Argon‐40‐argon‐39 chronology and petrogenesis along the eastern limb of the Moon from Luna 16, 20 and 24 samples

Abstract— Eighteen new lithic fragments from the Soviet Luna missions have been analyzed with electron microprobe and ⁴⁰Ar‐³⁹Ar methods. Luna 16 basalt fragments have aluminous compositions consistent with previous analyses, but have two distinct sets of well‐constrained ages (3347 ± 24 Ma, 3421 ± 30 Ma). These data, combined with other Luna 16 basalt ages, imply that there were multiple volcanic events filling Mare Fecunditatis. The returned basalt fragments have relatively old cosmicray exposure (CRE) ages and may have been recovered from the ejecta blanket of a young (1 Ga), nearby crater. A suite of highlands rocks (troctolites and gabbros) is represented in the new Luna 20 fragments. One fragment is the most compositionally primitive (Mg# = 91–92) spinel troctolite yet found. Both troctolites have apparent crystallization ages of 4.19 Ga; other rocks in the suite have progressively younger ages and lower Mg#s. The age and composition progression suggests that these rocks may have crystallized from a single source magma, or from similar sources mobilized at the same time. Within the new Luna 24 basalt fragments is a quench‐textured olivine vitrophyre with the most primitive composition yet analyzed for a Luna 24 basalt, and several much more evolved olivine‐bearing basalts. Both new and previously studied Luna 24 very low‐Ti (VLT) basalt fragments have a unimodal age distribution (3273 ± 83 Ma), indicating that most returned samples come from a single extrusive episode within Mare Crisium much later than the Apollo 17 VLT basalts (3.6–3.7 Ga).     

Argon-39-argon-40 “ages” and trapped argon in Martian shergottites,Chassigny, and Allan Hills 84001

Abstract— Argon-isotopic abundances were measured in neutron-irradiated samples of Martian meteorites Chassigny, Allan Hills (ALH) 84001, ALH 77005, Elephant Moraine (EET) 79001, Yamato (Y) 793605, Shergotty, Zagami, and Queen Alexandra Range (QUE) 94201, and in unirradiated samples of ALH 77005. Chassigny gives a ³⁹Ar-⁴⁰Ar age of 1.32 ± 0.07 Ga, which is similar to radiometric ages of the nakhlites. Argon-39-Argon-40 data for ALH 84001 indicate ages between 3.9 and 4.3 Ga. A more precise definition of this age requires detailed characterization of the multiple trapped Ar components in ALH 84001 and of ³⁹Ar recoil distribution. All six shergottite samples show apparent ³⁹Ar-⁴⁰Ar ages substantially older than the ～165–200 Ma range in ages given by other isotope dating techniques. Shergottites appear to contain ubiquitous Ar components acquired from the Martian atmosphere, the Martian mantle, and commonly terrestrial atmospheric contamination. Zagami feldspar also suggests inherited radiogenic ⁴⁰Ar. These data analyses indicate that the recent Martian atmospheric component trapped in shergottites has a ⁴⁰Ar/³⁶Ar ratio possibly as low as ～1750 and no greater than ～1900. These ratios are less than the value of 3000 ± 500 reported by Viking. The ⁴⁰Ar/³⁶Ar ratio for the Martian mantle component is probably <500 but is poorly constrained. The correlation between trapped ⁴⁰Ar/³⁶Ar and ¹²⁹Xe/¹³²Xe ratios in shergottite impact glasses and unirradiated samples of ALH 77005 shows considerable scatter and suggests that the ³⁶Ar/¹³²Xe ratio in the Martian components may vary. Resolution of Martian atmospheric ⁴⁰Ar/³⁶Ar ratio at different time periods (i.e., at ～4.0 and 0.2 Ga) is also difficult without an understanding of the composition of various trapped components.     

