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

Abstract

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.

Amorphous silicates as a record of solar nebular and parent body processes—A transmission electron microscope study of fine‐grained rims and matrix in three Antarctic CR chondrites

“…Although matrix and FGRs seem to be more altered according to this work and Abreu et al (2018), presolar O‐rich grain abundances are relatively high compared to other more altered CR chondrites such as GRO 95577 or Renazzo (Leitner et al 2016). Furthermore, investigations of OM in EET92 (Changela et al 2018) as well as Fe oxidation state synchrotron analyses of amorphous silicates in EET92 (Le Guillou et al 2015a) also detected pristine areas within this chondrite and therefore support this heterogeneity.…”

“…6). OM is a widespread constituent of CR chondrite matrices and has been investigated extensively in recent studies (e.g., Le Guillou et al 2014; Changela et al 2018; Vollmer et al 2019). The core of one globule consists of an altered Fe, Mg, Ca silicate, most likely a pyroxene.…”

Influence of phyllosilicates on the hydrothermal alteration of organic matter in asteroids: Experimental perspectives

“…Several studies tend to indicate that hydrothermal events have affected the molecular and physical nature of asteroidal OM (Alexander et al, 2007;Glavin et al, 2010;Cody et al, 2011;Kebukawa et al, 2013;Orthous-Daunay et al, 2013;Alexander et al, 2014;Yabuta et al, 2017;Changela et al, 2018;Vinogradoff et al, 2018). In carbonaceous chondrites, OM occurs as individual particles as well as diffuse OM physically embedded within the phyllosilicates at the nanometer scale (Zega et al, 2010;Vinogradoff et al, 2017;Changela et al, 2018). This association has been interpreted as the result of the formation of phyllosilicates in a fluid carrying organic compounds (Le .…”

“…The parent asteroids of CC have experienced hydrothermal alteration after accretion (Brearley, 2006). The alteration of primary mineral phases has been documented (e.g., formation of phyllosilicates, carbonates, sulfates…), but the amplitude of OM processing remains an open field of investigations (Alexander et al, 2007;Cody et al, 2011;Orthous-Daunay et al, 2013;Vollmer et al, 2014;Remusat, 2016;Elsila et al, 2016;Aponte et al, 2016;Changela et al, 2018;Vinogradoff et al, 2018).…”

“…Studies have pointed that bulk IOM transformation occurred inside parent bodies, and that SOM content and chemistry also evolves with increasing hydrothermal alteration (quantity of water, duration, temperature…), as for instance for the Tagish Lake chondrite (Herd et al, 2011;Glavin et al, 2012;Hilts et al, 2014;Alexander et al, 2014). In-situ observations of this OM have revealed at least two different population of OM: sub-micrometric OM particles and diffuse OM finely interspersed within phyllosilicates (Busemann et al, 2006;Garvie and Buseck, 2007;De Gregorio et al, 2013;Vollmer et al, 2014;Vinogradoff et al, 2017;Changela et al, 2018). Some of the chemical and morphological signatures of this OM could hence be related to asteroidal alteration, including hydrothermal reactions with minerals.…”