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| Detailed Reference Information |
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Farley, K.A. (1995). Rapid cycling of subducted sediments into the Samoan mantle plume. Geology 23(6): 531-534. |
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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 (sub 2) and were apparently formed at the base of the crust. They have high (super 3) He/ (super 4) He ratios (12 times the atmospheric ratio), inconsistent with the radiogenic values expected for recycled sediments. Furthermore, inclusion C/ (super 3) He ratios of approximately 3X10 (super 9) are indistinguishable from typical mantle values and are far lower than those in sediments (>10 (super 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 (super 3) He/ (super 4) He plume melt and recycled sediments. The retention of high (super 3) He/ (super 4) He ratios in such a mixture requires very rapid cycling of the sedimentary component through the mantle (probably <10 (super 7) 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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Keywords
alkaline earth metals; carbon; cycles; East Pacific; fluid, inclusions; geochemistry; geologic thermometry; harzburgite;, He-4/He-3; helium; hot spots; igneous rocks; inclusions; isotope, ratios; isotopes; lava; lead; mantle plumes; marine sediments;, metaigneous rocks; metals; metamorphic rocks; metasomatism;, microthermometry; neodymium; noble gases; Oceania; Pacific Ocean;, peridotites; plate tectonics; plutonic rocks; Polynesia; rare, earths; Samoa; Savai'i Island; sediments; South Pacific; Southeast, Pacific; Sr-87/Sr-86; stable isotopes; strontium; subduction;, Tonga Trench; ultramafics; West Pacific; xenoliths, 02C Geochemistry of rocks, soils, and sediments; 18 Solid-earth, geophysics; 07 Oceanography |
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The Geological Society of America
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