LU-HF AND SM-ND GEOCHRONOLOGICAL CONSTRAINTS ON THE INFLUENCE OF SUBDUCTION METAMORPHISM IN CONTROLLING THE HF-ND TERRESTRIAL ARRAY: EVIDENCE FROM THE WORLD'S OROGENIC BELTS

To better understand the influence of subduction metamorphism on the Hf–Nd Terrestrial Array, all available Lu–Hf and Sm–Nd garnet isochron ages from the world’s orogenic eclogites and amphibolites are examined. The garnet isochron y-intercept values provide a record of each sample’s epsilon Hf (Ɛ_Hf) and Nd (Ɛ_Nd) at the age recorded by the isochron, which in the case of most orogenic eclogites and amphibolites is the time of prograde metamorphism during subduction. It is further possible to assess the extent to which each sample’s Lu–Hf and Sm–Nd isotope systematics have remained undisturbed since the age recorded by the isochron by comparison of the bulk-rock isotope systematics with the isochron initial value. This approach provides a means of understanding the Lu–Hf and Sm–Nd systematics of post-subduction metamorphic oceanic crust. On a global scale, the coupled Lu–Hf and Sm–Nd isotope systematics of this crust closely align with the Hf–Nd Terrestrial Array and have average ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁴⁷Sm/¹⁴⁴Nd ratios within error of the chondritic uniform reservoir values. This is an important finding because it suggests that a reservoir of deeply subducted oceanic crust, possibly residing at the core mantle boundary, is unlikely to be significantly fractionated from the Hf–Nd Terrestrial Array. In contrast to the samples of subduction metamorphosed oceanic crust, examination of garnet isochron initials and bulk-rock Lu–Hf and Sm–Nd isotope systematics from metamorphic rocks with a continental crustal protolith indicate significant decoupling from the Hf–Nd Terrestrial Array. However, because this material is not readily subducted, this strongly decoupled signature is unlikely to be transferred into deep Earth. Mixing calculations aimed at developing a clearer understanding of what factors control decoupling of a sample’s Lu–Hf and Sm–Nd systematics from the Hf–Nd Terrestrial Array highlight the complexity of mass transfer regimes operating in the subduction environment.

*The full published version of this work can be found in Geosphere 15(3): doi.org/10.1130/GES02051.1