Zircons galore: an Indo-Pacific Heritage

Gemstones are an attractive field of natural history pursued by Australian Museum mineralogists for decades. From diamonds through to opals, gemstones have the ability to capture people’s imaginations in a different way to flora and fauna.

Zircon is a naturally occurring mineral, and is mined as a by-product in New South Wales and Queensland sapphire fields. It is a silicate mineral in which atoms of the element zirconium are held within clusters of silicon and oxygen atoms. Natural colours of zircon are highly variable and can be colourless, yellow-golden, red, brown, blue or green. Zircon as a gemstone has very bright lustre and in colourless pyramidal crystals is often mistaken for diamond by fossickers. It is not to be confused with cubic zirconia, a synthetic zirconium oxide used as a cheaper substitute for diamond.

This study aimed to collect and analyse zircon brought up from depths in volcanic basalt fields from many places along the western Pacific continental margins. It used modern age-dating and chemical techniques to pin-point their deep geological origin. Implications for this research include a wider understanding of the time lines and geological evolution of the mantle and crust below continents. This also benefits exploration for new ore and gem deposits.

Lin Sutherland and an assembled network of scientists have delved into the crystallising behaviour of these gem zircons. They used facilities from several institutions, including the Australian Museum, University of New South Wales, University of Western Sydney, Macquarie University, Melbourne University, University of Tasmania, Australian National University, Australian Gemmological Society, Centre of National Research, Nancy, France and Far East Geological Institute, Vladivostok, Russia.

Zircons are invaluable for dating Earth’s inner workings, from young volcanoes to oldest rocks on Earth. Traces of uranium in its structure acts as a natural clock. A zircon’s formation age is set by uranium-lead isotope dating. If brought up later in hot lava, its earlier radiation scars become healed and only start again after cooling. These two effects give birth time and surface arrival time. The Museum study not only dated volcanic eruptions, but also showed some zircons surfaced soon after birth, but others only after tens of millions years.

The Australian Museum research on large zircon gem crystals is now published in a tribute volume to a Professor of Geology, who promoted diamond research in Australia. The new Museum study covers a 12,000 km long gem belt, mined for natural and treatable stones, which supply a global jewellery market for great economic benefit.

More Information:

• Sutherland L., Graham I., Yaxley G., Armstrong R., Giuliani G., Hoskin P., Nechaev V. and Woodhead J. (2016): Major zircon megacryst suites of the Indo-Pacific lithosphere margin (ZIP) and their petrogenetic and regional implications. Mineralogy and Petrology. DOI 10.1007/s 00710-015-0421-3.