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. This is typical Hadean zircons show chondrite-normalized REE patterns with for igneous zircon and reflects the expected changes in the melt light REE (LREE) depletion, heavy REE (HREE) enrichment, a composition driven by melt fractionation and changing zircon par- positive Ce anomaly, and a negative Eu anomaly (Fig. 1), con- tition coefficients during zircon growth (14, 15). However, all sistent with an igneous source (23). analyses on the same grain fall into the same group. This shows that, To evaluate the compositions and compositional variability while there were the expected compositional variations during in- among GSB zircons, it is important to focus on element ratios for dividual zircon growth, the general compositional differences be- comparisons between the zircons, rather than absolute abun- tween the two groups remain. dances, to minimize the effect of the melt temperature on mineral The Ti abundance in zircon can be used to calculate the tem- partition coefficients (Kds) (16). Previous studies have shown that perature of the melt at which the zircon crystallized (Txlln)(24). cation ratios including U, Th, Nb, Sc, Ce, and Yb provide the most However, this calculation requires knowledge of the αSiO2 and compositional distinction among zircons from different tectono- αTiO2 of the parental magma. The absence of a petrologic context magmatic settings (16). To understand compositional differences and of identifiable mineral inclusions on the polished surfaces of between the zircons, we performed a simple principal component the Hadean GSB zircons means that it is currently not possible to analysis (PCA) using trace element ratios and plotted bivariate directly constrain αTiO2 and αSiO2. While the presence of quartz diagrams (Fig. 2). The results of the PCA show a clear correlation can generally be assumed (i.e., αSiO2 is usually close to unity), between Sc/Yb, Th/U, and Gd/Yb values. In turn, these values αTiO2 is variable but generally >0.6 for igneous systems (25, 26). xlln show a moderate negative correlation to the other ratios, U/Nb, For T calculations, we used an αTiO2 and αSiO2 of 1, which Th/Yb, Ce/Yb, and Hf/Ti. In general, U/Yb directly correlates assumes the coexistence of zircon with quartz and rutile in the with Th/Yb, and the majority of zircons fall on broad trends of melt. These temperatures represent a minimum estimate unless increasing U/Yb with increasing Nb/Yb and Nb/Sc and decreasing co-crystallization with rutile can be established, leading to the Sc/Yb and Gd/Yb. While there are several general trends between possibility that temperatures are underestimated by up to 50 °C many trace element proxies, there is no coherent trend between (24). For consistency, we calculated Txlln for zircons from different age and geochemical proxies (e.g., U/Yb), as would be expected if environments and locations using the same assumptions (Fig. 3).
A BC 10000 3.55 Ga Theespruit Rhyolite Group I zircon 3.51 Ga Steynsdorp Granodiorite 1000 3.45 Ga Hooggenoeg Dacite 3.55 Ga Bien Venue Rhyodacite 100 3.23 Ga Kaap Valley Tonalite 100 Group I average model melt Group II average model melt 10
1 10 Group II zircon REE / chondrite Group I 0.1 Group II
0.01 1 La Ce Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 20 μm
Fig. 1. Chondrite-normalized REE abundance of Hadean zircons from the GSB. (A) REE patterns of Hadean Group II show depletion in MREE relative to Group I. (B) Average model melt composition derived using Ti-calibrated Kds (17) compared to known Paleoarchean felsic igneous rocks in the Barberton greenstone belt (74–76). Group I Hadean zircons show similar model melt compositions to rhyolite of the 3.55 Ga Theespruit and rhyodacite of the 3.25 Ga Bien Venue Formations; Group II shows a similarly low HREE abundance to the 3.45 Ga Hooggenoeg dacite and the 3.23 Ga Kaap Valley Tonalite. LREEs were not evaluated because their low abundances and the potential influence of minute inclusions makes the LREEs inherently unreliable (16). (C) Representative CL images of zircons from Group I and Group II.
2of9 | PNAS Drabon et al. https://doi.org/10.1073/pnas.2004370118 Heterogeneous Hadean crust with ambient mantle affinity recorded in detrital zircons of the Green Sandstone Bed, South Africa Downloaded by guest on September 26, 2021 AB
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Fig. 2. (A) PCA in zircon trace element ratios. Principal component 1 accounts for 51.1% and principal component 2 for 16.1% of the variability. (B and C) Zircon trace element plots modified after Grimes et al. (16). (B) U/Yb ratio versus Hf for zircons from the GSB. (C) U/Yb versus Nb/Yb plot with inset showing the effect of open-system (Rayleigh) fractionation of select minerals on the displayed system (16). (D) U/Yb versus Nb/Sc plot. Colored fields represent a global compilation of zircons from different Phanerozoic tectono-magmatic settings. The outer contour line is shown at the 95% level, which represents the amount of the distribution inside the contour. Green circles represent Group I zircons; red circles represent Group II.
