Earth history in miniature: Zircon under the microscope

Dr Robert Sturm (Austria)

The mineral zircon (more correctly, orthosilicate zircon or ZrSiO₄) is an important accessory mineral in various rocks of the earth’s crust, but most of all of igneous rocks with the mineral composition of granite. An accessory mineral is a mineral comprising less than about 10% of a rock and which therefore plays little or no role in naming or classifying that rock.

As well as its ubiquitous appearance in magmatic, metamorphic and sedimentary rocks, zircon is a remarkable mineral due to its high resistance to mechanical and chemical processes within the earth crust. Therefore, it is very useful as a protolith indicator in different types of crustal rocks (Speer, 1982). (A protolith indicator is a mineral in a metamorphic assemblage that provides information on the chemistry of the host rock, within which it had originally grown.)

As a result of the repeated formation of magmatic overgrowths around older ‘inherited’ zircon cores (like the rings of an onion), evidence of several stages of earth history are preserved within a single grain and can be scientifically analysed (Sturm, 1999, 2004). Once these overgrowths have been identified, the phases of crystallisation included in accessory zircon can be attributed to geological time periods. This is achieved by radiometric dating, based on the mineral’s content of radioactive uranium and thorium, and the redistribution of these isotopes and their daughter products.

Another important characteristic of zircon is its ‘zoning’ (that is, the presence of concentric zones of different chemical composition that reflect its growth history). In general, three types of zoning can be distinguished (Speer, 1982):

1. Chemical zoning caused by the substitution of Zr and Si by the elements listed in the box. It is best described as ‘oscillatory’ (that is, it has a sequence of bright and dark growth zones). However, elements other than Zr are often enriched towards the mineral’s rim.
2. Growth zoning typically results from physical and chemical changes in the melt or solution, which serve as sources for zircon crystallisation. Single growth zones are very narrow and indicate a fast, unbalanced element exchange between crystal and melt or solution.
3. Passive zoning is a kind of chemical zoning caused by the removal or addition of elements without new growth.

In the rest of this article, I will discuss some impressive examples of zircon crystals, which are characterised by just such an ‘inherited’ core and overgrowth, and also significant zoning. In addition, I will briefly describe the preparation technique used to get detailed imaging of single grains.

How to prepare the grains for internal analyses

This preparation method is very time-consuming and needs a lot of experience! It has taken me several years to perfect the technique and to obtain satisfactory crystal sections suitable for electron microprobe analysis and imaging.

Crystal preparation consists of three steps:

1. First, pre-selected, equally sized crystals are mounted on a glass slide (typically 20 x 40mm), so that the grains have a uniform orientation. After being fixed on the slide, the crystals are embedded in a 2mm thick layer of epoxy resin.
2. Second, the hardened resin layer with the zircon crystals enclosed is continuously abraded using medium-to-fine-grained silicon carbide powder (SiC). This is continued until sections across the middle of the grains have been produced.
3. Third, the zircon sections exposed out of the resin layer are polished with diamond pastes (grain sizes: 1µm and 0.25µm). At this point, a plain crystal surface has been produced, which can be examined under an electron microprobe.

Next, the samples are covered with a carbon layer that allows for the conduction of electrons accumulated on the surface of the sample during microprobe work. Imaging of crystal sections is best achieved by using the ‘backscattered electron mode’, in which all electrons reflected from the surface are detected.

The internal shape of zircon

By using the above method, I have achieved some (to my mind) very impressive results! In addition, the important role of this mineral as a protolith indicator has now become very clear, because all of the grains shown in Fig. 3 contain an older ‘inherited’ core (indicated by a 1) and a newer overgrowth (indicated by a 2).

The older parts of the crystals crystallised during the formation of a protolith (that is, an igneous rock that was part of the original material for metamorphic processes in the earth’s crust). From the chemical composition of the inherited core material, the trace element chemistry of the former host rock can be estimated with great accuracy. In addition, the number and size of the growth zones within the core provide some information of the cooling history of the protolithic magma, in which the studied crystal had been formed. It is also possible to detect low to intermediate width growth bands in the crystals pictured, which indicates a rather fast cooling of the magma.

The overgrowth zoning is subject to remarkable variations, ranging from highly oscillatory growth zoning (with narrow growth increments) to chemical zoning (where single growth increments are much wider). The overgrowths crystallised during a later event of crustal melting, for example, due to anatexis (that is, the melting of the earth crust due to tectonic processes or the intrusion of a magmatic body) and are characterised by a composition of trace elements very different from those in the zircon cores.

The chemistry of zircon

Naturally occurring zircon crystals are tetragonal in shape (and, for the even more technically minded, belong to the space group I41/amd). The chemistry of pure zircon consists of 67.1% ZrO2 and 32.9% SiO4. However, for practical purposes, this ideal composition is not found in nature, because it is characterised by having many shapes according to the formula ABO4. Position A, in this formulas, is mainly occupied by zirconium (Zr), hafnium (Hf), uranium (U), thorium (Th), yttrium (Y), lanthanum (La), and rare earth elements. Position B is occupied by silicon (Si), aluminium (Al) and phosphorus (P).

According to radiometric dating, the cores of the crystals pictured were formed about 320mya, during the intrusion of Variscan granite batholiths. However, but crystallisation of the overgrowth probably happened during late-Variscan orogenesis and metamorphism 280mya or during a later metamorphic event. Therefore, two important events of Central European earth history are reflected in miniature in the grains in the picture.

What can we learn from this?

Finally, I want to present a simple, three-stage model of the formation of these zircon crystals, containing two phases of different ages (Fig. 4).

1. The first stage (a) includes the intrusion of deep crustal melt into higher levels of the earth’s crust, which typically occurs in combination with a subduction event by which an oceanic plate is thrust downwards into the mantle. During the cooling of the magma, the first crystal phase is formed.
2. The second stage (b) is marked by the complete subduction of the oceanic crust and the subsequent collision of two continental plates causing extensive deformation of the crust. Systems of shear zones (that is, vertical or horizontal planes, along which adjacent crustal blocks are dislocated, for example, the San Andreas fault) are generated, which also affect the granite bodies and their mineral components. Ductile and (above all) brittle deformations cause the fracture of single zircon grains.
3. In the third stage (c), one lithological block is overthrusted by the other, starting a sequence of metamorphic processes. A dramatic increase in temperature causes the local melting of the superimposed crustal unit and, as a result, the new crystallisation of major and accessory minerals. During this event, the overgrowths are formed.

Did you know?

Blue Zircon is the birthstone for December. It resembles diamond in luster and fire, and colourless zircons have been mistaken for diamonds, even by experienced jewellers. Although generally found in browns, it ran be heat-treated to get beautiful blue and golden colours. It is also one of the ‘Biblestones’, being a gemstone that was in the third row of the Breastplate of Aaron, where it is called a ‘ligure’.

References

Speer, J.A. 1982. Zircon. In: Ribbe, P. H. (ed.), Orthosilicates, MSA, Washington DC, 67-112.

Sturm, R. 1999. Longitudinal and cross section of zircon: a new method for the investigation of morphological evolutional trends. Swiss Bulletin of Mineralogy and Petrography 79: 309-316.

Sturm, R. 2004. Imaging of growth banding of minerals using 2-stage sectioning: application to accessory zircon. Micron 35: 681-684.