J. C. Huneke

40Ar-39Ar ages and cosmic ray exposure ages of Apollo 14 samples†

Abstract We have used the 40 Ar- 39 Ar dating technique on eight samples of Apollo 14 rocks (14053, 14310), breccia fragments (14321), and soil fragments (14001, 14167). The large basalt fragments give reasonable 40 Ar/ 39 Ar release patterns and yield well defined crystallization ages between 3.89–3.95 aeons. Correlation of the 40 Ar/ 39 Ar release patterns with 39 Ar/ 37 Ar patterns showed that the low temperature fractions with high radiogenic argon loss came from K-rich phases. A highly shocked sample and fragments included in the breccia yield complex release patterns with a low temperature peak. The total argon age of these fragments is 3.95 AE. Cosmic ray exposure ages on these samples are obtained from the ratio of spallogenic 38 Ar to reactor induced 37 Ar and show a distinct grouping of low exposure ages of ∼ 26 my correlated with Cone crater. Other samples have exposure ages of more than 260 my and identify material with a more complex integrated cosmic age exposure history.

Isotopic and chemical investigations on Angra dos Reis

We report on extensive isotopic studies of Pb, Sr and Xe and on chemical abundance measurements of K, Rb, Sr, Ba, Nd, Sm, U and Th for total meteorite and mineral separates of the Angra dos Reis achondrite. U-Pb, Th-Pb and Pb-Pb ages are concordant at 4.54 AE for the total meteorite and for high-purity whitlockite in Angra dos Reis. This establishes Angra dos Reis as an early planetary differentiate which has not been disturbed for these systems since 4.54 AE ago. Measured ^(87)Sr/^(86)Sr in pyroxene and whitlockite for Angra dos Reis (ADOR) are distinctly below BABI by two parts in 10^4 and only one part in 10^4 above the lowest ^(87)Sr/^(86)Sr (ALL) measured in an Allende inclusion. The difference in ADOR-ALL corresponds to an interval of condensation in the solar nebula of ∼3 m.y. If ^(26)Al was the heat source for the magmatism on the parent planets of Angra dos Reis and the basaltic achondrites (BABI) then the relatively large difference in ^(87)Sr/^(86)Sr, BABI - ALL, must be the result of planetary evolution rather than condensation over ∼10 m.y. Xe isotopic measurements confirm the presence of large amounts of ^(244)Pu-produced fission Xe and show that ^(244)Pu was enriched in the whitlockite relative to the pyroxene by a factor of ∼18. We present chemical element enrichment factors between the whitlockite and the fassaitic pyroxene in Angra dos Reis. The enrichment factors demonstrate close analogy between the rare earth elements and their actinide analogs. The enrichment factor for Pu is intermediate to the enrichment factors of Nd and Sm.

Ages, Irradiation History, and Chemical Composition of Lunar Rocks from the Sea of Tranquillity

The 87Rb-87Sr internal isochrons for five rocks yield an age of 3.65 �0.05 x 109 years which presumably dates the formation of the Sea of Tranquillity. Potassium-argon ages are consistent with this result. The soil has a model age of 4.5 x109 years, which is best regarded as the time of initial differentiation of the lunar crust. A peculiar rock fragment from the soil gave a model age of 4.44 x 109 years. Relative abundances of alkalis do not suggest differential volatilization. The irradiation history of lunar rocks is inferred from isotopic measurements of gadolinium, vanadium, and cosmogenic rare gases. Spallation xenon spectra exhibit a high and variable 131Xe/126Xe ratio. No evidence for 129I was found. The isotopic composition of solar-wind xenon is distinct from that of the atmosphere and of the average for carbonaceous chondrites, but the krypton composition appears similar to average carbonaceous chondrite krypton.

Isotopic composition of xenon and krypton in the lunar soil and in the solar wind

Xe and Kr analyses of three lunar samples, the Murray meteorite and Xe from the terrestrial atmosphere are presented. The isotopic compositions of surface-correlated (solar wind?) and lunar soil spallation xenon and krypton are derived from the lunar soil data alone. The lunar soil spallation Xe is similar to that in lunar rocks and meteorites, but the lunar soil spallation Kr has higher (84Kr/83Kr) and82Kr/83Kr). We have no adequate explanation for this Kr spectrum, although independent evidence for such a component comes from stepwise heating data. The surface-correlated Xe (SUCOR) is distinct from both AVCC and terrestrial Xe. However, SUCOR Xe cannot be directly identified with the solar wind, but may contain an admixture of gases from the lunar atmosphere implanted on the grain surfaces by ion pumping processes. The general fractionation trend in SUCOR Xe relative to the atmosphere presumably reflects the solar wind composition. SUCOR Kr appears to be totally ascribable to the solar wind. Solar wind and terrestrial Kr are related by fractionation, but opposite to that of Xe.

