CONSTRAINTS ON MELTING AND OSMIUM ISOTOPIC SOURCES IN THE OPHIOLITIC UPPER MANTLE: EVIDENCE FROM RU-OS SULFIDES AND OS-IR-RU ALLOYS

Abstract

Primary Os-rich platinum-group minerals (PGM) (e.g., laurite-erlichmanite series (RuS2-OsS2) and Os-Ir-Ru alloys) are potentially the best tracers of mantle melting events, which may be unequivocally recognised during ultramafic protoliths formation. The advantage of the Re-Os system, applied to Os-rich minerals, is that PGM contain Os as a main or trace element, while at the same time almost lack Re. This feature permits accurate initial Os isotope ratios to be determined, assuming that the Os isotopic composition of PGM has not changed after their formation and, therefore, reflects that of the source. The extensive data set of bedrock and detrital PGM grains from Early Palaeozoic and Late Proterozoic ophiolite complexes (the Speik Complex, Austria and the Kunar Complex, Russia, respectively), permits, for the first time, the evaluation of Os isotopic composition of Ru-Os sulfides and Os-Ir-Ru alloys by (1) laser ablation (LA) attached to multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) and (2) negative thermal ionization mass spectrometry (NTI-MS). Ru-Os sulfides of podiform chromitites from ophiolitic mantle sections at Kraubath and Hochgrössen (Speik Complex) are characterised by 'unradiogenic' 187Os/188Os and γOs(T=0) values, indicative of a subchondritic mantle source of PGE. These values yield a very wide range of 187Os/188Os (0.1158 ± 0.0015 to 0.1244 ± 0.0005, n=18, NTI-MS and LA MC-ICP-MS data) and γOs(T=0) (-9.08 to -2.35), which is almost identical to Os isotopic composition of detrital Os-Ir-Ru alloys (NTI-MS data, n=19) derived from the Kunar Complex (Taimyr Peninsula, Russia). These results are consistent with the model involving a prolonged melting-event history of parent ultramafic source rocks in the residual mantle. In contrast, Ru-Os sulfides from banded chromitite, which has been recognised closely above the typical mantle section at Kraubath, show high 187Os/188Os (0.1308 ± 0.0001 to 0.1321 ± 0.0006) and γOs (2.70-3.73) values, indicative of a suprachondritic source of PGE. This signature is interpreted as an evidence of a 'radiogenic' crustal component, which was introduced during a subduction-related event. The high Os-isotopic heterogeneity obtained imply that comprehensive sets of Os isotope data can only provide a valid understanding of Os-isotopic behaviour in the ophiolitic mantle. Therefore, a restricted number of Os-isotope analyses should be treated with caution. Os isotopic composition of PGM can be employed to test the validity of various petrological models.