Allan Hills 77081—an unusual stony meteorite

Allan Hills (ALHA) 77081 is achondritic in texture while the mineral composition and the chemistry are chondritic with the exception of a few elements. An assignment to one specific group of ordinary chondrites is therefore difficult. In many respects this meteorite is similar to the unusual stone meteorite Acapulco. The REE pattern of ALHA 77081 is essentially flat and the distribution ratios of siderophile elements between metal and silicates are high compared to ordinary chondrites.Gas retention ages are 3.5±0.5 AE for U, Th-He and 4.50±0.15 AE for K-Ar. In spite of the high degree of recrystallisation the meteorite contains trapped noble gases in amounts comparable to type 4 chondrites.Cosmic ray tracks and spallogenic noble gases indicate a small preatmospheric radius of about 2–3 cm. Spallogenic nuclides produced by solar cosmic rays or stopped solar flare ions may be present.     

Noble gases in the St. Mesmin chondrite: Implications to the irradiation history of a brecciated meteorite

Helium, neon and argon were analysed in matrix samples and in different clasts of the polymict-brecciated LL-chondrite St. Mesmin. All clasts have high K-Ar ages with a mean value of4.40 ± 0.26Ga. One exotic H-group xenolith, however, has a K-Ar age of only1.36 ± 0.05Ga. The low age indicates that the St. Mesmin breccia was compacted to its present structure relatively late in its history and that the St. Mesmin meteorite developed from regolith material on the meteorite's parent body. This is further demonstrated by the high concentrations of solar noble gases in the matrix and the cosmic ray pre-exposure of one individual clast.     

The registration-temperature dependence of heavy-ion track-etch rates and annealing sensitivity in crystals: implications for cosmic ray identification and fission track dating of meteorites

Etch rates and etchable lengths of cosmic ray tracks in meteoritic crystals have been used by several workers to derive the charge spectrum of ancient cosmic rays. This is done by comparing the fossil cosmic ray track record with fresh accelerator-produced calibration tracks. These calibration tracks are generally produced at room temperature, while meteorites spend a high proportion of their lifetimes orbiting at large distances from the Sun ( 3–5 AU) and are, consequently, at much lower temperatures (typically 100–150 K) during most of their cosmic ray exposure ages. We have irradiated crystals of apatite, olivine, enstatite and diopside held at 77, 293, 473 and 573 K, with 2 MeV/nucleon⁸¹Br ions, and then etched them. We find that their track etching properties are dependent upon the temperature of the mineral during registration. The track etch velocity generally increases with registration temperature up to 300 or 500 K (the upper limit depending upon the type of crystal). Our results also indicate that the annealing sensitivity of fission tracks in fluorapatite may be influenced by the registration temperature. This temperature dependence has important implications not only for cosmic ray particle identification but also for fission track dating of meteorites in view of the fact that the meteorite parent bodies were at elevated temperatures at the beginning af their life when²⁴⁴Pu fission tracks were being generated abundantly.     

14C content of ten meteorites measured by tandem accelerator mass spectrometry

¹⁴C has been measured in three North American and seven Antarctic meteorites with the Chalk River MP tandem accelerator. In most cases cosmogenic¹⁴C, which is tightly bound, was separated from absorbed atmospheric radiocarbon by stepwise heating extractions. Terrestrial ages obtained by comparing cosmogenic¹⁴C in the meteorite to that in Bruderheim are (7.2 ± 0.6) × 10³ years for Yamato 7304, (11.6 ± 0.4) × 10³ years for Estacado, and range from (32.7 ± 0.5) × 10³ to (41.0 ± 0.8) × 10³ years for six meteorites recovered at Allan Hills and its vicinity. The present upper limit to age determination by the accelerator method varies from 50 × 10³ to 70 × 10³ years depending upon mass and carbon content of the sample. The natural limit caused by cosmic ray production of¹⁴C in silicate rocks at 2000 m elevation is estimated to be (55 ± 5) × 10³ years. “Weathering ages” were estimated for the Antarctic meteorites from the specific activity of loosely-bound CO₂ considered to be absorbed from the terrestrial atmosphere on weathering. The accelerator measurements are in accordance with previous low-level counting measurements but have higher precision and sensitivity.     