The Blue Angel. I - The mineralogy and petrogenesis of a hibonite inclusion from the Murchison meteorite

A detailed mineralogie, chemical, and petrologic study of the Blue Angel, a relatively large (~1.5 mm) hibonite-containing inclusion from the Murchison meteorite, was performed in an attempt to understand the mechanisms of formation and modification of hibonite-rich inclusions. The Blue Angel inclusion is composed of roughly equal amounts of hibonite and calcite, with minor amounts of spinel, perovskite, diopside, and an Fe-rich silicate. The inclusion can be divided into three roughly concentric zones—a hibonite-rich core, a calcite-rich mantle, and a spinel-rich layered rim. The mineral chemistry and petrography of the Blue Angel are consistent with a three-stage formation history: (1) an early stage of nebular condensation which produced the hibonite, perovskite, and spinel; (2) a moderate temperature stage of aqueous alteration and metamorphism occurring on a small planetary body containing CO_2 and H_2O during which calcite was formed in the inclusion; and (3) the final emplacement of the Blue Angel into its present position in Murchison. The study of the Blue Angel indicates that extensive alteration of Ca-Al-rich inclusions (CAI) may have occurred by aqueous alteration and thermal metamorphism followed by explosive mixing processes on a parent body. Such metamorphic reactions may involve formation and destruction of phases, such as melilite and diopside, which have been previously thought to be primary condensates. The mechanisms proposed for the formation and modification of the Blue Angel help to explain the secondary phases and oxygen isotope anomalies found in many CAI and eliminate the need for invoking kinetically-complicated back-reactions at very low pressures with a cool part of the solar nebula. The contribution of planetary metamorphism in the formation and alteration of CAI must be considered along with nebular processes in order to understand the formation of carbonaceous chondrites.

Outline of a lunar chronology

We present here an outline of lunar chronology and evolution based on analyses of the isotopic parent-daughter systems 87Rb-87Sr, U-Th-206Pb-207Pb-208Pb and 40K-40Ar. An overview of the chronology will first be given, followed by an outline of the observational basis. A more complete discussion of 40K-40Ar results and their interpretation is presented in the paper by G. Turner in this volume. While the body of data on lunar materials is limited, the chronology for lunar evolution appears to be rather well defined. The samples which have been investigated represent mare basalts [returned by the Apollo missions (11, 12, 15 and 17) and by the Soviet Luna 16 mission] and terra rocks, which include non-mare basalts, anorthosites, troctolites and norites but are predominantly comprised of complex breccias [returned by Apollo 12, 14, 16 and 17 and Luna 20]. The mare basalts are associated with the late stage lava flows which covered the mare basins. These flood basalts have been broken up by impact processes but for the most part are associated with the local areas and have not been subject to major transport or metamorphism by impact. The highland rocks predate the mare lava flows but are not clearly associated with a particular magmatic or impact process. They may have been excavated from considerable depths and transported over wide distances. Impact metamorphism is certainly one of the critical stages in their development.

FUN with PANURGE: High mass resolution ion microprobe measurements of Mg in Allende inclusions

The performance characteristics of PANURGE, a modified CAMECA IMS3F ion microprobe, have been studied at a mass resolving power of 5000 for the purpose of determining isotopic ratios at a precision level approaching that of counting statistics using beam switching. The techniques used for this type of measurement are described. Using this approach, the isotopic composition of Mg and Si and the atomic ratio of Al/Mg in minerals from the Allende inclusion WA and the Allende FUN inclusion Cl have been measured with the ion microprobe at high mass resolving power. Enrichments in ^(26)Mg of up to 260%. have been found. Mg and Al/Mg measurements on cogenetic spinel inclusions and host plagioclase crystals yield Mg-Al isochrons in excellent agreement with precise mineral isochrons determined by thermal emission mass spectrometry. The measurements confirm the presence of substantial excess (^(26)Mg in WA ^(26)Mg^*/^(27)Al = 5 X 10^(-5)) and its near absence in Cl (^(26)Mg^*/^(27)Al < 4 X 10^(-6)). In WA plagioclase, data for which ^(27)Al/^(24)Mg = 300 to 1000 define a linear array with ^(26)Mg^*/^(27)Al = 3 X 10^5 and with initial ^(26)Mg/^(24)Mg composition 30%. greater than in high Mg phases. This suggests a metamorphic reequilibration of Mg in Allende plagioclase at least 0.6 my after WA formation. There were no variations in detected ^(26)Mg^*/^(27)Al in WA plagioclase associated with concentration of ^(26)Mg^∗ into isolated clusters. We have confirmed by ion probe measurements that the Mg composition in Allende Cl is highly fractionated and is uniform among pyroxene, melilite, plagioclase, spinel crystals and spinel included in melilite and plagioclase crystals. Likewise, the Si composition is mass fractionated and is the same in pyroxene, melilite and plagioclase.

