F. Begemann

Rare gases and 36Cl in stony-iron meteorites: cosmogenic elemental production rates, exposure ages, diffusion losses and thermal histories

Metal and silicate portions from 13 mesosiderites, one pallasite, Bencubbin (“unique”) and Udei Station (‘iron with silicate inclusions’) have been analysed for their content of He, Ne and Ar; in most cases 36Cl could be determined as well. 36Cl-36Ar cosmic ray exposure ages fall between 10 and 160 Myr. Half of the metal samples show a deficit of spallogenic 3He (up to 30%) which we ascribe to a loss of tritium. The observed depletion of 3He in the silicates is correlated with their mineralogical composition: feldspar has lost its 3He in all cases, pyroxene definitely in one and possibly in five others, while olivine has been affected in only two meteorites. The thermal histories during their exposure to the cosmic radiation have been different for different meteoroids. Nevertheless, with the exception of Veramin, the data are compatible with the assumption of a continuous diffusion loss during a considerable fraction of the exposure era. For Veramin, however, an episodic event late in the exposure history is required. The exceptionally high 39Ar36Cl ratio in the metal, which is due to a high 39Ar activity, indicates that the event occurred during the last 500,000 years or so and resulted in an extremely excentric orbit (large aphelion). Production rates of 38,39Ar from Ca and 21,22Ne from Mg are given. The ratio P38CaP21Mg is close to unity. The ratios P38CaP38Fe vary between 20 and 50, and are not correlated with the absolute production rate of 38Ar from metal. The 22Ne21Ne production ratio from Mg is found to be close to but below unity. Of the mesosiderites only Veramin shows unambiguous evidence for primordial rare gases with larger amounts and a higher 20Ne36Ar ratio in the olivine, suggesting in situ fractionation to have at least been partly responsible for the abundance pattern found. Bencubbin contains large amounts of strongly fractionated primordial gases, but again part of the fractionation may have occurred in situ. Udei Station shows an excess of (3.5 ± 0.6) × 10−10 cm3 STP 129Xe/g in the non-magnetic portion.

Shock induced thermal metamorphism and mechanical deformations in the Ramsdorf chondrite

The grey hypersthene chondrite Ramsdorf—which has short and concordant 4He-U/Th- and 40Ar-40K-gas retention ages—shows unambiguous evidence for severe thermal and mechanical alterations. 1. (1) The distribution of metal and troilite is very inhomogeneous. Both are found together in rather large, millimeter-sized patches near slickenside “veins”. 2. (2) The normal separation of the metal into individual grains of Ni-poor kamacite and Ni-rich taenite is lacking. Instead, it occurs as rounded globules (d ≲ 1 mm) with an interior Ni-content of about 11 per cent and a Ni-rich rim. We believe these rims not to be due to solid state lattice diffusion but to crystallization of metal from the melt. 3. (3) The silicate portion is extensively “recrystallized”: only occasionally chondrule remnants can be discovered; the feldspar (12 per cent An) has been transformed into a clear isotropic glass, which must have been a liquid melt proper. 4. (4) The pyroxenes are clinopyroxenes and are high (up to 16·3 per cent) and variable in CaO, the highest values being found at the pyroxene-glass interfaces. In about 30 per cent of all cases the CaO-content falls into the immiscibility gap of the enstatite-ferrosilite-diopside-hedenbergite system. 5. (5) Chromite occurs as small euhedral crystals in the glassy silicate matrix and as larger badly “corroded” crystals at the silicate-troilite boundaries. 6. (6) Some pyroxene crystals have been broken and the fragments displaced over short distances. In some metal globules shearing forces have shifted portions of the globules over distances up to 300 microns. At these shearing planes the Ni-rich rim is missing indicating the mechanical deformation to be a late feature acquired after the thermal alterations. It is proposed that the observed features came about by the following sequence of events: Shock heating with a mean residual temperature of about 1200°C, homogenization and redistribution of a metal + troilite melt by acceleration forces, rapid cooling to about 900°C within less than one month, a secondary collision causing the mechanical deformations. The rapid cooling indicates Ramsdorf to have had a radius of less than 2 m—more probably less than about 30 cm—at that time. At present it cannot be decided unambiguously whether this sequence of events occured 4 Ma ago-the cosmic ray exposure age-or 400 Ma ago—the radiogenic gas retention age.