Although the calculation of absolute temperatures is not possible, oceanic islands (16). Hence, this positive correlation may be related a comparison under similar assumptions remains meaningful, to differences in the original source compositions of the parental particularly in the case of the rare Hadean zircons. The two groups melts and/or relative melt evolution (Fig. 2C). show some overlap. Group I zircons have an average Ti value of Zircons of Group I show lower U/Yb and Nb/Yb ratios and less 8.6 ± 3.3 parts per million (ppm), reflecting a Txlln of 723 ± 44 °C spread in values compared to those of Group II (Fig. 2C). Thus, (Fig. 3). In contrast, Group II, while showing a wider spread in the elevated values indicate relatively more evolved melts for temperatures, has a comparatively lower average Ti value of 5.1 ± Group II compared to Group I as well as more spread, which we 3.8 ppm, reflecting a Txlln of 668 ± 57 °C. interpret to reflect some degree of heterogeneity of the crustal sources of melts with otherwise generally similar characteristics. A Compositional Variability and Possible Petrogenetic few zircons of Group II fall off this trend toward higher U/Yb and Implications relatively lower Nb/Yb values relative to mantle-derived zircon. In The trace element compositions of the two zircon groups show general, a U/Nb >40 indicates a clear hydrous melting signature certain differences that reflect the different natures of the parental caused by flux melting in modern arc settings (16). However, a magmas (Fig. 2). This is exemplified in the U–Nb systematics. The U/Nb value of >40 is only reached in one of the zircons (with two relationship between U/Yb and Nb/Yb in zircon from modern analyses), and the other zircons show values predominantly in magmatic systems is influenced by variations in mantle source agreement with a nonarc, mantle origin. composition (as today between modern mid-oceanic ridge and Sc in zircon also tracks mineral fractionation behavior and can plume magmas), aqueous fluid input (as today in arc magmatism), detect flux melting (16). Sc in zircon can track the Sc depletion of and crustal assimilation (16). In general, with increasing fraction- the melt during open-system fractionation of ferromagnesian ation of a melt, the U/Yb and Nb/Yb ratios will increase as a re- minerals involving amphibole and/or clinopyroxene, driving the Sc/ flection of increasing fractionation of zircon, apatite, and garnet out Yb value down (16, 27, 28). In general, with increasing fraction- of the magma (Fig. 2). Consequently, U/Yb and Nb/Yb values in ation of a melt during cooling, Sc and Sc/Yb in zircon decrease zircon increase with increasing melt crystallization, and more while U/Yb and Hf increase (16, 28). Overall, Group II zircons evolved parental melts in general tend to have zircons with higher show lower Sc/Yb values (average of 0.05 ± 0.02) at lower Txlln and values. Group I and many of Group II zircons define a positive higher U/Yb values compared to Group I zircons (average 0.08 ± trend between U/Yb and Nb/Yb—that is, parallel to the vector 0.04), suggesting that Group II zircons experienced a higher degree characterizing the fractional crystallization of zircon and apatite. of fractionation. Furthermore, a lower degree of fractionation is That trend parallels today’s zircon mantle array, which was based on necessary to reach zircon saturation under hydrous melting con- a qualitative survey of zircon derived from mid-oceanic ridges and ditions in continental arc magmas, and, hence, no extensive basalt
Drabon et al. PNAS | 3of9 Heterogeneous Hadean crust with ambient mantle affinity recorded in detrital zircons of https://doi.org/10.1073/pnas.2004370118 the Green Sandstone Bed, South Africa Downloaded by guest on September 26, 2021 and Y abundances (16, 29). A similar trend is seen in model melt calculations of zircon derived from highly fractionated pockets in low-H2O and low-pressure modern oceanic settings, such as pla- giogranites in mid-oceanic ridge environments. These tend to have higher HREE contents and shallower MREE-HREE slopes than those in continental settings and tonalite-trondhjemite-granodiorite (TTGs) (30). Model melt calculations of Group II zircons show depletion in the chondrite-normalized HREE (below 10), possibly indicating crystallization in the presence of garnet ± amphibole. Although the average model melt composition does not show the charac- teristic steep pattern, the HREE depletion resembles that of Ar- chean TTGs around the globe and in the Barberton greenstone belt area itself (31), whose HREE depletion is commonly inter- preted to reflect dehydration melting with garnet in the residue (or, equivalently, hydrous fractional crystallization with garnet in the cumulate) at pressures higher than 10 to 15 kilobars (32, 33). Hence, Group II zircons may have formed at deep crustal levels, but alternative shallower melting scenarios that allowed for the presence of garnet are also possible (34). In contrast, model melt calculations for Group I zircons essentially lack depletion in the HREE, suggesting an origin at shallower depths and lower pres- sures (i.e., lower than 10 to 15 kilobars). Finally, the presence of a perhaps more juvenile and a more evolved source component is supported by the Ti and hence the Txlln. In general, Txlln of zircon decreases from mafic to felsic rocks (35), but exact temperatures vary