Cosmic-ray exposure ages of the ordinary chondrites and their significance for parent body stratigraphy

Abstract Concentrations and isotopic ratios of noble gases are reported for nineteen Japanese chondrites. Among those, Nio (H3-4) is a solar-gas-rich meteorite.U/Th - He ages are younger than K - Ar ages for all meteorites studied. Six of the nine L-chondrites give significantly young K-Ar ages, suggesting gas loss by impact shock heating. The remaining three L-chondrites and seven of the ten H-chondrites have K-Ar ages older than 4 Ga. The L-chondrite Nogata and the H-chondrites Numakai, Ogi and Higashi-Koen have concordant ages.Cosmic-ray exposure ages for six of the H-chondrites show clustering around the 6-Myr peak in the distribution of exposure ages, while those for the L-chondrites, ranging from 8.2 to 64 Myr, do not show clustering.Fukutomi (L4) contains trapped 36Ar in excess, 3.5 times enriched compared to the highest value so far reported for type-4 ordinary chondrites except solar-gas-rich chondrites. The 36Ar/132Xe and 84Kr/132Xe ratios fit along a mixing line between a planetary and a sub-solar (or argon-rich) component found in separates of E-chondrites [43], The Xe isotopic composition is identical with that in Abee and Kenna. The isotopic signatures suggest that this meteorite may contain mineral fragments bearing the noble gas component found in E-chondrites or ureilites. Fukutomi also contains 80Kr, 82Kr and 128Xe produced by epithermal neutron captures on 79Kr, 81Kr and 127I, respectively. From the neutron-produced Kr, the preatmospheric minimum radius is estimated to be 20 cm with an assumption of a spherical meteoroid.

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The Monahans chondrite and halite: Argon-39/argon-40 age, solar gases, cosmic-ray exposure ages, and parent body regolith neutron flux and thickness

Meteoritics and Planetary Science ◽

10.1111/j.1945-5100.2001.tb01813.x ◽

2001 ◽

Vol 36 (1) ◽

pp. 107-122 ◽

Cited By ~ 17

Author(s):

Donald D. BOGARD ◽

Daniel H. GARRISON ◽

Jozef MASARIK

Keyword(s):

Neutron Flux ◽

Cosmic Ray ◽

Parent Body ◽

Exposure Ages ◽

Cosmic Ray Exposure Ages

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Der Mond als Mutterkörper der Bronzit-Chondrite

Zeitschrift für Naturforschung A ◽

10.1515/zna-1966-1-215 ◽

1966 ◽

Vol 21 (1-2) ◽

pp. 93-110 ◽

Cited By ~ 1

Author(s):

H. Wänke

Keyword(s):

Age Distribution ◽

Cosmic Ray ◽

Parent Body ◽

Gas Content ◽

Rare Gas ◽

Rare Gases ◽

The Mean ◽

Gas Measurements ◽

Exposure Ages ◽

Cosmic Ray Exposure Ages

Knowing the cosmic ray exposure ages of a sufficiently large number of meteorites and using the earth as analyser with special assumptions, criteria can be found to distinguish between a lunar or asteroidal origin of meteorites. Several of the following arguments are based on new and unpublished results of rare gas measurements by HINTENBERGER, SCHULTZ und WÄNKE70.Bronzite-chondrites :1. Arguments for an origin near the surface of the parent body. a) Porosity of the chondrites 0—20%. b) Many bronzite-chondrites contain light primordial rare gases, originating from the exposure of the single meteorite grains to the solar wind. c) Primordial rare gas content always connected with light-dark structure. d) In the distribution of the cosmic ray exposure ages certain groupings can be distinguished. The age distribution of bronzite-chondrites with light primordial rare gases is identical with the distribution of the cosmic ray exposure ages of all bronzite-chondri-tes. The bronzite-chondrites containing primordial gas therefore are probably coming from the very upper layers, and the other bronzite-chondrites from somewhat deeper layers of their parent body.2. Arguments for an origin close to the earth’s orbit. a) Bronzite-chondrites with high cosmic ray exposure ages show a slight tendency to fall in the afternoon (noon until midnight) . b) For the bronzite-chondrites, which are morning falls (midnight until noon), diffusion losses of 3He and 4He are higher and more frequent compared to the afternoon falls. The reason for this can be found in a closer approach to the sun of the first ones. Hypersthene-chondrites do not show this effect. c) Bronzite-chondrites with light primordial rare gas content concentrate among the afternoon falls. d) The mean cosmic ray exposure age of the bronzite-chondrites is considerably lower than that of the hypersthene-chondrites.3. Arguments concerning the size of the parent body. Light primordial rare gas and their connection with light-dark structure indicate a parent body of the size of the moon or a large asteroid.None of these arguments are strictly conclusive. In some cases they are based on observations, which can only be obtained by using statistical methods. Most of these effects are close to the mean error. Adding, however, all observations together, a lunar origin of the bronzite-chondrites becomes nearly undoubtable. A lunar origin of stone meteorites was in recent times first proposed by URET 3.Hypersthene-chondrites :Hypersthene-chondrites with low cosmic ray exposure ages are rare among the morning falls. Their parent body therefore probably has to be found outside the earth’s orbit. Their distribution of the cosmic ray exposure ages may also lead to this conclusion. As proposed by ANDERS 4, the Mars asteroids could possibly be the parent bodies for the hypersthene-chondrites. Mars itself might however be considered also. A lunar origin of the hypersthene-chondrites seems to be completely out of question.

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