Paleomagnetism Indicates That Primary Magnetite in Zircon Records

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Downloaded by guest on October 2, 2021 lke .Smirnov V. Aleksey a Erve ’t van Olaf geodynamo Tarduno A. Hadean John strong a records in zircon magnetite primary that indicates Paleomagnetism a eepce orcr n rsreapiaymagnetiza- primary a www.pnas.org/cgi/doi/10.1073/pnas.1916553117 preserve and record to behavior an expected domain-like only be single that with can indicate grains 12) magnetic classical of (11, ther- assemblage time sili- and by relaxation imposed a (10) of limits considerations that statistics statistical crucial Maxwell–Boltzmann the is of modynamics. because It measured inclusions (9). magnetic be many signature containing specimen magnetic crystal-sized analy- cate net SCP interrogated are their employs inclusions for also magnetic carrying approach crystals where This younger and ses, (2). metamorphism in escaped change minerals have chemical examine might that to rocks is explore sedimentary field to means pre-Paleoarchean alternative An the (8). reset minerals obscure magnetic thermally that primary transformations either thwarted chemical has caused been or which have magnetizations metamorphism rocks ubiquitous older recover even by to in Efforts sup- 7). records also (6, is directions paleointensity times magnetic (5). of these Africa studies at by field South ported geomagnetic of the of dacites presence (Paleoarchean) The extant 3.45-Gy-old of from analyses to strength (SCP) 3.4 field paleointensity of single-crystal record from oldest comes of evolution rocks The thermal 4). the (3, about planet us condi- inform the core can past they probing and thus (1). of and means establishment tions, a wind of the provide Records also solar (2). to geodynamo habitability the central the planetary is terrestrial by of field maintenance removal magnetic from the Hence, water and atmosphere E geodynamo Hadean Earth’s represent in may that precipitation core. (Ga) chemical iron ago liquid to Hadean years related billion field convection 4.0 geomagnetic efficient to high the 4.1 of robust of period at fidelity a strength meeting suggest the and for zircons record evidence magnetic additional Pb-Pb further provide and from criteria paleointensity selection data New age formation. radiometric their magnetic escaped since have zircons resetting geochemical Hadean new select rema- and that zircons magnetic indicating these data primary in a magnetite of by presence carried nence the microscope to electron attest and that paleomagnetic Australia). analyses new (Western million Hills provide Jack hundred we the paleo- Herein, few of from zircons come a of has analyses just impact, magnetic geomag- giant old, lunar-forming a the (Gy) for after billion-year years Evidence 4.2 conditions. field core a provides early netic also of wind. field geomagnetic gauge solar the the unique of by presence erosion or from absence paramount atmosphere The the of 2019) shields 24, is because field September planet field review the the of for geomagnetic evolution (received the 2019 the understanding 12, for December of importance approved and age CA, the Jolla, La Determining Diego, San California of University Tauxe, Lisa by Edited 49931 MI and Houghton, 53201-0413; University, Technological WI Michigan Milwaukee, Wisconsin–Milwaukee, of University Australia; Geoscience, 6982, WA Bentley, Group, Research 20375; DC Washington, Laboratory, Research Japan; 305-8567, Canada; Tsukuba (AIST), Technology and Science 14627; NY Rochester, Rochester, eateto at n niomna cecs nvriyo ohse,Rcetr Y14627; NY Rochester, Rochester, of University Sciences, Environmental and Earth of Department eas fterl h edpasi rtcigteplanet’s the protecting in plays field importance the fundamental role of the is of field because magnetic ancient arth’s e eateto elgclSine,Uiest fMntb,Wnie,M 3 N,Canada; 2N2, R3T MB Winnipeg, Manitoba, of University Sciences, Geological of Department f a,b,1 | paleointensity rni Nimmo Francis , k oyD Cottrell D. Rory , n rcG Blackman G. Eric and , c eerhIsiueo elg n eifrain elgclSre fJpn ainlIsiueo dacdIndustrial Advanced of Institute National Japan, of Survey Geological Geoinformation, and Geology of Institute Research | al at conditions Earth early g g i eateto at n lntr cecs nvriyo aiona at rz A95064; CA Cruz, Santa California, of University Sciences, Planetary and Earth of Department eateto ple hsc,Cri nvriyo ehooy et,W 01 Australia; 6001, WA Perth, Technology, of University Curtin Physics, Applied of Department etoHuang Wentao , a ihr .Bono K. Richard , b d elgclSre fCnd,GCCnrlCnd,Gohoooy taa NKA0E8, K1A ON Ottawa, Geochronology, Canada, GSC-Central Canada, of Survey Geological a rcR Thern R. Eric , a,2 doi:10.1073/pnas.1916553117/-/DCSupplemental. at online information supporting contains article This 2 have 1 repository, study MagIC this the in in analyzed deposited and/or been during generated datasets Magnetic deposition: Data is ulse aur 1 2020. 21, January published First BY-NC-ND) (CC 4.0 NoDerivatives License distributed under is article access open This Submission. Direct PNAS a is article This interest.y competing no declare authors microstructural The interpretation. provided magnetic G.M. rock J.A.T., provided interpretations; A.V.S. studies; and geodynamic field interpretation; performed performed contri- S.F. W.J.D., with E.G.B. and E.R.T., paper H.O., and R.K.B., the F.N. J.A.T., R.K.B., wrote coauthors; J.A.T. R.D.C., the data; J.A.T., from analyzed butions E.G.B. research; M.F., and A.V.S., performed W.J.D., G.M., S.F. H.O., F.N., O.v.’t.E., R.K.B., and R.D.C., E.R.T., J.A.T., W.H., research; O.v.’t.E., designed J.A.T. contributions: Author by compromised been have (1). which metamictization zircons magne- exclude remanent to of natural and importance net tization vig- measured the the examine Thus, for to oxides fluids. followed secondary of be introduction must can protocols the decay orous for dated radioactive pathways lead accurately crack to crys- create be uranium the accompanying can of lattice expansion anal- they the because tal SCP However, because record geochronology. for and geomagnetic U-Pb using targets ancient durability greater attractive most their times are the of relaxation recover Zircons to have (13). yses also Gy a 1 can than in which grains “single-domain–like” pseudo–single- domain use most single-vortex the including we explore sense, phenomenological Here to needed geodynamo. timescales ancient billion-year the on tion iouiOda Hirokuni , rsn drs:Goants aoaoy nvriyo iepo,LvrolL93GP, L69 Liverpool Liverpool, Kingdom. y of United University Laboratory, Geomagnetism address: [email protected] y Email: addressed. be may correspondence whom To k htceia rcptto ntecr a oeigthe powering was core the in geodynamo. precipitation chemical at that solar zircons the these from by field, atmosphere geomagnetic the the of wind, zircons. of shielding presence select associated the in and support inclusions data magnetite These primary of the presence indicate challenging data paleointensity electron most and paleomagnetic, geochemical, the microscope, New antiquity amongst paleomagnetism. the in are endeavors of magnetizations their determination ter- of the atmosphere. known and oldest materials—and core zircons—the restrial the Hadean of of measurement evolution But the pro- into can field insight geomagnetic Earth’s vide of history early and age The Significance eateto elgcladMnn niern n Sciences, and Engineering Mining and Geological of Department . ilo er g.Arltvl togfil recorded field strong relatively A ago. years billion ∼4.2 h,i eata Fearn Sebastian , PNAS b eateto hsc n srnm,Uiest of University Astronomy, and Physics of Department c ila .Davis J. William , | f aeil cec n ehooyDvso,Naval Division, Technology and Science Materials eray4 2020 4, February ilo er g a easignal a be may ago years billion ∼4 . y y http://earthref.org/MagIC/16680 raieCmosAttribution-NonCommercial- Commons Creative j atmMitra Gautam , y | d https://www.pnas.org/lookup/suppl/ o.117 vol. otf Fayek Mostafa , | y o 5 no. j eatetof Department h Geochron a , . y | 2309–2318 e ,

EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES −12 2 Tarduno et al. (1) reported the first paleointensity measure- 10 A m . These specimens were demagnetized with a CO2 ments of zircons, which were collected from the Discovery out- laser (15), which has the important advantage of heating over crop (14) of the Jack Hills of Western Australia (SI Appendix, very short time intervals (e.g., 90 s) compared to paleomagnetic Fig. S1). After robust selection criteria, less than 2% of the zir- ovens (generally 20× longer), greatly reducing the possibil- cons isolated were suitable for paleointensity recording. These ity of laboratory-induced alteration, an issue crucial for reliable select zircons yielded a paleointensity history starting at 3.2 Ga, paleointensity determination (9). The magnetization isolated at with values similar to those recorded by SCP analyses from high unblocking temperatures from the specimens (in this case extant rocks (15), to more variable intensities ranging as old as >550 ◦C to 580 ◦C and indicative of a magnetite carrier) passed 4.2 Ga. The latter values extend the record of the geodynamo a test for randomness (20), excluding scenarios for remagnetiza- by more than 750 million years (My) into the Hadean Eon, just tion after deposition of the Jack Hills conglomerate at ∼3 Ga a few hundred million years after the giant impact that formed (18). As we will discuss, these data are also inconsistent with the the Moon (16). characteristic magnetizations being secondary and acquired after Closely following the Tarduno et al. (1) study, paleointensi- zircon formation but prior to deposition. ties from ∼5-My-old zircons that agreed with global data were While the zircon specimens were oriented with respect to reported (17), further supporting the fidelity of this crystal type each other in the Tarduno et al. (1) study, the rock from which as a paleomagnetic recorder. However, in light of the importance they were cut was not geographically oriented. Therefore, there of evidence for a Hadean geodynamo, there has also been an was no means of comparing the paleomagnetic directions from intense effort to disprove the results. Herein, we address these the zircons with any global magnetization that might have been critiques and further our definition of the earliest geodynamo. imparted during peak metamorphism. Furthermore, the mag- We first return to the basic test used to explore the impact netization direction at the time of peak metamorphism at the of secondary magnetizations in Jack Hills zircons, the micro- Discovery Site was unknown. Subsequently, a constraint on conglomerate test. We expand this test to define the secondary the magnetization associated with metamorphism was derived magnetization direction that is recorded by the Jack Hills zircons by Cottrell et al. (21) who studied the metamorphic chrome- at low unblocking temperatures and show how this differs from mica fuchsite [K(Al,Cr)2(AlSi3O10)(OH)2]. Oriented fuchsite the magnetization used to define the Eoarchean to Hadean grains were isolated from a field-oriented sample adjacent to geodynamo. We show how the microconglomerate test results, the one yielding zircons from the Tarduno et al. (1) study. and considerations of statistical requirements for recording the The magnetic carriers in the fuchsite are relict Fe-Cr grains field, are inconsistent with the interpretation that secondary (21) clearly imaged in SEM analyses with compositions con- magnetic particles carry an important natural remanent magne- strained by energy-dispersive X-ray spectroscopy (EDS). We tization signal. Instead, microconglomerate tests further support note that a critique claiming that the magnetic carrier in the the presence of a magnetization carried by magnetite in zircons fuchsite is pyrrhotite (22) is erroneous: Unblocking tempera- that predates deposition. These results, together with new opti- tures exceed the sharp 320 ◦C Curie temperature of terrestrial cal (reflected light) microscopy, scanning electron microscopy pyrrhotite (12) and EDS data sufficient for the identification of (SEM), focused ion beam (FIB) analyses, magneto-optical Kerr pyrrhotite [Fe1−xS (x = 0 to 0.17)] (23) show that there is no effect (MOKE) measurements, and transmission electron micro- sulfur in the Fe-Cr particles (21). Fuchsite specimens yielded scope (TEM) analyses, indicate the presence of primary mag- coherent magnetizations at unblocking temperatures between netite inclusions in Jack Hills zircons. We show how new Pb and ∼270 and 340 ◦C that allow for the rejection of the conglom- Li analyses support the interpretation that select Hadean Jack erate test’s null hypothesis of randomly oriented magnetizations Hills zircons have not seen thermal processing between their for- (20, 21). Because the magnetizations come from a secondary mation and incorporation into the conglomerate that would have (metamorphic) mineral phase, this constitutes a positive inverse reset their primary high-temperature magnetizations. Zircons conglomerate test useful for further gauging overprint directions compromised by metamictization/alteration studied by others carried by the zircons. Bono et al. (24) found that the fuchsite are not representative of those used in our SCP analyses. We magnetization passed a fold test at the 90% confidence level present new paleomagnetic, paleointensity, and Pb-Pb radiomet- with an overprint magnetization isolated in cobble-sized con- ric age data on zircons meeting selection criteria (1) that provide glomerates ∼1 km from the Discovery Site (25). Bono et al. (24) further evidence for continuity of the geodynamo and attendant also isolated this magnetization in data from a study claiming magnetic shielding of the terrestrial atmosphere since Hadean that the Jack Hills were affected by a “pervasive” 1-Ga remag- times. Finally, the new data provide emerging evidence for a netization (26). This issue is important because pervasive late period of high fields near 4.0 to 4.1 Ga which imply a vigorously thermochemical and/or thermoviscous remagnetizations are well convecting Hadean core that may be driven by effects in addition known in younger terrains (12); if the Jack Hills had been sim- to purely thermal buoyancy (e.g., chemical precipitation). ilarly affected, their zircons would not be expected to preserve primary magnetizations and there would be no point in pur- Magnetic Overprint at the Jack Hills Discovery Site suing measurements of their natural remanent magnetizations. The Jack Hills have been affected by greenschist grade metamor- However, the analyses of Cottrell et al. (21) and Bono et al. phism that reached peak temperatures of approximately 420 to (24) conclusively demonstrate that the claim of a pervasive 1-Ga 475 ◦C at an age of 2,655 ± 5 million-years ago (Ma) (18, 19); magnetization in the Jack Hills is incorrect. Moreover, a 1-Ga this event corresponds with Pb loss at ∼2.65 Ga, the most signif- direction has not been reproduced after resampling of Jack Hills icant postdeposition event seen in zircons analyzed by Tarduno monzogranites (27). In data from sediments near the Discov- et al. (1) from the Discovery Site. To test whether the zircons ery Site, the reported 1-Ga direction (26) has been shown to be isolated from the microconglomerate were magnetically reset by artifactual (28, 29). the metamorphism, Tarduno et al. (1) conducted a microcon- Rather than a late remagnetization, the magnetic data from glomerate test. Thin sections ∼300 to 500 µm thick were cut the Jack Hills support a straightforward interpretation consistent from the conglomerate and individual zircons (150 to 250 µm with local and regional geology: A low to intermediate unblock- in size) surrounded by clean quartz (for a total size of 500 to ing temperature magnetic overprint was acquired during peak 800 µm diameter), oriented relative to the slide, were isolated. metamorphism at 2,655 ± 5 Ma (18, 19) coincident with, and These measurements, and those of single zircons, are among likely caused by, intrusion of the Jack Hills monzogranites dated the most challenging in paleomagnetism because of the low at 2,654 ± 7 Ma (30) (SI Appendix, Fig. S1). With the foun- natural remanent magnetization (NRM) moments, often <1 × dation of the true overprint magnetization at the Jack Hills

