Early mantle differentiation and its thermal consequences

Abstract

Initial positive ϵ Nd values of early Archean continental crust imply derivation from depleted (high Sm Nd ratio) mantle sources. A balancing enriched (low Sm Nd ratio) region is therefore required to exist contemporaneously somewhere on the Earth during the early Archean. The essentials of the mass balance involved are presented. It is shown that the enriched reservoir must contain a large proportion (∼ 30–70%) of the total Nd budget of the differentiated system, even if it was formed only shortly after the accretion of the Earth. This amount of Nd is comparable to or greater than the present-day continental inventory relative to the upper mantle. If this “enriched” reservoir was “continental crust,” the small proportion of surviving early Archean continent requires that nearly all of it was destroyed. However, surviving Archean metasediments and granites, in general, have primitive or depleted initial Sr and Nd isotopic compositions, and this does not suggest the existence of a large continental crust at this time. If the “enriched” reservoir was the result of early mantle melting, it is more likely to have consisted predominantly of basalt. It is shown that partial melting can yield appropriate Sm Nd fractionations and Nd depletions for the Archean depleted mantle if the melting was dominated by episodes with melt fractions in the range ∼2–10%. Such degrees of melting are far less than occur beneath spreading ridges today; for a hotter Archean mantle, melt fractions would be even greater than today. Therefore, there is considerable difficulty reconciling this with a plate tectonic regime in which isentropic melting due to plate spreading occurred to the near surface. Instead, we propose that no significant plate spreading occurred. In this case, an alkalic crust of basalt would be produced because any isentropic melting in the mantle would be of much more limited extent than occurs beneath ocean ridges. A crust built up serially in this way, rather than laterally, appears the most likely candidate for the enriched reservoir balancing the positive ϵ Nd values of the depleted mantle in the early to middle Archean. The existence of such a layer has important implications for the thermal and tectonic history of the Earth. Highly incompatible elements, including the heat-producing elements (K, Th, and U) would be concentrated there. Therefore, in spite of the greater overall global heat production, intrinsic radiogenic heating of the depleted mantle would have been low even during the earliest Archean. The present-day Urey ratio of the Earth (the ratio of radioactive heat production to heat loss from the Earth today) of 0.48 ± 0.12 therefore requires (1) sources of radiogenic heat in the deep mantle and (2) storage of this heat for > 1 Ga. Consequently, the mantle must be chemically layered with inefficient advective transport of material from the lower mantle to shallow levels.