Re-Os isotope systematics in Samoan shield lavas and the use of Os-isotopes in olivine phenocrysts to determine primary magmatic compositions

Samoan shield-stage lavas (from the islands of Ta'u, Savai'i, and Ofu and the seamounts of Vailulu'u and Malumalu) with Os concentrations >30 ppt have Os-187/Os-188 ratios that exhibit a narrow range of values between 0.128 and 0.132. Lavas with <= 30 ppt Os show more radiogenic Os-187/Os-188 ratios, in some cases as high as 0.191, suggesting that the Os-187/Os-188 ratios of the extreme Samoan EM2 (enriched mantle 2) lavas likely have been compromised by assimilation of altered oceanic crust. The Os-187/Os-188 ratios for rejuvenated-stage lavas from Savai'i are lower than shield lavas, and they exhibit some of the lowest Os-187/Os-188 ratios in the global ocean island basalt database (Hauri and Hart, 1993). The difference may owe to contamination of the rejuvenated lavas with unradiogenic Os from disaggregated xenoliths from the mantle lithosphere, and their low Os isotopic composition does not reflect the EM2 mantle source of magmas. The limited range in (OS)-O-187/(OS)-O-188 ratios of the higher Os-abundance shield lavas (0.128-0.132), coupled with a tremendous range of Sr-87/Sr-86 (0.7045-0.7114), are characteristics of the EM2 source that can be explained by mixing a continental crustal sediment characterized by a high Sr/Os (similar to 10(7)) with a mantle peridotite that has low Sr/Os (similar to 10(4)). Os abundances for nine whole rocks and their olivine phenocrysts show opposite trends with respect to whole rock MgO content: olivines have less than 0.1 ppb Os at 15 wt.% MgO and increase to 1-2 ppb Os in lavas with 8 wt.% MgO. Even though the Os isotopic composition of an evolved lava is susceptible to crustal assimilation, olivine phenocrysts with high Os concentrations preserve the pre-assimilation magmatic Os-187/Os-188 ratio. For example, the range of Os-187/Os-188 ratios measured on magmatic olivines from Samoan shield lavas (0.127 to 0.130) is lower (and narrower than) the range estimated using Os-rich (>30 ppt) whole rock lavas (0.128-0.132). (C) 2011 Elsevier B.V. All rights reserved.