Formation of lunar highlands anorthosites

1Xiaoqing Xu,1,2Hejiu Hui,3Wei Chen,4Shichun Huang,5Clive R.Neal,1XishengXu
Earth and Planetary Science Letters 536, 116138 Link to Article []
1State Key Laboratory of Mineral Deposits Research & Lunar and Planetary Science Institute, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China
sCAS Center for Excellence in Comparative Planetology, Hefei 230026, China
3State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
4Department of Geoscience, University of Nevada, Las Vegas, NV 89154, United States
5Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, IN 46556, United States
Copyright Elsevier

The lunar magma ocean (LMO) model was proposed after the discovery of anorthosite in Apollo 11 samples. However, the chemical and isotopic compositions of lunar anorthosites are not fully consistent with its LMO origin. We have analyzed major and trace elements in anorthositic clasts from ten lunar feldspathic meteorites, which are related to the solidification of the LMO. The plagioclase rare earth element (REE) abundances and patterns are not correlated with the Mg# of coexisting mafic minerals in anorthosites, implying that mafic minerals and plagioclase may not be in chemical equilibrium, consistent with their textural differences. The REE abundances in plagioclase range approximately fortyfold, which cannot be produced by fractional crystallization of a single magma. Combining plagioclase trace element data from Apollo and meteoritic anorthosites, we propose that plagioclases derived from the LMO floated to the surface to form the primordial crust, which then may have been metasomatized by incompatible-element-rich KREEP (potassium, rare earth element, phosphorus) melts and mantle-derived partial melts. The lunar anorthosites may represent this metasomatized crust rather than solely a derivative from the LMO. Furthermore, silicate melts similar to the metasomatic agents may also have melted the crust to form the Mg-suite rocks. This hypothesis is consistent with overlapping ranges of age and initial εNd between lunar anorthosites and Mg-suite rocks. These events are consistent with an overturn event of the cumulate mantle very early after primordial crust formation to produce the partial melts that metasomatized the crust.

Spectral investigation of Ceres analogue mixtures: In-depth analysis of crater central peak material (ccp) on Ceres

1A.Galiano,1F.Dirri,1,2E.Palomba,1 A.Longobardo,3B.Schmitt,3P.Beck
Icarus (in Press) Link to Article []
1INAF-IAPS, Rome, Italy
2SSDC-ASI, Rome, Italy
3Université Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France
Copyright Elsevier

The dwarf planet Ceres is an airless body composed of Mg-phyllosilicates, NH4-phyllosilicates, Mg/Ca-carbonates and a dark component. The subsurface of Ceres, investigated by the material composing the peak of complex craters (ccp, crater central peak material; Galiano et al., 2019), reveals a composition similar to the surface, with an increasing abundance of phyllosilicates in the interior. A moderate trend between age of craters’ formation and spectral slope of ccps suggests that younger ccps show a negative/blue slope and older ccps are characterized by positive/red slope. To investigate the causes of different spectral slope in ccps, different grain-sized Ceres analogue mixtures were produced and spectrally analysed. First, the end-members of the Ceres surface (using the antigorite as Mg-phyllosilicate, the NH4-montmorillonite as NH4-phyllosilicate, the dolomite as carbonate and the graphite as dark component), were mixed, obtaining mixtures with different relative abundance, and identifying the mixture with the reflectance spectrum most similar to the average Ceres spectrum. The selected mixture was reproduced with grain size of 0–25 μm, 25–50 μm and 50–100 μm. The three mixtures were heated and spectrally analysed, both with an acquisition temperature of 300 K (room temperature) and 200 K (typical for surface Ceres temperature during VIR observations).

The best analogue Ceres spectrum is coincident with a mixture composed of 18 M% (mass percentage) of Dolomite, 18 M% of Graphite, 36 M% of Antigorite and 28 M% of NH4-montmorillonite, after experiencing a heating process.

The heating process produces: 1) a darkening and reddening of spectrum, as consequence of the devolatilization of OH group in phyllosilicates and a more dominant effect of opaque phase; 2) a deepening in the intensity of the 3.4 and 4.0 μm band, as well as the 2.7 and the 3.1 μm band, likely due to the loss of absorbed atmospheric water; 3) narrowing of 3.1 μm band and the shift of band center toward longer wavelength (i.e. at 3.06 μm) coincident with mean Ceres spectrum, related to the loss of absorbed atmospheric water.

