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Abstract:
Geochemical thinking about crust-mantle evolution has historically been dominated by the idea of crustal growth and complementary mantle depletion, as tracked by Nd-Sr-Hf isotopes [1] and incompatible element abundances [2]. This approach has been invalidated by the realization that recycling destroyed much of the ancient continents so there may have been no net crustal growth during most of Earth history [3], and because much of the observed trace element and parent-daughter differentiation was affected by ocean crust recycling [4,5]. Net crustal growth cannot be constrained by counting crustal zircons, because an unknown number of zircons is lost during crustal recycling [3]. But the relative size of the Archean continental crust can be estimated from the (Nb/U)n ≅ 1.46 of Archean komatiites compared to the modern (MORB+OIB)n ≅ 1.68, indicating a late Archean crustal mass of at least 70% of the modern crust [6,7]. Beyond that, the mantle has largely “forgotten” its Archean and Hadean history:
The residual mantle after Hadean crust extraction was left with heterogeneous 142Nd/144Nd ratios and variably elevated Nb/U ratios, both of which were subsequently homogenized. Current mantle memory of continental recycling is limited to extreme EM-type OIBs with late-Archean or younger recycling ages. Post-Archean mantle differentiation was dominated by ocean-crust recycling (6 ×1022 kg Gyr-1), which differentiated the Sm/Nd ratios of the average MORB reservoir (ε(Nd) = 8.6±2.3) from the average OIB reservoir ε(Nd) = 4.4±2.3) without differentiating their Nb/U ratios. Continental recycling (0.5×1022 kg Gyr-1) of late Archean and younger crust was subordinate and is traceable only in extreme EM-type OIBs. This general mantle amnesia contrasts sharply with the preservation of near-primordial W and noble gas signatures which must have survived in isolated mantle reservoir(s) or in the core.
[1] Jacobsen, S.B. & Wasserburg, G.J., JGR 84, 7411-7427 (1979).
[2] Hofmann, A.W. EPSL 90, 297-314 (1988).
[3] Korenaga, J., Phil. Trans.Royal. Soc. A., 378, 20170408 (2018)
[4] Campbell, I.H. GCA 66, 1651 (2002).
[5] Hofmann, A.W., Class, C., Goldstein, S. L., G3 submitted (2022).
[6] Campbell, I.H., Am. J. Sci. 303, 319-351 (2003).
[7] Hofmann, A.W., Puchtel, I., Chauvel, C. Goldschmidt Abs. 2020, https://doi.org/10.46427/gold2020.1047
