Magic Crystals - Tales Zircons Tell

The picture on the left is of our oldest earthly possessions. They are sand grains, but special ones, each an individual crystal of zircon, the silicate of the element zirconium. Zircon may be the December birthstone, but these crystals were born 4.4 billion years ago - only a hundred million years or so after the Earth was formed, and are our oldest terrestrial material. They are found in sandstones of the Jack Hills in Western Australia, the tiny grains, less than a millimeter in size, painstakingly extracted one-by-one; the sandstones themselves are only around 3 billion years old, deposited by rivers that carried the debris from the erosion of far older rocks, of which the zircons are the only survivors. And survivors they are: tough as old boots, zircons are essentially indestructible - the original crystal shapes of the Jack Hills grains are still clearly visible - and they survive to tell extraordinary tales. They preserve a record of the ancient chemistry of the earth, and they tell time, the small amounts of radioactive uranium and thorium that they contain decaying constantly to provide a radiometric clock. Formed from the molten maelstrom of the newly-formed Earth, the Jack Hills zircons tell a story of cooling and the formation of solid rocks - and therefore in all likelihood, crust - far earlier than had been previously thought possible. And their chemistry suggests the presence of liquid water, demonstrating that we still have a lot to learn about the childhood of our planet. Looked at closely, using cathodoluminescence and other kinds of clever analysis, zircons commonly show internal zoning, like tree rings or the layers of an onion, recording successive stages in the growth of the crystal, each layer revealing a different chapter in the story of the crystal's growth, and its own age (photo, right - the numbers are millions of years).

Zircons are not major ingredients of igneous, originally molten, rocks, but they are common and they are the toughest, surviving to form constituent grains in sandstones of all ages and from all parts of the world - which makes them invaluable story-tellers, the troubadours of minerals. Look around the geological research that is being published worldwide on any given day, and, where there are biographies of mountain belts to be written, erosional sagas to recounted, or plate movements to be reconstructed, you will find zircons telling their tales. I'll select here just two fascinating pieces of work, tales of two rivers, published in Geology in the last year that illustrate the powers of these magic crystals to reveal extraordinary details of our planet.

The Brahmaputra River is one of the world's mightiest, both in terms of its length and the amount of water and sediment that it carries. It is the third greatest sediment-carrying river, surpassed only by the Yellow River and the Amazon; perhaps a couple of billion tons - the equivalent of eighty million large dump trucks - of sediment pass down the Brahmaputra every year. And where does all this sediment come from? Well, the erosion of the Himalayas, of course. Yes, but exactly where - surely it's not an evenly distributed process? The answer is no, it's not, but it's far more localized than you would think. The river originates in far southwestern Tibet as the Yarlung Zangbo that flows unswervingly eastwards for 1700 kilometers until it takes a very sharp right turn (see the satellite image below) and carves its way across the Himalayas via what is probably the deepest canyon in the world.

 

In the corner of that turn, the "Big Bend," is the region of the mountain of Namche Barwa, a mere 7782 meters high. And that's where the Brahmaputra collects most of its sediment. Researchers from the University of Washington, Lehigh University, and the Australian National University in Canberra (a pleasingly typical international collaboration) have demonstrated the extraordinary rate of erosion in this small areas of the Himalayas - using zircons. Now zircons contain not one, but two clocks. As the uranium decays, the fragments emitted blaze, literally, tiny trails of damage through the crystal, leaving fission tracks that can be quite easily analyzed. These radioactivity "contrails" are highly susceptible to temperature and are only preserved after the mineral has cooled below a certain temperature; this is complex, but for zircons it's around 230 degrees centigrade. For a zircon, crystallized from a molten rock, fission track analysis reveals when the rock cooled beyond this critical temperature, and therefore records the journey of that rock up into the cooler near-surface regions of the earth's crust. Now, we know that Himalayas are still rising as the Indian/Asian plate collision continues, and that rocks that had been deep and hot eventually reach the surface as erosion shovels away the overlying layers. That uplift and cooling sets the fission track age for a zircon crystal. But the region around the Big Bend is rising incredibly rapidly - the zircons in the sands pouring out of the mountains only cooled off 600,000 years ago. This implies that the rocks are being exhumed - carried up to the surface to be exposed to erosion - at rates of up to 20 millimeters every year - which is astonishing. The zircons tell the tale of sediment being supplied to the Brahmaputra at an incredible rate from the small area around Namche Barwa - 50% of the river system's sediment load comes from approximately 2% of its total drainage area. And this is not only a fascinating fact for geologists - as the authors of the paper point out, it has huge implications for flood management and hydroelectric development in the region. 

