Zircon survival in the mantle
A grand challenge in Earth Science is to unravel the forces driving our planet’s unique mode of mantle convection, including plate tectonics and volcanism, that shaped the evolution of its surface, climate, and life. Exciting new discoveries in geochronology and isotope geochemistry reveal much larger heterogeneities (memories of past geological processes) in the upper and lower mantle than hitherto thought. For example, the finding of surviving zircon grains in young lavas at the Central and South Atlantic ridges, in island arc volcanoes (Rojas-Agramonte et al, 2016; 2017), ‘supra-subduction zone’ ophiolites that formed from the mantle above subduction zones as well as on the young Galapagos (Rojas-Agramonte et al, 2022), and Hawaii Islands.
These new findings have the potential to unravel mixing processes in the upper mantle. In addition, the mere presence of these grains may explain that enigmatic isotopic compositions in hotspots (e.g., continental sediment-derived signatures of EM1-type hotspots, or isotopic asymmetry in hotspot islands) could result from upper mantle contamination. If it does, this would significantly change our interpretation of (lower) mantle composition and mantle dynamics.
Together with project partner Dr. Stephan Buhre from Mainz University we plan to conduct a series of piston-cylinder and multi-anvil P,T,t diffusion experiments to determine the closure temperature of the U/Pb system in zircon and reidite (HP polymorph) and establish the depth interval in the upper mantle under which zircon minerals can survive and retain their isotopic signatures. Here we plan to use SIMS inverse depth-profiling of Pb in zircon and reidite. In addition, I want to Identify the sources and pathways of very old zircons with continental and oceanic signatures in oceanic environments. This will be achieved by using isotopic compositional studies in zircon (U-Pb/Hf/O/He/REE by SIMS and LA-ICP-MS), Raman spectroscopy and mineral inclusions in zircon and through a combination of whole-rock geochemical studies of trace elements and radiogenic isotopes (Sr-Nd-Pb-Hf by LA-MC-ICP-MS).
Project partners: Douwe J.J. van Hinsbergen (Utrecht University), Arwen Deuss (Utrecht University), Janne Koornneef (VU University Amsterdam), Antonio García-Casco (Granada University), Stephan Buhre (Mainz University)