Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.421r&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 421
Other
Carbon-14, Cosmic Rays, Exposure Ages, Cosmogenic Nuclides, Mesosiderites, Radionuclides, Vaca Muerta
Scientific paper
We have studied the production of 5730-yr ^14C in samples of the VM10 mass of the mesosiderite Vaca Muerta. This ~165-kg mass was buried with only one surface exposed at ground level. Because of heavy corrosion, it could only be excavated in pieces ranging in size up to 4 kg. Our samples ranged from the exposed surface to about 25 cm depth. These samples were analyzed for ^14C content at the University of Arizona by accelerator mass spectrometry, and their elemental compositions were measured. The results are compared to calculations of ^14C in Vaca Muerta produced by cosmic rays, and constraints on the recent exposure record of Vaca Muerta, such as terrestrial age and preatmospheric size, are obtained. Five of the samples were fairly pure silicate, and three other samples were about 40-50% silicate. The major target for ^14C, oxygen, is ~43% by weight in the silicate. The results of surface silicate inclusions give ^14C of 37-43 dpm/kg. Samples of mesosiderite, ~45% metal, are about 23 dpm/kg ^14C at the same surface. Production rates of ^14C were calculated using the Los Alamos Monte Carlo LAHET Code System (LCS). LCS has yielded calculated production rates for ^14C and other nuclides that are in good agreement with cosmogenic-nuclide measurements in meteorites [1,2]. Earlier calculations showed that the bulk composition of a meteorite affects production rates of cosmogenic nuclides [1]. In the calculations done for Vaca Muerta, spherical objects with radii of 50 and 70 cm, a density of 5 g/cm^3, and typical bulk mesosiderite compositions were irradiated with galactic-cosmic-ray protons. Layers 2.5 cm thick were used. The calculated fluxes for each layer were then multiplied by the relevant cross sections and integrated over energy to get ^14C production rates for pure silicate and for the other samples. The calculated production profiles increased from the surface to depths of ~20-30 cm. In the 50-cm object, the profile was fairly flat to the center, but the profile dropped slightly for depths >30 cm in the 70-cm object. Production rates for ^14C were about 60 atoms/(min kg) in pure silicate but only a few atoms/(min kg) (not well determined because of the lack of cross sections) in metal. The rates for a 50-cm radius were ~10% higher than for the 70-cm radius. The ^14C measurements and the calculated production rates are only consistent with a meteoroid of short terrestrial age, as practically none of the ^14C can have decayed. The ^14C must have been produced in a body not substantially larger than a radius of 70 cm and probably not more than ~100 cm. This indicates a lack of significant erosion of the meteoroid in the last 20,000 years. If the current sampled surface were actually at some depth in the preatmospheric meteoroid, then some terrestrial age of up to about 2500 years is possible using these data. However, data from deeper samples argue against any significant terrestrial age. With some additional studies of samples at additional depths and other cosmic-ray-produced isotopes, particularly ^36Cl and Ne isotopes to constrain shielding, we may be able to derive a better model for the preatmospheric irradiation history. Acknowledgments: This work was supported by NASA, and the work at Los Alamos was done under the auspices of the U.S. Department of Energy. References: [1] Masarik J. and Reedy R. C. (1993) LPS XXIV, 937. [2] Reedy R. C. et al. (1993) LPS XXIV, 1195.
Donahue Douglas J.
Masarik Jozef
Reedy Robert C.
Timothy Jull A. J.
Wasson John T.
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