between tectonic environments as a function of magma composition and crystallization pressure. The higher Txlln for Group I zircons (723 ± 44 °C) shows greater overlap with zircon formed in highly fractionated pods in mafic rocks (758 ± 56 °C) than with felsic source rocks (Fig. 3) (35). In contrast, the low Fig. 3. Ti (ppm) and Txlln box-and-whisker plots (the ends of the boxes are Txlln in Group II zircons (668 ± 57°C)aresimilartowhatistypically the upper and lower quartiles, vertical lines mark the median, and hori- observed within zircon from Phanerozoic felsic rocks (average: zontal lines extend to the highest and lowest observations) of Hadean zir- 670 ± 82 °C) (35) and Archean TTGs (average: 678 ± 40 °C) (36), cons of the GSB in comparison to those of zircons derived from the Jack Hills (37), TTGs of the Acasta Gneiss Complex (36), TTGs of the Barberton although there are some examples of zircon from mafic melts greenstone belt, mafic and felsic rocks (35), and different tectono-magmatic reaching Ti abundances as low as seen in Group II (35). environments in the Phanerozoic (16). All temperatures were calculated with In summary, the geochemical signatures of the two groups of
αSiO2 and αTiO2 of 1. GSB Hadean zircons record crustal heterogeneity likely related to an origin from two different sets of parental magma compositions: Group I zircons show evidence for crystallization of a relatively more primitive magma at low pressure and elevated temperatures, fractionation occurs prior to zircon fractionation (16). As a con- > while the majority of Group II zircon compositions are consistent sequence, Sc/Yb values are clearly elevated (generally 0.1) in en- with crystallization from a cooler, more evolved source magma of vironments that experienced flux melting compared to mantle TTG-like composition that possibly assimilated a small amount of environments dominated by basaltic melts where extensive frac- hydrated crust. tionation needs to occur prior to zircon crystallization (16). The low Sc/Yb together with the low U/Nb values of Hadean zircons suggest Comparison to Hadean Zircons of the Jack Hills that flux melting was not a major process in the GSB source. The geochemistry of Hadean Jack Hills zircons (37, 38) and the The differences seen between Group I and Group II zircons are GSB zircons are similar, especially for the Group II zircons. furthermore corroborated by their REE geochemistry. Group II (Fig. 4). Jack Hills zircons are statistically indistinguishable from zircons show a depletion in the middle REE (MREE) (i.e., lower Group II zircons in terms of U/Yb, Nb/Yb, U/Nb, Gd/Yb, Eu/Eu*, A B Gd/Yb) compared to Group I (Figs. 1 and 4 ). The observed U, Y, Hf, and Ti based on the Welch’s t test. Furthermore, Sc general trend of decreasing Gd/Yb together with increasing U/Yb values are similarly low in both sections. Sc values for the Jack is typical for fractionation trends caused by crystallization of min- Hills have only been reported by Maas et al. (39). They report Sc erals that sequester MREE over HREE (e.g., amphibole, titanite, abundances of which half are below detection limit (<17 ppm) or, ilmenite, monazite, and apatite) conjointly with the decreasing for the remaining grains, averaging 35 ppm. This is consistent with temperature’s effect on the Kds (16, 29). To eliminate the effect of the Hadean zircons from the GSB, which are all <54 ppm (av- temperature on the REE, we used Ti-calibrated Kds (17) to esti- eraging 17.4 ppm). mate the compositions of the melts that would have been in Similar to the GSB zircons of Group II, the geochemistry of the equilibrium with the zircons. (Fig. 1B). Preliminary testing has Jack Hills zircons has been interpreted to reflect a source of the shown that these Kds are sufficiently robust to differentiate dif- parental melt relatively undepleted in incompatible elements that ferent crustal settings (30). Model melt for zircons of Group I show possibly experienced some wet melting based on high U/Yb val- about half an order of magnitude enrichment in the REE com- ues, relative MREE depletion, and low Ti values (29). Similar to pared to those of Group II as well as higher Y abundances. Higher the GSB, many of the Jack Hills zircons plot on the mantle array temperature and lower pressure melts tend to contain minerals in the U/Yb versus Nb/Yb diagram and may have been derived by (olivine, pyroxenes, feldspar, mostly plagioclase, and oxides ± remelting of a mafic, mantle-derived crust. However, while the quartz, unless clinopyroxene is unusually abundant) in which REEs GSB has only two analyses on one zircon that plot above the and Y are incompatible. As a consequence, fractionated melt mantle array and show a clear arc-like flux melting signature with pockets from such melts will crystallize zircon with high MREE U/Nb >40, for the Jack Hills, about 50% of zircons are slightly
4of9 | PNAS Drabon et al. https://doi.org/10.1073/pnas.2004370118 Heterogeneous Hadean crust with ambient mantle affinity recorded in detrital zircons of the Green Sandstone Bed, South Africa Downloaded by guest on September 26, 2021 AB
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Fig. 4. Geochemical comparison between Jack Hills (3, 37, 38) (gray dots) and GSB Hadean zircons. (A) U/Yb versus Hf, (B) U/Yb versus Gd/Yb, (C) U/Yb versus Nb/Yb, and (D) Th/U versus Ti (ppm). Fields for Phanerozoic zircons are from Grimes et al. (16).
