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Andesite rocks are blends of old and new volcanic material

29 October 2009, by Sara Coelho

Geology textbooks need a rewrite after findings published yesterday in Nature show that andesite rocks, erupted in arc-volcanoes around the world, do not come from pure molten rock – instead, they are blends of old crystal residues and silica-rich melts.

Mt St Helens

Andesites are the most common volcanic rock where oceanic plates slide beneath other tectonic plates - these areas are called subduction zones, and the volcanoes of the Andean Cordillera in South America are the classic example. Geologists tell volcanic rocks apart by their chemistry, and especially by the amount of silica molecules each rock type has. Andesites are notable for their intermediate silica composition, as opposed to silica-poor basalts and silica-rich granite-like rocks.

'In a sense, the silica-rich liquid cannibalises its ancestors to become an andesite.'
Professor Jon Blundy,
University of Bristol

Textbook geology says that andesite liquid - that is, molten rock with andesitic composition – forms when basalt is extracted from the Earth's mantle, settles temporarily in an underground chamber and acquires more silica as minerals crystallise as the liquid cools. So in a sense, andesite magmas which erupt in volcanoes across the world are thought to be the natural descendents of basalt parents.

Professor Jon Blundy from the University of Bristol and Dr Olivier Reubi from ETH Zurich in Switzerland challenge this view.

They argue that molten rock with a medium amount of silica is too rare to explain how andesites rocks form. Instead, they suggest that the andesite's distinct chemistry forms when silica-poor crystals from ancestral magmas mix with silica-rich liquids.

'Paradoxically, even though andesite is the dominant volcanic magma type to form above subduction zones, very few erupted andesites were ever true molten rock,' says Blundy.

The conclusion comes after a review of the chemical composition of hundreds of frozen droplets of liquid - called melt inclusions - trapped inside mineral crystals. Melt inclusions are small, little wider than a human hair, 'but can provide clues as to the composition of the molten volcanic rocks that exist beneath andesite volcanoes,' says Blundy.

The analysis of melt inclusions shows that 'the liquid sampled in the inclusions is either silica-poor or silica-rich,' he explains 'but rarely andesitic'.

One possible explanation for the mystery of the missing molten andesites is that the host crystals might be influencing the inclusion's composition with their own chemistry. 'But if this was the case, the composition would change according to the type of crystal in which the inclusions are found and that is not what we see,' says Blundy.

The most likely explanation is mixing - andesite magmas are a blend of silica-rich liquid with an old residue, left over when the magmas that previously occupied the chamber erupted.

Molten basalt rock comes out of the mantle and usually pond for a while at 30 to 40km deep, under the base of the continental crust. At this point, some crystals appear and the melt turns into a more silica-enriched magma before restarting its ascent. On its upward journey this silica-rich liquid encounters the crystalline residues of early magmas. The resulting blend is the andesite magma so characteristic of subduction zone volcanoes.

'Successive batches of liquid follows the same route and interact repeatedly with the residues of previous magmas. In a sense, the silica-rich liquid cannibalises its ancestors to become an andesite,' says Blundy.

Olivier Reubi and Jon Blundy. A dearth of intermediate melts at subduction zone volcanoes and the petrogenesis of arc andesites. Nature, 29 October 2009. doi:10.1038/nature08510.

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