The analysis of the best Ceres analogue mixture, reproduced at different grain size and after heating process, reveals a weakening of 2.7, 3.1, 3.4 and 4.0 absorption bands in coarser samples, likely related to large size of dark grains which reduce the spectral contrast. Furthermore, spectra of coarser mixtures are more red-sloped, suggesting that this trend is more affected by the dark component.

The best analogue Ceres mixture produced in this work is almost coincident with the mean spectrum of Haulani ccp, the youngest ccps on Ceres and therefore representative of less altered material on Ceres.

The redder spectral slope observed in the older ccps is probably the consequence of the space weathering effects on the original material composing the peak.

Toward the geological significance of hydrated silica detected by near infrared spectroscopy on Mars based on terrestrial reference samples

1M.PineauaL.Le Deit,2B.Chauviré,3J.Carter,1B.Rondeau,1N.Mangold
Icarus (in Press) Link to Article []
1Laboratoire de Planétologie et Géodynamique, CNRS UMR 6112, Université de Nantes, Université d’Angers, Nantes, France
2Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, Grenoble, France
3Institut d’Astrophysique Spatiale, CNRS UMR 8617, Université Paris-Sud, Orsay, France
Copyright Elsevier

Hydrated silica detected on the martian surface, from both orbital and in-situ data, is an indicator of past aqueous conditions. On Earth, several near infrared (NIR) spectral criteria can be used to discriminate silica phases (e.g. opal-A, opal-CT and chalcedony) and their formation processes. We have applied these spectral criteria to Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data in order to investigate the geological origin of hydrated silica on Mars. We used two spectral criteria: (i) the crystallinity spectral criteria on the 1.4- and 1.9 μm absorption bands to distinguish between amorphous (opal-A and hydrated glasses) and more crystalline (opal-CT and chalcedony) varieties of silica, and (ii) the Concavity-Ratio-Criterion (CRC) to differentiate opals of hydrothermal origin from weathering origin. We first adapted the CRC measurements on terrestrial samples to make them comparable to CRISM measurements on Mars: we resampled our terrestrial spectra down to the CRISM resolution, and tested the martian pressure effect on spectral signatures. Then, we selected several areas over nine sites where hydrated silica has been detected on Mars, on the basis of good quality detections. Our results show that two main types of spectra can be distinguished, and these are consistent with two distinct geomorphological contexts proposed by Sun and Milliken (2018): amorphous and/or dehydrated silica-bearing bedrock deposits, and more crystalline and/or hydrated silica-bearing aeolian deposits. The concavity criterion also indicates silica origins that are in agreement with most of the hypothesized geological origins proposed in the literature. Although these results need further strengthening, they are promising for the use of NIR signatures as means of investigating the processes of hydrated silica on Mars.

The Properties of Planetesimal Collisions under Jupiter’s Perturbation and the Application to Chondrule Formation via Impact Jetting

1,2Shoichi Oshino,3Yasuhiro Hasegawa,1,4Shigeru Wakita,1,5,6Yuji Matsumoto
The Astrophysical Journal 884, 37 Link to Article [DOI]
1Center for Computational Astrophysics, National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
2Institute for Cosmic Ray Research, University of Tokyo, Hida, Gifu 506-1205, Japan
3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
4Earth-Life Science Institute, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8550, Japan
5Planetary Exploration Research Center, Chiba Institute of Technology, Narashino, Chiba 275-0016, Japan
6Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 10617, Taiwan

