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[SGC36-P03] Estimation of highly siderophile element abundances in the primitive mantle: Insights from peridotites of Timor-Tanimber Ophiolite
Keywords:highly siderophile element, platinum group element, primitive mantle, Indonesia, East Timor
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 evolutional 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 (lherzolites) [1], which are abundant in "melt components" such as Al2O3, because the lherzolites are believed to retain their pristine compositions close to that of PM. However, since most of the lherzolites show chemical modification by basaltic melt infiltration (= refertilization) from refractory peridotites (harzburgites) [2], the lherzolites are not ideal materials for the estimation of HSEPM. To be more precise, we propose an estimation of the HSEPM by calculating the HSE composition of harzburgite–basalt mixture (i.e., the pyrolite model [3]). However, the harzburgites are generally more variable in HSE compositions than those of lherzolites [4], suggesting that HSE of harzburgites are more likely to be modified by secondary processes like metasomatism than lherzolites. Therefore, the estimation of HSEPM by applying the pyrolite model requires HSE data of the harzburgites that preserve their pristine HSE compositions.
Timor-Tanimbar Ophiolite (TTO) in the eastern Indonesia locates in the forearc of Sunda-Banda volcanic arc, where the Australian plate subducts beneath the Eurasian plate. TTO includes several peridotite bodies (harzburgite~lherzolite) supposed to originate from the lithosphere of the Eurasian plate. Petrographic textures, mineral compositions and whole-rock major and trace element compositions of TTO peridotites [5] suggest that they preserve their pristine HSE compositions. In this study, we reassess the HSEPM estimation by applying the pyrolite model using HSE composition data of the TTO peridotites. Compositional variations in HSE concentration of TTO peridotites are relatively small except for Pt, Pd and Re. Calculated HSE concentrations are as low as quarter of those of PM calculated by Becker et al. [1] except for Re that are 4 times more enriched, resulting in our results far different from chondrite. On the other hand, if we assume that the HSE variations in TTO peridotites were produced solely by simple melt extraction, calculated HSE concentrations of the source peridotite are broadly similar to those of PM by Becker et al. [1], but the HSE concentrations of the extracted melt are unrealistically high except for Pt. Our model calculations suggest that the source peridotite for the TTO harzburgite was depleted in HSE except for Re, which in turn predict that HSE concentrations of the mantle are, among various regions and tectonic setting, much more variable than major-element compositions. It is desired to elucidate the cause of such global-scale HSE variation in the mantle for better estimation of HSEPM.
[1] Becker H. et al. 2006, Geochim. Cosmochim. Acta 70, 4528-4550.
[2] Le Roux V. et al. 2007, Earth Planet. Sci. Lett. 259, 599-612.
[3] Ringwood T.E. 1962, J. Geophys. Res. 67, 57-866.
[4] Aulbach A. et al. 2016, Rev. Mineral. Geochem. 81, 239-304.
[5] Ishikawa A. et al. 2007, Gondwana Res. 11, 200-217.
Timor-Tanimbar Ophiolite (TTO) in the eastern Indonesia locates in the forearc of Sunda-Banda volcanic arc, where the Australian plate subducts beneath the Eurasian plate. TTO includes several peridotite bodies (harzburgite~lherzolite) supposed to originate from the lithosphere of the Eurasian plate. Petrographic textures, mineral compositions and whole-rock major and trace element compositions of TTO peridotites [5] suggest that they preserve their pristine HSE compositions. In this study, we reassess the HSEPM estimation by applying the pyrolite model using HSE composition data of the TTO peridotites. Compositional variations in HSE concentration of TTO peridotites are relatively small except for Pt, Pd and Re. Calculated HSE concentrations are as low as quarter of those of PM calculated by Becker et al. [1] except for Re that are 4 times more enriched, resulting in our results far different from chondrite. On the other hand, if we assume that the HSE variations in TTO peridotites were produced solely by simple melt extraction, calculated HSE concentrations of the source peridotite are broadly similar to those of PM by Becker et al. [1], but the HSE concentrations of the extracted melt are unrealistically high except for Pt. Our model calculations suggest that the source peridotite for the TTO harzburgite was depleted in HSE except for Re, which in turn predict that HSE concentrations of the mantle are, among various regions and tectonic setting, much more variable than major-element compositions. It is desired to elucidate the cause of such global-scale HSE variation in the mantle for better estimation of HSEPM.
[1] Becker H. et al. 2006, Geochim. Cosmochim. Acta 70, 4528-4550.
[2] Le Roux V. et al. 2007, Earth Planet. Sci. Lett. 259, 599-612.
[3] Ringwood T.E. 1962, J. Geophys. Res. 67, 57-866.
[4] Aulbach A. et al. 2016, Rev. Mineral. Geochem. 81, 239-304.
[5] Ishikawa A. et al. 2007, Gondwana Res. 11, 200-217.