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/3-He ratios of ∼3 × 109 are indistinguishable from typical mantle values and are far lower than those in sediments (>1011). 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 <10 (7thpower) yr), rather than the billions of years often envisaged. This time scale indicates involvement of material recently returned to the mantle at the nearby Tonga Trench, and pelagic sediments near the trench have approximately the requisite geochemical signature. The unique isotopic characteristics of the Samoan plume likely arise from its unusual tectonic setting at the northern terminus of subduction in the Tonga Trench.

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