elevated above the mantle array, and three Jack Hills zircons zircon age clusters seen in the GSB are not found as either auto- display a clear arc-like hydrous melting signature. The presence of or xenocrysts in felsic igneous rocks (49) nor as detrital zircon in some Nb-depleted zircons (Fig. 4C) together with mildly elevated sedimentary units within the greenstone belt (11). Additionally, δ18O of 6.3 to 7.5‰ (2, 38, 40–43) suggests that the mantle- the dominant Cr-spinel population from the GSB does not overlap derived mafic crust experienced some subsequent reworking that in its geochemistry with those of associated komatiitic flow se- included crust that had interacted with water at low temperatures quences analyzed to date (SI Appendix,Fig.S5) (50). These results (44). Lastly, recent studies have suggested that Jack Hills Hadean suggest that at least the zircon and spinel components of the GSB zircons may have been derived from TTG-like crust based on may not have been derived from the immediate vicinity of the model melt calculations (30) and similarities in their low Ti greenstone belt but possibly from relatively distant source rocks, abundances and elevated U/Yb values to Paleoarchean TTGs of perhaps a source that was also linked to the Yilgarn Craton. the Acasta Gneiss Complex (36). These similarities suggest that The results show that not only was the nature of source rocks zircons from Group II in the GSB and those of the Jack Hills were for the Hadean zircons of Group II similar to those of the Jack derived from more evolved magmas that had rather similar com- Hills but also the similarities in zircon age peaks offer the prospect positions at zircon saturation. of a common source. It is possible that an ancient crustal terrane Intriguingly, Byerly et al. (10) noted similarities between the existed in the Hadean and contributed sediments to the Jack Hills, zircon ages of the GSB and the Jack Hills. Both sequences reveal Maynard Hills, and GSB areas with different detritus being added a major detrital age peak at 3.38 Ga, a broad range of zircon ages during their transport. Nonetheless, the similarities in geochem- between 3.38 Ga to 3.90 Ga, and a minor cluster of late Hadean istry and detrital zircon age peaks may be coincidental, and the zircons (Fig. 5). Accessory peaks, however, appear to be unique lower abundance of Hadean zircons in the GSB and the absence to each of the locations. The GSB contains peaks at 3.65 Ga and of Group I-type Hadean zircons in the Jack Hills mark clear dif- 3.31 Ga that are absent in the Jack Hills but lacks the 3.47 Ga age ferences between the two sequences. Additional geochemical and peak found in the Jack Hills. Another area with reported Hadean isotopic characterization of the zircons is clearly necessary for an zircons in the Yilgarn Craton that is thought to have had the same unequivocal correlation. crustal source as the Jack Hills is the Maynard Hills greenstone belt (45–47), ∼300 km southeast of the Jack Hills. Its zircon distribution Comparison to Zircons from Phanerozoic Crustal Settings displays the entire range of zircon age clusters and could represent a Grimes et al. (16) proposed a series of trace element discrimina- link between the two sequences. The zircon age distribution seen in tion diagrams to distinguish zircons derived from different modern these three sequences is not common in other Archean sequences tectono-magmatic settings. These can be used to examine the (SI Appendix,Fig.S4). Hence, it does not appear to represent a possible sources of Hadean zircons from the GSB. While the exact global zirconforming and -preserving event such as major peaks in nature of crustal processes during the Hadean is still controversial, global zircon compilations seen during Phanerozoic supercontinent it is possible that several different magmatic sources and crustal formation (48). settings somewhat akin to modern crustal settings may have Lastly, the GSB provenance suggests that they may have been existed in the Hadean. Besides “primordial” crust (i.e., mafic crust sourced from crust outside the Barberton greenstone belt. The derived from a relatively undepleted mantle), some form of more
