Robert T. Pidgeon

Oxygen-isotope evidence from ancient zircons for liquid water at the Earth's surface 4,300 Myr ago

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 Earths 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 Earths 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 Earths surface. These data are therefore consistent with the presence of a hydrosphere interacting with the crust by 4,300 Myr ago.

Nature of the Earth's earliest crust from hafnium isotopes in singledetrital zircons

Continental crust forms from, and thus chemically depletes, the Earths mantle. Evidence that the Earths mantle was already chemically depleted by melting before the formation of todays 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.

Recrystallisation of oscillatory zoned zircon: some geochronological and petrological implications

Oscillatory zoning is a common feature in zircons from acid igneous rocks and is believed to form during crystallisation of zircons from a magma by a mechanism which is not yet understood. Many zircons with oscillatory zoning also show a patchwork replacement of zoned by unzoned zircon. The unzoned zircon occurs as rounded, transgressive patches distributed throughout the zoned zircon and as areas of transitional replacement where zoned zircon is progressively replaced by unzoned zircon such that only faint traces of original zones remain. This structure is interpreted as a progressive recrystallisation of the oscillatory zoned zircon made unstable by the incorporation of high concentrations of contaminant elements during magmatic crystallisation. Recrystallisation overprints oscillatory zones and appears to have occurred after completion of primary crystallisation. It is accompanied by loss of U, Th and Pb and the removal of oscillatory zones. The recrystallised unzoned zircon is extremely stable with respect to later Pb loss and tends to retain a concordant or slightly discordant U−Pb age. Recrystallisation provides a mechanism for resetting zircon U−Pb ages which is independent of the degree of radiation damage of the zircon lattice. This differs from other models of discordance which involve a leaching of radiogenic Pb as a consequence of a progressive breakdown of the zircon structure through time-integraded radiation damage further enhanced by high concentrations of trace-element contaminants. The U−Pb age of the unzoned zircon may date the recrystallisation event, which may be close to the age of primary crystallisation or reflect a later metamorphism.

Experimental hydrothermal alteration of partially metamict zircon

Abstract 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 ion 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.

Low-temperature hydrothermal alteration of natural metamict zircons from the Eastern Desert, Egypt

Abstract The chemical and structural alteration of metamict zircon crystals from a 619±17 (2σ) Ma old, post-tectonic granite in the southern part of the Eastern Desert, Egypt was studied. The crystals show simple oscillatory growth zones with metamictization-induced fractures, which provided pathways for fluid infiltration. Electron and ion microprobe analyses reveal that metamict, i.e. U and Th-rich, areas are heavily enriched in Ca, Al, Fe, Mn, LREE, and a water species, and have lost Zr and Si as well as radiogenic Pb. These chemical changes are the result of an intensive reaction with a low-temperature (120-200°C) aqueous solution. The chemical reactions probably occurred within the amorphous regions of the metamict network. During the zircon-fluid interactions the metamict structure was partially recovered, as demonstrated by micro-Raman and -infrared measurements. A threshold degree of metamictization, as defined empirically by an α-decay dose, Dc, was necessary for zircons to undergo hydrothermal alteration. It is proposed that Dc marks the first percolation point, where the amorphous domains start to form percolating clusters in the metamict network and where bulk chemical diffusion is believed to increase dramatically. The time of the hydrothermal alteration is determined by a lower intercept age of a U-Pb SHRIMP discordia of 17.9+6.9-7.4 (2σ) Ma, which is in good agreement with an apatite fission track age of 22.2+5.4-4.8 (2σ) Ma. The hydrothermal alteration event occurred contemporaneously with the main rifting phase of the Red Sea and widespread low-temperature mineralizations along the Red Sea coast.

Transport of uranium, thorium, and lead in metamict zircon under low-temperature hydrothermal conditions

Abstract Understanding the stability of radiation-damaged zircon under low-temperature hydrothermal conditions is crucial to the application of zircon for U–Pb geochronology and as a host phase for the disposal of plutonium waste. We report the results of an investigation of the stability of partially metamict zircon by leaching experiments at 175 °C with a 2 M AlCl 3 and a 1 M HCl–CaCl 2 solution as hydrothermal fluids for 1340 h. Cathodoluminescence (CL) and backscattered electron (BSE) images show that the zircon grains have developed a reaction rim several micrometers thick or deeply penetrating reticulated alteration zones with sharp boundaries to unaltered metamict zircon. These zones have experienced severe loss of Si, U, Th, and Pb, and gain of Al or Ca, and a water species as revealed by electron microprobe, sensitive high-resolution ion microprobe (SHRIMP) analyses, and infrared spectroscopy. Micro-Raman and infrared measurements on the altered areas show that disordered crystalline remnants of the partially metamict zircon structure were partially recovered, whereas recrystallization of the embedding amorphous phase was not observed. No detectable structural or chemical changes were detected inside the unaltered areas. Intensive fracturing, which was most intense in the HCl–CaCl 2 experiment, occurred inside the altered areas due to the volume reduction associated with the recovery of the disordered crystalline material and probably with the leaching reactions. We explain the formation of deep penetrating alteration zones by a percolation-type diffusion model, which is based on the assumption that percolating interfaces or areas of low atomic density between crystalline and amorphous regions as well as between amorphous domains exist along which fast chemical transport is possible. The idea of the existence of fast diffusion interfaces is supported by the sharp chemical gradients at the margin to unreacted zircon. The model was used to estimate for the first time diffusion coefficients for U, Th, and Pb diffusion in amorphous zircon at 175 °C by assuming that volume diffusion inside the amorphous domains is the loss rate-limiting process. These estimates show that volume diffusion in metamict zircon can cause significant loss of U, Th, and especially loss of radiogenic Pb over geological time scales, even at temperatures as low as 175 °C. Our results show that recent Pb loss discordias can be generated (1) by predominate Pb loss from metamict zircon through volume diffusion at low temperatures where thermal healing of the structure is insignificant, and (2) by leaching of Pb (and U and Th) from metamict zircon through an external fluid.

