Search

Here, we report small randomly-distributed crystalline lead (Pb) nanospheres occurring in detrital zircon grains obtained from a weakly metamorphosed Archean conglomerate at Jack Hills, Western Australia, making this the third known global example of this phenomenon. They form in zircon crystals ranging from Hadean (> 4 billion years—Ga) to Eoarchean (> 3.6 Ga) in age, but are absent from Paleoarchean (~ 3.4 Ga) crystals. Unlike previous discoveries of nanospheres in zircon from Precambrian gneisses in Antarctica and India, detrital zircon from Jack Hills shows no evidence of ever undergoing ultra-high temperature (UHT) metamorphism, either before or after deposition, therefore implying that nanospheres can form at temperatures lower than ca. 900 °C. The nanospheres are composed of radiogenic Pb released by the breakdown of uranium (U) and thorium (Th) and are present in zircon irrespective of its U, Th and water contents, its oxygen isotopic composition, and the degree of discordance due to Pb loss or gain. The nanospheres pre-date annealed cracks in the crystals, showing that, once formed, they effectively 'freeze' radiogenic Pb in the zircon structure, precluding any further interaction during subsequent geological processes. Both Pb nanoclusters and nanospheres are now reported from Jack Hills, and it appears likely the former is a precursor stage in the formation of the latter. Although the precise mechanism for this transition remains unresolved, a later thermal event is required, but this likely did not reach UHT conditions at Jack Hills. [ABSTRACT FROM AUTHOR]

The recent discovery of Pb* (radiogenic lead)-enriched domains (PEDs) in zircon using high-resolution scanning ion imaging, atom probe tomography and transmission electron microscopy indicates that the U-Pb isotopic system in zircon can be affected not only by Pb* loss but also by intra-crystalline Pb* enrichment. However, the formation mechanism of PEDs and the consequences for in-situ U-Pb dating remain elusive. To further understand these issues, scanning ion imaging and U-Pb dating were carried out on 51 Hadean detrital zircon grains from the Jack Hills, Western Australia. Of these, 8 grains were found to contain micrometer-scale PEDs with elevated 207Pb and 206Pb in the ion images. Isotope profiles across these PEDs confirm that they represent unsupported Pb* resulting from intra-grain mobilization and local enrichment of Pb* during ancient overprinting events. The PEDs are rare in most grains, with only 1–3 Pb* hotspots present in any ∼70 μm × 70 μm imaged area, and are located either in dark-CL, high-U growth zones, or associated with bright-CL, low-Th recrystallized domains, suggesting that they were likely formed by local-scale processes. The PEDs located in dark-CL, high-U growth zones likely resulted from Pb* trapped in local, interconnected pathways produced by radiation damage during thermal overprinting events. Those associated with bright-CL, low-Th domains are interpreted as Pb* concentrated in nanoscale pores formed during fluid-present recrystallization of non-metamict zircon. Our interpretations therefore imply that intra-crystalline enrichment of Pb* in zircon can occur at various scales and through diverse mechanisms. Age calculations based on the ion images demonstrate that incorporation of PEDs in conventional in-situ SIMS spots produces spuriously old ages that are either reversely discordant or concordant, depending on the time of Pb* mobilization and the analytical precision. The implication is that careful investigation of intra-grain distribution of U-Th-Pb isotopes is required in order to better understand the U-Pb isotopic system and to obtain reliable crystallization ages for ancient zircons affected by complex Pb* mobilization and redistribution. • Pb*-enriched domains (PEDs) are found in 8 Hadean zircon grains from the Jack Hills. • These PEDs were likely formed at various scales through diverse mechanisms. • A new model involving fluid-present recrystallization is proposed for PED formation. • Incorporating PEDs in in-situ SIMS U-Pb spot can result in spuriously old ages. [ABSTRACT FROM AUTHOR]

