9. Proposed Sources of Hadean Zircons

Any successful geodynamic or environmental model for early Earth must be consistent with ten robust lines of evidence derived from geochemical and petrologic observations of Hadean Jack Hills zircons. These are: (1) a zircon sub-population enriched in ¹⁸O and depleted in ³⁰Si relative to mantle values; (2) low crystallization temperatures; (3) the presence of primary hydrous mineral inclusions; (4) the predominance of magmatic muscovite, quartz, and biotite inclusions; (5) zircon formation in relatively low heat flow environments; (6) sub-chondritic initial ¹⁷⁶Hf/¹⁷⁷Hf ratios consistent with source isolation as early as 4.50 Ga; (7) fission Xe isotope compositions indicating variable fractionation of Pu from U; (8) the absence of ultra-high pressure mineral inclusions; (9) zircon formation under a wide range of redox conditions; and (10) geochemical signatures diagnostic of felsic continental crust. Numerous models have been proposed to explain these characteristics, including an origin similar to Icelandic rhyolites or lunar KREEP terranes, crystallization from mafic igneous rocks, formation in impact melts or sagduction, plate boundary and heat pipe tectonic environments, and multi-stage scenarios involving several of these mechanisms. While an origin of Jack Hills Hadean zircons in felsic and intermediate granitoids in a plate-boundary-type setting is consistent with all ten geochemically-derived constraints, competitor models are either only partially consistent or inconsistent with the evidence.