Ir is a sensitive tracer of extraterrestrial matter because of its strong depletion in the Earth
crust (
0.05 ng/g) compared to chondritic abundance (
500 ng/g). Most of the
Earth Ir is probably stored along with Fe in the core. The positive Ir anomaly supporting the
impact of a 
10 km asteroid or comet at the end of the Cretaceous [ Alvarez et al.,
1980] is now detected at > 100 marine and continental KT boundary sections all around the
world. The Ir is probably present at the KT boundary in extremely fine particles, but the carrier
phase has so far not been identified. The finding of Ir at the KT boundary has led to searches
for Ir anomalies throughout the Phanerozoic, in particular at stratigraphic horizons associated
with mass extinctions.
So far, considering its magnitude and worldwide distribution, the KT boundary Ir anomaly appears to be a unique event in the sedimentary record. Small Ir anomalies (< 0.5 ng/g) have been detected at stratigraphic boundaries such as the Precambrian-Cambrian, Late Devonian, Devonian-Carboniferous, Permo-Triassic, Cenomanian-Turonian, but there is insufficient evidence at this point to clearly distinguish between terrestrial and impact enrichment processes. In many of these cases the platinoid elements are not in chondritic proportions and in some cases the Ir enrichments appear related to anoxic conditions. Orth et al. [1990] have summarized in detail the searches and potential explanations for Ir anomalies in the rock record.
Cenozoic tektite layers were believed not to be enriched in Ir. However, Schmidt et al.
[1993] have recently found a small but detectable Ir enrichment (
0.2 ng/g) in Ocean
Drilling Project core 758B from the Ninetyeast Ridge (Eastern Indian Ocean) and in Ocean
Drilling Project core 769 in the Sulu Sea at the level where the Australasian microtektites are
found. Based on Ir concentration and microtektite abundance they proposed that the Australasian
impactor could have excavated a crater between 15 and 19 km in diameter.