Open AccessOsmium isotopic compositions of Os‐rich platinum group element alloys from the Klamath and Siskiyou Mountains
Author(s) -
Meibom Anders,
Frei Robert,
Sleep Norman H.
Publication year - 2004
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2003jb002602
Subject(s) - radiogenic nuclide , geology , geochemistry , metasomatism , partial melting , mantle (geology) , osmium , platinum group , peridotite , ophiolite , placer deposit , lithology , mineralogy , placer mining , platinum , paleontology , tectonics , chemistry , biochemistry , ruthenium , catalysis
We present new measurements of 186 Os/ 188 Os and 187 Os/ 188 Os in 10 Os‐rich platinum group element (PGE) alloys from placer deposits formed by the mechanical erosion of peridotite‐bearing ophiolites in the Klamath and Siskiyou Mountains in northern California and southwestern Oregon. These data nearly double our database of high‐precision 186 Os/ 188 Os measurements on such samples. Together with previously published data, our new results reinforce the conclusion that the radiogenic 186 Os/ 188 Os compositions of these PGE alloys are very difficult to reconcile with a derivation of their Os from the outer core. Such a model requires extremely early growth of the inner core to its present size, within several hundred million years after accretion of the Earth, which is geophysically implausible. Collectively, our data suggest instead that partial melting or metasomatic processes in the upper mantle play a primary role in controlling the Os isotopic systematics of these Os‐rich PGE alloys and suggest the existence of upper mantle components characterized by radiogenic 186 Os/ 188 Os ratios. Pyroxene‐rich lithologies are possible candidates.