Stellar age and mass determinations from kinematic data

Assuming that some exchange of energy between stars in the galactic disk may be at work, I have generalized the formulae expressing the growth of the velocity variance with time by adding a mass term (M/M )^– with =0.35, which is a good fit to the kinematic data for young stars. The generalized formulae (5a) and (5b) and the evolutionary mass-age relation (Iben, 1967) were used jointly to derive mass and age values for different stellar species. The results, presented in Table IV, are followed by a discussion.     

Halogens in tektites and impact glasses

Abstract— Impact glasses, tektites and some related basement rocks were analyzed for F, Cl, Br and I. The tektite and impact glasses show similar abundance patterns within the groups. Muong Nong-type tektites indicate that the halogens have been depleted in the order I > Br > Cl > F in their melt under oxidizing conditions. For Darwin Glass selective volatilization of F from the melt is a major depleting process. Cl, Br and I are lost to a lesser extent.     

Shocked rocks and impact glasses from the El'gygytgyn impact structure,Russia

Abstract— The El'gygytgyn impact structure is about 18 km in diameter and is located in the central part of Chukotka, arctic Russia. The crater was formed in volcanic rock strata of Cretaceous age, which include lava and tuffs of rhyolites, dacites, and andesites. A mid‐Pliocene age of the crater was previously determined by fission track (3.45 ± 0.15 Ma) and ⁴⁰Ar/³⁹Ar dating (3.58 ± 0.04 Ma). The ejecta layer around the crater is completely eroded. Shock‐metamorphosed volcanic rocks, impact melt rocks, and bomb‐shaped impact glasses occur in lacustrine terraces but have been redeposited after the impact event. Clasts of volcanic rocks, which range in composition from rhyolite to dacite, represent all stages of shock metamorphism, including selective melting and formation of homogeneous impact melt. Four stages of shocked volcanic rocks were identified: stage I (≤35 GPa; lava and tuff contain weakly to strongly shocked quartz and feldspar clasts with abundant PFs and PDFs; coesite and stishovite occur as well), stage II (35–45 GPa; quartz and feldspar are converted to diaplectic glass; coesite but no stishovite), stage III (45–55 GPa; partly melted volcanic rocks; common diaplectic quartz glass; feldspar is melted), and stage IV (>55 GPa; melt rocks and glasses). Two main types of impact melt rocks occur in the crater: 1) impact melt rocks and impact melt breccias (containing abundant fragments of shocked volcanic rocks) that were probably derived from (now eroded) impact melt flows on the crater walls, and 2) aerodynamically shaped impact melt glass “bombs” composed of homogeneous glass. The composition of the glasses is almost identical to that of rhyolites from the uppermost part of the target. Cobalt, Ni, and Ir abundances in the impact glasses and melt rocks are not or only slightly enriched compared to the volcanic target rocks; only the Cr abundances show a distinct enrichment, which points toward an achondritic projectile. However, the present data do not allow one to unambiguously identify a meteoritic component in the El'gygytgyn impact melt rocks.     

Laser argon-40-argon-39 age determinations of Luna 24 mare basalts

Abstract— The ages of seven rock fragments from the soil fraction of the Luna 24 core have been determined using a laser ⁴⁰Ar-³⁹Ar stepped heating technique. The investigated lithologies include fragments of fine-grained ophitic basalt, coarse-grained basalt, metabasalts and a regolith breccia. Most of the samples contain nonradiogenic Ar components of variable ³⁶Ar/⁴⁰Ar composition. These surface-correlated trapped components are predominantly released at low temperature and can be distinguished from volume-correlated radiogenic and cosmogenic components released at higher temperature during stepped heating. Binary mixtures of radiogenic and cosmogenic Ar components give linear correlations on ³⁶Ar_t/⁴⁰Ar-³⁹Ar/⁴⁰Ar diagrams from which the age and ³⁶Ar/⁴⁰Ar value of trapped Ar can be determined. The ages obtained span a narrow range between 3.18-3.28 Ga with an average of 3.22 ± 0.04 Ga. This is interpreted as being the age of the basalts at the Luna 24 sampling site. Systematic age differences between lithologies were not detected; however, a single age of 2.93 Ga obtained from a coarse-grained basalt hints at the possibility of younger volcanism. The results of this work effectively triple the chronological information available for Mare Crisium and are within the range of radiometric age measurements of Luna 24 mare basalts obtained previously.     