Laser argon‐40‐argon‐39 age studies of Dar al Gani 262 lunar meteorite

Abstract— The laser ⁴⁰Ar‐³⁹Ar dating technique has been applied to the Dar al Gani (DaG) 262 lunar meteorite, a polymict highland regolith breccia, to determine the crystallisation age and timing of shock events experienced by this meteorite. Laser stepped‐heating analyses of three dominantly feldspathic fragments (DaG‐1, DaG‐2, and DaG‐3) revealed the presence of trapped Ar, mostly released at intermediate and high temperatures, with an ⁴⁰Ar/³⁶Ar value of ～2.8. Trapped Ar is most likely released from melt glass present as small veins within the fragments. The ⁴⁰Ar‐³⁹Ar ages determined for the three fragments are ～3.0 Ga for DaG‐1 and DaG‐2 and 2.0 Ga for DaG‐3 and probably relate to major impact events. Laser spot analyses were performed on a feldspathic clast, an impact crystalline melt basalt (ICMB), and the matrix in a polished section of DaG 262. The feldspathic and ICMB clasts have low contents of trapped Ar compared with that in the matrix. The feldspathic clast shows a wide range of ages from 3.0 to 1.7 Ga similar to those obtained by stepped heating. The younger age is interpreted as a minimum age for the last major event that assembled this meteorite. The ICMB shows two age clusters at 3.37 and 3.07 Ga, where the older age may be that of the impact event that formed the impact melt. Several cosmic‐ray exposure (CRE) ages were obtained as expected for a polymict regolith breccia. The CRE ages are 106 and 141 Ma for the feldspathic clast and the ICMB, respectively. One of the feldspathic fragments, DaG‐2, shows a range between 200–400 Ma. These CRE ages, which are similar to those determined for returned samples of the lunar regolith, indicate that the different components of DaG 262 experienced preexposure prior to assemblage of the meteorite.     

Earliest high-Ti volcanism on the Moon: 40Ar-39Ar,Sm-Nd,and Rb-Sr isotopic studies of Group D basalts from the Apollo 11 landing site

Abstract High-Ti basalts from the Apollo collections span a range in age from 3.87 Ga to 3.55 Ga. The oldest of these are the common Apollo 11 Group B2 basalts which yield evidence of some of the earliest melting of the lunar mantle beneath Mare Tranquillitatis. Rare Group D high-Ti basalts from Mare Tranquillitatis have been studied in an attempt to confirm a postulated link with Group B2 basalts (Jerde et al., 1994). The initial Sr isotopic ratio of a known Group D basalt (0.69916 ± 3 at 3.85 Ga) lies at the lower end of the tight range for Group B2 basalts (⁸⁷Sr/⁸⁶Sr = 0.69920 to 0.69921). One known Group D basalt and a second postulated Group D basalt yield indistinguishable initial ?_Nd (1.2 ± 0.6 and 1.2 ± 0.3) and again lie at the lower end of the range for the Group B2 basalts from Apollo 11 (+2.0 ± 0.4 to +3.9 ± 0.6, at 3.85 Ga). A third sample has isotopic (⁸⁷Sr/⁸⁶Sr = 0.69932 ± 2; ?_Nd = 2.5 ± 0.4; at 3.59 Ga; as per Snyder et al., 1994b) and elemental characteristics similar to the Group A high-Ti basalts returned from the Apollo 11 landing site. Ages of ⁴⁰Ar-³⁹Ar have been determined for one known Group D basalt and a second postulated Group D basalt using step-heating with a continuous-wave laser. Suspected Group D basalt, 10002, 1006, yielded disturbed age spectra on two separate runs, which was probably due to ³⁹Ar recoil effects. Using the “reduced plateau age” method of Turner et al. (1978), the ages derived from this sample were 3898 ± 19 and 3894 ± 19 Ma. Three separate runs of known Group D basalt 10002, 116 yielded ⁴⁰Ar/³⁹Ar plateau ages of 3798 ± 9 Ma, 3781 ± 8 Ma, and 3805 ± 7 Ma (all errors 2σ). Furthermore, this sample has apparently suffered significant ⁴⁰Ar loss either due to solar heating or due to meteorite impact. The loss of a significant proportion of ⁴⁰Ar at such a time means that the plateau ages underestimate the “true” crystallization age of the sample. Modelling of this Ar loss yields older, “true” ages of 3837 ± 18, 3826 ± 16, and 3836 ± 14 Ma. These ages overlap the ages of Group B2 high-Ti basalts (weighted average age = 3850 ± 20 Ma; range in ages = 3.80 to 3.90 Ga). The combined evidence indicates that the Group D and B2 high-Ti basalts could be coeval and may be genetically related, possibly through increasing degrees of melting of a similar source region in the upper mantle of the Moon that formed >4.2 Ga ago. The Group D basalts were melted from the source first and contained 3–5×more trapped KREEP-like liquid than the later (by possibly only a few million years) Group B2 basalts. Furthermore, the relatively LREE- and Rb-enriched nature of these early magmas may lend credence to the idea that the decay of heat-producing elements enriched in the KREEP-like trapped liquid of upper mantle cumulates, such as K, U, and Th, could have initiated widespread lunar volcanism.     