Source of lead in Central American and Caribbean mineralization

The Zerga meteorite, an LL6 ordinary chondrite found at Aouelloul crater in 1973, is a small fragment of a larger mass whose pre-atmospheric radius was most likely between 20 and 125 cm. A typical amphoterite, it is a monomict breccia that has undergone at least one recrystallization episode.³He and²¹Ne contents define a shielding-corrected, cosmic ray exposure age of 21–24 × 10⁶ years and the²⁶Al content is consistent with a terrestrial age ?500,000 years (2σ limit). The K—Ar gas-retention age is 3.1 × 10⁹ years. The meteorite's areal association with the impact crater is merely coincidental. A new K-Ar age of the glassy impactite found at Aouelloul dates the crater at 3.1 ± 0.3 × 10⁶ years, sensibly indistinguishable from the recently determined age of nearby Tenoumer crater (2.5 ± 0.5 × 10⁶years). The similar ages of these two impact craters, and their almost perfect linear alignment with a third, morphologically similar crater (Temimichat Ghallaman) over a distance of 600 km, suggests a simultaneous triple impact occasioned by the disruption of a large meteorite moving on a very shallow atmospheric trajectory. If so, the concomitant low impact angles may be responsible for the unusually shallow original depths inferred for two of the craters from gravity data.     

Dating of meteorite falls using thermoluminescence: application to Antarctic meteorites

The date of fall of a meteorite may be estimated from its thermoluminescence (TL) and in this paper the principle of a method of utilising TL to determine the terrestrial ages of eight Antarctic meteorites (Allan Hills-77) is described. The TL in a meteorite is primarily induced by cosmic ray irradiation in space and once the meteorite is on the Earth's surface, it is shielded from further cosmic ray irradiation. Under these conditions the TL will decay at a rate governed by the thermal stability of the TL and by the environmental temperature.An estimate of the decay rate may be arrived at by using recently calculated data for the trapping parameters associated with meteorite TL. A major problem is the environmental storage temperature. The “effective” storage temperature of the Antarctic meteorites is unknown, but appears to be greater than the mean annual temperature of the region of the meteorite find.Only upper limits to the terrestrial ages can be calculated because the TL at the time of the fall to Earth is highly variable from sample to sample.     

Variations in the Fe/O value resulting from changes in the ion energy in flows of accelerated solar particles

Qualitative estimates of the relative iron and oxygen ions (Fe/O) in flows of solar cosmic rays from impulsive and long-duration solar flares are obtained for different ion energy ranges. The Fe/O value serves as a measuring unit for the known FIP effect in the solar atmosphere. It is shown that the FIP effect is most evident (maximum Fe/O values) in impulsive events for ions at energies <2 MeV/n. In long-duration events, the Fe/O value gradually decreases in parallel with ion energy and its maximum values are observable in the area of relatively low energies. The comparison of some flare models provided grounds for a qualitative explanation of the Fe/O behavior in response to changes in ion energies for both classes of solar cosmic ray (SCR) events.     

Solar proton event in December 2006

The variations in the rigidity spectrum and anisotropy of cosmic rays in December 2006 have been studied based on the surface measurements of the cosmic ray intensity at the global network of stations, using the method of global spectrographic survey. It has been indicated that the highest degree of anisotropy (to ～50%) with the maximal intensity of particles with a rigidity of 4 GV in the direction from the Sun (an asymptotic direction of about ?25° and 160°) was observed at 0400 UT on December 13. The parameters of the cosmic ray rigidity spectrum, which reflect the electromagnetic characteristics of the heliospheric fields during the studied period, have been determined when the surface and satellite measurements of protons in the energy range from several megaelectronvolts to several tens of gigaelectronvolts were jointly analyzed. The observed anisotropy and variations in cosmic rays in a wide energy range have been explained based on an analysis of the results.     

A new index of solar activity: An index of cosmic ray scintillation

An index of cosmic ray scintillation introduced previously is verified. This procedure has been performed within the scope of the long-term full-scale monitoring of galactic cosmic rays (GCRs) in the real time regime. The 5-min data of the global network of high-latitude neutron monitors at Tixie Bay (Apatity) and Oulu (Finland) stations during the last four solar cycles (cycles 20–23), i.e., during the entire period of data registration with a high (5 min) resolution, have been used. The relationship between the amplitude-frequency-time structure of a precursor in the GCR scintillation index and the soliton-like structure of the heliospheric current sheet during the disturbed period has been established. This indicates that the precursor is of a coherent origin. Only the presence of a coherent process—quasi-week variation—makes it fundamentally possible to predict heliospheric storms. Finally, the justifiability of the effective prediction of heliospheric storms (～80%) has been obtained based on the long-term cosmic ray monitoring during cycle 23.     