Argon 40-argon 39 chronology of four calcium-rich achondrites☆

Abstract Results are presented from thermal-release argon 40-argon 39 dating experiments on four calcium-rich achondrites. Pasamonte shows an apparent age of 4.1 AE over 70 per cent of its gas release indicating a degassing event at this age. At higher temperatures, the apparent age for Pasamonte rises to 4.51 AE, defining a lower limit gas retention age which is identical to that of the chondrites St. Severin and Guarena. Petersburg has a high-temperature gas retention age of 4.40 ± 0.03AE, 100 million years younger than St. Severin, in agreement with previously reported Pu-Xe and K-Ar gas retention ages and the absence of Xe 129 from the decay of I 129 . Stannern has a complicated pattern of apparent age with gas release which cannot be accounted for in detail by simple diffusive loss following a well-defined initiation of gas retention. In broad scale, however, the Stannern pattern is relatively flat and confined to the range 3.5–3.9 AE, indicating that this meteorite was strongly degassed in this interval. The nakhlite Lafayette has an exceedingly young gas-retention age in the range 1.4–1.7 AE. These results indicate that as a group the calcium-rich achondrites have experienced a more diverse and extensive thermal history than have the chondritic meteorites.

Xe and Kr analyses of silicate inclusions from iron meteorites

Abstract The Xe and Kr contents of silicate inclusions from the iron meteorites Copiapo, Four Corners, Linwood, Pine River, Weekeroo Station and Woodbine ( Xe 132 = 3–40, Kr 84 = 5–100 × 10 9 atoms/g; Kr 84 Xe 132 = 2 ) are comparable to chondritic values. The isotopic compositions show trapped gas of both chondritic composition (dominant in Pine River) and atmospheric composition (dominant in Linwood). Large spallation effects (Krsp84 = 4 × 109, Xesp126 = 2 × 108 atoms/g) occur in Weekeroo Station and Four Corners. The spallation Xe and Kr spectra in Weekeroo differ from those reported for stone meteorites. A re-analysis of literature data shows that systematic variations also exist among stone meteorite spallation spectra which can be qualitatively understood in terms of target element abundance and shielding variations. Very large neutron capture effects on Br and I occur in several meteorites (Kr80/Kr83/Kr86 = 100/2/3 × 1010 atoms/g in Linwood). The ( Kr 80 Kr 82 ) n ≅ 2·8 ratios from neutron capture on Br for Linwood and Copiapo are distinct from that found in stone meteorites. All samples have pronounced Xe129 excesses (3–50 × 109 atoms/g) which apparently indicate differences in formation times from chondrites of less than about 108 yr; however, the presence of trapped Xe132 in silicates which were enclosed in molten Fe-Ni and cooled slowly prove that they were not entirely outgassed; thus, some of the Xe129 excess may also be trapped. No discernable fission Xe was observed.

Gas retention and cosmic-ray exposure ages of a basalt fragment from Mare Fecunditatis.

Abstract An40Ar-39Ar gas retention age and an38Ar-37Ar cosmic ray exposure age have been determined on a total rock sample of the basalt fragment B-1 returned from Mare Fecunditatis by the Luna 16 mission. This sample shows large low-temperature loss of radiogenic40Ar but defines a reasonably good high-temperature plateau at 3.45 ± 0.04 AE. This is presumed to date a period of igneous activity in Mare Fecunditatis. This activity is later than that at Mare Tranquillitatis but earlier than at Oceanus Procellarum and Mare Imbrium. The cosmic ray exposure age is 475 m.y.