The chemical composition and homogeneity of the Allende matrix

Planetary and Space Science (in Press) Link to Article []
1University of Bern, Physics Institute, Space Research and Planetary Sciences, Sidlerstrasse 5, CH – 3012 Bern, Switzerland
2University of Bern, Institute of Geological Sciences, Baltzerstrasse 3, CH – 3012 Bern, Switzerland

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1,2V.G.Shevchenko et al. (>10)
Planetary and Space Science (in Press) Link to Article []
1Institute of Astronomy of V.N. Karazin Kharkiv National University, Kharkiv 61022, 4 Svobody sq., Ukraine
2Department of Astronomy and Space Informatics of V.N. Karazin Kharkiv National University, Kharkiv 61022, 4 Svobody sq., Ukraine

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Formation of rims around chondrules via porous aggregate accretion

1Yuji Matsumoto,2Yasuhiro Hasegawa,3Nozomi Matsuda,3Ming-Chang Liu
Icarus (in Press) Link to Article []
1Institute of Astronomy and Astrophysics, Academia Sinica, No.1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan
2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
3Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, USA
Copyright Elsevier

Chondrules are often surrounded by fine-grained rims or igneous rims. The properties of these rims reflect their formation histories. While the formation of fine-grained rims is modeled by the accretion of dust grains onto chondrules, the accretion should be followed by the growth of dust grains due to the shorter growth timescale than the accretion. In this paper, we investigate the formation of rims, taking into account the growth of porous dust aggregates. We estimate the rim thickness as a function of the chondrule fraction at a time when dust aggregate accretion onto chondrules is switched to collisions between these chondrules. Our estimations are consistent with the measured thicknesses of fine-grained rims in ordinary chondrites. However, those of igneous rims are thicker than our estimations. The thickness of igneous rims would be enlarged in remelting events.

Lunar meteorite Northwest Africa 11962: A regolith breccia containing records of titanium‐rich lunar volcanism and the high alkali suite

1Andreas Bechtold,2Franz Brandstätter,2Lidia Pittarello,2Ludovic Ferrière,3Richard C. Greenwood,1Christian Koeberl
Meteoritics & Planetary Science (in Press) Link to Article []
1Department of Lithospheric Research, University of Vienna, Althanstrasse 14, Vienna, 1090 Austria
2Natural History Museum Vienna, Burgring 7, Vienna, 1010 Austria
3Planetary and Space Sciences, School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA UK
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 11962 is a lunar regolith breccia composed of a wide range of different clasts. The lunar origin of this meteorite is confirmed by oxygen isotope analysis and the Fe/Mn ratio in pyroxene and olivine. In the present study, the clasts and the matrix of NWA 11962 are characterized by optical and electron microscopy along with electron microprobe analyses and micro‐Raman spectroscopy. The meteorite has a glassy impact melt matrix, which accounts for 35% of the surface area in the two thin sections examined, and which contains a very large variety of different lithic clasts, monomineralic clasts, and glass fragments. The presence of volcanic and impact‐related glass spherules led to the classification of this meteorite as a regolith breccia. Lithic clasts include numerous fragments of quartz monzogabbro and lunar felsite, which are quite rare lithologies in the lunar alkali suite. However, the most abundant components in the breccia are gabbroic clasts. The mineral chemistry of the pyroxenes in the gabbroic clasts and the chemistry of various types of glass fragments in the breccia indicate an origin of the regolith from an area with low‐Ti to high‐Ti mare basalt volcanism. In addition to the peculiar petrographic characteristics of NWA 11962, the possible pairing relationships with other lunar meteorites are discussed.

Metal‐rich nodules in anomalous EL3 chondrite Northwest Africa (NWA) 8785

1,2Margrit A. Rindlisbacher,2,3,4Michael K. Weisberg,2,4Denton S. Ebel,2,4Samuel P. Alpert
Meteoritics & Planetary Science (in Press) Link to Article []
1Department of Geology, Mount Holyoke College, South Hadley, Massachusetts, 01075 USA
2Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, 10024 USA
3Department of Physical Science, Kingsborough College CUNY, Brooklyn, New York, 11235 USA
4Department of Earth and Environmental Science, CUNY Graduate Center, New York, New York, 10016 USA
Published by arrangement with John Wiley & Sons

