ALBERT

Description: The concentrations of cosmogenic nuclides were studied as a function of shielding on samples from a cross section of the 293 kg main fragment of the L5 chondrite Knyahinya. The stone broke into two nearly symmetrical parts upon its fall in 1866. The planar cross section has diameters between 40 and 55 cm. He, Ne, and Ar were measured on about 20 samples by mass spectrometry and the 10-Be activities on aliquots of 10 selected samples were determined by AMS. The 10-Be data are presented and the abundances of spallogenic nuclides are compared with the model calculations reported by Reedy for spherical L chondrites. The 10-Be production rates in Knyahinya are shown versus the shielding parameter 22-Ne/21-Ne.

Description: Measurements of stable and radioactive spallation products in meteorites allow to investigate their histories, especially with respect to the exposure to galactic cosmic ray particles and the pre-atmospheric size of the object. While the concentrations of spallation products lead to the determination of exposure and terrestrial ages, production rate ratios are characteristic for the location of the sample in the meteorite. So, one of the aims of this investigation on meteorites is to obtain depth indicators from suitable pairs of cosmogenic nuclides. Because of the different depth profiles for nuclide productions it is necessary to determine the concentrations of a larger number of spallation products in aliquots of a single small sample. Such same sample measurements of Be-10 and light noble gases were performed on 15 ordinary chondrites (7 H- and 8 L-chondrites. Be-10 was determined by accelerator mass spectrometry and the noble gases were measured by static mass spectrometry. The results are summarized and discussed.

Description: Among the lunar soil constituents, monomineralic grains deserve special attention. Noble gases of carefully prepared mineral separates from lunar bulk soils were studied. The major results and conclusions of these investigations are summarized, in the context of both the regolith evolution and the history of the solar corpuscular radiation. With regard to the most abundant noble gas component in the regolith samples (the solar gases) the mineral grains have mainly two properties giving these particles among all soil constituents the best characteristics as sensors for solar gases, despite the fact, that the noble gas concentrations in a mineral separate are 10 to 60 times lower that those in a bulk sample of the same grain size. The first of these properties is the mineral dependent retentivity of the light gases He and Ne, the second property concerns the relatively short time during which a mineral grain acquires it solar gases. These two points are briefly discussed.

Description: Solar He, Ne, and Ar in a Fe-Ni separate from the chondrite Fayetteville are analyzed by closed system stepped oxidation. We report here data of the first 15 steps comprising 55 percent of the total solar gases. He-4/Ar-36 and Ne-20/Ar-36 are quite constant at values about 20 percent below those of present day solar wind (SWC). In this, Fe-Ni differs from lunar ilmenites where He-4/Ar-36 and Ne-20/Ar-36 in the first steps are several times below SWC. Thus, metal retains SW-noble gases even better than ilmenite, almost without element fractionation. Nevertheless, the isotopic composition of SW-He, -Ne, and -Ar in the first steps of the metal sample are identical to those found in a recently irradiated lunar ilmenite, indicating that ilmenites and chondritic metal both contain isotopically unfractionated SW noble gases. A preliminary analysis of a smaller Fayetteville metal separate shows Ne from solar energetic particles (SEP-Ne) with Ne-20/Ne-22 less than or equal to 11.5.

Description: The cosmogenic radionuclides Be(10), Al(26), and Mn(53) and noble gases were determined in more than 28 meteorites from Antarctica by nuclear analytical techniques and static mass spectrometry, respectively. The summarized results are listed. The concentrations of Al(26) and Mn(53) are normalized to the repective main target elements and given in dpm/kg Si sub eq and dpm/kg Fe. The errors stated include statistical as well as systematical errors. For noble gas concentrations estimated errors are 5% and for isotopic ratios 1.5%. Cosmic ray exposure ages T sub 21 were calculated by the noble gas concentrations and the terrestrial residence time (T) on the basis of the spallogenic nuclide Al(26). The suggested pairing of the LL6 chondrite RKPA 80238 and RKPA 80248 and the eucrites ALHA 76005 and ALHA 79017 is confirmed not only by the noble gas data but also by the concentrations of the spallation produced radionuclides. Futhermore, ALHA 80122, clasified as an H6 chondrite, has a noble gas pattern which suggest that this meteorite belongs to the ALHA 80111 shower.

Description: The concentrations of solar wind implanted Ar-36 in mineral grains extracted from lunar soils show that they were exposed to the solar wind on the lunar surface for an integrated time of 10E4 to 10E5 years. From the bulk soil 61501 plagioclase separates of 8 grain size ranges was prepared. The depletion of the implanted gases was achieved by etching aliquot samples of 4 grain sizes to various degrees. The experimental results pertinent to the present discussion are: The spallogenic Ne is, as in most plagioclases from lunar soils, affected by diffusive losses and of no use. The Ar-36 of solar wind origin amounts to (2030 + or - 100) x 10E-8 ccSTP/g in the 150 to 200 mm size fraction and shows that these grains were exposed to the solar wind for at least 10,000 years. The Ne-21/Ne-22 ratio of the spallogenic Ne is 0.75 + or - 0.01 and in very good agreement with the value of this ratio in a plagioclase separate from rock 76535. This rock has had a simple exposure history and its plagioclases have a chemical composition quite similar to those studied. In addition to the noble gases, the heavy particle tracks in an aliquot of the 150 to 200 mm plagioclase separate were investigated and found 92% of the grains to contain more than 10E8 tracks/sq cm. This corresponds to a mean track density of (5 + or - 1) x 10E8 tracks/sq cm. The exploration of the exposure history of the plagioclase separates from the soil 61501 do not contradict the model for the regolith dynamics but also fail to prove it.

