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Secrets of the ocean floor

Researchers analysing volcanic rocks from the ocean floor, called basalts, have uncovered a pattern in their chemical composition that prompts a new understanding of how the crust on the ocean floor forms.

The findings, published today in Nature, are the result of the authors’ analytical program on a global range of basalt samples.

Lead researcher Professor Hugh O’Neill from the Research School of Earth Sciences in the ANU College of Physical and Mathematical Sciences said “Ocean-floor basalts are erupted along mid-ocean ridges where the earth’s tectonic plates are rifting apart. Hot mantle welling up into the gap begins to melt. The resulting magma rises to the surface, where some of it crystallizes in magma chambers and the rest erupts as basalt. This forms the new oceanic crust.

“I’ve long been interested in identifying the abundance of rare elements in the earth’s mantle because of what they can tell us about how the earth formed, and this was the reason why we analysed basalts from across the globe. These basalts should give us a good picture of the mantle’s composition because they are so widespread. But to get back to the composition of the original magma coming out of the mantle, first we have to allow for the effects of the crystallization.

“Looking at the basalts on a global scale gave us a surprise. We couldn’t account for the chemistry of the basalts if we assumed that they evolved solely by the process of fractional crystallization, as previously thought. The process is more complex than that.

“What actually happens is that the magma chambers, which sit on top of the mid-ocean ridges, are periodically replenished by new magma from below. The new magmas mixes with old magma, and it is this hybrid magma that then erupts. Then magma left behind in the chambers partly crystallizes as it cools, until the next batch of replenishing magma arrives. This occurs over and over again. One consequence is that many of the rare elements that are not removed by crystallization build up cycle after cycle.

“Our findings have widespread implications for everything to do with basalts, and every inference that’s been made about what they mean for the process of plate tectonics, the composition of the earth’s mantle and how partial melting of the mantle occurs. For one thing, the mantle appears to be poorer in many rare elements than previously thought, because what we see in the basalts are abundances built up over many cycles.

“This study is a really good example of looking for one thing and finding something completely different, which you weren’t thinking about. We set out to improve our understanding of how the Earth formed, and ended up discovering this fascinating insight into the process of plate tectonics”.

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