A 2-billion-year window into the Earth
An international team of researchers has discovered 2 billion-year-old unaltered rocks on the ocean floor during two North Pole expeditions. Their findings led them to conclude that the Earth' mantle history is more complex than previously thought. The researchers were astonished to find rocks that had not been remixed inside the mantle for two billion years. The rocks found beneath Gakkel Ridge, a 1,000-mile-long underwater mountain range between Greenland and Siberia, were precisely dated because of the presence of isotopes of osmium. As said the lead researcher, 'I just about fell off my chair. We can't exaggerate how important these rocks are -- they're a window into that deep part of the Earth.' But read more...
You can see above a beautiful section of one of these 2 billion-year-old peridotite rocks. (Credit: Max-Planck-Gesellschaft news release, Germany, March 20, 2008) Here is a link to this news release -- in German -- "Uraltes aus dem Erdmantel" (PDF format, 2 pages, 200 KB). And if you want to know more about these rocks, here is another link to peridotite provided by Wikipedia.
According to the UH news release, this research work has been led by Jonathan Snow, assistant professor in the Geosciences Department of the UH. But he was not alone. He was helped by other scientists from China, Germany and other U.S. researchers in Hawaii. By the way, here is a link about other pictures of Jonathan Snow discovering ancient mantle rocks.
Now, what have these researchers found during their two trips to the North Pole? Unaltered rocks. "Venturing out aboard a 400-foot-long research icebreaker, Snow and his team sifted through thousands of pounds of rocks scooped up from the ocean floor by the ship’s dredging device. The samples were labeled and cataloged and then cut into slices thinner than a human hair to be examined under a microscope. That is when Snow realized he found something that, for many geologists, is as rare and fascinating as moon rocks – mantle rocks devoid of sea floor alteration. Analysis of the isotopes of osmium, a noble metal rarer than platinum within the mantle rocks, indicated they were two billion years old. The use of osmium isotopes underscores the significance of the results, because using them for this type of analysis is still a new, innovative and difficult technique."
For more information, this research work has been published in Nature under the title "Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean" (Volume 452, Number 7185, Pages 311-316, March 30, 2008). You can see below one of the figures featured in the article. (Credit: All the authors/Nature)
Here is a caption for the above figure. "Both enriched and refractory mantle are distributed among the 'matrix' of normally depleted mantle. The enriched mantle and the normally depleted mantle can be sampled by E-MORB and N-MORB, respectively. However, it is hard to sample the refractory mantle with inherited ancient depletion signals because it contributes little to the genesis of MORB. Therefore, they are not a part of the 'MORB source' sensu stricto, although the latter has been generally taken to be synonymous with asthenospheric mantle." Please note that MORB stands for "Mid-ocean Ridge Basalts." The "E" prefix means "enriched" while the "N" one means "normal."
Here are two links to a larger version of this illustration and to the abstract of the paper, which starts like this. "The Earth's mantle beneath ocean ridges is widely thought to be depleted by previous melt extraction, but well homogenized by convective stirring. This inference of homogeneity has been complicated by the occurrence of portions enriched in incompatible elements. Here we show that some refractory abyssal peridotites from the ultraslow-spreading Gakkel ridge (Arctic Ocean) have very depleted 187Os/188Os ratios with model ages up to 2 billion years, implying the long-term preservation of refractory domains in the asthenospheric mantle rather than their erasure by mantle convection."
Sources: University of Houston news release, April 10, 2008; and various websites
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