Water in tektites and impact glasses by fourier-transformed infrared spectrometry

Abstract— To improve the scarce data base of H₂O content in tektites and impact glasses, we analyzed 26 tektites from all four strewn fields and 25 impact glass samples for their H₂O content. We used the fourier-transformed infrared (FTIR) spectrometry method, which permits measurement of areas of ～40 μm in diameter. Our results show that the tektites have H₂O contents ranging from 0.002 to 0.030 wt% (average 0.014 ± 0.008 wt%). Ivory Coast tektites have the lowest H₂O abundances (0.002–0.003 wt%), and Muong Nong-type indochinites and some North American tektites having the highest contents (up to ～0.03 wt%). Impact glass samples (from the Zhamanshin, Aouelloul, and Rio Cuarto craters) yielded H₂O contents of 0.008 to 0.13 wt% H₂O. Typical impact glasses from the Aouelloul and Zhamanshin craters have low H₂O contents (0.008 to 0.063 wt%). Libyan Desert Glasses and Rio Cuarto glasses have higher H₂O contents (～0.11 wt%). We also analyzed glasses of unknown origin (e.g., urengoites; glass fragments from Tikal), which showed very low H₂O contents, in agreement with an origin by impact. Our data confirm that all tektites found on land have very low H₂O contents (<0.03 wt% H₂O), while impact glasses have slightly higher H₂O contents. Both glass types are very dry compared to volcanic glasses. This study confirms that the low H₂O contents (<0.05 wt%) of such glasses can be considered good evidence for an origin by impact.     

Infrared,Raman, and cathodoluminescence studies of impact glasses

Abstract— We studied the infrared reflectance (IR), Raman, and cathodoluminescence (CL) spectroscopic signatures and scanning electron microscope‐cathodoluminescence (SEM‐CL) images of three different types of impact glasses: Aouelloul impact glass, a Muong Nong‐type tektite, and Libyan desert glass. Both backscattered electron (BSE) and CL images of the Muong Nong‐type tektite are featureless; the BSE image of the Libyan desert glass shows only weak brightness contrasts. For the Aouelloul glass, both BSE and CL images show distinct brightness contrast, and the CL images for the Libyan desert glass show spectacular flow textures that are not visible in any other microscopic method. Compositional data show that the SiO₂ composition is relatively higher and the Al₂O₃ content is lower in the CL‐bright areas than in the CL‐dark regions. The different appearance of the three glass types in the CL images indicates different peak temperatures during glass formation: the tektite was subjected to the highest temperature, and the Aouelloul impact glass experienced a relatively low formation temperature, while the Libyan desert glass preserves a flow texture that is only visible in the CL images, indicating a medium temperature. All IR reflectance spectra show a major band at around 1040 to 1110 cm^?1 (antisymmetric stretching of SiO₄ tetrahedra), with minor peaks between 745 and 769 cm^?1 (Si‐O‐Si angle deformation). Broad bands at 491 and 821 cm^?1 in the Raman spectra in all samples are most likely related to diaplectic glass remnants, indicating early shock amorphization followed by thermal amorphization. The combination of these spectroscopic methods allows us to deduce information about the peak formation temperature of the glass, and the CL images, in particular, show glass flow textures that are not preserved in other more conventional petrographic images.     

A Paleozoic age for the Tunnunik impact structure

We report paleomagnetic directions from the target rocks of the Tunnunik impact structure, as well as from lithic impact breccia dikes that formed during the impact event. The target sedimentary rocks have been remagnetized after impact‐related tilting during a reverse polarity interval. Their magnetization is unblocked up to 350 °C. The diabase dikes intruding into these sediments retained their original magnetization which unblocks above 400 °C. The impact breccia records a paleomagnetic direction similar to that of the overprints in the target sedimentary rocks. The comparison of the resulting virtual geomagnetic pole for the Tunnunik impact structure with the apparent polar wander path for Laurentia combined with biostratigraphic constraints from the target sedimentary rocks is most consistent with an impact age in the Late Ordovician or Silurian, around 430–450 Ma, soon after the deposition of the youngest impacted sedimentary rocks. Our results from the overprinted sedimentary rocks and diabase dikes imply that the postimpact temperature of the studied rocks was about 350 °C.     

