¹A. K. Speck, ^2,3K. M. Pitman, ⁴A. M. Hofmeister
¹Department of Physics and Astronomy, University of Missouri-Columbia, Columbia, MO 65211, USA
²Planetary Science Institute, Tucson, AZ 85719, USA
³Space Science Institute, Boulder, CO 80301, USA
⁴Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA

“Astronomical” or “circumstellar” silicate optical functions (real and imaginary indices of refraction $n(\lambda )$ and $k(\lambda )$) have previously been derived from compositionally and structurally disparate samples; past values were compiled from different sources in the literature, and are essentially kluges of observational, laboratory, and extrapolated or interpolated values. These synthetic optical functions were created because astronomers lack the quantitative data on amorphous silicates at all wavelengths needed for radiative transfer modeling. This paper provides optical functions that (1) are created with a consistent methodology, (2) use the same sample across all wavelengths, and (3) minimize interpolation and extrapolation wherever possible. We present electronic data tables of optical functions derived from mid-ultraviolet to far-infrared (FIR) laboratory transmission spectra for two materials: iron-free glass with chondritic/solar atmospheric abundances, and metallic iron. We compare these optical functions to other popular n, k data used to model amorphous silicates (e.g., “astronomical” or “circumstellar” silicate), both directly and in application to a simple system: the dust shell of the post-AGB star HD 161796. Using the new optical functions, we find that the FIR profile of model spectral energy distributions are significantly affected by the ratio of glass to iron. Our case study on HD 161796 shows that in modeling with our new optical functions, the mineralogy is markedly different from that derived using synthetic optical functions and suggests a new scenario of crystalline silicate formation.

Reference
Speck AK, Pitman KM, Hofmeister AM (2015) Better Alternatives to “Astronomical Silicate”: Laboratory-Based Optical Functions of Chondritic/Solar Abundance Glass with Application to HD161796. The Astrophysical Journal 809
Link to Article [http://dx.doi.org/10.1088/0004-637X/809/1/65]

¹T.E. Johnson, ^1,2G.K. Benedix, ¹P.A. Bland
¹Department of Applied Geology, The Institute for Geoscience Research (TIGeR), Curtin University, GPO Box U1987, Perth, WA 6845, Australia
²Department of Earth and Planetary Sciences, Western Australia Museum, 49 Kew Street, Welshpool, WA 6986, Australia

Constraining the metamorphic pressures (P) and temperatures (T) recorded by meteorites is key to understanding the size and thermal history of their asteroid parent bodies. New thermodynamic models calibrated to very low P for minerals and melt in terrestrial mantle peridotite permit quantitative investigation of high-T metamorphism in ordinary chondrites using phase equilibria modelling. Isochemical P–T phase diagrams based on the average composition of H, L and LL chondrite falls and contoured for the composition and abundance of olivine, ortho- and clinopyroxene, plagioclase and chromite provide a good match with values measured in so-called equilibrated (petrologic type 4–6) samples. Some compositional variables, in particular Al in orthopyroxene and Na in clinopyroxene, exhibit a strong pressure dependence when considered over a range of several kilobars, providing a means of recognising meteorites derived from the cores of asteroids with radii of several hundred kilometres, if such bodies existed at that time. At the low pressures (<1 kbar) that typify thermal metamorphism, several compositional variables are good thermometers. Although those based on Fe–Mg exchange are likely to have been reset during slow cooling, those based on coupled substitution, in particular Ca and Al in orthopyroxene and Na in clinopyroxene, are less susceptible to retrograde diffusion and are potentially more faithful recorders of peak conditions. The intersection of isopleths of these variables may allow pressures to be quantified, even at low P, permitting constraints on the minimum size of parent asteroid bodies. The phase diagrams predict the onset of partial melting at 1050–1100 °C by incongruent reactions consuming plagioclase, clinopyroxene and orthopyroxene, whose compositions change abruptly as melting proceeds. These predictions match natural observations well and support the view that type 7 chondrites represent a suprasolidus continuation of the established petrologic types at the extremes of thermal metamorphism. The results suggest phase equilibria modelling has potential as a powerful quantitative tool in investigating, for example, progressive oxidation during metamorphism, the degree of melting and melt loss or accumulation required to produce the spectrum of differentiated meteorites, and whether the onion shell or rubble pile model best explains the metamorphic evolution of asteroid parent bodies in the early solar System.

