THE EUROPIUM ANOMALIES IN BANDED IRON FORMATIONS AND THE THERMAL HISTORY OF THE OCEANIC-CRUST

The REE (rare-earth element) patterns in banded iron formations (BIF's) from various sites in North America, two sites in Finland and three drillcores of the Hamersley Basin in Western Australia have been investigated using ICP-MS (inductively coupled-plasma mass spectrometry) and INAA (instrumental neutron activation analysis). The results of this study as well as existing data show trends in the Eu anomalies: BIF's older than 2.5 Ga exhibit positive Eu anomalies while younger ones exhibit none. In chemical sediments between 2.5 and 2.3 Ga both trends can be found. Because of the worldwide distribution of BIF's this feature can be considered as a global change in the conditions controlling REE mobilization. Consideration of the redox conditions of BIF formation in comparison with the calculated and measured Eu redox equilibria suggests that a Eu anomaly points to a dominantly hydrothermal source of REE's with solution temperatures above 175-degrees-C. We suggest that the disappearance of the Eu anomaly towards the Proterozoic marks a change from mainly high-temperature hydrothermal to dominantly low-temperature alteration.