New impact‐melt rock from the Roter Kamm impact structure,Namibia: Further constraints on impact age,melt rock chemistry,and projectile composition

Abstract— A new locality of in situ massive impact‐melt rock was discovered on the south‐southwestern rim of the Roter Kamm impact structure. While the sub‐samples from this new locality are relatively homogeneous at the hand specimen scale, and despite being from a nearby location, they do not have the same composition of the only previously analyzed impact‐melt rock sample from Roter Kamm. Both Roter Kamm impact‐melt rock samples analyzed to date, as well as several suevite samples, exhibit a granitic‐granodioritic precursor composition. Micro‐chemical analyses of glassy matrix and Al‐rich orthopyroxene microphenocrysts demonstrate rapid cooling and chemical disequilibrium at small scales. Platinum‐group element abundances and ratios indicate an ordinary chondritic composition for the Roter Kamm impactor. Laser argon dating of two sub‐samples did not reproduce the previously obtained age of 3.7 ± 0.3 (1s?) for this impact event, based on ⁴⁰Ar/³⁹Ar dating of a single vesicular impact‐melt rock. Instead, we obtained ages between 3.9 and 6.3 Ma, with an inverse isochron age of 4.7 ± 0.3 Ma for one analyzed sub‐sample and 5.1 ± 0.4 Ma for the other. Clearly a post‐5 Ma impact at Roter Kamm remains indicated, but further analytical work is required to better constrain the currently best estimate of 4–5 Ma. Both impactor and age constraints are clearly obstructed by the inherent microscopic heterogeneity and disequilibrium melting and cooling processes demonstrated in the present study.     

Fluorine and boron geochemistry of tektites,impact glasses,and target rocks

Abstract— We have analyzed fluorine and boron in nine tektites from all four strewn fields, and in a suite of impact glasses and target rocks from the Zhamanshin and Darwin impact craters, as well as Libyan Desert Glass and Aouelloul impact glass samples. Fluorine and boron are useful indicators for the volatilization and temperature history of tektites and impact glasses. Tektites from different strewn fields show a limited range of F and B contents and have F/B ratios near unity. Most splash-form tektites have lower average F and B contents than Muong Nong type tektites, which is similar to the relation between irghizites and zhamanshinites. The F and B contents in target rocks from the Zhamanshin and Darwin impact craters are similar to normal terrestrial sediments. Fluorine in impact glasses and tektites is more depleted compared to their (known or inferred) target rocks than is boron, which is caused by the higher volatility of F. The F/B ratios therefore decrease with increasing temperature of formation (suggesting that irghizites were formed at a higher temperature than zhamanshinites, and Muong Nong type tektites at a lower temperature than splash-form tektites). Mixing of local country rocks together with partial loss of the volatiles F and B can reproduce the F and B contents of impact glasses.     

The age of the Kara impact structure,Russia

Abstract— For the ～65 km sized Kara impact structure close to the polar Ural, we report an age of 70.3 ± 2.2 Ma (2s?), defined by the mean of ⁴⁰Ar-³⁹Ar plateau ages for three glassy or crystalline impact melt rocks cleaned from mineral and rock clasts. The fine structure of the age spectra of these samples can quantitatively be simulated by modeling taking into account ³⁹Ar recoil effects, without assuming the presence of excess Ar. The calculations corroborate our age results by showing that ³⁹Ar recoil does not affect the plateau fractions. Previously, Kara has been proposed as a probable K/T impact site or was related to the Campanian-Maastrichtian boundary at 73 Ma. At the 2s? level, both suggestions are ruled out by the well-constrained age for the Kara impact structure.     

