Hadean zircon
Hadean zircon is the oldest-surviving crustal material from the Earth's earliest geological time period, the Hadean eon, about 4 billion years ago. Zircon is a mineral that is commonly used for radiometric dating because it is highly resistant to chemical changes and appears in the form of small crystals or grains in most igneous and metamorphic host rocks.
Hadean zircon has very low abundance around the globe because of recycling of material by plate tectonics. When the rock at the surface is buried deep in the Earth it is heated and can recrystallise or melt. In the Jack Hills, Australia, scientists obtained a relatively comprehensive record of Hadean zircon crystals in contrast to other locations. The Jack Hills rocks are from the Archean eon, about 3.6 billion years old. However, the zircon crystals there are older than the rocks that contain them. Many investigations have been carried out to find the absolute age and properties of zircon, for example the isotope ratios, mineral inclusions, and geochemistry of zircon. The characteristics of Hadean zircons show early Earth history and the mechanism of Earth's processes in the past. Based on the properties of these zircon crystals, many different geological models were proposed.
Background
Importance
Deeper understanding of Earth history
The geological history of the Hadean eon of early earth is poorly known due to the lack of rock record older than 4.02 Ga. Most scientists accept that the plate recycling mechanism has melted almost all pieces of Earth's crust. However, some tiny parts of the crust have not been melted, as some rare Hadean zircon grains included in much younger host rock were discovered. The examination of Hadean detrital or inherited grains of zircon can give evidence of geophysical conditions of the early earth.Scientific contribution
Since there is no strong evidence depicting the early Earth's true environment, many models are generated to explain early Earth history. The high value of Hadean heat production and impact flux proved that continental crust did not exist, which is very different from the modern process. In the absence of large amount of undistributed data and within the constraints of analytical methods, calculation on geophysics and planetary science has been rapidly developed to explore this new area of knowledge.Abundance
Less than 1% of zircons detected around the world are over four billion years old. The probability of discovering at least a single over four billion-year-old zircon is very low. The abundance of over four billion-year-old zircon in the Jack Hills is anomalously high for most Archean quartzites and thus abundances probabilities of other spots are extremely low.By adopting uranium-lead dating together with other analytical methods, more geochemical information can be obtained. Only 3% out of over 200,000 detritial zircon grains dated by U-Pb analysis are over four billion years old.
concentration ratio versus yttrium concentration plot shows different trace elements signatures of zircon sources. Stars are the data for Kimberlite zircon, triangles are Hadean Jack Hills zircon and circles are ocean crust zircon.
Types
Due to different content of uranium and trace element concentration, four clusters of zircons are identified as below- Lunar and meteorite zircon
- Detrital grains zircon
- Kimberlite zircon
- Ocean crust zircon
Properties
The unspecified samples used for analyses below were Jack Hills zircon in Australia because of the high abundances and data available.Age distribution
in the U-Pb zircon system has long been viewed as the crustal geochronometer because zircon is chemically resistant and enriched in U and Th compared to the daughter product Pb. Trace element and isotopic composition of zircon is important to determine the crystallisation environment.Results from detrital zircons from the Erawondoo Hill discovery site conglomerate generally show the zircons to have a bimodal age distribution with major peaks at ca. 3.4 and 4.1 Ga.
However, zircon is sensitive to radiation damage and can degrade into amorphous material. The Hadean zircon with original uranium concentrations greater than 600 ppm is challenged by the effect of post-crystallization alteration.