2310 | www.pnas.org/cgi/doi/10.1073/pnas.1916553117 Tarduno et al. Downloaded by guest on October 2, 2021 Downloaded by guest on October 2, 2021 adn tal. et Tarduno Site Discovery surrounding material initial the criticism the by a carried of address are we test magnetizations microconglomerate directions, the the that of pro- claiming analysis Before an Inc.). device to Goree, ceeding S. interference (William magnetometer quantum (SQUID) superconducting magnetically the direct-current (field three-layer ther- Rochester assess room of additional better University shielded car- the that to in were out Analyses exception added ried magnetization. of the temperature were unblocking those with steps low follow (1) demagnetization test al. mal et microconglomerate the Tarduno for Procedures Test Microconglomerate an Improved An of analyses field. of the new in limitations by oriented rock (1) the host test adjacent address microconglomerate now original we the established, Site Discovery to relative 575 scale at in pTRM change a order-of-magnitude Note of application after only quartz 575 at pTRM a of and for used (Materials and information (C section Methods). orientation thin with from measurement extracted quartz paleomagnetic surrounding microconglomer- and for zircon samples of isolate Cr to test. used ate section thin 300-µm-thick of 1. Fig. B A Photomicrograph (A) magnetization. zircon Site Discovery Hills Jack m ITSMmaueeto icnadqat fe application after quartz and zircon of measurement SSM AIST ) hoemc uhie t,qat;Z,zro.(B zircon. Zr, quartz; Qtz, fuchsite; chrome–mica , ◦ ITSMmaueetof measurement SSM AIST D) ( field. applied 61-µT a in C 0 T sn h ultrahigh-sensitivity the using nT) <200 ◦ D C ple field. applied 61-µT a in C C. Specimen ) n n rmr opnnsa oe nlcigtemperatures unblocking lower at 2 (Fig. components more or one magnetization and of component temperature below. high-unblocking described clear work the in in and study used (1) materials al. mounting et the Tarduno of critique the cleanliness the the disprove verify thus and data (31) microscope These SQUID the 1D). for (Fig. used measurements) cover plastic on the and or to nonrepeatable quartz are attributable the signals of magnetization minor region (any the slide in the seen zir- be can an the magnetization and icant where slide, glass to 575 a (i.e., on adjacent pTRM mounted identical was quartz obtained Next, was specimen 1C). con signature (Fig. magnetic clear observed A subse- is Methods). and and Japan (Materials AIST (32) at magnetometer (SSM) SQUID field scanning 61-µT AIST the a with measured of quently to presence the screened thermorema- in 1B) partial laser 575 (Fig. at A slide (pTRM) remanence. magnetization glass magnetic nent a study negligible (1) in al. itself. a it et zircon have mounted Tarduno the the and from than 1A) specimens rather (Fig. the of (1), such one minerals selected used opaque We quartz lacks microcrystalline it is that which (31), zircons the hw sgetcrl t(oi/ahdln)t o/nemdaeunblocking low/intermediate to line) (solid/dashed fit magnetizations. circle great is shown chrome–mica low/intermediate the of Cr lower from (triangle, projection direction fuchsite symbols, with stereonet together Solid area magnetizations (nega- Equal unblocking fits. hemisphere (D) upper component inclination). symbols, tive principal open inclination); of (positive hemisphere values deviation angu- lar maximum (characteris- are Uncertainties unblocking magnetizations. characteristic unblocking high unblocking/intermediate (C arrows, low (inclination) magnetization. arrows, Red projection temperature Purple squares. magnetization. vertical remanent open circles; tic) with solid shown with is shown is specimens. (declination) microconglomerate are of labeled demagnetization Temperatures thermal of plot vector i.2. Fig. C AB eantzto fnwzrosi-urzseiesyed a yields specimens zircons-in-quartz new of Demagnetization A akHlsmcoogoeaets.(A test. microconglomerate Hills Jack and B and m rmCtrl ta.(1.Cnetosaea in as are Conventions (21). al. et Cottrell from ) PNAS .Tehigh-temperature The S2). Fig. Appendix, SI ◦ .Hrzna rjcino h magnetization the of projection Horizontal C. | ◦ eray4 2020 4, February ed a ple.N signif- No applied. was field) 61-µT C, qa raseentpoeto fhigh of projection stereonet area Equal ) D ◦ a ple sn a using applied was C and | o.117 vol. xml orthogonal Example B) | o 5 no. | Also C. CO 2311 2

EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES ◦ component (isolated between 565 and 580 C) passes a statis- AB tical test for randomness (i.e., the null hypothesis cannot be 5 m rejected at the 5% signiﬁcance level; SI Appendix, Table S1 and Fig. 2C). The high-unblocking temperature data correspond with a uniform (random) distribution with positive support in the Bayesian approach of Heslop and Roberts (33), whereas no viable data clusters that might indicate partial magnetic overprinting are seen using the approach of Bono et al. (24) which searches for clusters in data that might represent partial magnetic overprinting (SI Appendix). C While there is no evidence for a remagnetization in the 5 m high-unblocking temperature magnetizations using these three tests, there is, arguably, a nuanced incompleteness. Because the zircons have seen low-grade metamorphism at ∼2.65 Ga, they should retain some vestige of this magnetization if the zircons contain a range of magnetic inclusion sizes, including larger particles with lower unblocking temperatures and relaxation times. To examine this issue, we now turn our attention to the low unblocking temperature magnetizations. A central challenge in D this analysis is that for a distribution of magnetic carriers with 3 dominantly single-domain–like behavior, the number of grains reset and carrying the secondary magnetization is expected to be 5 small. Nevertheless, a coarse low-temperature component is seen 4 1 50 m 2 in the same nine samples (typically isolated between 100 and 1 m ◦ BSD 400 C) (Fig. 2D and SI Appendix, Table S1). E Interestingly, the three approaches to the conglomerate tests Spectra #1 O Spectra #2 160 O 15 again suggest that a random distribution cannot be excluded Fe 120 Zr (SI Appendix). However, visual inspection reveals that the low- Si Zr temperature components clearly differ in their distribution from 80 Si 0 6.2 6.4 6.6 6.8 the high-unblocking component. The former trend along a great Counts 40 Fe circle between the magnetization directions from fuchsite (21) Al Fe and the low- and intermediate-temperature component mode A 0 Spectra #3 Spectra #5 isolated from cobble remanences (24) and its antipode (Fig. 2D). SiZr O 120 O Zr This reveals a limitation of the aforementioned approaches to Si the conglomerate test: Their probative power is in comparing 80

unimodal and uniform distributed data. Other distributions of Counts geomagnetic and geologic significance, such as a great circle 40 distribution between two polarities, could appear to be uni- 0 0246810 formly distributed without additional inspection of the data. The keV 160 O Spectra #4 observed distribution suggests that the low unblocking tempera- 15 120 Fe ture component recorded by the zircons was acquired while the Zr geomagnetic field was reversing during peak metamorphism of 80 Si 0 6.2 6.4 6.6 6.8 the Jack Hills. Hence, the presence of the expected overprint Counts and its difference from the high unblocking temperature char- 40 Fe Fe acteristic remanence are foundational observations defining the 0 0246810 fidelity of the Jack Hills (JH) zircons as magnetic records. With keV this context, we next discuss the nature of the magnetic carriers in the zircons at multiple scales. Fig. 3. Optical (reflected light) and SEM analyses of large (1 to 3 µm) magnetite inclusions in JH zircon. (A) Reflected light image (zircon RZ64). (B and Evidence for Primary JH Magnetic Inclusions C) Partially oxidized magnetite inclusions in crack. Other bright areas (at base) are other reflectors (i.e., not Fe). (D and E) SEM backscatter detector We recall that Tarduno et al. (1) noted that “Sources of mag- (BSD) image (D) of region C with EDS analysis spots (1–5) and correspond- netite within the JH zircons include isolated inclusions trapped ing spectra (E). Also shown is possible stress orientation (σ) in A, suggesting in the zircon matrix, vapor phase magnetite associated with voids, cracks emanating from inclusion in B and C were formed under the influence and magnetite inclusions within other silicates trapped in the of the same stress field. zircons” (ref. 1, p. 4, SI Appendix). Preliminary data bearing on these occurrences were presented by Bono et al. (34) and Tarduno et al. (35) and are expanded upon here. We examined used to constrain paleointensity because these analyses employ 95 JH zircons using reflected light and SEM analyses (Mate- MD tests (1). However, these inclusions can contribute to the rials and Methods). We start with a consideration of relatively low unblocking temperature magnetization (Fig. 2) and they are large (∼1 to 3 µm) iron oxide inclusions that are seen in about most likely the largest representatives of a grain size distribu- 10% of the zircons (Fig. 3). This is an underestimate of the per- tion that extends down to sizes including pseudo–single-domain centage of zircons with such inclusions because each of these (PSD) and single-domain (SD) particles. The large inclusions analyses examined only a single cut through a given crystal, observed are commonly found along cracks. While most of the with a viewing area <<1% of the total zircon volume. Inclu- zircons examined would not be used for paleointensity analy- sions of this size are found throughout the zircons and are not ses because of the rigorous selection criteria (1) that exclude restricted to edges (cf. ref. 31). Some of these particles are physically altered crystals, the relationship between cracks and probably in the multidomain (MD) state; they are not principal inclusions provides critical insight into the primacy of Jack Hills contributors to the high unblocking temperature magnetization zircon magnetic inclusions. Specifically, magnetite inclusions, so

