Osmium isotope constraints on contrasting sources and prolonged melting in the Proterozoic upper mantle: evidence from ophiolitic Ru–Os sulfides and Ru–Os–Ir alloys

Abstract

This study presents the first extensive data set of Os isotopic compositions of primary Ru–Os–Ir alloys and Ru–Os sulfides derived from two Neoproterozoic ophiolite-type ultramafic massifs (Kunar in Northern Taimyr, Russia, and Kraubath in Eastern Alps, Austria). The study employed a multi-technique approach and utilized a number of analytical techniques, including hydroseparation, electron microprobe analysis, negative thermal ionization mass-spectrometry (N-TIMS) and laser ablation attached to multiple collector-inductively coupled plasma-mass spectrometry (LA MC-ICP-MS). The results identify two different Os isotope sources (subchondritic and suprachondritic) for bedrock Ru–Os sulfides at Kraubath, whereas detrital Ru–Os–Ir alloys at Kunar are characterized by ‘unradiogenic’ 187Os/ 188Os values, indicative of a chondritic or subchondritic mantle source of platinum-group elements (PGE). The ‘unradiogenic’ 187Os/ 188Os values yield a very wide range of 187Os/ 188Os (0.1094±0.0003 to 0.1241±0.0004, n=19, N-TIMS data), which is almost identical to the Os isotope composition of bedrock laurite, (Ru,Os)S 2, in podiform chromitite within an ophiolitic mantle section at Kraubath (0.11249±0.00062 to 0.12437±0.00051, n=17, N-TIMS and LA MC-ICP-MS data). These results are consistent with a model in which a prolonged history of melting events of parent ultramafic source rocks took place in the oceanic upper mantle. In contrast, erlichmanite and laurite from banded chromitite within the transition zone at Kraubath show high 187Os/ 188Os (0.13080 to 0.13212) values, indicative of a suprachondritic source of PGE. This signature might be interpreted as either evidence of an enriched mantle source or an indication of a ‘radiogenic’ crustal component, which was introduced during a subduction-related event. The Os isotope results obtained support the conclusion that the Re–Os system, represented by PGM, has remained unchanged from the time of formation of the PGM until now, despite later thermal events, which occurred in the vicinity of ophiolite-type complexes.