Cratering process and morphological features of the Xiuyan impact crater in Northeast China

By using crater impact morphological theory and mathematics modeling,the paper studies the cratering process and morphological features of the Xiuyan Crater in Northeast China based on remote sensing imagery,digital elevation model(DEM)and drilling and measuring data.The results show that:1)Simulated calculation indicates the diameter of the transient crater was1406±12 m and the depth was 497±4 m.The diameter of the final crater was 1758±15 m and the depth was 374.5±3.5 m;the thickness of the breccia lens was 188.5±0.5 m.The data is basically the same as previous drilling data.2)Preliminary determination of the size of Xiuyan impactor:The diameter at about 55 m for an iron meteorite,and about 115 m for a stony meteorite.3)The depth-to-diameter ratio is 0.143,similar to the typical simple crater.The circularity index of the Xiuyan Crater is0.884,indicating erosion and degradation.4)The distribution pattern of radial fractures has priority in NW-SE and WNW-ESE and these fractures control the development and evolution of drainage inside the crater.     

Cosmogenic radioisotopes in the Dhajala chondrite: implications to variations of cosmic ray fluxes in the interplanetary space

Activities of a suite of radioisotopes ranging in half-life from 5.6 days (⁵²Mn) to 3.7 m.y. (⁵³Mn) have been measured in the Dhajala chondrite. The results show that all the radioactivities are close to the expected levels except⁵⁴Mn and ²²Na. Their activities are higher than those based on the interplanetary fluxes at 1 A.U. near the ecliptic, expected immediately before the fall of Dhajala, corresponding to the time of solar minimum. Furthermore, activity ratios of ⁵⁴Mn/⁵³Mn and ²²Na/²⁶Al are higher by 30–50% than expected. The departure from the expected values is discussed in terms of spatial variations of cosmic rays based on the computed orbital parameters of the meteoroid. If the galactic cosmic ray fluxes in the equatorial region (±15°) are assumed to be the same as in the ecliptic plane then these results suggest higher fluxes by 33 ± 7% at heliographic latitudes 15–40°S, during solar minimum.     

Cosmic-ray-produced53Mn in thirty-one meteorites

Cosmic-ray-produced⁵³Mn (t_½ = 3.7 × 10⁶years) has been determined by neutron activation analysis in twenty-two chondrites including three Antarctic meteorites: Yamato-7301 (j), -7305 (k) and -7304 (m).⁵³Mn was also measured in four mesosiderites, three iron meteorites, Bencubbin (unique) and Udei Station (iron with silicate inclusions). In addition, preliminary results for¹⁰Be (1.6 × 10⁶ years) were obtained in the Yamato meteorites using a low-background needle GM counter. Based on published values of rare gas ages, corrections were made for undersaturation; the average specific saturation activities of⁵³Mn were found in the range 450 ± 90dpm⁵³Mn/kg Fe in most of the chondrites and 490 ± 75dpm⁵³Mn/kg Fe in the mesosiderites. Two meteorites had extremely low contents of⁵³Mn: 102 ± 6 in Yamato-7301 and 48 ± 3dpm⁵³Mn/kg Fe in Bondoc. The Bondoc mesosiderite was already known to have a low concentration of cosmogenic radionuclides due to its large pre-atmospheric size. Several possible mechanisms are discussed to explain the low⁵³Mn activity in Yamato-7301: (1) long terrestrial age of about 7 m.y.; (2) low production rate of⁵³Mn due to heavy pre-atmospheric shielding (>70cm); (3) multi-stage irradiation history resulting in an undersaturation of⁵³Mn; and (4) a mechanism in which two or three of the above factors are combined. The ratio of⁵³Mn production rate in Ni to that in Fe has been estimated to be 1/3, based on the measurements of⁵³Mn in the metallic and silicate phases of St. Séverin meteorite, as well as on published results of some high-energy bombardment experiments.     

Formation of a local atmospheric electric field

We have estimated the variations in the atmospheric electrostatic field (AEF, E _Z(0)) strength in the surface layer caused by variations in conductivity due to radon influences, cosmic ray intensity, changes in the balance of light and heavy ions during sunset and sunrise, and under the effect of the ionospheric electric current potential on the AEF potential. It is shown that the air conductivity varies due to ionization under the effect of radon emanations and is determined by the radon exhalation and turbulent diffusion of the surface air layer, while the cosmic ray intensity affects the surface air conductivity through changes in the ion recombination conditions. A decrease in the air conductivity due to a decrease in the cosmic ray intensity (Forbush decrease) also decreases E _Z(0), while a decrease in radon fluxes results in an increase in E _Z(0). We have estimated the effect of illumination conditions on the AEF due to variations in the relative concentration of heavy and light ions under the influence of photodetachment and photoattachment processes. The work has been done on the basis of data received from the Paratunka observatory (Kamchatka).     

