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Temporal relationships among the three most common suites of lunar crustal rocks have been investigated by obtaining new high precision ages on Felsic/Alkali-suite Quartz monzodiorite Clast B from breccia 15405 and Magnesian-suite norite 78235/6/8/55/56 and comparing them to previously dated ferroan anorthosite sample 60025. The weighted average age of 4337.19 ± 0.49 Ma of 15405 Clast B is defined by zircon U-Pb and Pb-Pb ages as well as mineral isochron Sm-Nd and Nd-Nd ages. It is identical to the weighted average age for Apollo 17 norite 78235/6/8/55/56 of 4334.1 ± 3.5 Ma which is defined by Pb-Pb ages measured onmore » baddeleyites in this investigation and less precise Pb-Pb and Sm-Nd ages reported in the literature. Both ages are ∼ 25 Ma younger than the weighted average of Sm-Nd and Pb-Pb ages reported in the literature on ferroan anorthosite 60025 of 4359.3 ± 2.3 Ma. The fact that ages of all three samples are defined by multiple U-Pb, Pb-Pb, Sm-Nd, and ¹⁴²Nd-¹⁴³Nd chronometers provide confidence that they record the igneous crystallization history of the samples and do not represent disturbances or mixing lines with no temporal significance. Here, the extent to which these three ages represent broader scale magmatism is difficult to evaluate. Nevertheless, the age defined for 15405 Clast B, 78235/6/8/55/56, and 60025 are contemporaneous with the peak of ages observed in detrital zircons from the Apollo 12, 14, 15, and 17 landing sites (4340 ± 20 Ma), a Mg-suite Sm-Nd whole rock isochron defined by samples from Apollo 14, 15, 16, and 17 landing sites (4348 ± 25 Ma), and a Ferroan Anorthosite-suite Sm-Nd whole rock isochron defined by samples from the Apollo 15 and 16 landing sites (4354 ± 29 Ma). This implies that Ferroan Anorthosite-suite magmatism is temporally distinct and earlier than magmatism associated with the Mg-suite and the Felsic/Alkali-suite, as predicted by the lunar magma ocean model of lunar differentiation. The short 35 ± 10 Ma interval between primary ferroan anorthosite magmatism and secondary magmatism suggests that the lunar crust formed over a limited period of time. Although heat from decay of long-lived isotopes, large impacts, tidal heating associated with interactions between the Earth and Moon, and density driven overturn of the magma ocean have all been invoked to explain production of ancient secondary crustal magmatism, only tidal heating and cumulate overturn are consistent with the apparent short duration of secondary crustal magmatism and the great depth of crystallization implied for some Mg-suite samples. The initial ε¹⁴³Nd values derived from the 15405 Clast B and 78238 Mg-suite norite isochrons, as well as a Mg-suite whole rock isochron are −0.23 ± 0.11, −0.27 ± 0.74, and −0.25 ± 0.09, respectively. They are identical within uncertainty indicating that Mg-suite and Felsic/Alkali-suite magmas were derived from materials that had the same time averaged Sm/Nd ratios since the formation of the solar system. This, combined with the contemporaneous nature of 15405 Clast B and 78235/6/8/55/56 Mg-suite norite, is consistent with evolution of both samples, and likely both magma suites, from a common source through closed system fractional crystallization or partial melting processes.« less

Trapped xenon isotopes in two Apollo 17 mare basalt fragments are similar to those in primitive meteorites. Xenon would have been effectively excluded from the Moon in the aftermath of its formation in a giant impact. As such, lunar mantle xenon trapped in the mare basalts is best explained as being derived from late accretion occurring before or concurrent with the formation of the lunar crust. Here, the isotopic composition indicates that a combination of comets and carbonaceous chondrites delivered this xenon to the lunar mantle. The inferred mass of accreted cometary ice would have delivered significantly less than amore » part per million of water to the lunar mantle. The inferred mass of accreted carbonaceous chondrites would have supplied at least a half of a part per million of water. The data further indicate that enstatite chondrites are unlikely to have supplied the majority of late accreted mass.« less

