Jack Hills Zircons Research Articles

Sediment subduction in Hadean revealed by machine learning

Due to the scarcity of rock samples, the Hadean Era predating 4 billion years ago (Ga) poses challenges in understanding geological processes like subaerial weathering and plate tectonics that are critical for the evolution of life. The Jack Hills zircon from Western Australia, the primary Hadean samples available, offer valuable insights into magma sources and tectonic genesis through trace element signatures. However, a consensus on these signatures has not been reached. To address this, we developed a machine learning classifier capable of deciphering the geochemical fingerprints of zircon. This allowed us to identify the oldest detrital zircon originating from sedimentary-derived "S-type" granites. Our results indicate the presence of S-type granites as early as 4.24 Ga, persisting throughout the Hadean into the Archean. Examining global detrital zircon across Earth's history reveals consistent supercontinent-like cycles from the present back to the Hadean. These findings suggest that a significant amount of Hadean continental crust was exposed, weathered into sediments, and incorporated into the magma sources of Jack Hills zircon. Only the early operation of both subaerial weathering and plate subduction can account for the prevalence of S-type granites we observe. Additionally, the periodic evolution of S-type granite proportions implies that subduction-driven tectonic cycles were active during the Hadean, at least around 4.2 Ga. The evidence thus points toward an early Earth resembling the modern Earth in terms of active tectonics and habitable surface conditions. This suggests the potential for life to originate in environments like warm ponds rather than extreme hydrothermal settings.

I-type and S-type granites in the Earth’s earliest continental crust

The composition and origin of Earth’s earliest continental crust remains enigmatic due to the absence of Hadean (>4 Ga) age rocks. Here we address this question by using machine learning to examine the provenance of the 4.4–3.3 Ga Jack Hills zircons, which constitute the best archive of Earth’s earliest continental crust. Our results reveal that although some Jack Hills zircons may be derived from trondhjemite-tonalite-granodiorite series rocks, which were common during the Archean (4–2.5 Ga), most (as high as ~70%) are sourced from igneous (I-) and sedimentary (S-) type granites. This finding provides clear evidence for rocks other than the trondhjemite-tonalite-granodiorite suite in the Earth’s earliest continental crust. Considering that I- and S-type granites are typical of modern convergent plate margins, the presence of a high proportion of Jack Hills zircons from these rocks supports the operation of a horizontal, mobile-lid tectonic regime in the early Earth.

Eoarchean and Hadean melts reveal arc-like trace element and isotopic signatures

Constraining the lithological diversity and tectonics of the earliest Earth is critical to understanding our planet’s evolution. Here we use detrital Jack Hills zircon (3.7 − 4.2 Ga) analyses coupled with new experimental partitioning data to model the silica content, Si+O isotopic composition, and trace element contents of their parent melts. Comparing our derived Jack Hills zircons’ parent melt Si+O isotopic compositions (−1.92 ≤ δ30SiNBS28 ≤ 0.53 ‰; 5.23 ≤ δ18OVSMOW ≤ 9.00 ‰) to younger crustal lithologies, we conclude that the chemistry of the parent melts was influenced by the assimilation of terrigenous sediments, serpentinites, cherts, and silicified basalts, followed by igneous differentiation, leading to the formation of intermediate to felsic melts in the early Earth. Trace element measurements also show that the formational regime had an arc-like chemistry, implying the presence of mobile-lid tectonics in the Hadean. Finally, we propose that these continental-crust forming processes operated uniformly from 4.2 to at least 3.7 Ga.

Direct age constraints on the magnetism of Jack Hills zircon

A potential record of Earth's magnetic field going back 4.2billion years (Ga) ago is carried by magnetite inclusions in zircon grains from the Jack Hills. This magnetite may be secondary in nature, however, meaning that the magnetic record is much younger than the zircon crystallization age. Here, we use atom probe tomography to show that Pb-bearing nanoclusters in magnetite-bearing Jack Hills zircons formed during two discrete events at 3.4 and<2 Ga. The older population of clusters contains no detectable Fe, whereas roughly half of the younger population of clusters is Fe bearing. This result shows that the Fe required to form secondary magnetite entered the zircon sometime after 3.4 Ga and that remobilization of Pb and Fe during an annealing event occurred more than 1 Ga after deposition of the Jack Hills sediment at 3 Ga. The ability to date Fe mobility linked to secondary magnetite formation provides new possibilities to improve our knowledge of the Archean geodynamo.

