[meteorite-list] Meteorites Illuminate Mystery of Chromium in Earth's Core

From: JoshuaTreeMuseum <joshuatreemuseum_at_meteoritecentral.com>
Date: Sun, 27 Feb 2011 15:14:48 -0500
Message-ID: <A6237EF5ED434605AF1F6BF3EBCA747A_at_ET>

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 http://www.universetoday.com/83452/meteorites-illuminate-mystery-of-chromium-in-earths-core/----------------------------------Meteorites Illuminate Mystery of Chromium in Earth's Coreby Anne Minard on February 24, 2011It's generally assumed that the Earth's overall composition is similar tothat of chondritic meteorites, the primitive, undifferentiated buildingblocks of the solar system. But a new study in Science Express led byFrederic Moynier, of the University of California at Davis, seems to suggestthat Earth is a bit of an oddball.Thin section of a chondritic meteorite. Credit: NASAMoynier and his colleagues analyzed the isotope signature of chromium in avariety of meteorites, and found that it differed from chromium's signaturein the mantle."We show through high-precision measurements of Cr stable isotopes in arange of meteorites, which deviate by up to ~0.4? from the bulk silicateEarth, that Cr depletion resulted from its partitioning into Earth's corewith a preferenti
 al enrichment in light isotopes," the authors write."Ab-initio calculations suggest that the isotopic signature was establishedat mid-mantle magma ocean depth as Earth accreted planetary embryos andprogressively became more oxidized."Chromium's origins. New evidence suggests that, in the early solar nebula(A), chromium isotopes were divided into two components, one containinglight isotopes, the other heavy isotopes. In the early Earth (B), thesecomponents formed a homogeneous mixture. During core partitioning (C), thecore became enriched with lighter chromium isotopes, and the mantle withheavier isotopes. Courtesy of Science/AAASThe results point to a process known as "core partitioning," rather than analternative process involving the volatilization of certain chromiumisotopes so that they would have escaped from the Earth's mantle. Corepartitioning took place early on Earth at high temperatures, when the coreseparated from the silicate earth, leaving the core with a distin
 ctcomposition that is enriched with lighter chromium isotopes, notes WilliamMcDonough, from the University of Maryland at College Park, in anaccompanying Perspective piece.McDonough writes that chromium, Earth's 10th most abundant element, is namedfor the Greek word for color and "adds green to emeralds, red to rubies,brilliance to plated metals, and corrosion-proof quality to stainlesssteels." It is distributed roughly equally throughout the planet.He says the new result "adds another investigative tool for understandingand documenting past and present planetary processes. For the cosmochemistryand meteoritics communities, the findings further bolster the view that thesolar nebula was a heterogeneous mixture of different components."Source: Science. The McDonough paper will be published online today by thejournal Science, at the Science Express website.----------------------------------Two questions: How do they know what kind of chormium is in the Earth's coreand is 0.4% a
  statistically significant number?Phil Whitmer
Received on Sun 27 Feb 2011 03:14:48 PM PST