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.
Continental crust forms from, and thus chemically depletes, the Earth's
mantle. Evidence that the Earth's mantle was already chemically depleted by
melting before the formation of today's oldest ...surviving crust has been presented
in the form of Sm-Nd isotope studies of 3.8-4.0 billion years
old rocks from Greenland and Canada.
But this interpretation has been questioned because of the possibility that
subsequent perturbations may have re-equilibrated the neodymium-isotope compositions
of these rocks. Independent and more robust evidence for the
origin of the earliest crust and depletion of the Archaean mantle can potentially
be provided by hafnium-isotope compositions of zircon, a mineral whose age
can be precisely determined by U-Pb dating, and which can survive metamorphisms. But the amounts of hafnium in single zircon grains are too small
for the isotopic composition to be precisely analysed by conventional methods.
Here we report hafnium-isotope data, obtained using the new technique of multiple-collector
plasma-source mass spectrometry, for 37 individual grains of
the oldest known terrestrial zircons (from the Narryer Gneiss Complex, Australia,
with U-Pb ages of up to 4.14 Gyr (
10-13). We find that none of the grains has a depleted mantle
signature, but that many were derived from a source with a hafnium-isotope
composition similar to that of chondritic meteorites. Furthermore, more than
half of the analysed grains seem to have formed by remelting of significantly
older crust, indicating that crustal preservation and subsequent reworking
might have been important processes from earliest times.
Granitoid gneisses and supracrustal rocks that are 3,800-4,000 Myr
old are the oldest recognized exposures of continental crust.
To obtain insight into conditions at the Earth's surface more than 4 ...Gyr
ago requires the analysis of yet older rocks or their mineral remnants. Such
an opportunity is presented by detrital zircons more than 4 Gyr old
found within 3-Gyr-old quartzitic rocks in the Murchison District of Western
Australia. Here we report in situ U-Pb and
oxygen isotope results for such zircons that place constraints on the age
and composition of their sources and may therefore provide information about
the nature of the Earth's early surface. We find that 3,910-4,280 Myr
old zircons have oxygen isotope (δ18O) values ranging
from 5.4 ± 0.6‰ to 15.0 ± 0.4‰.
On the basis of these results, we postulate that the ∼4,300-Myr-old zircons
formed from magmas containing a significant component of re-worked continental
crust that formed in the presence of water near the Earth's surface. These
data are therefore consistent with the presence of a hydrosphere interacting
with the crust by 4,300 Myr ago.
– The microstructures of lunar zircon grains from breccia samples 72215, 73215, 73235, and 76295 collected during the Apollo 17 mission have been characterized via optical microscopy, ...cathodoluminescence imaging, and electron backscatter diffraction mapping. These zircon grains preserve deformation microstructures that show a wide range in style and complexity. Planar deformation features (PDFs) are documented in lunar zircon for the first time, and occur along {001}, {110}, and {112}, typically with 0.1–25 μm spacing. The widest PDFs associated with {112} contain microtwin lamellae with 65°/ misorientation relationships. Deformation bands parallel to {100} planes and irregular low‐angle (<10°) boundaries most commonly have misorientation axes. This geometry is consistent with a dislocation glide system with {010} during dislocation creep. Nonplanar fractures, recrystallized domains with sharp, irregular interfaces, and localized annealing textures along fractures are also observed. No occurrences of reidite were detected. Shock‐deformation microstructures in zircon are explained in terms of elastic anisotropy of zircon. PDFs form along a limited number of specific {hkl} planes that are perpendicular to directions of high Young’s modulus, suggesting that PDFs are likely to be planes of longitudinal lattice damage. Twinned {112} PDFs also contain directions of high shear modulus. A conceptual model is proposed for the development of different deformation microstructures during an impact event. This “shock‐deformation mechanism map” is used to explain the relative timing, conditions, and complexity relationships between impact‐related deformation microstructures in zircon.
