Cosmic ray exposure ages of iron meteorites by the Ne²¹/Al²⁶method

Abstract

The Al26 and Ti44 contents of 10 iron meteorites were determined by γ‐γ coincidence counting of separated fractions. The Al26 contents, ranging from 0 to 2.70 dpm/kg, were lower by factors of 1.5 to 3 than literature values obtained by β‐counting. This discrepancy is apparently caused by a β‐emitting impurity that interferes in β‐counting but not in the γ‐γ coincidence method. The concentrations of He3, He4, Ne21, Ar36, and Ar38 were determined mass spectrometrically. From the Ne21 and Al26 content of these meteorites, the following cosmic ray exposure ages, in millions of years, were calculated (relative to Norfolk = 600 m.y.): Arispe = 120, Aroos = 1000, Duchesne = 200, Grant = 590, Odessa, 360, Sandia Mountains ≤ 210, Santa Apolonia = 840, Sikhote‐Alin = 320, Treysa = 530. In most cases, these values agree rather well with previously determined K41/K40 and Ar36Cl36 ages. The agreement with the K41/K40 ages, in particular, casts doubt on the notion that the exposure ages of iron meteorites are strongly affected by space erosion. Sandia Mountains and Arispe were found to contain noble gases in anomalous proportions. Apparently both these meteorites contain small amounts of radiogenic or primordial He4. Arispe, moreover, seems to have suffered a secondary breakup some 100 m.y. ago. Except for the observed falls, Aroos, Sikhote‐Alin, and Treysa, none of the above meteorites contained any detectable Ti44 activity. Evidently, the terrestrial ages of these meteorites are long in comparison with the half‐life of Ti44, 48 years. The absence of Ti44 activity in Norfolk shows that it cannot have been associated with a 1907 fireball, as had been previously supposed. Sandia Mountains appears to have a low Al26 activity, possibly due to a terrestrial age of the order of 106 years.

The constancy of galactic cosmic rays as recorded by cosmogenic nuclides in iron meteorites

“…Also, our CRE age of 385 ± 24 Ma for Sikhote‐Alin is in agreement with an earlier result of 320 ± 150 Ma (Lipschutz et al. ).…”

“…Published CRE ages for Grant range from 426 AE 9 Ma to 640 AE 100 Ma (cf. Lipschutz et al 1965;Lavielle et al 1999;Ammon et al 2008;Shankar et al 2011;Schaeffer and Heymann 1965). Here, we determined a CRE age of 642 AE 39 Ma.…”

Calibration of cosmogenic noble gas production in ordinary chondrites based on 36Cl‐36Ar ages. Part 1: Refined produced rates for cosmogenic 21Ne and 38Ar

“…) and the latter is constant at 15 ± 2 (Signer and Nier ; Lipschutz et al. ; Schultz and Hintenberger ). Based on this relationship, we conclude that the low 3 He c / 21 Ne c ratios in Mocs, La Criolla, and Richardton are due to high shielding conditions (see also above), while the low 3 He/ 21 Ne ratios in Mount Browne and Aumale are due to diffusive loss of 3 He, as shown by the low 3 He c (bulk)/ 38 Ar c (metal) ratio of approximately 10 for Mount Browne and a low 3 He c (metal)/ 38 Ar c (metal) ratio of between approximately 4–8 for Aumale.…”

Propagation of cosmic rays in the galaxy

“…has been the same as that averaged over 5 x 105 yrs within ±10%. Furthermore, the gross cosmic ray intensity has also remained unchanged within a factor two over extended periods in the past up to 109 yrs (Voshage and Hintenberger, 1963;Anders, 1965;Lipschutz et al 1965).…”