Stephen M. Elardo* and Charles K. Shearer Jr.

Institute of Meteoritics, Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.

Oscillatory zoning in silicate minerals, especially plagioclase, is a common feature found in volcanic rocks from various terrestrial tectonic settings, but is nearly absent in the lunar environment. Here we report backscattered electron images, quantitative wavelength-dispersive spectrometry (WDS) analyses, and qualitative WDS elemental X-ray maps that reveal oscillatory zoning of Mg, Ca, Fe, Ti, Al, Cr, and Mn in euhedral pyroxene phenocrysts, and faint oscillatory zoning of P in olivine phenocrysts in basaltic lunar meteorite Northwest Africa (NWA) 032. This is only the third known occurrence of oscillatory zoning in lunar silicate minerals. Zoning bands in pyroxene range from ~3–5 μm up to ~60 μm in width, but are typically ~10–20 μm in width. Oscillatory bands are variable in width over short distances, often within a single grain. Most oscillatory bands preserve a euhedral form and have sharp edges; however some bands have jagged or uneven edges indicative of resorption surfaces. The short-scale oscillatory nature of the zoning in pyroxene is overprinted on longer-scale core to rim normal magmatic zoning from pigeonite to augite compositions. Oscillatory zoning of P in olivine is faint and only resolvable with high beam current (400 nA) mapping. Bands of higher P are typically only a few micrometers in width, and although they preserve a euhedral form, they are not traceable around the full circumference of a grain and have variable spacing.
Resorption surfaces, longer-scale normal magmatic zoning, and relatively thick oscillatory bands are indicative of the formation of these chemical oscillations as a result of variable magma composition. Pyroxenes likely experienced variable liquid compositions as a result of convection in a differentially cooling, chemically stratified magma chamber. Periodic replenishments of progressively decreasing volumes of primitive parental magma are also permissible and may have enabled convection. In a convection model, Mg-rich bands reflect growth in the lower, warmer, more crystal-poor regions of the chamber, whereas Ca-Al-Ti-Cr-rich bands reflect growth in the upper, cooler, more crystal-rich regions of the chamber. The limited duration of crystallization in the magma chamber and the slow diffusion rates of multiple elements among multiple crystallographic sites in clinopyroxene, combined with fast cooling upon eruption, act to preserve the oscillatory zoning. Oscillatory zoning of P in olivine is a product of solute trapping resulting from the slow diffusion of P in silicate melts and minerals, and relatively fast magma cooling rates that may be related to magma chamber convection. Differential cooling of the chamber and the fast cooling rates within the chamber are likely a product of the thermal state of the lunar crust at 2.93 Ga when NWA 032, which is currently the youngest dated lunar igneous rock, erupted onto the surface of the Moon.

Reference
Elardo SM and Shearer Jr. CK (2014) Magma chamber dynamics recorded by oscillatory zoning in pyroxene and olivine phenocrysts in basaltic lunar meteorite Northwest Africa 032. American Mineralogist 99:355-368.
[doi:10.2138/am.2014.4552]
Copyright: The Mineralogical Society of America

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Richard S. Miller^a, David J. Lawrence^b, Dana M. Hurley^b

^aDepartment of Physics, University of Alabama in Huntsville, Huntsville, AL 35899, USA
^bJohns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA

Bulk surface hydrogen enhancements have been identified within the Moon’s Shackleton crater. Using an analysis of fast and epithermal neutron data from the Lunar Prospector mission, the permanently shadowed region (PSR) within this crater has a surface concentration of 0.72±0.13 wt.% water equivalent hydrogen (WEH). In contrast, hydrogen enhancements within other polar PSRs such as Cabeus are likely buried under more than 10 cm of hydrogen-poor regolith. Subsurface hydrogen absent a surficial counterpart implies an episodic delivery mechanism. The burial depth suggests the epoch of hydrogen deposition was at least 100 million years ago if impact gardening is the dominant mechanism for volatile transport to depth. Shackleton crater’s surface enhancement may be related to its thermal environment, ~30 K warmer than other South Pole PSRs, in which thermal processes control the vertical migration of hydrogen within Shackleton but inhibit migration in colder regions.

