Heterogeneous Hadean crust with ambient mantle affinity recorded in detrital zircons of the Green Sandstone Bed, South Africa Nadja Drabona,1,2, Benjamin L. Byerlyb,3, Gary R. Byerlyc, Joseph L. Woodend,4, C. Brenhin Kellere, and Donald R. Lowea aDepartment of Geological Sciences, Stanford University, Stanford, CA 94305; bDepartment of Earth Sciences, University of California, Santa Barbara, CA 93106; cDepartment of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803; dPrivate address, Marietta, GA 30064; and eDepartment of Earth Sciences, Dartmouth College, Hanover, NH 03755 Edited by Albrecht W. Hofmann, Max Planck Institute for Chemistry, Mainz, Germany, and approved January 4, 2021 (received for review March 10, 2020) The nature of Earth’s earliest crust and the processes by which it While the crustal rocks in which Hadean zircon formed have formed remain major issues in Precambrian geology. Due to the been lost, the trace and rare earth element (REE) geochemistry of absence of a rock record older than ∼4.02 Ga, the only direct re- these zircons can be used to characterize their parental magma cord of the Hadean is from rare detrital zircon and that largely compositions. Zircon crystallizes as a ubiquitous accessory mineral from a single area: the Jack Hills and Mount Narryer region of in silica-rich, differentiated magmas formed in a number of crustal Western Australia. Here, we report on the geochemistry of environments. Since zircon compositions are influenced by varia- Hadean detrital zircons as old as 4.15 Ga from the newly discov- tions in melt composition, coexisting mineral assemblage, and trace ered Green Sandstone Bed in the Barberton greenstone belt, element partitioning as a function of magmatic processes, temper- South Africa. We demonstrate that the U-Nb-Sc-Yb systematics ature, and pressure (14–17), zircon geochemistry provides valuable of the majority of these Hadean zircons show a mantle affinity constraints on magmatic compositions and processes at the time of as seen in zircon from modern plume-type mantle environments zircon saturation. Consequently, trace element compositions of ig- and do not resemble zircon from modern continental or oceanic arcs. The zircon trace element compositions furthermore suggest neous and detrital zircon are a powerful tool that can be used to magma compositions ranging from higher temperature, primitive track melt evolution of individual magma bodies (18, 19) and EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES to lower temperature, and more evolved tonalite-trondhjemite- changes within a tectono-magmatic system (20) or across an entire granodiorite (TTG)-like magmas that experienced some reworking continent (21). Grimes et al. (16) pioneered the use of zircon trace of hydrated crust. We propose that the Hadean parental magmas of element geochemistry to differentiate modern tectono-magmatic the Green Sandstone Bed zircons formed from remelting of mafic, environments based on differences in source compositions (e.g., mantle-derived crust that experienced some hydrous input during depleted versus undepleted mantle) and magmatic process (flux melting but not from the processes seen in modern arc magmatism. melting in arc settings) by using a family of discrimination diagrams. They used a compilation of ∼5,300 zircons from various crustal Hadean | zircon | early Earth | crustal evolution environments across the globe and found that, despite internal heterogeneity due to petrological processes, bivariate diagrams us- nderstanding the nature of Earth’s earliest crust and con- ing U, Nb, Sc, Yb, Gd, and Ce can be used to distinguish between Ustraining the timing of early continent formation are crucial to modeling the evolution of the geodynamic system and the Significance compositions of Earth’s early atmosphere and hydrosphere. Consensus over the nature of the earliest crust remains elusive. The nature of Earth’s earliest crust is enigmatic due to the lack General models range from those suggesting an onset of continent of a rock record for most of Earth’s first ∼600 My, the Hadean formation (1, 2) and possible formation of arcs in a plate tectonic Eon. Studies have thus turned to scarce sites where Hadean regime similar to that of today (3) shortly following Earths for- detrital zircons have been discovered. The geochemistry of mation to studies suggesting that the earliest crust was overall Hadean detrital zircon from a newly discovered site in South more mafic than modern curst in composition and continents ei- Africa suggests that the parental melts formed from variably ther rare or absent (4–7). Detrital zircons provide the only direct hydrous melting of crust derived from the ambient mantle and record of Earth’s first 500 million years of history. The most sig- show little evidence for an origin in arc-like settings. These nificant