Gary Lofgren^a

^a1905 Trail View, Friendswood, Texas 77546, U.S.A.

New data based on a detailed analysis of pyroxene zoning strongly suggests that convection is an important process in lunar magmas. Elardo and Shearer (2014) carefully document irregular oscillatory zoning that is best explained by movement of pyroxene crystals in a convecting magma. Lunar samples that contain such data are rare, but this study should inspire more extensive efforts to further document magmatic processes.

Reference
Lofgren G (2014) New data on lunar magmatic processes. American Mineralogist 99:561.
[doi:10.2138/am.2014.4803]
Copyright: The Mineralogical Society of America

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Valérie Malavergne^a,bet al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aUniversité Paris Est Marne La Vallée, Laboratoire des Géomatériaux et Environnement, Champs-sur-Marne Cedex, 77454, France
^bEcole Normale Supérieure, Laboratoire de Géologie, 24 rue Lhomond, 75005 Paris, France

Mercury is notorious as the most reduced planet with the highest metal/silicate ratio, yet paradoxically data from the MESSENGER spacecraft show that its iron-poor crust is high in sulfur (up to ∼6 wt%, ∼80× Earth crust abundance) present mainly as Ca-rich sulfides on its surface. These particularities are simply impossible on the other terrestrial planets. In order to understand the role played by sulfur during the formation of Mercury, we investigated the phase relationships in Mercurian analogs of enstatite chondrite-like composition experimentally under conditions relevant to differentiation of Mercury (∼1 GPa and 1300–2000 °C). Our results show that Mg-rich and Ca-rich sulfides, which both contain Fe, crystallize successively from reduced silicate melts upon cooling below 1550 °C. As the iron concentration in the reduced silicates stays very low (≪1 wt%), these sulfides represent new host phases for both iron and sulfur in the run products. Extrapolated to Mercury, these results show that Mg-rich sulfide crystallization provides the first viable and fundamental means for retaining iron as well as sulfur in the mantle during differentiation, while sulfides richer in Ca would crystallize at shallower levels. The distribution of iron in the differentiating mantle of Mercury was mainly determined by its partitioning between metal (or troilite) and Mg–Fe–Ca-rich sulfides rather than by its partitioning between metal (or troilite) and silicates. Moreover, the primitive mantle might also be boosted in Fe by a reaction at the core mantle boundary (CMB) between Mg-rich sulfides of the mantle and FeS-rich outer core materials to produce (Fe, Mg)S. The stability of Mg–Fe–Ca-rich sulfides over a large range of depths up to the surface of Mercury would be consistent with sulfur, calcium and iron abundances measured by MESSENGER.

Reference
Malavergne et al. (2014) How Mercury can be the most reduced terrestrial planet and still store iron in its mantle. Earth and Planetary Science Letters 394:186.
[doi:10.1016/j.epsl.2014.03.028]
Copyright Elsevier

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Timothy A. Goudge¹et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹Department of Geological Sciences, Brown University, Providence, Rhode Island, USA

We present new observations of pyroclastic deposits on the surface of Mercury from data acquired during the orbital phase of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. The global analysis of pyroclastic deposits brings the total number of such identified features from 40 to 51. Some 90% of pyroclastic deposits are found within impact craters. The locations of most pyroclastic deposits appear to be unrelated to regional smooth plains deposits, except some deposits cluster around the margins of smooth plains, similar to the relation between many lunar pyroclastic deposits and lunar maria. A survey of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval. Measurements of surface reflectance by MESSENGER indicate that the pyroclastic deposits are spectrally distinct from their surrounding terrain, with higher reflectance values, redder (i.e., steeper) spectral slopes, and a downturn at wavelengths shorter than ~400 nm (i.e., in the near-ultraviolet region of the spectrum). Three possible causes for these distinctive characteristics include differences in transition metal content, physical properties (e.g., grain size), or degree of space weathering from average surface material on Mercury. The strength of the near-ultraviolet downturn varies among spectra of pyroclastic deposits and is correlated with reflectance at visible wavelengths. We suggest that this interdeposit variability in reflectance spectra is the result of either variable amounts of mixing of the pyroclastic deposits with underlying material or inherent differences in chemical and physical properties among pyroclastic deposits.

Reference
Goudge et al. (in press) Global inventory and characterization of pyroclastic deposits on Mercury: New insights into pyroclastic activity from MESSENGER orbital data. Journal of Geophysical Research: Planets
[doi:10.1002/2013JE004480]
Published by arrangement with John Wiley & Sons

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