Rapid cycling of subducted sediments into the Samoan mantle plume

Harzburgite xenoliths from the Samoan hotspot carry extraordinarily enriched Sr-Nd-Pb isotopic ratios previously attributed to metasomatism by a carbonatitic fluid derived from recycled sediments. I describe here the gas composition and microthermometric properties of fluid inclusions that have trapped this unusual agent. The inclusions are dominantly pure CO_2 and were apparently formed at the base of the crust. They have high ^3He/^4He ratios (12 times the atmospheric ratio), inconsistent with the radiogenic values expected for recycled sediments. Furthermore, inclusion C/^3He ratios of ∼3 × 10^9 are indistinguishable from typical mantle values and are far lower than those in sediments (>10^(11)). These striking observations imply that little of the helium and carbon in the metasomatic fluid was derived from the recycled source required by the Sr-Nd-Pb isotopes. The metasomatic fluid is probably a product of mixing within the Samoan hot spot of a volatile-rich high ^3He/^4He plume melt and recycled sediments. The retention of high ^3He/^3He ratios in such a mixture requires very rapid cycling of the sedimentary component through the mantle (probably