Reference
Johnson TE, Benedix GK, Bland PA (2015) Metamorphism and partial melting of ordinary chondrites: Calculated phase equilibria. Earth and Planetary Science Letters 433, 21–30
Link to Article [doi:10.1016/j.epsl.2015.10.035]
Copyright Elsevier

¹Audrey Bouvier, ²Janne Blichert-Toft, ³Maud Boyet,²Francis Albarède
¹Department of Earth Sciences, Centre for Planetary Science and Exploration, University of Western Ontario, London, ON, Canada
²Laboratoire de Géologie de Lyon, CNRS UMR 5276, Ecole Normale Supérieure de Lyon and Université Claude Bernard Lyon 1, Lyon, France
³Laboratoire Magmas et Volcans, CNRS UMR 6524, Université Blaise Pascal, Clermont-Ferrand, France

Comparative planetary geochemistry provides insight into the origin and evolutionary paths of planetary bodies in the inner solar system. The eucrite and angrite achondrite groups are particularly interesting because they show evidence of early planetary differentiation. We present 147Sm-143Nd and 176Lu-176Hf analyses of eight noncumulate (basaltic) eucrites, two cumulate eucrites, and three angrites, which together place new constraints on the evolution and differentiation histories of the crusts of the eucrite and angrite parent bodies and their mantle mineralogies. The chemical compositions of both eucrites and angrites indicate similar evolutionary paths and petrogenetic models with formation and isolation of differentiated crustal reservoirs associated with segregation of ilmenite. We report a 147Sm-143Nd mineral isochron age for the Moama cumulate eucrite of 4519 ± 34 Ma (MSWD = 1.3). This age indicates protracted magmatism within deep crustal layers of the eucrite parent body lasting up to about 50 Ma after the formation of the solar system. We further demonstrate that the isotopic compositions of constituent minerals are compromised by secondary processes hindering precise determination of mineral isochron ages of basaltic eucrites and angrites. We interpret the changes in geochemistry and, consequently, the erroneous 147Sm-143Nd and 176Lu-176Hf internal mineral isochron ages of basaltic eucrites and angrites as the result of metamorphic events such as impacts (effects from pressure, temperature, and peak shock duration) on the surfaces of the eucrite and angrite parent bodies.

Reference
Bouvier A, Blichert-Toft J, Boyet M, Albarède F (2015) 147Sm-143Nd and 176Lu-176Hf systematics of eucrite and angrite meteorites. Meteoritics & Planetary Science (in Press).
Link to Article [DOI: 10.1111/maps.12553]
Published by arrangement with John Wiley & Sons

¹Piero D’Incecco et al. (>10)*
¹Institute of Planetary Research, German Aerospace Center, Rutherfordstrasse 2, D-12489 Berlin, Germany
*Find the extensive, full author and affiliation list on the publishers website

