40Ar-39Ar ages of Dellen,Jänisjärvi,and Sääksjärvi impact craters

Abstract— ⁴⁰Ar-³⁹Ar age measurements were made for three whole rock melt samples produced during impact events which formed the Dellen, Jänisjärvi, and Sääksjärvi craters on the Baltic Shield. An age of 109.6 ± 1.0 Ma was obtained for the Dellen sample based on an age spectrum plateau. The age spectrum shows a small (7%) loss of radiogenic ⁴⁰Ar from low temperature fractions. Ages of 698 ± 22 Ma and 560 ± 12 Ma were obtained from isochrons for the Jänisjärvi and Sääksjärvi samples, respectively. Data obtained by laser degassing support the Sääksjärvi result. The presence of excess ⁴⁰Ar is indicated in lower temperature fractions for both samples and is correlated with K concentrations in the Sääksjärvi sample. Models explaining these results may require a change in the local “atmospheric” Ar isotopic composition as cooling of melt rocks proceeded. However, it cannot be excluded that devitrification and/or alteration changed the Ar budget. A crater production rate on the Baltic Shield based on measured ages of 6 craters is (0.3 ± 0.2) · 10^?14 20-km-and-larger craters per km² per year, in satisfactory agreement with previous estimates.     

Ion microprobe uranium‐lead dating of zircons from the Lappajärvi impact crater,western Finland

Abstract— The lake Lappajärvi impact crater lies in Paleoproterozoic Svecofennian metasedimentary rocks, on the western side of the Central Finland granitoid complex (～1.9 Ga). Two conflicting ages have been reported for the meteorite impact: an age of 77.3 ± 0.4 Ma on the basis of Ar‐Ar whole‐rock data from impact melt samples and a paleomagnetic age of 195 Ma. During studies on impact crater indicator minerals at Lappajärvi, zircons with an atypical appearance were found in suevite boulders. These zircons seemed to have been affected by impact shock metamorphism and it was considered that they would be good candidates for ion microprobe U‐Pb dating, allowing a new and independent age estimate for the impact event at Lappajärvi. Four spot analyses on two black‐coated zircons plotted close to the upper intercept end of the concordia curve giving an approximate age of 1.8 Ga for the source rock. Seventeen analyses were done on three dull zircon grains showing patchy impact‐related partial recrystallization. Most of these data fell fairly well on a single discordia line with intercept ages of 73.3 ± 5.3 Ma and 1854 ± 51 Ma. However, five of the data spots near the lower intercept end fell on the younger side of the line. This was interpreted to indicate post‐impact loss of lead. Importantly, the new ion microprobe U‐Pb age of 73.3 ± 5.3 Ma is in a very good agreement with the previously reported Ar‐Ar age.     

A Middle‐Late Triassic 40Ar/39Ar age for the Paasselkä impact structure (SE Finland)

Abstract– ⁴⁰Ar/³⁹Ar dating of recrystallized feldspar glass particles separated from clast‐rich impact melt rocks from the approximately 10 km Paasselkä impact structure (SE Finland) yielded a Middle to Late Triassic (Ladinian‐Karnian) pseudo‐plateau age of 228.7 ± 3.0 (3.4) Ma (2σ). This new age makes Paasselkä the first known Triassic impact structure dated by isotopic methods on the Baltic Shield. The new Paasselkä impact age is, within uncertainty, coeval with isotopic ages recently obtained for the Lake Saint Martin impact structure in Canada, indicating a new Middle to Late Triassic impact crater population on Earth. The comparatively small crater size, however, suggests no relationship between the Paasselkä impact and a postulated extinction event at the Middle/Late Triassic boundary.     

