NanoSIMS isotopic investigation of xenolithic carbonaceous clasts from the kapoeta howardite

Abstract

We report on the investigation 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 consist of one CM-like and 13 CI-like clasts, which are classified based on their mineralogies. The clasts from this study are likely sourced from an ice-bearing parent body, either an icy asteroid or a comet, originating from the outer solar system according to the following mineralogical observations: (1) 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; (2) all of the C-clasts, especially the 13 CI-like clasts, likely originated from the same parent body, because (i) the 14 C-clasts are clustered in the thin section, and (ii) the clasts show a much higher ratio of CI-like to CM-like clasts with respect to those reported in the literature. 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 sub-micron to micron-sized D and 15N hotspots, indicating the presence of primitive organic material. Comparison of the abundances and isotopic compositions of presolar grains and OM in these C-clasts with literature data for different samples of primitive extraterrestrial material provides support for (1) the genetic linkage of xenolithic C-clasts to highly aqueously altered but minimally heated carbonaceous chondritic materials and (2) a homogeneous distribution of circumstellar and interstellar materials in the protoplanetary disk. The low amounts of heat experienced by the C-clasts suggest that they arrived at Vesta and/or Vestoids at low speeds after the late heavy bombardment in the inner solar system ∼3.5–4.0 Gyr ago.