¹Richard D. Starr et al. (>10)
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2017.08.001]
¹Physics Department, The Catholic University of America, Washington, DC 20064, USA

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¹M.E. Varela, ²S-L. Hwang, ³P. Shen, ⁴H-T. Chu, ⁵T-F. Yui, ⁵Y. Iizuka, ⁶F. Brandstätter, ⁷Y.A. Abdu
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.027]
¹Instituto de Ciencias Astronómicas de la Tierra y del Espacio (ICATE), Avenida España 1512 sur, J5402DSP, San Juan, Argentina
²Department of Materials Science and Engineering, National Dong Hwa University, Hualien, Taiwan, ROC
³Department of Materials and Optoelectronic Science, National Sun Yat-sen University, Kaohsiung, Taiwan, ROC
⁴Central Geological Survey, PO Box 968, Taipei, Taiwan, ROC
⁵Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, ROC
⁶Mineralogisch-Petrographische Abteilung, Naturhistorisches Museum, Burgring 7, 1010 Wien, Austria
⁷Department of Applied Physics and Astronomy, University of Sharjah, P.O.Box 27272, Sharjah, United Arab Emirate
Copyright Elsevier

Olivinites, together with olivine megacrysts, are the most magnesian phases found in angrites. Their chemical composition (mg# 90) is out of equilibrium with the groundmass and far away from that of possible precipitates from angrite parent melts. Therefore olivinites, as well as olivine megacrysts, were considered as xenoliths and xenocrysts. We report here a detailed study of five olivinites from the angrite D’Orbigny. Our results indicate that D’Orbigny experienced metasomatic alteration processes, which led to enrichments in FeO and MnO (relative to the original composition), changing the initial Mg-rich composition of the olivines to the one seen now. As this process took place in equilibrium with a chondritic reservoir (e.g., Fe/Mn ratios spreading around primitive values), the primitive (Mg-rich) olivine chemical composition was changed towards a more fayalitic one while preserving a chondritic signature. This chondritic signature was preserved in the Fe/Mn ratio of the olivinites, olivine megacrysts, augite grains in olivinites and groundmass olivine of D’ Orbigny. Therefore the fayalite content of about 35 mol.% that characterizes the groundmass olivine of this rock – as well as other angrites- does not correspond to its original composition but may be the result of a late metasomatic process that affected these rocks. If so, olivinites and Mg-rich olivines might not be compositionally exotic phases but are an early constituent phase that retained the pristine more reducing conditions that have been preserved in some angrites, where they form either a small part of the rock (e.g., Asuka 881371 and D’Orbigny) or the majority of it (NWA 8535).

^1,2Stepan M. Chernonozhkin, ³Mona Weyrauch, ^1,2Steven Goderis, ³Martin Oeser, ²Seann J. McKibbin, ³Ingo Horn, ⁴Lutz Hecht, ³Stefan Weyer, ²Philippe Claeys, ¹Frank Vanhaecke
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.022]
¹Ghent University, Department of Analytical Chemistry, Campus Sterre, Krijgslaan, 281 – S12, 9000 Ghent, Belgium
²Vrije Universiteit Brussel, Analytical, Environmental, and Geo- Chemistry, Pleinlaan 2, 1050 Brussels, Belgium
³Leibniz Universität Hannover, Institute of Mineralogy, Callinstrasse 3, 30167 Hannover, Germany
⁴Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Invalidenstrasse 43, 10115 Berlin, Germany
Copyright Elsevier