40Ar‐39Ar chronology of lunar meteorites Northwest Africa 032 and 773

Abstract— The ⁴⁰Ar‐³⁹Ar dating technique has been applied to the lunar meteorites Northwest Africa 032 (NWA 032), an unbrecciated mare basalt, and Northwest Africa 773 (NWA 773), (composed of cumulate and breccia lithologies), to determine the crystallization age and timing of shock events these meteorites may have experienced. Stepped heating analyses of several different samples of NWA 032 gave complex age spectra but indistinguishable total ages with a mean of 2.779 ± 0.014 Gyr. Possible causes of the complex age spectra obtained from NWA 032 include recoil of ³⁹Ar, or the presence of pre‐shock ⁴⁰Ar incorporated into shock‐melt veins. The effects of shock veins were investigated by laser fusion of 20 small samples expected to contain varying proportions of the shock veins. The laser ages show a narrow age distribution between 2.61–2.86 Gyr and a mean of 2.73 ± 0.03 Gyr, identical to the total age of ?2.80 Gyr obtained for the bulk sample. Diffusion calculations based on the stepped heating data indicate that Ar release can be reconciled by release from feldspar (and possibly shock veins) at low temperatures followed by pyroxene at higher temperatures. The exposure age of NWA 032 is 212 ± 11 Myr, and it contains low trapped solar Ar. Stepped heating of cumulate and breccia portions of NWA 773 also give a relatively young age of 2.91 Gyr. The presence of trapped Ar in the breccia makes the age determination of this component less precise, but release of Ar appears to be from the same mineral phase, assumed to be plagioclase, in both lithologies. A marked difference in exposure age between the 2 lithologies also exists, with the breccia having spent 81 Myr longer at the lunar surface; this finding is consistent with the higher trapped Ar content of this lithology. Assuming that 2.80 Gyr and 2.91 Gyr are the crystallization ages of NWA 032 and NWA 773 respectively, these two meteorites are the youngest lunar mare basalts available for study.     

Origin and significance of cosmogenic signatures in vesicles of lunar basalt 15016

Lunar basalt 15016 (~3.3 Ga) is among the most vesicular (50% by volume) basalts recovered by the Apollo missions. We investigated the possible occurrence of indigenous lunar nitrogen and noble gases trapped in vesicles within basalt 15016, by crushing several cm-sized chips. Matrix/mineral gases were also extracted from crush residues by fusion with a CO₂ laser. No magmatic/primordial component could be identified; all isotope compositions, including those of vesicles, pointed to a cosmogenic origin. We found that vesicles contained ~0.2%, ~0.02%, ~0.002%, and ~0.02% of the total amount of cosmogenic ²¹Ne, ³⁸Ar, ⁸³Kr, and ¹²⁶Xe, respectively, produced over the basalt's 300 Myr of exposure. Diffusion/recoil of cosmogenic isotopes from the basaltic matrix/minerals to intergrain joints and vesicles is discussed. The enhanced proportion of cosmogenic Xe isotopes relative to Kr detected in vesicles could be the result of kinetic fractionation, through which preferential retention of Xe isotopes over Kr within vesicles might have occurred during diffusion from the vesicle volume to the outer space through microleaks. This study suggests that cosmogenic loss, known to be significant for ³He and ²¹Ne, and to a lesser extent for ³⁶Ar (Signer et al. 1977 ), also occurs to a negligible extent for the heaviest noble gases Kr and Xe.     

The Chelyabinsk meteorite: Thermal history and variable shock effects recorded by the 40Ar-39Ar system

We studied three lithologies (light and dark chondritic and impact melt rock) differing in shock stage from the LL5 chondrite Chelyabinsk. Using the ⁴⁰Ar-³⁹Ar dating technique, we identified low- and high-temperature reservoirs within all samples, ascribed to K-bearing oligoclase feldspar and shock-induced jadeite–feldspar glass assemblages in melt veins, respectively. Trapped argon components had variable ⁴⁰Ar/³⁶Ar ratios even within low- and high-temperature reservoirs of individual samples. Correcting for trapped argon revealed a lithology-specific response of the K-Ar system to shock metamorphism, thereby defining two distinct impact events affecting the Chelyabinsk parent asteroid (1) an intense impact event ~1.7 ± 0.1 Ga ago formed the light–dark-structured and impact-veined Chelyabinsk breccia. Such a one-stage breccia formation is consistent with petrological observations and was recorded by the strongly shocked lithologies (dark and impact melt) where a significant fraction of oligoclase feldspar was transformed into jadeite and feldspathic glass; and (2) a young reset event ~30 Ma ago particularly affected the light lithology due to its low argon retentivity, while the more retentive shock-induced phases were more resistant against thermal reset. Trapped argon with ⁴⁰Ar/³⁶Ar ratios up to 1900 was likely incorporated during impact-induced events on the parent body, and mixed with terrestrial atmospheric argon contamination. Had it not been identified via isochrons based on high-resolution argon extraction, several geochronologically meaningless ages would have been deduced.     