Plate tectonic–like cycles since the Hadean: Initiated or inherited?

Abstract Interpretation of Earth's oldest preserved crustal archive, the Jack Hills zircon of Western Australia, has been controversial in terms of the onset of plate tectonics. We conduct time-series analysis on hafnium isotopes of the Jack Hills zircon and reveal an array of statistically significant cycles that are reminiscent of plate-tectonic subduction. At face value, such cycles may suggest early Earth conditions similar to those of today—the uniformitarian hypothesis that plate tectonics was essentially operational since “day one”. On the other hand, in the context of expected secular changes due to planetary evolution and geological observations, the cycles could instead imply that modern plate-tectonic subduction inherited mantle convective harmonics already facilitated by an early phase of stagnant-lid delamination—the “lid-to-plates” hypothesis. Either way, any model for the nature of plate tectonics must incorporate conditions operating during Hadean time, either by initiation of plate tectonics then or by later inheritance of preexisting cycles of mantle convection.

Hadean zircon formed due to hydrated ultramafic protocrust melting

Abstract Hadean zircons, from the Jack Hills (Western Australia) and other localities, are currently the only window into the earliest terrestrial felsic crust, the formation of which remains enigmatic. Based upon new experimental results, generation of such early crust has been hypothesized to involve the partial melting of hydrated peridotite interacting with basaltic melt at low pressure (&amp;lt;10 km), but it has yet to be demonstrated that such liquids can indeed crystallize zircons comparable to Jack Hills zircon. We used thermodynamic and geochemical modeling to test this hypothesis. The predicted zircon saturation temperatures of &amp;lt;750 °C, together with the model zircon Th, U, Nb, Hf, Y, and rare earth element (REE) contents at 700 °C, δ18OVSMOW (Vienna standard mean ocean water) signatures, and co-crystallizing mineral assemblage were compared to those of the Jack Hills zircon. This comparison was favorable with respect to crystallization temperature, most trace-element contents, and mineral inclusions in zircon. The discrepancy in δ18OVSMOW signatures may be explained by hotter conditions of Hadean protocrust hydration. Our work supports the idea that felsic magma generation at shallow depths involving a primordial weathered ultramafic protocrust and local basaltic intrusions is indeed a viable mechanism for the formation of felsic crust on early Earth.

Paleomagnetism of 3.5-4.0 Ga zircons from the Barberton Greenstone Belt, South Africa

The existence of a core dynamo during the first billion years of Earth history is closely related to the thermal state of the Earth's interior and composition of the early atmosphere. The scarcity of well-preserved rock units older than 3.5 billion years (Ga) has motivated the paleomagnetic analysis of detrital zircons. Studies of zircons from Jack Hills, Australia, however, have found the pervasive occurrence of secondary ferromagnetic minerals, casting doubt on the ability of these zircons to record a >3.5 Ga geomagnetic field. Here we report paleomagnetic analyses on a set of 19 zircons with crystallization age 3.5-4.0 Ga from the Barberton Greenstone Belt (BGB) of South Africa, which have undergone lower grade metamorphism compared to all other known >3.5 Ga detrital zircon localities. We find that BGB zircons have magnetic moments nearly one order of magnitude weaker than Jack Hills zircons, precluding the retention of primary paleomagnetic information. This result corroborates findings from the Jack Hills and other Archean zircon populations that primary ferromagnetic inclusions are readily eliminated from zircons during sedimentary transport and metamorphism, likely facilitated by radiation damage-induced permeability. Paleomagnetic determination of geodynamo activity prior to 3.5 Ga may require investigation of other detrital grains with lower radiation damage potential or whole-rock samples that have escaped high degree metamorphism.

