¹Maria Eugenia Varela
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.1353]
¹Instituto de Ciencias Astronómicas de la Tierra y del Espacio (ICATE)‐CONICET, Avenida España 1512 sur, J5402DSP San Juan, Argentina
Published by arrangement with John Wiley & Sons

Five silica‐rich objects (SRO) from Acfer 182 were studied. They have cryptocrystalline textures characterized by micro‐emulsion and amoeboid patterns that point toward the coexistence of pyroxene‐ and silica‐normative liquids that were quenched. Both objects have variable contents of refractory lithophile elements. Their positive Yb versus La correlation around primordial values suggests a cosmochemical process (e.g., a gas/liquid condensation) as responsible for SRO formation. The bulk trace element abundances of amoeboid‐ and emulsion‐type SRO as well as their fractionation do not support an origin through high temperature processes. Conversely, their formation might have taken place while cooling of the nebular gas in two different chondrule‐forming regions characterized by having different evolution paths. Cooling of these dust‐enriched regions might lead to the condensation of pyroxene‐rich liquids first, followed by formation of Mg‐rich and SiO₂‐rich liquids, provided irradiation and annealing were active in these regions. Irradiation could be the process involved both in the formation of cristobalite (with annealing ~1200 K) and in triggering a spinoidal decomposition causing unmixing of the enstatite liquid into two coexisting phases, such as Mg‐rich and SiO₂‐rich liquids, the precursors of the SRO in Acfer 182. Formation of emulsion‐ and amoeboid‐type objects may be the result of exposing those chondrule‐forming regions to different degrees of radiation.

¹G.Alemanno,¹A.Maturilli,¹J.Helbert,¹M.D’Amore
Earth and Planetary Science Letters 546, 116424 Link to Article [https://doi.org/10.1016/j.epsl.2020.116424]
¹Institute for Planetary Research, German Aerospace Center DLR, Rutherfordstr. 2, 12489 Berlin, Germany
Copyright Elsevier

Recent orbital data revealed the presence of hydrated minerals on the surfaces of asteroids, mainly through the identification and the study of the 3-μm spectral absorption band (Hamilton et al., 2019; Kitazato et al., 2019). The presence of an absorption feature around 3-μm on planetary bodies’ surfaces is indicative of the presence of OH-bearing minerals. This band has been widely detected on carbonaceous chondrites but its appearance and its shape are diverse indicating different composition and/or the occurrence of subsequent alteration events. In this work, we present the results of laboratory experiments performed at the Planetary Spectroscopy Laboratory (PSL) of the German Aerospace Center (DLR) to study the spectral behaviour of the 3-μm spectral features in the Mg-OH minerals with thermal variation. It has been suggested that thermal alteration processes, can darken the surfaces of carbonaceous chondrites, thus decreasing the appearance and visibility of the spectral features around 3 μm. Thermal alteration processes are consistent with the scenario currently proposed to explain the formation of 162173 Ryugu asteroid (Sugita et al., 2019). The Near Infrared Spectrometer (NIRS3) on the Hayabusa2 mission detected a weak and narrow absorption feature centred at 2.72 μm across the entire observed surface of the C-type asteroid (Kitazato et al., 2019). However, the collected spectra from the Ryugu surface show no other absorption features in the 3-μm region. To investigate this point further and analyze the variation of the spectral features around 3-μm with thermal alteration, we studied the Mg-rich phyllosilicates serpentine and saponite in two different situations: 1) thermal alteration at increasing temperature – the samples were heated at steps of 100 °C, starting from 100 °C up to 700 °C, for 4 hours each; 2) long time heating at constant temperature – samples were kept constantly at ∼250 °C for 1 month (1st step), then cooled down and measured in reflectance. This long heating process has been repeated at the same temperature of 250 °C for 2 months (2nd step). The results obtained show an important variation of phyllosilicates spectral bands with temperature and provide useful data for the interpretation of past and future mission small bodies collected surface spectra.