Presolar grains in primitive ungrouped carbonaceous chondrite Northwest Africa 5958

1Larry R. Nittler,2Rhonda M. STROUD, 1Conel M. O’D. ALEXANDER, 1,3Kaitlin HOWELL
Meteoritics & Planetary Science (in Press) Link to Article [doi: 10.1111/maps.13397]
1Department of Terrestrial Magnetism, Carnegie lnstitution of Washington, Washington,District of Columbia 20015, USA
2U.S. Naval Research Laboralory, Codle 6366, 4555 Overlook Ave. SW, Washington, District of Columbia 20375, USA
3Present address: School of Engineering, Ecole Polytechnique Federale de Lausanne, Lausanne CH-1015, Switzerland
Published by arrangement with John Wiley & Sons

We report a correlated NanoSIMS-transmission electron microscopy study of theungrouped carbonaceous chondrite Northwest Africa (NWA) 5958. We identified 10presolar SiC grains, 2 likely presolar graphite grains, and 20 presolar silicate and/or oxidegrains in NWA 5958. We suggest a slight modification of the commonly used classificationsystem for presolar oxides and silicates that better reflects the grains’ likely stellar origins.The matrix-normalized presolar SiC abundance in NWA 5958 is 18þ1510ppm (2r) similar tothat seen in many classes of unmetamorphosed chondrites. In contrast, the matrix-normalized abundance of presolar O-rich phases (silicates and oxides) is 30:9þ17:813:1ppm (2r),much lower than seen in interplanetary dust particles and the least-altered CR, CO, andungrouped C chondrites, but close to that reported for CM chondrites. NanoSIMS mappingalso revealed an unusual13C-enriched (d13C100–200&) carbonaceous rim surrounding a1.4lm diameter phyllosilicate grain. Transmission electron microscopy (TEM) analysis oftwo presolar grains with a likely origin in asymptotic giant branch stars identified one asenstatite and one as Al-Mg spinel with minor Cr. The enstatite grain amorphized rapidlyunder the electron beam, suggesting partial hydration. TEM data of NWA 5958 matrixconfirm that it has experienced aqueous alteration and support the suggestion of Jacquetet al. (2016) that this meteorite has affinities to CM2 chondrites.

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