On neutron-induced and other noble gases in Allende inclusions

Abstract Noble gases have been measured in one bulk sample, six light inclusions, and one black xenolith from four specimens of the carbonaceous chondrite Allende which cover a range in their 60Co content of about a factor of ten. The release pattern obtained upon heating the samples at different temperature steps shows: 1. 1.) The parent nuclides of radiogenic 4He are finely dispersed throughout the inclusions in host minerals with grain sizes smaller than the typical range of α-particles within them; 2. 2.) Neon from the inclusions contains a component deficient in 20,21Ne which is most conspicuous at low temperatures. The apparent excess 22Ne∗ is attributed to spallation reactions on sodium although the reasons for excluding NeE are weak; 3. 3.) 36Ar/38Ar ratios up to 90 are observed at release temperatures around 800°C. For inclusions from the same specimens the excess 36Ar∗ is proportional to the chlorine content, for different specimens the chlorine-normalized amounts of 36Ar∗ tend to correlate with 60Co; 4. 4.) 80,82Kr are found enriched about 80 fold, the ratio of the excess amounts 80 Kr ∗ 82 Kr ∗ is 2.68 ± 0.06; 5. 5.) Xenon is dominated by 129Xe, the overabundances are accompanied by much smaller excesses of 128Xe. The ratio of the excess amounts 128 Xe ∗ 129 Xe ∗ is constant for inclusions from the same specimens while for different specimens there is a range of a factor of four. The abundance anomalies in Ar and Kr as well as that of 128Xe can be explained as being due to the capture of neutrons by Cl, Br, and I, respectively, during the exposure of the meteoroid to the cosmic radiation. There is no compelling evidence for a contribution from an early neutron irradiation nor, in the case of 36Ar, for a contribution from extinct 36Cl. The ratio of the specific excesses 36 Ar ∗ 128 Xe ∗ as well as the 80 Kr ∗ 82 Kr ∗ ratio indicate no more than ~20 percent of the neutrons to have been thermal neutrons.

PuXe dating of eucrites

Abstract We report the results of Xe analyses of the eucrites Bereba, Cachari, Caldera, Camel Donga, Chervony Kut, Ibitira, Jonzac, Juvinas, Milbillillie, Moore County, Padvarninkai, Pasamonte, Pomozdino, Sioux County, and Vetluga, and the howardite Petersburg. The obtained data together with data from the literature were utilized for the calculation of PuXe ages using a modified method where the concentration of the reference element Nd (that is used in the traditional PuXe dating) is replaced by that of spallation 126 Xe originating from rare earth elements. The fraction of spallation 126 Xe from REE in total spallation 126 Xe is calculated via the Ba/REE ratios that are very similar in different samples of a given meteorite. In comparison with the traditional PuXe dating the present method avoids problems of sampling heterogeneity and uncertainties in absolute instrument sensitivity and can be used when the concentration of Nd in a sample is not known. The PuXe ages for different samples of individual meteorites show good reproducibility. Ages of the eucrites Bouvante, Chervony Kut, Ibitira, Juvinas, Moore County, Pasamonte, PCA82502, Pomozdino, of the howardite Petersburg, and of the angrite LEW86010 are old and most likely date the time of crystallization at ∼4.56 Ga. Almost complete retention of fission Xe in these meteorites is in a stark contrast with the apparent ∼1‰ retention of radiogenic 129 Xe. The eucrites Bereba, Cachari, Caldera, Camel Donga, Jonzac, Millbillillie, Neuvo Laredo, Padvarninkai, Sioux County, Stannern, and Vetluga are distinctly younger. This can be interpreted in different ways: the ages may represent crystallization and/or metamorphic ages or may simply reflect the different extent of diffusive fission Xe losses during later events. Comparison of the PuXe ages with those from the SmNd and PbPb isotope chronometers yields, in most cases, a reasonable agreement. In some meteorites, however, the PbPb chronometer appears to have been reset during secondary events while the PuXe isotope system is seen to be more resistant to such events.

ISOTOPIC COMPOSITION OF XENON AND KRYPTON IN SILICATE-GRAPHITE INCLUSIONS OF THE EL TACO, CAMPO DEL CIELO, IAB IRON METEORITE

