News in Science

Rock shows earth got off to a hot start

A geological controversy over how a 2700-million-year-old rock was formed has been solved using synchrotron technology, an international team reports.

The team reports in the latest Nature journal that a rare form of magmatic rock known as komatiite was formed in the earth's mantle at temperatures around 1700°C in the Archaean age, more than 2700 million years ago.

Australian co-author Associate Professor Leonid Danyushevsky, at the Australian Research Council Centre of Excellence in Ore Deposits at the University of Tasmania, says the finding settles a long-disputed controversy over the volcanic rocks origin.

Danyushevsky says there are two opposing theories on the origin of komatiites, which were generally formed in the first half of the earth's history.

The first "hot-melting" model assumes the komatiite was formed through the melting of a mantle source at temperatures up to 500°C hotter than today's mantle temperature.

Under the second "wet-melting" theory komatiites were formed at temperatures only slightly higher than today's mantle temperature and cooled by the presence of water.

Well-preserved sample

Danyushevsky says a difficulty in solving the debate has been the rarity of well-preserved samples of komatiite.

However, in 1992 fresh komatiite lava flows were found in Zimbabwe that had inclusions of the original melt preserved in crystals.

He says a new technique, using synchrotron x-ray technology, developed by co-authors Professor Hugh O'Neill of the Research School of Earth Sciences at the Australian National University and Dr Andrew Berry at Imperial College, London, has allowed them to look at the chemistry of the volcanic rock.

He says by examining oxidation rates they have shown the mantle has cooled several hundred degrees since the Archaean.

Danyushevsky says the knowledge is important for "our understanding of the earth's evolution".

It could help determine the rate at which the earth's interior has been cooling, how the forces affecting the earth's crust have changed over time, and the heating produced by radioactive decay, the researchers say.

O'Neill says by looking at the water content and oxidation ratio of the komatiite samples they were able to discount the wet-melting theory.

He says the water content of 0.3% per weight in the inclusion "was almost certainly the original water content".

Using the synchrotron x-ray technique the team could determine the oxidation ratio in situ in the sample.

It found about 10% oxidation in the 2700-million-year-old sample, a ratio that is "similar to that of present-day mantle", the paper says.

O'Neill says their approach will have a "flow-on effect for a whole suite" of things including minerals exploration.