Rare gases and 36Cl in stony-iron meteorites: cosmogenic elemental production rates, exposure ages, diffusion losses and thermal histories
Abstract
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 {39Ar}/{36Cl} 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 {P38Ca}/{P21Mg} is close to unity. The ratios {P38Ca}/{P38Fe} vary between 20 and 50, and are not correlated with the absolute production rate of 38Ar from metal. The {22Ne}/{21Ne} 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 {20Ne}/{36Ar} 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.- Publication:
-
Geochimica et Cosmochimica Acta
- Pub Date:
- March 1976
- DOI:
- 10.1016/0016-7037(76)90213-1
- Bibcode:
- 1976GeCoA..40..353B
- Keywords:
-
- Chlorine;
- Iron Meteorites;
- Meteoritic Composition;
- Rare Gases;
- Stony Meteorites;
- Stony-Iron Meteorites;
- Argon Isotopes;
- Cosmic Rays;
- Helium Isotopes;
- Neon Isotopes;
- Radiation Dosage