Drabon et al. PNAS | 5of9 Heterogeneous Hadean crust with ambient mantle affinity recorded in detrital zircons of https://doi.org/10.1073/pnas.2004370118 the Green Sandstone Bed, South Africa Downloaded by guest on September 26, 2021 rock types, each individual zircon potentially requiring different A Kds. Overall, the comparison between the Hadean GSB zircons and those from modern tectono-magmatic setting shows that all of Group I zircons and a large number of Group II zircons fall into the field that represents a relatively undepleted mantle source signature that is exemplified today by zircons from ocean islands like Iceland and Hawaii, rather than into fields for depleted mid- oceanic ridge or arc environments (Fig. 2). In contrast, only two analyses on one Hadean GSB grain clearly fall into the field of magmatic arc associations as delineated by Grimes et al. (16). As seen in Fig. 2, Group I zircons plot in the lower end of the field for relatively undepleted mantle zircon signatures using the U/Yb, Nb/Yb, and Nb/Sc criteria. While Group I zircons display uniquely low U/Yb ratios that have not been previously reported in Hadean zircons (Figs. 2C and 4), the GSB Hadean zircons B have U/Yb = 0.15 and Nb/Yb = 0.014 values that are significantly different to those of zircon derived directly from Phanerozoic depleted mantle represented by today’s mid-oceanic ridge envi- ronments (U/Yb <0.1 and Nb/Yb <0.005) (Fig. 2) (16). There- fore, these Hadean zircons were derived from magmas that were not as depleted as most zircon from modern mid-oceanic ridges but instead were more enriched and similar to modern zircon found on Iceland and Hawaii. Yet, it is notable that Group I zircons overlap only with the lower end of the Ti spectrum of zircon from plume sources (Fig. 3). Because most GSB zircons do not show an obvious Nb depletion and U/Yb enrichment that would indicate that slab-derived fluids promoted zircon forma- tion at lower temperatures, it is possible that the zircons formed late in the crystallization sequence of relatively dry magmas and, thus, under slightly lower temperatures compared to modern environments. One possible explanation for the relatively low temperatures is that secular trends in average magma zirconium Fig. 5. Probability density plots of detrital zircons from (A) the base and the content and magma polymerization state caused delayed Zr satu- top of the GSB in the Kaapvaal Craton and (B) the Maynard Hills greenstone ration in magmas during early Earth history (53). In general, de- belt (47), and a representative set of detrital zircon dates from the Jack Hills creasing bulk [Zr] by 30 ppm will lower the onset of crystallization area (77) in the Yilgarn Craton. Prominent age clusters (colored) were cal- by 11 °C, all other conditions being equal (26). This caused an onset σ culated using weighted mean ages and overlap within 2 -level. of zircon crystallization on average at lower temperatures during early Earth history (SI Appendix,Figs.S6andS7). Based on zircon saturation modeling of 52,300 continental whole-rock analyses (53, evolved felsic, perhaps continental crust, may have been present 54), the average calculated zircon crystallization temperature (nor- (51, 52). For example, the isotopic and geochemical characteristics malized to 6 kilobar and 3 weight % H2O) was 34 °C lower in the of zircons from the Jack Hills have been interpreted to represent Paleoarchean (804 ± 1 °C), and likely even lower in the Hadean, continental crust by some researchers (25, 38, 39, 43). If felsic and/ compared to today (838 ± 1°C)(SI Appendix,Figs.S6andS7). or mafic crust was being removed and not remixed with the mantle While we cannot constrain the exact Zr abundance of the parental during the Hadean, a complementary depleted mantle may have crustal rocks from which the GSB zircons were derived, lower ap- also existed. Thus, a comparison of the Hadean zircon geochemistry parent Txlln (i.e., Ti values) may at least in part be explained by a to that of Phanerozoic zircon from different tectono-magmatic delay in zircon saturation in low-ZrmagmasearlyinEarthhistory. settings can highlight possible similarities and differences in the Group II zircons share most characteristics with Phanerozoic nature of melt sources. undepleted mantle; however, some show similarities to arc-derived Although trace element abundances likely varied through time continental crust. Today, continental crust is shaped by a complex because of the secular cooling of the mantle (53), the use of some series of tectonic processes, including arc magmatism, terrane ac- trace element ratios as proposed by Grimes et al. (16) reduces the cretion, crustal reworking, and late-stage formation of potassic temperature effect. A global database of 52,300 whole-rock sam- granitic plutons (55). As a consequence, the hallmarks of zircon ples with both major and trace element data, weighted by how derived from Phanerozoic continental arcs include relative deple- much zircon each rock saturates, shows that continental crust ∼4 tion in Nb, low Ti content, and enrichment in Sc (16). Low zircon Ti Ga ago likely had U/Yb, Nb/Yb, and Nb/Sc ratios that vary by less abundances (and hence, low Txlln) in arcs today are related to hy- than a factor of two from those in the Phanerozoic (SI Appendix, drous melting conditions (25). Txlln values of many Group II zircons, Fig. S6) (54). If this global database is sufficiently representative of even when adjusted for a delay in zircon saturation, are lower than preserved continental crust