The Archean rock record of late accretion – New constraints from highly siderophile elements
Monday, November 23, 2020 - 11:00
Mario Fischer-Gödde, Universität Münster
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Highly siderophile elements (HSE) are key elements for studying core formation and late accretion processes on planetary bodies. Recent isotopic investigation of the highly siderophile element ruthenium (Ru) in Archean ultramafic rocks provided new insights not only into the nature and origin of the materials that were added to the Earth as a late veneer, but also about the timescales and efficiencies of mixing the late veneer into Earth’s mantle.
Based on evidence from 182W and HSE concentrations previous studies proposed that Archean rocks from southwest Greenland and the Pilbara craton (Australia) derive from mantle sources that did not receive the full complement of late veneer material.
In this regard, ultramafic rocks from both of these localities display distinct mass-independent Ru isotope compositions with a relative 100Ru excess of 22 parts per million compared with the modern mantle value. The 100Ru excess in these rocks indicates that the Archean mantle source of these rocks already contained a substantial fraction of Ru before the late veneer. The preservation of this pre-late veneer Ru isotope signature requires that Ru was not completely sequestered into the core and provides strong evidence that the mantle domains beneath southwest Greenland and Pilbara had not yet fully equilibrated with the late veneer.
In conjunction with additional constraints from meteorites, the distinct Ru isotope composition of the Archean mantle is best explained by late mixing of carbonaceous chondrite-like late veneer fraction into Earth’s mantle. Towards the end of the Archean eon, the transition from a stagnant lid mode to a plate tectonic regime with deep reaching subduction caused efficient mixing of the late veneer component, resulting in a homogenous modern mantle composition as recorded by the Ru isotope composition of post-Archean mantle rocks.