Early Mantle Evolution and the Late Veneer – New Perspectives from Highly Siderophile Elements

Wednesday, 16 December 2015
Poster Hall (Moscone South)
Jude Ann Coggon1, Ambre Luguet1, Jean-Pierre Lorand2, Raúl Fonseca3, Peter Appel4, Sisir Kanti Mondal5, Stefan Peters6, Geoffrey M Nowell7 and J Elis Hoffmann8, (1)University of Bonn, Bonn, Germany, (2)LPGN Laboratoire de Planétologie et Géodynamique de Nantes, Nantes Cedex 03, France, (3)Univeristy of Bonn, Bonn, Germany, (4)Geological Survey of Denmark and Greenland, Petrology and Economic Geology, Copenhagen, Denmark, (5)Jadavpur University, Kolkata, India, (6)University of Göttingen, Göttingen, Germany, (7)University of Durham, Durham, United Kingdom, (8)Free University of Berlin, Berlin, Germany
Numerous studies show that core – mantle differentiation should have fractionated the highly siderophile elements (HSE) into Earth’s core during its formation, leaving them almost entirely depleted in the mantle. It is widely held that later addition of chondritic material (a.k.a. the “late veneer”) can account for the disparity between modelled and observed HSE concentrations in the upper mantle. Recent experimental data (Médard et al., 2015) indicate that addition of ~0.6 % of the mass of the Earth could re-enrich the mantle HSE budget sufficiently to satisfy these observations. However, debate remains strong regarding the absolute timing, duration and nature of the re-enrichment. Chondrite-normalised HSE patterns (Coggon et al., 2015) of massive chromitites from the >3.811 Ga Ujaragssuit nunât layered ultramafic body, Greenland, are strikingly similar in both shape and abundance to the patterns of Phanerozoic chromitites from ultramafic layered intrusions. These data suggest that late veneer re-enrichment had already occurred prior to 3.811 Ga (Bennett et al., 2002; Coggon et al., 2013). Furthermore, Pt-Os model ages for these samples indicate that a late veneer component may have been present in Earth’s mantle as early as 4.1 – 4.3 Ga (Coggon et al., 2013). HSE inter-element ratios demonstrate distinct differences between this chromitite sample suite and younger chromitites from analogous tectonic settings. It remains unclear whether late veneer addition was already complete at 3.82 Ga and how long it took for this material to be accreted and homogenised within the upper mantle. We will address these issues using HSE and Os isotope data from Ujaragssuit nunât, Greenland, and the Singhbum Craton, India.