NWA 8785 is a remarkable, recently identified, unequilibrated enstatite chondrite. It was classified as an EL3 but contains highly unusual characteristics not observed in any other EL3, including a high abundance of FeO‐rich matrix and metal‐rich nodules that are texturally and mineralogically different from those in other EL3s. We characterized the mineral assemblages and compositions of metal‐rich nodules in a thin section of NWA 8785 and compared them to nodules in other EL3s to evaluate models for formation of metal‐rich nodules in EL3s. Of a total of 40 metal‐rich nodules, 10 were selected for detailed study. These metal‐rich nodules vary in their physical structure, texture, and mineral assemblages. Some contain the rare Al‐poor, alkali‐rich silicate mineral roedderite, a first discovery in an EL3, as well as the Cl‐bearing sulfide djerfisherite. The diversity of metal‐rich nodules in NWA 8785 suggests each nodule formed independently and supports their origin by primary processes prior to accretion. The high abundance of FeO‐rich matrix and the unique qualities of its metal‐rich nodules call into question classification of NWA 8785 as an EL3, but the Si content in its kamacite and Cr and Ti content in its troilite, and the presence of alabandite, support its classification as an EL3; thus, it is an EL3‐anomalous. Although alternative hypotheses exist, the presence of roedderite, as well as a magnetite‐rich matrix and sodalite, may provide the first evidence of extensive metasomatic alteration on the EL3 parent body.

Investigating the roles of magmatic volatiles, ground ice and impact-triggering on a very recent and highly explosive volcanic eruption on Mars

1,2Pranabendu Moitra,2,3David G.Horvath,2Jeffrey C.Andrews-Hanna
Earth and Planetary Science Letters 567, 116986 Link to Article []
1Department of Geosciences, University of Arizona, AZ, USA
2Lunar and Planetary Laboratory, University of Arizona, AZ, USA
3Planetary Science Institute, Tucson, AZ, USA
Copyright Elsevier

Volcanic activity on Mars has been dominantly effusive. The existence of a young (∼0.05-1 Ma) and well-preserved possible pyroclastic deposit along a segment of the Cerberus Fossae fissures, overlying the effusive lava flows making up the bulk of Elysium Planitia, provides the motivation and opportunity to explore the dynamics of explosive volcanic eruptions on Mars. Here we investigate the subsurface magmatic processes that may have led to magma fragmentation and the explosivity of the eruption forming the deposit. Using numerical models of magma ascent in a volcanic fissure, we show that the dissolved magmatic water with or without suspended crystals is capable of driving the inferred explosive magma fragmentation and the formation of the deposit. We also explore an alternative eruption scenario and show that an intruded dike explosively interacting with melted ground ice might also have generated the deposit. The close proximity of the proposed pyroclastic deposit (15-35 km) to the similarly aged Zunil impact crater suggests the possibility of an impact-triggered volcanic eruption scenario. Using scaling analysis, we find that the high seismic energy density associated with the impact may have been sufficient to trigger a volcanic eruption if a magma chamber was present in the subsurface. These findings have implications for the generation of similar explosive eruptions on Mars and other bodies, as well as the possibility of ongoing magmatic activity on Mars.

Paleointensity and Rock Magnetism of Martian Nakhlite Meteorite Miller Range (MIL) 03346: Evidence for Intense Small Scale Crustal Magnetization on Mars

1Michael Volk,1Roger Fu,2Anna Mittelholz,3James M.D. Day
Journal of Geophysical Research Planets (in Press) Link to Article []
1Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, 02138
2Institute of Geophysics, ETH Zuerich, 8092 Zuerich, Switzerland
3Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, 92093‐0244 USA
Published by arrangement with John Wiley & Sons

