Jijin Yang^a,e,*, Joseph I. Goldstein^a, Edward R.D. Scott^b, Paul G. Kotula^c, Ansgar Grimberg^d, Ingo Leya^d

^aDept. of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA 01003, USA
^bHawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI 96822, USA
^cMaterials Characterization Dept., Sandia National Laboratories, Albuquerque, NM 87185, USA
^dPhysical Institute , Universität Bern, CH-3012 Bern, Switzerland
^eCarl Zeiss Microscopy, One Zeiss Drive, Thornwood, NY 10594, USA

Tishomingo is a chemically and structurally unique iron with 32.5 wt.% Ni that contains 20% residual taenite and 80% martensite plates, which formed on cooling to between -75 and -200 °C, probably the lowest temperature recorded by any meteorite. Our studies using transmission (TEM) and scanning electron microscopy (SEM), x-ray microanalysis (AEM) and electron backscatter diffraction (EBSD) show that martensite plates in Tishomingo formed in a single crystal of taenite and decomposed during reheating forming 10-100 nm taenite particles with ~50 wt.% Ni, kamacite with ~4 wt.% Ni, along with martensite or taenite with 32 wt.% Ni. EBSD data and experimental constraints show that Tishomingo was reheated to 320-400 °C for about a year transforming some martensite to kamacite and to taenite particles and some martensite directly to taenite without composition change. Fizzy-textured intergrowths of troilite, kamacite with 2.7 wt.% Ni and 2.6 wt.% Co, and taenite with 56 wt.% Ni and 0.15 wt.% Co formed by localized shock melting. A single impact probably melted the sub-mm sulfides, formed stishovite, and reheated and decomposed the martensite plates. Tishomingo and its near-twin Willow Grove, which has 28 wt.% Ni, differ from IAB-related irons like Santa Catharina and San Cristobal that contain 25-36 wt.% Ni, as they are highly depleted in moderately volatile siderophiles and enriched in Ir and other refractory elements. Tishomingo and Willow Grove therefore resemble IVB irons but are chemically distinct. The absence of cloudy taenite in these two irons shows that they cooled through 250 °C abnormally fast at >0.01 °C/yr. Thus this grouplet, like the IVA and IVB irons, suffered an early impact that disrupted their parent body when it was still hot. Our noble gas data show that Tishomingo was excavated from its parent body about 100 to 200 Myr ago and exposed to cosmic rays as a meteoroid with a radius of ~50-85 cm.

Reference
Yang J, Goldstein JI, Scott ERD, Kotula PG, Grimberg A and Leya I (in press) Thermal and Collisional History of Tishomingo: More evidence for early disruption of differentiated planetesimals. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.09.023]
Copyright Elsevier

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N. Dalcher¹, M. W. Caffee², K. Nishiizumi³, K. C. Welten³, N. Vogel^4,†, R. Wieler⁴, I. Leya^1,*

¹Space Sciences and Planetology, University of Bern, Bern, Switzerland
²Department of Physics, PRIME Laboratory, Purdue University, West Lafayette, Indiana, USA
³Space Sciences Laboratory, University of California, Berkeley, California, USA
⁴Institute of Geochemistry and Petrology, ETH Zurich, Switzerland
^†EAWAG, Dübendorf, Switzerland