Target rocks,impact glasses,and melt rocks from the Lonar impact crater,India: Petrography and geochemistry

Abstract— The Lonar crater, India, is the only well‐preserved simple crater on Earth in continental flood basalts; it is excavated in the Deccan trap basalts of Cretaceous‐Tertiary age. A representative set of target basalts, including the basalt flows excavated by the crater, and a variety of impact breccias and impact glasses, were analyzed for their major and trace element compositions. Impact glasses and breccias were found inside and outside the crater rim in a variety of morphological forms and shapes. Comparable geochemical patterns of immobile elements (e.g., REEs) for glass, melt rock and basalt indicates minimal fractionation between the target rocks and the impactites. We found only little indication of post‐impact hydrothermal alteration in terms of volatile trace element changes. No clear indication of an extraterrestrial component was found in any of our breccias and impact glasses, indicating either a low level of contamination, or a non‐chondritic or otherwise iridium‐poor impactor.     

Spectral properties of simulated impact glasses produced from martian soil analogue JSC Mars-1

To simulate the formation of impact glasses on Mars, an analogue of martian bright soil (altered volcanic soil JSC Mars-1) was melted at relevant oxygen fugacities using a pulsed laser and a resistance furnace. Reduction of Fe³⁺ to Fe²⁺ and in some cases formation of nanophase Fe⁰ in the glasses were documented by Mössbauer spectroscopy and TEM studies. Reflectance spectra for several size fractions of the JSC Mars-1 sample and the glasses were acquired between 0.3 and 25 μm. The glasses produced from the JSC Mars-1 soil show significant spectral variability depending on the method of production and the cooling rate. In general, they are dark and less red in the visible compared to the original JSC Mars-1 soil. Their spectra do not have absorption bands due to bound water and structural OH, have positive spectral slopes in the near-infrared range, and show two broad bands centered near 1.05 and 1.9 μm, typical of glasses rich in ferrous iron. The latter bands and low albedo partly mimic the spectral properties of martian dark regions, and may easily be confused with mafic materials containing olivine and low-Ca pyroxene. Due to their disordered structures and vesicular textures, the glasses show relatively weak absorption features from the visible to the thermal infrared. These weak absorption bands may be masked by the stronger bands of mafic minerals. Positive near-infrared spectral slopes typical of fresh iron-bearing impact or volcanic glasses may be masked either by oxide/dust coatings or by aerosols in the Mars' atmosphere. As a result, impact glasses may be present on the surface of Mars in significant quantities that have been either misidentified as other phases or masked by phases with stronger infrared features. Spectrometers with sufficient spatial resolution and wavelength coverage may detect impact glasses at certain locations, e.g., in the vicinity of fresh impact craters. Such dark materials are usually interpreted as accumulations of mafic volcanic sand, but the possibility of an impact melt origin of such materials also should be considered. In addition, our data suggest that high contents of feldspars or zeolites are not necessary to produce the transparency feature at 12.1 μm typical of martian dust spectra.     

The age of the Saltpan impact crater,South Africa

Abstract— The 1.13-km-diameter Pretoria Saltpan impact crater is located about 40 km NNW of Pretoria, South Africa. The crater is situated in 2.05 Ga old Nebo granite of the Bushveld Complex that is locally intruded by about 1.3 Ga old volcanic rocks. In 1988, a borehole was drilled in the center of the crater. At depths >90 m, breccias were found that contained minerals with characteristic shock-metamorphic features, thus confirming the impact origin of the crater. Fragments of impact glass were recovered from the melt breccias and several hundred sub-millimeter-sized glass fragments were subjected to fission track analysis. The measurements were complicated by the inhomogeneous composition of the impact glasses, but analysis of a large number of tracks yielded an age of 220 ± 52 ka for the Saltpan crater.     

Re‐evaluating the age of the Haughton impact event

Abstract— We have re‐evaluated the published age information for the Haughton impact structure, which was believed to have formed ?23 Ma ago during the Miocene age, and report new Ar/Ar laser probe data from shocked basement clasts. This reveals an Eocene age, which is at odds with the published Miocene stratigraphic, apatite fission track and Ar/Ar data; we discuss our new data within this context. We have found that the age of the Haughton impact structure is ?39 Ma, which has implications for both crater recolonization models and post‐impact hydrothermal activity. Future work on the relationship between flora and fauna within the crater, and others at high latitude, may resolve this paradox.     

