Oxidized lava may help explain Earth's evolution

July 30, 2009 By RANDOLPH E. SCHMID , AP Science Writer
Lava pours out of Montserrat during an eruption. Image: University of Arkansas

(AP) -- Material from volcanoes where the Earth's plates squeeze together is more oxidized than in regions where the seafloor splits apart, a finding that helps shed light on some of the basic processes in the planet's mantle.

Using highly sensitive X-ray techniques researchers were able to measure the amount of reaction with oxygen that had occurred in minerals in various situations.

Oxidation, best known as rust when it affects metals, was low in materials erupting from mid-ocean ridges where the seafloor spreads apart, Katherine A. Kelley of the University of Rhode Island and Elizabeth Cottrell of the Smithsonian's National Museum of Natural History report in Friday's edition of the journal Science.

But higher rates were found in produced by arc volcanoes, which occur in areas where the Earth's collide, with one sliding below another, they found.

"The seafloor is kind of like a rust conveyor belt," Cottrell said in a telephone interview. As material moves over millions of years from the mid-ocean ridges to the subduction zones it becomes increasingly oxidized.

This indicates that what happens on the Earth's surface influences what goes on beneath, Cottrell said, and things that are happening in geology today probably have also done so in the past.

"This is a step in looking at the long-term evolution of the planet," she said.

The finding firmly establishes that subduction oxidizes the source of magma for arc volcanoes, "but the long-term consequences for the evolution of Earth remain poorly understood," commented Marc M. Hirschmann of the University of Minnesota, who was not part of the research team.

The researchers made their calculations by studying water trapped in tiny particles, Cottrell said, but that doesn't mean that the is causing the oxidation. While water causes rust in the atmosphere it is much less effective at doing that under high pressure such as on the .

The research was supported by the National Science Foundation and the Department of Energy.


On the net:

Science: http://www.sciencemag.org

©2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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1 / 5 (2) Jul 30, 2009

This is an interesting story.

As I recall the noble gas record in lavas from arc volcanoes is also distinct from that in MORBs (Mid-Ocean Ridge Basalts).

MORBs contain primordial He and Ne (leaking from the lower mantle) mixed with highly radiogenic Ar-40 from the decay of K-40, radiogenic Xe-129 from the decay of extinct I-129, and fissiogenic Xe-136 from the decay of U-238 and extinct Pu-244 in the upper, depleted mantle.

See: 1. "The xenon record of extinct radioactivities in the Earth," Science 174 (1971) 1334-1336 http://tinyurl.com/nnjh4v and
2. "The noble gas record of the terrestrial planets", Geochemical Journal 15 (1981) 247-267 http://tinyurl.com/2k8ds3

With kind regards,
Oliver K. Manuel
1 / 5 (1) Jul 31, 2009
Sea floor spreading is new crust. Subduction is old crust. Nothing new here.

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