Aluminum-magnesium isotopic analysis of plagioclase in the eucrite Piplia Kalan, with 27Al/24Mg ratio between 2000 to 7000, reveals the presence of excess magnesium-26 resulting from in situ decay of the short-lived nuclide aluminum-26. This observation confirms aluminum-26 as a plausible heat source for melting and differentiation of the eucrite parent body in particular and for planetesimals in general. The inferred initial abundance of 26Al/27Al of (7.5 +/- 0.9) x 10(-7) indicates that melting, differentiation, and crust formation in the parent body of Piplia Kalan was complete within 5 million years of the formation of the solar system.

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The nature of large scale stratospheric circulation is studied using the cosmic ray produced isotopes Par, Be", Sa6 and Nas2 as tracers. Supplementary information obtained from observations of the distribution of the bomb-produced Na** and radongenic Pbp10 is taken into account. The activities of these tracer elements have been measured in the stratospheric air, up to altitudes of 20 km, during 1960-64. Data are fairly extensive for studying the characteristics of the mean circulation in the stratosphere as well as seasonal changes in patterns of mixing/transport of air in certain regions of the stratosphere.The interpretation of the data on cosmic ray tracers is based on a comparison of their observed activities with the expected production rates due to cosmic rays. For this purpose, the work of La1 & Peters is extended to evaluate the variations in the relative production rates of the isotopes Par, Be', S" and Nag* in the atmosphere. These have to be taken into account when isotope data are compared for different altitudes and latitudes in the stratosphere where relative isotope production rates are different because of the markedly different prevailing energy spectrum of nucleons.The analysis allows us to distinguish three zones in the lower stratosphere (below 20 km), well separated from the tropopause, having distinct circulation patterns. These regions are separately well mixed either vertically or horizontally; the mean time of residence of aerosols in these regions differs appreciably too. The most stable region in the Stratosphere is found to be 18-20 km region at 0-30" latitude, where apparent residence times are of the order of twenty months. Polar regions are observed to exhibit an enhanced vertical mixing during November-February. Combining these results with the observations of dispersion of bomb-produced Naxz, which appeared in significant amounts from early 1962 onwards all over the stratosphere, we deduce that in the polar regions, vertical mixing occurs rapidly during November-February so that any activity injected in this region a t 20 km or so mixes downwards a t the rate of about 1.6 km month-'. It is concluded that the observed spring peaks in the troposphere are merely the consequence of this phenomena which is triggered in upper levels (above 20 km) of the stratosphere during October-November.The observations of concentrations of PbP1O in the stratosphere are discussed. The analysis reveals that an appreciable gravitational settling of Pb'l0 seems to have occurred, a t least during the period over which data were collected, from the stratospheric air between 56'-75' latitude. These observations imply that the residence time of air in this region of the atmosphere is appreciably higher than that deduced from tracers which attach themselves to aerosols. Lastly, the Pbglo data indicate that appreciable amounts of tropospheric radon presumably enter the equatorial stratosphere; this conclusion rests on the Observation that Pbz10 concentrations are higher in this region compared to that in the ...

Abstract-We measured cosmic-ray products-noble gases, radionuclides, thermoluminescence, and nuclear tracks-and trace element contents and mineralogy of samples ofthree orthogonal and mutually intersecting cores (41-46 em long) of a 101.6 kg Ghubara individual (1958,805) at The Natural History Museum, London. The xenoliths, like the host, have high concentrations of trapped solar gases and are heavily shocked. While contents of noble gases and degree of shock-loading in this individual and three others differ somewhat, the data indicate that Ghubara is a two-generation regolith breccia. Contents of cosmogenic 26A I and lOBe and low track densities indicate that the Ghubara individuals were located more than 15 em below the surface of an 85 em met eoroid. Because of its large size, Ghubara's cosmic-ray exposure age is poorly defined to be 15-20 Ma from cosmogenic nuclides. Ghubara's terrestrial age , based on l4C data, is 2-3 ka . Not only is Ghubara the first known case of a two-generation regolith breccia on the macroscale , it also has a complicated thermal and irradiation history.