A Novel Approach to Identifying Mantle‐Equilibrated Zircon by Using Trace Element Chemistry

AbstractOne of the requirements for inferring mantle redox states via zircon Ce/Ce* oxybarometry and Ti‐in‐zircon thermometry is to select mantle‐equilibrated zircon derived from melts that have not interacted with the hydrosphere nor assimilated exogenous materials. Traditional protocols for identifying mantle‐equilibrated zircon require a comprehensive examination of whole rock geochemistry in addition to zircon morphology, texture, and trace element and isotopic measurements. This study proposes a simple filter scheme for selecting mantle‐equilibrated zircon using trace element systematics alone (P, Ti, Y, Nb, rare earth elements [REE], Hf, Th, and U) to provide the geological context of zircon from three perspectives: (1) mineral inclusions; (2) source melt petrogenesis; and (3) zircon saturation context. The filter scheme presented here, comprising of 13 criteria, is established based on detailed classifications of 2,173 zircon analyses from 30 independent references and is shown to distinguish between nonmagmatic zircon, magmatic zircon with significant inclusions and/or sourced from highly enriched source melts, and mantle‐equilibrated zircon. The filter scheme is validated with known, well‐characterized mantle‐equilibrated Jack Hills zircon (n = 53) and derives an average mantle redox state of (sd) circa 4,400 Ma, in agreement with previously published results. Further application of the filter scheme to zircon dataset (n = 76) from three metasedimentary rocks successfully reveals the mantle‐equilibrated zircon in the dataset, and implies a mantle redox state of (sd) circa 2,950 Ma. This filter scheme can be applied to studies of out‐of‐context detrital and/or xenocrystic zircon in the future.

Lu-Hf systematics of 4.0 – 2.3 Ga old zircons from the Turee Creek Group (Pilbara Craton, W. Australia): Implications on the rise of atmospheric oxygen and global glaciation during the Paleoproterozoic

We investigated the Hf isotopic systematics of detrital zircons in a succession of siliciclastic sediments and glacial diamictites from the early Paleoproterozoic sequence of the Boolgeeda Iron Formation (Hamersley Group) and overlying Turee Creek Group of the Pilbara Craton, Western Australia. About 400 detrital zircons yielding >95% concordant U-Pb ages were analyzed for Hf isotopes to constrain their magmatic sources. 70% of the analyzed zircons display super-chondritic initial Hf isotopic compositions, demonstrating crystallization in mantle-derived magmas. Most of the data are comprised between model age lines at ~2.5 and 3.2 Ga, which suggests a sub-continuous crust generation by extraction from the depleted mantle during this time period. A single grain yields a 4.0 Ga age, which represents the first Hadean age for a zircon from the Pilbara Craton. Our results are significantly different from zircon Hf isotope data of the Glenburgh Terrane, in the southern border of the Turee Creek Group, or older successions of the Pilbara, Kaapvaal and Superior cratons, but show overlap with some of the Yilgarn Craton. This together with the occurrence of a Hadean zircon crystal preserved in the Boolgeeda glacial diamictite with similar Hf isotopic signature than the Jack Hills zircons makes the Yilgarn Craton a possible source material for the Boolgeeda glacial horizon. Alternatively, the majority of the zircons analyzed show ages which are consistent with those of the underlying 2.45–2.78 Ga Hamersley and Fortescue groups, formed by sedimentary successions interleaved with thick subaerial volcanic sequences associated with the emplacement of Large Igneous Provinces. Such subaerial volcanic rocks could account for the relatively juvenile character of the zircon analyzed. A local provenance of the siliciclastic material delivered to the Turee Creek Basin would support the role of large subaerial magmatic provinces as triggers of the rise of atmospheric oxygen and the onset of glaciations at the beginning of the Proterozoic.

Reevaluating the evidence for a Hadean-Eoarchean dynamo.

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.