source of Hadean zircons are Archean sedimentary rocks results suggest that crust derived from ambient mantle played in the Jack Hills and Mount Narryer region in Western Australia. an important role during crust formation in the Hadean. Isolated Hadean zircons have also been found in a dozen other locations worldwide (8, 9). Of these locations, the Green Sand- Author contributions: N.D. and D.R.L. designed research; N.D., B.L.B., and G.R.B. per- stone Bed (GSB) in the Barberton greenstone belt, South Africa, formed research; N.D. and J.L.W. analyzed data; C.B.K. did zircon saturation modeling, stands out with a total of 33 Hadean zircons discovered to date and N.D. wrote the paper. (10) (Datasets S1 and S2 and SI Appendix,Fig.S1). The GSB is The authors declare no competing interest. dated by the S6 spherule layer that lies 2 m below the GSB and has This article is a PNAS Direct Submission. a depositional age of ∼3.31 Ga (11). The detrital zircons in the Published under the PNAS license. GSB show a major age peak at 3.38 Ga and include a significant 1To whom correspondence may be addressed. Email: [email protected]. number of zircons older than the oldest known igneous rocks and 2Present address: Department of Earth and Planetary Sciences, Harvard University, strata in the 3.55 to 3.22 Ga Barberton greenstone belt (10). In Cambridge, MA 02138. total, 0.5% of the analyzed zircons from the GSB are Hadean in age 3Present address: Enterprise Services, Thermo Fisher Scientific, Waltham, MA 02451. (10). Throughout its geological history, the GSB has experienced 4Retired author. only lower greenschist-grade metamorphism (12, 13) and remains This article contains supporting information online at https://www.pnas.org/lookup/suppl/ essentially unsheared. As a result, it contains well-preserved primary doi:10.1073/pnas.2004370118/-/DCSupplemental. mineral grains. Published February 18, 2021. PNAS 2021 Vol. 118 No. 8 e2004370118 https://doi.org/10.1073/pnas.2004370118 | 1of9 Downloaded by guest on September 26, 2021 different tectono-magmatic settings, even if the crust experienced the zircons formed by simple fractionation from a single magma subsequent reworking within the same crustal setting. composition (SI Appendix,Fig.S3). In this study, we use the geochemistry of GSB Hadean zircons Besides these general characteristics, there is an apparent to characterize the compositions and possible sources of their clustering in the data. The U/Yb data show a separation of the parental magmas, highlight similarities and differences to zircon zircon compositions into two groups (here termed as Group I and from Phanerozoic tectono-magmatic settings, and compare the Group II) that, based on a PCA and bivariate plots, also translates results to the geochemistry of Hadean zircons from the Jack Hills to general differences in other trace element ratios (Figs. 2 and 3 to make broader inferences about the nature of Hadean crust and and Dataset S3). It is important to note that due to the low ultimately determine whether they originated from an arc or a number of zircons, it is difficult to evaluate if these groups rep- relatively undepleted mantle environment. resent distinct source magmas or what is actually a continuum of compositions. However, discussing these as separate composi- Geochemistry of Hadean Zircons tional types of zircons makes it easier to compare and contrast the Zircon texture and geochemistry have long been used to differ- different possible processes or sources that could produce the entiate igneous from metamorphic zircon (22). Hadean zircons observed variations. Zircons of Group I (5 zircons with 11 analy- from the GSB show oscillatory and sector zoning typical of igne- ses) shows less spread in its compositional averages with lower ± ± ous zircon (10) (Fig. 1 and SI Appendix,Fig.S2). A total of five U/Yb (0.15 0.015), higher Th/U (0.91 0.16), higher Gd/Yb ± ± ± zircons show igneous cores with recrystallization rims indicative of (0.09 0.01), lower Nb/Yb (0.01 0.01), lower Nb/Sc (0.29 ± ± metamorphic overgrowth. These rims were not analyzed for this 0.06), higher Ti (8.6 3.3 ppm), and elevated Sc/Yb (0.08 0.04). study. None of the Hadean zircons found to date display a spongy In contrast, Group II (10 zircons with 16 analyses) shows a wider ± ± texture indicative of fluid-dominated recrystallization. In Phaner- spread with elevated U/Yb (0.61 0.61), lower Th/U (0.54 ± ± ozoic zircon, a Th/U ratio <0.1 serves as an indicator for meta- 0.21), lower Gd/Yb (0.05 0.02), lower Ti (5.1 3.8 ppm), higher Nb/Yb (0.04 ± 0.04), higher Nb/Sc (1.0 ± 0.02), and lower Sc/Yb morphic zircon (22). In the GSB zircons, Th/U ratios vary from ± 0.18 to 1.15, supporting a primary igneous origin. Similarly, GSB (0.05 0.02). There is some variability in the geochemical com- position between different spots on the same grain.