Evolution Of The Archean Continental Crust And Link To The Great Oxidation Event Traced By Mg,Fe Isotopes

The composition change and redox state of the early continental crust rocks is critical to understand the role of the deep solid earth processes in the formation of the habitable environment of the Earth.The early crust is comprised with Archean Na-rich tonalite-trondhjemite-granodiorite(TTGs),however,decreased in abundance at the late Archean.The crustal composition transformed from being Na-rich to K-rich accompanied by the increased potassic granites during the late Archean,marking the enhanced crust maturity.Although,how continental crust become more potassic remains poorly constrained.Whether such compositional change has genetically linked to the rise of atmospheric O2 remains poorly known.To address these questions,we investigated Mg and Fe isotopes of Archean metabasalts,TTGs and potassic granites from the North China Carton(NCC),owing to the advantages of Mg isotopes in tracing the source nature of igneous rocks and the sensitivity of Fe isotopes on magma redox state.Combined with their petrographic geochemistry characteristics and 4,066 data compiled from worldwide Archean igneous felsic rocks,we explored the mechanism on the transformation of continental crust from being Na-rich TTG-dominated to K-rich granite-dominated,the redox state of relevant granitoids and their contribution to the rise of atmospheric O2.A few conclusions have been drawn as follows:(i)TTGs have similar ?26Mg of-0.39‰?-0.13‰ to those of metabasalts(?26Mg=-0.27‰‰?-0.09‰),consistent with the derivation by partial melting of(meta)basalts at different depth/pressure.Potassic granites can be subdivided into I-and S-type.I-type Archean potassic granites have mantle-like ?26Mg values of-0.28‰to-0.22‰,are derived from pre-existing TTGs or high-K basalts.While,S-type potassic granites have substantially distinctive ?26Mg of-0.65‰ to+0.91‰.Given the limited fractionation of Mg isotopes during magmatic differentiation,such varied?26Mg of these granites reflect that their sources are sediments with large variation of?26Mg caused by surficiai low-temperature processes.(ii)Based on the 4066 statistical data of worldwide Archean felsic igneous rocks,the S-type potassic granites are present in the middle Archean(3.5-3.0Ga)and widespread in in the late Archean(3.0-2.5 Ga).Considering sediments as the source of S-type potassic granites are produced by continental weathering,temporal distribution of S-type granites reflects the onset of continental weathering may begin from the early Archean(4.0-3.5 Ga).This study estimated the proportion of Archean I-,S-type potassic granites and their contribution to making continental crust more potassic,which reflects the synergy of intracrustal melting and surficial weathering in crust compositional change from being-sodic to being-potassic,by which the former contributed more quantitatively,while the latter contributed more efficiently.(iii)Archean metabasalts have 856Fe of 0.044‰-0.120‰,which are identical to those of Eoarchean and modern basalts,mirroring constant redox-state of basalt from the Archean.TTGs and I-type potassic granites show ?56Fe of 0.075‰-0.201‰ and 0.135‰-0.230‰,respectively,which can be explained by differentiation of magmatic melt compositions.While,S-type potassic granites show higher ?56Fe of 0.178‰-0.506‰,suggesting anoxic continental weathering caused the loss of Fe2+ in supracrustal materials and melting of such clastic sediments could produce more oxidized melts.Consequently,as S-type potassic granites increased in abundance at the late Archean,the more oxidized volcanic gases equilibrated with them are enhanced,thus promote the rise of atmospheric O2.