Re–Pt–Os Isotopic and Highly Siderophile Element Behavior in Oceanic and Continental Mantle Tectonites

Abstract

Tectonically emplaced mantle rocks, such as ophiolites, abyssal peridotites, and orogenic peridotite massifs, provide a principle constraint on the composition of and processes in the Earth’s upper mantle (Bodinier and Godard 2003). In the past, these ‘mantle tectonites’ have sometimes received different names because their history and origin has been unclear. Mantle tectonites are now understood to reflect a range of geologic environments regarding their emplacement and their origin (e.g., Dilek and Furnes 2014). The advantage of these rocks compared to mantle xenoliths is the large-scale exposure of textural and compositional relations between different rock types that can be used to identify processes such as melting, magma or fluid transport, chemical reactions, mixing or deformation at a range of spatial scales. A disadvantage of most mantle tectonites is that they commonly display substantial chemical modification of some elements, resulting from widespread serpentinization at low temperatures. In some cases, this may also affect abundances of several of the highly siderophile elements (HSE: Re, Au, PGE: Os, Ir, Ru, Rh, Pt, Pd), however, this can be tested by comparison with unaltered rocks of similar composition. As is discussed in Luguet and Reisberg (2016, this volume), Harvey et al. (2016, this volume) and Aulbach et al. (2016, this volume), peridotite xenoliths have their own alteration issues regarding sulfides and chalcophile elements. Numerous studies have obtained Os isotope and/or highly siderophile element abundance data on many different types of mantle tectonites. Some of these studies have focused on large-scale chemical and isotopic variations, others on grain size-scale compositional variations to understand small-scale distribution processes. These studies have, together, significantly advanced the understanding of the processes that fractionate the HSE in the mantle at different spatial scales and have provided insights into the behavior of sulfide in the mantle—the phase that typically …