Monday, February 11, 2008

Hawaiian lava--down to the core?

It's the one of the sad truths about science that someone develops a theory of why a thing occurs, and then someone else rejects the theory based on new information.

It's good for science, but can be tough on the person who put a lot of time and effort into the original theory.

Recently, there's been a controversy over the source of the Hawaiian Islands.

There is general consensus that the molten rock for the Hawaiian volcanoes comes from a hot spot that drives deep in the Earth.

But how deep?

At the Earth's surface is the crust, which is a few miles to a few dozen miles thick, depending on where you're checking. The mantle extends roughly another 2,000 miles down, and the core 2,000 miles more, until you reach the legendary “Center of the Earth.” (It would be hard to reach that center, since it's believed to be in the middle of an immense solid iron sphere.)

At the least, the plume of magma that forms the Hawaiian hot spot goes down into the planet's mantle.

The elements that make up different parts of the planet's interior come in different mixtures, and some folks have suggested that the detection of certain chemical signatures in Hawaiian lava indicate the plume may extend all the way down to the boundary between the mantle and the core.

One of the elements on which these calculations are made is Osmium, a hard, heavy metal often found in conjunction with iron—the main component of the planet's core.

The theory: If there is a fair amount of Osmium in certain proportions in Hawaiian lavas, it must be coming from somewhere near the planet's core, and this suggests that the plume reaches all the way down to the edge of the core.

Tossing a little cold water on that hot topic, an international team of researchers found that it's possible to get that proportion of Osmium isotopes from within the upper mantle—that you don't need to go all the way down to the core to get it.

The scientists, whose work was published in the journal Science, include Ambre Luguet, Graham Pearson, Geoff Nowell, Scott Dreher, Judith Coggon and Stephen Parman of the Northern Centre for Isotopic and Elemental Tracing at the University of Durham in the United Kingdom, and Zdislav Spetsius of the Russian Yakutian Research and Design Institute of Diamond Mining Industry, a part of the ALROSA Joint-Stock Company.

But they concede that the idea of using lavas to try to prove where the hot spot originates is cool.

“The possibility of observing the chemical signature of core-mantle interaction in magmas erupted at the Earth's surface is one of the most exciting prospects in mantle geochemistry,” they write, in their article, “Enriched Pt-Re-Os Isotope Systematics in Plume Lavas Explained by Metasomatic Sulfides.”

But their research shows it is possible for the melting of existing rocks within the Earth's upper mantle to produce the same Osmium chemical signatures found in Hawaiian lavas on Hualalai, the submarine volcano Loihi and Mauna Loa.

It doesn't mean that the Hawaiian plume isn't reaching down to the interface between mantle and core, but just that this particular bit of Osmium-based information isn't sufficient to prove it.

The Osmium signature “observed in plume-related lavas can have an upper-mantle origin,” the writers say. And as a result, it “cannot be taken to be a unique signature of core-mantle interaction.”

As often happens in science, a theory bites the dust, but provides ample ground for new scientific inquiry.

© 2007 Jan W. TenBruggencate