through time, then secular cooling of what is typically seen in zircon from modern, relatively undepleted the mantle has not greatly influenced the relevant trace element mantle sources and may similarly be related to hydrous conditions. ratios of zircon at the scale of the Grimes discrimination plots (16), However, Phanerozoic arc-like Nb depletion occurred in only one and the locations of tectono-magmatic fields in log-ratio space are of the zircons. Zircon formed in modern ambient mantle environ- still applicable. One advantage of the direct use of zircon trace el- ments (mid-oceanic ridge or ocean island settings) typically has ements for comparison to tectono-magmatic settings is that it U/Nb values <20, whereas arc-related and continental-crust– minimizes reported issues with model melt calculations for Paleo- derived zircon usually has U/Nb >40 (16). Similar to a dry mantle archean rocks (36). The direct approach is especially pertinent for environment, the vast majority of GSB zircons show a U/Nb value detrital zircons which may have originated from a range of different of <40 (average of all GSB zircons is 17.2), and only two analyses on
6of9 | PNAS Drabon et al. https://doi.org/10.1073/pnas.2004370118 Heterogeneous Hadean crust with ambient mantle affinity recorded in detrital zircons of the Green Sandstone Bed, South Africa Downloaded by guest on September 26, 2021 the same grain show values above 40. Furthermore, the Hadean The results only suggest that some process that at least mimics zircons from the GSB lack the characteristic high Sc and Sc/Yb fluid-assisted melting occurred at that time. 3) Lastly, it is critical values of continental arcs. Combining the two proxies, the U/Yb to emphasize that even adjusting for the changing nature of the versus Nb/Sc diagram (Fig. 2D) shows the strong similarity of the crust through time, none of the geochemical proxies used here GSB zircons to those from modern undepleted mantle environ- require an origin of the Hadean GSB zircons from fully formed ments and only a single analysis falls into the continental arc field Phanerozoic-style arc magmas. While at least some of the GSB (Fig. 2D). Overall, while some assimilation of hydrated crust ap- zircons were apparently derived from felsic, TTG-series rocks, the pears to have occurred, the lack of strong Nb depletion and Sc absence of major Nb depletion and Sc enrichment suggests that enrichment makes an original source of ambient mantle composi- these zircons do not necessitate an origin from fully formed tion compelling and an origin from flux melting, such as that seen in Phanerozoic-style continental arcs. a continental arc setting, unlikely. An origin from remelting of ambient mantle-derived sources Implications also appears possible for some of the Jack Hills zircons. While The present study using trace elements and REE of Hadean zircons Carley et al. (29) pointed out notable differences in the com- from the GSB suggests that the majority of zircons formed by the position of the Hadean Jack Hills zircons to those from plume- melting of mafic crust derived from the ambient mantle and neither derived magmas on Iceland and favored a continental arc origin, the mantle melting nor melting of the mafic crust involved modern they did not evaluate the parameters Grimes et al. (16) later arc magmatic processes. More specifically, the U-Nb-Sc-Yb sys- found to be particularly crucial in differentiating continental arc tematics of the GSB zircons suggest a relatively undepleted mantle from relatively undepleted mantle settings: relative Nb depletion origin as seen in modern plume-related settings. The Hadean zir- and Sc enrichment. Most published Jack Hills Nb data (3, 38) do cons’ geochemical compositions suggest derivation from at least two not display as extreme an Nb depletion as most modern arc types of melt: more mafic, moderate temperature melts and zircon, and a majority fall into the transitional area where the somewhat more evolved and lower temperature melts. Because only continental arc and relatively undepleted mantle fields overlap a single GSB zircon clearly suggests an arc-like flux melting signa- (U/Nb 20 to 40); hence, either origin may be possible (Fig. 4). In ture, it is unlikely that the GSB zircons reflect an origin from or addition, while Maas et al. (39) associate the reportedly low Sc in associated with a modern-style continental arc. Instead, because the Jack Hills zircon with growth in an Sc-poor continental envi- GSB zircons show a wide range in ages, exhibit diverse geochemical ronment, Grimes et al. (16) later showed that Sc and Sc/Yb features, and lack the coherence expected for a fractionation se- values are elevated in continental arc rocks compared to those quence, they are perhaps best interpreted as having protoliths de-
