Volatile-rich xenolithic clasts in different meteoritic breccias represent primitive solar system material. Here, we report the mineralogy and organic maturity of C1, C2, and CM-like clasts present within five polymict ureilites, two polymict eucrites, one howardite, and one CR chondrite. Of all the clasts, the C1s are the most abundant, and the majority have mineralogy and organic maturity similar to the CI chondrites. Two C1 clasts show the least altered polyaromatic organic matter, similar to Bells, Essebi, and a CR chondrite. A few C1 clasts have unique mineralogy with abundant Fe-rich olivine surrounding magnetite grains. Such clasts show a Raman signature of mild to moderate heating effects related to impact-shock processing. The Fe-rich olivine within these clasts formed around magnetite grains from an aqueous fluid at a water/rock ratio of 0.1-0.2, T > 150 degrees C, and high activity of SiO2 and Fe. Additionally, the Fe-rich olivine could have also formed through the reduction of magnetite by H-2 gas and SiO2(aq). The CM-like clasts are found only in the studied HED meteorites and have mineralogy and organic maturity similar to the CM chondrites. Only one CM-like clast shows unique Raman signatures, either due to thermal effects or the presence of unique organic precursor material. The C2 clasts show similarities with other C2 clasts in CH/CB chondrites. None of the studied clasts show signatures of high degrees of thermal metamorphism like the subtype >= 3.1 chondrites.
This study investigates the spatial distribution and spectral characteristics of the broad hydration feature near 3 mu m across the surface of asteroid (101955) Bennu using OVIRS (OSIRIS-REx Visible and InfraRed Spectrometer) data. We analyzed multiple global OVIRS data sets obtained at different times throughout the Bennu day, revealing variations in band depth with wavelength, as well as wavelength-dependent correlations with temperature and other surface properties. Our analysis distinguishes between the behavior of the 2.74-mu m and 3.0-mu m features, which contribute to a broad 3-mu m region absorption, suggesting they may have different origins or respond differently to surface conditions. The 2.74-mu m band depth shows strong negative correlation with temperature and positive correlation with albedo and hydroxyl, whereas the 3.0-mu m band depth exhibits primarily weak relationships with other spectral and surface parameters, such as temperature, albedo, and UV/visible spectral slope, indicating that these features probe different aspects of the surface materials. These findings demonstrate that space weathering likely plays a strong role within the hydration band. Specifically, temperature and time-of-day dependencies observed at 2.74-mu m point to an active hydroxyl cycle on Bennu's surface driven by solar wind hydrogen implantation, hydroxylation, and subsequent thermal desorption. Conversely, the distinct behavior of the 3.0-mu m band suggests it may serve as an indicator of surface composition, including the phosphate minerals recently found in the returned sample; however, the lack of correlation with other expected diagnostic spectral features, such as a bluer UV/visible slope, currently leaves this relationship inconclusive.
Abstract A transmission electron microscopy (TEM) study of four troilite grains from enstatite chondrites (Y‐691, Y‐791790, Y‐86004, A‐881314) and two grains from aubrites (Y‐793592, Mayo Belwa) addressed their crystal structures and nanoscale mineralogical relationships. The crystal structure of troilite in enstatite‐rich meteorites is predominantly consistent with the space group , except for the FeS phase from Mayo Belwa, which exhibits a complex modulated structure consistent with a trilling of the modulated N C pyrrhotite superstructure. TEM energy‐dispersive X‐ray spectroscopy (TEM‐EDS) analyses confirmed the incorporation of chromium and titanium into the troilite crystal structure, even in samples containing nanometer‐sized daubréelite exsolution lamellae (A‐881314, Y‐793592). Nanometer‐sized inclusions of (Mn,Mg,Fe)S monosulfide solid solution occur in troilite in Y‐791790. Crystallographic orientation relationships between troilite and associated sulfide phases (daubréelite and keilite) indicate semi‐coherent intergrowths at the nanoscale. The absence of nanometer‐sized daubréelite in Y‐86004 and Y‐791790 suggests rapid cooling of the Cr‐Fe sulfide solution and quenching of the chemical composition of the phase stable above 700°C, consistent with the impact melt origin of these meteorites. Additional weak, diffuse diffractions observed in selected area electron diffraction patterns for troilites from Y‐86004, Y‐791790, and Y‐793592 are attributed to the development of vacancy ordering with variable periodicity along the c ‐axis, corresponding to N C ‐type superstructures. The detailed nature of vacancy ordering and the contributions of Cr and Ti substitution to the observed superstructure features remain unresolved and require comprehensive structure analysis.
Abstract Eight different H, L, and LL chondrites with 11 lithologies were studied and show good evidence for progressive shock effects and compaction. In weakly shocked chondrites (Baszkόwka, Bjurböle, Saratov, NWA 7298 Lithology A and C), silicates were dominantly fractured, troilite and metal were more deformed than olivine and fluidized, and estimated stresses were ~4–7 GPa. In strongly shocked chondrites (NWA 7298 Lithology B, NWA 11315 Lithology A and B, Alfianello, NWA 8709, NWA 4860), olivine grains display more plastic deformation than in weakly shocked chondrites and were ultimately recrystallized; troilite, some metal, and some bulk areas were melted; and estimated stresses were >7 GPa. Three settings for lithification are inferred: (A) initial lithification during accretion of warm chondrule aggregates, (B) later lithification in warm, metamorphosing parent bodies that were being affected by mainly low‐velocity collisions, and (C) final lithification in cool parent bodies after the waning of metamorphic heat that were being affected by higher velocity collisions. Thus, collisions together with heat of different types (accretionary, metamorphic, and shock‐related) were probably the main ways in which unmelted planetesimals were lithified. Impact‐fluidized metal and troilite may have acted as important lithification binding agents even in weak shock events.
Abstract CI chondrite water‐leaching experiments are commonly used to investigate soluble inventories formed during parent‐body aqueous alteration, but terrestrial weathering may modify them. We performed water‐leaching experiments on pristine Bennu samples together with Orgueil (CI) and Murchison (CM). The pristine Bennu aggregate sample OREX‐800107‐127 released only 0.23 wt% cations, dominated by Na. In contrast, Bennu aggregate sample OREX‐800107‐0 (−114, −115, and −116), flash‐heated to 600 °C for 20 s under continuous helium flow, and a curated Orgueil sample released 2.0 and 2.2 wt% cations, respectively. Both thermally treated Bennu and Orgueil released much more Mg and Ca than untreated Bennu, while also releasing more Na in absolute terms. The thermally treated Bennu leachate was dominated by Mg (38 wt%), Na (23 wt%), Fe (17 wt%), and Ca (14 wt%), broadly resembling the Orgueil leachate, except for its higher Fe fraction than Orgueil (3.6 wt%). Mass‐balance models indicate that carbonate dissolution supplied mainly Ca and Mn, Mg came from carbonates and phyllosilicate‐associated reservoirs, and the increases in Fe and Ni record substantial sulfide mobilization. Broadly CI‐like materials can possess markedly different soluble inventories. Returned samples retain labile alteration products when stored under anaerobic conditions, whereas meteorites are unavoidably modified in oxidizing terrestrial environments.
