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 Taʹu, Savaiʹi, and Ofu and the seamounts of Vailuluʹu and Malumalu) with Os concentrations > 30 ppt have 187Os/188Os ratios that exhibit a narrow range of values between 0.128 and 0.132. Lavas with ≤ 30 ppt Os show more radiogenic 187Os/188Os ratios, in some cases as high as 0.191, suggesting that the 187Os/188Os ratios of the extreme Samoan EM2 (enriched mantle 2) lavas likely have been compromised by assimilation of altered oceanic crust. The 187Os/188Os ratios for rejuvenated-stage lavas from Savaiʹi are lower than shield lavas, and they exhibit some of the lowest 187Os/188Os 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 187Os/188Os ratios of the higher Os-abundance shield lavas (0.128–0.132), coupled with a tremendous range of 87Sr/86Sr (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 (~ 107) with a mantle peridotite that has low Sr/Os (~ 104). ndances 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 187Os/188Os ratio. For example, the range of 187Os/188Os 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).