Oxygen isotopic constraints on the origin of Mg-rich olivines from chondritic meteorites |
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| Authors: | Guy Libourel Marc Chaussidon |
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| Institution: | 1. Nancy-Université, CRPG-CNRS, 15 rue Notre-Dame-des-Pauvres, B.P. 20, F-54501 Vand?uvre lès Nancy, France;2. Nancy-Université, ENSG, Rue du Doyen Marcel Roubault, B.P. 20, F-54501 Vandoeuvre-lès-Nancy, France;1. Institute of Physics and Technology, Ural Federal University, Ekaterinburg, 620002, Russian Federation;2. Racah Institute of Physics, The Hebrew University, Jerusalem, Israel;3. Institute of Material Science and Metallurgy, Ural Federal University, Ekaterinburg, 620002, Russian Federation;1. Department of Geology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA;2. Université de Paris, Institut de physique du globe de Paris, CNRS, F-75005 Paris, France;1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;2. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;3. Department of Geoscience, University of Nevada, Las Vegas, NV 89154, USA;4. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China;1. CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China;2. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, The Chinese Academy of Sciences, Guangzhou 510640, China;3. University of Chinese Academy of Sciences, Beijing 100049, China |
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| Abstract: | Chondrules are the major high temperature components of chondritic meteorites which accreted a few millions years after the oldest solids of the solar system, the calcium–aluminum-rich inclusions, were condensed from the nebula gas. Chondrules formed during brief heating events by incomplete melting of solid dust precursors in the protoplanetary disk. Petrographic, compositional and isotopic arguments allowed the identification of metal-bearing Mg-rich olivine aggregates among the precursors of magnesian type I chondrules. Two very different settings can be considered for the formation of these Mg-rich olivines: either a nebular setting corresponding mostly to condensation–evaporation processes in the nebular gas or a planetary setting corresponding mostly to differentiation processes in a planetesimal. An ion microprobe survey of Mg-rich olivines of a set of type I chondrules and isolated olivines from unequilibrated ordinary chondrites and carbonaceous chondrites revealed the existence of several modes in the distribution of the ?17O values and the presence of a large range of mass fractionation (several ‰) within each mode. The chemistry and the oxygen isotopic compositions indicate that Mg-rich olivines are unlikely to be of nebular origin (i.e., solar nebula condensates) but are more likely debris of broken differentiated planetesimals (each of them being characterized by a given ?17O). Mg-rich olivines could have crystallized from magma ocean-like environments on partially molten planetesimals undergoing metal–silicate differentiation processes. Considering the very old age of chondrules, Mg-rich olivine grains or aggregates might be considered as millimeter-sized fragments from disrupted first-generation differentiated planetesimals. Finally, the finding of only a small number of discrete ?17O modes for Mg-rich olivines grains or aggregates in a given chondrite suggests that these shattered fragments have not been efficiently mixed in the disk and/or that chondrite formation occurred in the first vicinity of the breakup of these planetary bodies. |
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