Studies of brazilian meteorites: IV – Origin of a dark-colored, uniquilibrated lithic fragment in the Rio Negro chondrite

A dark-colored lithic fragment, approximately 5 mm in diameter, in the Rio Negro, L3-4 (gas-rich) chondrite, contains mineral fragments and a poikilitic chondrule, olivine (Fa 1-45 ), orthopyroxene (Fs 1-25 ), three high-Ca pyroxenes, an Fe-Mg silicate tentatively identified as a dehydrated layerJattice silicate, glass, chromite, kamacite, taenite, and troilite. The fine-grained matrix of the fragment is responsible for its dark appearence. In the host chondrite, Fe-Mg silicates also vary in composition, but they cluster mainly in the L-group compositional range. Compared to the Rio Negro host, Fe/Mg ratios in olivine and low-Ca pyroxene of the fragment show less equiliblation and indicate that the fragment is not simply shock-darkened or shock-melted host material, but a xenolith. T€xture, bulk and mineral compositions of the fragment resemble those of unequiliblated ordinary chondrites and carbonaceous chondrites of type III, although Some properties suggest a closer relationship to type III carbonaceous chondrites. It is concluded that the fragment was embedded into the Rio Negro host when the latter was part of a regolith on its parent body, and that thc fragment originated from either coexisting rocks on the Rio Negro parent body or is the residue of infalling projectile material.