Understanding chondrule formation provides invaluable clues about the origin of the solar system. Recent studies suggest that planetesimal collisions and the resulting impact melts are promising for forming chondrules. Given that the dynamics of planetesimals is a key in impact-based chondrule formation scenarios, we here perform direct N-body simulations to examine how the presence of Jupiter affects the properties of chondrule-forming collisions. Our results show that the absence/presence of Jupiter considerably changes the properties of high-velocity collisions whose impact velocities are higher than 2.5 km s−1. High-velocity collisions occur due to impacts between protoplanets and planetesimals for the case without Jupiter; for the case with Jupiter, the eccentricities of planetesimals are pumped up by the secular and resonant perturbations from Jupiter. We also categorize the resulting planetesimal collisions and find that most high-velocity collisions are classified as grazing ones for both cases. To examine the effect of Jupiter on chondrule formation directly, we adopt the impact-jetting scenario and compute the resulting abundance of chondrules. Our results show that for the case without Jupiter, chondrule formation proceeds in the inside-out manner, following the growth of protoplanets. If Jupiter is present, the location and timing of chondrule formation are determined by Jupiter’s eccentricity, which is treated as a free parameter in our simulations. Thus, the existence of Jupiter is the key parameter for specifying when and where chondrule formation occurs for impact-based scenarios.

Fingerprints of the Protosolar Cloud Collapse in the Solar System. II. Nucleosynthetic Anomalies in Meteorites

1Emmanuel Jacquet,1,2Francesco C. Pignatale,2Marc Chaussidon,2Sébastien Charnoz
The Astrophysical Journal 884, 32 Link to Article [DOI]
1Muséum national d’Histoire naturelle, UMR 7590, CP52, 57 rue Cuvier, F-75005, Paris, France
2Institut de Physique du Globe de Paris (IPGP), 1 rue Jussieu, F-75005, Paris, France

The isotopic heterogeneity of the solar system shown by meteorite analyses is more pronounced for its earliest objects, the calcium–aluminum-rich inclusions (CAIs). This suggests that it was inherited from spatial variations in stardust populations in the protosolar cloud. We model the formation of the solar protoplanetary disk following its collapse and find that the solid-weighted standard deviation of different nucleosynthetic contributions in the disk is reduced by one order of magnitude compared to the protosolar cloud, whose successive isotopic signatures are fossilized by CAIs. The enrichment of carbonaceous chondrites in r-process components, whose proportions are inferred to have diminished near the end of infall, is consistent with their formation at large heliocentric distances, where the early signatures would have been preferentially preserved after outward advection. We also argue that thermal processing had little effect on the (mass-independent) isotopic composition of bulk meteorites for refractory elements.

Fingerprints of the Protosolar Cloud Collapse in the Solar System. I. Distribution of Presolar Short-lived 26Al

1,2Francesco C. Pignatale,1Emmanuel Jacquet,2Marc Chaussidon,2Sébastien Charnoz
The Astrophysical Journal 884, 31 Link to Article [DOI]
1Muséum national d’Histoire naturelle, Institut de Minéralogie, Physique des Matériaux et de Cosmochimie, Département Origines et Evolution, UMR 7590, CP52, 57 rue Cuvier, F-75005, Paris, France
2Université de Paris, Institut de Physique du Globe de Paris, CNRS, 1 rue Jussieu, F-75005 Paris, France

The short-lived radionuclide 26Al is widely used to determine the relative ages of chondrite components and timescales of physical and thermal events that attended the formation of the solar system. However, an important assumption for using 26Al as a chronometer is its homogeneous distribution in the disk. Yet, the oldest components in chondrites, the Ca–Al-rich inclusions (CAIs), which are usually considered as time anchors for this chronometer, show evidence of 26Al/27Al variations independent of radioactive decay. Since their formation epoch may have been contemporaneous with the collapse of the parent cloud that formed the disk, this suggests that 26Al was heterogeneously distributed in the cloud. We model the collapse of such a heterogeneous cloud, using two different 26Al distributions (monotonic and nonmonotonic), and follow its redistribution in the first condensates and bulk dust that populate the forming disk. We find that CAIs inherit the 26Al/27Al ratio of the matter infalling at the time of their formation, so that variations of 26Al/27Al among primordial CAIs can be accounted for, independently of radioactive decay. The prevalence of a canonical ratio among them and its necessity for the differentiation of the first planetesimals suggest a (monotonic) scenario where 26Al sharply rose relatively close to the center of the protosolar cloud and essentially remained at a high level outward (rather than decreased since). As the 26Al abundance would be relatively homogeneous after cessation of infall, this would warrant the use of the Al–Mg chronometer from the formation of “regular” CAIs onward, to chondrules and chondrite accretion.