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 zircons are found in metamorphosed sediments that were initially deposited around 3 billion years ago, or during the Archean Eon. 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 so much as a single zircon over four billion years in age 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 detrital zircon grains dated by U–Pb analysis are over four billion years old.
File:Zircon type.png|thumb|upright=1.75|A uranium to ytterbium 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

Low crystallisation temperatures and trace element characteristics are the two major characteristics that differentiate mantle derived zircon or oceanic crust-derived zircon. Lunar and meteoritic zircons are unique because of their REE signature for example, lack of a cerium anomaly. The crystallization temperature ranges from 900 to 1100 °C. In contrast, terrestrial Hadean zircons are restricted to 600 to 780 °C. Hadean Jack Hills zircon has a wide range of oxygen fraction comparing to meteoritic zircons. No extraterrestrial zircons were found in any terrestrial locality. The textural characteristics like the growth zoning and inclusion mineralogy shows that Hadean zircon from the Jack Hills all come from igneous sources.

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 c. 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 δ¹⁸O values less than mantle 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 values of δ¹⁸O observed in Hadean Jack Hills zircons led to two different ideas about the source of Hadean zircon.	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 δ¹⁸O values while those derived by S type metasedimentary rocks have higher δ¹⁸O 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 E_Hf 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 c. 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
Plutonium-xenon	The initial Pu/U ratios of Jack Hills zircon ranges from c. 0.007 to zero.	Due to Xe loss during later metamorphism. Variation 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
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
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