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

Abstract

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.