39Ar‐40Ar chronology of the enstatite chondrite parent bodies

Ar‐Ar isochron ages of EL chondrites suggest closure of the K‐Ar system at 4.49 ± 0.01 Ga for EL5 and 6 chondrites, and 4.45 ± 0.01 Ga for EL3 MAC 88136. The high‐temperature release regimes contain a mixture of radiogenic ⁴⁰Ar* and trapped primordial argon (solar or Q‐type) with ⁴⁰Ar/³⁶Ar_TR ~ 0 , which does not affect the ⁴⁰Ar budget. The low‐temperature extractions show evidence of an excess ⁴⁰Ar component. The ⁴⁰Ar/³⁶Ar is 180–270; it is defined by intercept values of isochron regression. Excess ⁴⁰Ar is only detectable in petrologic types >4/5. These lost most of their primordial ³⁶Ar from low‐temperature phases during metamorphism and retrapped excess ⁴⁰Ar. The origin of this excess ⁴⁰Ar component is probably related to metamorphic Ar mobilization, homogenization of primordial and in situ radiogenic Ar, and trapping of Ar by distinct low‐temperature phases. Ar‐Ar ages of EH chondrites are more variable and show clear evidence of a major impact‐induced partial resetting at about 2.2 Ga ago or alternatively, prolonged metamorphic decomposition of major K carrier phases. EH impact melt LAP 02225 displayed the highest Ar‐Ar isochron age of 4.53 ± 0.01 Ga. This age sets a limit of about 25–45 Ma for the age bias between the K‐Ar and U‐Pb decay systems.     

A nitrogen and argon stable isotope study of Allan Hills 84001: Implications for the evolution of the Martian atmosphere

Abstract— The abundances and isotopic compositions of N and Ar have been measured by stepped combustion of the Allan Hills 84001 (ALH 84001) Martian orthopyroxenite. Material described as shocked is N-poor ([N] ～ 0.34 ppm; δ¹⁵N ～ +23%); although during stepped combustion, ¹⁵N-enriched N (δ¹⁵N ～ +143%) is released in a narrow temperature interval between 700 °C and 800 °C (along with ¹³C-enriched C (δ¹³C ～ +19%) and ⁴⁰Ar). Cosmogenic species are found to be negligible at this temperature; thus, the iso-topically heavy component is identified, in part, as Martian atmospheric gas trapped relatively recently in the history of ALH 84001. The N and Ar data show that ALH 84001 contains species from the Martian lithosphere, a component interpreted as ancient trapped atmosphere (in addition to the modern atmospheric species), and excess ⁴⁰Ar from K decay. Deconvolution of radiogenic ⁴⁰Ar from other Ar components, on the basis of end-member ³⁶Ar/¹⁴N and ⁴⁰Ar/³⁶Ar ratios, has enabled calculation of a K-Ar age for ALH 84001 as 3.5–4.6 Ga, depending on assumed K abundance. If the component believed to be Martian palaeoatmos-phere was introduced to ALH 84001 at the time the K-Ar age was set, then the composition of the atmosphere at this time is constrained to: δ¹⁵N ≥ +200%, ⁴⁰Ar/³⁶Ar ≤ 300 and ³⁶Ar/¹⁴N ≥ 17 × 10^?5. In terms of the petrogenetic history of the meteorite, ALH 84001 crystallised soon after differentiation of the planet, may have been shocked and thermally metamorphosed in an early period of bombardment, and then subjected to a second event. This later process did not reset the K-Ar system but perhaps was responsible for introducing (recent) atmospheric gases into ALH 84001; and it might mark the time at which ALH 84001 suffered fluid alteration resulting in the formation of the plagioclase and carbonate mineral assemblages.     

A noble gas and cosmogenic radionuclide analysis of two ordinary chondrites from Almahata Sitta

Abstract– We present the results of a noble gas (He, Ne, Ar) and cosmogenic radionuclide (¹⁰Be, ²⁶Al, ³⁶Cl) analysis of two chondritic fragments (#A100, L4 and #25, H5) found in the Almahata Sitta strewn field in Sudan. We confirm their earlier attribution to the same fall as the ureilites dominating the strewn field, based on the following findings: (1) both chondrite samples indicate a preatmospheric radius of approximately 300 g cm^?2, consistent with the preatmospheric size of asteroid 2008 TC₃ that produced the Almahata Sitta strewn field; (2) both have, within error, a ²¹Ne/²⁶Al‐based cosmic ray exposure age of approximately 20 Ma, identical to the reported ages of Almahata Sitta ureilites; (3) both exhibit hints of ureilitic Ar in the trapped component. We discuss a possible earlier irradiation phase for the two fragments of approximately 10–20 Ma, visible only in cosmogenic ³⁸Ar. We also discuss the approximately 3.8 Ga (⁴He) and approximately 4.6 Ga (⁴⁰Ar) gas retention ages, measured in both chondritic fragments. These imply that the two chondrite fragments were incorporated into the ureilite host early in solar system evolution, and that the parent asteroid from which 2008 TC₃ is derived has not experienced a large break‐up event in the last 3.8 Ga.     