The martian dynamo’s strength and duration are essential for understanding Mars’ habitability and deep interior dynamics. Although most northern volcanic terranes were likely emplaced after the martian dynamo ceased, recent data from the InSight mission show stronger than predicted crustal fields. Studying young volcanic martian meteorites offers a precise, complementary method to characterize the strength of the martian crustal field and examine its implications for past dynamo activity. We present the first rock and paleomagnetic study of nine mutually oriented samples from the martian Nakhlite meteorite MIL 03346, which is well‐suited for paleomagnetic analysis due to its well‐known age (1368 ± 83 Ma) and lack of significant aqueous, thermal, and shock overprinting. Rock magnetic analysis, including quantum diamond microscope (QDM) imaging, showed that the natural remanent magnetization (NRM) is carried by Ti‐magnetite crystals containing µm‐scale ilmenite exsolution lamellae, which can accurately record ancient magnetic fields. Demagnetization of the NRM revealed a high coercivity magnetization interpreted to date from the age of eruption based on its intensity, unidirectionality, and a passing fusion crust baked contact test. Paleointensities of four samples reveal a 5.1±1.5 µT paleofield, representing the most reliable martian paleointensity estimates to‐date and stronger than the 2 µT surface fields measured by InSight. Modeling shows that the observed fields can be explained by an older sub‐surface magnetized layer without a late, active dynamo and support a deeply buried, highly magnetized crust in the northern hemisphere of Mars. These results provide corroborating evidence for strong, small scale crustal fields on Mars.

High‐Temperature VIS‐IR Spectroscopy of NH4‐Phyllosilicates

1S. De Angelis,1M. Ferrari,1M.C. De Sanctis,2E. Ammannito,1A. Raponi,1M. Ciarniello
Journal of Geophysical Research Planets (in Press) Link to Article []
1INAF‐IAPS, Via Fosso del Cavaliere 100, 00133 Rome, (Italy)
2ASI ‐ Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Rome, (Italy)
Published by arrangement with John Wiley & Sons

Ammonium phyllosilicates have been identified on the dwarf planet Ceres, thanks to infrared telescopic and orbital data from the Dawn mission, by means of the 3.06 μm spectral feature. Nevertheless, it is not known which ammonium‐bearing phyllosilicate species are present, nor the thermal processing they underwent throughout Ceres history. Identifying the NH4+‐hosting mineral species is important for deciphering Ceres’ surface mineralogy, which provides a link to its interior and putative different evolutionary pathways. Ammoniated species can have formed in the presence of water/ammonia‐rich fluids in different conditions in the interior of the planet; in case of an exogenous outer Solar System origin, they can have undergone heating at depth.

In this work, we study the visible‐infrared spectra of several NH4‐treated/untreated phyllosilicates in the range 0.35‐5 μm, acquired in vacuum and at temperatures between 298‐723K. Previously NH4‐phyllosilicates have been mostly studied at ambient condition, preventing the characterization of the NH4+ band at 3.06 μm, due to overlapping bands of water. With this new set of measurements, we investigate how the NH4‐phyllosilicates spectra are modified when the mineral’s water is lost, and which temperature is the limit for the releasing of NH4+. We present the first high temperatures/high vacuum 3‐μm reflectance spectra of ammonium phyllosilicates.

Our measurements indicate that Mg‐phyllosilicates are the best candidates for the ammonium‐bearing species. Moreover, the almost complete disappearing of NH4+ absorption feature at ∼3.06 μm for ammoniated phyllosilicates heated at the highest temperatures, indicates that such species on Ceres could not have experienced temperatures higher than 623K.

Christiansen Feature Map from the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment: Improved Corrections and Derived Mineralogy

1Paul G. Lucey,2Benjamin Greenhagen,3Kerri Donaldson Hanna,4Neil Bowles,1Abigail Flom,5David A. Paige
Journal of Geophysical Research Planets (in Press) Link to Article []
1Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa
2The Johns Hopkins University Applied Physics Laboratory
3University of Central Florida
4Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, Oxford University
5University of California at Los Angeles
Published by arrangement with John Wiley & Sons

Maps of plagioclase, olivine and pyroxene at 1 km resolution are derived from a combination of data from the Diviner Lunar Radiometer on the Lunar Reconnaissance Orbiter and the Kaguya Multiband Imager. The Diviner instrument features three infrared bands designed to characterize a spectral feature of lunar soils that is sensitive to the average silica polymerization of the surface called the Christiansen Feature, which is directly sensitive to the presence of plagioclase, the dominant lunar silicate. Existing global mineral maps based on near‐IR data largely infer the presence of plagioclase from the bright mineral’s effect on total reflectance, excepting in rare locations where the surface is nearly pure plagioclase and a weak feature in the plagioclase near‐IR spectrum can be relied upon. By integrating both wavelength regions we produced more robust estimates of the abundance of the three dominant minerals. In the process of this work, we also improved the removal of space weathering effects from Christiansen Feature maps, and showed that silica rich compositional anomalies could be reliably detected by decorrelating Christiansen Feature and FeO maps. New silica‐rich locations are reported as are the global abundances of the three major silicates.

