-Press Release Number: PR-157-ANU-08-1
-Source: The Australian National University
-Date issued: October 23, 2008
-Contact: +61 2 6125 6125, media@anu.edu.au

Cooling Earth revealed in ancient magma

Scientists have discovered new evidence that the interior of the Earth has cooled considerably over the last three billion years, leading to fresh questions about whether plate tectonic movement is only a relatively recent phenomenon.

The international research team used synchrotron technology to study the chemical composition of komatiites – ancient volcanic rocks thrown up from the planet’s mantle.

Komatiites have long intrigued geologists because these magmas must have been created by unusually high degrees of melting of the mantle. The examples studied by the team are 2.7 billion years old, and were found in present-day Zimbabwe. Similar magmas are not produced on Earth today.

“Because the rocks are so old, they have been subject to alteration from erosion, weathering and metamorphism,” explains Dr Hugh O’Neill from the Research School of Earth Sciences at ANU. “But there are tiny drops of ancient magma trapped inside crystals in the komatiites that are protected from alteration, and by studying these we’ve been able to get a sense of what was going on inside the planet in its infancy.”

For years scientists have argued about whether or not komatiites formed at a time when the planet’s mantle was up to 500 degrees hotter than at present, or if the mantle was only marginally hotter than it is currently, and the unusual composition of komatiites was due to the presence of water. Water lowers the temperature at which rocks melt by many hundreds of degrees.

Dr O’Neill and his colleagues from Imperial College in London, the University of Tasmania and the University of Chicago found that there was no evidence of oxidation inside the melt inclusions, which means that the small amount of water trapped in the inclusions was likely all that ever existed at the time of the rocks’ formation.

The lack of water means that the mantle must have been much hotter billions of years ago than it is today, leading the researchers to conclude that the planet has cooled markedly.

“This might mean that we can’t assume that plate tectonics and continental drift occurred in the distant past, because there could have been a different mechanism needed to cool a hotter Earth,” Dr O’Neill said.

Dr O’Neill said the latest komatiite research was possible thanks to synchrotron technology, similar to the Australian Synchrotron facility that recently opened in Victoria.

Synchrotrons are giant rings around which electrons are fired to produce intense x-rays. Such a facility was used to reveal the elemental spectra of tiny melt inclusions, only one-twentieth of a millimetre wide, inside olivine crystals in the komatiites.

“With this research we’ve shown that synchrotron technology means we can now learn more about the chemical composition of very tiny samples. This is very promising for the future of this kind of research in Australia.”

The findings were published in the latest edition of the journal Nature.

###

The original news release can be found at: http://news.anu.edu.au/?p=778.

Also:
21st Century detective work reveals how ancient rock got off to a hot start (Imperial College London)
21st Century detective work reveals how ancient rock got off to a hot start (Argonne National Laboratory).

[Note: Published in Nature, 455, 960-963 (16 October 2008), doi:10.1038/nature07377; Andrew J. Berry, Leonid V. Danyushevsky, Hugh St C. O'Neill, Matt Newville & Stephen R. Sutton; "Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle".

Iron K-edge XANES spectra were recorded at the Advanced Photon Source (Argonne National Laboratory), beamline 13-ID-C (GSECARS). Argonne National Laboratory is funded by the U.S. Department of Energy's Office of Science. The authors also acknowledge the Australian National Beamline Facility (Photon Factory) for assistance with XANES experiments that provided the foundations for the present study.

The contents of all materials on lightsources.org are the sole responsibility of the authors of the materials and/or the facilities or institutions under whose auspices the materials were produced. -lightsources.org]

Share this page:

Email this page