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)).