Mark H. Thiemens

Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla, CA 92093-0356

Stable isotope ratio variations are regulated by physical and chemical laws. These rules depend on a relation with mass differences between isotopes. New classes of isotope variation effects that deviate from mass dependent laws, termed mass independent isotope effects, were discovered in 1983 and have a wide range of applications in basic chemistry and nature. In this special edition, new applications of these effects to physical chemistry, solar system origin models, terrestrial atmospheric and biogenic evolution, polar paleo climatology, snowball earth geology, and present day atmospheric sciences are presented.

Reference
Thiemens MH (2013) Introduction to Chemistry and Applications in Nature of Mass Independent Isotope Effects Special Feature. PNAS 110:17631-17637.
[doi:10.1073/pnas.1312926110]

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Subrata Chakraborty^a,*, Teresa L. Jackson^a, Musahid Ahmed^b, and Mark H. Thiemens^a

^aDepartment of Chemistry and Biochemistry, University of California at San Diego, La Jolla, CA 92093-0356
^bChemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

Select meteoritic classes possess mass-independent sulfur isotopic compositions in sulfide and organic phases. Photochemistry in the solar nebula has been attributed as a source of these anomalies. Hydrogen sulfide (H₂S) is the most abundant gas-phase species in the solar nebula, and hence, photodissociation of H₂S by solar vacuum UV (VUV) photons (especially by Lyman-α radiation) is a relevant process. Because of experimental difficulties associated with accessing VUV radiation, there is a paucity of data and a lack of theoretical basis to test the hypothesis of a photochemical origin of mass-independent sulfur. Here, we present multiisotopic measurements of elemental sulfur produced during the VUV photolysis of H₂S. Mass-independent sulfur isotopic compositions are observed. The observed isotopic fractionation patterns are wavelength-dependent. VUV photodissociation of H₂S takes place through several predissociative channels, and the measured mass-independent fractionation is most likely a manifestation of these processes. Meteorite sulfur data are discussed in light of the present experiments, and suggestions are made to guide future experiments and models.

Reference
Chakraborty S, Jackson TL, Ahmed M and Thiemens MH (2013) Sulfur isotopic fractionation in vacuum UV photodissociation of hydrogen sulfide and its potential relevance to meteorite analysis. PNAS 110:17650-17655.
[doi:10.1073/pnas.1213150110]

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Paula Lindgren^a, Mark C. Price^b, Martin R. Lee^a, Mark J. Burchell^b

^aSchool of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK
^bSchool of Physical Sciences, University of Kent, Canterbury, Kent CT2 7NZ, UK

Calcite twinning in carbonaceous chondrite meteorites can be used to reconstruct the deformation history and the parent body environment during and/or after aqueous alteration, but the shock pressure threshold at which the twins develop is unknown. Accordingly, the aim of this study is to determine the magnitude of shock pressure that is needed to generate calcite twins. This was done by measuring the depths of twinning beneath the resulting craters from experimental impacts in six calcite targets, combined with hydrocode modelling of the peak pressures at the corresponding depths within the targets. Brecciation, fracturing and calcite e-twinning occur below the floors of all the craters and results from the hydrocode modelling show that the twins start to form at shock pressures of ~110 to 480 MPa, which is at least a factor of ten higher than the 10 MPa that is considered to be required to produce calcite twins in low strain rate terrestrial settings. These pressures are equivalent to shock stage S1 as determined by olivine microstructures and consistent with calcite twinning in carbonaceous chondrites being a result of impact gardening in shallow levels of their asteroidal parent bodies.

Reference
Lindgren P, Price MC, Lee MR and Burchell MJ (in press) Constraining the pressure threshold of impact induced calcite twinning: Implications for the deformation history of aqueously altered carbonaceous chondrite parent bodies. Earth and Planetary Science Letters 384:71–80.
[doi:10.1016/j.epsl.2013.10.002]
Copyright Elsevier

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Noam R. Izenberg^a, Rachel L. Klima^a, Scott L. Murchie^a, David T. Blewett^a, Gregory M. Holsclaw^b, William E. McClintock^b, Erick Malaret^c, Calogero Mauceri^c, Faith Vilas^d, Ann L. Sprague^e, Jörn Helbert^f, Deborah L. Domingue^d, James W. Head III^g, Timothy A. Goudge^g, Sean C. Solomon^h,i, Charles A. Hibbitts^a, M. Darby Dyar^j

^aPlanetary Exploration Group, The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
^bLaboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA
^cApplied Coherent Technology Corporation, Herndon, VA 20170, USA
^dPlanetary Science Institute, Tucson, AZ 85719, USA
^eLunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
^fInstitute of Planetary Research, Deutsches Zentrum für Luft und Raumfahrt, D-12489 Berlin, Germany
^gDepartment of Geological Sciences, Brown University, Providence, RI 02912, USA
^hDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA
ⁱLamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA
^jDepartment of Astronomy, Mount Holyoke College, South Hadley, MA 01075, USA

The MESSENGER spacecraft’s Mercury Atmospheric and Surface Composition Spectrometer (MASCS) obtained more than 1.6 million reflectance spectra of Mercury’s surface from near-ultraviolet to near-infrared wavelengths during the first year of orbital operations. A global analysis of spectra in the wavelength range 300–1450 nm shows little regional variation in absolute reflectance or spectral slopes and a lack of mineralogically diagnostic absorptions. In particular, reflectance spectra show no clear evidence for an absorption band centered near 1 μm that would be associated with the presence of ferrous iron in silicates. There is, however, evidence for an ultraviolet absorption possibly consistent with a very low iron content (2–3 wt% FeO or less) in surface silicates and for the presence of small amounts of metallic iron or other opaque minerals in the form of nano- or micrometer-sized particles. These findings are consistent with MESSENGER X-ray and gamma-ray measurements of Mercury’s surface iron abundance. Although X-ray and gamma-ray observations indicate higher than expected quantities of sulfur on the surface, reflectance spectra show no absorption bands diagnostic of sulfide minerals. Whereas there is strong evidence of water ice in permanently shadowed craters near Mercury’s poles, MASCS spectra provide no evidence for hydroxylated materials near permanently shadowed craters.

Reference
Izenberg NR, Klima RL, Murchie SL, Blewett DT, Holsclaw GM, McClintock WE, Malaret E, Mauceri C, Vilas F, Sprague AL, Helbert J, Domingue DL, Head III JW, Goudge TA, Solomon SC, Hibbitts CA and Dyar MD (in press) The low-iron, reduced surface of Mercury as seen in spectral reflectance by MESSENGER. Icarus
[doi:10.1016/j.icarus.2013.10.023]
Copyright Elsevier

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