Are zircons forever? On the problem of survival of inherited zircons in juvenile crust and lithospheric mantle

• Applicants: Professor Dr. Alfred Kröner (†); Dr. Yamirka Rojas-Agramonte, Ph.D.
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2014 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 252610233

Earth sciences experienced a far-reaching revolution since ion microprobe technology (e.g. SHRIMP) and high-resolution ICP-MS were developed and routine in-situ U-Th-Pb analysis of domains in U-bearing minerals (e.g. zircon) became available. One of the mayor discoveries using SHRIMP was the occasional presence of inherited (xenocrystic) zircons, or cores of zircons, much older than the host rock. Such inherited grains were even found in juvenile igneous rocks generated and emplaced in intra-oceanic environments, far away from any continent. The presence of inherited zircons in these environments challenges our view of mantle geodynamics and suggests the existence of yet unknown recycling processes, possibly even involving the deep mantle. The problem of zircon inheritance is not yet fully understood and has not been seriously addressed despite its considerable implications for crustal evolution and mantle geodynamics. Therefore, resolving the origin and conditions of preservation of exotic zircon in seemingly juvenile, mantle-derived rocks is the target of this project. We will address this long-standing geological problem by taking into consideration various geodynamic settings (e.g., intra-oceanic fore-arc, arc, back-arc, mid-ocean ridge and ocean island) of different ages and by using rock samples from the Caribbean (Cuba and Grenada Islands), Galápagos, and Central Asia (Mongolia and Kyrgyzstan). We follow a novel approach in combining U/Pb age dating, Lu/Hf and O-isotopes in zircons, Re/Os isotopes in mantle-derived sulfides as well as chemical analyses (e.g. element/isotope distribution in zircons using SHRIMP II, LA/ICP/MS and EPMA) and the study of the chemical composition of mineral inclusions in zircon. The Hf and O isotope information will be obtained on SHRIMP-dated zircon grains. Whole rock geochemistry (major and trace elements) and mineral analysis will be routinely performed on rock samples from which the zircons are dated. We shall also study the detrital zircon population of intra-oceanic arc sediments from Cuba and Central Asia in order to compare the provenance ages with the age of inherited grains. These analytical methods will be complemented by experiments using piston cylinder and multi-anvil presses at Mainz and Frankfurt Universities, respectively. The Pb-diffusivity in reidite (high-pressure ZrSiO4 polymorph) will be investigated for the first time. The project will demonstrate how advanced small-domain geochronology, geochemistry and experimental petrology can be used to understand crust formation and evolution and mass-recycling at active plate margins.

• DFG Programme: Research Grants
• International Connection: Spain
• Cooperation Partners: Professor Antonio García-Casco, Ph.D.; Dr. Axel Gerdes; Professor Joaquín Antonio Proenza, Ph.D.