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

摘要

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' Os-187/Os-188 values, indicative of a chondritic or subchondritic mantle source of platinum-group elements PGE. The 'unradiogenic' Os-187/Os-188 values () g yield a very wide range Of Os-187/Os-188 (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 Os-187/Os-188 (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. (C) 2004 Elsevier B.V. All rights reserved.