A Late Mesoproterozoic 40Ar/39Ar age for a melt breccia from the Keurusselkä impact structure,Finland

Field investigations in the eroded central uplift of the ≤30 km Keurusselkä impact structure, Finland, revealed a thin, dark melt vein that intersects the autochthonous shatter cone‐bearing target rocks near the homestead of Kirkkoranta, close to the center of the impact structure. The petrographic analysis of quartz in this melt breccia and the wall rock granite indicate weak shock metamorphic overprint not exceeding ~8–10 GPa. The mode of occurrence and composition of the melt breccia suggest its formation as some kind of pseudotachylitic breccia. ⁴⁰Ar/³⁹Ar dating of dark and clast‐poor whole‐rock chips yielded five concordant Late Mesoproterozoic miniplateau ages and one plateau age of 1151 ± 10 Ma [± 11 Ma] (2σ; MSWD = 0.11; P = 0.98), considered here as the statistically most robust age for the rock. The new ⁴⁰Ar/³⁹Ar age is incompatible with ~1.88 Ga Svecofennian tectonism and magmatism in south‐central Finland and probably reflects the Keurusselkä impact, followed by impact‐induced hydrothermal chloritization of the crater basement. In keeping with the crosscutting relationships in the outcrop and the possible influence of postimpact alteration, the Late Mesoproterozoic ⁴⁰Ar/³⁹Ar age of ~1150 Ma should be treated as a minimum age for the impact. The new ⁴⁰Ar/³⁹Ar results are consistent with paleomagnetic results that suggested a similar age for Keurusselkä, which is shown to be one of the oldest impact structures currently known in Europe and worldwide.     

Rock magnetic and paleomagnetic study of the Keurusselkä impact structure,central Finland

Abstract– There are 31 proven impact structures in Fennoscandia—one of the most densely crater‐populated areas of the Earth. The recently discovered Keurusselkä impact structure (62°08′ N, 24°37′ E) is located within the Central Finland Granitoid Complex, which formed 1890–1860 Ma ago during the Svecofennian orogeny. It is a deeply eroded complex crater that yields in situ shatter cones with evidence of shock metamorphism, e.g., planar deformation features in quartz. New petrophysical and rock magnetic results of shocked and unshocked target rocks of various lithologies combined with paleomagnetic studies are presented. The suggested central uplift with shatter cones is characterized by increased magnetization and susceptibility. The presence of magnetite and pyrrhotite was observed as carriers for the remanent magnetization. Four different remanent magnetization directions were isolated: (1) a characteristic Svecofennian target rock component A with a mean direction of D = 334.8°, I = 45.6°, α₉₅ = 14.9° yielding a pole (Plat = 51.1°, Plon = 241.9°, A₉₅ = 15.1°), (2) component B, D = 42.4°, I = 64.1°, α₉₅ = 8.4° yielding a pole (Plat = 61.0°, Plon = 129.1°, A₉₅ = 10.6°), (3) component C (D = 159.5°, I = 65.4°, α₉₅ = 10.7°) yielding a pole (Plat = 21.0°, Plon = 39.3°, A₉₅ = 15.6°), and (4) component E (D = 275.5°, I = 62.0°, α₉₅ = 14.4°) yielding a pole (Plat = 39.7°, Plon = 314.3°, A₉₅ = 19.7°). Components C and E are considered much younger, possibly Neoproterozoic overprints, compared with the components A and B. The pole of component B corresponds with the 1120 Ma pole of Salla diabase dyke and is in agreement with the ⁴⁰Ar/³⁹Ar age of 1140 Ma from a pseudotachylitic breccia vein in a central part of the structure. Therefore, component B could be related to the impact, and thus represent the impact age.     

Argon‐40/argon‐39 age of the El'gygytgyn impact event,Chukotka, Russia

Abstract— Lake El'gygytgyn, Chukotka, Russia, lies in a ～18 km crater of presumably impact origin. The crater is sited in Cretaceous volcanic rocks of the Okhotsk‐Chukotka volcanic belt. Laser ⁴⁰Ar/³⁹Ar dating of impact‐melted volcanic rocks from the rim of Lake El'gygytgyn yields a 10‐sample weighted plateau age of 3.58 ± 0.04 Ma. The Ar step‐heating method was critical in this study in identifying inherited Ar in the samples due to incomplete degassing of the Cretaceous volcanic rocks during impact melting. This age is consistent with, but more precise than, previous K‐Ar and fission‐track ages and indicates an “instantaneous” formation of the crater. This tight age control, in conjunction with the presence of impactites, shocked quartz, and other features, is consistent with an impact origin for the structure and seems to discount internal (volcanogenic) origin models.     

