Highly Siderophile Element and Os-187 Signatures in Non-cratonic Basalt-hosted Peridotite Xenoliths: Unravelling the Origin and Evolution of the Post-Archean Lithospheric Mantle

摘要

The highly siderophile elements (HSE) consist of the Platinum Group Elements (PGE: Ru, Rh, Pd, Os, Ir, Pt) along with rhenium and gold. These transition elements show relative chemical inertness and high market values, which respectively earned them the additional names of noble metals and precious metals.The HSE show a very pronounced affinity for iron metal, which translates into metal/ silicate partition coefficients similar to or higher than 10,000 over large ranges of both pressure and temperature (e.g., O'Neill et al. 1995; Borisov and Palme 2000; Ertel et al. 1999, 2001, 2006, 2008; Fortenfant et al. 2003, 2006; Brenan et al. 2005; Cottrell and Walker 2006; Brenan and McDonough 2009; Laurenz et al. 2010; Mann et al. 2012; see Brenan et al. 2016, this volume for detailed review). Consequently, the HSE are thought to have been efficiently sequestered within the metallic core of our planet during the metal-silicate differentiation of Earth, leaving the silicate counterpart almost HSE-barren. Investigations of mantle peridotites since the 1970s revealed ng.g~(-1) level abundances as well as close-to-chondritic proportions of the HSE (Chou 1978; Jagoutz et al. 1979; Mitchell and Keays 1981; McDonough and Sun 1995; Becker et al. 2006; Fischer-Godde et al. 2011). Such abundances and inter-HSE fractionations are not predicted for the silicate Earth left after separation of the metallic core for low- or high-pressure core-mantle differentiation (see Brenan et al. 2016, this volume). The close agreement between the osmium isotopic compositions of fertile mantle peridotites and those of chondritic meteorites (Walker et al. 2002a), which requires nearly identical Re/Os ratios in these two reservoirs, provides particularly convincing evidence that the mantle's HSE content cannot simply represent the residue left after core formation. These observations have led to suggestions that the HSE systematics in the Earth's mantle could in fact reflect 1) inefficient core formation (Arculus and Delano 1981; Jones and Drake 1986), 2) repeated equilibrium-fractionation events, each involving only part of the mantle (Azbel et al. 1993), 3) core-mantle differentiation followed by core-mantle interaction (Snow and Schmidt 1998), and/or 4) a late accretionary event, also known as the "late veneer" hypothesis (e.g., Kimura et al. 1974; Chou 1978; Jagoutz et al. 1979; Morgan et al. 1981, 2001; Pattou et al. 1996). Among these, the last scenario, consisting of global-scale metal-silicate differentiation followed by late addition of an extraterrestrial component 4.2-3.8 Ga ago, after core formation has ceased, is currently the most favoured hypothesis, although some issues remain unresolved (Walker 2009; see review of Yokoyama and Walker 2016, this volume). It has also been proposed that a late heavy bombardment event of this type established the HSE abundances and 1870s signatures in the silicate Moon (Walker et al. 2004; Day et al. 2007; see detailed review in Day et al. 2016, this volume). If such a 2-fold scenario (core-mantle differentiation followed by late reintroduction of HSE) accounts for the HSE signatures in the Earth's mantle, the relative HSE- abundances within rocks from the terrestrial mantle (as well as in the lunar mantle) should hold the key to the nature of the extraterrestrial component, which "refertilized" the silicate Earth-Moon system in HSE. This late accreted material has also been postulated to have (re)-introduced volatiles and the organic molecules necessary for the emergence of life on our planet (Cooper et al. 2001; Kring and Cohen 2002). Therefore, understanding the origin of the HSE signatures may help us to understand when and how our planet acquired a favorable environment for the development of life. Identifying the nature of this late meteoritic bombardment would furthermore provide firm constraints on the origin of the impactors and thus on models of the formation and early evolution of the inner solar system.