¹F.Tosi et al.
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.08.012]
¹INAF-IAPS Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere, 100, I-00133 Rome, Italy
Copyright Elsevier

Ac-H-6 ‘Haulani’ is one of five quadrangles that cover the equatorial region of the dwarf planet Ceres. This quadrangle is notable for the broad, spectrally distinct ejecta that originate from the crater Haulani, which gives the name to the quadrangle. These ejecta exhibit one of the most negative (‘bluest’) visible to near infrared spectral slope observed across the entire body and have distinct color properties as seen in multispectral composite images. Besides Haulani, here we investigate a broader area that includes other surface features of interest, with an emphasis on mineralogy as inferred from data obtained by Dawn’s Visible InfraRed mapping spectrometer (VIR), combined with multispectral image products from the Dawn Framing Camera (FC) so as to enable a clear correlation with specific geologic features.

Our analysis shows that crater Haulani stands out compared to other surface features of the quadrangle. Albedo maps obtained in the near infrared range at 1.2 μm and 1.9 μm reveal that the floor and ejecta of Haulani are indeed a patchwork of bright and dark material units. Visible to near-infrared spectral slopes display negative values in crater Haulani’s floor and ejecta, which are indicative of a younger age. Spectral features centered at ∼2.7 μm and ∼3.1 μm, respectively diagnostic of magnesium-bearing phyllosilicates and ammoniated phyllosilicates, show a substantial decrease in band depth in crater Haulani’s floor and bright ejecta. Similar, but less prominent, spectral behavior is observed in other small craters of this quadrangle. There is a general trend in quadrangle Ac-H-6 for the two 2.7-μm and 3.1-μm band depths to increase from the northwest to the southeast. However, it is worth noting that the correlation between these two spectral parameters is generally strong in the Haulani crater’s area, but much weaker elsewhere, which indicates a variable degree of mixing between these two major mineral phases.

^1,2Agata M. Krzesińska
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12933]
¹Department of Earth Sciences, Natural History Museum, London, UK
²Institute of Geological Sciences, Polish Academy of Sciences, Wrocław, Poland
Published by arrangement with John Wiley & Sons

Three-dimensional X-ray tomographic reconstructions and petrologic studies reveal voluminous accumulations of metal in Pułtusk H chondrite. At the contact of these accumulations, the chondritic rock is enriched in troilite. The rock contains plagioclase-rich bands, with textures suggesting crystallization from melt. Unusually large phosphates are associated with the plagioclase and consist of assemblages of merrillite, and fluorapatite and chlorapatite. The metal accumulations were formed by impact melting, rapid segregation of metal-sulfide melt and the incorporation of this melt into the fractured crater basement. The impact most likely occurred in the early evolution of the H chondrite parent body, when post-impact heat overlapped with radiogenic heat. This enabled slow cooling and separation of the metallic melt into metal-rich and sulfide-rich fractions. This led to recrystallization of chondritic rock in contact with the metal accumulations and the crystallization of shock melts. Phosphorus was liberated from the metal and subsumed by the silicate shock melt, owing to oxidative conditions upon slow cooling. The melt was also a host for volatiles. Upon further cooling, phosphorus reacted with silicates leading to the formation of merrillite, while volatiles partitioned into the residual halogen-rich, dry fluid. In the late stages, the fluid altered merrillite to patchy Cl/F-apatite. The above sequence of alterations demonstrates that impact during the early evolution of chondritic parent bodies might have contributed to local metal segregation and silicate melting. In addition, postshock conditions supported secondary processes: compositional/textural equilibration, redistribution of volatiles, and fluid alterations.

¹Shu-Zhou Wang,^1,2Ai-Cheng Zhang,¹Run-Lian Pang,¹Jia-Ni Chen,³Li-Xin Gu,¹Ru-Cheng Wang
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12931]
¹State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing, China
²Lunar and Planetary Science Institute, Nanjing University, Nanjing, China
³Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 7755 is a newly found enriched lherzolitic shergottite. Here, we report its detailed petrography and mineralogy. NWA 7755 contains both poikilitic and non-poikilitic lithologies. Olivine has different compositional ranges in the poikilitic and non-poikilitic lithologies, Fa30–39 and Fa37–40, respectively. Pyroxene in the non-poikilitic lithology is systematically Fe-richer than that in the poikilitic lithology. The chromite grains in non-poikilitic lithology are highly Ti-richer than those in the poikilitic lithology. The chemical variations of olivine, pyroxene, and chromite between the poikilitic and non-poikilitic lithologies support a two-stage formation model of lherzolitic shergottites. Besides planar fractures and strong mosaicism in olivine and pyroxene, shock-induced melt veins and pockets are observed in NWA 7755. Olivine grains within and adjacent to melt veins and/or pockets have either transformed to ringwoodite, amorphous phase, or dissociated to bridgmanite plus magnesiowüstite. Merrillite in melt veins has completely transformed to tuite; however, apatite only has partially transformed to tuite, indicating a relatively sluggish transformation rate. The partial transformation from apatite to tuite resulted in fractional devolatilization of Cl and F in apatite. The fine-grained mineral assemblage in melt veins consists mainly of bridgmanite, minor magnesiowüstite, Fe-sulfide, Fe-phosphide, and Ca-phosphate minerals. The coexistence of bridgmanite and magnesiowüstite in these veins indicates a shock pressure of >~24 GPa and a temperature of 1800–2000 °C. Coesite and seifertite are probably present in NWA 7755. The presence of these high-pressure minerals indicates that NWA 7755 has experienced a more intense shock metamorphism than other enriched lherzolitic shergottites.