Inherited zircon and titanite UPb systems in an Archaean syenite from southwestern Australia: implications for UPb stability of titanite

Abstract Inherited zircon and titanite have been identified in a syenite from the Archaean of southwestern Australia. Conventional and SHRIMP analyses on euhedral zoned zircon and zoned rims on complex grains define a crystallisation age of 2654 ± 5 Ma for the syenite. In addition, SHRIMP analyses on zircon cores and unzoned subhedral zircons show that zircon has a ca. 3250 Ma inherited component. Conventional UPb ages on titanite also fall between ca. 3250 Ma and ca. 2650 Ma, demonstrating that inherited titanite as well as zircon is present in the syenite. The syenite has been affected by regional upper amphibolite facies metamorphism at an estimated temperature of 625–650°C. Retention of the inherited radiogenic Pb in the titanite is evidence that the closure temperature for titanite is greater than 650°C. The presence of inherited titanite and zircon also demonstrates a crustal source component for the syenite and indicates it originated by partial assimilation of crustal rocks by a potassic magma at

Hadean diamonds in zircon from Jack Hills, Western Australia.

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.

The distribution of 3.0 Ga and 2.7 Ga volcanic episodes in the Yilgarn Craton of Western Australia

Abstract The results of a conventional zircon U-Pb investigation of the ages of felsic volcanic rocks in a number of greenstone belts in the Yilgarn Craton of Western Australia indicate that volcanism took place during two distinct episodes at ca. 3.0 and ca. 2.7 Ga. Each episode spans a time period of about 75 Ma and the two ages of volcanism are spatially separated. 3.0 Ga volcanic rocks are found in greenstone belts in the northern part of the Western Gneiss Terrain and in the Murchison Province, whereas 2.7 Ga volcanic rocks are present in the southern part of the Western Gneiss Terrain and in the Southern Cross and Eastern Goldfields Provinces. There is no evidence of inheritance in the zircon U-Pb systems which would support an older crustal origin for these rocks. Present results, together with previous studies of the ages of granites and gneisses, suggest that the volcanic episodes were accompanied by broadly contemporaneous granite plutonism. It is suggested that the evolution of the Yilgarn Craton took place in three main stages. The first was the development of a stable 3.4 Ga granitic nucleus, which incorporated earlier generations of granitoid, a 3.7 Ga layered complex and possibly 4.2 Ga primitive crust. This was followed by the two separate granite-greenstone episodes, at ca. 3.0 and ca. 2.7 Ga., involving the addition of significant amounts of new crust. Each episode was characterised by the accretion of extensive granite-greenstone terranes at the margins of the stable craton, the intrusion of granite plutons into the evolving craton and the reworking of older cratonic rocks to form granites and gneisses with inherited isotopic systems.

Th–U–Pb isotopic systems and internal structures of complex zircons from an enderbite from the Pium Complex, Carajás Province, Brazil: evidence for the ages of granulite facies metamorphism and the protolith of the enderbite

Abstract SHRIMP U–Pb micro-analyses are reported for parts of complex zircons from a granulite facies enderbite from the Pium Complex in the Carajas Metallogenic Province of the Amazon Craton of Brazil. Zircons from the granulite consist of two types of grains — (a) those with oscillatory zoned cores surrounded by rims composed of two or more curved, overlapping, nebulously zoned domains distinguished by differences in cathodoluminescence intensity, and (b) grains which are composed entirely of such domains and have no obvious cores. The SHRIMP U–Pb age of 3002±14 Ma, determined for oscillatory-zoned zircon cores is interpreted as dating the parent rock of the enderbite. This age is resolved from the U–Pb age of 2859±9 Ma, determined on zircon rims, which is interpreted as a time point within the evolving granulite facies metamorphism. SHRIMP U–Pb ages of cores and rim-type zircon are generally concordant to slightly discordant where uranium contents are less than about 820 ppm, but once this value is exceeded the zircon becomes strongly discordant suggesting that a threshold value of radiation damage exists above which zircon stability with respect to Pb loss is sharply reduced. The results provide the first substantial age determination of the granulite metamorphism in the Carajas Province. The zircon U–Pb ages have not been influenced by the Transamazonian metamorphism dated at ca. 2.0–1.85 Ga from Ar–Ar and Rb–Sr mineral ages.