Variability of the Al26 production rate in ordinary chondrites

Gregory F. Herzog, Philip J. Cressy

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Abstract

15 ordinary chondrites for which unusually high spallogenic Ne22 Ne21 or He3 Ne21 ratios had been reported and one meteorite with marked shock characteristics were selected in order to investigate the relations between Ne22 Ne21 ratios, Al26 contents and depth. We report Al26 and K contents of 13 samples from 11 of these and-noble gas contents of 30 samples from all of these stones. A decrease in the Al26 production rate accompanies the increase of Ne22 Ne21 towards the pre-atmospheric surface: Alobs26 Alcalc26 = 3.2-2.0 Ne22 Ne21 for 1.08 ≤ Ne22 Ne21 ≤ 1.2. Large deviations from this relationship may indicate that a meteorite experienced an abnormal flux of cosmic rays. For Ne2 Ne21 > 1.2 this trend continues but the data scatter more, probably because of the steadily increasing influence of pre-atmospheric size. Ne22 Ne21 ratios increase most rapidly in the outermost few centimeters according both to a plot of Ne22 Ne21 vs (recovered mass) 1 3 and to track studies. The increase seems to derive from the enhanced importance of nuclear reactions on Si. Ne22 Ne21 < 1.08 defines a region where the Al26 production rates are less sensitive to depth and vanish in the limit of large shielding; the weak correlation between Ne22 Ne21 and Al26 in this region rules out the use of the Ne22 Ne21 ratio as a basis for a shielding correction to Al26.

Original languageEnglish (US)
Pages (from-to)1827-1841
Number of pages15
JournalGeochimica et Cosmochimica Acta
Volume38
Issue number12
DOIs
StatePublished - Dec 1974

All Science Journal Classification (ASJC) codes

  • Geochemistry and Petrology

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