Zircon is a widespread accessory mineral found in a variety of rocks. It can retain its original geochemical information due to its exceptionally stable physical and chemical characteristics. When rocks break down, zircon remains as stable detritus in sedimentary rocks, making it a valuable "time capsule" of early geological history. Hadean zircons have been found in many sedimentary rocks around the world, the most famous being the Jack Hills conglomerate in Australia. The conglomerate contains not only the oldest known zircons on Earth (up to 4.4 billion years old) (Wilde et al., 2001), but also a large number of ancient zircons with ages ranging from 4.4 billion to 3.3 billion years old, providing a rare continuous record for the study of the early evolution history of the Earth. Over the past 20 years, a variety of geochemical, geophysical, and mineralogical methods have been employed to investigate them (e.g., Ti thermometers, O-Hf- Li isotopes, U-Pb dating and trace elements) (Harrison et al., 2005; Peck et al., 2001; Tarduno et al., 2020; Turner et al., 2020; Wilde et al., 2001) and numerous research achievements has been. For the first time, the simultaneous analysis of water content and H-O isotopes of these ancient detrital zircons was performed using Secondary Ion Mass Spectrometry (SIMS) hosted in Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. The results show that the oxygen isotopes of these zircons are mostly in the range of 5-7permil, which is consistent with previous studies. The first water content data shows that the zircons mostly contain hundreds ppm of water, with a wide range of hydrogen isotopes ranging from -180 to +150 permil. What is remarkable is that such a large range of hydrogen isotopes occurs only in zircons that are about 3.9 billion years old. The rest of the zircons have significantly smaller ranges of hydrogen isotopes, ranging from -130 to +9. Raman spectroscopic analysis of zircons shows that these zircons have been metamict to some extent, and it is possible that water may have entered the zircons in the later period, resulting in increased water content in zircons and affecting hydrogen isotope composition to some extent. However, we believe that the late addition of water is not enough to completely change the hydrogen isotope of zircon. Combined with previous studies, the 3.9-billion-year zircon has a significantly different halogen composition from other zircons (Tang et al., 2019). We believe that the wide range of hydrogen isotope changes in the 3.9 billion zircons records a major event in Earth's history, the Late Heavy Bombardment (LHB). The "Late Heavy Bombardment" refers to a high-frequency collision event that affected the inner solar system 4 billion to 3.8 billion years ago. The most direct evidence for LHB is the widespread impact craters on the surfaces of other planets such as the Moon and Mars during this period and the melting age of meteorite impacts from the asteroid belt. Earth is also thought to have experienced a LHB impact event, but plate tectonic activity has erased most records including the impact craters. The impact was strong enough to melt most of the Earth's crust at the surface, leaving rich magmatic activities, and we think the meteorite impact carried a wealth of material with distinguishable H isotopic composition from the Earth itself, which has been preserved in the Jack Hills detrital zircons. 

Reference(s)

Harrison, T.M., Blichert-Toft, J., Muller, W., Albarede, F., Holden, P., Mojzsis, S.J., 2005. Heterogeneous Hadean hafnium: Evidence of continental crust at 4.4 to 4.5 Ga. Science 310, 1947-1950. 

Peck, W.H., Valley, J.W., Wilde, S.A., Graham, C.M., 2001. Oxygen isotope ratios and rare earth elements in 3.3 to 4.4 Ga zircons: Ion microprobe evidence for high delta O-18 continental crust and oceans in the Early Archean. Geochimica Et Cosmochimica Acta 65, 4215-4229. 

Tang, H., Trail, D., Bell, E.A., Harrison, T.M., 2019. Zircon halogen geochemistry: Insights into Hadean-Archean fluids. Geochemical Perspectives Letters 9, 49-53. 

Tarduno, J.A., Cottrell, R.D., Bono, R.K., Oda, H., Davis, W.J., Fayek, M., van't Erve, O., Nimmo, F., Huang, W.T., Thern, E.R., Fearn, S., Mitra, G., Smirnov, A.V., Blackman, E.G., 2020. Paleomagnetism indicates that primary magnetite in zircon records a strong Hadean geodynamo. Proceedings of the National Academy of Sciences of the United States of America 117, 2309-2318. 

Turner, S., Wilde, S., Worner, G., Schaefer, B., Lai, Y.J., 2020. An andesitic source for Jack Hills zircon supports onset of plate tectonics in the Hadean. Nature Communications 11, 5. Wilde, S.A., Valley, J.W., Peck, W.H., Graham, C.M., 2001. Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature 409, 175-178.