Shock melts in QUE 94411, Hammadah al Hamra 237, and Bencubbin: Remains of the missing matrix?
We have studied the CB carbonaceous chondrites Queen Alexandra Range (QUE) 94411, Hammadah al Hamra (HH) 237, and Bencubbin with an emphasis on the petrographical and mineralogical effects of the shock processing that these meteorite assemblages have undergone. Iron-nickel metal and chondrule silicates are the main components in these meteorites. These high-temperature components are held together by shock melts consisting of droplets of dendritically intergrown Fe,Ni-metal/sulfide embedded in silicate glass, which is substantially more FeO-rich (3040 wt%) than the chondrule silicates (FeO < 5 wt%). Fine-grained matrix material, which is a major component in most other chondrite classes, is extremely scarce in QUE 94411 and HH 237, and has not been observed in Bencubbin. This material occurs as rare, hydrated matrix lumps with major and minor element abundances roughly similar to the ferrous silicate shock melts (and Cl). We infer that hydrated, fine-grained material, compositionally similar to these matrix lumps, was originally present between the Fe,Ni-metal grains and chondrules, but was preferentially shock melted. Other shock-related features in QUE 94411, HH 237, and Bencubbin include an alignment and occasionally strong plastic deformation of metal and chondrule fragments.