2312 | www.pnas.org/cgi/doi/10.1073/pnas.1916553117 Tarduno et al. Downloaded by guest on October 2, 2021 Downloaded by guest on October 2, 2021 adn tal. et Tarduno E 90 and (e.g., surveys SEM as by small as particles, sized eetdlgtiaewt nlso eaaefo rcshighlighted cracks from separate inclusion (B with (box). image light Reflected (A) 4. Fig. Fe EDS lacking cracks nearest found 4 the (Fig. are with signatures inclusions cracks, selection magnetite from robust large separate apply Occasionally to need , Appendix (1). the (SI criteria metam- highlighting oxidation further by inclusion S3), associated promote Fig. disruption to appear as crystal to ictization However, leading and Appendix). strains cracks (SI (i.e., erroneous; expected, missed are processes were (31) past concentrations) cracks of stress in indicators inclusions are cogent JH some the that because assumption the secondary on are based critiques that conglomer- Thus, host deformation the ate. of and morphology stretched-pebble metamorphism the the created most during inclusions formed with associated likely zircons JH in con- Cracks metamorphic (36). quartz grade ditions in greenschist inclusions under opaque initiation deformed of rocks crack study from the of in type documented This been has cracking. and (39), concentration, pile-ups dislocation stress causing inclusions, stress scales by under obstructed crystal of be a Plastic can variety within 37). glide a (36, dislocation by stress at secondary (38) experienced deformation seen have with that formation materials crack inconsistent natural in of is indicative pat- this the is Moreover, tern fluids. cracks; Fe-enriched by the inclusions of formation in oscil- absent zircon and are internal 3 across (Fig. cut zoning that occur latory cracks Appendix), of (SI intersection character the light at reflected their by identified 3*10 F DE C AB .(F ). Counts 0 Length ( m) 1 2 0.02 0.06 0.1 0.2 0.6 10 3 012345678910 1 2 6 umr fprilsvsbeietfidi eetdlgtanalyses. light reflected in identified visible particles of Summary ) O . . . . 1 0.8 0.6 0.4 0.2 Fe ◦ eetdlgt E D,adsz fio nlsosi Hzircons. JH in inclusions iron of size and EDS, SEM light, Reflected (C Si and oaiain oe in Boxes polarization. ) Zr 0m 50 C

eetdlgtiae(1,000× image light Reﬂected ) SD - superparamagnetic - SD 1 m keV A–E Fe ,cnitn ihour with consistent ), S4 Fig. Appendix, SI and Fe .ESF signals Fe EDS S3). Fig. Appendix, SI 0 o30n,wr lodetected also were nm, 300 to ∼200 .Afwsubmicrometer- few A Appendix). SI 1 Axial Ratio(w/l) B 6*10 ihih ra fESaayi (D analysis EDS of areas highlight PSD-MD 2 4 0 2

012345678910 SD - two domains two - SD O Si Zr 1 m i meso)wt 0 with immersion) oil keV RZ64 RZ76 RZ46 RZ28 RZ71 ◦ and (B) IAppendix, (SI cuts initial in absent S6). and succes- were S5 zircons; they Figs. where JH zircons in inclusions in magnetite common micrometer-sized reveals are also polishing inclusions sive one melt shows be polishing these successive can that Furthermore, magnetite phases. included vapor-phase the inclusions; these of interpret melt We primary common. inclu- are as many voids that magnetic and confirms multiphase, the trenching are FIB to sions (1). contribute zircons hosts JH can are of inclusions signal which inclusions magnetic silicate primary Thus, to cracks. lacks surround- zircon the primary; ing clearly void is Importantly, inclusions 42). the is (9, surrounding as type space silicate inclusions, this magnetic in contains observed it commonly suggesting feldspars, the 5 of (Fig. feldspars and multicompositional quartz of cate and depth pristine a a at 5 structure (Materials revealed lift-out (Fig. This FIB light a Methods). with reflected and region in this often explored expression we are surface Next, A–C). roughness any to lacking corresponding grain signal area large of an a areas found we (Mate- Some However, polishing polished unavoidable. Methods). Si smooth a colloidal and for through rials need achieve the we limita- after is which well-known rotated technique surface, A MOKE is the (45). wave of mineral incoming tion magnetic an a of with polarization interaction of using plane structures the magnetic NanoMOKE3; dis- subsurface (using surface for MOKE avoid searched To particles. we these ruption, interpretation disruption of limits exploration However, removal) further aggre- space. and inclusion multicrystalline (i.e., void of polishing with part through be together to possibly appear gates, zircons JH in sions secondary a with magnetite below. equates that analyses space through assumption formation void Appendix ). larger with (SI occurrence the mechanism inclusion our challenge this in migration, we for for be problematic However, pathways the highly cannot in is 40 Fe study, of threshold ref. size absence in SD the the that highlighted below and is mineral size/volume that Fe its note because We the important voids. cases of most occurrence magnetic the in secondary is for (40) posited formation argument particle Appendix ) another (SI Yet formation that 44). zircon Fe (43, initial note vapor-phase during We resemble formed 40 have zircon. ref. could coprecipitated in particles of elongated involvement that the have without could with other temperature planes or crystallographic in high the at exsolution along elongated migration to particles Fe formed Similar 42), nonformula Li). 9, other K, (3, do silicates Mn, as (e.g., (41), elements zircons not trace to (type-C) and known pristine is expected in Iron is Appendix). occur (SI stress magnetite experienced secondary has of inclusions evidence that primary crystal In to a evidence. adjacent within as occurrence presence elongation dislocation the inclusion hypothet- contrast, or upon to dislocations calling particles adjacent processes, small of formation these the attributed secondary Instead, study. 40 ical (1) ref. al. of et order Tarduno authors first the to of of results predictions on identification relies the confirm still data, the cases temperature all and unblocking nearly in magnetic (1) magnetite as criteria particles small selection these of our zircons study pass these magnetite-like TEM of would the a none reproduced Although in also characteristics. identified which high-unblocking (40) been zircons have particles JH range three Magnetic present, size (1). be this analyses within paleomagnetic should by range predicted size the as within single-domain inclusions and smaller single-vortex even inclusions, magnetic primary pseudo–single-domain temperature unblocking 4F the high (Fig. the remanence in recording of are capable particles. range detected magnetic size of particles distribution small size The grain a about inferences esatb oigta ihscesv oihn,mn inclu- many polishing, successive with that noting by start We submicrometer and micrometer of presence the of Because PNAS ). ∼2 | eray4 2020 4, February Fg 5 (Fig. µm whereby Methods) and Materials F –H .Io a eetdi one in detected was Iron ). D and | o.117 vol. .ESdt indi- data EDS E). | o 5 no. | 2313

EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES A B Reflected Light C Kerr Rotation D

BSD 20 μm InLens 2 μm 9 μm 9 μm

E F 3000 C G 6000 Si Si

2000 O 4000 120 Fe 80 Zr O Al 40 Counts Counts 1000 Ga 2000 C 0 Ca Zr 6 6.4 6.8 Al K K keV Ga Ga K Ca Cu K Fe Cu Ga 0 0 0246810 0246810 H keV keV 12000 H Si G

F 8000

O Counts 4000

200 nm C InLens AlZr Ga K Cu 0 0246810 keV

Fig. 5. Primary iron-bearing Jack Hills inclusions. (A) SEM image (backscatter detector [BSD] at 20 keV) of zircon. (B) NanoMOKE reﬂected-light image of zircon surface. Green circle highlights area of interest (see C). (C) NanoMOKE parallel image showing strong Kerr angle response (red inside green reference circle, note scale on right) from area without any surface features (compare with B). (D and E) SEM images (InLens, 20 keV) of focused ion beam lift-out of area (C) showing strong NanoMOKE response reveal a buried multicomponent silicate inclusion and voids. (F–H) Energy-dispersive analysis reveals the presence of quartz (H) and two different compositions of feldspars (F and G). One of the feldspars (G) bears iron that is a likely source of the magnetic signal. Cu and Ga peaks are related to TEM grid and FIB analyses, respectively.

