Diffusive infiltration may explain strange behavior in magma

March 02, 2000

CHAMPAIGN, Ill. -- Recent experiments by a University of Illinois researcher have shed light on how glassy materials - melts that have been quickly frozen - are formed in exotic chunks of mantle called xenoliths, and how ascending magmas in the mantle can affect the lava output at Earth¹s surface through chemical, rather than thermal, reactions.

"Sodium in ascending magma can quickly diffuse into the surrounding mantle at lower pressures, fundamentally altering the process by which the mantle melts," said Craig Lundstrom, a UI professor of geology. "Sodium infiltration can account for the creation of silica-rich glasses in xenoliths, and for the anomalous mineralogical composition of mantle found beneath mid-ocean ridges."

Xenoliths are pieces of mantle that get ripped off and are carried to the surface during a volcanic eruption. Within the xenoliths are former melts - now found as glasses - which differ radically from the magma that typically emanates from the mantle, basalt. The glasses reside between the two primary silicate minerals of the uppermost mantle, olivine and orthopyroxene.

"Xenoliths have been found in alkali-rich magmas from ocean island and continental volcanic settings worldwide," Lundstrom said. "But the origin of the glassy regions, and why they have peculiar elemental compositions, have been poorly understood."

To study the potential chemical interaction, Lundstrom first synthesized samples of magma found deep in the mantle and of mantle lying much closer to the surface in a high-temperature, high-pressure apparatus. He then placed these samples together in the same apparatus and "cooked" them for 10 minutes. After cooling the samples, he separated them and analyzed their mineralogical compositions.

"We generally think of magmas as moving very slowly and geological processes as occurring over hundreds of thousands of years, so it¹s really amazing how much happens in just 10 minutes," Lundstrom said. "In that brief time, considerable migration occurred as sodium diffused through the melt between mineral grains, significantly altering the composition of the material."

In a positive-feedback loop, the infiltrating sodium causes orthopyroxene to break down into olivine plus the peculiar composition melt. This melt, in turn, causes even more sodium to be pulled in. The increase in olivine and the decrease in orthopyroxene within the diffusively infiltrated piece of mantle may explain another perplexing observation: the anomalous ratio of these minerals found in the shallow mantle region beneath mid-ocean ridges.

"We tend to think of volcanic edifices - like the Hawaiian Islands - as resulting from hot spots in the mantle," Lundstrom said. "But these results show that sodium-rich, silica-poor magma can cause the surrounding mantle to melt through a chemical reaction without invoking huge quantities of heat." The diffusion process could also be important to melting the cold lithospheric "plates" that cover the earth, said Lundstrom, who reported his findings in the Feb. 3 issue of the journal Nature.
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University of Illinois at Urbana-Champaign

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