Graphitic inclusions in zircon from early Phanerozoic S-type granite: Implications for the preservation of Hadean biosignatures

A biotic origin of isotopically light graphite in Hadean zircon remains contested, in part because it is unclear how biogenic carbon in sediments behaves during diagenesis, metamorphism and anatexis, and how it can be preserved as inclusions in zircon. Here, we report the discovery of graphitic inclusions in zircon from Rumburk granite in the Lusatian Block of eastern central Europe as the first example of such inclusion-bearing zircon in a well-constrained geological and petrological context. Most graphite inclusions are trapped at the interface between inherited zircon interiors of early Cambrian- Proterozoic age and late Cambrian (496.2 +/- 2.3 Ma; 95% confidence) zircon overgrowths that crystallized at-700 degrees C under highly reducing conditions (typically-4.6 log units relative to the fayalite-magnetite-quartz buffer). Zircon overgrowths are also enriched in xenotime component (up to 93 mu mol/g P) and 818O (average 818O = 7.7%0), both typical for S-type granitic melts. Raman microspectroscopy reveals crys-talline and disordered graphitic carbon, but because overgrowth temperatures reached >700 degrees C, sufficient for complete graphitization, disordering is presumably secondary and caused by in-situ irradiation from the zircon host. Because organic materials were avoided during sample preparation, and inclusions were excavated by ion beam sputtering, exposure to potential C-bearing contaminants can be dismissed. Inclusions range in 813C from-44.6 to-7.5%0 with a dominant mode at 813C =-34%0 that is positively skewed. Rayleigh-type graphite precipitation from a CO2-CH4 fluid with a starting composition equiva-lent to regional black shale (813C =-32%0) explains inclusion carbon isotopic range and distribution. Collectively, these observations suggest that graphite was initially trapped in voids formed via dissolution-reprecipitation of trace element enriched, and possibly metamict, detrital zircon in metased-imentary protoliths when exposed to C-O-H fluids during prograde metamorphism. Subsequently, graphite-filled voids in inherited zircon became enclosed when zircon rims crystallized from highly reduced S-type granitic melts. Besides partial disordering of graphitic carbon due to irradiation, there is no indication for post-entrapment alteration, demonstrating that graphitic inclusions in zircon can pre-serve isotopic biosignatures over hundreds of millions of years. (c) 2023 Elsevier Ltd. All rights reserved.