Ordinary Chondrite Micrometeorites from the Koronis Asteroids

Introduction: Micrometeorites (MMs) are large (>25 µm) interplanetary dust particles that survive atmospheric entry to be recovered from the Earth's surface. Amongst those particles that escaped significant heating during atmospheric entry two groups have been identified [1]: (i) fine-grained MMs (fgMMs), which are similar to the fine-grained matrices of hy-drated carbonaceous chondrites (CCs), and (ii) coarse-grained MMs (cgMMs), which are dominated by py-roxene, olivine and/or glass and often have igneous textures. Some cgMMs have selvages of fine-grained matrix and thus indicate that cgMMs and fgMMs may be derived from the same asteroidal parent bodies [2]. These composite particles suggest that the source objects of cgMMs were small mm-sized objects on their parent asteroids and were, therefore, probably chondrules. The CC-like nature of fgMMs implies that cgMMs likewise sample a CC-like chondrule population. Numerical models of the delivery of asteroidal dust to Earth, however, suggest that a significant fraction is derived from the S-type Koronis Family asteroids that cannot be CC-like materials [3]. In this study the minor element compositions of olivines, pyroxenes and accessory phases from 77 cgMMs are reported in order to identify their parent body associations. The results indictae that ~70% of cgMMs are derived from an ordinary chondrite (OC) chondrule population. Their parent body is, therefore, most likely to be the Koronis Family asteroids.