In this work, a femtosecond laser ablation (LA) system coupled to a multi-collector inductively coupled plasma-mass spectrometer (fs-LA-MC-ICP-MS) was used to obtain laterally resolved (30-80 μm), high-precision combined Ni and Fe stable isotope ratio data for a variety of mineral phases (olivine, kamacite, taenite, schreibersite and troilite) composing main group pallasites (PMG) and iron meteorites. The stable isotopic signatures of Fe and Ni at the mineral scale, in combination with the factors governing the kinetic or equilibrium isotope fractionation processes, are used to interpret the thermal histories of small differentiated asteroidal bodies. As Fe isotopic zoning is only barely resolvable within the internal precision level of the isotope ratio measurements within a single olivine in Esquel PMG, the isotopically lighter olivine core relative to the rim (Δ56/54Ferim-core = 0.059 ‰) suggests that the olivines were largely thermally equilibrated. The observed hint of an isotopic and concentration gradient for Fe of crudely similar width is interpreted here to reflect Fe loss from olivine in the process of partial reduction of the olivine rim. The ranges of the determined Fe and Ni isotopic signatures of troilite (δ56/54Fe of -0.66 to -0.09 ‰) and schreibersite (δ56/54Fe of -0.48 to -0.09 ‰, and δ62/60Ni of -0.64 to +0.29 ‰) may result from thermal equilibration. Schreibersite and troilite likely remained in equilibrium with their enclosing metal to temperatures significantly below their point of crystallization. The Ni isotopic signatures of bulk metal and schreibersite correlate negatively, with isotopically lighter Ni in the metal of PMGs and isotopically heavier Ni in the metal of the iron meteorites analyzed. As such, the light Ni isotopic signatures previously observed in PMG metal relative to chondrites may not result from heterogeneity in the Solar Nebula, but rather reflect fractionation in the metal-schreibersite system. Comparison between the isotope ratio profiles of Fe and Ni determined across kamacite-taenite interfaces (Δ56/54Fekam-tae = -0.51 to -0.69 ‰ and Δ62/60Nikam-tae = +1.59 to +2.50 ‰) and theoretical taenite sub-solidus diffusive isotopic zoning broadly constrain the cooling rates of Esquel, CMS 04071 PMGs and Udei Station IAB to between ∼25 and 500 °C/Myr.

¹A.A. Nemchin, ²H. Jeon, ³J.J. Bellucci, ¹N.E. Timms, ³J.F. Snape, ²M.R. Kilburn, ³M.J. Whitehouse
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.024]
¹Department of Applied Geology, Curtin University, Perth, WA 6845, Australia
²Centre for Microscopy Characterisation and Analysis, University of Western Australia, 35 Stirling Highway, Perth, WA 6009, Australia
³Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
Copyright Elsevier

Pb-Pb isochron ages of ca. 3.92 Ga for three K-feldspar-rich clasts from Apollo 14 breccias 14303 and 14083 were determined using Secondary Ion Mass Spectrometry (SIMS). These ages are interpreted to represent the resetting of the U-Pb system in the clasts as a result of brecciation during the Imbrium impact. One of the clasts contains zircon grains that record a significantly older crystallization age (ca. 4.33-4.35 Ga) for the rock represented by that clast. Initial Pb compositions determined for the clasts, combined with the previously measured Pb isotopic compositions of K-feldspar grains from several Apollo 14 breccia samples, constrain a range of initial Pb compositions in the ca. 3.9 Ga Fra Mauro formation at the Apollo 14 landing site. This range in initial Pb compositions indicates that the rocks represented by these clasts, or the sources of those rocks, evolved with a high 238U/204Pb (μ-value) for substantial periods of time, although the precise crystallization ages of the rocks represented by at least two of the clasts investigated here are unknown.

¹J. Moreau,^1,2T. Kohout,³K. Wünnemann
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12935]
¹Department of Physics, University of Helsinki, Helsinki, Finland
²Institute of Geology, The Czech Academy of Sciences, Prague, Czech Republic
³Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
Published by arrangement with John Wiley & Sons

We determined the shock-darkening pressure range in ordinary chondrites using the iSALE shock physics code. We simulated planar shock waves on a mesoscale in a sample layer at different nominal pressures. Iron and troilite grains were resolved in a porous olivine matrix in the sample layer. We used equations of state (Tillotson EoS and ANEOS) and basic strength and thermal properties to describe the material phases. We used Lagrangian tracers to record the peak shock pressures in each material unit. The post-shock temperatures (and the fractions of the tracers experiencing temperatures above the melting point) for each material were estimated after the passage of the shock wave and after the reflections of the shock at grain boundaries in the heterogeneous materials. The results showed that shock-darkening, associated with troilite melt and the onset of olivine melt, happened between 40 and 50 GPa with 52 GPa being the pressure at which all tracers in the troilite material reach the melting point. We demonstrate the difficulties of shock heating in iron and also the importance of porosity. Material impedances, grain shapes, and the porosity models available in the iSALE code are discussed. We also discuss possible not-shock-related triggers for iron melt.

