^1,2,3Andrew M. Davis, ^1,3Frank M. Richter, ³Ruslan A. Mendybaev, ⁴Philip E. Janney, ⁴Meenakshi Wadhwa, ⁵Kevin D. McKeegan
¹Chicago Center for Cosmochemistry, The University of Chicago, Chicago, Il 60637
²Enrico Fermi Institute, The University of Chicago, Chicago, Il 60637
³Department of the Geophysical Sciences, The University of Chicago, Chicago, Il 60637
⁴Department of Geology, The Field Museum, Chicago, IL 60605
⁵Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095

Magnesium isotope ratios are known to vary in solar system objects due to the effects of 26Al decay to 26Mg and mass dependent fractionation, but anomalies of nucleosynthetic origin must also be considered. In order to infer the amount of enhancement of 26Mg/24Mg due to 26Al decay or to resolve small nucleogenetic anomalies, the exact relationship between 26Mg/24Mg and 25Mg/24Mg ratios due to mass-dependent fractionation, the mass-fractionation “law”, must be accurately known so that the 25Mg/24Mg ratio can be used to correct the 26Mg/24Mg ratio for mass fractionation. Mass-dependent fractionation in mass spectrometers is reasonably well characterized, but not necessarily fully understood. It follows a simple power fractionation law, sometimes referred to as the “exponential law”. In contrast, mass fractionation in nature, in particular that due to high temperature evaporation that likely caused the relatively large effects observed in calcium-, aluminum-rich inclusions (CAIs), is reasonably well understood, but mass-fractionation laws for magnesium have not been explored in detail. The magnesium isotopic compositions of CAI-like evaporation residues produced in a vacuum furnace indicate that the slope on a log 25Mg/24Mg vs. log 26Mg/24Mg plot is ∼0.5128, and different from those predicted by any of the commonly used mass-fractionation laws. Evaporation experiments on forsterite-rich bulk compositions give exactly the same slope, indicating that the measured mass-fractionation law for evaporation of magnesium is applicable to a wide range of bulk compositions. We discuss mass-fractionation laws and the implications of the measured fractionation behavior of magnesium isotopes for 26Al-26Mg chronology.

Reference
Davis AM, Richter FM Mendybaev RA, Janney PE, Wadhwa M, McKeegan KD (2015) Isotopic mass fractionation laws for magnesium and their effects on 26Al-26Mg systematics in solar system materials. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.01.034]

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^1,2Hitesh G. Changela, ⁴George D. Cody, ⁴Conel M.O’D. Alexander, ³Z Peeters, ⁴Larry R. Nittler, ²Rhonda M. Stroud
¹George Washington University, Department of Physics, 725 21st Street, NW, Washington, DC, 20052
²Naval Research Laboratory, 4555 Overlook Ave. SW, Washington, DC, 20375
³Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd NW, Washington DC, 20015
⁴Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd NW, Washington DC, 20015

The major form of organic material delivered to earth from an extraterrestrial origin is Insoluble Organic Matter (IOM). A morphological study of IOM in the CR (Renazzo-type) and CM (Mighei-type) carbonaceous chondrites was performed in order to constrain its origins and processing history. IOM residues from the following CR chondrites: GRO 95577 (CR 1), Al Rais (CR 1/2), EET 92042 (CR 2), QUE 99177 (CR 3) and the CM chondrites: MET 01070 (CM 2.2), Cold Bokkeveld (CM 2.3), Murchison (CM 2.4) and QUE 97990 (CM 2.5) were studied using Annular Dark Field STEM imaging. Characteristic features of the IOM, organic nanoglobules, were manually identified and measured for their abundances and size distributions. The IOM residues were also compared holistically for their degree of average ‘roughness’ or ‘coarsening’ using fractal image analysis. Manually identified nanoglobules have abundances making up less than 10% of the total IOM, which is consistent with previous studies. Their measured abundances do not correlate with petrologic grade. Thus parent body processing did not systematically deplete their abundances. The IOM is however on average ‘smoother’ or ‘coarser’ in the more altered chondrites, demonstrated by a lower fractal dimension using fractal box counting (DB). The DB values for the IOM in the CR chondrites are distinctive: QUE 99177 has the largest DB value (average = 1.54 ± 0.004) and GRO 99577 has the lowest (average = 1.45 ± 0.011). Al Rais and EET 92042 have IOM with average DB values within this range (average, 1.46 ± 0.009 and 1.50 ± 0.006). The CMs record a similar but less distinctive trend in DB, with QUE 97990 having the largest value (1.52 ± 0.004), MET 01070 the lowest (1.45 ± 0.019), and Cold Bokkeveld (1.50 ± 0.011) and Murchison (1.49 ± 0.017) equivalent to one another within error. The identified nanoglobules in the IOM of the CM chondrites are on average larger than those in the CR chondrites. The ‘coarsening’ or ‘smoother’ texture of the IOM (lower DB) in the more altered chondrites coupled with a tentative increase in the size of large features (identified nanoglobules) demonstrates that the aqueous processes leading to the lower petrologic types also formed the overall IOM morphology. In addition, observations of fluid-like textures more frequently found in the more altered carbonaceous chondrite residues suggests that organic and aqueous fluids determined at least some of these morphologies. The polymerisation of organic solutions is consistent with these morphologies. Their formation conditions are more favourable under containment in carbonaceous chondrite parent bodies.

Reference
Changela HG, Cody GD, Alexander CMOD, Peeters Z, Nittler LR, Stroud RM (2015) Morphological Study of Insoluble Organic Matter from Carbonaceous Chondrites: Correlation with Petrologic Grade. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.02.007]

Copyright Elsevier