Presolar Grains in Primitive Ungrouped Carbonaceous Chondrite Northwest Africa 5958

Status Report From: e-Print archive
Posted: Tuesday, September 17, 2019

Larry R. Nittler, Rhonda M. Stroud, Conel M. O'D. Alexander, Kaitlin Howell

(Submitted on 16 Sep 2019)

We report a correlated NanoSIMS-transmission electron microscopy study of the ungrouped carbonaceous chondrite Northwest Africa (NWA) 5958. We identified 10 presolar SiC grains, 2 likely presolar graphite grains, and 20 presolar silicate and/or oxide grains in NWA 5958. We suggest a slight modification of the commonly used classification system for presolar oxides and silicates that better reflects the grains' likely stellar origins. The matrix-normalized presolar SiC abundance in NWA 5958 is 18+15−10 ppm (2σ), similar to that seen in many classes of unmetamorphosed chondrites. In contrast, the matrix-normalized abundance of presolar O-rich phases (silicates and oxides) is 30.9+17.8−13.1 ppm (2σ), much lower than seen in interplanetary dust particles and the least altered CR, CO and ungrouped C chondrites, but close to that reported for CM chondrites. NanoSIMS mapping also revealed an unusual 13C-enriched (δ13C∼ 100--200 permil) carbonaceous rim surrounding a 1.4 μm diameter phyllosilicate grain. TEM analysis of two presolar grains with a likely origin in asymptotic giant branch stars identified one as enstatite and one as Al-Mg spinel with minor Cr. The enstatite grain amorphized rapidly under the electron beam, suggesting partial hydration. TEM data of NWA 5958 matrix confirm that it has experienced aqueous alteration and support the suggestion of Jacquet et al. (2016) that this meteorite has affinities to CM2 chondrites.

Comments: Accepted in Meteoritics and Planetary Science (32 pages, 9 figures). In accordance with Wiley license restrictions, the submitted version is provided. Dedicated to the memory of Dr. Christine Floss

Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)

Cite as: arXiv:1909.07225 [astro-ph.EP] (or arXiv:1909.07225v1 [astro-ph.EP] for this version)

Submission history

From: Larry Nittler

[v1] Mon, 16 Sep 2019 14:16:14 UTC (7,726 KB)

Astrobiology, Astrochemistry

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