A Voyage From The Earth’s Crust To Its Mantle And Back Again b4in.org/hEgM Uranium isotopes leave a distinct ‘fingerprint’ in the sources of volcanic rocks, making it possible to gauge their age and origin. Geologists have gained a new understanding of how the Earth’s crust is recycled back into its interior based on these uranium isotopes. From the beginning of time, uranium has been part of the Earth and, thanks to its long-lived radioactivity, it has proven ideal to date geological processes and deduce Earth’s evolution. Natural uranium consists of two long-lived isotopes uranium-238 and the lighter uranium-235. A new study of the global cycle of these uranium isotopes brings additional perspectives to the debate on how the Earth has changed over billions of years as revealed in a recently published study in the journal Nature. A numerical simulation shows how Earth’s crust (blue) is subducted and transported into the mantle (orange). From early Earth history, the continental crust (the Earth’s thick solid outer skin that we live on) has accumulated mass from the underlying hot mantle. Most of the newly formed crust, however, is lost again. At mid-ocean ridges at the bottom ocean, where plates drift apart, new oceanic crust is constantly produced as basaltic rocks when hot volcanic lava emerges from the mantle and solidifies. The oceanic crust moves away from the mid-ocean-ridges and ultimately gets transported back into the underlying mantle through “subduction” at ocean trenches. Uranium is enriched in the rocks of the continental crust; however, at Earth’s surface, different environments over time have influenced its mobility. In an oxygen-free atmosphere, as prevailed on early Earth, uranium stayed immobile in rocks as tetravalent uranium (IV). Only after atmospheric oxygen was formed did uranium become oxidised to its mobile hexavalent uranium (VI). This more mobile uranium may then be released during the weathering and break-down of rocks and transported to the oceans in aqueous form. As the cooling oceanic crust moves away from the mid-ocean-ridges in the oceans, seawater eventually percolates through cracks in its rock and in the process uranium gets incorporated into the oceanic crust, in a similar way that a sponge takes up water. “The radioactive nature of uranium isotopes has long been key in reconstructing early Earth history, but we now see that they also have another story to tell” explains Morten Andersen, a geochemist in the Department of Earth Sciences at ETH Zurich. More beforeitsnews/science-and-technology/2015/01/a-voyage-from-the-earths-crust-to-its-mantle-and-back-again-2743766.html
Posted on: Tue, 20 Jan 2015 18:17:46 +0000
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