We have performed a combined geological and spectral analysis of two impact craters on Mercury: the 15 km diameter Waters crater (106 °W; 9 °S) and the 62.3 km diameter Kuiper crater (30 °W; 11 °S). Using the Mercury Dual Imaging System (MDIS) Narrow Angle Camera (NAC) dataset we defined and mapped several units for each crater and for an external reference area far from any impact related deposits. For each of these units we extracted all spectra from the MESSENGER Atmosphere and Surface Composition Spectrometer (MASCS) Visible-InfraRed Spectrograph (VIRS) applying a first order photometric correction. For all the mapped units, we analyzed the spectral slope in two wavelength ranges, 350–450 nm and 450–650 nm, and the absolute reflectance in the 700-750 nm range. Normalized spectra of Waters crater display a generally bluer spectral slope than the external reference area over both wavelength windows. Normalized spectra of Kuiper crater generally display a redder slope than the external reference area in the 350–450 nm window, while they display a bluer slope than the external reference area in the 450–650 nm wavelength range. The combined use of geological and spectral analyses enables reconstruction of the local scale stratigraphy beneath the two craters, providing insight into the properties of the shallower crust of Mercury. Kuiper crater, being ~4 times larger than Waters crater, exposes deeper layers with distinctive composition, while the result for Waters crater might indicate substantial compositional homogeneity with the surrounding intercrater plains, though we can’t exclude the occurrence of horizontal compositional heterogeneities in the shallow sub-surface.

Reference
D’Incecco P. et al. (2015) Shallow crustal composition of Mercury as revealed by spectral properties and geological units of two impact craters. Icarus (in Press)
Link to Article [doi:10.1016/j.pss.2015.10.007]

Copyright Elsevier

The most popular papers on Cosmochemistry Papers in October were:

1.Renne PR, Sprain CJ, Richards MA, Self S, Vanderkluysen L, Pande K (2015) State shift in Deccan volcanism at the Cretaceous-Paleogene boundary, possibly induced by impact. Science 6256:76-78. Link to Article [doi:10.1126/science.aac7549]

2.Defouilloy C, Cartigny P, Assayag N, Moynier F, Barrat J-A (2015) High-precision sulfur isotope composition of enstatite meteorites and implications of the formation and evolution of their parent bodies. Geochimica et Cosmochimica Acta (in Press) Link to Article [doi:10.1016/j.gca.2015.10.009]

3.Becker M, Hezel DC, Schulz T, Elfers B-M, Münker C (2015) Formation timescales of CV chondrites from component specific Hf–W systematics. Earth and Planetary Science Letters (in Press) Link to Article [doi:10.1016/j.epsl.2015.09.049]

4.Laurent B, Roskosz M, Remusat L, Robert F, Leroux H, Vezin H, Depecker C, Nuns N, Lefebvre J-M (2015) The deuterium/hydrogen distribution in chondritic organic matter attests to early ionizing Irradiation. Nature Communications 6, 8567 Link to Article [doi:10.1038/ncomms9567]

5. Goderis S, Brandon AD, Mayer B, Humayun M (2015) s-Process Os isotope enrichment in ureilites by planetary processing. Earth and Planetary Science Letters 431, 110–118 Link to Article [doi:10.1016/j.epsl.2015.09.021]

¹de Vet, S. J.
¹Earth Surface Science, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, the Netherlands

Two meteorites impacted in 1925 around the town of Serooskerke on the isle of Schouwen, the Netherlands. The largest mass is widely known as the “Ellemeet” diogenite, while a second mass, heavily weathered due to environmental exposure, also survived until the present day. This work aims to reconstruct the history of the 1925 fall and for the first time documents the second mass, known as the “Serooskerke,” by integrating a historical and experimental approach. The study of historical news archives and cadastral records redefined the 1925 impact site at N 51°42.086′ E 3°49.789′. Environmental exposure experiments reproducing the effects of rainfall and frost weathering identified the latter as the main cause for the second mass’ reported disintegration in the field sometime during the 1925–1926 winter. The bulk mineralogy of the second mass was established using XRD powder diffraction for a 2θ range of 3–70° and was found to be identical to an Ellemeet reference sample. UV/VIS/nIR spectroscopy (300–2500 nm) was subsequently used to broadly compare the second mass to HED clan meteorites Bouvante, EET87503, Johnstown and asteroid 4 Vesta in order to corroborate its vestan origin. The historical and geographic relationship of the two masses and the comparable bulk mineralogy supported the pairing of these two meteorites. This makes the Serooskerke a valuable legacy of the 1925 fall, especially as the location of ~50% of the remaining Ellemeet mass is presently unknown.

