G. F. Herzog

26Al and rare gases in Allende, Bereba and Juvinas: implications for production rates

Abstract The 26Al, light rare gas and major and minor element contents of Al-rich and poor samples separated from Allende. Bereba and Junivas have been measured. The production rate of 21Ne from Al (21PAl) is (1.9 ± 0.6) × 21 P Si and 22 21 P Al = 1.4 ± 0.4. The 3He, 21Ne and 38Ar exposure ages of the eucritic pyroxenes agree suggesting complete cosmogenic gas retention. The eucritic feldspars have lost virtually all 3He and most radiogenic 4He. The equation 26Al = 0.42 ± 0.41 Mg + 2.74 ± 0.21 Si + 4.92 ± 0.51 Al + 1.33 S + 0.24 Ca + 0.03 Fe reproduces within 15% our 26Al measurements and the average values measured in ordinary chondrites without recourse to unusual cosmic-ray effects.

40Ar39Ar ages of Allende

KAr and/or 40Ar39Ar plateau ages of Allende samples—whole rock, matrix, chondrules, white inclusions–range from 3.8 AE for matrix of ≳5 AE for some white inclusions, but cluster strongly near 4.53 AE. This age marks the dominant KAr resetting of Allende materials. Age spectra show disturbances due to 39Ar recoil or some other argon redistribution processes. Possible explanations for the apparent presolar ages (>4.6 AE) include: ≳20% loss of 39Ar; ≳40% loss of 40K ∼3.8 AE ago with no loss of 40Arl trapped argon of unique 40Ar/36Ar isotopic composition; admixture of “very old” presolar grains.

The reaction Mg(n,α)Ne at 14.1 and 14.7 MeV: cross sections and implications for meteorites

Abstract Mass spectrometric analyses of neutron-irradiated targets of natural magnesium yield cross sections of59 ± 14,160 ± 8, and11.0 ± 3.3mb for 20 Ne, 21 Ne, and 22 Ne, respectively, at 14.1 MeV and of94 ± 8,152 ± 12, and13.0 ± 2.0mb at 14.7 MeV. With the incorporation of these cross sections, calculations modeling cosmic-ray interactions in stony meteoroids of radii 20 and 26 cm predict that between the surface and center the 22 Ne/ 21 Ne ratio falls more than 10% while the 21 Ne production rate rises about 30%. The reaction 24 Mg(n,α) 21 Ne predominantly controls these trends: the 22 Ne/ 21 Ne ratio due to magnesium decreases over 15% while that due to silicon remains constant with increasing depth. The calculations are compared with published neon measurements for the Keyes and St. Severin meteorites.

Cosmic-ray constancy and cosmogenic production rates in short-lived chondrites

Abstract Five chondrites with short cosmic ray exposure ages ( 26Al and 53Mn saturation values deduced from short-lived chondrites alone agree within 20%, with those deduced from all chondrites. Values of 21p based on 53Mn and 26Al are 0.30 ± 0.03 and 0.46 ± 0.05, respectively, in H chondrites. Possible causes for the difference include half-life errors, data selection and a recent (≲2.5Myr) increase in the cosmic ray intensity.

Thermal metamorphism of primitive meteorites. VIII - Noble gases, carbon and sulfur in Allende /C3/ meteorite heated at 400-1000 C

Abstract Noble gases, C and S are lost from Allende samples heated for 1 week at temperatures of 400–1000°C in a low pressure environment. In the extreme, losses of 3 He and 4 He are ~ 100 × while for C. S and Ne, Ar and Kr isotopes and 132 Xe. these are ≤10 ×. Except for He, these losses are less severe than those of Bi or Tl from samples heated in the same runs. Significant He. Ne and Ar isotopic fractionation during heating indicates preferential outgassing of specific reservoirs. Apparent activation energies for all species generally indicate loss controlled by a diffusive process. Next to He, 40 Ar is the most labile of those species considered here but still less so than Bi or Tl. L-group (but not H- or LL-group) chondrites may have lost mobile elements like Tl while being outgassed after late impact-associated heating. A less likely alternative possibility involving a collateral relation between condensation conditions and depth in a parent object may also explain the L-group trend.

26AI in stony meteorites with gas losses

Abstract 26 Al contents of 16 meteorites with 3 He losses have the same average values as all chondrites indicating that gas losses did not occur in a recent, prolonged exposure to unusual cosmic-ray activity.