Noble gases, extracted by temperature-stepped pyrolysis from acid-resistant silicate, schreibersite, and graphite residues of inclusions from the IAB iron meteorite El Taco (Campo del Cielo), have been analyzed for their isotopic composition. The concentrations of spallogenic (light) gases agree with previous data on adjacent specimens. Nominal feldspar and pyroxene fractions have lost up to 80% of their 3He during, or towards the end of, the exposure of El Taco to the cosmic radiation. In olivine the 3Ne21Ne and NePNe ratios are as low as 2.18 ± 0.01 and 1.09 ± 0.03, respectively, presumably because the silicates were irradiated within a FeNi matrix under extremely heavy shielding. Argon, Kr, and Xe in the silicates are dominated by a trapped “planetary” component with 100 ≤ 36Ar132Xe ≤ 700 and 1.7≤ 84Kr132Xe ≤ 2.2. Xenon released from the silicates at extraction temperatures > 1000°C is isotopically close to ureilite Xe, while the high-T Xe from graphite and schreibersite is unlike any of the presently well-established types of Xe. The isotopic composition of this Xe can be generated as a mixture of mass-fractionated UXe, with light isotopes depleted by (2.2 ± 0.1) % per atomic mass unit, and 244PuXe. The fission component, 10% in the case of 136Xe, is not due to in situ-production; it must have been added prior to incorporation of the Xe into the El Taco parent body and presumably underwent the mass fractionation process together with UXe. From the silicates 127I(n, γ)-produced 128Xe∗ and radiogenic 129Xe∗ are released together, with the same 128Xe∗129Xe∗ ratio in nominal olivine, pyroxene, and feldspar although the maximum release from pyroxene occurs about 300°C higher than from feldspar. From this the cooling rate of silicates for the temperature range 1500-1200°C is deduced to be ≥100°C per million years. The 80Kr82Kr ratio of 79,81Br(n, γ)-produced 80,82Kr is 2.86 ± 0.04, in agreement with the value found in inclusions of the IAB Linwood but higher than observed in almost all stone meteorites.

On the isotopic composition of ruthenium in the Allende and Leoville carbonaceous chondrites

The isotopic composition of Ru has been measured mass spectrometrically in an acid resistant residue of the Allende meteorite and in one magnetic and three non-magnetic portions of type-B inclusions, two from Allende and one from Leoville. Normalized to 96Ru101Ru = 0.324851, all 98Ru101Ru and 99Ru101Ru ratios were found to be indistinguishable from the terrestrial values within 1.6 permil and 0.4 permil (3σ), respectively. Thus, in the sampled reservoirs we find no evidence for different relative amounts of radiogenic 98Ru and 99Ru from the decay of now extinct 4.2 Myr-98Tc and 0.21 Myr-99Tc. 100Ru101Ru and 104Ru101Ru were found to deviate from the terrestrial norm by more than 2σ (≤0.6 permil) in one and two cases, respectively. Since terrestrial ratios were occasionally compromised by molecular interferences to an even larger extent, these overabundances cannot be unambiguously attributed to nonlinear isotope anomalies in the samples. The 102Ru101Ru ratio is normal in two aliquants of the acid resistant Allende residue but higher than normal by up to 0.35 permil in all four inclusion samples. There is a possibility, although very remote, that these excesses are due to contamination by a 102Ru spike. A less conservative interpretation of the data and normalization to 104Ru101Ru rather than to 96Ru101Ru results in overabundances of the light p-process isotopes 96Ru and 98Ru in the non-magnetic portions of all three inclusions.

Comment on "The nature and origin of ureilites" by J. L. Berkley et al.

Abstract The record of primordial noble gases in ureilites imposes serious constraints on models of ureilite petrogenesis. It is argued that the inhomogeneous distribution between different mineral phases and the low 40 Ar 36 Ar ratio in particular can only be satisfied with difficulties in the model proposed by Berkley et al.

On the production by solar protons of 126Xe via Te (p,xn) reactions

Abstract Results are reported for cumulative thick-target yields of 126,128,130Xe upon irradiating metallic tellurium of natural isotopic composition with protons of ∼3.7 MeV, 10 MeV, 14 MeV, and 17 MeV, respectively. Combined with existing data we find, for a spectrum of solar protons as derived from lunar studies (R o = 100 MV), a spectrum-averaged yield η(126Xe)Te = 4.2 × 10−4126Xe atoms per incident proton and yield ratios η(128Xe)/η(126Xe) = 4.42 and η(126Xe)/η(126Xe) = 0.33. In order to account for the excesses of 126Xe occasionally observed, up to 3.0 × 10 −11 cm 3 STP/g in bulk lunar samples and 250 times smaller ones in stone meteorites, the product of tellurium concentration, in ppm, times the proton dose must have been 2 × 10 18 cm −2 and 8 × 10 15 cm −2 respectively. If the irradiated matter was a surface layer of a regolith, the 126Xe thus produced will be diluted, by mixing with deeper layers, to an extent that depends on time of exposure and rate of “gardening.” It remains to be shown, by analysing suitable samples for both xenon and tellurium, whether this production mode can quantitatively account for the excesses of 126Xe that are occasionally observed in lunar samples and gas-rich meteorites.