Description: The N determination in mg sized mineral separates from lunar soils by static mass spectrometry is an experimental break-through likely to contribute to the deciphering of the records left in the mineral grains by the exposure to the solar wind. In this discussion some comparisons of the results of N and noble gas analyses of the 71501 bulk soil and an ilmenite separate thereof are focussed on. Conclusions from noble gas data obtained on mineral separates from some 20 soils are summarized in a companion paper and are also discussed herein.

Description: Analysis of lunar surface samples for elements implanted therein by solar corpuscular radiation reveals evidence for the following compositional changes over a time period between 1.5 and 3 Gyr: 50-percent decreases in the ratios He-4/Ar-36 and Xe/Ar-36; a 20-percent increase in the ratio He-3/He-4; a 3-percent increase in the ratio Ne-20/Ne-22; and a 50-percent increase in the ratio N-15/N-14. The causes of these changes are not resolved at this time but may include (1) a change in acceleration conditions of the solar wind, (2) a change in flux of solar energetic particles relative to that of the solar wind, and (3) a change in composition of the solar convective zone. There is good evidence for a long-term decrease in the solar-wind flux.

Description: Solar noble gases in an ilmenite separate from breccia 79035 (antiquity greater than 1 Ga) were analyzed by closed system stepped etching (CSSE). All five gases show the familiar two-component structure: first solar-wind (SW) gases are released, followed by gases from solar energetic particles (SEP). Element patterns in 79035 are similar to those of 71501 ilmenite. SW-He-Ne were partly lost, but SEP-He-Ne-Ar are retained (nearly) unfractionated. Constant Ar/Kr/Xe ratios indicate that ilmenites contain an unfractionated sample of the heavy SW-SEP noble gases. Ar/Kr/Xe ratios in the solar corpuscular radiation are, however, different from 'solar system' values, whereby the Kr/Xe difference in 79035 is about twice as large as in 71501. We propose that Xe is less fractionated than Kr and Ar, though its first ionization potential (FIP) is higher than the 'cutoff' at approximately 11.5 eV, above which all elements in SEP are usually assumed to be depleted by a roughly constant factor. SW-Ne may be isotopically slightly heavier in the ancient SW trapped by 79035, as proposed earlier. In this work we extend our previous CSSE studies of solar noble gases including Kr and Xe to a lunar sample irradiated at least 1 Ga ago (breccia 79035, ilmenite separate, 42-64 microns). This sample was particularly gently etched in the first steps. Surprisingly, the first three steps, each releasing less than or equal to 0.5% of the total 36-Ar, showed an SEP-like trapped component plus relatively large concentrations of cosmogenic gases. Steps 4ff contain much less cosmogenic and more solar gas with a SW-like isotope pattern. Thus, a very minor easily etchable phase that has completely lost its SW-gases must be responsible for steps 1-3. We will not discuss these steps here and refer to the actual step 4 as the 'initial' etching step.

Description: Roosevelt County (RC) 075 was recovered in 1990 as a single 258-gram stone. Classification of this meteorite is complicated by its highly unequilibrated nature and its severe terrestrial weathering, but we favor H classification. This is supported by O isotopes and estimates of the original Fe, Ni metal content. The O isotopic composition is similar to that of a number of reduced ordinary chondrites (e.g., Cerro los Calvos, Willaroy), although RC 075 exhibits no evidence of reduced mineral compositions. Chondrule diameters are consistent with classification as an L chondrite, but large uncertainties in chondrule diameters of RC 075 and poorly constrained means of H, L and LL chondrites prevent use of this parameter for reliable classification. Other parameters are compromised by severe weathering (e.g., siderophile element abundances) or unsuitable for discrimination between unequilibrated H, L and LL chondrites (e.g., Co in kamacite delta C-13). Petrologic subtype 3.2 +/- 0.1 is suggested by the degree of olivine heterogeneity, the compositions of chondrule olivines, the thermoluminescence sensitivity, the abundances and types of chondrules mapped on cathodoluminescence mosaics, and the amount of presolar SiC. The meteorite is very weakly shocked (S2), with some chondrules essentially unshocked and, thus, is classified as an H3.2(S2) chondrite. Weathering is evident by a LREE enrichment due to clay contamination, reduced levels of many siderophile elements, the almost total loss of Fe, Ni metal and troilite, and the reduced concentrations of noble gases. Some components of the meteorite (e.g., type IA chondrules, SiC) appear to preserve their nebular states, with little modification from thermal metamorphism. We conclude that RC 075 is the most equilibrated H chondrite yet recovered and may provide additional insights into the origin of primitive materials in the solar nebula.