Return of oceanic crust to the mantle through subduction was explicitly part of plate tectonic theory (Isacks et al., 1968; Le Pichon, 1968), but it took longer for the idea that sediment, and therefore indirectly continental crust, could also be subducted into the mantle. Here again, Dick Armstrong along with Mitsunobu Tatsumoto were ahead of the crowd. Tatsumoto (1969) wrote that “a mixture of one to two percent of oceanic sediment with abyssal tholeiite can roughly make a balance sheet for the Pb, U, and Th concentrations in the Japanese tholeiite.” Armstrong (1971), comparing Pb isotope ratios...

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