Detrital zircon evidence for change in geodynamic regime of continental crust formation 3.7–3.6 billion years ago

The nature of the early terrestrial crust and how it evolved through time remains highly controversial. Whether conventional plate tectonics operated in the Hadean and early Archean and when it came into existence remains unclear. Here, we describe U-Pb ages, Hf isotope composition and trace element chemistry of 3.95–3.10 Ga old detrital zircons from the Singhbhum Craton in eastern India. The >3.7 Ga old zircons of this suite have crust-like Hf isotope compositions with strongly negative εHfi and their granitoid sources formed by intra-crustal reworking of a Hadean protolith that was extracted from primitive mantle at 4.4–4.5 Ga. The trace element and Hf isotope compositions of the zircons record a transition from higher Nb/Th (0.070 ± 0.010), Nb/U (0.045 ± 0.005), crust-like Hf isotope compositions, and longer crustal residence times of the protoliths prior to 3.7–3.6 Ga, to lower Nb/Th (0.032 ± 0.012), Nb/U (0.024 ± 0.009; 1σ), mantle-like Hf isotope compositions, and shorter protolith residence times post 3.7–3.6 Ga. The Nb/Th and Nb/U fractionation at 3.7 Ga seen in the detrital zircon record can be explained by transition to granitoid production at greater depths in an oceanic plateau-like regime. However, had that been the case, the crustal residence times of the protoliths of the granitoids from which the detrital zircons were sourced should have progressively increased with time, given the >1.1 billion years protracted history of granitoid magmatism in the craton, which is contrary to what is observed. We suggest that the changes in the granitoid chemistry recorded by the detrital zircons document a significant change in the depth of melting of the protoliths as well as in the tectonic setting of continental crust formation, and argue that it marks the transition to granitoid production from oceanic plateaus to arc-like tectonic environments. Broadly similar transitions at ca. 3.6 Ga have been documented in gneisses from the Acasta Gneiss Complex, the Jack Hills zircons and in detrital zircons from the Wyoming Province, which suggest that the end of the Eoarchean may have been marked by widespread transition in planetary tectonic regime.

An andesitic source for Jack Hills zircon supports onset of plate tectonics in the Hadean

The composition and origin of Earth’s early crust remains hotly debated. Here we use partition coefficients to invert the trace element composition of 4.3–3.3 Gyr Jack Hills zircons to calculate the composition of the melts from which they crystallised. Using this approach, the average SiO2 content of these melts was 59 ± 6 wt. % with Th/Nb, Dy/Yb and Sr/Y ratios of 2.7 ± 1.9, 0.9 ± 0.2 and 1.6 ± 0.7, respectively. Such features strongly indicate that the protolith for the Jack Hills zircons was not an intra-plate mafic rock, nor a TTG (tondjhemite-tonalite-granodiorite) or a Sudbury-like impact melt. Instead, the inferred equilibrium melts are much more similar to andesites formed in modern subduction settings. We find no evidence for any secular variation between 4.3 and 3.3 Gyr implying little change in the composition or tectonic affinity of the Earth’s early crust from the Hadean to Mesoarchaean.

Stepwise chemical abrasion–isotope dilution–thermal ionization mass spectrometry with trace element analysis of microfractured Hadean zircon

Abstract. The Hadean Jack Hills zircons represent the oldest known terrestrial material, providing a unique and truly direct record of Hadean Earth history. This zircon population has been extensively studied via high-spatial-resolution high-throughput in situ isotopic and elemental analysis techniques, such as secondary ionization mass spectrometry (SIMS), but not by comparatively destructive, high-temporal-precision (&lt;0.05 % two-sigma) thermal ionization mass spectrometry (TIMS). In order to better understand the lead loss and alteration history of terrestrial Hadean zircons, we conduct stepwise chemical abrasion–isotope dilution–thermal ionization mass spectrometry with trace element analysis (CA-ID-TIMS-TEA) on manually microfractured Hadean Jack Hills zircon fragments previously dated by SIMS. We conducted three successive HF leaching steps on each individual zircon fragment, followed by column chromatography to isolate U–Pb and trace element fractions. Following isotopic and elemental analysis, the result is an independent age and trace element composition for each leachate of each zircon fragment. We observe ∼50 Myr of age heterogeneity in concordant residues from a single zircon grain, along with a protracted history of post-Hadean Pb loss with at least two modes circa ∼0 and 2–4 Ga. Meanwhile, stepwise leachate trace element chemistry reveals enrichments of light rare earth elements, uranium, thorium, and radiogenic lead in early leached domains relative to the zircon residue. In addition to confirming the efficacy of the LREE-I alteration index and providing new insight into the mechanism of chemical abrasion, the interpretation and reconciliation of these results suggest that Pb loss is largely driven by low-temperature aqueous recrystallization and that regional thermal events may act to halt – not initiate – Pb loss from metamict domains in the Hadean Jack Hills zircons.

