A new metal-rich chondrite grouplet

A new grouplet of primitive, metal-rich chondrites, here called the CB (C, carbonaceous; B, bencubbinite) chondrites, has been recognized. It includes Bencubbin, Weatherford, Hammadah al Hamra (HH) 237 and Queen Alexandra Ran ge (QUE) 94411, paired with QUE 94627. Their mineral compositions, as well as their oxygen and nitrogen isotopic compositions, indicate that they are closely related to the CR and CH chondrites, all of which are members of th e more inclusive CR dan. CB chondrites have much greater metal/silicate rat ios than any other chondrite group, widely increasing the range of metal/si licate fractionation recorded in solar nebular processes. They also have th e greatest moderately volatile lithophile element depletions of any chondri tic materials. Metal has compositional trends and zoning patterns that sugg est a primitive condensation origin, in contrast with metal from other chon drite groups. CB chondrites, as well as other CR dan chondrites, have much heavier nitrogen (higher N-15/N-14) than that in other chondrite groups. Th e primitive characteristics of the CB chondrites suggest that they contain one of the best records of early nebular processes. Another chondrite, Grosvenor Mountains 95551, is petrographically similar t o the CB chondrites, but its mineral and oxygen and nitrogen isotope compos itions indicate that it formed from a different nebular reservoir.