¹Klaus Keil, ²Timothy J.McCoy
Chemie der Erde (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2017.04.004]
¹Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, HI 96822, USA
²Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560-0119, USA
Copyright Elsevier

Acapulcoites (most ancient Hf-W ages are 4,563.1 ± 0.8 Ma), lodranites (most ancient Hf-W ages are 4,562.6 ± 0.9 Ma) and rocks transitional between them are ancient residues of different degrees of partial melting of a chondritic source lithology (e.g., as indicated by the occurrence of relict chondrules in 9 acapulcoites), although the precise chondrite type is unknown. Acapulcoites are relatively fine- grained (∼150–230 μm) rocks with equigranular, achondritic textures and consist of olivine, orthopyroxene, Ca-rich clinopyroxene, plagioclase, metallic Fe,Ni, troilite, chromite and phosphates. Lodranites are coarser grained (540–700 μm), with similar equigranular, recrystallized textures, mineral compositions and contents, although some are significantly depleted in eutectic Fe,Ni-FeS and plagioclase- clinopyroxene partial melts. The acapulcoite-lodranite clan is most readily distinguished from other groups of primitive achondrites (e.g., winoanites/IAB irons) by oxygen isotopic compositions, although more than 50% of meteorites classified as acapulcoites currently lack supporting oxygen isotopic data. The heat source for melting of acapulcoites-lodranites was internal to the parent body, most likely 26Al, although some authors suggest it was shock melting. Acapulcoites experienced lower temperatures of ∼980–1170 °C and lower degrees of partial melting (∼1–4 vol.%) and lodranites higher temperatures of ∼1150–1200 °C and higher degrees (∼5 ≥ 10 vol.%) of partial melting. Hand-specimen and thin section observations indicate movement of Fe,Ni-FeS, basaltic, and phosphate melts in veins over micrometer to centimeter distances. Mineralogical, chemical and isotopic properties, Cosmic Ray Exposure (CRE) ages which cluster around 4–6 Ma and the occurrence of some meteorites consisting of both acapulcoite and lodranite material, indicate that these meteorites come from one parent body and were most likely ejected in one impact event. Whereas the precise parent asteroid of these meteorites is unknown, there is general agreement that it was an S-type object. There is nearly total agreement that the acapulcoite-lodranite parent body was <∼100 km in radius and, based on the precise Pb–Pb age for Acapulco of 4555.9 ± 0.6 Ma, combined with the Hf/W and U/Pb records and cooling rates deduced from mineralogical and other investigations, that the parent body was fragmented during its cooling which the U/Pb system dates at precisely 4556 ± 1 Ma. Hf-W chronometry suggests that the parent body of the acapulcoites-lodranites and, in fact, the parent bodies of all “primitive achondrites” accreted slightly later than those of the differentiated achondrites and, thus, had lower contents of 26Al, the heat producing radionuclide largely responsible for heating of both primitive and differentiated achondrites. Thus, the acapulcoite-lodranite parent body never experienced the high degrees of melting responsible for the formation of the differentiated meteorites, but arrested its melting history at relatively low degrees of ∼15 vol.%.

¹M. Melwani Daswani,¹E. S. Kite
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2017JE005319]
¹Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, USA
Published by arrangement wit John Wiley & Sons

Chloride-bearing deposits on Mars record high-elevation lakes during the waning stages of Mars’ wet era (mid-Noachian to late Hesperian). The water source pathways, seasonality, salinity, depth, lifetime, and paleoclimatic drivers of these widespread lakes are all unknown. Here we combine reaction-transport modeling, orbital spectroscopy, and new volume estimates from high-resolution digital terrain models, in order to constrain the hydrologic boundary conditions for forming the chlorides. Considering a T = 0 °C system, we find: (1) individual lakes were >100 m deep and lasted decades or longer; (2) if volcanic degassing was the source of chlorine, then the water-to-rock ratio or the total water volume were probably low, consistent with brief excursions above the melting point and/or arid climate; (3) if the chlorine source was igneous chlorapatite, then Cl-leaching events would require a (cumulative) time of >10 yr at the melting point; (4) Cl masses, divided by catchment area, give column densities 0.1 – 50 kg Cl/m2, and these column densities bracket the expected chlorapatite-Cl content for a seasonally-warm active layer. Deep groundwater was not required. Taken together, our results are consistent with Mars having a usually cold, horizontally segregated hydrosphere by the time chlorides formed.

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.