On nature of bimodal release of noble gases during pyrolysis of the meteoritic nanodiamonds

Analysis of noble gas proportions and their release kinetics during stepped pyrolysis and oxidation of meteoritic nanodiamonds, as well as their core-shell structure led to the following conclusions: (1) Noble gases of HL component with anomalous isotopic composition were presumably formed prior to implantation in the nanodiamonds owing to mixing of nucleosynthetic products of p- and r- process associated with explosion of type-II supernova with noble gases having “normal” isotopic composition; (2) isotopically normal P3 noble gases in the nanodiamonds grains are confined to the nondiamond (for instance, graphite-like) phase in the surface layer. The “layer” structure of nanodiamonds grains resulted from heating up to 800–900°C. Observed increase in contents of P3 noble gases with increasing grain sizes of meteoritic nanodiamonds is caused by the dependence of the degree of graphitization of the superfical layer at given temperature on the grain size and surface defect density; (3) bimodal release of noble gases during pyrolysis of the meteoritic nanodiamonds from weakly metamorphosed meteorites was caused by P3 and HL components, which are comparable in abundance but sharply differ in their release temperature.     

Tritium corrected 14C and atmospheric noble gas corrected 4He applied to deduce ages of mixed groundwaters: Examples from the Baden region,Switzerland

Mixing of recent water (post-1953) and old water (up to a limit of 25,000 years) can be identified by tritium and ¹⁴C values. The age of an old endmember may be deduced, applying a tritium correction to the measured ¹⁴C value.⁴He may be used as a second semi-quantitative age indicator of old groundwaters, overlapping the ¹⁴C range and extending beyond it. Tritium may be used to deduce the radiogenic ⁴He concentration in the old endmember. In addition, the atmospheric noble gases may be used to check He retention and to correct for losses.The corrected ¹⁴C and ⁴He values may be used to check concordance between computed ages in mixed and unmixed groundwaters. Concordant ages indicate that the basic dating premises were not violated, whereas discordant ages indicate that secondary processes were active.The suggested approach is demonstrated for a number of springs and wells studied in the regions of Baden and Yverdon, Switzerland.     

Geothermochronology based on noble gases: III. Migration of radiogenic He in the crystal structure of native metals with applications to their isotopic dating

It was shown that the behavior of ⁴He in native and technical metals is very similar owing to the symmetric and stable electron shells of its atoms, which cannot gain electrons from other atoms or donate their own electrons to metal atoms in a crystal lattice. Therefore, they rapidly migrate toward grain boundaries and dislocations, where they are released as vesicles or He clusters. It was found that the thermal desorption of radiogenic He occurring in the crystal lattice of native metals as gas clusters requires activation energies of 100 and even 180 kcal/mol up to the attainment of the melting temperature of the metal. The frequency factor is several orders of magnitude higher than the limiting value k ₀ ∼ 10¹³ s⁻¹ for the migration of single atoms in the crystal lattice. Near the melting temperature and tens-hundreds degrees above it, the character of the thermal desorption of radiogenic ⁴He changes fundamentally. The migration is strongly accelerated, and sharp narrow peaks appear on the kinetic curves of thermal desorption. A similar phenomenon was observed during the annealing of technical metals and is known as the burst-effect. The destruction of the crystal structure results in the disappearance of helium clusters (vesicles). At the very high temperature, He migrates as individual atoms relatively rapidly from the melt. The activation energy for He thermal desorption and the pre-exponential frequency factor acquire values characteristic of ordinary migration. Such peculiarities of radiogenic He provide unique opportunities for its preservation in the structure of gold and other native metals below their melting temperatures. The rapid advances of (U-Th)/He geochronology is still hampered by the experimentally established extremely heterogeneous distribution of U, He, and, probably, Th in the structure of gold and other natural metals. This difficulty can be circumvented by the development of a method for the determination of the contents of all the mentioned chemical elements in a single aliquot from each sample.     

Occurrence of larch (Larix) in Fennoscandia during the Eemian interglacial and the Brørup interstadial according to pollen analytical data

The occurrence of pollen and macrofossils of larch in Eemian deposits in northern Finland indicates that this species must have grown in the area during the last interglacial. Lark spread to Finland from the east, its date of arrival being deducible from the general vegetational succession. It probably did not grow in central or southern Finland during the interglacial, but is thought to have extended fairly far south in Sweden and Noway along the Fennoscandian mountain range. The Lark pollen found at the upper boundary of the interglacial deposits at Margreteberg and Stenberget in southern Sweden may suggest that it did reach southern Sweden by the very end of the Eemian, but it cannot be said for certain whethcr this pollen represents an influx of Larix from the north or from Central Europe.