A comparison between zircons from the Acasta Gneiss Complex and the Jack Hills region

The composition and origin of the earliest continental crust is intensely debated. The understanding of its composition relies heavily on the oldest vestiges of continental crust, which includes detrital zircon grains that are up to 4.36 billion-years-old (Ga) and <4.03 Ga zircon-bearing rocks. However, the interpretation of these two sample suites has thus far remained largely separate. Here, we demonstrate that the trace-element compositions of magmatic zircons from the Acasta Gneiss Complex compare favorably with those of ancient detrital zircons from western Australia. We combine these new data with existing oxygen and hafnium isotope datasets to show that the petrological processes that formed the Acasta Gneiss Complex, namely partial melting of mafic crust at various depths, are an appropriate analogue for the formation of the Hadean crust parental to the Jack Hills zircon grains. We also suggest that a transition from shallow-, to deep-crustal melting occurred in the JH source region ca. 3.8 Ga.

The accumulation of non-formula elements in zircons during weathering: Ancient zircons from the Jack Hills, Western Australia

In this contribution we describe the influx of non-formula elements (Fe, Ca, Al, Y, U and Th) into fractures and selected zone lamellae in zircons from Jack Hills during recent weathering and discuss the effects of this on overlapping SIMS U-Th-Pb and oxygen isotope analyses. Previous research has recognised the importance of fractures in the generation of anomalous U-Th-Pb and oxygen isotope systems. In this report we show that besides fractures specific zones in euhedrally zoned zircon can act as pathways for the influx of weathering solutions and contain a similar range of trace element materials as do the fractures. Whereas zero-age discordant U-Pb systems of Jack Hills zircons have been explained by many authors in terms of Pb loss, present results confirm conclusions of our previous study that the main discordance mechanism of Jack Hills zircons is U-Th gain, due to overlap of SIMS analyses with mineralized fractures and zone lamellae with excess weathering-fluid-deposited U and Th. We explain the anomalously light and heavy oxygen isotopes and significant OH in SIMS analyses that overlap fractures and mineralized zones as due to the presence in the fractures of Ca, Fe, Al oxides and hydroxides with complexly fractionated oxygen isotopic systems. There is a suggestion in some of the elemental maps that there has been minor dispersion of trace elements away from fractures. But SIMS U-Th-Pb and oxygen isotope analyses on parts of the zircon away from fractures and mineralized zones show no evidence of interaction with weathering-fluid, indicating that penetration of weathering fluids into the body of the zircon at the location of the SIMS spots has not occurred. Results of this study have implications for other SIMS U-Th-Pb and oxygen isotope studies of zircons from rocks that have been subjected to weathering and also for early TIMS U-Pb measurements of bulk zircon samples that show zero Ma U-Pb discordance.

Secondary magnetic inclusions in detrital zircons from the Jack Hills, Western Australia, and implications for the origin of the geodynamo

The time of origin of Earth’s dynamo is unknown. Detrital zircon crystals containing ferromagnetic inclusions from the Jack Hills of Western Australia have the potential to contain the oldest records of the geodynamo. It has recently been argued that magnetization in these zircons indicates that an active dynamo existed as far back as 4.2 Ga. However, the ages of ferromagnetic inclusions in the zircons are unknown. Here we present the first detailed characterization of the mineralogy and spatial distribution of ferromagnetic minerals in Jack Hills detrital zircons. We demonstrate that ferromagnetic minerals in most Jack Hills zircons are commonly located in cracks and on the zircons’ exteriors. Hematite is observed to dominate the magnetization of many zircons, while other zircons also contain significant quantities of magnetite and goethite. This indicates that the magnetization of most zircons is likely to be dominantly carried by secondary minerals that could be hundreds of millions to billions of years younger than the zircons’ crystallization ages. We conclude that the existence of the geodynamo prior to 3.5 Ga has yet to be established.

