Detrital zircon geochronology

Zircon is a common accessory or trace mineral constituent of most granite and felsic igneous rocks.

Due to its hardness, durability and chemical inertness, zircon persists in sedimentary deposits and is a common constituent of most sands.

Zircons contain trace amounts of uranium and thorium and can be dated using several modern analytical techniques.

Detrital zircon geochronology has become increasingly popular in geological studies from the 2000s mainly due to the advancement in radiometric dating techniques.

[4] Detrital zircons are part of the sediment derived from weathering and erosion of pre-existing rocks.

[3] Detrital zircons usually retain similar properties as their parent igneous rocks, such as age, rough size and mineral chemistry.

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.

In some cases, the sedimentary rock type and depositional setting can significantly affect the result.

[3] Examples include: After rock samples are collected, they are cleaned, chipped, crushed and milled through standardized procedures.

[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 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).

In this case, BSE and CL imagery are applied to select the best spot on a zircon grain for acquiring reliable age.

[25] Depending on the detrital zircon study, there should be different variables included for analysis.

[18][34] These can be evidence for provenance studies, by correlating the zircon's melt condition with similar igneous province.

For a large dataset, however, data with high U-Pb age discordance (>10 – 30%) are filtered out numerically.

Three or more data overlapping within ±2σ uncertainty would be classified as a valid age population of a particular source origin.

[19] There are no set limit for age uncertainty and the cut-off value varies with different precision requirement.

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]) 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] Apart from analytical methods, researchers would isolate core or rim ages for analysis.

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.

This provides useful age information to rock strata where fossils are unavailable, such as the terrestrial successions during Precambrian or pre-Devonian times.

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.

Fig. 1 – Zircon grains in real life (Coin for scale)
Fig. 2 – Simple diagram illustrating the formation of igneous zircon, the processes of them becoming detrital zircons and the differences between igneous and detrital zircons
Fig. 3 – Schematic images of 3 zircons under different imaging instruments. Modified from Corfu et al. (2003), Nemchin and Pidgeon (1997) and J.M. Hanchar
Fig. 4 – Laser ablation pit (Spot analysis in LA-ICPMS) on a zircon grain
Fig. 5 – Diagram illustrating two major forms of zircon and their sets with miller indices, with reference to Corfu et al. (2003) and Wang and Zhou (2001)
Fig. 6 – Global detrital zircon age distribution in a frequency versus geological age diagram. Modified from Voice et al. (2011)
Fig. 7 – Schematic diagram showing the source rock nature and their proximity to the sedimentary basins in multiple tectonic settings. Modified from Cawood et al. (2012)
Fig. 8 – Graph illustrating the generalized zone for cumulative proportional curves of CA-DA in convergent basins. Modified from Cawood et al. (2012)
Fig. 9 – Graph illustrating the generalized zone for cumulative proportional curves of CA-DA in collisional basins. Modified from Cawood et al. (2012).
Fig. 10 – Graph illustrating the generalized zone for cumulative proportional curves of CA-DA in extensional basins. Modified from Cawood et al. (2012)