How Recycling of Sediments and Oceanic Crust Have Changed the Nd-Hf Isotopic Composition of the Mantle through Time

Abstract:

The Nd and Hf isotopic compositions of modern mantle-derived rocks such as mid-ocean ridge basalts (MORB) and ocean island basalts (OIB), define a linear trend known as Nd-Hf mantle array. This array is thought to reflect mixing between mantle from which crust has previously been extracted and enriched recycled oceanic crustal components of variable ages - including sediments with radiogenic Hf isotopic compositions (Chauvel et al., Nature Geoscience, 2008). Thus the present-day mantle as sampled by MORB and OIB is pervasively contaminated by recycled material. We here develop a model to quantify how the continuous incorporation of recycled material changed the Nd-Hf isotopic composition of the mantle through time.

The Nd-Hf isotopic compositions of Archean sediments were decoupled - high ε_Hf for a given ε_Nd - due to the contribution of radiogenic Hf from cherts. From these observations, the possible compositions for recycled material currently into the mantle can be estimated. Assuming that modern MORB and OIB contain ~3-5% and ~10-15% recycled material, respectively; we calculate that a mantle reservoir free of recycled material would be significantly more radiogenic than the most extreme MORB and would lie well below the Nd-Hf mantle array. Such a Nd-Hf isotopic composition is not sampled by modern mantle-derived rocks. Crustal rocks formed at ~3.8 Gyr (compilation of granites and TTG), however, have low ε_Hf for a given ε_Nd, consistent with our estimated composition for the mantle without recycled material. We thus suggest that Eoarchean mantle-derived rocks sampled depleted mantle that was not yet contaminated by recycled material and that the slope of the Nd-Hf mantle array changed through time due to continuous recycling of sedimentary material in subduction zones.