The Cycladic blueschist unit (CBU) in the central Aegean Sea comprises a pre‐Alpidic crystalline basement that is overlain by thrust sheets of a metamorphosed volcanosedimentary sequence. Mélanges ...are widespread and enclose a wide variety of meta‐igneous rock fragments in a serpentinitic and/or metasedimentary matrix. Previous geochronology has documented the general temporal framework for the Alpidic metamorphic history but did not produce well‐constrained protolith ages for the main volcanosedimentary succession and for mélange blocks. This issue is addressed here in an ion probe U‐Pb zircon study focusing on the islands of Andros, Sifnos, and Ios. The new results indicate a consistent regional pattern of Triassic ages (ca. 237–245 Ma) for the magmatic precursors of metatuffaceous and metavolcanic rocks, which occur as intercalations within metasediments. This narrow range of igneous crystallization ages records an important period of volcanic activity in the larger study area. On Andros, such ages were also determined for a disrupted tabular body of meta‐acidic gneisses (several hundred meters in length) that has been considered an olistolith within a meta‐olistostrome. Field observations are compatible with the interpretation that this fragment represents a layer of the main succession that has been disrupted by large‐scale boudinage. A metagabbro and a meta‐acidic gneiss from a high‐pressure mélange on Andros yielded Jurassic206Pb/238U ages of
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Ma, respectively, that correspond very well to ages reported for ophiolites from mainland Greece, the Balkan region, and Crete. The new results and literature data document that remnants of both Jurassic and Cretaceous meta‐ophiolites are included in the mélanges of the CBU. It is currently not known whether both age groups occur within the same mélange or at different lithostratigraphic and/or tectonic levels.
We present the results of a series of hydrothermal experiments on grains from two partially metamict zircon samples from Sri Lanka in the temperature range 350 to 650°C and with different solutions ...(2 M AlCl3, 2 M CaCl2, pure H2O, and a multi-cation solution). Under these conditions, sharply bounded reaction fronts penetrated into the zircon grains and developed complex lobate and rim structures that resemble structures found in natural zircon systems. The reaction zones are characterized by a marked increase in the cathodoluminescence intensity, a decrease of the back-scattered electron emission, and an increased degree of structural order, as revealed by micro-Raman and infrared spectroscopy. Sensitive high-resolution microprobe and electron microprobe measurements revealed that the altered areas gained solvent cations (e.g., Ca2+, Ba2+, Mg2+, Al3+) from the solution and lost variable amounts of Zr, Si, Hf, the REE, U, Th as well as radiogenic Pb. A comparison between "dry" and "hydrothermal" annealing trends shows that the kinetics of structural recovery, including recrystallization of the amorphous phase in metamict zircon, is strongly enhanced under hydrothermal conditions. This finding suggests that water "catalyzes" structural recovery processes in metamict zircon. We found that the structure of the reacted areas does not resemble that of crystalline zircon, i.e., is still characterized by a temperature-dependent degree of disorder, which would not be expected if the reaction is controlled by a coupled dissolution and re-precipitation process. Instead, the alteration process can be described best by a diffusion-reaction-recrystallization model. In this model, it is postulated that the diffusion of water into the metamict structure is the driving force for moving recrystallization fronts. We found that the rate and the extent of solid-state recrystallization of the amorphous phase is an important factor in determining the mobility of trace elements. This interpretation is indicated by the observation that trace elements, including U and Th, were preferentially lost during the reaction with a fluid at low temperatures, where recrystallization of the amorphous material was slow or not activated at all. The observed chemical alteration patterns are believed to reflect a competition between the kinetics of long-range diffusion and ion exchange and the kinetics of the short-range diffusion necessary for the recrystallization process.
We report the results of a differential scanning calorimeter (DSC) study of the annealing of a metamict Sri Lankan zircon. Raman measurements on most chips of the powdered zircon starting material, ...Sri Lankan zircon (WZ19), showed no evidence of a crystalline structure, whereas a few chips retained residual Raman bands typical of highly radiation damaged zircon. DSC runs on aliquots of the powdered sample were heated to 850 and 1000 °C at rates of 2 and 10 °C/min and to 1500 °C at a rate of 10 °C/min. Raman spectroscopy was used to investigate the crystallinity of grains at selected temperature stages. Exothermal peaks were observed at about 910 and 1260 °C during the DSC run to 1500 °C. The 910 °C peak was demonstrated by Raman spectroscopy to mark the crystallization of tetragonal zirconia and the exothermic peak at about 1260 °C was demonstrated to represent the reaction of zirconia and amorphous silica to form crystalline zircon. The degree of crystallinity of these grains was almost identical to that of highly crystalline zircons from recent gem gravels from New South Wales. A small number of experimental chips from DSC analyses under 1000 °C were found to have zircon Raman bands that indicated they had undergone partial annealing. The present experimental results suggest that reconstitution of amorphous zircon to the crystalline state by dry annealing will rarely occur in terrestrial geological settings, even under extreme metamorphic conditions.
