¹L. M. Barge,¹E. Flores,²D. VanderVelde,¹J. M. Weber,³M. M. Baum,³A. Castonguay
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2020JE006423]
¹NASA Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109 USA
²Department of Chemistry, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA, 91125 USA
³Department of Chemistry, Oak Crest Institute of Science, 128‐132 W. Chestnut Ave., Monrovia, CA, 91016 USA
Published by arrangement with John Wiley & Sons

Geological conditions play a significant role in prebiotic / abiotic organic chemistry, especially when reactive minerals are present. Previous studies of the prebiotic synthesis of amino acids and other products in mineral‐containing systems have shown that a diverse array of compounds can be produced, depending on the experimental conditions. However, these previous experiments have not simulated the effects of varying geochemical conditions, in which factors such as pH, iron redox state, or chemical concentrations may vary over time and space in a natural environment. In geochemical systems that contain overlapping gradients, many permutations of individual conditions could exist and affect the outcome of an organic reaction network. We investigated reactions of pyruvate and glyoxylate, two compounds that are central to the emergence of metabolism, in simulated geological gradients of redox, pH, and ammonia concentration. Our results show that the positioning of pyruvate/glyoxylate reactions in this environmental parameter space determines the organic product distribution that results. Therefore, the distribution pattern of amino acids and alpha‐hydroxy acids produced prebiotically in a system reflects the specific reaction conditions, and would be distinct at various locations in an environment depending on local geochemistry. This is significant for origin of life chemistry in which the composition and function of oligomers could be affected by the environmentally‐driven distribution of monomers available. Also, for astrobiology and planetary science where organic distribution patterns are sometimes considered as a possible biosignature, it is important to consider environmentally‐driven abiotic organic reactions that might produce similar effects.

¹Shaofan Che,¹Adrian J.Brearley
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.10.031]
¹Department of Earth and Planetary Sciences, MSC03-2040, 1 University of New Mexico, Albuquerque, NM 87131-0001, USA
Copyright Elsevier

Type C CAIs are a rare group of refractory inclusions in carbonaceous chondrites that are compositionally and isotopically distinct from the more commonly observed igneous CAIs (i.e., Type As and Type Bs). We have investigated a forsterite-bearing Type C CAI ALNH-04 from the Allende CV3 chondrite. This CAI has both textural and compositional similarities to some of the Type C CAIs previously reported; however, there are notable differences that imply that ALNH-04 may have formed from a different precursor from other Type C inclusions. Based on the bulk composition of ALNH-04 and the minor element contents of forsterite, we suggest that the forsterite grains were inherited from a Forsterite-bearing Type B CAI (FoB) precursor. The presence of augite on the periphery of ALNH-04 implies a re-melting event that probably occurred in a chondrule-forming region.

Another interesting feature of ALNH-04 is the secondary iron-alkali-halogen zoning sequence as manifested by varying proportions of nepheline, sodalite, fayalitic olivine, and sulfides in different regions of the CAI. Nepheline ± sodalite have replaced anorthite in the outer part of the inclusion, giving way to the presence of ubiquitous sodalite with minor nepheline, partially replacing anorthite at grain boundaries and fractures in the interior of the inclusion. Sulfides and Fe-bearing olivine form an iron-rich alteration zone. The textural relationships between nepheline and sodalite show no evidence of a direct replacement relationship between the two phases. Combined with the SEM observations, the microstructures are most consistent with a two-stage fluid alteration process: (1) nepheline replaced anorthite in the outer part of the CAI via a fluid with within the stability range of nepheline; (2) a later-stage fluid, with elevated that could preferentially stabilize sodalite, penetrated further into the CAI interior, replacing anorthite with sodalite. The lack of a nepheline-sodalite replacement relationship indicates that the conditions and fluid chemistry were suitable for nepheline and/or sodalite to be stable. Together with other Fe-rich secondary phases, fayalitic olivine may have recorded an increase in pH and of the fluid. These changes were probably induced by the extensive alteration of the outer part of the CAI by feldspathoids. The observed alteration microstructures are consistent with a coupled dissolution-precipitation alteration mechanism. The fluid alteration was also responsible for the formation of Na- and Ca-rich halos in the matrix surrounding the CAI.

We compared ALNH-04 with other CAIs and chondrules showing alkali-halogen-(iron) zoning sequences in Allende, and found that the observed zoning structures are consistent with the two-stage fluid-assisted metasomatic process mentioned above. The different distribution patterns of nepheline and sodalite in plagioclase-rich CAIs, chondrules, and melilite-rich CAIs may be explained by different chemical potential gradients in SiO2 in the fluid. Precipitation of nepheline and sodalite may require a higher SiO2 activity compared to grossular and dmisteinbergite (±secondary anorthite), which controlled the formation location of sodalite during the second fluid alteration event. Fluids with different compositions may be produced by fluid percolation along different directions and pathways, changing convection patterns, or release of water from a differentiated asteroidal interior.