A late Triassic age for the Rochechouart impact structure,France

Abstract— Argon-40/Argon-39 laser spot fusion dating of pseudotachylyte from the ～25 km diameter Rochechouart impact structure of western-central France yields a matrix age of 214 ± 8 Ma (2s?). Field evidence indicates that the pseudotachylyte was generated during the modification stage of the impact process, probably during transient cavity collapse. This new age is considerably older than the previously accepted age of 186 ± 8 Ma for this structure, which was obtained from hydrothermally-altered melt sheet samples. The new age is in accordance with earlier paleomagnetic and fission track data, which indicated that Rochechouart was formed during the late Triassic. Moreover, the new determination is in agreement with the regional geological setting and field relations of the structure. The new age of 214 ± 8 Ma falls within the Norian stage of the Triassic system.     

40Ar‐39Ar step‐heating of impact glasses from the Nördlinger Ries impact crater—Implications on excess argon in impact melts and tektites

Seven impact melts from various places in the Nördlinger Ries were dated by ⁴⁰Ar‐³⁹Ar step‐heating. The aim of these measurements was to increase the age data base for Ries impact glasses directly from the Ries crater, because there is only one Ar‐Ar step‐heating spectrum available in the literature. Almost all samples display saddle‐shaped age spectra, indicating the presence of excess argon in most Ries glass samples, most probably inherited argon from incompletely degassed melt and possibly also excess argon incorporated during cooling from adjacent phases. In contrast, moldavites usually contain no inherited argon, probably due to their different formation process implying solidification during ballistic transport. The plateau age of the only flat spectrum is 14.60 ± 0.16 (0.20) Ma (2σ), while the total age of this sample is 14.86 ± 0.20 (0.22) Ma (isochron age: 14.72 ± 0.18 [0.22] Ma [2σ]), proofing the chronological relationship of the Ries impact and moldavites. The total ages of the other samples range between 15.77 ± 0.52 and 20.4 ± 1.0 Ma (2σ), implying approximately 2–40% excess ⁴⁰Ar (compared to the nominal age of the Ries crater) in respective samples. Thus, the age of 14.60 ± 0.16 (0.20) (2σ) (14.75 ± 0.16 [0.20 Ma] [2σ], calculated using the most recent suggestions for the K decay constants) can be considered as reliable and is within uncertainties indistinguishable from the most recent compilation for the age of the moldavite tektites.     

An integrated approach to understanding Apollo 16 impact glasses: Chemistry,isotopes, and shape

Abstract The major‐ and minor‐element abundances were determined by electron microprobe in 1039 glasses from regoliths and regolith breccias to define the compositional topology of lunar glasses at the Apollo 16 landing site in the central highlands of the Moon. While impact glasses with chemical compositions similar to local materials (i.e., Apollo 16 rocks and regoliths) are abundant, glasses with exotic compositions (i.e., transported from other areas of the Moon) account for up to ?30% of the population. A higher proportion of compositionally exotic, angular glass fragments exists when compared to compositionally exotic glass spherules. Ratios of non‐volatile lithophile elements (i.e., Al, Ti, Mg) have been used to constrain the original source materials of the impact glasses. This approach is immune to the effects of open‐system losses of volatile elements (e.g., Si, Na, K). Four impact glasses from one compositionally exotic group (low‐Mg high‐K Fra Mauro; lmHKFM) were selected for ⁴⁰Ar/³⁹ Ar dating. The individual fragments of lmHKFM glass all yielded ages of ?3750 ± 50 Ma for the time of the impact event. Based on the petrography of these individual glasses, we conclude that the likely age of the impact event that formed these 4 glasses, as well as the possible time of their ballistic arrival at the Apollo 16 site from a large and distant cratering event (perhaps in the Procellarum KREEP terrain) (Zeigler et al. 2004), is 3730 ± 40 Ma, close to the accepted age for Imbrium.     

