Recent studies on iron meteorites. IV The origin of meteoritic helium and the age of meteorites
Abstract
The rate of generation of helium in iron, by nuclear evaporation processes initiated by cosmic radiation, is recalculated with particular reference to the problem of the helium content of iron meteorites. The absence of a significant "depth effect" (postulated by BAUER to arise through the exponential absorption of the primary radiation), is shown to be explicable for small meteorites in terms of helium production by secondary particles (mostly π mesons). Curves are presented showing the calculated helium content as a function of radial distance, and these are in good quantitative agreement with the experimental data of CHACKETT, REASBECK and WILSON. The meteorite (Morden) with the highest known helium content is shown to require a (cosmic-ray flux) × (time) product about three times the product (present day flux in neighbourhood of earth's orbit) × ("age" of helium-poorest meteorite, as deduced from radioactive age calculations). This may represent a genuine spread in "ages", although it seems more reasonable to explain it in terms of a theory of cosmic ray production such as BAADE and ZWICKY'S super nova mechanism which would require the mean flux to be several times the flux measured (as to-day) in the absence of an active "local" super nova. A comparison of the actual sizes of meteorites with the pre-atmospheric sizes necessary to explain the observed helium contents is shown to be in good agreement with the astro-ballistic data of THOMAS and WHIPPLE. In particular, the Treysa meteorite, for which observational data are available concerning the fall, provides a most satisfactory test of the present calculations.
- Publication:
-
Geochimica et Cosmochimica Acta
- Pub Date:
- June 1953
- DOI:
- 10.1016/0016-7037(53)90037-4
- Bibcode:
- 1953GeCoA...3..288M