H. Hintenberger

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.

Noble gases in the Haveröureilite

Abstract The rare gases He, Ne, Ar, Kr and Xe were measured in two bulk samples and one of dark inclusions from the Haveroureilite. The3He- and21Ne-exposure model ages are found to be 18 my and 13.5 my, respectively. In addition to a cosmogenic component, the bulk samples contain heavy primordial gases of the fractionated or ‘planetary’ type in amounts typical of ureilites. It is shown that they consist of at least two components, one of which has been found to reside in the dark inclusions. In these graphite-diamond-kamacite intergrowths primordial Ar is more abundant by a factor of 29, Kr and Xe about 20 fold. While the elemental abundance pattern clearly shows the gases to be of the fractionated type, the20Ne/22Ne-ratio is close to the solar value;36Ar/38Ar = 5.23 and40/36Ar ≤ 0.15. Furthermore, the depletion factors of the rare gases show a striking correlation with the respective ionisation energies.

Primordial gases in graphite-diamond-kamacite inclusions from the Haveröureilite

Abstract Stepwise heating experiments on separated graphite-diamond-kamacite aggregates have revealed a pronounced difference in the release patterns of spallogenic 3 He and trapped gases. About half the 3 He is released at T ≲ 920°C , without being accompanied by significant amounts of primordial gases; the latter, together with the remaining 3 He, is given off only at T ≳ 1200°C . Acid treatment of an aliquant dissolved about 2/3 of the total Fe in the sample but did not cause a significant change in the gas concentrations. It is concluded that (a) there is no evidence for a loss of spallogenic 3 He from the graphite-diamond-kamacite aggregates, (b) one major constituent of the aggregates - graphite - is almost void of trapped gases, (c) kamacite is not a main carrier of the gases. This leaves diamond as the most probable site of the primordial gases. The elemental abundance pattern in the noble gases is essentially as reported previously. In particular, the excellent correlation between relative depletion factors, normalized to the cosmic abundance ratios, and the respective ionisation energies is confirmed. Other important features of the trapped gases are a 20 Ne/ 22 Ne ratio of 12.3 ± 0.6, intermediate between solar wind and solar flare implanted Ne, 36 Ar/ 38 Ar = 5.20 ± 0.06 and a measured 40 Ar/ 36 Ar ratio (before blank correction) of 0.0076. Possible modes of trapping of the noble gases are discussed.

Chemical composition and rare gas content of four new detected Antarctic meteorites

Abstract Within a region of 5 km × 10 km on a downhill slope of the Yamato Mounties, in 1969 the Japanese Expedition Team collected many stones. 9 of them were recognized as meteorites. On 4 of these findings we determined the chemical composition and the rare gas content. According to the mineralogical and the chemical composition, Yamato (a) is an enstatite chondrite, Yamato (b) a Ca-poor achondrite, Yamato (c) seems to be a carbonations chondrite Type III and Yamato (d) a olivine bronzite chondrite. Exposure ages are 1.7, 31, 25 and 4.3 my respectively.

Rare Gases, Hydrogen, and Nitrogen: Concentrations and Isotopic Composition in Lunar Material

The concentrations and isotopic abundances of the rare gases have been investigated in fines and three types of rocks. The results obtained from different grain-size fractions and from samples etched to different degrees with nitric and hydrofluoric acids demonstrate the strong concentrations of the solar-wind component in the surface layers of the grains. Exposure ages as well as gas retention ages have been determined in different types of Apollo 11 material. Hydrogen, nitrogen, and other gases have been analyzed by a high-resolution mass spectrometer. As compared with that in terrestrial water, deuterium is depleted by at least a factor of 3 in the investigated type C rocks.

The concentrations of the heavy metals in the four new Antarctic meteorites Yamato (a), (b), (c) and (d) and in Orgueil, Murray, Allende, Abee, Allegan, Mocs and Johnstown

In the four meteorites Yamato (a), (b), (c) and (d) recently found in Antarctica, the trace elements W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Th and U were determined by spark source mass spectrometry. The concentrations of these elements were also measured in the meteorites Orgueil (C1), Murray (C2), Allende (C3), Abee (E4), Allegan (H5), Mocs (L6) and Johnstown (Ah). The trace element concentrations confirm the preliminary classification of the Yamato meteorites already given earlier: Yamato (a) = enstatite chondrite, Yamato (b) = Ca-poor achondrite, Yamato (c) = carbonaceous chondrite of type 3, Yamato (d) = ordinary chondrite. The concentrations of Au and Hg are antipathetically correlated. In the achondrite Yamato (b) the concentrations of the siderophile elements W, Re, Os, Ir, and Pt depend strongly on the boiling points of these metals.

The Antarctic meteorite Yamato 74123 — a new ureilite

The rare gases He, Ne, Ar, Kr and Xe were measured in bulk samples of Yamato 74123. The 3He and 21Ne exposure ages are found to be 5.50 Ma and 2.83 Ma, respectively. In addition to the cosmogenic component the samples contain primordial rare gases of the fractionated type in amounts typical of ureilites. In a three-isotope plot neon turns out to be a mixture of planetary neon and cosmogenic neon. The elements Na, Mg, Al, Si, P, S, K, Ca, Cr, Mn, Fe, Co, and Ni have been determined by spark source mass spectrometry in Yamato 74123 and for comparison in the ureilites Haveroand Kenna. The chemical composition as well as the noble gas abundance pattern identify Yamato 74123 as an ureilite.

Massenspektrometrische Bestimmung der Spurenelemente in Steinmeteoriten

In den letzten Jahren wurde ein Verfahren zur Bestimmung aller schweren Spurenelemente von W bis U durch Festkorper-Massenspektrometrie mit Funkenionenquellen erarbeitet. Zur Bestimmung der Empfindlichkeitsfaktoren wurde der Allende-Standardmeteorit herangezogen. Die Konzentrationen der siderophilen schweren Elemente W, Re, Os, Ir, Pt und Au sind in gewohnlichen und kohligen Chondriten untereinander streng korreliert, wahrend Bi zu den siderophilen Elementen antikorreliert ist. Enstatitchondrite verhalten sich etwas anders.