Target rocks,impact glasses,and melt rocks from the Lonar crater,India: Highly siderophile element systematics and Sr‐Nd‐Os isotopic signatures

The Lonar crater is a ~0.57‐Myr‐old impact structure located in the Deccan Traps of the Indian peninsula. It probably represents the best‐preserved impact structure hosted in continental flood basalts, providing unique opportunities to study processes of impact cratering in basaltic targets. Here we present highly siderophile element (HSE) abundances and Sr‐Nd and Os isotope data for target basalts and impactites (impact glasses and impact melt rocks) from the Lonar area. These tools may enable us to better constrain the interplay of a variety of impact‐related processes such as mixing, volatilization, and contamination. Strontium and Nd isotopic compositions of impactites confirm and extend earlier suggestions about the incorporation of ancient basement rocks in Lonar impactites. In the Re‐Os isochron plot, target basalts exhibit considerable scatter around a 65.6 Myr Re‐Os reference isochron, most likely reflecting weathering and/or magma replenishment processes. Most impactites plot at distinctly lower ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios compared to the target rocks and exhibit up to two orders of magnitude higher abundances of Ir, Os, and Ru. Moreover, the impactites show near‐chondritic interelement ratios of HSE. We interpret our results in terms of an addition of up to 0.03% of a chondritc component to most impact glasses and impact melt rocks. The magnitude of the admixture is significantly lower than the earlier reported 12–20 wt% of extraterrestrial component for Lonar impact spherules, reflecting the typical difference in the distribution of projectile component between impact glass spherules and bulk impactites.     

L‐chondrite asteroid breakup tied to Ordovician meteorite shower by multiple isochron 40Ar‐39 Ar dating

Abstract— Radiochronometry of L chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 Myr ago. Fossil meteorites from Sweden indicate a highly increased influx of extraterrestrial matter in the Middle Ordovician ～480 Myr ago. An association with the L‐chondrite parent body event was suggested, but a definite link is precluded by the lack of more precise radiometric ages. Suggested ages range between 450 ± 30 Myr and 520 ± 60 Myr, and can neither convincingly prove a single breakup event, nor constrain the delivery times of meteorites from the asteroid belt to Earth. Here we report the discovery of multiple ⁴⁰Ar‐³⁹Ar isochrons in shocked L chondrites, particularly the regolith breccia Ghubara, that allow the separation of radiogenic argon from multiple excess argon components. This approach, applied to several L chondrites, yields an improved age value that indicates a single asteroid breakup event at 470 ± 6 Myr, fully consistent with a refined age estimate of the Middle Ordovician meteorite shower at 467.3 ± 1.6 Myr (according to A Geologic Time Scale 2004). Our results link these fossil meteorites directly to the L‐chondrite asteroid destruction, rapidly transferred from the asteroid belt. The increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate, which implies severe environmental stress.     

