Isotopically Heavy Micrometeorites—Fragments of CY Chondrite or a New Hydrous Parent Body?

Cosmic dust grains sample a diverse range of solar system small bodies. This includes asteroids that are not otherwise represented in our meteorite collections. In this work we obtained 3D images of micrometeorite interiors using tomography before collecting destructive high-precision oxygen isotope measurements. These data allow us to link textures in unmelted micrometeorites to known chondrite groups. In addition to identifying particles from ordinary chondrites, CR and CM chondrites we report two micrometeorites derived from an anomalous 16O-poor source (δ 17O: +16.4‰, δ 18O: +28.4‰, and Δ 17O: +1.4‰). Their compositions overlap with a previously reported micrometeorite (TAM50-25) from Suttle et al. (2020), https://doi.org/10.1016/j.epsl.2020.116444 (EPSL: 546:116444). These particles represent hydrated carbonaceous chondrite material derived either from a new group or from the CY chondrites (thereby extending the isotopic range of this group). In either scenario they demonstrate close petrographic and isotopic connections to the CO-CM chondrite clan. Furthermore, their position in O-isotope space makes them the most likely candidate for the parent body of the anomalous “group 4” cosmic spherules previously reported by Suavet et al. (2010), https://doi.org/10.1016/j.epsl.2010.02.046 (EPSL: 293:313-320) and several subsequent isotopic studies. We conclude that the “group 4” cosmic spherules originate from hydrated C-type asteroid parents.