Abstract: Rare felsic volcanic rocks of dacitic to rhyolitic composition occur in the central part of the Jack Hills metasedimentary belt in the Narryer Terrane of Western Australia, interleaved with clastic sedimentary rocks and amphibolite. Representative samples of the four identified felsic volcanic units reveal a similar complex pattern of zircon age distribution, with all samples containing zircon populations at ∼3.3–3.4, ∼3.0–3.1, ∼2.6 and ∼1.8–1.9Ga. The ∼3.3–3.4Ga zircons show well-developed oscillatory zoning in cathodoluminescence (CL) images and are interpreted as inherited igneous zircon derived from granitic precursors, similar to the ∼3.3Ga trondhjemitic granitoids currently exposed along the northern and southern margins of the belt. The ∼3.0–3.1Ga zircons also reveal well-developed oscillatory zoning in CL and are most likely derived from granitoid and/or volcanic rocks of this age, as recorded in the Murchison domain to the south and possibly also present in the Narryer Terrane. The ∼2.6Ga population matches the age of nearby late Archean granitoids intruding the Jack Hills belt and their oscillatory zoning and U–Th chemistry is consistent with their origin from such a source. The youngest discrete group of zircon grains, with ages ranging from ∼1970 to ∼1775Ma, show strong oscillatory zoning and average Th/U ratios of 0.76, features consistent with an igneous origin. These younger zircons are therefore interpreted as defining the age of crystallisation of the volcanic rocks. These results establish that the Jack Hills metasedimentary belt contains significant post-Archean components. Taken together with similar results obtained from zircon occurring as detrital grains in clastic sedimentary rocks at Jack Hills, these results overturn the generally-accepted view that the belt is entirely Archean in age and that sedimentation was completed around 3.0Ga ago. Instead, there is a distinct possibility that much of the material currently exposed in the Jack Hills belt formed in the Proterozoic. A further implication of this study is that the metamorphism affecting these rocks also occurred in the Proterozoic and consequently the rocks should not be considered as forming an Archean greenstone or metasedimentary belt. The paucity of zircons >4Ga in the known Proterozoic sedimentary rocks and their total absence in the felsic volcanic rocks suggests that such ancient source rocks were no longer present in the area. [Copyright &y& Elsevier]

Abstract: Rare heavy mineral bands within quartzite cobbles were identified in two conglomerate units within the Jack Hills belt, Western Australia. Seven zircon-bearing cobbles were analysed from one location (site 152) and three from another (site 154), both approximately 1km west of the site where zircons in excess of 4Ga are abundant (W74 ‘discovery’ site). Individual pebbles from the 152 site reveal three distinctive features, containing either zircons >3.0Ga in age, <1.9Ga in age or a range of ages from ∼1.2 to ∼3.6Ga. Those from site 154 are more uniform, containing only zircons with ages between 3.1 and 3.9Ga. Only one grain >4Ga was discovered from the entire suite of pebbles, in contrast to the well-studied W74 site. A single detrital zircon with an age of 1220±42Ma from location 152 is the youngest grain so far reported from sedimentary rocks at Jack Hills. It shows magmatic oscillatory zoning and thus implies at least two sedimentary cycles within the Proterozoic; requiring erosion of an igneous precursor, incorporation into a clastic sediment, induration and subsequent erosion and transport to be hosted in the conglomerate. The nearest source for rocks of this age is the Bangemall Supergroup in the Collier Basin, ∼100km northeast in the Capricorn Orogen. This would imply tectonic interleaving of originally more extensive Bangemall rocks, possibly related to activity along the Cargarah Shear Zone that traverses the Jack Hills belt. The lack of >4.1Ga zircons in the pebbles is highly significant, suggesting the immediate source of ancient zircons was no longer present at the Earth''s surface. This equates with a general lack of ancient crystals noted in rocks that contain Proterozoic zircons from previous studies and implies that such grains diminish in number as earlier sedimentary rocks were successively recycled. [Copyright &y& Elsevier]