Mineral inclusion assemblage and detrital zircon provenance

Mineral inclusions are common in magmatic zircon and a potentially rich source of petrologic information. Controls on the relative proportions of inclusion phases, specifically early-crystallizing minerals such as apatite and late-crystallizing phases such as quartz, K-feldspar, and muscovite, have not been systematically studied. For instance, apatite dominates many magmatic zircon inclusion suites, and selective replacement of apatite over other phases has been proposed as a mechanism for generating apatite-poor inclusion assemblages in detrital zircons. However, the extent to which apatite inclusion abundance is influenced by source rock composition has not been established. The preservation of characteristic minerals in granite series, such as differences in magnetite and ilmenite abundances due to varying redox, have also not been systematically explored as inclusion phases in zircon. We surveyed zircon inclusion assemblages in Phanerozoic granitoids of a range of compositions and found a broadly inverse relationship between the presence of apatite in the inclusion suite and whole-rock SiO2 content. Selective loss of apatite is evident from deficits in apatite content among inclusions in contact with cracks in both detrital zircons and some granitoid zircons with independent evidence for fluid ingress (i.e., secondary phases filling open cracks). In cases where microstructural observations can identify primary inclusion assemblages (versus those impacted by fluid ingress), the relative occurrence of apatite can be used to broadly predict source rock SiO2 content. There is little relationship between whole-rock chemistry and the abundance of late-crystallizing phases or the relative proportions of quartz, plagioclase, and alkali feldspar. The presence of ilmenite not in contact with cracks in the host zircon likely points to origins in ilmenite-series (i.e., reduced) magmas, but the presence of magnetite is less diagnostic. Using the apatite abundance metric, detrital Hadean zircons from Jack Hills (Western Australia), appear to derive from felsic rather than intermediate or mafic granitoids and detrital Eoarchean zircons from Nuvvuagittuq Supracrustal Belt (Quebec) may derive from intermediate to felsic granitoids. No Fe-Ti oxides have been identified in Nuvvuagittuq zircons, but rare ilmenite in Jack Hills zircons points to origins of at least some part of the population in ilmenite-series magmas.

Cluster analysis on a sphere: Application to magnetizations from metasediments of the Jack Hills, Western Australia

Metasediments of the Jack Hills contain the oldest known terrestrial minerals in the form of zircons nearly 4.4 billion years old. Paleointensity data from these zircons provide evidence for a Hadean geodynamo as old as 4.2 billion years old. Given the importance of these zircons for constraining the earliest history of the core, it is vital to understand the fidelity of the zircon record. A fundamental aspect providing context for the preservation of primary magnetic signals is the nature of overprints predicted to have been imparted on rocks of the Jack Hills due to Archean to Proterozoic metamorphic events. To be viable magnetic records of a Hadean geodynamo, zircon magnetization directions should differ from these secondary magnetizations. To evaluate these secondary magnetizations, we report paleomagnetic analyses of a comprehensive sampling of 68 quartzite cobble-sized clasts from the Jack Hills metasediments ∼0.5 to 1.0 km from the Discovery Site (which has yielded the oldest zircons and paleofield estimates). While application of standard paleomagnetic tests suggests that the ensemble of cobble directions cannot be distinguished from those drawn from a random distribution, a new cluster analysis of directions on a sphere and non-parametric resampling approaches reveal significant directions amongst subsets of the data. One, isolated at the lowest temperature analyzed [200 to 300 °C, Declination (Dec.) = 316.8°, Inclination (Inc.) =−51.1°] appears to be dominated by the present day field. Another, isolated at higher (but still relatively low unblocking temperatures that we call “intermediate”, of ∼350–500 °C, Dec. = 243.8°, Inc. = 9.5°) agrees with a magnetic overprint isolated from the secondary Cr–Fe mica fuchsite isolated from the Jack Hills Discovery site, passing a field test at the 80% confidence level. No evidence is found in our data, or in the data of others collected on similar Jack Hills lithologies, for a widespread 1 Ga remagnetization event. Instead, we interpret the most prevalent secondary magnetization of the quartzite (i.e., intermediate unblocking) and the fuchsite characteristic remanent magnetization to be ∼2.65 Ga in age, coincident with peak metamorphism (as high as ca. 475 °C) of the Jack Hills. The presence of this distinct secondary magnetization, its difference from that recorded by Jack Hills zircons at high unblocking temperatures, and the lack of a dominant remagnetization direction at high unblocking temperatures in the cobble data (the expected result for a primary magnetization), lends further support to the fidelity of the Hadean geomagnetic record. The presence of the secondary magnetization also lends support to the conclusion that most of the Jack Hills metasediments were deposited in the Archean, with only minor reworking and potential tectonic interleaving of Proterozoic components. Overall, the application of the new directional cluster analysis presented here has the potential to reveal magnetic directions in highly scattered data sets, typical of weakly magnetized coarse-grained sedimentary rocks

