Hafnium/rare earth element fractionation in the sedimentary system and crustal recycling into the Earth's mantle

Details Hafnium/rare earth element fractionation in the sedimentary system and crustal recycling into the Earth's mantle Hafnium/rare earth element fractionation in the sedimentary system and crustal recycling into the Earth's mantle

Aug 1984

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984e%26psl..69..365p&link_type=abstract

Earth and Planetary Science Letters, Volume 69, Issue 2, p. 365-378.

Computer Science

71

Scientific paper

Among long-lived radioactive parent-daughter element pairs, the ratio Lu/Hf is strongly fractionated relative to constant Sm/Nd in the Earth's sedimentary system. This is caused by high resistance to chemical weathering of the mineral zircon (Zr,Hf)SiO4. Zircon-bearing sandy sediments on and near continents have very low Lu/Hf, while deep-sea clays have up to three times the chondritic Lu/Hf ratio. Turbidity currents mechanically carry the low-Lu/Hf sandy material onto the ocean floor. The results are important for the crust-to-mantle recycling discussion, where most recycled materials would be subducted oceanic sediments. Such sediment should be capable of explaining the Hf-Nd mantle isotopic variation by mixing with peridotite, but in fact any average pelagic sediment has Nd/Hf and Lu/Hf too high to allow mixing curves to pass through the Hf/Nd isotopic array. The array could only be reproduced by subduction of turbidite sandstone with pelagic sediment in the approximate ratio 1.2 to 1, and by maintaining a good mixture between the two components. At least today, turbidites are available for subduction only at locations quite different and distant from those where pelagic sediments may be recycled; furthermore, mantle isotopic variation shows that the mantle often cannot mix itself well enough to homogenize these widely-separated sedimentary components to the degree required. The Lu/Hf fractionations place a severe restriction on the ability of recycled sediments to explain mantle isotopic patterns.

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