NanoSIMS isotopic investigation of xenolithic carbonaceous clasts from the Kapoeta howarditeOPEN ACCESS 

Nan Liu, Ryan C. Ogliore, Lionel G. Vacher

Geochimica et Cosmochimica Acta
In Press, Journal Pre-proof, Available online 2 June 2020


Kapoeta images


“We report on the investigation of the inventories of presolar grains and organic matter (OM) in 14 xenolithic carbonaceous clasts (C-clasts) identified in the Kapoeta howardite based on high-resolution NanoSIMS hydrogen, carbon, nitrogen, and oxygen isotopic imaging data. The 14 C-clasts are ∼50–200 μm in size and consisted of one CM-like and 13 CI-like clasts, according to the mineralogy-based classification scheme adopted in the literature. All of the 14 C-clasts are located along one edge of the thin section. In two CI-like clasts, embayments of magnetite grains between the C-clast and the host howardite point to aqueous alteration occurring on Vesta as a result of melting the ice embedded in the C-clasts. It also strongly suggests that all of the C-clasts, especially the 13 CI-like clasts, are originated from the same parent body, because of their localized distribution across the thin section and the much higher ratio of CI-like to CM-like clasts with respect to the ratios reported in the literature. Thus, taking the two pieces of evidence together implies that the clasts from this study are sourced from an ice-bearing parent body, either an icy asteroid or a comet, originated from the outer solar system. Four presolar silicon carbide (SiC) and two presolar silicate grains were identified in the C-clasts. In addition, all the C-clasts contain moderate bulk D- and 15N-enrichments with the presence of micron to submicron-sized D and 15N hotspots, indicating the presence of primitive organic material. Comparison of our data with the literature data for a wide range of extraterrestrial materials for their inventories of presolar grains and OM, provides support to (1) the genetic linkage of xenolithic C-clasts to highly aqueously altered but minimally heated carbonaceous chondritic materials and (2) homogeneous distribution of circumstellar and interstellar materials in the protoplanetary disk. The low amounts of heat experienced by the C-clasts suggest their rather late arrival at Vesta and/or Vestoids at low speeds after the occurrence of late heavy bombardment in the inner solar system during ∼3.5–4.0 Gyr ago.”