Combined Whole-Rock to Nano-Scale Investigations Reveal Contrasting Response of Pt-Os and Re-Os Isotope Systematics During Magmatic and Post-Magmatic Processes

Abstract:

Advances in single-grain and micro- and nano-analytical techniques in recent years have been particularly important to the study of highly siderophile elements (HSE) and have contributed significantly to our knowledge and understanding of their host phases and behaviour. Furthermore, whole-rock- to nano-scale studies provide new perspectives for investigation of HSE isotope systematics. Recent multi-scale ¹⁸⁷Re-¹⁸⁷Os and ¹⁹⁰Pt-¹⁸⁶Os studies facilitate comparison, to a previously unattainable degree, of the differing responses of these two decay systems to magmatic and post-magmatic processes. It is well established that mafic-ultramafic melts are sensitive to disturbance of their Re-Os isotope systematics by crustal assimilation, due to the incompatibility and resulting enrichment of Re in crustal lithologies. In contrast the very long half-life and extremely low atomic abundance of ¹⁹⁰Pt, combined with relatively low Pt concentrations in crustal rocks, generally render the Pt-Os isotope system insensitive to modification during assimilation. However, using new single chromite grain data (Coggon et al., 2015) from the >3.811 Ga Ujaragssuit nunât layered ultramafic body, Greenland, we show that it is possible to distinguish two distinct episodes of ¹⁸⁷Os/¹⁸⁸Os modification; Country rock contamination of the parent melt was followed by later metamorphic disturbance of the isotope system. The Pt-Os data (Coggon et al., 2013) from the same samples show no evidence of crustal assimilation, but preserve signatures of mantle melting at ~4.1 Ga as well as disturbance during metamorphism. Macro- to micro-petrographic study clearly demonstrates that Pt, Re and Os are hosted by different mineral phases, of different origins, in these samples. This, together with the physical parameters of the decay systems reported above, leads to the dissimilar behaviour and response of the ¹⁸⁷Re-¹⁸⁷Os and ¹⁹⁰Pt-¹⁸⁶Os isotope systems during both magmatic and post-magmatic processes and consequently the facility for the two decay systems collectively to record a more complete geological history than can be gleaned from one alone, or from study at only one scale.