A search for evidence of superheavy-element fission in chondritic metal

There is evidence for the early melting of the Moon1,2, and Runcorn2–4 has concluded from palaeomagnetic data that a fluid core dynamo must have been present for ∼109 yr and that the decay of a siderophile superheavy element (SHE) with a half life of ∼108 yr may have been responsible for these phenomena3–5 Libby and co-workers6 have suggested that SHE-decay could have produced certain trace-element abundance patterns in iron meteorites. Several searches7–9 have failed to detect evidence for SHE decay in iron meteorites. We describe here a search for SHE in metal fragments within ordinary chondrites. The absence of excesses of tracks along the boundaries between olivines and metal grains allows upper limits to the SHE content of the metal of ∼2.5×10−13 to ∼2.5×10−15 kg per kg to be set at a time ∼4×109 yr ago, determined from the 244Pu fission track densities in chondritic phosphates.