Now, let's travel north to another of the world's great rivers, the Lena of Siberia. The Lena rises, unexpectedly, only a few kilometers west of Lake Baikal (see map below) and, rather than having anything to do with the lake and its drainage system, sets off northeastwards on its own journey towards the Verkhoyansk Mountains.

 

However, it doesn't cross them but turns left and heads up to the Arctic Ocean into which it deposits a magnificent delta. The Lena's journey covers around 4400 kilometers (it's the tenth longest in the world) and it drains a total area four times the size of Texas. Now I suppose that it's no real surprise that a river will sometimes turn when it encounters a mountain range, but the surprise here is when and exactly why this happened. Again a pleasing collaboration of researchers, this time from the Russian Academy of Sciences, West Virginia University, Stanford University and the University of Arizona, have read the stories that zircons tell about the Lena, a very old river. Three hundred and fifty million years ago, where the Verkhoyansk Mountains are today was the northeastern edge of the old Siberian continent, and rivers flowed to the sea (red arrows), depositing - and depositing and depositing - huge thicknesses of sediment along the continental edge. And then, over the northeastern horizon, appeared another continental fragment, being driven inexorably into Siberia. The great collision occurred, welding together eastern Russia as we now know it, bulldozing up the pile of sediment, and raising the Verkhoyansk Mountains. The bulldozed pile of sand, deposited over a period of 200 million years, is up to 15 kilometers thick - and contains plenty of zircons that record the erosion of the old Central Asian highlands to the southwest. The zircon records show that the rivers, including the ancestor of the Lena, harvested an unchanging selection of eroding rocks from those highlands for that 200 million year period. The Lena changed its course 150 million years ago because it encountered the growing mountainous terrain of a plate collision. Of course today's Lena is the descendant of its ancestral river system and has changed its precise location and itinerary - but the zircons show that it is the latest member of a family of rivers that lasted for 200 million years and, in its upper reaches, for the last 350 million years. As the paper states, "Thus, the life span of major transcontinental drainage systems can be comparable to that of the plate boundaries that surround them." Tiny crystals telling huge stories.

[R.J.Stewart et al., Brahmaputra sediment flux dominated by highly localized rapid erosion from the easternmost Himalaya, Geology 2008;36;711-714; A.V. Prokopiev et al., The paleo-Lena River -- 200 m.y. of transcontinental zircon transport in Siberia, Geology 2008;36;699-702. For accounts of the Jack Hills zircons, see http://www.geology.wisc.edu/zircon/Valley2005SciAm.pdf, http://www.geology.wisc.edu/zircon/Cavosie2004.PR.pdf, (source of the zoned zircon image)and http://serc.carleton.edu/NAGTWorkshops/earlyearth/teaching_refs.html (source of the photos of zircon grains, Aaron Cavosie, Image credit: Aaron Cavosie, University of Puerto Rico, Mayaguez). Satellite image http://visibleearth.nasa.gov/view_rec.php?id=2680. For the immensely complex and fascinating topic of sediment flux, Brian, at Clastic Detritus, has a provocative post at http://clasticdetritus.com/category/sediment-flux/]

 

Comments

[Callan Bentley]

Excellent post, as usual!

[Jules]

Amazing complex information from tiny grains of rock. Even the most mundane and miniscule has a story to tell us when we "dig" deep enough!

[BrianR]

Nice stuff ... I have a paper in review right now about detrital zircons from the Magallanes Basin in Patagonia ... these little grains are indeed incredibly useful for reconstructing the ancient Earth.