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The advantage of the Re-Os system, applied to Os-bearing platinum-group minerals (PGMs), is that these minerals contain Os as a main or trace element in their crystal structures, while at the same time almost lack Re. Consequently, primary Os-rich PGMs (e.g., laurite-erlichmanite series (RuS2-OsS2) and Os-Ir-Ru alloys), frequently the earliest minerals in ultramafic mantle rocks, are (1) the best tracers of mantle melting events, and (2) promising targets to constrain initial 187Os/188Os ratios of different mantle environments.
This contribution summarizes the extensive data set of Os isotope compositions of bedrock and detrital Ru-Os sulphides and Os-Ir-Ru alloys derived from continental (Guli, Kondyor and Nizhny Tagil, Russia) and oceanic (Kunar, Russia; Kraubath and Hochgrossen, Austria and Shetland, U.K.) ultramafic massifs and the Late Archean paleoplacers of the Witwatersrand Basin (South Africa). The compositionally diverse PGMs have been studied by a number of modern techniques including SEM, EMPA, N-TIMS and LA MC-ICP-MS. PGM grains were extracted from bedrock samples using the innovative non-destructive concentration technique at NATI Research JSC, Russia (http://www.natires.com).
Os-rich alloy grains from continental clinopyroxenite-dunite massifs show a relatively narrow range of 'unradiogenic' 187Os/188Os values (median 0.12452, n=205), indicative of a subchondritic mantle source of PGE. Similar Os isotope composition has been determined in laurite-dominated PGM assemblages from podiform chromitite at Harold's Grave, Shetland (median 0.12444, n=105). The Os isotope results are consistent with earlier findings (Malitch et al. 2002), when a narrow range of subchondritic 187Os/188Os values has been proposed to indicate a single-stage formation of PGMs in ultramafic massifs.
In contrast, Ru-Os sulphides from podiform chromitite at Kraubath and Hochgrossen yield a very wide range of subchondritic 187Os/188Os values (0.1125-0.1244, n=17, Malitch 2004), which is almost identical to that of Ru-Os-Ir alloy grains at Kunar (0.1094-0.1241, n=45). The Os isotope system of PGMs thus unequivocally recorded temporally extended melting events in the Proterozoic Eastern Alpine and Chelyuskin parent ultramafic protoliths. This variability is probably controlled by deep-geodynamic processes implying that source rocks in the residual mantle have a more complex geological history than is commonly assumed.
The 187Os/188Os value in single PGM grains of different composition (i.e. ruthenium, osmium, Ru-Os-Ir-Pt alloy and Ru-Os sulphide) from the Evander Goldfield at Witwatersrand was found to range from 0.10482±0.00002 to 0.10894±0.0006 (n=55), which is in general agreement with previous results (Malitch & Merkle 2004). The model 187Os/188Os ages for the main set of the PGMs studied (3290-2795 Ma) favor a scenario in which the primary PGMs were incorporated into the Witwatersrand Basin by their mechanical release during weathering of parent source rocks.
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