Overall, the lack of a high unblocking temperature overprint, Lithium Diffusion in Magnetized Hadean Zircons the physics of chemical remanent magnetization (CRM) acqui- To utilize zircons as records of the Eoarchean to Hadean geo- sition, and Maxwell–Boltzmann statistical limits on magnetic magnetic field we must investigate whether they have experi- recording are a priori inconsistencies with hypothetical models enced reheating after crystallization but prior to incorporation (cf. ref. 40) invoking secondary magnetite as an important into the conglomerate. Tarduno et al. (1) approached this in two remanence carrier in JH zircons (SI Appendix). In addition, ways. First, U-Pb data were reviewed for evidence of Pb loss reported observations on zircons compromised by metamicti- that might have accompanied hypothetical reheating. Second, zation/alteration (31, 40) are inappropriate representations of nonsystematic changes in Pb abundance during sensitive high- zircons selected for SCP analyses (SI Appendix), and mag- resolution ion microprobe (SHRIMP) analyses, seen in studies netizations measured using high fields (31) are artificial and of other zircons that have experienced amphibolite to granulite inappropriate representations of the relevant high unblocking grade metamorphism and thought to be a signal of Pb redistribu- temperature magnetization (SI Appendix). But even if section (46), were investigated. These checks were incorporated into ondary magnetite formed in some JH zircons, a critique claiming selection criteria (1). We note that another critique claimed that that this “precludes analysis of the Hadean dynamo” (ref. 40, instrumentation bias would exclude detection of nonsystematic p. 407) is mistaken because it misses the relevant question: changes (47). But this criticism omitted key information: Data presence or absence of the most ancient geodynamo (1). To collection on JH zircons (1) utilized the same SHRIMP instru- indicate presence of a core geomagnetic field, paleointensity val- ment for which nonsystematic changes had been documented for ues must be above a threshold set by an external field induced zircons that had experienced high-grade metamorphism. While by the solar wind interacting with a hypothetical Earth with- it is well known that SHRIMP cannot distinguish Pb abundance out an internal core dynamo (1, 2). Because of the nature of on an atomic scale, we show below that it has recorded nonsys- a hypothetical secondary CRM (SI Appendix), any bias will be tematic changes in Pb in some JH zircons which we deselect in conservative, toward the inability of paleointensity data to dis- our analyses. tinguish between internal and external fields. We will revisit this A different approach to the problem has been developed by issue later with a new dataset. But first we further investigate Trail et al. (48) who suggested that Li zoning could be used methods to constrain postformation temperature exposure of JH as a qualitative indicator of the maximum postcrystallization zircons. temperature experienced by a zircon. This has in turn been

2314 | www.pnas.org/cgi/doi/10.1073/pnas.1916553117 Tarduno et al. Downloaded by guest on October 2, 2021 Downloaded by guest on October 2, 2021 adn tal. et Tarduno greenschist Hills Jack with consistent associated are that S6 ) esti- than and metamorphism. and S5 greater Figs. zircon no Appendix, 4.2-Ga reheating (SI for with grains values Hadean slope other (Materials and for (48) width mates Band geospeedometry superimposed (D) using with Methods). formation image and since CL heating (B) spots. maximum paleoﬁeld analysis 12.8-µT age (C a are image. yielding pits zircon Dark of (1). image (CL) Cathodoluminescence 6. Fig. 475 ods of heating metamorphic peak band- Li measured For minimum Manitoba. is the with (1)], of Ga University [4.2 Z565-12 the zircon JH at (SIMA) other spectrometry mass and JH with needed in are proceed processes studies We additional zircons. charge-coupling–limited zircons. that study caveat some to the to with applicable method the that is indicating (50), method document mobility clearly the REE’s of (48) (Tanzania), independent al. diffusion et xenolith Li Trail grade However, granulite Canada). (REE) Li Province, a (Superior elements of sanukitoid from a earth limitations and zircons tonalite–trondhjemite–granodiorites, on rare in focus with Y who coupling and (49) charge al. by et diffusion Tang by challenged D A CL 50 m ecollected We

S (%/μm) 1000 0.01

0 100 0.1 10 and 1 50 m .10111 0 1000 100 10 1 0.1 0.01 ∼3.3 ihu rﬁigi antzd42G akHlszro.(A) zircon. Hills Jack 4.2-Ga magnetized in proﬁling Lithium ) .Pa eprtr predictions temperature Peak S3 ). Table Appendix, SI 7 iiae rnet n adwdh n lpsue oassess to used slopes and widths band and transect, image, Li Fg 6 (Fig. µm Band width Slope B 7 idt sn aeaIS7 secondary-ion 7f IMS Cameca using data Li a-b hc orsod oa estimated an to corresponds which A–C) b-c a Z565-12: 4.2Ga Myr

Percent Li ◦ 100 20 60 aeil n Meth- and (Materials (48) C

0102030

a b

a-b a c 700 °C 700 ~50 b ~3 Microns b-c μ μ m profile

m profile

520 °C 520 580 °C 580 600 °C 600 475 °C 475 500 °C 500 °C 400 300 °C 300 c 10 10 10 10 1 0.1 CL C 2 3 4 7 Li total profile length (μm) i.S n al S3 diffu- Table and is Li S9 temperature enhanced Fig. much peak indicate yields estimated age may the ambiguous which sion; of bands loss and Pb defined with S8 highlighted less-well (1) grain and al. Hadean et S7 one Tarduno that Figs. by note Appendix, we evidence However, (SI lack S3). loss that Table (1) Pb al. predepositional et Tarduno for by studied zircons Hadean peak consistent yield bands 6D). zoning (Fig. Li values temperature between slope the using n etrs(i.8.Ffypreto h aahv gswithin ages have data the interest- those of some percent with yield Fifty combined 8). analyses, data, (Fig. paleointensity features new zircon ing first These the 2). of (1, threshold ence zircon Ga, (∼4.3 core old exceptionally an with NZ zircon at a in event (51), an 7B). is (Fig. includes as episodes latter data reheating more SHRIMP The or these one for in evidence observed age are Pb-Pb Nonsys- Pb deposition. in to changes prior tematic but note, formation We after (1). may reheating that criteria characteristics mark selection have zircons our rejected of some Pb part that in however, as changes use nonsystematic we the evidence that lack counts lack well as data the and event, with select morphic associated These that beyond S6). disturbance, Table of Appendix, (SI ples 565 the for responsible unfounded. is the is heating signal characteristic by Thus, paleointensity produced the intensity. magnetite to that its claim masking opposed over- remanence, partially low-temperature high-temperature and directions complex predicted of with that removal prints, of the 0.007% expected, to only is attributable is ( intensity observed data increase in the 565 largest increase in the absent huge of but a step determinations, hematite, applied-field sity (e.g., first the phase During parent A ferric 0.4 hypothetical a of (M magnetization saturation an ture (31) But metamictization by Appendix). compromised fer- zircons (SI be JH would in source seen potential iron only formation ric the magnetite and Conjectural precursor, a (15). requires use limited is The alteration experi- data. that the and of physics, a features mineral heating of salient during magnetic the created protocol, neglects but mental by criticism intrinsic detected not This magnetite is (31). that (1) claiming data we critique unblocking However, final Methods). one 565 and both address (Materials at checks using determinations tail paleointensity and multidomain analyses evaluate Thellier–Coe We full the conducted. tests which microconglomerate been on rock inverse have host and same zir- microconglomerate the and JH from prior evaluate methods zircons further analyze to the (1) We al. follow cons. et now Tarduno prior of we criteria reheating selection deposition, minimal for conglomerate evidence to further provided Having Geodynamo Paleoarchean–Hadean a Defining Data New 48). 29, high 28, 24, their 23, 21, reset (1, would reheated magnetizations temperature that not unblocking temperatures were zircons to JH crystallization sup- select after 19), that conclusion (18, the event porting peak metamorphic the to greenschist similar remarkably postdepositional yield are (1) that criteria estimates selection reheating all peak passing grains Hadean the method, 207 S5 and iia eprtr siae r vial rmother from available are estimates temperature Similar l ftenwploneste r bv h ednm pres- geodynamo the above are paleointensities new the of All twenty-three and analysis Thellier–Coe full one from Data the on caveats aforementioned the notwithstanding Thus, Pb/ S59). ◦ alsS4 Tables Appendix, (SI criteria selection met analyses C CO 206 m .SRM ecrnlgclaaye yield analyses geochronological SHRIMP 7A). Fig. and 2 2 bae between ages Pb k)t agtrmgeie(M magnetite daughter to /kg) ae et vrsc hr iecls(.. 0s) 90 (i.e., timescales short such over heats laser PNAS ). 3 ie nraei omtempera- room in increase times ∼230 | .The S11). and S10 Figs. Appendix, SI eray4 2020 4, February .5ad41 afrteesam- these for Ga 4.13 and ∼3.35 . a oe npirstudies prior in noted Ga, ∼3.4 s copne h conversion the accompanies ) | ∼560 s o.117 vol. of ◦ .5G meta- ∼2.65-Ga 2A ∼92 IAppendix (SI C ◦ paleointen- C | o 5 no. ◦ m with C M | 2 /kg). s 2315 of ,

EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES A uniform laboratory TRM. This strong TRM, relative to the lowered NRM, results in artificially low paleointensities, inconsistent with observations (Fig. 8). The emerging pattern of paleointensity values is intriguing. Because each zircon paleointensity value averages at least several tens of thousands of years (1), the trends are unlikely to represent only an intrinsic variability similar to that of the mod- ern dynamo (although a different dynamo variability cannot be excluded with the small number of paleointensity determinations currently available). The oldest paleointensity value at 4.2 Ga is based on a single value, but this grain has both Pb-Pb and Li characteristics supporting a primary recording. Core cooling sufficient to drive convection at 4.2 Ga may have been dominated by a heat pipe mode of heat transport through the mantle (52). One hundred million years later, field values are approximately two times higher. In the Eoarchean, the field drops by a factor of ∼2. The available data suggest only minor variations hence- B forward, until ∼3.2 Ga. Olson and Christensen (53) used the results of dynamo numerical simulations to suggest that dipole moment (MD ) increases with convective buoyancy flux (F ) as 1/3 MD ∼ F . Thus, the factor of 2 change in field strength in the late Hadean suggests a factor of 8 change in cooling across the core mantle boundary. This change seems unrealistically large for the Hadean core given the likelihood of a basal magma ocean C (54). The rapid change in Hadean field strengths could instead reflect the onset of MgO exsolution, which is expected to provide a sudden boost to the dynamo (55, 56). The pattern is consistent with a core that is initially undersaturated in MgO, with precipitation commencing after several hundred million years of cooling. Temporal changes in core SiO2 precipitation (57) might also be recorded by these data. The drop in field strength in the Eoarchean correlates in time with the Late Heavy Bombardment (LHB), which is opposite to the Hadean field strength–impact flux relationship proposed by O’Neill et al. (58). Instead, the Fig. 7. New geochronological analyses on Jack Hills zircons. (A) Concordia potential drop in field intensity may reflect the development diagram showing SHRIMP geochronological data for samples meeting selec- of core–mantle boundary heterogeneity associated with cooling tion criteria (SI Appendix, Table S6). (B) Zircon NZ-S59 has an exceptionally old of a basal magma ocean. This could have driven core-mantle core (4.3 Ga, spot 2, Inset) but the data fail criteria established by Tarduno et al. (1). The data show evidence of multiple potential reheating events. boundary (CMB) heat flow, the core velocity field, and the result- Moreover, the data in B show nonsystematic changes with time (spot 2, C). ing magnetic field to smaller length scales, ultimately resulting in a lower net surface magnetic-field strength. Alternatively, if final solidification of a basal magma ocean occurred in the 50 My of 3.4 Ga, further highlighting the importance of this Eoarchean, this would likely result in a strong drop in CMB heat magmatism recorded at the Discovery Site. The mean paleoin- flow and an attendant drop in magnetic-field intensity. While tensity of this age group (n = 34) is 10.8 ± 6.3 µT. If we the Hadean/Eoarchean history is tentative because of the small exclude the one zircon with Pb loss and Li diffusion charac- number of acceptable paleointensity analyses, the available data teristics that could reflect reheating, there are three Hadean nevertheless highlight the potential of zircon magnetizations to paleointensity values between 4.02 and 4.13 Ga (24.5 ± 6.5 µT) track evolution of fundamental core processes. that are more than two times greater than the 3.4 Ga value. This pattern yields further support for the primary nature of the rema- Materials and Methods nence. Specifically, the 3.4-Ga data cluster clearly defines this Paleomagnetism and Paleointensity. Procedures follow those outlined in age as the most important magmatic event that could poten- Tarduno et al. (1) and Cottrell et al. (21). Below, we briefly review salient tially overprint our Discovery Site zircons prior to deposition. points of these techniques and include new methods applied at AIST. For If the 4.02- to 4.13-Ga Hadean zircons had been magnetically the preparation of oriented zircons, the geographic orientation line of the field-collected hand sample was transcribed onto the cut billet from which reset, their paleointensity values should be lower than observed. the 300- to 500-µm thick sections were derived. All documentation pho- In addition, conjectural secondary magnetite, as discussed ear- tographs were taken relative to the projection of the down dip azimuth lier, would further bias the field to low, not high, values (SI plane (vertical). Zircons in quartz were isolated from the same sections that Appendix). This hypothetical underestimate has its origin in were studied by Cottrell et al. (21) for paleomagnetic directions held by several complementary processes: Chemical remanent magne- fuchsite. Excess material was etched away using a sodium silicate–alumina tizations carried by secondary grains are inefficient relative to powder to leave a small pillar of zircon in quartz on the oriented micro- TRMs, and select areas of conjectural secondary magnetic grain scope slide. The slide was then trimmed to an area ∼2 mm × 2 mm in size with the pillar of zircon in quartz centered on the subslide. To study growth within a zircon are unlikely to hold remanences on ◦ billion-year timescales because of Maxwell–Boltzmann statistics. the TRM imparted at 565 C on zircon in quartz versus quartz alone, a sample originally measured by Tarduno et al. (1) was removed from the Even if enough secondary grains to hold a remanence formed, microscope subslide with the use of acetone and distilled water in an oscil- the vector addition of such a magnetization with the primary lating water bath. The sample was then remounted in a 27-mm × 27-mm × direction would result in an overall lower apparent NRM inten- 2-mm fused quartz slide that had been precision drilled with a diamond tip sity unless the primary and secondary directions were exactly to produce a 600-µm diameter well, ∼500 µm deep. Both quartz and zir- aligned. However, all secondary grains will tend to acquire a con in quartz from the same slide (slide1-z1) and quartz of approximately

2316 | www.pnas.org/cgi/doi/10.1073/pnas.1916553117 Tarduno et al. Downloaded by guest on October 2, 2021 Downloaded by guest on October 2, 2021 adn tal. lift-out pro- et a Tarduno to milling preparing FIB by to signal prior deposited MOKE was strong layer JHZ5-4X- platinum a zircon A of in 60). source (59, trench lamella a the mill examine to to used 7 was Research) Materials for Center TEM. and FIB, magneto- SEM, and optical between discrimination simulta- features. the collected surface) optical were grain for images to effect normal allowing Kerr parallel applied neously, and applied (field light (field laser polar Reflected longitudinal both modes. to and using 1 surface) imaged grain of and to order mT) the (∼500 field on netic depths at features 10 magnetic microm- submicrometer detecting of to 0.5 capable eter to is NanoMOKE3) alumina Designs (Quantum with employed (first polished mounted and were 0.05 analyses Grains stubs MOKE Laboratories. epoxy assembly. Research camera Naval on digital the trans- color at CCD both performed 1,000× 4MP were with maximum Insight Spot a microscope a capabilities, LV100POL and light Eclipse reflected and Nikon mitted a used MOKE. Rochester) and Microscopy Optical 565 sam- results. at Thellier–Coe the obtained reheating values by paleointensity performed 565 was to check ple multidomain 565 a Finally, to field. heating second 100 A at min was 1 this 565 500 space; for to field-free temperature increments in the ture heated measurement holding gradually After was by alteration. sample accomplished the laboratory-induced NRM, of the effects of the limit 565 to simplified as The calculated scan. is rent slide a of surface ueet S esrmns(2 eecnutdi cnigae of area scanning a mea- for in SSM conducted AIST mm were the 6 (32) to CO measurements shields, Synrad 40-W Mu-metal SSM a Heavy using surement. with transferred, Late TRM and LHB: a AIST diamonds. given from at then wind (5) were solar pink dimensions the and same by the (5), imparted tan field (15), external on yellow based (15), assum- line) values green green equivalent and by (dashed (equatorial value shown results presence field crystal field are equatorial silicate geomagnetic rocks Single mean (2). for extant (1). al. is threshold y et of line above Tarduno 800,000 studies solid are past Pink prior data the from All field: from data Bombardment. Recent field) of value. dipole-dominated resampling reset bootstrap a 1σ potentially a ing at from excluding region) results plotted shaded paleointensity are (blue zircon SD uncertainties of Age average (1). running 100-My bounds two temperature of maximum indicating factor a are minations 565 565 circles, red (1); 8. Fig. eet h oihdsrae ape eeepsdt mag- a to exposed were Samples surface. polished the beneath µm ◦ ◦ olia iia oeps h ri neir.Teinstrument The interiors. grain the expose to silica) colloidal µm aeitniydtriain ti td) hlirCeplonest netite r lte t1σ at plotted are uncertainties paleointensity Thellier–Coe study). (this determinations paleointensity C o nadtoa i n hnalwdt ol o o5min. 5 to 3 for cool, to allowed then and min 1 additional an for C × aeitniyv.tm rmJc il icn.Zro aeitniyrsls lebxs hlirCeplonest eut rmTruoe al. et Tarduno from results paleointensity Thellier–Coe boxes, blue results: paleointensity Zircon zircons. Hills Jack from time vs. Paleointensity ma 50 at mm 6 ◦ nteasneo ed nlssi e.1idct that indicate 1 ref. in Analyses field. a of absence the in C I eupe na E taa40lctda Cornell at located 400 Strata FEI an in (equipped FIB A ◦ aeitniydtriain rmTruoe l 1;prl o,Tele–o aeitniyrsl ti td) upetriangles, purple study); (this result paleointensity Thellier–Coe box, purple (1); al. et Tarduno from determinations paleointensity C rdsaig h itneo h esrt the to sensor the of distance The spacing. grid µm ◦ .Teefe,tetmeauewsicesdto increased was temperature the Thereafter, C. ◦ a efre eti h rsneo a of presence the in next performed was C