Reference
de Vet SJ (2015) The 1925 meteorite fall near Ellemeet and Serooskerke, the Netherlands. Meteoritics & Planetary Science (in Press)
Link to Article [doi: 10.1111/maps.12554]
Published by arrangement with John Wiles & Sons

¹David T. Blewett et al. (>10)*
¹Planetary Exploration Group, Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd., Laurel, MD 20723, USA
*Find the extensive, full author and affiliation list on the publishers website

Exposure to ion and micrometeoroid bombardment in the space environment causes physical and chemical changes in the surface of an airless planetary body. These changes, called space weathering, can strongly influence a surface’s optical characteristics, and hence complicate interpretation of composition from reflectance spectroscopy. Prior work using data from the Dawn spacecraft ( Pieters et al., 2012) found that accumulation of nanophase metallic iron (npFe0), which is a key space-weathering product on the Moon, does not appear to be important on Vesta, and instead regolith evolution is dominated by mixing with carbonaceous chondrite (CC) material delivered by impacts.
In order to gain further insight into the nature of space weathering on Vesta, we constructed model reflectance spectra using Hapke’s radiative-transfer theory and used them as an aid to understanding multispectral observations obtained by Dawn’s Framing Cameras (FC). The model spectra, for a howardite mineral assemblage, include both the effects of npFe0 and that of a mixed CC component. We found that a plot of the 438-nm/555-nm ratio vs. the 555-nm reflectance for the model spectra helps to separate the effects of lunar-style space weathering (LSSW) from those of CC-mixing. We then constructed ratio-reflectance pixel scatterplots using FC images for four areas of contrasting composition: a eucritic area at Vibidia crater, a diogenitic area near Antonia crater, olivine-bearing material within Bellicia crater, and a light mantle unit (referred to as an “orange patch” in some previous studies, based on steep spectral slope in the visible) northeast of Oppia crater. In these four cases the observed spectral trends are those expected from CC-mixing, with no evidence for weathering dominated by production of npFe0. In order to survey a wider range of surfaces, we also defined a spectral parameter that is a function of the change in 438-nm/555-nm ratio and the 555-nm reflectance between fresh and mature surfaces, permitting the spectral change to be classified as LSSW-like or CC-mixing-like. When applied to 21 fresh and mature FC spectral pairs, it was found that none have changes consistent with LSSW.
We discuss Vesta’s lack of LSSW in relation to the possible agents of space weathering, the effects of physical and compositional differences among asteroid surfaces, and the possible role of magnetic shielding from the solar wind.

Reference
Blewett DT et al. (2015) Optical Space Weathering on Vesta: Radiative-transfer Models and Dawn Observations. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.10.012]
Copyright Elsevier

¹Vladimir V. Svetsov, ¹Valery V. Shuvalov
¹Institute for Dynamics of Geospheres, Russian Academy of Sciences, Leninskiy Prospekt 38-1, Moscow, 119334, Russia

