Detrital zircon geochronology

Origin

Detrital zircons are part of the sediment derived from weathering and erosion of pre-existing rocks. Since zircons are heavy and highly resistant at Earth's surface,^[5] many zircons are transported, deposited and preserved as detrital zircon grains in sedimentary rocks.^[3]

Properties

Detrital zircons usually retain similar properties as their parent igneous rocks, such as age, rough size and mineral chemistry.^[6][7] However, the composition of detrital zircons is not entirely controlled by the crystallization of the zircon mineral. In fact, many of them are modified by later processes in the sedimentary cycle. Depending on the degree of physical sorting, mechanical abrasion and dissolution, a detrital zircon grain may lose some of its inherent features and gain some over-printed properties like rounded shape and smaller size.^[5] On a larger scale, two or more tribes of detrital zircons from different origins may deposit within the same sedimentary basin. This give rise to a natural complexity of associating detrital zircon populations and their sources.^[3]

Zircon is a strong tool for uranium-lead age determination because of its inherent properties:^[8]

1. Zircon contains high amount of uranium for machine recognition, commonly 100–1000 ppm.^[8]
2. Zircon has a low amount of lead during crystallization, in parts per trillion.^[8] Thus, lead found in zircon can be assumed as daughter nuclei from parent uranium.
3. Zircon crystals grow between 600 and 1100 °C, while lead is retained within the crystal structure below 800 °C (see Closure temperature). So once zircon has cooled below 800 °C it retains all the lead from the radioactive decay. Therefore, U-Pb age can be treated as the age of crystallization,^[8] if the mineral/sample itself has not undergone high temperature metamorphism after formation.
4. Zircon commonly crystallizes in felsic igneous rocks, with greater than 60% silica (SiO₂) content.^[4] These rocks are generally less dense and more buoyant. They sit high in the Earth's (continental crust), and have good preservation potential.
5. Zircon is physically and chemically resistant, so it is more likely to be preserved in the sedimentary cycle.^[8]
6. Zircon contains other elements which gives supplementary information, such as hafnium (Hf), uranium/thorium (U/Th) ratio.^[8]

Qualitative analysis

Qualitative approach examines all the available detrital zircons individually regardless of their abundance among all grains.^[13][14] This approach is usually conducted with high precision thermal ionization mass spectrometry (TIMS) and sometimes secondary ion mass spectrometry (SIMS).^[3] Optical examination and classification of detrital zircon grains are commonly included in qualitative studies through back-scatter electrons (BSE) or cathodoluminescence (CL) imagery,^[3] despite the relationship between the age and optical classification of detrital zircon grains is not always reliable.^[15]

Quantitative analysis

Quantitative approach requires large number of grain analyses within a sample rock in order to represent the overall detrital zircon population^[3] statistically (i.e. the total number of analyses should achieve an appropriate level of confidence).^[16] Because of the large sample size, secondary ion mass spectrometry (SIMS) and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) are used instead of thermal ionization mass spectrometry (TIMS). In this case, BSE and CL imagery are applied to select the best spot on a zircon grain for acquiring reliable age.^[17]

By U-Pb age discordance

Before applying detrital zircon ages, they should be evaluated and screened accordingly. In most cases, data are compared with U-Pb Concordia graphically. For a large dataset, however, data with high U-Pb age discordance (>10 – 30%) are filtered out numerically. The acceptable discordance level is often adjusted with the age of the detrital zircon since older population should experience higher chances of alteration and project higher discordance.^[19] (See Uranium–lead dating)

By choosing the best age

Because of the intrinsic uncertainties within the three yield U-Pb ages (²⁰⁷Pb/²³⁵U, ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²⁰⁶Pb), the age at ~1.4 Ga has the poorest resolution. An overall consensus for age with higher accuracy is to adopt:

• ²⁰⁷Pb/²⁰⁶Pb for ages older than 0.8 – 1.0 Ga
• ²⁰⁶Pb/²³⁸U for ages younger than 0.8 – 1.0 Ga^[14][36]

By data clustering

Given the possibility of concordant yet incorrect detrital zircon U-Pb ages associated with lead loss or inclusion of older components, some scientists apply data selection through clustering and comparing the ages. Three or more data overlapping within ±2σ uncertainty would be classified as a valid age population of a particular source origin.^[19]

