Fossils from the mantle

Researchers find two billion years old remnants of enamel formation in the Earth's mantle

Thin section of a peridotite rock. This rock sample is extraordinary because the minerals are completely fresh, as they occur in the mantle, are preserved. Usually, such mantle peridotites are highly converted. © MPI for Chemistry
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Revolutions of the mantle promote the deep Earth's interior to the surface and constantly generate new ocean floor to the mid-ocean ridge. This produces volcanic melts, which almost always produce a basalt layer several kilometers thick and cover the mantle rocks. Researchers now report in Nature that they have unearthed fresh rock samples directly from the Earth's mantle on the Arctic Ocean Ridge, where basalt cover is absent over long stretches. Using isotope measurements, they were able to show that these rocks were melted two billion years ago.

The Gakkel Ridge extends under the Arctic Sea over 1, 800 kilometers from northern Greenland to Siberia. It is the northernmost spur of the mid-ocean ridge system, that vast 75, 000-kilometer-long volcanic mountain range below the sea, where rising sea-magma creates new seabed, oceanic crust. The Gakkel Ridge is of particular interest to geoscientists because at one centimeter per year it is the slowest expanding ocean back on earth, 20 times slower than, for example, the much better researched East Pacific Ridge.

Explored interface of continental shift

In order to explore this interface of continental drift in the Arctic Ocean and its formation, scientists from the Max Planck Institute for Chemistry participated in two international expeditions with the German icebreaker "FS Polarstern". They were able to collect rock samples that came directly from the upper part of the mantle.

The largest part of the mantle consists of the stone peridotite. The collected rock samples were investigated by scientists in Mainz on the basis of very accurate measurements of isotope abundances of natural radioactive decay systems. Natural decay of the radioactive element rhenium to an isotope of the element osmium continuously changes the isotopic composition of osmium. When a peridotite rock of the earth's mantle melts, the mother element rhenium is extracted from the rock with the melt, so that the isotope abundance of osmium is frozen in the mantle at the time of melt formation.

"Fossil" osmium

Mainz found such "fossil" osmium in the exceptionally well-preserved rock samples of the Arctic seabed, and they calculated that smelting and depletion must have occurred about two billion years ago. Strangely enough, no such fossil indication has been found in today's smelting products of such rocks of the mantle, the normal ocean bottom basalts

Osmium.

Mainz geochemists conclude that the fossil mantle rocks have lost their fusible constituents already two billion years ago and therefore would no longer produce basalts today. Either way, the peridotites of the Arctic ocean floor have opened a new window on the development history of the mantle.

(idw - Max Planck Institute for Chemistry, 20.03.2008 - DLO)