Nitrogen and argon release profiles in Luna 16 and Luna 24 regolith samples: The effects of regolith reworking

Abstract— Fines, microbreccias and agglutinates from the Luna 16 mature regolith 1635 and fines from the immature/submature Luna 24 regolith have been analysed for N and argon isotopes in order to understand the origin of isotopically distinct N released at different temperatures. All high‐resolution runs reveal a similarity in the release of ³⁶Ar, ⁴⁰Ar and N over a wide temperature interval. The similarity in the ⁴⁰Ar and ³⁶Ar releases and the near coincidence in the 1635 agglutinates implies that the implanted species were redistributed and homogenised during regolith processing such that, regardless of the huge difference in ion implantation energy between solar ³⁶Ar and non‐solar ⁴⁰Ar, their present distribution and their release temperatures are now essentially equal. A small amount of ⁴⁰Ar released in the lower temperature steps with elevated ⁴⁰Ar/³⁶Ar is considered to be trapped after reworking. While such mixing and homogenisation may also be expected for N components of different origins, to date all known stepped runs regularly demonstrate a reproducible variation in δ¹⁵N, suggesting no homogenisation. We consider regolith N to be a mixture of several components trapped at different times, and some nitrogen that was not involved in the reworking. Relatively heavy N released around 500 °C appears to be the most pure form of the component trapped after reworking, probably from accreted meteoritic matter. Middle‐temperature isotopically lighter N appears to be a mixture of solar and non‐solar N largely homogenised, and therefore solar N can not be seen in its pure form. Bulk δ¹⁵N as well as formally deconvoluted δ¹⁵N thermal profiles imply that the non‐solar N has a variable δ¹⁵N value. Several non‐solar N sources are considered with their input resulting in increasing regolith δ¹⁵N with time. Because N from meteorites and interplanetary dust particles appear to be dominant, a mechanism is required to reduce the C/N ratio typical of meteoritic matter to that approaching the low value observed in the lunar regolith. Preferential loss of methane appears to be a viable explanation, following generation either by proton sputtering or in reducing vapour plumes.     

Exposure history of glass and breccia phases of lunar meteorite EET87521

Abstract— Glass-rich separates were prepared from a sample of the basaltic lunar meteorite EET87521 rich in dark glass. Noble gas isotopic abundances and ²⁶Al and ¹⁰Be activities were measured to find out whether shock effects associated with lunar launch helped to assemble these phases. Similar ¹⁰Be and ²⁶Al activities indicate that all materials in EET87521 had a common exposure history in the last few million years before launch. However, the glass contains much higher concentrations of trapped gases and records a much longer cosmic-ray exposure, 100 Ma–150 Ma, in the lunar regolith than does the bulk sample. The different histories show that the glass existed long before the ejection of EET87521. The trapped ⁴⁰Ar/³⁶Ar ratio of 1.6 ± 0.1 implies that the lunar exposure that produced most of the stable cosmogenic noble gases began 500 Ma ago. Cosmogenic and trapped noble gas components correlate strongly in various temperature-release fractions and phases of EET87521, which is probably because the glass contains most of the gas. The trapped solar ratios, ²⁰Ne/²²Ne = 12.68 ± 0.20 and ³⁶Ar/³⁸Ar = 5.24 ± 0.05 can be understood as resulting from a mixture consisting of ～60% solar wind and 40% solar energetic particles (SEP). All EET87521 phases show a ⁴⁰K-⁴⁰Ar gas retention age of ～3300 Ma, which is in the range of typical lunar mare basalts.     

