Hydrothermal evolution of the morphology,molecular composition,and distribution of organic matter in CR (Renazzo-type) chondrites

The morphology, molecular composition, and distribution of organic matter (OM) were investigated in a suite of CR chondrites to better constrain its hydrothermal evolution. Multiple focused ion beam sections were extracted from the matrices of seven CR chondrites. Scanning transmission X-ray microscopy and transmission electron microscopy reveal OM ubiquitously distributed across the CR matrices. OM mainly occurs as either discrete submicron rounded or irregularly shaped vein-like particles. Two spectral populations of organic particles were identified by carbon K-edge X-ray absorption near edge structure (XANES): the most abundant one, similar to insoluble organic matter (IOM) residues, contains aromatic, carbonyl, and carboxylic groups. The second population is more aromatic-rich and lacks a distinctive carbonyl peak. An additional, ubiquitous organic component occurs associated with amorphous silicates and phyllosilicates. Less aromatic but aliphatic- and carboxylic-rich, this diffuse OM is interpreted as the result of the redistribution of organic compounds by aqueous fluids. The most altered CR1 GRO 95577 contains a more mature OM and highly aliphatic- and carboxylic-rich diffuse OM. This evolution, from the CR2s to the CR1, is comparable to that of terrestrial gas shale maturation involving cracking reactions, releasing bitumen-like, aliphatic-, and carboxylic-rich compounds, and aromatic residues. Our observations support the accretion of soluble OM and its later polymerization to IOM, as well as the maturation of IOM and its partial oxidation, releasing mobile compounds. The differences in GRO 95577 are clearly attributable to the hydrothermal episode(s), but the relative role of water and temperature on the evolution of OM remains elusive.     

Ancient geologic events on Mars revealed by zircons and apatites from the Martian regolith breccia NWA 7034

Zircons and apatites in clasts and matrix from the Martian breccia NWA 7034 are well documented, timing ancient geologic events on Mars. Furthermore, in this study, zircon trace elemental content, apatite volatile content, and apatite volatile isotopic compositions measured in situ could constrain the evolution of those geologic events. The U‐Pb dates of zircons in basalt, basaltic andesite, trachyandesite igneous clasts, and the matrix are similar (4.4 Ga) suggesting intense volcanism on ancient Mars. However, two metamict zircon grains found in the matrix have an upper intercept date of ~4465 Ma in crystalline, whereas amorphous areas have a lower intercept date of 1634 ± 93 Ma. The younger date is consistent with the date of apatites (1530 ± 65 Ma), suggesting a metamorphic event that completely reset the U‐Pb system in both the amorphous areas of zircon and all apatites. δD values in all apatites negatively correlate with water content in a two‐endmember mixing trend. The D (δD up to 2459‰) and ³⁷Cl heavy core (3.8‰) of a large apatite grain suggest a D‐, ³⁷Cl‐rich fluid during the metamorphic event ~1.6 Ga ago, consistent with the trace elements Y, Hf and Ti and P in zircons. The fluid was also therefore P‐rich. The D‐, ³⁷Cl‐poor H₂O‐rich rim (<313‰) suggests the degassing of water from the Martian Cl‐poor interior at a later time. This D‐, ³⁷Cl‐poor Martian mantle reservoir could have derived from volcanic intrusions postdating the younger metamorphic event recorded in NWA 7034.     

Iron oxides in comet 81P/Wild 2

Abstract– We have used synchrotron Fe‐XANES, XRS, microRaman, and SEM‐TEM analyses of Stardust track 41 slice and track 121 terminal area slices to identify Fe oxide (magnetite‐hematite and amorphous oxide), Fe‐Ti oxide, and V‐rich chromite (Fe‐Cr‐V‐Ti‐Mn oxide) grains ranging in size from 200 nm to ～10 μm. They co‐exist with relict FeNi metal. Both Fe‐XANES and microRaman analyses suggest that the FeNi metal and magnetite (Fe₂O₃FeO) also contain some hematite (Fe₂O₃). The FeNi has been partially oxidized (probably during capture), but on the basis of our experimental work with a light‐gas gun and microRaman analyses, we believe that some of the magnetite‐hematite mixtures may have originated on Wild 2. The terminal samples from track 121 also contain traces of sulfide and Mg‐rich silicate minerals. Our results show an unequilibrated mixture of reduced and oxidized Fe‐bearing minerals in the Wild 2 samples in an analogous way to mineral assemblages seen in carbonaceous chondrites and interplanetary dust particles. The samples contain some evidence for terrestrial contamination, for example, occasional Zn‐bearing grains and amorphous Fe oxide in track 121 for which evidence of a cometary origin is lacking.