◦ Field strength (microTesla) 10 15 20 25 30 35 40 45 50 55 60 aeitniyapoc 1 a applied was (1) approach paleointensity C pia irsoysuis(nvriyof (University studies microscopy Optical 0 5 0030 4030 8040 4200 4000 3800 3600 3400 3200 3000 ∼240 <475 Recent field Mesoarchean ◦ ◦ ae napeiinln cur- line precision a on based µm r ihnafco f2o full of 2 of factor a within are C .Dse iewt arrow: with line Dashed C. olwdby followed µm Paleoarchean magnification, ◦ tempera- C 2 7 ipoln ihihigptnilfrmgei eetn.Prl imnsadline: and diamonds Purple resetting. magnetic for potential highlighting profiling Li laser Age (Ma) de upr rmcr rgasa h aa eerhLbrtr.We McIntyre Laboratory. L. acknowl- B. Research preparation, O.v.’t.E. Naval specimen J.A.T.). in the Kloc at (to Gerald from programs fellowship help for core acknowledge (JSPS) Society from Japan Science support a edges of and EAR1656348 Promotion and the EAR1520681 Grants (NSF) dation repository, MagIC ACKNOWLEDGMENTS. the in available are MagIC/16680. study this in Availability. Data in noted where except (1) al. S6. et Table Tarduno of section methods mean calculated. Geochronology. smoothed slopes and this assigned were From Li approximately of calculated. maxi- troughs 20 were and 1σ a peaks profile, profile of profile, obtain mean mean profiles A to smoothed Li considered. analyzed and were follows: were tracks Assigned as spaced (Z565-5) uniformly slopes. estimate zircon and temperature one widths mum in band bands estimate diffuse to an an assume used with temperatures and smoothed filter were values window Li Along-profile analysis. Profiles zonation. sectoral and/or oscillatory kV) Analyses. 200 Li emission, (field Univer- S/TEM spectrometer. the X-ray G2 energy-dispersive F20 at of EDAX Tecnai conducted University an FEI with were the an equipped TEMs using at SEM. Rochester conducted of Auriga other also sity Zeiss in were a signal analyses using overall FIB Rochester the to and contributes SEM typically surround- analyses. the which from matrix contribution SEM zircon the improved ing removing for by allowed elec- characterization lamella EDS to prepared and thinned the the Nevertheless, further of sample. fragility not the the ( was to thicknesses lamella Due zir- transparency grid. sample tron the copper the from TEM inclusion, a removed to multipart was mounted inclusion and multipart An matrix damage. con a FIB from containing region lamella target the thick above surface grain the tect ag pnbakboxes: black open Large . Eoarchean aeaIS7 imp hwnro ad orsodn to corresponding bands narrow show maps Li 7f IMS Cameca nltclpoeue r ealdi h aeil and materials the in detailed are procedures Analytical antcdtst eeae uigado analyzed and/or during generated datasets Magnetic PNAS LHB hswr a upre yNtoa cec Foun- Science National by supported was work This -ydrto fooei etn.Temore The heating. orogenic of duration ∼1-My | netite o 565 for uncertainties ; 7 ipoln esedmtycntans(54) constraints geospeedometry profiling Li eray4 2020 4, February 0 m,peldn ute E td of study TEM further precluding nm), <100 Hadean limit Detection ∼1 | iewr eetdfor selected were wide µm o.117 vol. ◦ aeitniydeter- paleointensity C http://earthref.org/ ∼ | sliding- 1-µm o 5 no. IAppendix SI uncertainty, ∼1-µm | 2317 ,

EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES and R. Wiegandt on electron microscopy analyses, T. Pestaj in the SHRIMP Cornell Center for Materials Research Shared Facilities which are sup- laboratory, and A. Katayama on AIST SSM measurements. We thank the ported through the NSF Materials Research Science and Engineering Centers anonymous reviewers for helpful comments. This work made use of the (MRSEC) program (DMR-1719875).