Ar‐Ar and I‐Xe ages and the thermal history of IAB meteorites

Abstract— Studies of several samples of the large Caddo County IAB iron meteorite reveal andesitic material enriched in Si, Na, Al, and Ca, which is essentially unique among meteorites. This material is believed to have formed from a chondritic source by partial melting and to have further segregated by grain coarsening. Such an origin implies extended metamorphism of the IAB parent body. New ³⁹Ar‐⁴⁰Ar ages for silicate from three different Caddo samples are consistent with a common age of 4.50‐4.51 Gyr. Less well‐defined Ar‐Ar degassing ages for inclusions from two other IABs, EET (Elephant Moraine) 83333 and Udei Station, are ?4.32 Gyr, whereas the age for Campo del Cielo varies considerably over about 3.23‐4.56 Gyr. New ¹²⁹I‐¹²⁹Xe ages for Caddo County and EET 83333 are 4557.9 ± 0.1 Myr and 4557–4560 Myr, respectively, relative to an age of 4562.3 Myr for Shallowater. Considering all reported Ar‐Ar degassing ages for IABs and related winonaites, the range is ?4.32‐4.53 Gyr, but several IABs give similar Ar ages of 4.50‐4.52 Gyr. We interpret these older Ar ages to represent cooling after the time of last significant metamorphism on the parent body and the younger ages to represent later ⁴⁰Ar diffusion loss. The older Ar‐Ar ages for IABs are similar to Sm‐Nd and Rb‐Sr isochron ages reported in the literature for Caddo County. Considering the possibility that IAB parent body formation was followed by impact disruption, reassembly, and metamorphism (e.g., Benedix et al. 2000), the Ar‐Ar ages and IAB cooling rates deduced from Ni concentration profiles in IAB metal (Herpfer et al. 1994) are consistent if the time of the postassembly metamorphism was as late as about 4.53 Gyr ago. However, I‐Xe ages reported for some IABs define much older ages of about 4558–4566 Myr, which cannot easily be reconciled with the much younger Ar‐Ar and Sm‐Nd ages. An explanation for the difference in radiometric ages of IABs may reside in combinations of the following: a) I‐Xe ages have very high closure temperatures and were not reset during metamorphism about 4.53 Gyr ago; b) a bias exists in the ⁴⁰K decay constants which makes these Ar‐Ar ages approximately 30 Myr too young; c) the reported Sm‐Nd and Rb‐Sr ages for Caddo are in error by amounts equal to or exceeding their reported 2‐sigma uncertainties; and d) about 30 Myr after the initial heating that produced differentiation of Caddo silicate and mixing of silicate and metal, a mild metamorphism of the IAB parent body reset the Ar‐Ar ages.     

81Kr‐Kr age and multiple cosmic‐ray exposure history of the Vaca Muerta mesosiderite

Abstract– Noble gas isotopic compositions were measured for a eucritic pebble and bulk material of a silicate–metal mixture from the Vaca Muerta mesosiderite as well as pyroxene and plagioclase separated from the eucritic pebble by total melting and stepwise heating methods. Trapped noble gases were degassed completely by a high‐temperature thermal event, probably at the formation of the Vaca Muerta parent body (VMPB). The presence of fissiogenic Xe isotopes from extinct ²⁴⁴Pu in the bulk samples might be a result of rapid cooling from an early high‐temperature metamorphism. High concentrations of cosmogenic noble gases enabled us to determine precise isotopic ratios of cosmogenic Kr and Xe. Spallogenic Ne from Na and unique Ar isotopic compositions were observed. The ⁸¹Kr‐Kr exposure age of 168 ± 8 Myr for the silicate pebble is distinctly longer than the age of 139 ± 8 Myr for the bulk samples. The precursor of the pebble had been irradiated on the surface of the VMPB for more than 60 Myr (first stage irradiation), with subsequent incorporation into bulk materials approximately 4 Gyr ago. The Vaca Muerta meteorite was excavated from the VMPB 140 Myr ago (second stage irradiation). Relative diffusion rates among the cosmogenic Ar, Kr, and Xe based on data obtained by stepwise heating indicate that Kr and Xe can be partially retained in pyroxene and plagioclase under the condition that resets the K‐Ar system. This result supports the presence of fission Xe and of excess concentration of cosmogenic Kr, which could have survived the thermal event approximately 3.8 Gyr ago.     