In this paper, we compare the U‐Pb zircon age distribution pattern of sample 14311 from the Apollo 14 landing site with those from other breccias collected at the same landing site. Zircons in ...breccia 14311 show major age peaks at 4340 and 4240 Ma and small peaks at 4110, 4030, and 3960 Ma. The zircon age patterns of breccia 14311 and other Apollo 14 breccias are statistically different suggesting a separate provenance and transportation history for these breccias. This interpretation is supported by different U‐Pb Ca‐phosphate and exposure ages for breccia 14311 (Ca‐phosphate age: 3938 ± 4 Ma, exposure age: ~550–660 Ma) from the other Apollo 14 breccias (Ca‐phosphate age: 3927 ± 2 Ma, compatible with the Imbrium impact, exposure age: ~25–30 Ma). Based on these observations, we consider two hypotheses for the origin and transportation history of sample 14311. (1) Breccia 14311 was formed in the Procellarum KREEP terrane by a 3938 Ma‐old impact and deposited near the future site of the Imbrium basin. The breccia was integrated into the Fra Mauro Formation during the deposition of the Imbrium impact ejecta at 3927 Ma. The zircons were annealed by mare basalt flooding at 3400 Ma at Apollo 14 landing site. Eventually, at approximately 660 Ma, a small and local impact event excavated this sample and it has been at the surface of the Moon since this time. (2) Breccia 14311 was formed by a 3938 Ma‐old impact. The location of the sample is not known at that time but at 3400 Ma, it was located nearby or buried by hot basaltic flows. It was transported from where it was deposited to the Apollo 14 landing site by an impact at approximately 660 Ma, possibly related to the formation of the Copernicus crater and has remained at the surface of the Moon since this event. This latter hypothesis is the simplest scenario for the formation and transportation history of the 14311 breccia.
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.
CO2 fluid inclusions in Jack Hills zircons Menneken, Martina; Geisler, Thorsten; Nemchin, Alexander A. ...
Contributions to mineralogy and petrology,
08/2017, Letnik:
172, Številka:
8
Journal Article
Recenzirano
The discovery of Hadean to Paleoarchean zircons in a metaconglomerate from Jack Hills, Western Australia, has catalyzed intensive study of these zircons and their mineral inclusions, as they ...represent unique geochemical archives that can be used to unravel the geological evolution of early Earth. Here, we report the occurrence and physical properties of previously undetected CO
2
inclusions that were identified in 3.36–3.47 Ga and 3.80–4.13 Ga zircon grains by confocal micro-Raman spectroscopy. Minimum P–T conditions of zircon formation were determined from the highest density of the inclusions, determined from the density-dependence of the Fermi diad splitting in the Raman spectrum and Ti-in-zircon thermometry. For both age periods, the CO
2
densities and Ti-in-zircon temperatures correspond to high-grade metamorphic conditions (≥5 to ≥7 kbar/~670 to 770 °C) that are typical of mid-crustal regional metamorphism throughout Earth’s history. In addition, fully enclosed, highly disordered graphitic carbon inclusions were identified in two zircon grains from the older population that also contained CO
2
inclusions. Transmission electron microscopy on one of these inclusions revealed that carbon forms a thin amorphous film on the inclusion wall, whereas the rest of the volume was probably occupied by CO
2
prior to analysis. This indicates a close relationship between CO
2
and the reduced carbon inclusions and, in particular that the carbon precipitated from a CO
2
-rich fluid, which is inconsistent with the recently proposed biogenic origin of carbon inclusions found in Hadean zircons from Jack Hills.
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