Mineralogy of carbonaceous chondritic microclasts in howardites: Identification of C2 fossil micrometeorites

Seventy-one carbonaceous chondritic microclasts of average size 150 gm have been found in three howardites (Yamato-793497, Jodzie, Kapoeta). All carbonaceous chondritic microclasts are made of a fine-grained phyllosilicate-rich matrix supporting a variety of minerals such as olivine, pyroxene, spinel, iron oxides, iron-nickel sulfides, and calcium carbonates. Such a mineralogy is typical of chondritic C2 matter. Half of the carbonaceous chondritic microclasts are tochilinite-rich, and have been tentatively called CM2 microclasts. The other half are magnetite-rich, and have been tentatively called CR2 microclasts. The absence of a correlation between the CM2/CR2 ratio in carbonaceous chondritic microclasts and in numerous millimeter-sized clasts found in the same sections argues for carbonaceous chondritic microclasts being true micrometeorites rather than fragments of larger objects. Dynamical simulations show that it is possible for asteroidal dust to encounter Vesta (the howardite's putative parent-asteroid) at velocity low enough (<1 km.s(-1)) to prevent fragmentation. Because the micrometeorite flux in the inner Solar System has been decreasing with time, we argue that carbonaceous chondritic microclasts have been trapped in Vesta's regolith early in the history of the Solar System and are fossil micrometeorites. Because both microclasts and clasts found in howardites are related to C2 chondritic matter, we propose that C2 matter represents the bulk, or at least a significant fraction of the primordial howardite parent-asteroid. Considering the abundance of C2 matter among fossil micrometeorites, we speculate that the C2 fossil micrometorites are the so far unidentified agent of the late chondritic veneer that endowed the Earth's mantle with an excess of siderophile elements relative to the contents predicted by the core-mantle separation models. The discovery that C2 fossil micrometeorites are similar to C2 modern Antarctic micrometeorites supports recent models proposing a micrometeoritic origin for the Earth's oceans and volatile species.