Reference
Miller RS, Lawrence DJ and Hurley DM (in press) Identification of Surface Hydrogen Enhancements Within the Moon’s Shackleton Crater. Icarus
[doi:10.1016/j.icarus.2014.02.007]
Copyright Elsevier

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O. Panić^1,2, Th. Ratzka³, G. D. Mulders⁴, C. Dominik^5,6, R. van Boekel⁷, Th. Henning⁷, W. Jaffe⁸ and M. Min⁵

¹Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK
²European Southern Observatory, Karl Schwarzschild Strasse 2, 85748 Garching, Germany
³Universitaets-Sternwarte Muenchen, Ludwig-Maximilians-Universitaet, Scheinerstr. 1, 81679 Muenchen, Germany
⁴Lunar and Planetary Laboratory, The University of Arizona, 1629 E. University Blvd., Tucson AZ 85721, USA
⁵Astronomical Institute Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁶Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, the Netherlands
⁷Max-Planck Institute for Astronomy, Koenigstuhl 17, 69117 Heidelberg, Germany
⁸Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherland

Context. A region of roughly half of the solar system scale around the star HD 100546 is known to be largely cleared of gas and dust, in contrast to the outer disc that extends to about 400 AU. However, some material is observed in the immediate vicinity of the star, called the inner disc. Studying the structure of the inner and the outer disc is a first step to establishing the origin of the gap between them and possibly link it to the presence of planets.
Aims. We answer the question of how the dust is distributed within and outside the gap, and constrain the disc geometry.
Methods. To discern the inner from the outer disc, we used the VLTI interferometer instrument MIDI to observe the disc in the mid-infrared wavelength regime where disc emission dominates in the total flux. Our observations exploited the full potential of MIDI, with an effective combination of baselines of the VLTI 1.8 m and of 8.2 m telescopes. With baseline lengths of 40 m, our long baseline observations are sensitive to the inner few AU from the star, and we combined them with observations at shorter, 15 m baselines, to probe emission beyond the gap at up to 20 AU from the star. We modelled the mid-infrared emission using radial temperature profiles, informed by prior works on this well-studied disc. The model is composed of infinitesimal concentric annuli emitting as black bodies, and it has distinct inner and outer disc components.
Results. Using this model to simulate our MIDI observations, we derived an upper limit of 0.7 AU for the radial size of the inner disc, from our longest baseline data. This small dusty disc is separated from the edge of the outer disc by a large, ≈10 AU wide gap. Our short baseline data place a bright ring of emission at 11 ± 1 AU. This is consistent with prior observations of the transition region between the gap and the outer disc, known as the disc wall. The inclination and position angle are constrained by our data toi = 53 ± 8° and PA = 145 ± 5°. These values are close to known estimates of the rim and disc geometry and suggest co-planarity. Signatures of brightness asymmetry are seen in both short and long baseline data, unequivocally discernible from any atmospheric or instrumental effects.
Conclusions. Mid-infrared brightness is seen to be distributed asymmetrically in the vicinity of the gap, as detected in both short and long baseline data. The origin of the asymmetry is consistent with the bright disc wall, which we find to be 1–2 AU wide. The gap is cleared of micron-sized dust, but we cannot rule out the presence of larger particles and/or perturbing bodies.

Reference
Panić O, Ratzka Th, Mulders GD, Dominik C, van Boekel R, Th. Henning Th, Jaffe W and Min M (2014) Resolving HD 100546 disc in the mid-infrared: Small inner disc and asymmetry near the gap.  Astronomy & Astrophysics 562:A101.
[doi:10.1051/0004-6361/201219223]
Reproduced with permission © ESO

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