Re-Os geochronology of base metal sulfides from cratonic mantle xenoliths : Case study from Somerset Island (Canada) and method development

• Main Author: Bragagni, Alessandro
• Other Authors: Luguet, Ambre, Fonseca, Raúl O. C.
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Universitäts- und Landesbibliothek Bonn 2016
• Subjects: ddc:550; Canada; Rae; Somerset Island
• Online Access: https://hdl.handle.net/20.500.11811/6755

Robust and reliable time constraints are necessary to infer the formation and evolution of the cratonic mantle. Due to the large fractionation of Re from Os during mantle melting, the Re-Os decay system has been largely used for dating the melting event that led to the formation of the subcontinental lithospheric mantle (SCLM). In mantle rocks Re, Os and the other highly siderophile elements (HSE: Ru, Rh, Pd, Re, Os, Ir, Pt, and Au) are controlled by base metal sulfides (BMS), which can be residual phases of partial melting processes or can be re-introduced in mantle rocks during metasomatism. The present study aimed at the improvement and enhancement of our ability to use the Re-Os system and the HSE to unravel geological processes recorded in BMS. The contribution of this work is twofold because it provides new data on natural samples as well as a novel analytical technique for future applications. In the first part of this dissertation (Chapter 1) some basic concepts are introduced to make the reader more familiar with the topics encountered in the next sections. This includes an overview of the HSE behavior in terrestrial reservoirs and the explanations of the geochemical tools that will be used in the following sections. The second part of this dissertation (Chapter 2) is focused on the investigation of partial melting and metasomatic processes recorded in four mantle xenoliths from Somerset Island (Rae craton, Canada). After textural and mineralogical investigations, individual BMS grains were micro-sampled and analyzed for 187 Os/ 188 Os. The two xenoliths with the most metasomatic HSE signature (e.g. suprachondritic Pd/Pt) are distinguished for the high BMS modal abundance, the occurrence of large interstitial BMS grains, and the extreme 187 Os/ 188 Os variation measured in BMS grains ( 187 Os/ 188 Os = 0.172-0.108). Archean Re-depletion model ages (T RD ) are recorded in BMS grains from three different xenoliths, suggesting a main formation of the SCLM at 2.7-2.8 Ga, in association with the local Rae ...