26Al in iron meteorites and the constancy of cosmic ray intensity in the past

Cosmic-ray-produced²⁶Al in iron meteorites has been measured by low-level γγ-coincidence counting. The²⁶Al activities, in dpm/kg, are: Aroos3.0 ± 1.0, Braunau2.6 ± 0.5. Kayakent4.6 ± 1.5, N'Goureyma4.4 ± 1.1, Okahandja3.6 ± 0.9, Treysa4.0 ± 0.5. Exposure ages based on²⁶Al/²¹Ne are in agreement, within experimental error(±20%), with those based on³⁶Cl/³⁶Ar and³⁹Ar/³⁸Ar but the ages based on⁴⁰K/⁴¹K are higher by about 50%. The difference in exposure ages is probably caused by a real change of the cosmic ray intensity in the inner solar system.     

Cosmogenic and radiogenic noble gases in the Dhajala chondrite

Whole rock and chondrules of the Dhajala chondrite were analyzed for Ne, Ar, Kr and Xe by total melting as well as by stepwise heating techniques. The cosmic ray exposure ages for the whole rock and the chondrules are6.2 ± 0.8 and6.3 ± 1.0m.y. as determined by the²¹Ne method and4.8 ± 1.5 and4.2 ± 2.0m.y. by the³⁸Ar method, respectively. The K-Ar age of the whole rock is4.2 ± 0.4b.y. The elemental composition of the trapped gas in this chondrite is of “planetary” type. The radiogenic¹²⁹Xe contents in the whole rock and chondrules are similar and this component is very retentively sited in the chondrules.     

Annual effect of cosmic rays on ionization in the high-latitude troposphere

The seasonal effect of the daily variations in the cosmic ray intensity on the conductivity of the Earth-high-conductivity layer column has been analyzed based on the observations of the cosmic ray intensity, atmospheric current, and electric field vertical component, performed from summer 2006 to spring 2007 at Apatity station. The method for correcting the measurements of the atmospheric current and electric field vertical component under complex tropospheric conditions by numerically simulating the spatial structure of the current and field lines in the observation region has been proposed. It has been indicated that cosmic rays are the main source of ions in the winter polar lower atmosphere and are responsible for the type of daily variations in the conductivity, whereas the daily variations in the atmospheric current more depends on the conductivity rather than on the vertical electric field.     

NanoSIMS imaging method of zircon U-Pb dating

We report an imaging method of zircon U-Pb dating with NanoSIMS 50 L, which overcomes the significant U-Pb fractionation as the pit was sputtered deeper during conventional spot mode analysis and can be applied to irregular small grains or heterogeneous areas of zircon. The U-Pb and Pb-Pb ages can be acquired simultaneously for 2 μm×2 μm(for small grains) or 1 μm×9 μm(for zoned grains), together with Zr, Y and other trace elements distributions. Using zircon M257 as standard, the U-Pb ages of other zircon standards, including Qinghu, Plesovice, Temora and 91500, were measured to(2σ) as158.8±0.8, 335.9±3.4, 412.0±12 and 1067±12 Ma, respectively, consistent with the recommended values within the analytical uncertainties. Tiny zircon grains in the impact melt breccia of the lunar meteorite SaU 169 were also measured in this study,with a Pb-Pb age of 3912±14 Ma and a U-Pb age of 3917±17 Ma, similar to previous results reported for the same meteorite.The imaging method was also applied to determine U-Pb age of the thin overgrowth rims of Longtan metamorphic zircon, with a Pb-Pb age of 1933±27 Ma and a U-Pb age of 1935±25 Ma, clearly distinct from the Pb-Pb age of 2098±61 Ma and the U-Pb age of 2054±40 Ma for detrital cores.     

The abundance of barium in stony meteorites

The concentration of Ba in 7 carbonaceous chondrites, 18 ordinary chondrites, 3 achondrites and 1 stony-iron meteorite has been determined by the stable isotope dilution technique using solid source mass spectrometry. Analysis of the C1 chondrite Orgueil indicates a small adjustment of the “cosmic” abundance of Ba to 4.2 on the Si=10⁶ abundance scale. The present work provides a more complete coverage of a number of meteorite classes than has so far been available for the abundance of Ba in stony meteorites.     

Effects of cosmic rays on the Earth's environment

Cosmic rays are considered as a possible link coupling solar activity to atmospheric processes. The most intense types of cosmic ray flux modulations are briefly described as a source of variability in the ion production rate in the atmosphere. The ions may affect atmospheric phenomena through (1) charge-dependent chemical reactions, (2) charge-dependent droplet and ice crystal formation, and (3) as their influence on the current flowing in the global electric circuit. However, the latter two have still to be proved as the causes of correlations between changes in the large-scale meteorological parameters and cosmic ray flux variations.