Thermal and irradiation history of lunar meteorite Dhofar 280

Dhofar 280 recorded a complex history on the Moon revealed by high‐resolution ⁴⁰Ar‐³⁹Ar dating. Thermal resetting occurred less than 1 Ga ago, and the rock was exposed to several impact events before and afterwards. The cosmic ray exposure (CRE) age spectrum indicates a 400 ± 40 Ma CRE on the lunar surface. A unique feature of this lunar sample is a partial loss of cosmogenic ³⁸Ar, resulting in a (low‐temperature) CRE age plateau of about 1 Ma. This was likely caused by the same recent impact event that reset the (low‐temperature) ⁴⁰Ar‐³⁹Ar age spectrum and preceded the short transit phase to Earth of ≤1 Ma. Dhofar 280 may be derived from KREEP‐rich lunar frontside terrains, possibly associated with the Copernicus crater or with a recent impact event on the deposits of the South Pole–Aitken basin. Although Dhofar 280 is paired with Dhofar 081, their irradiation and thermal histories on the Moon were different. An important trapped Ar component in Dhofar 280 is “orphan” Ar with a low ⁴⁰Ar/³⁶Ar ratio. It is apparently a mixture of two components, one endmember with ⁴⁰Ar/³⁶Ar = 17.5 ± 0.2 and a second less well‐constrained endmember with ⁴⁰Ar/³⁶Ar ≤10. The presence of two endmembers of trapped Ar, their compositions, and the breccia ages seem to be incompatible with a previously suggested correlation between age or antiquity and the (⁴⁰Ar/³⁶Ar)_trapped ratio (Eugster et al. 2001; Joy et al. 2011a). Alternatively, “orphan” Ar of this impact melt breccia may have an impact origin.     

The antiquity indicator argon‐40/argon‐36 for lunar surface samples calibrated by uranium‐235‐xenon‐136 dating

Abstract— Several solar gas rich lunar soils and breccias have trapped ⁴⁰Ar/³⁶Ar ratios >10, although solar Ar is expected to yield a ratio of <0.01. Radiogenic ⁴⁰Ar produced in the lunar crust from ⁴⁰K decay was outgassed into the lunar atmosphere, ionized, accelerated in the electromagnetic field of the solar wind, and reimplanted into lunar surface material. The ⁴⁰Ar loss rate depends on the decreasing abundance of ⁴⁰K. In order to calibrate the time dependence of the ⁴⁰Ar/³⁶Ar ratio in lunar surface material, the period of reimplantation of lunar atmospheric ions and of solar wind Ar was determined using the ²³⁵U‐¹³⁶Xe dating method that relies on secondary cosmic‐ray neutron‐induced fission of ²³⁵U. We identified the trapped, fissiogenic, and cosmogenic noble gases in lunar breccia 14307 and lunar soils 70001‐8, 70181, 74261, and 75081. Uranium and Th concentrations were determined in the 74261 soil for which we obtain the ²³⁵U‐¹³⁶Xe time of implantation of 3.25^+0.38_‐0.60 Ga ago. On the basis of several cosmogenic noble gas signatures we calculate the duration of this near surface exposure of 393 ± 45 Ma and an average shielding depth below the lunar surface of 73 ± 7 g/cm². A second, recent exposure to solar and cosmic‐ray particles occurred after this soil was excavated from Shorty crater 17.2 ± 1.4 Ma ago. Using a compilation of all lunar data with reliable trapped Ar isotopic ratios and pre‐exposure times we infer a calibration curve of implantation times, based on the trapped⁴⁰ Ar/³⁶Ar ratio. A possible trend for the increase with time of the solar ³He/⁴He and ²⁰Ne/²²Ne ratios of about 12%/Ga and about 2%/Ga, respectively, is also discussed.     

Textural variations and impact history of the Millbillillie eucrite

Abstract— We have investigated 10 new specimens of the Millbillillie eucrite to study its textures and mineral compositions by electron probe microanalyser and scanning electron microscope. Although originally described as having fine-grained texture, the new specimens show diversity of texture. The compositions (Mg/Fe ratios) of the host pigeonites and augite lamellae are homogeneous, respectively, in spite of the textural variation. In addition to their chemical homogeneity, pyroxenes in coarse and fine-grained clasts are partly inverted to orthopyroxene. Chemical zoning of plagioclase during crystal growth is preserved. This eucrite includes areas of granulitic breccias and impact melts. Large scale textures show a subparallel layering suggesting incomplete mixing and deposition of impact melt and lithic fragments. An ³⁹Ar-⁴⁰Ar age determination for a coarse-grained clast indicates a strong degassing event at 3.55 ± 0.02 Ga. We conclude that Millbillillie is among the most equilibrated eucrites produced by thermal annealing after impact brecciation. According to the classification of impact breccias, Millbillillie can be classified as a mixture of granulitic breccias and impact melts. The last significant thermal event is characterized by network-like glassy veins that run through clasts and matrices. Consideration of textural observations and requirements for Ar-degassing suggests that the ³⁹Ar-⁴⁰Ar age could in principle date either the earilier brecciation and annealing event or the event which produced the veins.     