Establishing a 14.6 ± 0.2 Ma age for the Nördlinger Ries impact (Germany)—A prime example for concordant isotopic ages from various dating materials

Abstract– ⁴⁰Ar/³⁹Ar dating of recrystallized K‐feldspar melt particles separated from partially molten biotite granite in impact melt rocks from the approximately 24 km Nördlinger Ries crater (southern Germany) yielded a plateau age of 14.37 ± 0.30 (0.32) Ma (2σ). This new age for the Nördlinger Ries is the first age obtained from (1) monomineralic melt (2) separated from an impact‐metamorphosed target rock clast within (3) Ries melt rocks and therewith extends the extensive isotopic age data set for this long time studied impact structure. The new age goes very well with the ⁴⁰Ar/³⁹Ar step‐heating and laser probe dating results achieved from mixed‐glass samples (suevite glass and tektites) and is slightly younger than the previously obtained fission track and K/Ar and ages of about 15 Ma, as well as the K/Ar and ⁴⁰Ar/³⁹Ar age data obtained in the early 1990s. Taking all the ⁴⁰Ar/³⁹Ar age data obtained from Ries impact melt lithologies into account (data from the literature and this study), we suggest an age of 14.59 ± 0.20 Ma (2σ) as best value for the Ries impact event.     

40Ar‐39Ar age determinations of lunar basalt meteorites Asuka 881757, Yamato 793169, Miller Range 05035, La Paz Icefield 02205, Northwest Africa 479, and basaltic breccia Elephant Moraine 96008

Abstract— ⁴⁰Ar‐³⁹Ar data are presented for the unbrecciated lunar basaltic meteorites Asuka (A‐) 881757, Yamato (Y‐) 793169, Miller Range (MIL) 05035, LaPaz Icefield (LAP) 02205, Northwest Africa (NWA) 479 (paired with NWA 032), and basaltic fragmental breccia Elephant Moraine (EET) 96008. Stepped heating ⁴⁰Ar‐³⁹Ar analyses of several bulk fragments of related meteorites A‐881757, Y‐793169 and MIL 05035 give crystallization ages of 3.763 ± 0.046 Ga, 3.811 ± 0.098 Ga and 3.845 ± 0.014 Ga, which are comparable with previous age determinations by Sm‐Nd, U‐Pb Th‐Pb, Pb‐Pb, and Rb‐Sr methods. These three meteorites differ in the degree of secondary ⁴⁰Ar loss with Y‐793169 showing relatively high Ar loss probably during an impact event ?200 Ma ago, lower Ar loss in MIL 05035 and no loss in A‐881757. Bulk and impact melt glass‐bearing samples of LAP 02205 gave similar ages (2.985 ± 0.016 Ga and 2.874 ± 0.056 Ga) and are consistent with ages previously determined using other isotope pairs. The basaltic portion of EET 96008 gives an age of 2.650 ± 0.086 Ga which is considered to be the crystallization age of the basalt in this meteorite. The Ar release for fragmental basaltic breccia EET 96008 shows evidence of an impact event at 631 ± 20 Ma. The crystallization age of 2.721 ± 0.040 Ga determined for NWA 479 is indistinguishable from the weighted mean age obtained from three samples of NWA 032 supporting the proposal that these meteorites are paired. The similarity of ⁴⁰Ar‐³⁹Ar ages with ages determined by other isotopic systems for multiple meteorites suggests that the K‐Ar isotopic system is robust for meteorites that have experienced a significant shock event and not a prolonged heating regime.     

