186Os-187Os systematics of Hawaiian picrites revisited: New insights into Os isotopic variations in ocean island basalts

Eighteen picrites (MgO > 13 wt.%) and three related basalts from six Hawaiian volcanoes were analyzed for Os-187/Os-188 and Os-186/Os-188. Variations in these ratios reflect long-term Re/Os and Pt/Os differences in the mantle source regions of these volcanoes. Os-187/Os-188 ratios vary from similar to 0.12(to 0.136, consistent with the range defined by previous studies of Hawaiian picrites and basalts. Samples with lower Os-187/Os-188 are mainly from Kea trend volcanoes (Mauna Kea and Kilauea), and the more radiogenic samples are mainly from Loa trend volcanoes (Mauna Loa, Hualalai, Koolau and Loihi). As previously suggested, differences in Os-187/Os-188 between volcanic centers are most consistent with the presence of variable proportions of recycled materials and/or pyroxenitic components in the Hawaiian source. Os-186/Os-188 ratios vary from 0.1198332 +/- 26 to 0.1198480 +/- 20, with some samples having ratios that are significantly higher than current estimates for the ambient upper mantle. Although the range of Os-186/Os-188 for the Hawaiian suite is consistent with that reported by previous studies, the new data reveal significant heterogeneities among picrites from individual volcanoes. The linear correlation between Os-187/Os-188 and Os-186/Os-188 reported by a previous study is no longer apparent with the larger dataset. The postulated recycled materials and pyroxenites responsible for the dominant variations in Os-187/Os-188 are likely not responsible for the variations in Os-186/Os-188. Such materials are typically characterized by both insufficiently high Os concentrations and Pt/Os to account for the Os-186/Os-188 heterogeneities. The lack of correspondence between Os-186/Os-188 variations and the Kea and Loa trends supports this conclusion. The primary cause of Os-186/Os-188 variations are evaluated within the framework of two mixing scenarios: (1) metasomatic transport of Pt and/or Os-186-rich Os into some portions of the Hawaiian source, and (2) interaction between an isotopically complex plume source with a common, Os- and Os-186-enriched reservoir (COs). Both scenarios require large scale, selective transport of Pt, Re and/or Os. Current estimates of HSE concentrations in the mantle source of these rocks, however, provide little evidence for either process, so the dominant cause of the Os-186/Os-188 variations remains uncertain. (C) 2011 Elsevier Ltd. All rights reserved.