The time of origin of the geodynamo has important implications for the thermal evolution of the planetary interior and the habitability of early Earth. It has been proposed that detrital zircon grains from Jack Hills, Western Australia, provide evidence for an active geodynamo as early as 4.2 billion years (Ga) ago. However, our combined paleomagnetic, geochemical, and mineralogical studies on Jack Hills zircons indicate that most have poor magnetic recording properties and secondary magnetization carriers that postdate the formation of the zircons. Therefore, the existence of the geodynamo before 3.5 Ga ago remains unknown.
Determining the history of the geodynamo before 3.5 Ga ago is limited by the lack of a well-preserved Archean-Hadean rock record. However, the discovery of Hadean detrital zircon grains in metasediments of the Jack Hills, Western Australia (1), opens up the possibility of studying the magnetic history of Earth during its first billion years. In particular, primary ferromagnetic inclusions (e.g., magnetite) in the zircons may contain a thermoremanent magnetization (TRM) that records the paleointensity of the ancient field during primary cooling (2–5).
To preserve such a record, magnetite-bearing zircon crystals must have avoided being heated above magnetite’s 580°C Curie temperature over their subsequent histories (3, 6). Furthermore, obtaining accurate paleointensity studies with well-determined ages for bulk zircon grains requires that the grains’ natural remanent magnetization (NRM) be dominated by a TRM rather than a secondary crystallization remanent magnetization (CRM) carried by ferromagnetic inclusions formed or altered during aqueous alteration events after zircon crystallization (3, 6).
Two recent studies (7, 8) using single-crystal paleointensity analyses of Jack Hills zircon grains suggested that a geodynamo existed as early as 4.2 Ga ago with a surface field ~0.1 to 1 times that of present-day Earth. However, those studies (7, 8) had three main limitations: (i) The ages of the NRMs in the grains analyzed are unknown (3, 9); (ii) the grains were not shown to contain a TRM rather than a secondary CRM (6, 10, 11); (iii) the studies’ grain selection criteria, which targeted grains with NRM intensities >10−12 Am2, might inadvertently have excluded zircons that would have recorded the absence of a dynamo (i.e., that carry no magnetization). In addition, there have been no independent studies corroborating their paleomagnetic measurements. The latter issue is particularly important because Jack Hills zircons have some of the weakest magnetic NRMs measured in the history of paleomagnetism and therefore require exceptionally sensitive magnetometry techniques and stringent contamination controls. To further evaluate the evidence of an early dynamo and address these limitations, we conducted coupled paleomagnetic, geochemical, and mineralogical analyses on Jack Hills detrital zircon grains.
We extracted the zircon crystals from the pebble conglomerate that we sampled in 2012 at the Hadean zircon discovery locality at Erawandoo Hill [site W74 (3, 9)] using nonmagnetic techniques (see Materials and Methods). From these samples, 3754 zircons were washed with HCl acid and mounted in nonmagnetic epoxy, polished to approximately their midplanes, and dated using U-Pb chronometry. Grains found to have U-Pb ages older than 3.5 Ga (a total of 250) were analyzed using backscattered scanning electron (BSE) microscopy, cathodoluminescence (CL) imaging, and Li-ion imaging. BSE and CL images were used to assess the likelihood of secondary CRM by identifying zircon overgrowths, recrystallization zones, metamictization, cracks, and secondary deposits of minerals in void spaces (12). The goal of Li-ion imaging was to constrain the possibility of secondary TRM by providing estimates of the peak metamorphic temperatures experienced by zircons (11).