39Ar‐40Ar ages of eucrites and thermal history of asteroid 4 Vesta

Abstract— Eucrite meteorites are igneous rocks that derived from a large asteroid, probably 4 Vesta. Past studies have shown that after most eucrites formed, they underwent metamorphism in temperatures up to ≥800°C. Much later, many were brecciated and heated by large impacts into the parent body surface. The less common basaltic, unbrecciated eucrites also formed near the surface but, presumably, escaped later brecciation, while the cumulate eucrites formed at depths where metamorphism may have persisted for a considerable period. To further understand the complex HED parent body thermal history, we determined new ³⁹Ar‐⁴⁰Ar ages for 9 eucrites classified as basaltic but unbrecciated, 6 eucrites classified as cumulate, and several basaltic‐brecciated eucrites. Precise Ar‐Ar ages of 2 cumulate eucrites (Moama and EET 87520) and 4 unbrecciated eucrites give a tight cluster at 4.48 ± 0.02 Gyr (not including any uncertainties in the flux monitor age). Ar‐Ar ages of 6 additional unbrecciated eucrites are consistent with this age within their relatively larger age uncertainties. By contrast, available literature data on Pb‐Pb isochron ages of 4 cumulate eucrites and 1 unbrecciated eucrite vary over 4.4–4.515 Gyr, and ¹⁴⁷Sm‐¹⁴³Nd isochron ages of 4 cumulate and 3 unbrecciated eucrites vary over 4.41–4.55 Gyr. Similar Ar‐Ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites, as was previously proposed. We suggest that these cumulate and unbrecciated eucrites resided at a depth where parent body temperatures were sufficiently high to cause the K‐Ar and some other chronometers to remain as open diffusion systems. From the strong clustering of Ar‐Ar ages at ?4.48 Gyr, we propose that these meteorites were excavated from depth in a single large impact event ?4.48 Gyr ago, which quickly cooled the samples and started the K‐Ar chronometer. A large (?460 km) crater postulated to exist on Vesta may be the source of these eucrites and of many smaller asteroids thought to be spectrally or physically associated with Vesta. Some Pb‐Pb and Sm‐Nd ages of cumulate and unbrecciated eucrites are consistent with the Ar‐Ar age of 4.48 Gyr, and the few older Pb‐Pb and Sm‐Nd ages may reflect an isotopic closure before the large cratering event. One cumulate eucrite gives an Ar‐Ar age of 4.25 Gyr; 3 additional cumulate eucrites give Ar‐Ar ages of 3.4–3.7 Gyr; and 2 unbrecciated eucrites give Ar‐Ar ages of ?3.55 Gyr. We attribute these younger ages to a later impact heating. Furthermore, the Ar‐Ar impact‐reset ages of several brecciated eucrites and eucritic clasts in howardites fall within the range of 3.5–4.1 Gyr. Among these, Piplia Kalan, the first eucrite to show evidence for extinct ²⁶Al, was strongly impact heated ?3.5 Gyr ago. When these data are combined with eucrite Ar‐Ar ages in the literature, they confirm that several large impact heating events occurred on Vesta between ?4.1–3.4 Gyr ago. The onset of major impact heating may have occurred at similar times for both Vesta and the moon, but impact heating appears to have persisted for a somewhat later time on Vesta.     

The Suavjärvi impact structure,NW Russia

Abstract– We present the geology and interpreted shock features of the Suavjärvi circular structure. Suavjärvi is a circular feature (illustrated by satellite imagery, topography, and magnetic data) located in the central part of the Karelian Craton (lat. 63°07′N, long. 33°23′E). To date, little information on the geologic and impact features of the Suavjärvi structure is available in the literature. The structure is characterized by gravity and magnetic lows and disruption of the regional magnetic fabric. In the northeastern and southwestern parts of the structure, several erosional remnants of highly disturbed rocks occur referred to as monomict and polymict megabreccia. These comprise blocks of both basement granitoids and supracrustal greenstone rocks. The impact origin of polymict megabreccia and therefore of the Suavjärvi structure is confirmed by observations of closely spaced planar microstructures at angles consistent with planes that have Miller indices indicative of impact shock effects, mostly of ω{10¯13}. The Suavjärvi is considered to be a remnant of a deeply eroded and metamorphosed impact structure, which has a diameter of 16 km and was formed during the Paleoproterozoic (older than 2.2 Ga); this is inferred from the age of the overlying volcanic‐sedimentary Jatulian sequence. Suavjärvi underwent regional metamorphism that resulted in obliteration or transformation of shock metamorphic effects. Massive sulfides occur within megabreccia; originating probably from postimpact redeposition of pre‐existing mineralization.     

