C. M. Hohenberg

Xenon-iodine dating: sharp isochronism in chondrites.

Measurements of the accumulation of Xel29 from radioactive decay of extinct 1129 in meteorites show that the 1129/ 1127 ratio in high-temperature minerals in diverse chondrites was 10-4 at the time of cooling. The uniformity in the ratio indicates that the minerals cooled simultaneously within 1 or 2 million years.

IXe dating of the shallowater achondrite

Abstract Xenon from a pile irradiated sample of the Shallowater enstatite achondrite was analysed in a step-wise heating experiment. Below 1200°C very little radiogenic 129 Xe was observed. Above this temperature radiogenic 129 Xe was released in excellent correlation with 128 Xe, from neutron capture on iodine, satisfying the requirements for IXe dating. The ratio of 129 I to 127 I when Shallowater began retaining xenon is the same within experimental error as the ratio in 10 chondrites previously measured, indicating that Shallowater and these chondrites began to retain xenon simultaneously to within one or two million years. Calculated values of the formation interval for these bodies lie between 60 and 220 million years depending on the model for nucleosynthesis assumed.

Fission xenon from extinct 244Pu in 14301

Xenon extracted in step-wise heating of lunar breccia 14 301 contains a fission-like component in excess of that attributable to uranium decay during the age of the solar system. There seems to be no adequate source for this component other than 244Pu. Verification that this component is in fact due to the spontaneous fission of extinct 244Pu comes from the derived spectrum which is similar to that observed from artificially produced 244Pu. It thus appears that 244Pu was extant at the time lunar crustal material cooled sufficiently to arrest the thermal diffusion of xenon. Subsequent history has apparently maintained the isotopic integrity of plutonium fission xenon. Of major importance are details of the storage itself. Either the fission component is the result of in situ fission of 244Pu and subsequent storage in 14 301 material, or the fission xenon was stored in an intermediate reservoir before incorporation into 14301. In the former case accurate dating of this material is possible which would place formation of lunar crustal material early in primitive solar system history (nearly simultaneous with meteorite parent-body formation). In the latter case accurate dating is not possible, but the implied re-implantation from a source rich in 244fission xenon still demands that the moon was once primitive enough to contain large amounts of now extinct 244Pu and at the same time cool enough to prevent xenon isotopic mixing.

Isotopic Analysis of Rare Gases from Stepwise Heating of Lunar Fines and Rocks

Highlights of a first effort in sorting out rare gases in lunar material are solar wind rare gases in abundance; variable 20Ne/22Ne but constant 21Ne/ 22Ne ratios in fractions of the trapped neon; cosmogenic rare gases similar to those found in meteorites, except for copious 131Xe in one rock but not in another; at Tranquillity Base a rock 4.1 x 109 years old which reached the surface 35 to 65 million years ago, amid soil whose particles have typically been within a meter of the surface for 109 years or more.

Implanted solar helium, neon, and argon in individual lunar ilmenite grains: Surface effects and a temporal variation in the solar wind composition

Abstract Solar implanted helium, neon, and argon were extracted from individual ilmenite grains from lunar soil 71501 and soil breccia 79035 using laser vaporization and analyzed by static mass spectrometry. Clear differences were observed for these two grain populations, one from a contemporary soil and the other from an ancient soil exposed on the lunar surface approximately 1 Ga ago. The different trends between the 20 Ne 22 Ne ratios and the He Ne ratios do not simply reflect differences either in regolith gardening or in diffusive losses but rather suggest a greater relative helium abundance in the ancient solar wind by a factor of about 1.8. The majority of the grains are enriched in solar energetic particle (SEP) Ne relative to solar wind (SW) Ne in a manner that increases with surface exposure. The progressive enrichment in retained SEP Ne relative to SW Ne is explained by a combination of diffusion and nonfractionating losses of the less deeply implanted SW component. The neon isotopic differences observed in various analyses of ilmenite separates from these two soils and previously attributed to a secular variation of either the SW SEP flux ratio or the SW Ne isotopic composition may alternatively be a natural consequence of greater SW losses which accompany an enhanced helium flux in the ancient solar wind.

Noble gases in the howardites Bholghati and Kapoeta

Analyses of noble gases in whole rock samples of the howardites Bholghati and Kapoeta and grain-size separates of Kapoeta yield evidence for excesses of the Xe isotopes {sup 129}Xe, {sup 131}Xe, {sup 132}Xe, {sup 134}Xe, and {sup 136}Xe in a low-temperature component, similar to lunar excess fission Xe. Such a component may be able to provide chronometric information if the relative abundances of radioactive progenitors ({sup 129}I, {sup 244}Pu, and {sup 238}U) can be determined, but the isotopic spectra we obtain are not sufficiently precise to do so. Eucritic clast BH-5 in Bholghati contains Xe produced in situ by the decay of {sup 244}Pu. Calculated fission Xe retention ages are 30-70 Ma after the formation of the solar system, consistent with the apparent presence of {sup 146}Sm decay products. Both the clast and the matrix of Bholghati have K-Ar ages of about 2 Ga, suggesting a common thermal event at least that recently.

The I-Xe chronometer

Abstract129Xe, from the decay of the now-extinct 16.7 Ma129I, accumulates in iodine-bearing sites and since most iodine host phases are secondary, the I-Xe system is typically a chronometer for post-formational processes. The validity of the I-Xe chronometer is confirmed by comparison with Pb-Pb ages on phosphate and feldspar separates from twelve meteorites. Phosphate separates are found to be concordant with Pb-Pb for all six samples in which useful I-Xe data were obtained. Feldspar is a better iodine host than apatite in H chondrites, typically providing good I-Xe isochrons. These too are concordant with the Pb-Pb ages of the corresponding phosphates for five out of six feldspar separates. The exception is Allegan whose feldspar yields one of the oldest I-Xe ages observed, similar to those for CI and CM magnetites. We attribute this to a more primary mineralization, predating the secondary phosphate from which the comparison Pb-Pb age was obtained. Absolute I-Xe ages, found using the reported Pb-Pb age of Acapulco phosphate provide an absolute I-Xe age of 4.566 ± 0.002 Ga for both Shallowater and Bjurböle irradiation standards. This allows relative I-Xe ages to be interpreted in the context of absolute ages.