Isotope geochemistry
Stable isotope data, indicating that the original host rocks to the zircon related to a significant amount of material formed on or near the Earth's surface and subsequently transferred to a middle- to lower-crustal level where they melted to generate the host magmas from which zircon crystallised.| Data Type | Observation | Interpretation | Limitation |
| Oxygen isotope ratios | Granitoids with lower ઠ18O values | There were hydrothermal interactions with meteoric water instead of weathering. | There is a lack of comprehensive record of the analysed areas within the grains, which leads to difficulty in relating ages of specifically dated parts of the zircon grains to their oxygen and hafnium isotope systematics and trace element concentrations. |
| Oxygen isotope ratios | Ratios of oxygen isotopes have been measured in Hadean zircons. High value of 18ઠSMOW observed in Hadean Jack Hills zircons led to two different ideas about the source of Hadean zircon. 18O-enriched clay minerals were found in the host rock of zircon grains. | Water was present on the Earth's surface around 4.3 Ga. | There is a lack of comprehensive record of the analysed areas within the grains, which leads to difficulty in relating ages of specifically dated parts of the zircon grains to their oxygen and hafnium isotope systematics and trace element concentrations. |
| Oxygen isotope ratios | Hadean Jack Hills zircons contain more 18O-enrichments than the mantle zircon about 5.3%. I-type granitoids protoliths give relatively low ઠ18O values while those derived by S type metasedimentary rocks have higher ઠ18O values. | The presence in the protolith of recycled crustal material that had interacted with liquid water under surface, or near surface, condition. | There is a lack of comprehensive record of the analysed areas within the grains, which leads to difficulty in relating ages of specifically dated parts of the zircon grains to their oxygen and hafnium isotope systematics and trace element concentrations. |
| Lutetium-hafnium | The ratio of isotopes of hafnium 176Hf/177Hf data in crustal rocks being consistent with the formation of crust since 4.5 Ga. | Lu-Hf systematics potentially indicating existence of an early formed reservoir, similar to continental crust in its degree of Lu depletion relative to Hf. | Most of the data matches the formation of crust at 4.5 Ga while some zircon data is unreasonable requiring the removal of protolith from chrondritic uniform reservoir. Since these extra findings, studies cannot conform the positive value of EHf due to the complication of Hf isotope analysis and lack of U-Pb date being simultaneously available. |
| Lutetium-hafnium | Cluster of results along a line corresponding to a Lu/Hf ~0.01, low reservoir at ~4Ga | The data is consistent with either early extraction of very felsic crust or by remelting of a primordial basaltic reservoir, but in either case extrapolation of this trend yields a present-day εHf of approximately -100 | A recycling event ca. 3.9-3.7 Ga which resembles the Hf isotope evolution of modern subduction-related orogens and so may have additional tectonic significance. |
| Plutonium-xenon | Some Hadean zircon grains originally contained plutonium, an element that has since disappeared from the natural environment. In the meteorite record, the abundance ratio of initial plutonium to uranium was about 0.007 and 244Pu was present in the early Solar System. | The result of the ratio can be interpreted as xenon loss during later metamorphism. Uranium became oxidised to soluble uranyl ion while the solubility of plutonium compounds is low, variations in Pu/U are regarded as an effective indicator of aqueous alteration in Jack Hills protoliths. | Only Nd/U has correlations expected from aqueous processes excluding analysis of Xe isotopic ratios, U-Pb age, trace element contents, and δ18O |
| Plutonium-xenon | The initial Pu/U ratios of Jack Hills zircon ranges from ~0.007 to zero. | Due to Xe loss during later metamorphism. Variations in Pu/U has been suggested as a potential indicator of aqueous alteration in the Jack Hills zircon protoliths | Only Nd/U has correlations expected from aqueous processes excluding analysis of Xe isotopic ratios, U-Pb age, trace element contents, and δ18O |
| Plutonium-xenon | High-Nd/U zircons display only low Pu/U, while Nd/U zircons show more heterogeneous Pu/U | High-Nd/U group appears less magmatically evolved than other Hadean zircons, has REE patterns suggestive of some degree of alteration, either by hydrothermal fluid interaction or phosphate replacement, and consists of solely low-Pu/U zircons with a range of Hadean to Proterozoic U-Xe ages | Only Nd/U has correlations expected from aqueous processes excluding analysis of Xe isotopic ratios, U-Pb age, trace element contents, and δ18O |
