Evidence for an ancient osmium isotopic reservoir in Earth.
Iridosmine grains from placer deposits associated with peridotite-bearing ophiolites in the Klamath mountains have extremely radiogenic 186Os/188Os ratios and old Re-Os minimum ages, from 256 to 2644 million years. This indicates the existence of an ancient platinum group element reservoir with a supra-chondritic Pt/Os ratio. Such a ratio may be produced in the outer core as a result of inner core crystallization that fractionates Os from Pt. However, if the iridosmine Os isotopic compositions are a signature of the outer core, then the inner core must have formed very early, within several hundred million years after the accretion of Earth.
Meibom
Anders
A
Geological and Environmental Sciences, 320 Lomita Mall, Stanford University, CA 94305-2115, USA. meibom@pangea.stanford.edu
Frei
Robert
R