale mineralogical inhomogeneity or to significant variation in the degree of degassing of minerals during shock, although the presence of trapped argon with a high 40 Ar 36 Ar ratio and heterogeneous distribution cannot be ruled out. The present study does not provide evidence for or against an origin of Shergotty on Mars.

Noble gases and nitrogen released from lunar soils by acid etching

A stepwise acid-etching technique similar to the closed system stepwise etching (CSSE) method developed at ETH Zurich was used to examine the solar wind reservoirs of lunar soil grains. Samples were treated with weak acids (H2O, H2SO3) to facilitate the release of the most shallowly implanted gases. Noble gas abundances and isotopic compositions, including Kr and Xe in some cases, and a few nitrogen data were obtained for mineral or grain-size separates of three lunar soils (plagioclase from 60051, pyroxene from 75081, and <25 ym bulk 79035). The 60051 plagioclase grains, considered to be a possibly unique resource for determining the modern-day solar wind composition, show unusually low contents of solar wind He, Ne, and particularly Ar, but do not otherwise possess any characteristics clearly attributable to a modem-day solar wind exposure. Initial water and acid treatments of the grains, however, release an apparently pure SEP component. The 75081 pyroxene and the size separate of bulk 79035 both yield Kr and Xe compositions in initial etch steps that are characteristic of undiffused solar wind, significantly increasing the database for measurements of solar wind Kr and Xe where possible laboratory thermal diffusion and fractionation effects are not a concern. Pyroxene in particular appears to be a suitable alternative to ilmenite for the purpose of making measurements of this kind. Nitrogen release by acid etching is not at present quantitative, and while it appears possible to obtain reasonable isotopic ratios for solar wind N, we are unable to use the technique to determine solar nitrogen to noble gas ratios. Light noble gases in all three soil separates, other than the aforementioned behavior of 60051, appear to behave in accord with expectations based on acid-etching analyses performed by the Zuirich group.