The recent discovery of diamond–graphite inclusions in the Earth’s oldest zircon grains (formed up to 4,252 Myr ago) from the Jack Hills metasediments in Western Australia provides a unique opportunity to investigate Earth’s earliest known carbon reservoir. Here we report ion microprobe analyses of the carbon isotope composition of these diamond–graphite inclusions. The observed δ13CPDB values (expressed using the PeeDee Belemnite standard) range between -5 per mil and -58 per mil with a median of -31 per mil. This extends beyond typical mantle values of around -6 per mil to values observed in metamorphic and some eclogitic diamonds that are interpreted to reflect deep subduction of low-δ13CPDB biogenic surface carbon. Low δ13CPDB values may also be produced by inorganic chemical reactions, and therefore are not unambiguous evidence for life on Earth as early as 4,250 Myr ago. Regardless, our results suggest that a low-δ13CPDB reservoir may have existed on the early Earth. [ABSTRACT FROM AUTHOR]

Detrital zircons more than 4 billion years old from the Jack Hills metasedimentary belt, Yilgarn craton, Western Australia, are the oldest identified fragments of the Earth’s crust and are unique in preserving information on the earliest evolution of the Earth. Inclusions of quartz, K-feldspar and monazite in the zircons, in combination with an enrichment of light rare-earth elements and an estimated low zircon crystallization temperature, have previously been used as evidence for early recycling of continental crust, leading to the production of granitic melts in the Hadean era. Here we present the discovery of microdiamond inclusions in Jack Hills zircons with an age range from 3,058 ± 7 to 4,252 ± 7 million years. These include the oldest known diamonds found in terrestrial rocks, and introduce a new dimension to the debate on the origin of these zircons and the evolution of the early Earth. The spread of ages indicates that either conditions required for diamond formation were repeated several times during early Earth history or that there was significant recycling of ancient diamond. Mineralogical features of the Jack Hills diamonds—such as their occurrence in zircon, their association with graphite and their Raman spectroscopic characteristics—resemble those of diamonds formed during ultrahigh-pressure metamorphism and, unless conditions on the early Earth were unique, imply a relatively thick continental lithosphere and crust–mantle interaction at least 4,250 million years ago. [ABSTRACT FROM AUTHOR]

A suite of potassium‐bearing minerals from the Walgidee Hills lamproite intrusion in the Kimberley region of Western Australia was selected for 39Ar/40Ar dating. These included wadeite, jeppeite, priderite, potassium richterite, and phlogopite. All recorded excellent plateau ages, with the mean age of the combined data set being 17.3±0.3 Ma. Phlogopite recorded the largest uncertainty, whereas, of the other minerals, wadeite gave the best precision. Although rare to absent in common magmatic rocks, these minerals are widely distributed in alkaline complexes and in lamproite, kimberlite and orangeite intrusions. The results indicate this suite of minerals is excellent for 39Ar/40Ar dating and that they can be used singly or in combination to obtain the precise magmatic crystallization ages of ultra‐alkaline rocks. Because of the stability of potassium richterite at mantle depths, 39Ar/40Ar dating of MARID (mica‐amphibole‐rutile‐ilmenite‐diopside) xenoliths should be a more widely‐applied technique to investigating mantle geodynamics. [ABSTRACT FROM AUTHOR]

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 timeseries 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-toplates" 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. [ABSTRACT FROM AUTHOR]

