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[SGC33-02] Estimation of highly siderophile element abundances in the primitive mantle from the perspective of the pyrolite model
Keywords:primitive mantle, highly siderophile element, platinum group element, early Earth
Highly siderophile elements (HSE: Ru, Rh, Pd, Re, Os, Ir, Pt and Au) are strongly concentrated in the metallic core in the Earth, and therefore their abundances in the silicate mantle are quite low (~µg/g). Just because of the low abundances, HSE in the mantle are the key index for deciphering chemical processes involving metallic phases, such as the core-mantle segregation and subsequent core-mantle chemical interactions during the Earth's history. In particular, HSE abundances in the mantle at the time of core-mantle segregation, that is, the HSE abundances in the primitive mantle (PM), are one of the crucial constraints to know the initial condition of the chemical evolution of the mantle.
The HSE abundances in the primitive mantle (HSEPM hereafter) have been estimated based on HSE concentrations of fertile peridotites, which are rich in "melt components" such as Al2O3, because fertile peridotites are thought to retain their pristine compositions. However, recent studies (e.g., [1]) have revealed that most of the fertile peridotites were formed by addition of basaltic melt to refractory peridotites (= refertilization). Therefore, more precise estimation of the HSEPM can be done by calculation of supposed mixture of refractory peridotites (harzburgites) and refertilizing agents with basaltic composition. This calculation is just identical to the "pyrolite model" of Ringwood (e.g., [2]), which was proposed for estimating the major element composition of the primitive mantle. In fact, the major element composition of the pyrolite has widely been accepted as the representative of the Earth's mantle. This would be due to the facts that compositions of natural harzburgites are rather homogeneous and that major element compositions of the refertilizing agents (coincidentally?) resemble those of supposed melts extracted from the primitive mantle. On the other hand, HSE compositions of natural harzburgites are very variable (up to 2 orders of magnitude), which makes it difficult to select the composition of harzburgite to be used for the "pyrolitic" estimation of HSEPM. Despite such difficulty, we can estimate the pyrolitic HSE abundances using compositions of the harzburgites that were not remarkably affected by secondary processes if we can properly select suitable harzburgite samples based on detailed petrography. Our preliminary estimation of the pyrolitic HSE abundances demonstrates non-chondritic relative ratios with significant depletion of Pd and Pt. This is contrary to the conventional view that the HSEPM is chondritic.
[1] Le Roux V., Bodinier J.-L., Tommasi A., Alard O., Dautria J.-M., Vauchez A., Riches A.J.V. 2007. The Lherz spinel lherzolite: Refertilized rather than pristine mantle. Earth Planet. Sci. Lett., 259, 599-612.
[2] Ringwood T.E. 1962. A model for the upper mantle. J. Geophys. Res. 67, 57-866.
The HSE abundances in the primitive mantle (HSEPM hereafter) have been estimated based on HSE concentrations of fertile peridotites, which are rich in "melt components" such as Al2O3, because fertile peridotites are thought to retain their pristine compositions. However, recent studies (e.g., [1]) have revealed that most of the fertile peridotites were formed by addition of basaltic melt to refractory peridotites (= refertilization). Therefore, more precise estimation of the HSEPM can be done by calculation of supposed mixture of refractory peridotites (harzburgites) and refertilizing agents with basaltic composition. This calculation is just identical to the "pyrolite model" of Ringwood (e.g., [2]), which was proposed for estimating the major element composition of the primitive mantle. In fact, the major element composition of the pyrolite has widely been accepted as the representative of the Earth's mantle. This would be due to the facts that compositions of natural harzburgites are rather homogeneous and that major element compositions of the refertilizing agents (coincidentally?) resemble those of supposed melts extracted from the primitive mantle. On the other hand, HSE compositions of natural harzburgites are very variable (up to 2 orders of magnitude), which makes it difficult to select the composition of harzburgite to be used for the "pyrolitic" estimation of HSEPM. Despite such difficulty, we can estimate the pyrolitic HSE abundances using compositions of the harzburgites that were not remarkably affected by secondary processes if we can properly select suitable harzburgite samples based on detailed petrography. Our preliminary estimation of the pyrolitic HSE abundances demonstrates non-chondritic relative ratios with significant depletion of Pd and Pt. This is contrary to the conventional view that the HSEPM is chondritic.
[1] Le Roux V., Bodinier J.-L., Tommasi A., Alard O., Dautria J.-M., Vauchez A., Riches A.J.V. 2007. The Lherz spinel lherzolite: Refertilized rather than pristine mantle. Earth Planet. Sci. Lett., 259, 599-612.
[2] Ringwood T.E. 1962. A model for the upper mantle. J. Geophys. Res. 67, 57-866.