Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 242

of ice and dust. Spinel associated with the K--T impact, however, suggests it was
formed by ablation of a large object in the lower atmosphere suggesting that
this was anasteroid rather than a comet.
Iridium and other platinum-group elements (PGE) as indicators
of extraterrestrial impacts
Iridium is one of the platinum-group elements. Iridium concentrations
are very low in the Earth's crust, and high iridium values in crustal rocks can
be assumed to be an indicator of a bolide impact. The most famous iridium-rich
anomaly is associated with the Cretaceous--Tertiary boundary but iridium-rich
anomalies have also been associated with other boundaries in the stratigraphic
record. Other element anomalies have also been detected in the platinum-group
elements. The platinum-group elements can be divided into two groups, the Ir
group (Iridium, Ir; Ruthenium, Ru; Osmium, Os) and the Pt group (Platinum,
Pt; Rhodium, Rh; Palladium, Pd; Gold, Au). There is a continual input of Ir
and other PGE from micrometeorites as a relatively constant background accu-
mulation rate occurs in marine sediments. However, at times the stratigraphic
record shows that this background is modified periodically by large asteroids and
comets. The anomalies assumed to arise from impacts of asteroids and comets
are particularly evident in deep-sea sediments, and may be detected worldwide.
Extraterrestrial material may not only supply Ir to seawater in detrital form, but
also as a result of vaporisation of dust in the atmosphere and shock vaporisation
of impacting large asteroids. Material enriched in Ir and other PGE does not only
originate from the asteroid or comet itself, but from the impact ejecta. Whereas
oceanic impacts result in relatively 'pure' extraterrestrial material, terrestrial
impacts include both extraterrestrial and target material in the impact ejecta
from the centre of the developing crater. Thus, in these cases, the abundance
of Ir and other PGE depends not only on the asteroid composition, but also on
the type oftargetrockand the relative masses of asteroid and ejecta materials
(Sawlowicz, 1993).
Although it has been assumed that iridium anomalies are related to asteroid
impacts, it has been suggested that natural processes also have the potential to
cause similar anomalies. Volcanism in particular is a major process of Ir enrich-
ment. Enormous amounts of basalt, known as the Deccan Traps, were erupted
throughout western India around the time of the K--T boundary. Although such
events can possibly supply enough Ir to cause a marked enrichment, the con-
tent of Ir in the Deccan Basalts is low, and it is unlikely, therefore, to have
contributed sufficient Ir to produce the widespread increase in Ir at this time.
Notall volcanic products, however, are necessarily low in Ir. Ir can be released