The radiogenic isotopic compositions of basaltic Martian meteorites (shergottites) and clinopyroxene/olivine cumulate meteorites (nakhlite/chassignites) are used to define the global evolution of Mars. However, the two main groups of meteorites demonstrate that their sources underwent divergent styles of magmatic evolution. The shergottites portray a planet that differentiated ~4.52 billion years ago via solidification of a magma ocean, producing incompatible element-depleted and -enriched reservoirs that remained isolated until melt production. In contrast, the reservoir from which the nakhlite/chassignites derive may have formed earlier, produced melts that fractionated Sm/Nd and Hf/W differently, was compositionally less variable, and experienced a significantly more complexmore » history following primordial differentiation than the shergottite sources. The disparate histories recorded by these two groups of meteorites elucidate important questions that could be addressed by acquiring additional samples. Obtaining samples that shared the isotopic systematics of the shergottites would provide confidence that extrapolating the primordial differentiation history of Mars from shergottite radiogenic isotope systematics is reasonable. Returned samples from Mars will also constrain the physical locations of the meteorite source regions, providing insights into the general structure of the Martian mantle. In addition, they will help constrain the phases present in the martian mantle during melting and the conditions under which they are stable. Finally, identifying an evolved lithology that satisfies the geochemical and isotopic constraints placed on the incompatible element-enriched endmember observed in the shergottites would define the nature of magmatic evolution on Mars and whether it is more akin to processes on the Earth or the Moon.« less

Here we present a methodology for absolute activity counting of long-lived isotopes based on cryogenic Decay Energy Spectroscopy. A 146Sm source was produced at the TRIUMF Laboratory and then processed and purified at Lawrence Livermore National Laboratory, yielding a pure sample. The source was embedded within a 4π thermal absorber coupled to a magnetic microcalorimeter achieving nearly 100% counting efficiency. Experimental uncertainties were studied and modeled, including thermal coupling of the source to the absorber, pulse pile-up, trigger, and event selection efficiencies. The absolute activity of the pure 146Sm source was measured to better than 1% uncertainty.

Defining the age of the Moon has proven to be an elusive task because it requires reliably dating lunar samples using radiometric isotopic systems that record fractionation of parent and daughter elements during events that are petrologically associated with planet formation. Crystallization of the magma ocean is the only event that unambiguously meets this criterion because it probably occurred within tens of millions of years of Moon formation. There are three dateable crystallization products of the magma ocean: mafic mantle cumulates, felsic crustal cumulates, and late-stage crystallization products known as urKREEP (uniform residuum K, rare earth elements, and P). Althoughmore » ages for these materials in the literature span 200 million years, there is a preponderance of reliable ages around 4.35 billion years recorded in all three lunar rock types. This age is also observed in many secondary crustal rocks, indicating that they were produced contemporaneously (within uncertainty of the ages), possibly during crystallization and overturn of the magma ocean. •The duration of planet formation is key information in understanding the mechanisms by which the terrestrial planets formed. •Ages of the oldest lunar rocks range widely, reflecting either the duration of Moon formation or disturbed ages caused by impact metamorphism. •Ages determined for compositionally distinct crust and mantle materials produced by lunar magma ocean differentiation cluster near 4.35 Gyr. •The repeated occurrence of 4.35 Gyr ages implies that Moon formation occurred late in Solar System history, likely by giant impact into Earth.« less

Significance Understanding the history of volatile species such as water in the Earth–Moon system is a major objective of planetary science. In this work, we use the moderately volatile element Rb, which has a long-lived isotope ( ⁸⁷ Rb) that decays to ⁸⁷ Sr, to show that lunar volatile element depletion was not caused by the Moon-forming impact. The Rb–Sr systematics of lunar rocks mandate that the bodies involved in the impact that formed the Earth–Moon system were depleted in volatile elements relative to the bulk solar system prior to the impact. As such, Earth’s relatively small proportion of watermore » is either primarily indigenous or was added after the Giant Impact from a source that contained essentially no moderately volatile elements.« less

Here, in this study, we present new Ga isotope data from a suite of 28 mare basalts and lunar highland rocks. The δ⁷¹Ga values of these samples range from -0.10 to +0.66‰ (where δ⁷¹Ga is the relative difference between the ⁷¹Ga/⁶⁹Ga ratio of a sample and the Ga-IPGP standard), which is an order of magnitude more heterogeneous than δ⁷¹Ga values in terrestrial magmatic rocks. The cause of this isotopic heterogeneity must be established to estimate the bulk δ⁷¹Ga value of the Moon. In general, low-Ti basalts and ferroan anorthosite suite (FAS) rocks have δ⁷¹Ga values that are lower than high-Timore » basalts and KREEP-rich rocks. The observation that rocks derived from later forming LMO cumulates have higher δ⁷¹Ga values suggests that Ga isotopes are fractionated by processes that operate within the chemically evolving LMO, rather than localized degassing or volatile redistribution. Correlations between indices of plagioclase removal from the LMO (e.g. Eu/Eu*) with Ga isotope ratios suggest that a Δ⁷¹Ga_{plagioclase-melt} of -0.3‰, (where Δ⁷¹Ga_{plagioclase-melt} is the isotopic fractionation associated with crystallization of plagioclase from a melt), could drive the observed isotopic fractionation in high-Ti mare basalts and KREEP-rich rocks. This would be consistent with the observation that FAS rocks have δ⁷¹Ga values that are lower than mare basalts. However, the addition of KREEP-like material into the mare basalt source regions would not contribute enough Ga to perturb the isotopic composition outside of analytical uncertainty. Thus, basalts derived from early formed LMO cumulates such as those from Apollo 15, would preserve light Ga isotopic compositions despite containing modest amounts of urKREEP. We estimate that the δ⁷¹Ga value of the LMO was ~0.14‰ prior to the onset of plagioclase crystallization and extraction. Whether this δ⁷¹Ga value is representative of the initial BSM cannot be ascertained from the current dataset. It remains plausible that the Moon accreted with a heavier Ga isotopic composition than the Earth. Alternatively, the Moon and Earth could have accreted with similar isotopic compositions (BSE = 0.00 ± 0.06‰, Kato et al., 2017) and volatile loss drove the LMO to higher δ⁷¹Ga values prior to formation of the lunar crust.« less

