Xenon compositions of magmatic zircons in 3.64 and 3.81 Ga meta-granitoids from Greenland - A search for extinct ²⁴⁴Pu in ancient terrestrial rocks

In order to solve the fundamental problem as to whether the extinct radioactive isotope ²⁴⁴Pu was present in the early Earth, we have analysed xenon in magmatic zircons from three early Archaean meta-granitoids from Greenland. Two samples, a granite (sample G97/111) and a ferrogabbro/ferrodiorite (sample G97/112), are from the same ca 3.64 Ga composite intrusion at the mouth of Ameralik fjord. The third sample (sample G97/018) is a ca 3.81 Ga tonalite from a locality south of the Isua supracrustal belt. The 3.81 Ga tonalite (sample G97/018) and possibly the 3.64 Ga granite (sample G97/111) magmatic zircons appear to have small excesses of fission xenon different from the ²³⁸U-derived spontaneous fission xenon. This excess fission xenon is consistent with that expected for ²⁴⁴Pu-derived spontaneous fission, and it may indicate that these zircons incorporated ²⁴⁴Pu when they crystallised. If this is true, it suggests the existence of ²⁴⁴Pu early in the Earth's history. Furthermore, our preliminary investigation supports views that the Earth's primitive material had a near-chondritic ²⁴⁴Pu/²³⁸U abundance ratio and that fissiogenic xenon in the atmosphere is of plutonic origin.