Abstract: In situ Li analyses of 4348 to 3362 Ma detrital zircons from the Jack Hills, Western Australia by SIMS reveal that the Li abundances (typically 10 to 60 ppm) are commonly over 10,000 times higher than in zircons crystallized from mantle-derived magmas and in mantle-derived zircon megacrysts (typically <2 ppb). High Li concentrations in zircons (10 to 250 ppm) have also been found in igneous zircons from three continental parent rocks: granites, Li-rich pegmatites, and migmatites in pelitic metasediment. The substitution of trivalent cations (REEs and Y) in zircon correlates with Li+1 and P+5, suggesting that an interstitial site for Li, as well as the xenotime substitution for P, provides charge balance for REEs. Li is thus fixed in the zircon structure by coupled substitutions, and diffusive changes in [Li] composition are rate-limited by slow diffusion of REEs. The Jack Hills zircons also have fractionated lithium isotope ratios (δ 7Li=−19 to+13‰) about five times more variable than those recorded in primitive ocean floor basalts (2 to 8‰), but similar to continental crust and its weathering products. Values of δ 7Li below −10‰ are found in zircons that formed as early as 4300 Ma. The high Li compositions indicate that primitive magmas were not the source of Jack Hills zircons and the fractionated values of δ 7Li suggest that highly weathered regolith was sampled by these early Archean magmas. These new Li data provide evidence that the parent magmas of ancient zircons from Jack Hills incorporated materials from the surface of the Earth that interacted at low temperature with liquid water. These data support the hypothesis that continental-type crust and oceans existed by 4300 Ma, within 250 million years of the formation of Earth and the low values of δ 7Li suggest that weathering was extensive in the early Archean. [Copyright &y& Elsevier]

Four decades have now passed since Schidlowski et al. [1] proposed that a light carbon isotope signature in metasedimentary rocks, as found in the 3.7 Ga Isua Greenstone Belt (IGB) of southwest Greenland, might be used to indicate the presence of life in some of the earliest preserved geology. Isotopic evidence was relied upon because the host rocks have been metamorphosed and deformed to a degree that precludes preservation of unambiguous biomorphic features, such as those seen in the ca. 3.4 Ga algal mats at Strelley Pool, Western Australia [2]. Since Schidlowski et al.'s seminal work, a number of claims have been made for low δ13C indicating life in the Eoarchean, including those from Isua [3] and Akilia [4] in west Greenland, and the Nuvvuagittuq Greenstone Belt [5] and Saglek region, Labrador [6] in northern Canada. Such cases, often hailed at the time of publication as evidence for 'the Earth's oldest life', attract great publicity and debate, and are remarkable not least because of the overlap with a time when the rate of meteorite bombardment was considerably greater [7], with a concomitantly increased "impact frustration" to the establishment of terrestrial life [8].A number of key criteria, required for the credibility of claims for early life, were proposed in a review paper by Whitehouse and Fedo (2007) [9] and have recently been reiterated by Whitehouse et al. [10]. Along with the veracity of the carbon isotopic evidence itself, upon which most Eoarchean life claims rely, these criteria include (1) the suitability of host rocks as possible environments in which life might have developed and (2) temporal constraints provided either directly by the isotopic dating of the host rock or through field relationships with well-dated lithologies. Using the case studies cited above, this presentation will stress the important of both unambiguous geological relationships and geochronology in the quest for finding evidence of Earth's earliest biogenicity, and will review the status of such claims. It is noteworthy that the preservation of fossil evidence in all claims prior to the Paleoarchean is severely limited by the effects of tectonism and metamorphism, and sole reliance on chemical or isotopic evidence cannot provide an unambiguous basis for biogenicity.References: [1] Schidlowski et al. (1979) Geochim. Cosmochim. Acta 43, 189. [2] Allwood et al. (2007) Precam. Res. 158, 198. [3] Rosing (1999) Science 283, 674. [4] Mojzsis et al. (1996) Nature 384, 55. [5] Dodd et al. (2017) Nature 543, 60. [6] Tashiro et al. (2017) Nature 549, 516. [7] Bottke and Norman (2017) Annu Rev Earth Pl Sci 45, 619. [8] Maher & Stevenson, (1988) Nature 331, 612. [9] Whitehouse & Fedo (2007) Developments in Precambrian Geology (Elsevier) 15, 841. [10] Whitehouse et al. (2019). Precam. Res, doi.org/10.1016/j.precamres.2019.01.001. [ABSTRACT FROM AUTHOR]