CONSTRAINTS ON LU-HF AND NB-TA SYSTEMATICS IN GLOBALLY SUBDUCTED OCEANIC CRUST FROM A SURVEY OF OROGENIC ECLOGITES AND AMPHIBOLITES

In the past 20 years, the Lu-Hf system has been widely applied to orogenic eclogites and amphibolites (OEAs) to reveal their metamorphic histories in relation to the tectonic evolution of plate boundaries. Because many of these OEAs are fragments of oceanic crust that have undergone subduction-related changes before being exhumed to the surface (instead of continuing their journey into the deeper parts of the mantle), they also have the potential to provide insights into the geochemistry of the significant quantity of oceanic crust that has been subducted over Earth’s history. This talk explains how Lu-Hf geochronologic data from mountain belts can also be used to understand the role of subducted oceanic crust in a variety of terrestrial geochemical mass-balances, with a focus on the problem of Earth’s ‘missing’ Nb. This is achieved by evaluating all available Lu-Hf garnet isochron ages and initial ε_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 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 towards higher Lu/Hf, or towards lower Lu/Hf that can also be associated with unradiogenic ε_Hf values. However, evaluation of potential mechanisms for fractionating Nb from Ta is more complicated because many of the OEA’s have Nb-Ta systematics that are decoupled from Lu-Hf and the behavior of K, Zr, and Ti. Nonetheless, the global dataset demonstrates that the association between unradiogenic ε_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 un-subducted oceanic crust and is therefore not the solution to the problem of the Earth’s ‘missing’ Nb.