D. S. Burnett

A 15N-Poor Isotopic Composition for the Solar System As Shown by Genesis Solar Wind Samples

The solar atmosphere is about 40% enriched in the heavy nitrogen-15 isotope compared with the Sun and Jupiter. The Genesis mission sampled solar wind ions to document the elemental and isotopic compositions of the Sun and, by inference, of the protosolar nebula. Nitrogen was a key target element because the extent and origin of its isotopic variations in solar system materials remain unknown. Isotopic analysis of a Genesis Solar Wind Concentrator target material shows that implanted solar wind nitrogen has a 15N/14N ratio of 2.18 ± 0.02 × 10−3 (that is, ≈40% poorer in 15N relative to terrestrial atmosphere). The 15N/14N ratio of the protosolar nebula was 2.27 ± 0.03 × 10−3, which is the lowest 15N/14N ratio known for solar system objects. This result demonstrates the extreme nitrogen isotopic heterogeneity of the nascent solar system and accounts for the 15N-depleted components observed in solar system reservoirs.

Experimental determination of U and Th partitioning between clinopyroxene and natural and synthetic basaltic liquid

Clinopyroxene-silicate liquid partition coefficients for U and Th have been determined by particle track radiography from 1 atm crystallization experiments at controlled fO_2. Two natural basaltic and one synthetic composition were studied at fO_2 values from the Ni—NiO oxygen buffer to 1 log unit more oxidizing than Fe—FeO (IW+ 1). Over the range of fO_2 values and compositions studied, D_U^(cpx/liq) = 0.0034–0.015,D_(Th)^(cpx/liq) = 0.008–0.036, and D_(Th)/D_U= 3.4–1.1. With decreasing fO_2, D_(Th)/D_U can decrease by up to a factor of 3 for a given composition, primarily from an increase in D_U^(cpx/liq), which we interpret as resulting from an increase in the proportion of tetravalent U in the system with decreasing fO_2. This demonstrates that crystal-liquid U—Th fractionation is fO_2 dependent and that U in terrestrial magmas is not entirely tetravalent. D_(Th)^(cpx/liq) appears to decrease in the two basalts at the lowest fO_2, possibly as a result of changes in composition with fO_2. n nOur data show the sense of U—Th fractionation by clinopyroxene-liquid partitioning is consistent with previous experimental determinations, in that D_(Th)^(cpx)/D_U^(cpx)> 1 in all cases. This indicates that, during partial melting, the liquid will have a Th/U ratio less than the clinopyroxene in the source. The observed ^(238)—U^(230)Th disequilibrium in MORB requires that the partial melt should have a Th/U ratio greater than the bulk source, and, therefore, cannot result from clinopyroxene-liquid partitioning. Further, the magnitudes of the measured partition coefficients are too small to generate significant U—Th fractionation in either direction. Assuming that clinopyroxene contains the bulk of the U and Th in MORB source, our results indicate that ^(238)U—^(230)Th disequilibrium in MORB may not be caused by partial melting at all.

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.

Rb--Sr AGES OF LUNAR ROCKS FROM THE SEA OF TRANQUILLITY.

Abstract 87 Rb- 87 Sr internal isochrons have been determined for six crystalline rocks from the Sea of Tranquillity, and all yield ages within the narrow range3.65±0.06 × 10 9 yr. Differences in initial ( 87 Sr/ 86 Sr) I define at least two main rock groups which coincide with the two groups indicated by alkali content of the rocks. This demonstrates that at least two magnetic reservoirs were involved in the 3.65 × 10 9 yr event on the moon. The soil yields a model Rb-Sr age of 4.6 × 10 9 yr which suggests that it consists of an average over a variety of rock systems of different ages which represent a closed total system 4.6 × 10 9 yr old or the widespread existence of rocks of 4.6 × 10 9 yr age. A single peculiar rock fragment from the soil gives a model age of 4.4 × 10 9 yr. Initial Sr compositions for the lunar rocks and for soil fragments define a narrow range (0.6990–0.6994) whose lower limit is equal to the basaltic achondrite initial Sr. The moon, earth and basaltic achondrites represent planets with Rb/Sr much lower than in the sun. The moon and the basaltic achondrites must have separated from the solar nebula within a time interval of less than 4 × 10 6 yr. From consideration of Sr-Rb relations, if the moon formed by fission from the earth, it must have occurred prior to 4.4 × 10 9 yr ago.

