Mineralogy of Carbonaceous Chondritic Microclasts in Hed Achondrites: Discovery of C2 Fossil Micrometeorites and Implications

摘要

Abundant carbonaceous chondritic microclasts with sizes ranging from 25 to 800 microns across have been found in 3 howardites (Kapoeta, Jodzie, Yamato-793497) and one eucrite (Lewis Cliff-87295) thin sections. Half of the microclasts are mineralogically related to CM2 chondrites, and the other half to CR2 chondrites. CM2 microclasts consist of a phyllosilicate-rich fine-grained matrix supporting olivine-pyroxene- serpentine-sulfide aggregates, chondrules and CAIs (rare), isolated olivine and pyroxene grains (abundant), spinel (rare), chromite (very rare), metal (rare), sulfides (abundant), tochilinite (abundant), COPS (very rare) and calcium carbonate (rare) . CR2 carbonaceous microclasts consist of a very abundant fine-grained phyllosilicate-rich matrix supporting a variety of minerals such as olivine, pyroxene (both common), coarse-grained phyllosilicates (common), magnetite (abundant), spinel (rare), iron sulfides (common), barringerite (very rare), metal (rare), COPS (very rare) and calcium carbonate (rare). When directly compared to the CM2 and CR2 chondrites group, microclasts exhibit a handful of mineralogical difference1l, indicating they come from distinct parent-bodies. Comparison of microclasts with larger millimeter-sized clasts found by Zolensky et al.(1996b) in the same sections, suggest that micro clasts are not mere fragments of larger objects but true micrometeorites. Numerical simulations of dust dynamics indicate that microclasts found in Vesta (probable parent-asteroid of the HED achondrites) can have originated from asteroidal bodies. A cometary origin is also possible. We discuss different possibilities for the timing of accretion of microclasts onto Vesta, and conclude that the most likely epoch is within the solar system first million years. Carbonaceous chondritic microclasts are the inner solar system fossil micrometeorites. C2 fossil micrometeorites bear many mineralogical similarities to C2 contemporary micrometeorites collected in the Antarctic ice cap: therefore the micrometeorites' composition has been constant over inner solar system history. The importance of C2 matter, also recorded in the (micro)meteoritic contamination of lunar soils, lead us to speculate that C2 matter may have contributed to the formation of large planetary bodies such as Vesta, or the Earth. Our findings also support the model proposed by Maurette et al. (2000) of a micrometeorite origin for some of the Earth's volatiles.