James A. Van Orman^a,*, Daniele J. Cherniak^b and Noriko T. Kita^c

^aDepartment of Earth, Environmental and Planetary Sciences, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA
^bDepartment of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
^cDepartment of Geoscience, University of Wisconsin–Madison, Madison, WI 53706, USA

Experimental data are reported on Mg diffusion in plagioclase crystals with a range of anorthite content (x_An), at temperatures between 800 and 1150 °C. Oriented and polished single crystals of anorthite (x_An=0.93), labradorite (x_An=0.67), andesine (x_An=0.43) and oligoclase (x_An=0.23) were each embedded in powered source material enriched in natural MgO or ²⁵MgO and suspended in a furnace at constant temperature. Diffusion profiles in quenched samples were measured from the polished surface using SIMS depth profiling. The diffusion coefficient does not depend significantly on the Mg concentration gradient, and little anisotropy is observed between the b and c directions in labradorite. Diffusion coefficients increase systematically with decreasing x_An, and the entire data set is described by , where R is the gas constant, T is absolute temperature, and the diffusion coefficient D is in m²/s. ²⁶Al–²⁶Mg ages in albite-rich plagioclase are much more easily reset than in anorthite, with closure temperatures up to 120–150 K lower.

Reference
Van Orman JA, Cherniak DJ and Kita NT (2013) Magnesium diffusion in plagioclase: Dependence on composition, and implications for thermal resetting of the ²⁶Al–²⁶Mg early solar system chronometer. Earth and Planetary Science Letters 385:79–88.
[doi:10.1016/j.epsl.2013.10.026]
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C. Koike¹, R. Noguchi², H. Chihara^3,5, H. Suto⁴, O. Ohtaka², Y. Imai², T. Matsumoto² and A. Tsuchiyama³

¹Department of Physics, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
²Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
³Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo, Kyoto 606-8052, Japan
⁴National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
⁵Currently at College of General Education, Osaka Sangyo University, Daito, Osaka 574-8530, Japan.

The existence of silica within several debris disks has been suggested. Data on both the spectroscopy and annealing conditions of the various polymorphs of silica need to be investigated, as these data are lacking and incomplete in the literature. We investigate the annealing conditions of silica and prepare various types of silica, including α-cristobalite, α-quartz, coesite, stishovite, and fused quartz, which are natural, synthetic, or commercial samples. This paper presents a new study of both the spectroscopy of relevant silica polymorphs and the conditions under which they form. We compare the results to previous studies and find that there are discrepancies. The interesting result of features similar to those of forsterite should be highlighted, where α-cristobalite and coesite showed similar peaks at 16, 33, and 69 μm as forsterite. The 69 μm band for α-cristobalite is especially very broad and strong and shifts largely to a shorter wavelengths under cooling to low temperatures. The band for coesite, however, is very sharp and shifts only a small amount to longer wavelengths under cooling to low temperatures. We discuss the possibility of silica detection around debris disks.

Reference
Koike C, Noguchi R, Chihara H, Suto H, Ohtaka O, Imai Y, Matsumoto T and Tsuchiyama A (2013) Infrared Spectra of Silica Polymorphs and the Conditions of Their Formation. The Astrophysical Journal 778:60.
[doi:10.1088/0004-637X/778/1/60]

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Yonaton Goldsmith^a,b,c, Mordechai Stein^a,b, Yehouda Enzel^a

^aThe Fredy and Nadine Herrmann Institute of Earth Sciences, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem 91904, Israel
^bGeological Survey of Israel, 30 Malkhei Israel St., Jerusalem 95501, Israel
^cLamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA

Chemical compositions of rock varnish from the Negev Desert of Israel and local settled dust were used to constrain the mechanisms of varnish formation and patterns of Mn enrichment and accumulation in the varnish. Rock varnish was sampled from coeval, undisturbed prehistoric flint artifacts along a south–north climatic transect (∼30 to 120 mm/yr of rain). Our analyses indicate that Mn, Ba and Pb in the varnish are significantly enriched (∼100×) in respect to the local settling dust and that Mn content systematically fluctuates with depth in the varnish. The varnish and settled dust data combined with basic thermodynamic and kinetic reasoning are used to constrain the following geochemical model of rock varnish formation: dust accumulates in micro-basins on exposed rock surfaces, under pH∼8 (common Negev value) and during wetting by dew and rain, Mn in the dust is mobilized and leached to a depth of ∼5 μm under the varnish surface where Hollandite Mn-oxides precipitate and are adsorbed onto and between the porous clay minerals that comprise most of the varnish. During its mobile phase Mn-oxide is negatively charged and adsorbs rare earth elements. Once the solution dries abrasion removes the upper, weakly cemented dust sediment, which contains mainly Si, Al and Fe (which are not mobile at pH ∼ 8). Ca is also removed in large quantities. Mn, Ba, Pb and the REE are deposited at a depth and thus, protected from erosion. Reoccurrences of these processes result in a noticeable accumulation of these elements, but not of Si, Al or Fe. The alternating Mn-rich and Mn-poor laminas form as a result of a competition between the leaching rate of Mn and the adhesion rate of the clay minerals. When moisture is high (low), lamina with high (low) Mn/clay mineral ratio forms.

Reference
Goldsmith Y, Stein M and Enzel Y (in press) From dust to varnish: Geochemical constraints on rock varnish formation in the Negev Desert, Israel. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.10.040]
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Patrick Höhener^a,* and Olivier Atteia^b

^aAix-Marseille Université – Laboratoire Chimie Environnement, FRE 3416 –CNRS, Marseille, France
^bENSEGID, 1 allée Daguin 33607 Pessac, France

In isotope geochemistry, the Rayleigh equation describes the evolution of isotope ratios in a parent compound as a function of reaction progress, and associated equations describe isotope ratios in an instantaneous product and an accumulated product. The Rayleigh equation is commonly used for fitting fractionation factors of processes undergoing kinetic isotope fractionation such as biochemical reactions. This work extends the equations associated with the Rayleigh equation for describing the isotope ratios in intermediate products in a chain of reacting species degrading with first-order kinetics. A general solution is presented for decay chains of any length, and explicit examples are presented for the biodegradation of a substrate or a mixture of substrates through 3 intermediate products to a final product. Applications of these analytical solutions for the fitting of enrichment factors for intermediate compounds in laboratory experiments are demonstrated with a spreadsheet. This avoids separate experiments to measure each intermediate product. The utility of the equations for the assessment of slopes in dual isotope plots is furthermore illustrated, and limitations of its use are critically discussed.

Reference
Höhener P and Atteia O (in press) Rayleigh equation for evolution of stable isotope ratios in contaminant decay chains. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.10.036]
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