We defined a set of selection criteria that enables the identification of detrital zircon grains minimally affected by secondary TRM and CRM overprints (Fig. 1): (1) U-Pb age discordance <10% (see Materials and Methods); (2) lack of visible cracks, metamictization, and secondary deposits in BSE images and the presence of zonation in CL images interpreted as a primary igneous texture; and (3) presence of detectable primary Li zoning with thickness of <20 μm as observed by Li-ion imaging (11). Criterion (3) indicates the absence of TRM overprints acquired during ≳1 million years (Ma) long, ≳550°C metamorphic events under the assumption that natural Li diffusivity is similar to experimentally determined values (13). Note that these three criteria are based on measurements that only probe the polished surface of the grain (i.e., do not survey the full grain volume). Furthermore, the analytical methods used for criterion (2) are unable to resolve the <1-μm-diameter single-domain magnetite grains that would carry stable primary magnetization. Thus, these criteria likely are necessary but not sufficient requirements for identifying a zircon with primary NRM.
(A to C) Example of a zircon grain (7-13-20; 3.973 ± 0.001 Ga) that passes all selection criteria: U-Pb age discordance <10%, presence of zonation in CL (A), no signs of secondary deposits on the exposed surface from BSE (B), and <20-μm-thick Li zonation banding (black arrow), indicating that the sample may not have been fully thermally remagnetized since crystallization (C). (D to F) Example of a zircon grain (12-2-8; 3.666 ± 0.004 Ga) that passes some of the selection criteria: U-Pb age discordance <10%, presence of zonation in CL (D), no signs of secondary deposits on the exposed surface from BSE (E), and no observed Li zonation (F). (G to I) Example of a zircon grain (15-18-8; 3.527 ± 0.007 Ga) that fails most of the selection criteria: U-Pb age discordance <10%, absence of igneous zonation (G), presence of secondary mineral filling cracks at the lower right side of the grain (white arrow) (H), and no observed Li zonation (I).
Of a total of 250 zircon grains, only 3 grains passed all of the above selection criteria. We selected these 3 grains, along with 53 grains that failed one or more criteria (including 13 subsamples from 6 grains; see Materials and Methods), for subsequent paleomagnetic studies. As a control to confirm that our polishing and ion and electron microprobe measurements do not fundamentally alter the zircons’ NRMs, we also analyzed an additional 21 grains in their natural unpolished forms from the same host rocks using nonmagnetic methods, 4 of which were acid-washed. We conducted paleomagnetic analyses on a total of 77 grains.
Given the weak NRMs of the zircons (ranging between 6.05 × 10−15 and 4.15 × 10−12 Am2 before demagnetization), their magnetic moments were analyzed using superconducting quantum interference device (SQUID) microscopy (see Materials and Methods) (14, 15). Following methods previously developed for the Bishop Tuff zircons (2), we obtained paleointensity estimates for the 77 grains using the in-field zero-field zero-field in-field (IZZI) double-heating protocol (16) with partial TRM (pTRM) alteration checks at every other heating step starting at 300°C.
We defined paleomagnetic quality criteria that are permissive compared with those of typical paleointensity studies of younger rocks (see the Supplementary Materials). This is because the overall goal of this study was to establish the presence or absence of a geodynamo at >3.5 Ga ago, which only requires paleointensities with order-of-magnitude uncertainties. Therefore, paleointensity estimates were considered acceptable when a sample (a) had a difference ratio sum ≤25% (17) and (b) gained a moment in the direction of the laboratory field during in-field steps with a maximum angular deviation ≤15o (18). Criterion (a) indicates that minimal thermochemical alteration occurred during the paleointensity experiments, while criterion (b) provides evidence that the sample can record an ancient field’s direction and intensity (while not requiring the presence or absence of such a field when the zircon acquired its magnetic record). In summary, samples that pass our initial selection criteria and paleomagnetic criteria are candidates for providing a robust constraint on the dynamo at the time of their crystallization. Conversely, samples with unstable NRM would either indicate the absence of a dynamo (if the sample passes the selection and paleomagnetic criteria) or that the sample is unsuitable for paleointensity experiments (either because of poor magnetic recording properties and/or sample alteration during laboratory heating). Following the paleointensity experiments, we analyzed selected grains with quantum diamond magnetometry (QDM) (19) coupled with transmission electron microscopy (TEM) to elucidate the origin of the magnetic sources within the grains.