| Lithium | Lithium isotopes significantly vary in Hadean zircon. The 7Li isotope result of Hadean Jack Hills zircons gave highly negative values. | The environment of forming zircon as highly weathered. | A high lithium diffusion rate in zircon at low temperature and exchange with hydrogen during metamorphism are two examples of subsequent variations to lithium that may limit the usefulness of the measurements |
| Lithium | Li is homogeneously distributed within single growth zones of the zircons. Jack Hills zircons are zoned in both 7Li and Li concentration. | These values correlate with igneous growth zoning. | A high lithium diffusion rate in zircon at low temperature and exchange with hydrogen during metamorphism are two examples of subsequent variations to lithium that may limit the usefulness of the measurements |
Mineral inclusions
The development of textural criteria for identifying primary inclusions opens up possibilities for recognising zircons' changing provenance with time and investigating their post-depositional alteration history. There are two common inclusion assemblages that are consistent with their forming in "I-type" and "S-type" granitoids. Dominated by quartz with less abundant K-feldspar, plagioclase, muscovite, biotite, and phosphates, that are interpreted to have formed under relatively low geothermal gradient similar to that pertaining to modern subduction zones.| Data Type | Observation | Interpretation |
| Muscovite | Quartz and muscovite are the principal inclusion phases, potentially pointing to an aluminous granitic source. Hopkins used a thermodynamic solution model for celadonite substitution in muscovite to estimate pressures for muscovite inclusions in magmatic zircons. The result revealed that the pressure conditions for over 1700 inclusion samples is greater than 5 kbar, coupled with a relatively low host zircons crystallization temperature. | Muscovite inclusion coupled with a thermodynamic model implied that the Earth surface had a remarkably low heat flow. This result led scientists to suggest formation in an underthrust or subduction-like environment as found on the modern Earth |
| Iron oxides | Cerium anomaly of a zircon is a quantitative investigation for host magma fO2. Hadean Jack Hills zircons show a range in fO2 with an average near the fayalite-magnetite-quartz buffer. | The Hadean geological setting is similar to the modern upper mantle |
| Biotite | Composition of biotite differ among granitoids. FeO, MgO and Al2O3 content isolate calc-alkaline, peraluminous and alkaline granitoids. | The nature of Hadean melt compositions can be known. |
| Biotite | Sulfide and carbonaceous phases have been revealed in Hadean zircon though the number of cases is small. | The rare phases have deleted the volatile contents in Hadean magmas and source materials. |
| Graphite | By knowing the occurrence of carbon, existence of life can be revealed in the aspect of timing, conditions and mechanisms. | Isotopic result identified the zircon was 3.8 to 3.5 billion years age and metabolism has occurred within the host microbiota. |
Zircon geochemistry
By analysing the content of zircon, some zircon show the presence of titanium, rare earth minerals, lithium, aluminium and carbon. Certain ratio and normal distribution give evidence of zircon's origin and the source of magma.| Data Type | Observation | Interpretation | Limitation |
| Titanium | The content of Ti-in-zircon serves as a crystallization thermometer given knowledge of the melt aSiO2 and aTiO2. The Ti measurements were applied to grains ranging from 3.91 to 4.35 Ga and the majority of the data plots a normal distribution graph. | Crystallisation of Ti-in-zircon grains are from evolved melts | It yielded an extremely high temperature 680±25 °C. Since the crystallisation of rutile is unknown, researchers can only estimate the temperature by calculation. |
| Rare earth mineral | Cerium anomaly of a zircon is a quantitative investigation for host magma fO2. The result showed low value of Ce/Ce* ratio. | Diversity of source materials | REE signatures in some zircon grains that have been interpreted to indicate crystallisation of these grains from evolved melts. |