Impact glasses in fallout suevites from the Ries impact structure,Germany: An analytical SEM study

Abstract— t‐Impact‐generated glasses from fallout suevite deposits at the Ries impact structure have been investigated using analytical scanning electron microscopy. Approximately 320 analyses of glass clasts were obtained. Four glass types are distinguished on the basis of composition and microtextures. Type 1 glasses correspond to the aerodynamically shaped glass bombs studied previously by many workers. Major oxide concentrations indicate the involvement of granitic rocks, amphibolites, and minor Al‐rich gneisses during melting. Type 2 glasses are chemically heterogeneous, even within individual clasts, with variations of several wt% in most of the major oxides (e.g., 57–70 wt% SiO₂). This suggests incomplete mixing of: 1) mineral‐derived melts or 2) whole rock melts from a wide range of lithologies. Aluminium‐rich clinopyroxene and Fe‐Mg‐rich plagioclase quench crystals are present in type 1 and 2 glasses, respectively. Type 3 glasses contain substantial amounts of H₂O (?12–17 wt%), low SiO₂ (50–53 wt%), high Al₂O₃ (17–21 wt%), and high CaO (5–7 wt%) contents. This suggests an origin due to shock melting of part of the sedimentary cover. Type 4 glasses form a ubiquitous component of the suevites. Based on their high SiO₂ content (?85–100 wt%), the only possible protolith are sandstones in the lowermost part of the sedimentary succession. Calcite forms globules within type 1 glasses, with which it develops microtextures indicative of liquid immiscibility. Unequivocal evidence also exists for liquid immiscibility between what are now montmorillonite globules and type 1, 2, and 4 glasses, indicating that montmorillonite was originally an impact melt glass. Clearly, the melt zone at the Ries must have incorporated a substantial fraction of the sedimentary cover, as well as the underlying crystalline basement rocks. Impact melts were derived from different target lithologies and these separate disaggregated melts did not substantially mix in most cases (type 2, 3, and 4 glasses and carbonate melts).     

Apollo 17 regolith, 71501,262: A record of impact events and mare volcanism in lunar glasses

Abstract— Thirteen glasses from Apollo 17 regolith 71501,262 have been chemically analyzed by electron microprobe and isotopically dated with the ⁴⁰Ar/³⁹Ar dating method. We report here the first isotopic age obtained for the Apollo 17 very low titanium (VLT) volcanic glasses, 3630 ± 40 Ma. Twelve impact glasses that span a wide compositional range have been found to record ages ranging from 102 ± 20 Ma to 3740 ± 50 Ma. The compositions of these impact glasses show that some have been produced by impact events within the Apollo 17 region, whereas others appear to be exotic to the landing site. As the data sets that include compositions and ages of lunar impact glasses increase, the impact history in the Earth‐Moon system will become better constrained.     

New abundance determinations in z < 1.5 QSO absorbers: seven sub-DLAs and one DLA

We present chemical abundance measurements from high-resolution observations of seven subdamped Lyα (sub-DLA) absorbers and one DLA system at   z < 1.5  . Three of these objects have high metallicity, with near or supersolar Zn abundance. Grids of cloudy models for each system were constructed to look for possible ionization effects in these systems. For the systems in which we could constrain the ionization parameter, we find that the ionization corrections as predicted by the cloudy models are generally small and within the typical error bars (∼0.15 dex), in general agreement with previous studies. The Al  iii to Al  ii ratio for these and other absorbers from the literature are compared, and we find that while the sub-DLAs have a larger scatter in the Al  iii to Al  ii ratios than the DLAs, there appears to be little correlation between the ratio and   N _{H  i}   . The relationship between the metallicity and the velocity width of the profile for these systems is investigated. We show that the sub-DLAs that have been observed to date follow a similar trend as DLA absorbers, with the more metal rich systems exhibiting large velocity widths. We also find that the systems at the upper edge of this relationship with high metallicities and large velocity widths are more likely to be sub-DLAs than DLA absorbers, perhaps implying that the sub-DLA absorbers are more representative of massive galaxies.