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 Genesis Discovery Mission: Return of Solar Matter to Earth

The Genesis Discovery mission will return samples of solar matter for analysis of isotopic and elemental compositions in terrestrial laboratories. This is accomplished by exposing ultra-pure materials to the solar wind at the LI Lagrangian point and returning the materials to Earth. Solar wind collection will continue until April 2004 with Earth return in Sept. 2004. The general science objectives of Genesis are to (1) to obtain solar isotopic abundances to the level of precision required for the interpretation of planetary science data, (2) to significantly improve knowledge of solar elemental abundances, (3) to measure the composition of the different solar wind regimes, and (4) to provide a reservoir of solar matter to serve the needs of planetary science in the 21st century. The Genesis flight system is a sun-pointed spinner, consisting of a spacecraft deck and a sample return capsule (SRC). The SRC houses a canister which contains the collector materials. The lid of the SRC and a cover to the canister were opened to begin solar wind collection on November 30, 2001. To obtain samples of O and N ions of higher fluence relative to background levels in the target materials, an electrostatic mirror (‘concentrator’) is used which focuses the incoming ions over a diameter of about 20 cm onto a 6 cm diameter set of target materials. Solar wind electron and ion monitors (electrostatic analyzers) determine the solar wind regime present at the spacecraft and control the deployment of separate arrays of collector materials to provide the independent regime samples.

Lunar neutron stratigraphy.

Abstract Variations in the isotopic ratios158Gd/157Gd and150Sm/149Sm in the Apollo 15 deep drill stem show that the neutron fluence is a smoothly varying function of depth with a relatively symmetric peak at a depth of 190 g/cm2. The peak fluence is about 60% greater than the surface value. The observed peak is at a depth comparable to the theoretically calculated peak in the neutron flux. These data may be quantitatively explained by a model in which a blanket of pre-irradiated material was deposited rapidly 450 × 106 yr ago which has remained essentially undisturbed since, i.e., the material was deposited during terrestrial Cambro-Ordovician time. The upper several cm of the drill stem represents soil which has been mixed and transported to the drill site by shallow impacts. Other models quantitatively compatible with the data are those for a regolith continuously accreting for a time as long as 400 × 106 yr and subsequently irradiated for ∼ 500 × 106 yr as an unmixed section. This would allow the base of the drill stem to be as old as 900 × 106 yr. These results show that at this site the turnover time to 2.5 m depth is significantly greater than about 500 my. No positive evidence of ray material from Aristillus or Autolycus was observed in the fluence data. Studies of a variety of cosmic-ray nuclei from deep cores from several sites should permit the determination of detailed depositional histories. If deeper core samples could be obtained, it should be possible to study over a billion years of lunar stratigraphy. However, the determination of absolute stratigraphic ages will require that neutron capture rates be determined in situ by accurate methods.

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.

Actinide crystal-liquid partitioning for clinopyroxene and Ca3(PO4)2

Using fission and alpha track radiography techniques, we have measured partition coefficients (D) for the actinide elements Th, U and Pu between diopsidic clinopyroxene, whitlockite [β-Ca_3 (PO_4)_2] and silicate liquid at 20kbar. Equilibrium partitioning at the crystal-liquid interface is assumed, and corrections for actinide zoning have been applied to the measured D values. Reproducibility for both actinide and minor element D values is carefully examined as a criterion for crystal-liquid interface equilibrium. The data are mostly compatible with interface equilibrium except for experiments at high cooling rates ( ⪸ 30 deg/hr). Partition coefficients for Th/U/Pu of about 0.002/0.002/0.06 are measured for clinopyroxene and 1.2/0.5/3.4 for whitlockite. At an oxygen fugacity of 10^(−8.5), Pu is much more readily incorporated into the crystalline phases than is U or Th because of the importance of trivalent Pu. The D_(Pu)(cpx) is similar to D(cpx) of the light rare earths supporting the concept of Pu/(rare earth) dating.

Neutron capture on149Sm in lunar samples

High precision isotopic composition measurements of Sm have been carried out for two terrestrial and seven lunar samples from three Apollo sites. The lunar samples, selected to show a wide variation in cosmic ray exposure ages, have a wide range of enrichments in150Sm/154Sm (up to 0.8%) and depletions in149Sm/154Sm which are due to neutron capture. The ratio of the number of neutrons captured per atom by149Sm to157Gd is 0.9 and reflects a hardened lunar neutron spectrum. This ratio is in reasonable but not exact agreement with that obtained from the theoretical lunar neutron energy spectrum of Lingenfelter, Canfield and Hampel. The average composition for terrestrial samarium is144Sm :147Sm :148Sm :149Sm :150Sm :152Sm :154Sm = 3.074 : 14.995 : 11.243 : 13.820 : 7.380 : 26.739 : 22.749%.