40Ar-39Ar Ages of Types 3 and 4, L and H Chondrites from Antarctica

Abstract— This is a report on ⁴⁰Ar-³⁹Ar studies of 7 low petrographic type L and H chondrites from Antarctica. From petrographic similarities it has been argued that the L3 chondrites ALHA77015, ?77167, ?77249, and ?77260 are pieces from a common fall (McKinley et al., 1981). Our results now confirm this supposition: The four meteorites have identical characteristic Ar-degassing patterns, very similar K, Ca, Cl, and ³⁶Ar_trapped contents, and similar ⁴⁰Ar-³⁹Ar ages of <4 Ga which are rather unusual for ordinary chondrites and might be due to shock. The undulating age patterns could be due to weathering or to ³⁹Ar recoil. The L4 chondrite ALHA77230 shows no age plateau and only a lower limit for the time of a severe degassing, 4.0 Ga, can be given. ALHA77226 and RKPA78002, two H4 chondrites, exhibit reasonably well defined age plateaus at about 4.3 and 4.4 Ga. Two individual chondrules from RKPA78002 have the same age as the whole rock sample.     

Cosmic-ray produced,radiogenic, and solar noble gases in lunar meteorites Queen Alexandra Range 94269 and 94281

Abstract— We measured the noble gas isotopic abundances in lunar meteorite QUE 94269 and in bulk-, glass-, and crystal-phases of lunar meteorite QUE 94281. Our results confirm that QUE 94269 originated from the same meteorite fall as QUE 93069: both specimens yield the same signature of solar-particle irradiation and also the cosmogenic noble gases are in agreement within their uncertainities. Queen Alexandra Range 93069/94269 was exposed to cosmic rays in the lunar regolith for ～1000 Ma, and it trapped 3.5 × 10^?4 cm³STP/g solar ³⁶Ar, the other solar noble gases being present in proportions typical for the solar-particle irradiation. The bulk material of QUE 94281 contains about three times less cosmogenic and trapped noble gases than QUE 93069/94269 and the lunar regolith residence time corresponds to 400 ± 60 Ma. We show that in lunar meteorites the trapped solar ²⁰Ne/²²Ne ratio is correlated with the trapped ratio ⁴⁰Ar/³⁶Ar, that is, trapped ²⁰Ne/²²Ne may also serve as an antiquity indicator. The upper limits of the breccia compaction ages, as derived from the trapped ratio ⁴⁰Ar/³⁶Ar for QUE 93069/94269 and QUE 94281 are ～400 Ma and 800 Ma, respectively. We found very different regolith histories for the glass phase and the crystals separated from QUE 94281. The glass phase contains much less cosmogenic and solar noble gases than the crystals, in contrast to the glasses of lunar meteorite EET 87521, that were enriched in noble gases relative to the crystalline material. The QUE 94281 phases yield a ⁴⁰K-⁴⁰Ar gas retention age of 3770 Ma, which is in the range of that for lunar mare rocks.     

Shergottites Dhofar 019, SaU 005, Shergotty,and Zagami: 40Ar‐39Ar chronology and trapped Martian atmospheric and interior argon

Abstract— We report a high‐resolution ⁴⁰Ar‐³⁹Ar study of mineral separates and whole‐rock samples of olivine‐phyric (Dhofar 019, Sayh al Uhaymir [SaU] 005) and basaltic (Shergotty, Zagami) shergottites. Excess argon is present in all samples. The highest (⁴⁰Ar/³⁶Ar)_trapped ratios are found for argon in pyroxene melt inclusions (?1500), maskelynite (?1200), impact glass (?1800) of Shergotty and impact glass of SaU 005 (?1200). A high (⁴⁰Ar/³⁶Ar)_trapped component‐usually uniquely ascribed to Martian atmosphere‐can also originate from the Martian interior, indicating a heterogeneous Martian mantle composition. As additional explanation of variable high (⁴⁰Ar/³⁶Ar)_trapped ratios in shocked shergottites, we suggest argon implantation from a “transient atmosphere” during impact induced degassing. The best ⁴⁰Ar‐³⁹Ar age estimate for Dhofar 019 is 642 ± 72 Ma (maskelynite). SaU 005 samples are between 700–900 Ma old. Relatively high ⁴⁰Ar‐³⁹Ar ages of melt inclusions within Dhofar 019 (1086 ± 252 Ma) and SaU 005 olivine (885 ± 66 Ma) could date entrapment of a magmatic liquid during early olivine crystallization, or reflect unrecognized excess ⁴⁰Ar components. The youngest ⁴⁰Ar‐³⁹Ar age of Shergotty separates (maskelynite) is ?370 Ma, that of Zagami is ?200 Ma. The ⁴⁰Ar‐³⁹Ar chronology of Dhofar 019 and SaU 005 indicate >1 Ga ages. Apparent ages uncorrected for trapped (e.g., Martian atmosphere, mantle) argon components approach 4.5 Ga, but are not caused by inherited ⁴⁰Ar, because excess ⁴⁰Ar is supported by ³⁶Ar_trapped. Young ages obtained by ⁴⁰Ar‐³⁹Ar and other chronometers argue for primary rather than secondary events. The cosmic ray exposure ages calculated from cosmogenic argon are 15.7 ± 0.7 Ma (Dhofar 019), 1.0–1.6 Ma (SaU 005), 2.1–2.5 Ma (Shergotty) and 2.2–3.0 Ma (Zagami).     

