The distribution and geochemistry of platinum-group elements as event markers in the Phanerozoic

The discovery of the platinum-group element (PGE) enrichment in the Cretaceous-Tertiary (K-T) boundary impact layer popularized the use of PGEs as geochemical event markers and prompted an explosion of interest in these elements whose geochemical behavior is poorly understood. Instead of concentrating on the more easily analyzed PGEs (e.g., iridium), many studies now include the entire group. Analytical methods have been improved to the point where small samples (less than or equal to 5 g) can be analyzed confidently at the sub-ppb level, allowing the determination of the distribution of PGEs in the oceans, atmosphere and biosphere, The presence or absence of a PGE anomaly is often used as a criterion for determining whether an event horizon is impact-related, however, to date, only the K-T boundary yields a distinct PGE anomaly with positive interelement correlations and solar interelement ratios. Terrestrial sources such as volcanic emissions are also being investigated as potential contributors to PGE anomalies at other Phanerozoic event horizons. Primary signatures (e.g., interelement ratios), characteristic of the PGE source, may be altered during various high and;or low temperature processes. For example, fractionation during impact melting, vaporization/condensation, remobilization, redistribution and diagenesis can obscure original signatures and make determination of the PGE source difficult, Future research might be directed towards establishing the behavior of the PGEs during such processes. (C) 1997 Elsevier Science B.V.