Evidence for sodium-rich alkaline water in the Tagish Lake parent body and implications for amino acid synthesis and racemization

Abstract

Understanding the timing and mechanisms of amino acid synthesis and racemization on asteroidal parent bodies is key to demonstrating how amino acids evolved to be mostly left-handed in living organisms on Earth. It has been postulated that racemization can occur rapidly dependent on several factors, including the pH of the aqueous solution. Here, we conduct nanoscale geochemical analysis of a framboidal magnetite grain within the Tagish Lake carbonaceous chondrite to demonstrate that the interlocking crystal arrangement formed within a sodium-rich, alkaline fluid environment. Notably, we report on the discovery of Na-enriched subgrain boundaries and nanometer-scale Ca and Mg layers surrounding individual framboids. These interstitial coatings would yield a surface charge state of zero in more-alkaline fluids and prevent assimilation of the individual framboids into a single grain. This basic solution would support rapid synthesis and racemization rates on the order of years, suggesting that the low abundances of amino acids in Tagish Lake cannot be ascribed to fluid chemistry.

Keywords: Tagish Lake; amino acid; atom probe tomography; framboidal magnetite.

Fig. 1.

Secondary electron images of magnetite framboids in Tagish Lake. The larger spherical to hexagonal structure within the thin section (A) is constructed of numerous framboidal aggregates, as imaged during FIB analysis of the sample (B). The abundance of carbonaceous material within the framboids is correlated with the tightness of packing, with deformed regions incorporating higher quantities of carbon. For reference, the lift-out location for atom probe microtip preparation is highlighted in A.

Fig. 2.

APT analysis of a decorated subgrain boundary between magnetite framboids in the Tagish Lake meteorite (cryogenically prepared microtip R47_02314). A curved, Mg-enriched boundary, which runs between two Fe₃O₄ (magnetite) grains, is densely decorated by ∼10-nm-wide Na-rich clusters. Dislocation loops, decorated by Mg, Na, and Mn, can also be observed in association with the boundaries. For reference, the mass-to-charge spectrum for the microtip is shown (0 Da to 200 Da), with major peak families highlighted, including the Na, Mg, and Fe peaks integrated into the reconstruction.