We measured the concentrations and isotopic compositions of He, Ne, and Ar in bulk samples and metal separates of 14 ordinary chondrite falls with long exposure ages and high metamorphic grades. In addition, we measured concentrations of the cosmogenic radionuclides ¹⁰Be,²⁶Al, and ³⁶Cl in metal separates and in the nonmagnetic fractions of the selected meteorites. Using cosmogenic ³⁶Cl and ³⁶Ar measured in the metal separates, we determined ³⁶Cl-³⁶Ar cosmic-ray exposure (CRE) ages, which are shielding-independent and therefore particularly reliable. Using the cosmogenic noble gases and radionuclides, we are able to decipher the CRE history for the studied objects. Based on the correlation ³He/²¹Ne versus ²²Ne/²¹Ne, we demonstrate that, among the meteorites studied, only one suffered significant diffusive losses (about 35%). The data confirm that the linear correlation ³He/²¹Ne versus ²²Ne/²¹Ne breaks down at high shielding. Using ³⁶Cl-³⁶Ar exposure ages and measured noble gas concentrations, we determine ²¹Ne and ³⁸Ar production rates as a function of ²²Ne/²¹Ne. The new data agree with recent model calculations for the relationship between ²¹Ne and ³⁸Ar production rates and the ²²Ne/²¹Ne ratio, which does not always provide unique shielding information. Based on the model calculations, we determine a new correlation line for ²¹Ne and ³⁸Ar production rates as a function of the shielding indicator ²²Ne/²¹Ne for H, L, and LL chondrites with preatmospheric radii less than about 65 cm. We also calculated the ¹⁰Be/²¹Ne and ²⁶Al/²¹Ne production rate ratios for the investigated samples, which show good agreement with recent model calculations.

Reference
Dalcher N, Caffee MW, Nishiizumi K, Welten KC, Vogel N, Wieler R and Leya I (in press) Calibration of cosmogenic noble gas production in ordinary chondrites based on ³⁶Cl-³⁶Ar ages. Part 1: Refined produced rates for cosmogenic ²¹Ne and ³⁸Ar. Meteoritics & Planetary Science
[doi:10.1111/maps.12203]
Published by arrangement with John Wiley & Sons

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Sabyasachi Sen^a, Scarlett J. Widgeon^a,b, Alexandra Navrotsky^a,b,*, Gabriela Mera^c, Amir Tavakoli^b, Emanuel Ionescu^c, and Ralf Riedel^c

^aDepartment of Chemical Engineering and Materials Science and
^bPeter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture, and Technology Organized Research Unit, University of California, Davis, CA 95616
^cInstitut für Materialwissenschaft, Technische Universität Darmstadt, D-64287 Darmstadt, Germany

Amorphous silicon oxycarbide polymer-derived ceramics (PDCs), synthesized from organometallic precursors, contain carbon- and silica-rich nanodomains, the latter with extensive substitution of carbon for oxygen, linking Si-centered SiO_xC_4-x tetrahedra. Calorimetric studies demonstrated these PDCs to be thermodynamically more stable than a mixture of SiO₂, C, and silicon carbide. Here, we show by multinuclear NMR spectroscopy that substitution of C for O is also attained in PDCs with depolymerized silica-rich domains containing lithium, associated with SiO_xC_4-x tetrahedra with nonbridging oxygen. We suggest that significant (several percent) substitution of C for O could occur in more complex geological silicate melts/glasses in contact with graphite at moderate pressure and high temperature and may be thermodynamically far more accessible than C for Si substitution. Carbon incorporation will change the local structure and may affect physical properties, such as viscosity. Analogous carbon substitution at grain boundaries, at defect sites, or as equilibrium states in nominally acarbonaceous crystalline silicates, even if present at levels at 10–100 ppm, might form an extensive and hitherto hidden reservoir of carbon in the lower crust and mantle.

Reference
Sen S Widgeon SJ, Navrotsky A, Mera G, Tavakoli A, Ionescu E and Riedel R (in press) Carbon substitution for oxygen in silicates in planetary interiors. PNAS
[doi:10.1016/j.icarus.2013.09.020]

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Marina A. Ivanova^1,*, Cyril A. Lorenz¹, Ian A. Franchi², Andrei Y. Bychkov³, Jeffrey E. Post⁴

¹Vernadsky Institute of Geochemistry of Russian Academy of Sciences, Moscow, Russia
²Planetary and Space Sciences, The Open University, Milton Keynes, UK
³Moscow State University, Moscow, Russia
⁴National Museum of Natural History, Smithsonian Institution, Washington, District of Columbia, USA