Of the 77 zircon grains analyzed for paleointensity estimations, only a total of 6 grains passed the two paleomagnetic criteria. We found that 63 of the 77 samples failed paleomagnetic criterion (a), indicating alteration during our experiments. In addition, we found that 54 samples have poor magnetic recording properties, as indicated by their failure of paleomagnetic criterion (b). Among the six grains that passed both paleomagnetic criteria, only two passed all five combined selection and paleomagnetic criteria (Fig. 2). Even if we were to exclude Li zonation as one of the selection criteria, there would be no additional grains that would pass the other selection and paleomagnetic criteria (13). In addition, our analyses of the unpolished control grains confirm that polishing the grains did not increase the incidence of alteration during experiments or the magnetic recording quality (see the Supplementary Materials).
Each circle shows the number of zircon grains remaining after each selection step. The histogram on the top right shows the measured age distribution of the 3754 grains. From the 250 grains that were older than 3.5 Ga, we selected all grains that passed all the selection criteria (3 grains) and an additional set of 53 grains. The histograms at the bottom left show the number of grains that satisfy the various selection criteria [(1) U-Pb age discordance <10%; (2) lack of visible cracks, metamictization, and secondary deposits; and (3) detectable primary Li zoning with thickness of <20 μm] and paleomagnetic criteria [(a) the NRM component had a difference ratio sum ≤25%, and (b) the sample gained a moment in the direction of the laboratory field during in-field steps with a maximum angular deviation ≤15o over the same temperature range as the NRM component] for the 56 grains selected for paleomagnetic analysis. Only two grains pass all the selection and paleomagnetic criteria. In addition to the 56 polished grains shown here, 21 whole grains were also analyzed paleomagnetically as a control. No grain showed evidence for a Hadean-Eoarchean dynamo.
The two grains that passed the five combined criteria were sample 7-13-20, with a U-Pb age of 3.973 ± 0.001 Ga, and sample 8-2-11, with a U-Pb age of 3.979 ± 0.007 Ga. Figure 2 summarizes the selection process starting from the initial 3754 grains and ending at these 2 grains. Figures 3 and 4 show BSE, CL, Li, and paleomagnetic data for these two grains. The two grains each have at least two NRM components. Sample 7-13-20 (Fig. 3) has a low-temperature component that unblocked between room temperature and 200°C, a medium-temperature component that unblocked between 200° and 300°C, and a high-temperature component that unblocked between 300° and 580°C. Sample 8-2-11 (Fig. 4) has a low-temperature component that unblocked between room temperature and 510°C and a high temperature component that unblocked between 510° and 580°C. The 580°C peak demagnetization temperature of the NRMs for both samples indicates that the high-temperature components are carried by nearly pure magnetite.
Figure 5 shows an example of a grain that passes all of the selection criteria but fails all of the paleomagnetic criteria. Most of our grains present NRM demagnetization similar to the one in Fig. 5: unstable demagnetization, thermochemical alteration in the laboratory, and no in-field acquisition of remanence.
(A) Orthographic projection of NRM vector endpoints during thermal demagnetization. Closed symbols show the X–Y projection of the magnetization; open symbols show Z–Y projection of the magnetization. Selected demagnetization steps are labeled. (B to D) Out-of-the-page magnetic field component (Bz) maps measured at a height of ~360 μm above the grains obtained with the SQUID microscope for the NRM, 500°C, and 575°C steps. We use a “1” subscript on X1, Y1, and Z1 to denote the fact that the grain orientations during the thermal demagnetization and paleointensity experiments are different from the grain orientations during the BSE, CL, and Li measurements and during the QDM measurements (Fig. 6). (E) Vector-subtracted NRM from the 300°C step versus pTRM grained during progressive laboratory heating. Blue triangles show pTRM checks. The red line shows the measurements used to compute paleointensity values (300° to 580°C). (F to H) CL, BSE, and Li images of the grains.