A search for evidence of a fissionable nuclide in iron meteorites and implications on heat sources in planetary cores

Abstract

It is shown that nuclear fission could not have significantly contributed as a heat source to drive an early lunar core dynamo. Furthermore, contributions of any fissionable nuclide to the energetics of the earth's core today are very unlikely in comparison to other sources of energy. These conclusions are inferred from searches for records of fission events in the metal phase of IA and IVB meteorites. We obtain strict upper limits for the concentration level of Nd and Sm in the Canyon Diablo (IA), Pin˜on (anom.), and Santa Clara (IVB) meteorites which are not distinguishable from the levels of the procedural blank and exhibit a normal isotopic composition. No contribution from fission to the unshielded isotopes could be detected. Upper limits for contributions by fission are 10 8 atoms g −1 per isobar. Assuming a low fission yield of 10 −4, we obtain 10 12 fission events g −1. It is reasonable to assume no losses of REE fission products due to late stage recrystallization or outgassing. This limit would correspond to a maximum deposition of 30 J g −1 in the earth's core due to fission. From earlier experiments on Kr and Xe in other IA iron meteorites, upper limits for contributions by fission of 3 × 10 5 and 2 × 10 6 fission events g −1 per isobar can also be estimated for the unshielded isotopes. Recent studies on nuclear tracks in silicate inclusions of IA iron meteorites set upper limits of 10 9–10 10 fission events g −1, the detected tracks being most plausibly explained by spontaneous fission of 244Pu. We can find no evidence in support of a fissionable, siderophile superheavy element and all limits for fission events are too low to support fission as a heat source in planetary cores. Furthermore, by Nd and Sm studies on two IVB iron meteorites, we demonstrate that Ag anomalies found earlier in meteorites of this class cannot have been produced by in situ fission of any known nuclide. This further supports the interpretation that extinct 107Pd is the source of the 107Ag anomalies.