We have conducted hydration–dehydration experiments on terrestrial olivine to investigate the behavior of oxygen isotopic fractionation to test the hypothesis that multiple cycles of aqueous and thermal processing on a parent asteroid comprise a genetic relationship between CM2s and metamorphosed carbonaceous chondrites (MCCs). Two experiments were undertaken. In the first experiment, serpentine was obtained by hydrating terrestrial olivine (Fo_90.9) in the laboratory. During this experiment, olivine was reacted with isotopically heavy water (δ¹⁸O 21.5‰) at T = 300 °C, P_H2O = 300 bar, for 100 days. The oxygen isotopic composition of the experimental serpentine was enriched in ¹⁸O (by 10 ‰ in δ¹⁸O) due to exchange of oxygen isotopes between olivine and the ¹⁸O-rich water. Dehydrated serpentine was then produced during laboratory heating experiment in vacuum, at T = 930 °C, for 1 h. The oxygen isotopic composition of the dehydrated serpentine was enriched in ¹⁸O by a further 7 ‰. The net result of the hydration–dehydration process was an enrichment of ¹⁸O in the final material by approximately 17‰. The new experimental results suggest that the oxygen isotopic compositions of MCCs of the Belgica-like group, including Dhofar 225 and Dhofar 725, could be derived from those of typical CM2 chondrites via several cycles of hydration–dehydration caused by aqueous alteration and subsequent thermal metamorphism within their parent asteroids.

Reference
Ivanova MA, Lorenz CA, Franchi IA, Bychkov AY and Post JE (in press) Experimental simulation of oxygen isotopic exchange in olivine and implication for the formation of metamorphosed carbonaceous chondrites. Meteoritics & Planetary Science
[doi:10.1111/maps.12204]
Published by arrangement with John Wiley & Sons

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Mikhail Yu. Zolotov

School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA

Dwarf planet Ceres is the largest body in the main asteroid belt with a rocky surface and uncertain internal structure. Spectra of Ceres in near- and mid-infrared wavelengths are consistent with the occurrence of brucite, Mg-bearing carbonates, and an Fe-rich phyllosilicate cronstedtite. Spectra of 10 Hygiea and 324 Bamberga imply similar compositions. Here, we considered stabilities of these minerals to constrain their origin. Cronstedtite is most stable at the temperature of ~0 °C at moderately oxidizing aqueous conditions, and at high water/rock ratios. Although cronstedtite could form on planetesimals, the apparent lack of serpentine may indicate its formation by Ceres’ temporary surface solutions. Brucite forms at a low activity of dissolved SiO₂, at a low fugacity of CO₂, and at highly alkaline pH. Brucite and cronstedtite do not form together and may not form deep in the Ceres’ interior. The absence of Mg serpentine from Ceres’ surface materials and the unlikely occurrence of very olivine-rich rocks do not indicate a formation of brucite through serpentinization of such rocks. Brucite could form by transient near-surface fluids which do not equilibrate with silicates. Temporary fluids could deposit Mg carbonates before, after, or together with brucite at near-surface conditions that favor CO₂ degassing. Regardless of Ceres’ internal structure, internal thermal and aqueous processes may not affect cold near-surface layers. Percolation of interior fluids is not consistent with the lack of detection of low-solubility salts. However, impacts of ice-rich targets during the Late Heavy Bombardment could account for transient aqueous environments and unusual surface mineralogies of Ceres, Hygiea, and Bamberga. Brucite and Mg carbonates could have formed through hydration and carbonation of MgO evaporated from silicates. Apparently abundant carbonates may indicate an ample impact oxidation of organic matter, and the occurrence of brucite with cronstedtite may reflect turbulent and disequilibrium environments. Clay-like homogeneous surface materials on Ceres could be gravitationally sorted deposits of impact clouds.

Reference
Zolotov MY (in press) Formation of brucite and cronstedtite-bearing mineral assemblages on Ceres. Icarus
[doi:10.1016/j.icarus.2013.09.020]
Copyright Elsevier

Link to Article