Ages determined for magnesian and ferroan anorthosite crustal rock suites overlap, suggesting they formed contemporaneously about 4.3–4.5 Ga. A notable exception is the Sm-Nd age previously determined on Mg-suite gabbronorite 67667 which is at least 100 Ma younger than the youngest ferroan anorthosite. New chronologic measurements of 67667 presented here yield concordant Sm-Nd and Rb-Sr mineral isochron ages of 4349 ± 31 Ma and 4368 ± 67 Ma, suggesting the sample is older than previous estimates. Furthermore, a whole rock Sm-Nd isochron of Mg-suite rocks from the Apollo 14, 15, 16, and 17 landing sites yields an age of 4348more » ± 25 Ma, indicating that Mg-suite magmatism was widespread and roughly contemporaneous on the lunar nearside. Here, analysis of Sm-Nd internal isochron ages confirms that Mg-suite magmatism was restricted to a period between about 4.33 and 4.35 Ga at the Apollo 14, 15, 16, and 17 landing sites and was synchronous with magmatism at the Apollo 16 site associated with the ferroan anorthosite suite between 4.35 and 4.37 Ga. Magnesian- and ferroan anorthosite suite rocks with ages younger than ~4.33 Ga appear to have experienced slow cooling in the deep lunar interior, so that the ages record when the samples cooled below the closure temperature of the Sm-Nd isotopic system and not the time they crystallized.« less

Uranium ore is mined and milled to produce uranium ore concentrate (UOC), a regulated product of the nuclear fuel cycle. Additionally, diversion of UOC from the fuel cycle into possible weapons production is a key concern in global nonproliferation efforts. As such, the ability to trace the origin of seized nuclear materials is imperative to law enforcement efforts. Although isotopic signatures of UOCs have proven fruitful to pinpoint sample provenance, new isotopic signatures are needed because most existing isotopic signatures are not indicative of the original ore body from which the U is derived. In this work, we developed amore » new method to separate samarium (Sm) from a U-rich sample matrix and report the first Sm isotope compositions of 32 UOCs derived from a variety of worldwide uranium mines. Relative to terrestrial standards, approximately half the UOCs have resolved and anticorrelated ¹⁴⁹Sm-¹⁵⁰Sm isotope compositions, consistent with the capture of thermal neutrons by ¹⁴⁹Sm in the ore body. The UOCs with anomalous Sm isotope compositions tend to derive from older (~>1.5Ga) and higher-grade ore bodies, although other factors, such as the presence of neutron moderators like water, also play a role. Nonetheless, the Sm isotope compositions of UOCs directly reflects the neutron fluence over the history of the original ore body and can be used to discern different geologic conditions associated with that ore body. Overall, this work demonstrates the potential use of Sm isotopes as a novel nuclear forensics signature for origin assessment of UOCs.« less

Renewed interest in gallium isotope systematics has stemmed from the fact that Ga is moderately volatile and is hypothesized to undergo kinetic fractionation during evaporation. In this work, we present the first Ga isotope data from terrestrial volatile depleted samples including a suite of experimentally heated rhyolitic soils, fallout melt glass, and splash-form tektites from the Australasian strewn field (hereafter termed australite tektites). The Ga in these samples is isotopically heavy compared to Ga in terrestrial basalts and estimates for the composition of the bulk silicate Earth (BSE). For each sample suite the isotopic fractionation of Ga scales with themore » degree of Ga depletion, consistent with isotopic fractionation caused by evaporation.« less