Sunday, September 23, 2007

Geochemistry: Argon retention in terrestrial planets

The Earth’s mantle may not be as efficient as previously thought at expelling gases, such as argon, suggests a paper in Nature this week. The study challenges the assumption that most argon escapes into the atmosphere through partial melting when mantle rocks are brought near the Earth’s surface, and concludes that atmospheric argon may therefore not be such a reliable indicator of the vigour of planetary convection.

Stable noble gases such as xenon, helium and argon are generally used as tracers of mantle degassing. It has generally been thought that the Earth releases a large percentage of the gases contained in rocks during episodes of partial melting in the mantle, followed by ascent of the melt to the surface where the gas can move into the atmosphere. That is, scientists believe that noble gases behave as ‘incompatible’ elements during mantle melting — partitioning into a melt as soon as melting begins.

Bruce Watson and colleagues question this theory with data showing that argon doesn’t actually behave in this way. They find that argon is much more compatible in mantle minerals than previously thought, and that the diffusion of argon through mantle minerals is much slower. They conclude that an alternative to magmatism is needed to explain the abundance of argon in the atmosphere, pointing to the hydration of oceanic crust containing argon-rich minerals as a possible source.

Because the mantles of the other terrestrial planets are dominated by similar minerals, the authors note that their results will affect theories of degassing on Mars and Venus as well as Earth.

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