| Rare earth mineral | In EDS analysis, magnetite was dominant in the inclusion instead of ilmenite in granitoids. | Hydrothermal alternation of zircon is often determined by high, flat light rare-earth mineral pattern. | REE signatures in some zircon grains that have been interpreted to indicate crystallisation of these grains from evolved melts. |
| Lithium | Li zoning in zircon serve as a peak temperature indicator while examining retention of primary remanent magnetic signals. Jack Hills zircon containing ~5up-wide Li concentration band which required below 500 °C peak heating temperature of zircon. | The grains can be applied to study primary magnetism because it didn't exceed the Curie temperature which is 585 °C for magnetite. | The metaconglomerates at Erawondoo Hill didn't experience temperature greater than 500 °C. The result showed that there's variation of data and thermal history in different occurrence. |
| Aluminium | Peraluminous granitoids contain around 10 ppm aluminium in Jack Hills zircon while the I-type and A-type zircon obtained average 1.3 ppm. The molar value of Al2O3/ greater than 1. | The origin is from recycled pelitic material. | Small amount of sample zircon contains high Al contents suggests that metaluminous crustal rocks is more common than peraluminous rocks in the Hadean. However, the ~20% overlap of low Al in S-type zircons somewhat obscures this inference. |
| Aluminium | Some of the grains show high aluminium content | Metaluminous crustal rocks may be more common than peraluminous rocks in the Hadean. | Small amount of sample zircon contains high Al contents suggests that metaluminous crustal rocks is more common than peraluminous rocks in the Hadean. However, the ~20% overlap of low Al in S-type zircons somewhat obscures this inference. |
| Carbon | Scientists measured concentration of carbon in the form of graphite in zircon by using secondary ion mass spectrometer. Detecting Hadean crustal carbon could ensure that there was a transfer of carbon from mantle reservoirs | Allow the selection among models of the early earth. | Quite a few early earth models contain this property which can't confirm which model is correct |
Analytic method
Ion microprobe analysis
and uranium-thorium-lead geochronology are two common methods to measure isotope in specific time interval.Highly precise in situ SIMS measurements of oxygen isotopes and OH/O ratios, laser-ablation inductively-coupled mass spectrometry determination of hafnium isotopes, and atom-probe tomography. LA-ICP-MS is the most common method to date using isotopes but it lacks capacity to measure 204Pb. Therefore, there is a possibility that the occurrences of single zircons over 4 billion years old could be due to inclusion of non-radiogenic Pb.
U-Pb dating, delta 18O and Ti measurements can be tested by CAMECA ims 1270 ion microprobe. Epoxy are applied on the sample. A flat surface of sample is needed to conduct an analysis. U-Pb dating and T measurement uses a primary O− beam with low intensity. U-Pb age standard AS3 was used for dating studies. The concentration of Ti can be determined based on analysis of Jack Hills zircon and NIST610 glass.
Electron microprobe analysis
For inclusions investigation, JEOL 8600 electron microprobe analyzer were used to chemically analyze zircon. It is used to analyze the chemical composition of material. Electron beams are emitted to the mineral's surface and blow off ions and estimate the abundance of the elements within a very small sized sample. Many isotopes can be measured at once in this analysis for example Ti and Li.Occurrence
Australia
North America
Asia
South America
Proposed mechanisms for forming Hadean Jack Hills zircons
Plate tectonic theory is widely accepted for the generation of crust. However, it's still an unknown how the early Earth was formed. With the Hadean rock record, most of the scientists concluded that the belief of a hellish early Earth devoid of ocean is wrong. Scientists have constructed different models to explain the thermal history in early history including continental growth model, Icelandic rhyolites, intermediate igneous rocks, mafic igneous rocks, sagduction, impact melt, heat pipe tectonics, terrestrial KREEP and multi-stage scenarios.The most famous one is continental growth model which is similar to the modern tectonic dynamics. Relatively low crystallisation temperature and some are enriched in heavy oxygen, contain inclusion similar to modern crustal processes and show evidence of silicate differentiation at ~4.5 Ga. Early terrestrial hydrosphere, early felsic crust in which granitoids were produced and later weathered under high water activity conditions and even the possible existence of plate boundary interactions.