derived from mid-oceanic ridge or ocean island settings. Finally, rived from an ambient mantle, and this mantle-derived crust, EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES the delay in zircon saturation during the Hadean due to the subsequently, experienced reworking in the presence of some higher heat flux from the mantle may account for some, though amount of water or was a hydrated crust. Overall, although these certainly not the majority, of the low Ti zircons from the Jack zircons can only reveal information about their specific sources, our Hills (SI Appendix, Figs. S6 and S7), and assimilation of hydrated results may suggest that more mafic, relatively undepleted mantle- crust appears to have occurred here too. Hence, some Jack Hills derived melts played an important role during crust formation in zircons, similar to at least one Group II GSB zircon, appear to the Hadean. clearly display an arc affinity based on the relative Nb depletion In addition, the GSB Hadean zircons also suggest a more im- and are possibly supported by the presence of primary muscovite portant role for a previously poorly represented, unrecognized inclusions in some zircons indicating peraluminous protoliths compositional component in the Hadean crust that can produce (56, 57). Yet, an overall match of the zircon geochemical sig- higher Ti (8 to 16 ppm) and low U/Yb zircons. Based on trace nature with continental arc must be tentative, and a contribution element studies, the Jack Hills zircons and the few other Hadean of some zircons derived from crustal rocks with mantle affinity, zircons that have been studied in detail are dominated by lower similar to the majority of GSB zircons, should be considered. temperature undepleted crustal sources (8, 59–62). This apparent In light of these results, some general statements can be made homogeneity in the inferred source(s) of non-GSB zircons may be when comparing the geochemistry of the Hadean GSB zircons to related to a bias in the zircon record because silica-rich, differ- those formed in Phanerozoic source environments: 1) Ti con- entiated magmas are the most zircon fertile and will crystallize centrations in Hadean zircons may be ∼35 °C lower due to the substantially more zircon than more mafic magmas. To date, only higher heat flow from the mantle and its effect on zircon satura- a single Hadean zircon from the Cathaysia Block from southern tion (54, 58). While this change translates to a similarity of Group China (8) has been reliably shown to have Ti abundances of 53 ± 3 I zircons to those formed in modern relatively undepleted mantle ppm in its core, suggesting a Txlln of 910 °C. Xing et al. (8) in- settings, the Group II zircon’s adjusted Txlln is still lower than in terpret the zircon to have been derived from granitoids that modern mantle-related settings and likely requires some rework- formed under hot and dry melting conditions. This single high- ing of hydrated crust to form zircons at such low temperatures. 2) temperature zircon together with Group I moderate-temperature The compositional features of the GSB zircons may be explained GSB zircons suggest that there was a greater diversity of Hadean by varied melting processes acting on perhaps more mafic, ocean- crustal sources than previously recognized and that such zircons, if type crust that was itself derived by the melting of sources with a found more widely, offer the potential to better characterize mantle affinity for the majority of GSB zircons. Although rela- crustal compositions and magmatic processes in the Hadean. tively undepleted mantle sources are associated with deep plume (e.g., Iceland or Hawaii) and rift environments in modern tec- Materials and Methods tonics, those tectonic environments are not implied for the Samples were obtained from the GSB-type locality (S25°54′33.11″ and E31° 2′ Hadean, as the mantle structure was likely different. The GSB 42.35″) and a second location (S25°53′56.50″ and E31° 2′2.13″). Zircon grains zircon compositions form an array within and parallel to the were extracted from 6- to 30-kg sandstone samples using standard size, modern mantle Nb/Yb versus U/Yb array, and only a few of the hydrodynamic, and density mineral separation techniques at Stanford Uni- GSB analyses indicate some form of arc-like wet melting. How- versity. The samples were crushed in a jaw crusher into rock chips and then ground in a disk grinder to medium-grained sand size. Light and heavy ever, it is important to emphasize that the weak to moderate Nb- mineral fractions were separated utilizing a Gemini table, and, subse- depletion does not provide conclusive evidence for some, perhaps quently, the magnetic minerals were removed from the heavy fraction uti- less efficient, subduction processes already operating in the lizing a Frantz isodynamic magnetic separator. The residual minerals were Hadean, as these data could perhaps be explained by the burial of split using methylene iodide to remove any remaining minerals less dense hydrated crust through volcanic resurfacing or sagduction (5, 6). than 3.32 g/cm3. Zircons were mounted onto 1″ epoxy mounts together with