Geochronology of Hadean zircon grains from the Jack Hills, Western Australia constrained by quantitative scanning ion imaging

Five Hadean (>4Ga) aged zircon grains from the Jack Hills metasedimentary belt have been investigated by a secondary ion mass spectrometry scanning ion image technique. This technique has the ability to obtain accurate and precise full U-Pb systematics on a scale <5μm, as well as document the spatial distribution of U, Th and Pb. All five of the grains investigated here have complex cathodoluminescence patterns that correlate to different U, Th, and Pb concentration domains. The age determinations for these different chemical zones indicate multiple reworking events that are preserved in each grain and have affected the primary crystalized zircon on the scale of <10μm, smaller than conventional ion microprobe spot analyses. In comparison to the spot analyses performed on these grains, these new scanning ion images and age determinations indicate that almost half of the spot analyses have intersected several age and chemical domains in both fractured and unfractured parts of the individual crystals. Some of these unfractured, mixed domain spot analyses have concordant ages that are inaccurate. Thus, if the frequency of spot analyses intersecting mixed domains here is even close to representative of all other studies of the Jack Hills zircon population, it makes the interpretation of any trace element, Hf, or O isotopic data present in the literature tenuous. Lastly, all of the grains analysed here preserve at least two distinguishable 207Pb/206Pb ages. These ages are preserved in core-rim and/or complex internal textural relationships in unfractured domains. These secondary events took place at ca. 4.3, 4.2, 4.1, 4.0, 3.7, and 2.9Ga, which are coincident with previously determined statistically robust age peaks present in this zircon population.

Multi-mode Li diffusion in natural zircons: Evidence for diffusion in the presence of step-function concentration boundaries

Micron- to submicron-scale observations of Li distribution and Li isotope composition profiles can be used to infer the mechanisms of Li diffusion in natural zircon. Extreme fractionation (20–30‰) within each single crystal studied here confirms that Li diffusion commonly occurs in zircon. Sharp Li concentration gradients frequently seen in zircons suggest that the effective diffusivity of Li is significantly slower than experimentally determined (Cherniak and Watson, 2010; Trail et al., 2016), otherwise the crystallization/metamorphic heating of these zircons would have to be unrealistically fast (years to tens of years). Charge coupling with REE and Y has been suggested as a mechanism that may considerably reduce Li diffusivity in zircon (Ushikubo et al., 2008; Bouvier et al., 2012). We show that Li diffused in the direction of decreasing Li/Y ratio and increasing Li concentration (uphill diffusion) in one of the zircons, demonstrating charge coupling with REE and Y. Quantitative modeling reveals that Li may diffuse in at least two modes in natural zircons: one being slow and possibly coupled with REE+Y, and the other one being fast and not coupled with REE+Y. The partitioning of Li between these two modes during its diffusion may depend on the pre-diffusion substitution mechanism of REE and Y in the zircon lattice. Based on our results, sharp Li concentration gradients are not indicative of limited diffusion, and can be preserved at temperatures >700 °C on geologic timescales. Finally, large δ7Li variations observed in the Hadean Jack Hills zircons may record kinetic fractionation, rather than a record of ancient intense weathering in the granite source materials.