We present a global compilation of major element, as well as Re-Os isotope, data on mantle xenoliths from continental lithosphere to constrain the secular evolution of mantle depletion since the early Archean. Whereas a temporal dichotomy in the degree of mantle depletion has long been recognized in previous regional studies of mantle xenoliths, this global compilation reveals, for the first time, a smooth secular trend in mantle depletion, which is in remarkable agreement with what is expected from the secular cooling of the ambient mantle as inferred from the petrology of non-arc basalts. Depleted mantle now composing continental lithosphere is likely to have been originally formed beneath mid-ocean ridges or similar spreading environments, and a greater degree of depletion in the past can be seen as a corollary of the secular cooling of the mantle.

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