Are There Cosmochemical Fingerprints in the Earth Upper Crust?

The Earth's upper continental crust is the Earth's minor repository of the compatible noble metals Os, Ir, Ru, Pt, Rh, and Pd. However, the noble metal composition is essential for understanding its origin and the fundamental processes by which it formed. An estimation of the average noble metal composition is a difficult challenge because of the geological complexity and therefore heterogeneous composition of the exposed Earth's crust as well as the low contents in the pg/g to ng/g range. Preliminary estimates of Os, Ir, Ru, Pt, Rh, Pd, and Ni contents of the upper crust show that these elements are highly fractionated, compared to Earth's upper mantle and CI-chondrites. The abundance distribution apparently indicates two groups of elements; (1) Os and Ir and (2) Ru, Pt, Rh, Pd, and Ni. Interestingly, the fractionation pattern from IIIAB iron meteorites and pallasites are comparable with a compilation of the present upper continental crustal data suggesting processing of the Earth's crust by large impact events. One possible explanation is that the noble metal and Ni systematics of the continental upper crust were established during magmatic crystallisation, resulting in fractionation of the compatible from the more incompatible elements during cooling of IIIAB iron and/or pallasite parent bodies. The upper continental crust probably preserves an imprint of some of the major fractionation processes which have occurred in these types of meteorites.