1. J. A. Tarduno, R. D. Cottrell, W. J. Davis, F. Nimmo, R. K. Bono, A Hadean to Pale- 29. R. K. Bono, J. A. Tarduno, R. D. Cottrell, Primary pseudo-single and single-domain oarchean geodynamo recorded by single zircon crystals. Science 349, 521–524 magnetite inclusions in quartzite cobbles of the Jack Hills (western Australia): (2015). Implications for the Hadean geodynamo. Geophys. J. Int. 216, 598–608 (2019). 2. J. A. Tarduno, E. G. Blackman, E. E. Mamajek, Detecting the oldest geodynamo and 30. R. T. Pidgeon, S. A. Wilde, The interpretation of complex zircon U-Pb systens in attendant shielding from the solar wind: Implications for habitability. Phys. Earth Archaean granitoids and gneisses from the Jack Hills, narryer gneiss terrane, western Planet. Inter. 233, 68–87 (2014). Australia. Precambrian Res. 91, 309–332 (1998). 3. R. K. Bono, J. A. Tarduno, F. Nimmo, R. D. Cottrell, Young inner core inferred 31. B. P. Weiss et al., Secondary magnetic inclusions in detrital zircons from the Jack Hills, from Ediacaran ultra-low geomagnetic field intensity. Nat. Geosci. 12, 143–147 Western Australia, and implications for the origin of the geodynamo. Geology 46, (2019). 427–430 (2018). 4. P. Driscoll, Geodynamo recharged. Nat. Geosci. 12, 83–84 (2019). 32. H. Oda et al., Scanning SQUID microscope system for geological samples: System 5. J. A. Tarduno et al., Geodynamo, solar wind and magnetopause 3.45 billion years ago. integration and initial evaluation. Earth Planets Space 68, 179 (2016). Science 327, 1238–1240 (2010). 33. D. Heslop, A. P. Roberts, A Bayesian approach to the paleomagnetic conglomerate 6. Y. Usui, J. A. Tarduno, M. K. Watkeys, A. Hofmann, R. D. Cottrell, Evidence for test. J. Geophys. Res. 123, 1132–1142 (2018). a 3.45-billion-year-old magnetic remanence: Hints of an ancient geodynamo 34. R. K. Bono, J. A. Tarduno, R. D. Cottrell, M. S. Dare, The paleomagnetic record of from conglomerates of South Africa. Geochem. Geophys. Geosyst. 10, Q09Z07 Eoarchean to Hadean zircons: Magnetic hysteresis, imaging magnetic sources, micro- (2009). magnetic modeling and constraining post-formation heating. Abstract GP22A-07, 7. A. J. Biggin et al., Palaeomagnetism of Archaean rocks of the Onverwacht Group, American Geophysical Union 2016 Fall Meeting, 12–16 December, San Francisco, CA. Barberton Greenstone Belt (southern Africa): Evidence for a stable and potentially 35. J. A. Tarduno et al., Earth’s paleomagnetosphere and planetary habitability. Abstract reversing geomagnetic field at ca. 3.5 Ga. Earth Planet. Sci. Lett. 302, 314–328 P53E-2231, American Geophysical Union 2017 Fall Meeting, 11–15 December, New (2011). Orleans, LA . 8. J. A. Tarduno, Geodynamo history preserved in single silicate crystals: Origins and 36. G. Mitra, Ductile deformation zones and mylonites: The mechanical processes long-term mantle control. Elements 5, 217–222 (2009). involved in the deformation of crystalline basement rocks. Am. J. Sci. 278, 1057–1084 9. J. A. Tarduno, R. D. Cottrell, A. V. Smirnov, The paleomagnetism of single sili- (1978). cate crystals: Recording geomagnetic field strength during mixed polarity intervals, 37. D. D. Pollard, A. Aydin, Progress in understanding jointing over the past century. Geol. superchrons, and inner core growth. Rev. Geophys. 44, RG1002 (2006). Soc. Am. Bull. 100, 1181–1204 (1988). 10. T. Berndt, A. R. Muxworthy, K. Fabian, Does size matter? Statistical limits of paleo- 38. S. M. Reddy, N. E. Timms, W. Pantleon, P. Trimby, Quantitative characterization of magnetic field reconstruction from small rock specimens. J. Geophys. Res. 121, 15–26 plastic deformation of zircon and geological implications. Contrib. Mineral. Petrol. (2016). 153, 625–645 (2007). 11. G. Pullaiah, E. Irving, K. L. Buchan, D. J. Dunlop, Magnetization changes caused by 39. J. D. Eshelby, F. C. Frank, F. R. N. Nabarro, The equilibrium of linear arrays of burial and uplift. Earth Planet. Sci. Lett. 28, 133–143 (1975). dislocations. Philos. Mag. 42, 351–364 (1961). 12. D. J. Dunlop, O.¨ Ozdemir,¨ Rock Magnetism, Fundamentals and Frontiers (Cambridge 40. Tang et al., Secondary magnetite in ancient zircon precludes analysis of a Hadean University Press, New York, NY, 1997). geodynamo. Proc. Natl. Acad. Sci. U.S.A. 116, 407–412 (2018). 13. T. P. Almeida et al., Direct visualization of the thermomagnetic behavior of pseudo- 41. M. Takehara, K. Horie, T. Hokada, S. Kiyokawa, New insight into disturbance of U-Pb single-domain magnetite particles. Sci. Adv. 2, e1501801 (2016). and trace-element systems in hydrothermally altered zircon via SHRIMP analyses of 14. S. A. Wilde, J. W. Valley, W. H. Peck, C. M. Graham, Evidence from detrital zircons for zircon from the Duluth gabbro. Chem. Geol. 484, 168–178 (2018). the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature 409, 42. J. M. Feinberg, G. R. Scott, P. R. Renne, H.-R. Wenk, Exsolved magnetite inclusions 175–178 (2001). in silicates: Features determining their remanence behavior. Geology 33, 513–516 15. J. A. Tarduno, R. D. Cottrell, M. K. Watkeys, D. Bauch, Geomagnetic field strength 3.2 (2005). billion years ago recorded by single silicate crystals. Nature 446, 657–660 (2007). 43. R. Symonds, Scanning electron microscope observations of sublimates from Merapi 16. M. Barboni et al., Early formation of the moon 4.51 billion years ago. Sci. Adv. 3, Volcano, Indonesia. Geochem. J. 26, 337–350 (1993). e1602365 (2017). 44. J. P. Bradley, H. Y. McSween Jr., R. P. Harvey, Epitaxial growth of nanophase magnetite 17. M. Sato, Rock-magnetic properties of single zircon crystals sampled from the Tanzawa in Martian meteorite Allan Hills 84001: Implications for biogenic mineralization. tonalitic pluton, central Japan. Earth Planets Space 67, 150 (2015). Meteorit. Planet. Sci. 33, 765–773 (1998). 18. B. Rasmussen, I. R. Fletcher, J. R. Muhling, S. A. Wilde, In situ U-Th-Pb geochronol- 45. P. N. Argyres, Theory of the Faraday and Kerr effects in ferromagnetics. Phys. Rev. 97, ogy of monazite and xenotime from the Jack Hills belt: Implications for the age 334–345 (1955). of deposition and metamorphism of Hadean zircons. Precambrian Res. 180, 26–46 46. W. J. Davis, N. Rayner, T. Pestaj, “When good zircons go bad-redistribution of radio- (2010). genic Pb in granulite grade zircon, snowbird tectonic zone, Canada” in Abstract 19. B. Rasmussen, I. R. Fletcher, J. R. Muhling, C. J. Gregory, S. A. Wilde, Metamor- Volume, 4th International SHRIMP Workshop, Saint Petersburg, Russia, O. Petrov, phic replacement of mineral inclusions in detrital zircon from Jack Hills, Australia: S. Shevchenko, S. Sergeev, L. Mordberg, Eds. (VSEGEI Press, St. Petersburg, Russia, Implications for the Hadean earth. Geology 39, 1143–1146 (2011). 2008), pp. 42–44. 20. G. S. Watson, A test for randomness of directions. Geophys. J. Int. 7, 160–161 (1956). 47. T. M. Harrison, E. A. Bell, P. Boehnke, Hadean zircon petrochronology. Rev. Mineral. 21. R. D. Cottrell, J. A. Tarduno, R. K. Bono, M. S. Dare, G. Mitra, The inverse microcon- Geochem. 83, 329–363 (2017). glomerate test: Further evidence for the preservation of Hadean magnetizations in 48. D. Trail et al., Li zoning in zircon as a potential geospeedometer and peak metasediments of the Jack Hills, western Australia. Geophys. Res. Lett. 43, 4215–4220 temperature indicator. Contrib. Mineral. Petrol. 171, 25 (2016). (2016). 49. M. Tang, R. L. Rudnick, W. F. McDonough, M. Bose, Y. Goreva, Multi-mode Li dif- 22. B. P. Weiss et al., Reply to comment on “Pervasive remagnetization of detrital zircon fusion in natural zircons: Evidence for diffusion in the presence of step-function host rocks in the Jack Hills, Western Australia and implications for records of the early concentration boundaries. Earth Planet. Sci. Lett. 474, 110–119 (2017). dynamo”. Earth Planet. Sci. Lett. 450, 409–412 (2016). 50. J. T. Sliwinski et al., Controls on lithium concentration and diffusion in zircon. Chem. 23. M. S. Dare et al., Detrital magnetite and chromite in Jack Hills quartzite cobbles: Fur- Geol. 501, 1–11 (2018). ther evidence for the preservation of primary magnetizations and new insights into 51. J. W. Valley et al., Hadean age for a post-magma-ocean zircon confirmed by atom- sediment provenance. Earth Planet. Sci. Lett. 451, 298–314 (2016). probe tomography. Nat. Geosci. 7, 219–223 (2014). 24. R. K. Bono, J. A. Tarduno, M. S. Dare, G. Mitra, R. D. Cottrell, Cluster analysis on a 52. W. B. Moore, A. A. G. Webb, Heat-pipe Earth. Nature 501, 501–505 (2013). sphere: Application to magnetizations from metasediments of the Jack Hills, western 53. P. Olson, U. R. Christensen, Dipole moment scaling for convection-driven planetary Australia. Earth Planet. Sci. Lett. 484, 67–80 (2018). dynamos. Earth Planet. Sci. Lett. 250, 561–571 (2006). 25. J. A. Tarduno, R. D. Cottrell, Signals from the ancient geodynamo: A paleomagnetic 54. S. Labrosse, J. W. Hernlund, N. Coltice, A crystallizing dense magma ocean at the base field test on the Jack Hills metaconglomerate. Earth Planet. Sci. Lett. 367, 123–132 of the Earth’s mantle. Nature 450, 866–869 (2007). (2013). 55. J. G. O’Rourke, D. J. Stevenson, Powering Earth’s dynamo with magnesium precipita- 26. B. P. Weiss et al., Pervasive remagnetization of detrital zircon host rocks in the Jack tion from the core. Nature 529, 387–389 (2016). Hills, Western Australia and implications for records of the early geodynamo. Earth 56. J. Badro, J. Siebert, F. Nimmo, An early geodynamo driven by exsolution of mantle Planet. Sci. Lett. 430, 115–128 (2015). components from Earth’s core. Nature 536, 326–328 (2016). 27. W. Huang, J. A. Tarduno, R. D. Cottrell, R. K. Bono, E. R. Thern, Unraveling Archean 57. K. Hirose et al., Crystallization of silicon dioxide and compositional evolution of the magnetic histories by a comparison of whole rock and single crystal analyses of gran- Earth’s core. Nature 543, 99–102 (2017). ites from Western Australia. Abstract GP21B-0651,American Geophysical Union 2018 58. C. O’Neill, S. Marchi, S. Zhang, W. Bottke, Impact-driven subduction on the Hadean Fall Meeting, 10–14 December, Washington, DC. Earth. Nat. Geosci. 10, 793–797 (2017). 28. R. K. Bono, J. A. Tarduno, R. D. Cottrell, Comment on: Pervasive remagnetization of 59. L. A. Giannuzzi, F. A. Stevie, A review of focused ion beam milling techniques for TEM detrital zircon host rocks in the Jack Hills, Western Australia and implications for specimen preparation. Micron 30, 197–204 (1999). records of the early dynamo, by Weiss et al. (2015). Earth Planet. Sci. Lett. 450, 406– 60. J. Mayer, L. A. Giannuzzi, T. Kamino, J. Michael, TEM sample preparation and FIB- 408 (2016). induced damage. MRS Bull. 32, 400–407 (2007).

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