Detrital zircon geochronology

Detrital zircon geochronology is a special way scientists use to find out how old rocks and sediments are and where they came from. It looks at tiny minerals called zircons found in sand and other loose rocks. These zircons are very tough and don’t change easily, so they stay around for a long time.

Fig. 1 – Zircon grains (Coin for scale)

Scientists use a tool called mass spectrometry to measure certain elements inside the zircons, especially the uranium–lead ratio. This helps them figure out the age of the zircons. Since zircons are found in many types of rocks, like granite, this method helps tell us about the history of the Earth.

Since the 2000s, this method has become very popular because new ways to measure age have gotten better. By studying these zircons, scientists can learn when sediments were laid down, where the rocks came from, and even how big areas of the Earth’s surface moved and changed over time.

Detrital zircon

Origin

Detrital zircons are part of the sediment made from weathering and erosion of older rocks. Because zircons are heavy and very tough, many of them are moved, laid down, and kept safe as tiny grains in sedimentary rocks.

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

Properties

Detrital zircons usually keep similar traits as the rocks they came from, such as their age, rough size, and mineral makeup. But their makeup isn’t only from when they first formed. Many of them change later during the journey of sediment. Depending on how much they get sorted, worn down, or dissolved, a detrital zircon grain might lose some of its original looks and gain new ones like a rounded shape and smaller size. On a bigger scale, zircons from different places can end up in the same sedimentary basin. This makes it tricky to match detrital zircon groups with where they started.

Zircon is a great tool for finding uranium-lead ages because of its special traits:

Zircon holds a lot of uranium for machines to find, usually between 100 and 1000 parts per million.
Zircon has very little lead when it first forms, in parts per trillion. So any lead found in zircon can be thought of as coming from uranium.
Zircon crystals form between 600 and 1100 °C, but keep all the lead below 800 °C (see Closure temperature). Once zircon cools below 800 °C, it keeps all the lead from radioactive decay. So the U-Pb age can be seen as when it formed, if the mineral hasn’t been changed by high heat after it was made.
Zircon usually forms in felsic igneous rocks, which have more than 60% silica (SiO₂). These rocks are lighter and float higher in the Earth’s continental crust, and they can be kept for a long time.
Zircon is tough both physically and chemically, so it is more likely to stay during the sediment process.
Zircon also holds other elements that give extra clues, such as hafnium (Hf) and the uranium/thorium (U/Th) ratio.

Sample collection

Choosing samples for detrital zircon geochronology depends on the goals and size of the research. The type of sedimentary rock and where it was laid down can change the results.

For example, in the Vlamy Formation, a type of rock called matured quartz arenite has older and more varied ages because its zircons are well-rounded, suggesting they were moved and redeposited many times. In contrast, the Harmony Formation in the same area has younger and similar ages because its zircons are euhedral. This shows how the maturity of sedimentary rocks, like siltstone and mudstone, can affect the ages of zircons found.

In the Harts Pass Formation, turbidites show similar zircon ages, while the fluvial Winthrop Formation has many different age groups. Rocks that are quickly deposited, like turbidites, tend to have a narrower range of zircon ages. Slower-deposited rocks, such as marine mudstone, have more time to gather zircons from various places, leading to a wider range of ages.

Detrital zircon extraction

After collecting rock samples, scientists clean and break them into smaller pieces. They then separate tiny zircon grains from the rock powder using water, magnets, and special liquids. The zircons are also sorted by size, with the most commonly used size being similar to fine sand grains.

Type of detrital zircon analysis

There are two main ways scientists study tiny minerals called zircons in rocks: qualitative and quantitative analysis. Qualitative analysis looks at every zircon grain to find where the rock might have come from. Scientists use special tools like thermal ionization mass spectrometry and secondary ion mass spectrometry to study these grains. They also look at the zircons using special images.

Quantitative analysis needs many zircon grains to understand the whole sample. Scientists use tools like LA-ICPMS to study lots of zircons. They pick the best spots on each grain to get accurate age information.

Methods

In detrital zircon analysis, scientists use different tools and methods to get different results. Usually, they mention the tools and instruments they used in their studies. There are three main groups: the tools for analyzing zircons, the standards for checking these tools, and the tools for taking pictures of zircons. More details can be found in the table below.

Table 1. Different types of analytical methods in detrital zircon study

Type of instrument for zircon analysis

In modern research, common instruments for U-Pb analysis are sensitive high-resolution ion microprobe (SHRIMP), inductively coupled plasma mass spectrometry (LA-ICPMS) and thermal ionization mass spectrometry (TIMS). Ion microprobe (non-SHRIMP) and lead-lead evaporation techniques were more commonly used in older research.

Zircon calibration standards

Basically analytical machines need to be calibrated before use. Scientists use age-similar (comparable to the sampled zircons) and accurate zircons as their machine calibration standards. Different calibration standards may give slight deviation of the resulting ages. For example, there are at least twelve different standards catering for different sample zircons in Arizona Laserchron Center, primarily using Sri Lanka zircon, followed by Oracle.

Type of instrument for zircon imagery

Instruments	Usage
For macroscopic view (Gives the general appearance of the zircon, cannot identify internal zircon texture properly, especially when the zircon is neither zoned nor metamictized)
Binocular microscope (BM)	Can examine zircon grain as a whole: color, transparency, crystal morphology and form growth, inclusions, fractures and alterations.
Transmitted light microscopy (TLM)	Can examine zircon growth zoning and metamictization in cross-polarized light. Challenging for small zircon grains due to limited resolution. Difficult to identify zircon from other high-relief and high-birefringence minerals (such as monazite).
Reflected light microscopy (RLM)	Can examine zircon growth zoning, alteration and metamictization.
For zircon internal structure
Uranium Mapping (UM)	Induce fission tracks within the zircon by neutron flux reactor and record the tracks into an image. Has implications on the amount and distribution of radioactive elements (i.e. uranium) within the zircon grain.
Cathodoluminescence (CL)	One of the best resolution instruments. Induced CL by bombarding zircon with electrons, where U⁴⁺ ions and radiation damages suppress CL and give darker bands. Different colored CL emission may imply the presence of different element, such as blue (Y³⁺) and yellow (Ti⁴⁺ or U⁴⁺)
Back-scattered electron microscopy (BSM)	Also one of the best resolution instruments currently. Almost like a reversed CL imagery, as the brightness correlates to atomic number. The brightness/ color intensity in BSM is primarily due to hafnium (Hf), with uranium (U) being second.
Secondary electron microscopy (SEM)	See scanning electron microscope.

Detrital zircon data

Different studies of detrital zircons look at various details. The two main types of information are data from the zircons themselves, like measurements and pictures, and data about the sample where the zircons were found. More details can be found in Table 2.

Filtering detrital zircon data

All information collected should be checked and cleaned before use to avoid mistakes, usually by using a computer.

Before using the ages of zircons, they should be checked carefully. For large groups of data, those with big differences in age are removed. The level of difference allowed changes based on the age of the zircon.

To get the best age measurement, scientists use different methods depending on how old the zircon is. For older zircons, one method is used, and for younger ones, another method is used.

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)

Sometimes, scientists group ages together to make sure they are correct. If three or more ages are very close to each other, they are considered a valid group.

There are no fixed rules for how much uncertainty is allowed in age measurements. Scientists must balance removing uncertain data with keeping enough data to be reliable.

The difference between when zircons formed and when they were deposited can help understand past tectonic settings.

Table 3. Variable detrital zircon record in different tectonic setting.
	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.	Tectonically stable. Lack of syn-sedimentary magmatic generation
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	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,
Graphical representation
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.