Shock metamorphic features in quartz grains from the Saarijärvi and Söderfjärden impact structures,Finland

Shock metamorphic features at the Saarijärvi (D > 2 km) and Söderfjärden (D = 6.5 km) structures in Finland have so far only been studied tentatively, although both are considered to be proven impact structures. This work presents the first detailed universal stage study of planar deformation features (PDFs), feather feature lamellae (FFL), and planar fractures (PFs) in quartz grains from a polymict impact breccia dike from Söderfjärden, and from sedimentary crater‐fill rocks from Saarijärvi. Planar microstructures, particularly PDFs, are very rare and poorly developed or preserved in Saarijärvi, whereas in Söderfjärden they are much more common and well defined. Miller–Bravais indices of the planar microstructures in both Saarijärvi and Söderfjärden are indicative of relatively low‐shock pressure but high shear conditions, only compatible with an impact origin for these structures. Although a Proterozoic age for Saarijärvi cannot be ruled out, the observations of shock features throughout the sedimentary crater‐fill sequence and a brecciated sedimentary dike below the crater floor are more consistent with a Lower Cambrian (or younger) impact age.     

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.     

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).     

Kinetics of organic matter degradation in the Murchison meteorite for the evaluation of parent‐body temperature history

Abstract– To evaluate kinetic parameters for thermal degradation of organic matter, in situ heating experiments of insoluble organic matter (IOM) and bulk of Murchison (CM2) meteorite were conducted under Fourier transform infrared micro‐spectroscopy combined with a heating stage. Decreases of aliphatic C–H band area under Ar flow were well fitted with Ginstling‐Brounshtein three‐dimensional diffusion model, and the rate constants for decreases of aliphatic C–H were determined. Activation energies E_a and frequency factors A obtained from these rate constants at different temperatures using the Arrhenius equation were E_a = 109 ± 3 kJ mol^?1 and A = 8.7 × 10⁴ s^?1 for IOM, and E_a = 61 ± 6 kJ mol^?1 and A = 3.8 s^?1 for bulk, respectively. Activation energy values of aliphatic C–H decrease are larger for IOM than bulk. Hence, the mineral assemblage of the Murchison meteorite might have catalytic effects for the organic matter degradation. Using obtained kinetic expressions, the time scale for metamorphism can be estimated for a given temperature with aliphatic C–H band area, or the temperature of metamorphism can be estimated for a given time scale. For example, using the obtained kinetic parameters of IOM, aliphatic C–H is lost approximately within 200 years at 100 °C and 100 Myr at 0 °C. Assuming alteration period of 7.5 Myr, alteration temperatures could be calculated to be <15 ± 12 °C. Aliphatic C–H decrease profiles in a parent body can be estimated using time–temperature history model. The kinetic expression obtained by the infrared spectral band of aliphatic C–H could be used as an alternative method to evaluate thermal processes of organic matter in carbonaceous chondrites.     

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.     

Agate as an indicator of impact structures: An example from Sääksjärvi,Finland

Abstract— Mineralogical, petrographical and chemical determinations were made for 743 agate (banded variety of chalcedonic quartz) nodules (diameters from 5 mm to 5 cm) formed during postimpact, low-temperature hydrothermal activity as vesicle fillings in the melt rocks of the Sääksjärvi meteorite impact structure (diameter 5 km) in southwest Finland. Other hydrothermal vesicle fillings in the impact melt rocks include chlorite, mordenite, smectite and kaolinite. The agates were classified into two types, whose mineralogical properties and chemical compositions fall within the range of volcanic agates (basaltic and rhyolitic host rocks). The relatively high age (～510 Ma) of the Sääksjärvi impact melt rocks, however, is reflected by the presence of recrystallization textures, which are rare in younger volcanic agates. The Sääksjärvi structure was initially located after following the fortuitous discovery of agate “path-finders” in the glacial overburden. It is recommended that wherever volcanic type agates are found as float in Precambrian shield areas devoid of younger volcanic rocks, the possible presence of impact (or volcanic) craters in the vicinity should be considered.