Constraints on Lu‐Hf and Nb‐Ta systematics in globally subducted oceanic crust from a survey of orogenic eclogites and amphibolites

To further understand Lu‐Hf and Nb‐Ta systematics in globally subducted oceanic crust, this paper evaluates all available Lu‐Hf garnet isochron ages and initial ɛ Hf values in conjunction with present‐day bulk‐rock Lu‐Hf isotope and trace element (K, Nb, Ta, Zr, and Ti in addition to Lu‐Hf) data from the world's orogenic eclogites and amphibolites (OEAs). Approximately half of OEAs exhibit Lu‐Hf and Nb‐Ta systematics mimicking those of unsubducted oceanic crust whereas the rest exhibit variability in one or both systems. For the Lu‐Hf system, mixing calculations demonstrate that subduction‐related phase transformations, in conjunction with open system behavior, can shift subducted oceanic crust toward higher Lu/Hf, or toward lower Lu/Hf that can also be associated with unradiogenic ɛ Hf values. However, evaluation of potential mechanisms for fractionating Nb from Ta is more complicated because many of the OEAs have Nb‐Ta systematics that are decoupled from Lu‐Hf and the behavior of K, Zr, and Ti. Nonetheless, the global data set demonstrates that the association between unradiogenic ɛ Hf and elevated Nb/Ta observed in some kimberlitic eclogite xenoliths can be inherited from processes that occurred during subduction of their oceanic crustal protoliths. This allows for a geologically based estimate of the Nb concentration in a reservoir composed of deeply subducted oceanic crust. However, mass balance calculations confirm that such a reservoir, when considered as a whole, likely has a Nb concentration similar to unsubducted oceanic crust and is therefore not the solution to the problem of the Earth's “missing” Nb.