¹Michael Volk,¹Roger Fu,²Anna Mittelholz,³James M.D. Day
Journal of Geophysical Research Planets (in Press) Link to Article [https://doi.org/10.1029/2021JE006856]
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.

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 [https://doi.org/10.1029/2020JE006696]
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.

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 [https://doi.org/10.1029/2020JE006777]
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.

^1,2Ryoga Maeda,¹Steven Goderis,²Vinciane Debaille,²Hamed Pourkhorsandi,²Geneviève Hublet,¹Philippe Claeys
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.05.005]
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.