Zircons

Zircons are — more often than not — the reason why I travel. Zircon is a zirconium (number 40 on the periodic table) bearing silicate mineral (ZrSiO4). Zircon is an extremely important mineral for geologists because although Zr is the dominant cation, zircon also contains several other large cations such as uranium, hafnium, and rare earth elements (REE).

SEM image of zircon crystal (from SDSU)

Uranium in zircons is useful because over time it decays to lead, thus providing a radiometric clock to tell the age of a particular zircon (and often the rock from which it was extracted). Zircon is particularly useful in U-Pb dating because zircon hates Pb. That is, when the zircon is crystallizing uranium is readily incorporated into the crystal structure, but lead is not. Another thing that makes U-Pb dating so robust is that there are two isotopes of U (235 and 238) that decay to different isotopes of lead (207 and 206). Therefore if the ages of both isotopic systems match then we call it “concordant” or that it lies on the U-Pb concordia (see figure A below). When analyses lie beneath the concordia, this implies there has been some thermal event allowing some of the radiogenic lead to leak out of the crystal lattice (lead loss). Zircons from a sample that experienced lead loss lie on a line known as a ‘discordia’ where the intercept of the discordia and the concordia represent the ages of initial crystallization (upper intercept) and lead loss event (lower intercept) (see figure B below).

A: example of a sample with concordant zircon analyses from the Big Cottonwood Formation, Utah. In this case the youngest zircon represents the maximum depositional age.
B: example of a sample with discordant zircon analyses from the Munsiari Formation of the Lesser Himalayan Sequence, India. The formation age of the protolith is 1898 million years, however during the collision of India and Asia these zircons experienced lead loss at 39 Ma.

Hafnium in zircons is important because it tells us the age at which the material latter to crystalize as a zircon was extracted from the mantle (see below).

Schematic Hf isotope evolution diagram showing Hf fractionation from the time crust began forming (~4.55 Ga) with the depleted mantle and chondrite reservoirs diverging. At time 1 material is extracted from the mantle locking in its depleted mantle age with a zircon crystallizing from that extracted material at time 2.

And lastly, REE in zircon give us the ability to determine the times at which zircon and other minerals grew in relation to one another. For example, when a zircon grows in the presence of garnet, because garnet will consume the REE more than the zircon and when a zircon grows in the absence of garnet the REE concentrations will be much higher in the zircon. This allows us to decipher the timing at which garnet growth stopped (which is very important for metamorphic petrologists).

Rare Earth Element concentration of two hypothetical zircons: one growing without garnet and the other in the presence of garnet. Because garnet will incorporate REE in their crystal structure more readily than zircon, the REE concentration is dramatically lower (two orders of magnitude) when the two are growing synchronously.

Despite all of this analytical mumbo-jumbo, first things first: we must first collect the samples we hope to analyze. The samples are disaggregated and heavy minerals (zircon, apatite, magnetite, etc.) are separated using dense liquids (light mineral such as quartz floats and heavy minerals sink). Magnetic minerals are removed using a powerful magnet and lastly, the zircons are picked by hand using a binocular microscope. Here is the end result:

Zircon separate from South Africa (from SandAtlas.org)

These zircons are then mounted in resin and imaged using backscatter and cathodoluminescence (CL) imaging using an electron microprobe and scanning electron microscope.

Backscatter (left) and CL (right) images of a zircon extracted from the Torridon Group, Scotland
Zircons with inherited cores from the High Himalaya. The cores were formed in a volcanic arc and were transported over 2000 km (~1250 mi) and deposited off the coast of northern India ~800 Ma. These sandstones were melted during orogenesis (mountain building) and new zircons grew around the older detrital zircons around 470 Ma.
Highly altered zircon from East Timor. The outer rim of this zircon gave an age of 7.6 Ma which coincides with the age of the collision of Australia and Indonesia. The red specks are allanite (a REE-rich sorosilicate).

See my recent zircon related publications here:

Spencer et al., 2012. Constraining the timing and provenance of the Neoproterozoic Little Willow and Big Cottonwood Formations, Utah: Expanding the sedimentary record for early rifting of Rodinia. Precambrian Research, in press.

Spencer et al., 2012. Depositional provenance of the Himalayan metamorphic core of Garhwal region, India: Constrained by U–Pb and Hf isotopes in zircons. Gondwana Research, in press.


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