Plate Tectonics Coming of Age
The following news release was issued by the European Synchrotron Radiation Facility (ESRF), in Grenoble, France. Vincent De Andrade, at the U.S. Department of Energy’s Brookhaven National Laboratory, is a coauthor on the paper described in the release. An earth scientist, De Andrade joined Brookhaven Lab in 2010 to help develop SRX, a state-of-the-art spectroscopy beamline at Brookhaven Lab’s National Synchrotron Light Source II, now under construction. According to De Andrade, SRX will be able to provide high-resolution and high-precision chemical images of rocks, from which geodynamic models of the primitive Earth will be derived.
Plate tectonics in its current form is believed to have started one billion years ago. A study of two billion year old rocks from African gold mines has now shown that the same process of subduction we observe today as a by-product of these large-scale continental movements, was already present on Earth more than two billion years ago. Experiments with X-rays at the ESRF have contributed to this discovery which has been published on 20 November 2011 in Nature Geoscience.
The study was performed by an international team of scientists led by J. Ganne of the University of Toulouse and included scientists from Brookhaven National Laboratory (BNL), Monash University in Melbourne, the Universities of Cambridge, Grenoble, Lausanne and Ouagadougou, and the ESRF.
Some fifty years ago, plate tectonics, motions of large parts of the Earth's surface against each other, became a generally accepted theory to explain continental drift, along with many volcanic and earthquake zones, and seafloor spreading. The exact origin of the forces driving these large-scale movements of the continental plates is still a matter of scientific debate.
Today, some eight large plates move against each other and experience so-called plate subduction at their boundaries. In these subduction zones, one tectonic plate moves under another, lowering itself into the Earth's mantle. This is a slow process at a rate of a few centimetres per year. The shape of the continents suggests that 250 million years ago, the Earth’s land masses were united in a single continent, called Pangaea, from which today’s plates started to move away.
The high pressure and temperature in subduction zones are the main drivers for chemical elements to accumulate in high concentration in an ore. Metal ore mines therefore are remnants of past subduction events, even if today they are located far away from a plate boundary. The exact composition of the minerals constituting an ore is, like a fingerprint, representative of the pressures and temperatures experienced when the plate sank into the Earth’s mantle. Today, scientists can deduce these values, and their evolution in time, from the crystalline and elemental composition of a given ore.
For this study, the scientists investigated ore mineral samples aged between 2 and 2.2 billion years collected in West Africa in an area rich in gold mines. Using an electron microprobe, they established detailed maps of the spatial distribution of major chemical elements in the samples, notably of iron. However, the iron oxidation state (Fe2+ or Fe3+) cannot be measured with electron microprobes but can vary inside ore-bearing minerals. Thanks to the use of X-ray absorption near-edge spectroscopy (XANES) with a submicrometric beam at ESRF beamline ID21, important variations of the Fe3+ content in the minerals were shown, which had major repercussions for the pressure and temperature calculations.