By age uncertainty (±σ)

There are no set limit for age uncertainty and the cut-off value varies with different precision requirement. Although excluding data with huge age uncertainty would enhance the overall zircon grain age accuracy, over elimination may lower overall research reliability (decrease in size of the database). The best practice would be to filter accordingly, i.e. setting the cut-off error to eliminate reasonable portion of the dataset (say <5% of the total ages available^[6])

By applied analytical methods

Depending on the required analytical accuracy, researchers may filter data via their analytical instruments. Generally, researchers use only the data from sensitive high-resolution ion microprobe (SHRIMP), inductively coupled plasma mass spectrometry (LA-ICPMS) and thermal ionization mass spectrometry (TIMS) because of their high precision (1–2%, 1–2% and 0.1% respectively^[17]) in spot analysis. An older analytical technique, lead-lead evaporation,^[37] is no longer used since it cannot determine the U-Pb concordance of the age data.^[38]

By spot nature

Apart from analytical methods, researchers would isolate core or rim ages for analysis. Normally, core ages would be used as crystallization age as they are first generated and least disturbed part in zircon grains. On the other hand, rim ages can be used to track peak metamorphism as they are first in contact with certain temperature and pressure condition.^[39] Researchers may utilize these different spot natures to reconstruct the geological history of a basin.

Maximum depositional age

Some of the most important information we can get from detrital zircon ages is the maximum depositional age of the referring sedimentary unit. The sedimentary unit cannot be older than the youngest age of the analyzed detrital zircons because the zircon should have existed before the rock formation. This provides useful age information to rock strata where fossils are unavailable, such as the terrestrial successions during Precambrian or pre-Devonian times. ^[40][3] Practically, maximum depositional age is averaged from a cluster of youngest age data or the peak in age probability because the youngest U-Pb age within a sample is almost always younger with uncertainty.^[17]

Tectonic studies

Using detrital zircon age abundance

In a global scale, detrital zircon age abundance can be used as a tool to infer significant tectonic events in the past.^[4] In Earth's history, the abundance of magmatic age peaks during periods of supercontinent assembly.^[6] This is because supercontinent provides a major crustal envelop selectively preserve the felsic magmatic rocks, resulting from partial melts.^[41] Thus, many detrital zircons originate from these igneous provence, resulting similar age peak records.^[6] For instance, the peak at about 0.6–0.7 Ga and 2.7 Ga (Figure 6) may correlate the break-up of Rodinia and supercontinent Kenorland respectively.^[26]

Using difference between detrital zircons crystallisation ages and their corresponding maximum depositional age

Apart from the detrital zircon age abundance, difference between detrital zircons crystallisation ages (CA) and their corresponding maximum depositional age (DA) can be plotted in cumulative distribution function to correlate particular tectonic regime in the past. The effect of different tectonic settings on the difference between CA and DA is illustrated in Figure 7 and summarized in Table. 3.^[4]

More information Table 3. Variable detrital zircon record in different tectonic setting. ...

Table 3. Variable detrital zircon record in different tectonic setting.[4]
	Convergent Setting	Collisional Setting	Extensional Setting
Referred tectonic zone	Ocean-continent collision	Continent-continent collision	Spreading oceanic ridges
Magmatic activities	Syn-sedimentary magmatic activities is likely with continuous subduction induced partial melts	Magma generation is enveloped within a thick lithosphere.[41]	Tectonically stable. Lack of syn-sedimentary magmatic generation[42]
Associated basin	Arc-flanking basin	Foreland basin	Rift basin, passive margin
Main detrital zircon sources	Fed by juvenile generations of volcanic/magmatic rocks	Fed by syn-collisional magmatism and old units caught in the orogen	Fed by a large range of pre-existing old terraines
Resulting zircon record	Youngest detrital zircon grain is approximately the onset of sediment accumulation[35]	High, especially within periods of supercontinent	Youngest detrital zircon provide a maximum depositional age much older than the onset of sediment accumulation
Crystallization age – depositional age	Small	Medium, 10 – 50% within 150Ma	Large, < 5% within 150 Ma
Graphical representation
	The colored zones within Figure 8-10 are simply bounded by constructed cumulative proportion curves of their corresponding setting from all around the world.[4]

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