In this study we numerically simulated the impacts of asteroids and comets on the Moon in order to calculate the amount of condensate that can be formed after the impacts and compare the results with data for lunar samples. Using available equations of state for quartz and dunite, we have determined pressure and density behind shock waves in these materials for particle velocities behind the shock from 4 to 20 km/s and obtained release adiabats from various points on the Hugoniot curves to very low pressures. For shock waves with particle velocities behind the front below 8 km/s the release adiabats intersect the liquid branch of the two-phase curve and, during the following expansion, the liquid material vaporizes and does not condense, forming a two-phase mixture of melt and vapor. The condensate can appear during expansion of material compressed by a shock with higher (>8 km/s) velocities. Using our hydrocode SOVA, we have conducted numerical simulations of the impacts of spherical quartz, dunite, and water-ice projectiles into targets of the same materials. Impact velocities were 15-25 km/s for stony projectiles and 20-70 km/s for icy impactors, and impact angles were 45°and 90° to the target surface. Along with the masses of condensates we calculated the masses of vaporized and melted material. Upon the impact of a projectile consisting of dunite into a target of quartz at a speed of 20 km/s at an angle of 45°, vaporized and melted masses of the target are equal to 1.6 and 11 in units of projectile mass, respectively, and the mass of condensate is 0.19. Vaporized and condensed masses of the projectile are 0.16 and 0.02, the rest mass of the projectile is melted. The calculated ratio of vaporized to melted mass proved to be on the order of 0.1. However, we calculated that, at impact velocities below 20 km/s, the condensate mass is only a small fraction of the vaporized and melted masses and, consequently, the major part of vapor disperses in vacuum in the form of separate molecules or molecular clusters. At an impact velocity of 15 km/s, the abundance of silicate condensates relative to melt is 0.001 – 0.0001, in agreement with data from lunar samples. Should the observed condensate abundances be representative, the velocities of major asteroid impacts on the Moon could not substantially exceed 20 km/s. Comet impacts at the same velocities produce much smaller amounts of vapor condensate because the low densities of cometary material induce lower shock pressures in the target.

Reference
Svetsov VV, Shuvalov VV (2015) Silicate impact-vapor condensate on the Moon: Theoretical estimates versus geochemical data. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.10.019]
Copyright Elsevier

¹T.S. Peretyazhko, ¹B. Sutter, ²R.V. Morris, ³D.G. Agresti, ¹L. Le, ²D.W. Ming
¹Jacobs, NASA Johnson Space Center, Houston, TX 77058
²NASA Johnson Space Center, Houston, TX 77058
³University of Alabama, Birmingham, AL 35294

Phyllosilicates of the smectite group detected in Noachian and early Hesperian terrains on Mars have been hypothesized to form under neutral to alkaline conditions. These pH conditions would also be favorable for formation of widespread carbonate deposits which have not been detected on Mars. We propose that smectite deposits on Mars formed under moderately acidic conditions inhibiting carbonate formation. We report here the first synthesis of Fe/Mg smectite in an acidic hydrothermal system [200 °C, pHRT ∼4 (pH measured at room temperature) buffered with acetic acid] from Mars-analogue, glass-rich, basalt simulant with and without aqueous Mg or Fe(II) addition under N2-purged anoxic and ambient oxic redox conditions. Synthesized Fe/Mg smectite was examined by X-ray-diffraction, Mössbauer spectroscopy, visible and near-infrared reflectance spectroscopy, scanning electron microscopy and electron microprobe to characterize mineralogy, morphology and chemical composition. Alteration of the glass phase of basalt simulant resulted in formation of the Fe/Mg smectite mineral saponite with some mineralogical and chemical properties similar to the properties reported for Fe/Mg smectite on Mars. Our experiments are evidence that neutral to alkaline conditions on early Mars are not necessary for Fe/Mg smectite formation as previously inferred. Phyllosilicate minerals could instead have formed under mildly acidic pH conditions. Volcanic SO2 emanation and sulfuric acid formation is proposed as the major source of acidity for the alteration of basaltic materials and subsequent formation of Fe/Mg smectite.

Reference
Peretyazhko TS, Sutter B, Morris RV, Agresti DG, Le L, Ming DW (2015) Fe/Mg smectite formation under acidic conditions on early Mars. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.10.012]
Copyright Elsevier

¹Yasuhito Sekine et al. (>10*)
¹Department of Earth and Planetary Science, University of Tokyo, Bunkyo 113-0033, Japan
*Find the extensive, full author and affiliation list on the publishers website

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

Reference
Sekine Y et al. (2015) High-temperature water–rock interactions and hydrothermal environments in the chondrite-like core of Enceladus. Nature Communications 6, 8604 Link to Article [doi:10.1038/ncomms9604]