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

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.

High pressure minerals in the Château-Renard (L6) ordinary chondrite: implications for collisions on its parent body.

We report the first discoveries of high-pressure minerals in the historical L6 chondrite fall Château-Renard, based on co-located Raman spectroscopy, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy and electron backscatter diffraction, electron microprobe analysis, and transmission electron microscopy (TEM) with selected-area electron diffraction. A single polished section contains a network of melt veins from ~40 to ~200 µm wide, with no cross-cutting features requiring multiple vein generations. We find high-pressure minerals in veins greater than ~50 µm wide, including assemblages of ringwoodite + wadsleyite, ringwoodite + wadsleyite + majorite-pyropess, and ahrensite + wadsleyite. In association with ahrensite + wadsleyite at both SEM and TEM scale, we find a sodic pyroxene whose Raman spectrum is indistinguishable from that of jadeite but whose composition and structure are those of omphacite. We discuss constraints on the impact record of this meteorite and the L-chondrites in general.

Compressional pathways of α-cristobalite, structure of cristobalite X-I, and towards the understanding of seifertite formation.

In various shocked meteorites, low-pressure silica polymorph α-cristobalite is commonly found in close spatial relation with the densest known SiO2 polymorph seifertite, which is stable above ∼80 GPa. We demonstrate that under hydrostatic pressure α-cristobalite remains untransformed up to at least 15 GPa. In quasi-hydrostatic experiments, above 11 GPa cristobalite X-I forms-a monoclinic polymorph built out of silicon octahedra; the phase is not quenchable and back-transforms to α-cristobalite on decompression. There are no other known silica polymorphs, which transform to an octahedra-based structure at such low pressures upon compression at room temperature. Further compression in non-hydrostatic conditions of cristobalite X-I eventually leads to the formation of quenchable seifertite-like phase. Our results demonstrate that the presence of α-cristobalite in shocked meteorites or rocks does not exclude that materials experienced high pressure, nor is the presence of seifertite necessarily indicative of extremely high peak shock pressures.