Osmium isotopic compositions of Os-rich platinum group element alloys from the Klamath and Siskiyou Mountains

TY - JOUR

T1 - Osmium isotopic compositions of Os-rich platinum group element alloys from the Klamath and Siskiyou Mountains

AU - Meibom, Anders

AU - Frei, Robert

AU - Sleep, Norman H.

PY - 2004/2/10

Y1 - 2004/2/10

N2 - We present new measurements of 186Os/188Os and 187Os/188Os 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 186Os/188Os measurements on such samples. Together with previously published data, our new results reinforce the conclusion that the radiogenic 186Os/188Os 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 186Os/188Os ratios. Pyroxene-rich lithologies are possible candidates.

AB - We present new measurements of 186Os/188Os and 187Os/188Os 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 186Os/188Os measurements on such samples. Together with previously published data, our new results reinforce the conclusion that the radiogenic 186Os/188Os 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 186Os/188Os ratios. Pyroxene-rich lithologies are possible candidates.

KW - Isotopes

KW - Osmium

KW - Upper mantle

UR - http://www.scopus.com/inward/record.url?scp=2542482591&partnerID=8YFLogxK

M3 - Journal article

AN - SCOPUS:2542482591

SN - 0148-0227

VL - 109

JO - Journal of Geophysical Research

JF - Journal of Geophysical Research

IS - 2

ER -