The effects of Antarctic alteration and sample heterogeneity on Sm-Nd and Lu-Hf systematics in H chondrites

1,2Ryoga Maeda,1Steven Goderis,2Vinciane Debaille,2Hamed Pourkhorsandi,2Geneviève Hublet,1Philippe Claeys
Geochimica et Cosmochimica Acta (in Press) Link to Article []
1Analytical-, Environmental-, and Geo-Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, BE-1050 Brussels, Belgium
2Laboratoire G-Time, Université libre de Bruxelles, CP 160/02, 50, Av. F.D. Roosevelt, BE-1050, Brussels, Belgium
Copyright Elsevier

Long-lived radioactive isotope systematics, such as Sm-Nd and Lu-Hf, are useful tools as important chronometers and tracers for chemical differentiation processes. Even though Antarctic meteorites include rare meteorites such as ungrouped meteorites, the effects of Antarctic alteration on the Sm-Nd and Lu-Hf systems in chondrites have not yet been evaluated in detail. Moreover, the heterogeneity of Sm-Nd and Lu-Hf data in bulk chondrites prevents the determination of precise average Sm-Nd and Lu-Hf values (e.g., for individual chondrite groups). To examine the effects of Antarctic alteration and sample heterogeneity on the Sm-Nd and Lu-Hf isotope systematics, ten Antarctic H chondrites (HCs) and three HCs from hot deserts were characterized for their modal abundances, elemental abundances, and Sm-Nd and Lu-Hf isotopic compositions. Regardless of the classical weathering index for Antarctic meteorites and the normalized Rb abundance used as a chemical alteration indicator in this study, the modal and elemental abundances in Antarctic HCs appear to be in good agreement with those in non-Antarctic HCs. The Sm-Nd and Lu-Hf isotopic compositions of the characterized H chondrites fall within the range measured for both HC falls and for falls of other chondrite classes, except in the case of the most heavily altered samples. Consequently, the effects of Antarctic alteration processes on the Sm-Nd and Lu-Hf systematics in HCs appear to be limited, except in the case of Asuka 09516. The latter meteorite exhibits severe mineralogical and chemical alteration, with considerable losses of even the rare earth elements (REEs), which are considered relatively immobile. The 147Sm/144Nd, 143Nd/144Nd, 176Lu/177Hf, and 176Hf/177Hf of bulk HCs correlate with their P/Mg and Y/Mg. Furthermore, the Lu-Hf ratios correlate strongly with their P/Ca and Y/Ca as well as their P/Mg and Y/Mg. Thus, the distribution of the elements between constituent minerals in ordinary chondrites (OCs) may control the heterogeneity observed for the bulk Sm-Nd and Lu-Hf data. In this context, the weight ratio of Ca-phosphates to Ca-pyroxene, or at least that of Ca-phosphates to silicates, may be a key factor leading to the observed elemental and isotopic variations. This observation indicates that the nugget effect of Ca-phosphates in OCs as the result of insufficient homogenization or terrestrial alteration leads to the heterogeneities displayed by the Sm-Nd and Lu-Hf data. Moreover, it also indicates that the use of equilibrated OCs for the determination of Sm-Nd and Lu-Hf data is affected more by sample heterogeneity, especially with respect to Ca-phosphates, than is the case for unequilibrated OCs, based on the re-distribution of REEs during thermal metamorphism on their parent bodies. This study demonstrates that Antarctic meteorites commonly preserve their original Sm-Nd and Lu-Hf isotopic compositions as much as chondrite falls, although exceptions are possible in the case of severe alteration. Similar to previous studies, we recommend the use of unequilibrated chondrites, for which the re-distribution of REEs is less extensive, for the determination of well-constrained average Sm-Nd and Lu-Hf isotopic compositions for individual chondrite groups as well as their robust Chondritic Uniform Reservoir values.