Zircon: Nature’s best clock

How old is our planet Earth? When did the dinosaurs live? When were ancient monuments built? It is a proven fact that the oldest rock ever found on Earth is as old as 4.375 ± 0.006 Ga (Giga annum or Billion years) old. How did the scientists calculate this age? The answer to the above questions is dating!

One of the oldest and most reliable dating methods is the Uranium-Lead (U-Pb) dating. Uranium–Lead dating is a geological age-determination method that uses the radioactive decay of the unstable Uranium (U) (238U and 235U) into stable isotopes of Lead (Pb) (206Pb and 207Pb respectively). The two decay systems have half-lives of 4470 and 710 million years, respectively, which permit the dating of rocks as old as the age of the Earth (4.5 Ga) or just a few million years old. Techniques like secondary ion mass spectrometry (SIMS), laser-ablation induced coupled plasma–mass spectrometry (LA-ICP–MS) and thermal ionization mass spectrometry (TIMS) help us study trace element and isotopes signature in Zircon crystals and have made it possible to determine U–Pb ages with a high spatial resolution. For this technique, scientists use minerals like Monazite, Titanite, Baddeleyite and Zirconolite. Yet, the most common mineral used is Zircon.

Figure 1: Photomicrograph of
a single Zircon crystal.

Tiny crystals of Zircon occur in most of the granitic rocks as the primary accessory mineral, i.e., a mineral that is found in a small quantity and not used for naming the rock. It is also present in most of the rocks metamorphosed from igneous rocks like gneiss and is a common constituent of sands (sediments). Thus, Zircon can be considered to be a ubiquitous mineral. As their atomic structure remains stable over very long periods of geological time, they can provide a clear picture of the Earth’s history.

Zircon is a tetragonal (a type of crystal system) orthosilicate mineral in which isolated SiO4 tetrahedra (triangular pyramid) are linked by sharing their edges and corners with intervening ZrO8 dodecahedra (12 face polyhedron). These ZrO8 dodecahedra share edges to form zigzag chains along the b axis, whereas, along the c axis, edges are shared with the SiO4 tetrahedra to produce chains with alternating SiO4 and ZrO8 polyhedra. Zircon can incorporate many elements like P, Sc, Nb, Hf, Ti, U, Th, and Rare Earth Elements (REE) in trace (up to thousands of ppm) or minor (up to 3 wt%) amounts. Zr4+ is in 8-fold coordination

with an ionic radius of 0.084 nm while Si4+ is in 4-fold coordination with an ionic radius of 0.026 nm. (OH-)4 can replace SiO4 while Hf4+, U4+, Th4+, Sc3+, Lu3+, Y3+, Ce4+ and Ti4+ are accommodated in the Zr4+ site as substituents. U can reach wt% levels, although it is usually less than 5000 ppm. Now, due to the 8-fold coordination and an ionic radius of 0.129 nm, Pb2+ is not incorporated into growing Zircon crystals at more than ppb levels. Thus, any Pb found in trace amounts must be radiogenic (product of U-Pb decay). This is exploited by scientists to calculate the ages.

Figure 2: Zircon CL Image with dating
points and 207Pb/206Pb ages (Ma).

 This steady decay of U causes accumulation of radiogenic Pb, and this provides the basis for accurate and precise determination of a Zircon’s isotopic age. Adding to this, Zircon is a hard, refractory mineral that can remain intact even if its host rock is metamorphosed, melted, or mechanically weathered away. Furthermore, diffusion rates within Zircon for elements like U and Pb are very low, so it retains age and other isotopic information even when exposed to magmatic temperatures. All these characteristics make zircon “nature’s best clock“.

Figure 3: Structure of Zircon. (a) projected from a-axis showing
alternate SiO4 – ZrO8 (b) projected from a-axis with one ZrO8 unshaded to reveal its sub-structure consisting of two distorted ZrO4 tetrahedra.

 By,

Rahul Subbaraman

Department of Earth Science

IISER Kolkata

References:

Zircon Dating. (2020) from https://scienceaid.net https://geology.com/minerals/zircon.shtml/Zircon_Dating Zircon Tiny but Timely by Simon L. Harley and Nigel M. Kelly Turkina, Olga & Urmantseva, Lena & Berezhnaya, N. & Skublov, S. (2011). Formation and Mesoarchean metamorphism of hypersthene gneisses from the Irkut granulite-gneiss block ( Sharyzhalgai uplift in the southwestern Siberian craton ). Russian Geology and Geophysics.

 About The Author: Rahul is a 3rd year BS-MS student at IISER Kolkata and an INSPIRE Scholar majoring in Earth Science with a minor in Chemical Sciences. He is an amazing multitasker and a really curious person who loves to learn new things. He is a foodie who loves to cook by exploring different styles and ingredients. He is also an avid reader, a cinephile and now a budding writer.     

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