Comparison of cosmic‐ray exposure ages and trapped noble gases in chondrule and matrix samples of ordinary,enstatite, and carbonaceous chondrites

Abstract— We performed a comprehensive study of the He, Ne, and Ar isotopic abundances and of the chemical composition of bulk material and components of the H chondrites Dhajala, Bath, Cullison, Grove Mountains 98004, Nadiabondi, Ogi, and Zag, of the L chondrites Grassland, Northwest Africa 055, Pavlograd, and Ladder Creek, of the E chondrite Indarch, and of the C chondrites Hammadah al Hamra 288, Acfer 059, and Allende. We discuss a procedure and necessary assumptions for the partitioning of measured data into cosmogenic, radiogenic, implanted, and indigenous noble gas components. For stone meteorites, we derive a cosmogenic ratio ²⁰Ne/²²Ne of 0.80 ± 0.03 and a trapped solar ⁴He/³He ratio of 3310 ± 130 using our own and literature data. Chondrules and matrix from nine meteorites were analyzed. Data from Dhajala chondrules suggest that some of these may have experienced precompaction irradiation by cosmic rays. The other chondrules and matrix samples yield consistent cosmic‐ray exposure (CRE) ages within experimental errors. Some CRE ages of some of the investigated meteorites fall into clusters typically observed for the respective meteorite groups. Only Bath's CRE age falls on the 7 Ma double‐peak of H chondrites, while Ogi's fits the 22 Ma peak. The studied chondrules contain trapped ²⁰Ne and ³⁶Ar concentrations in the range of 10^?6–10^?9 cm³ STP/g. In most chondrules, trapped Ar is of type Q (ordinary chondritic Ar), which suggests that this component is indigenous to the chondrule precursor material. The history of the Cullison chondrite is special in several respects: large fractions of both CR‐produced ³He and of radiogenic ⁴He were lost during or after parent body breakup, in the latter case possibly by solar heating at small perihelion distances. Furthermore, one of the matrix samples contains constituents with a regolith history on the parent body before compaction. It also contains trapped Ne with a ²⁰Ne/²²Ne ratio of 15.5 ± 0.5, apparently fractionated solar Ne.     

Constraints on the flux of meteoritic and cometary water on the Moon from volatile element (N–Ar) analyses of single lunar soil grains,Luna 24 core

We report new nitrogen and argon isotope and abundance results for single breccia clasts and agglutinates from four different sections of the Luna 24 drill core in order to re-evaluate the provenance of N trapped in lunar regolith, and to place limits on the flux of planetary material to the Moon’s surface. Single Luna 24 grains with ⁴⁰Ar/³⁶Ar ratios <1 show δ¹⁵N values between ?54.5‰ and +123.3‰ relative to terrestrial atmosphere. Thus, low-antiquity lunar soils record both positive and negative δ¹⁵N signatures, and the secular increase of the δ¹⁵N value previously postulated by Kerridge (Kerridge, J.F. [1975]. Science 188(4184), 162–164. doi:10.1126/science.188.4184.162) is no longer apparent when the Luna and Apollo data are combined. Instead, the N isotope signatures, corrected for cosmogenic ¹⁵N, are consistent with binary mixing between isotopically light solar wind (SW) N and a planetary N component with a δ¹⁵N value of +100‰ to +160‰. The lower δ¹⁵N values of Luna 24 grains compared to Apollo samples reflect a higher relative proportion of solar N, resulting from the higher SW fluence in the region of Mare Crisium compared to the central near side of the Moon. Carbonaceous chondrite-like micro-impactors match well the required isotope characteristics of the non-solar N component trapped in low-antiquity lunar regolith. In contrast, a possible cometary contribution to the non-solar N flux is constrained to be ?3–13%. Based on the mixing ratio of SW to planetary N obtained for recently exposed lunar soils, we estimate the flux of micro-impactors to be (2.2–5.7) × 10³ tons yr^?1 at the surface of the Moon. Our estimate for Luna 24 agrees well with that for young Apollo regolith, indicating that the supply of planetary material does not depend on lunar location. Thus, the continuous influx of water-bearing cosmic dust may have represented an important source of water for the lunar surface over the past ～1 Ga, provided that water removal rates (i.e., by meteorite impacts, photodissociation, and sputtering) do not exceed accumulation rates.