A new way to date old rocks

January 2020

Over time, uranium decays into lead. Because the rate at which this decay takes place is known with great precision, uranium–lead dating (usually abbreviated by scientists to U–Pb dating), is the oldest and the most commonly used method for dating rocks and minerals.

(Actually U-Pb is two different types of dating involving separate decay chains. The uranium series from 238U to 206Pb, with has a half-life of 4.47 billion years, while the actinium series from 235U to 207Pb with its half-life of 710 million years is useful with - relatively - juvenile rocks.)

U-Pb is particularly useful in dating zircon since newly-formed zircon crystals start with no lead content. Therefore any lead accumulation in zircon crystals is due to the conversion of U to Pb. Since zircon often crops up with other minerals, if you can find some zircon, you can date the rock.

But what do you do if the rocks or minerals you want to date have no zircon and very little uranium of their own? How for example do you date the iron deposits of the eastern Awulale metallogenetic belt in Central Asia? The iron ore from this area is the most important source of high-grade iron in China, and finding out when and how these deposits were formed may be instrumental in locating new deposits for China's fast-growing economy.

The iron ore is contained alongside skarns - hard, coarse-grained metamorphic rocks- which are widespread around those iron-ore deposits. The skarns are rich in alcium-magnesium-iron-manganese-aluminium silicate minerals and are closely associated with iron mineralization. Accurately dating the ages and origins of the skarms has proven challenging. However, while the skarns don't have much in the way of uranium of zircon, they do contain garnets.

Recently a group of researchers from the University of Queensland took samples of garnet from skarns and dated the that garnet. They managed this by using laser ablation coupled with a mass spectrometer (ICP-MS) that measures isotopes at a very high speed as they are revealed by the laser ablation. Laser ablation imaging is a relatively new analytical technique that can visualize the distribution of elements of interest within solid samples in two or even three dimensions. It has been used successfully for a number of geological projects including magma elemental mapping (ref 2) and analysing the composition of gold samples (ref 3)

The advantage of dating rocks through garnet is that garnets are very common. They can be found in metamorphic and some igneous rocks. Indeed. garnet is not a single mineral but rather a family of silicates with a general formula X3Y2Si3O12 where X is usually either Ca, Mn, Mg or Fe²⁺ and Y is Al,Cr or Fe³⁺. Simple chemical substitutions are common in different garnet deposits.

"Garnet, which looks a little like the seeds of a pomegranate, is a challenging mineral to date but very common in volcanic mineral systems, ... It forms when hot magma enters the subsurface of the Earth's crust and it gives us clues about other magmatic formations in the same area. By understanding exactly when certain formations have occurred, it's much easier to pin down mineral-rich deposits," explained Professor Zhao.

The samples were dated using the Ma timescale (Ma means mega-annum, as in millions of years before the present). The nine skarn samples collected from the Chinese deposits fell into three age groups. These were 329-326 Ma (2 samples), 316-311 Ma (6 samples) and 295 Ma (1 sample), suggesting that over time there were three episodes of skarn alteration in these iron deposits.

The authors believe that the first and the second episodes of skarn formation occurred by contact metasomatism (the process of altering the composition of a rock, either by the addition or subtraction of chemical elements) between volcanic rocks and limestone in the same volcanic rocks. The third episode probably resulted from collision with granitic intrusions.

Finding new mineral rich deposits is always challenging, but the authors believe that understanding exactly when certain formations occurred will help with the future discovery of further deposits.

Journal Reference:

1. Shuang Yan, Renjie Zhou, He-Cai Niu, Yue-xing Feng, Ai Duc Nguyen, Zhen-hua Zhao, Wu-Bin Yang, Qian Dong, Jian-xin Zhao. LA-MC-ICP-MS U-Pb dating of low-U garnets reveals multiple episodes of skarn formation in the volcanic-hosted iron mineralization system, Awulale belt, Central Asia. GSA Bulletin (2019); LA-MC-ICP-MS-U-Pb-dating-of-low-U-garnets-reveals 2. Ubide, T.; Caulfield, J.; Brandt, C.; Bussweiler, Y.; Mollo, S.; Di Stefano, F.; Nazzari, M.; Scarlato, P. Deep Magma Storage Revealed by Multi-Method Elemental Mapping of Clinopyroxene Megacrysts at Stromboli Volcano. Frontiers in Earth Science (2019) https://www.frontiersin.org/articles/10.3389/feart.2019.00239/full 3. Olga Borovinskaya, David Banks, Robert Chapman. Fast Multi-Element LA-icpTOF Mapping of Gold Grains: Application Note. (2017) https://www.tofwerk.com/la-icp-ms_gold_mapping/