Redox Conditions in the Solar Nebula: Observational, Experimental, and Theoretical Constraints

Research Article| January 01, 2008 Redox Conditions in the Solar Nebula: Observational, Experimental, and Theoretical Constraints Lawrence Grossman; Lawrence Grossman Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago Chicago, Illinois 60637, U.S.A., yosi@midway.uchicago.edu Search for other works by this author on: GSW Google Scholar John R. Beckett; John R. Beckett Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A. Search for other works by this author on: GSW Google Scholar Alexei V. Fedkin; Alexei V. Fedkin Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois 60637, U.S.A. Search for other works by this author on: GSW Google Scholar Steven B. Simon; Steven B. Simon Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois 60637, U.S.A. Search for other works by this author on: GSW Google Scholar Fred J. Ciesla Fred J. Ciesla Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. 20015-1305, U.S.A. Search for other works by this author on: GSW Google Scholar Author and Article Information Lawrence Grossman Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago Chicago, Illinois 60637, U.S.A., yosi@midway.uchicago.edu John R. Beckett Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A. Alexei V. Fedkin Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois 60637, U.S.A. Steven B. Simon Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois 60637, U.S.A. Fred J. Ciesla Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. 20015-1305, U.S.A. Publisher: Mineralogical Society of America First Online: 03 Mar 2017 © The Mineralogical Society Of America Reviews in Mineralogy and Geochemistry (2008) 68 (1): 93–140. https://doi.org/10.2138/rmg.2008.68.7 Article history First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Lawrence Grossman, John R. Beckett, Alexei V. Fedkin, Steven B. Simon, Fred J. Ciesla; Redox Conditions in the Solar Nebula: Observational, Experimental, and Theoretical Constraints. Reviews in Mineralogy and Geochemistry 2008;; 68 (1): 93–140. doi: https://doi.org/10.2138/rmg.2008.68.7 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search Abstract Crystallization experiments on liquids with compositions similar to those of compact Type A, Type B1 and Type B2 refractory inclusions were conducted under controlled temperature and fO2 conditions. Application of the results to the compositions of coexisting Ti3+ -bearing fassaitic clinopyroxene + melilite pairs in natural inclusions shows that, if they crystallized at ~1509 K, they did so at log fO2 = −19.8 ± 0.9, only slightly below the equilibrium log fO2 of a partially condensed system of solar composition at the same temperature, −18.1−0.3+0.2, or IW-6.8. Fassaite is the only fO2 indicator that shows that anything in chondrites formed in a system that was close to solar in composition. Solar composition is so reducing that equilibrium calculations predict vanishingly small FeO/(FeO + MgO) ratios in the condensate until temperatures fall below 800 K, where significant oxidation of metallic iron and formation of fayalite in solid solution with previously condensed forsterite begin. The mechanism for the latter process is diffusion of Fe2+ through forsterite, but the diffusion rate is nearly zero at these temperatures. By comparison to what is achievable in a system of solar composition, the mean FeO/(FeO + MgO) ratio of the olivine in chondrules in unequilibrated ordinary chondrites (UOCs) is very high, ~0.15. Making such ratios in chondrule precursors by solar nebular processes requires sufficiently high fO2 for iron to become oxidized above temperatures where diffusion of Fe2+ becomes very slow. Two dynamic models for enrichment of oxygen relative to carbon and hydrogen were investigated quantitatively: radial transport of water ice-rich migrators across the snow line into the inner part of the solar nebula where the ice evaporates; and coagulation, vertical settling and evaporation of anhydrous dust in the median plane of the inner nebula. In both cases, the maximum achievable fO2, ~IW-4.5, produces a maximum XFa before diffusion ceases that is a factor of >7 less than would be required for UOC chondrule precursors, even for grains only 0.1 μm in radius and nebular cooling times as high as 106 yr. The same dynamic models are also incapable of creating environments sufficiently oxidizing to produce olivine with XFa = 0.15 during formation of chondrules by melting of FeO-poor precursors. If, instead, chondrule precursors were made of very FeO-rich, non-equilibrium condensates, reduction of chondrule melts by nebular gas may have been arrested before the mean XFa of chondrule olivine could fall below 0.15 because chondrules were hot for such a short time. A nebular origin for the mineral assemblage of unequilibrated enstatite chondrites (UECs) requires fO2 significantly below that of a system of solar composition. In particular, after fractionation of specific amounts of predicted high-temperature condensates, equilibrium condensation in a system whose Ptot = 10−4 atm and whose initial composition is solar except for a C/O ratio of 0.83 yields an assemblage characterized by a very large enstatite/forsterite ratio, the presence of oldhamite and niningerite, metallic nickel-iron containing several wt% Si, and small amounts of pure silica and albitic plagioclase, very similar to the mineral assemblage of EH3 chondrites. Log fO2 in this system varies from IW-8.9 at 1500 K to IW-13 at 900 K. The mechanisms proposed to date for fractionation of C, O and H from one another are quantitatively insufficient to produce the magnitude of nebular fO2 variations needed to account for primitive features of UOCs and UECs. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.