Drabon et al. PNAS | 7of9 Heterogeneous Hadean crust with ambient mantle affinity recorded in detrital zircons of https://doi.org/10.1073/pnas.2004370118 the Green Sandstone Bed, South Africa Downloaded by guest on September 26, 2021 Sri Lanka (563.5 ± 2.3 Ma), FC-1 (1099.0 ± 0.6 Ma), R33 (419.3 ± 0.4 Ma), and has been amorphized. Finally, we applied the LREE index to identify altered OG1 (3465.4 ± 0.6 Ma) standards and then polished. zircon grains (37). In total, 27 geochemical analyses on 15 Hadean zircons pass Initial age dating was done by Laser Ablation Inductively Coupled Plasma Mass all filters and likely represent primary geochemistry. A total of 12 analyses on 8 Spectrometry with a 15-μm spot diameter at the Arizona LaserChron Center zircons did not pass these filters. Subsequently, we conducted a PCA using the 204 using established techniques (63, 64) (Dataset S1). Grains with Pb >600 counts, factoextra R package written by Alboukadel Kassambara and Fabian Mundt. 206 204 low Pb/ Pb and poor precision, and, unless Hadean in age, concordance For a more complete discussion of PCAs, see Wackernagel (70). below 95% were excluded. For Hadean zircon, a cutoff of 80% was used. Variations in relevant geochemical ratios in magmas likely to saturate Subsequently, we analyzed the Hadean grains with the sensitive high- zircon were conducted using the dataset and bootstrap resampling approach resolution ion microprobe reverse geometry (SHRIMP-RG) at Stanford Uni- of Keller and Schoene (54) and weighted by the calculated mass of zircon versity to confirm the 207Pb/206Pb ages (Dataset S2) and to measure the trace saturated according to the methods of Keller et al. (53), normalized to 6 element geochemistry [the same instrument was used by Grimes et al. (16)] kilobars and 3 weight %. H O. In this approach, the composition of the in- (Dataset S3). Each grain was analyzed in up to three spots depending on 2 terstitial melt of a crystallizing igneous whole-rock sample is calculated available space. Zircon U-Pb dating was conducted following the methods of throughout the entire crystallization process using alphaMELTS (71, 72) and Premo et al. (65). AS3, MAD, and OG1 were run as standards. Data reduction zircon saturation state assessed using the zircon saturation calibration of was accomplished using SQUID and Isoplot (66). Ages are calculated using 204Pb-corrected 207Pb/206Pb ratios and 204Pb-corrected 206Pb/238Uand207Pb/235U Boehnke et al. (73). Concordia model ages. Major, trace, and REE geochemistry data were calibrated to primary standard MAD-599 (67) and secondary standard 91500 (68), following Data Availability. All study data are included in the article and/or supporting the procedure described by Grimes et al. (16). information. The geochemical signal can be biased by chemical alteration, especially in radiation-damaged zones. The beam was focused on sites with no obvious ACKNOWLEDGMENTS. We thank Sappi and the Mpumalanga Tourism and impurities and within the same crystallographic domain where the grain was Parks Agency for access to lands. Funding for travel expenses and analytical dated. Rims, either resulting from magmatic overgrowth or solid-state re- costs were kindly provided by the School of Earth, Energy, and Environmen- crystallization, were not analyzed. We excluded analyses where the analyt- tal Sciences. N.D. was supported through the Liebermann Fellowship. We also thank Emily Stoll, Anton Drabon, William Thompson-Butler, and Jake ical pits revealed inclusions or cracks during later microscopic imaging. Harrington for help during sampling. The staff of the Arizona LaserChron Furthermore, geochemical screening was applied to filter analyses that show Center at the University of Arizona and Matthew Coble at the SHRIMP-RG elemental enrichments of nonconstituent cations and thus signify contami- laboratory at Stanford University were very helpful during detrital zircon nation. Ca and P serve as screens for apatite inclusions, and we excluded analyses. Ruth Dawn and David Damby at the US Geological Survey in Menlo > > zircons with high Ca ( 50 ppm) and high P abundances ( 1,000 ppm). Sim- Park helped take Cathodoluminescence images. This paper benefited from ilarly, cracks commonly contain iron and titanium oxides (69), and thus we discussions with Marty Grove. We gratefully acknowledge editor Albrecht excluded analyses with elevated Fe (>100 ppm). Grains with high Al abun- Hofmann and three reviewers who provided many helpful suggestions and dance (>100 ppm) were excluded as well, as they signify zones where zircon thoughtful insights.
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