Mineralogy, petrology, and thermal evolution of the Benton LL6 chondrite

Abstract

The Benton LL6 chondrite is a brecciated meteorite that was observed to fall on January 16, 1949 in Benton, New Brunswick, Canada. Internally, the meteorite comprises light-colored, subangular to subrounded clasts embedded in a dark grey-colored matrix. Clasts comprise the same mineral phases as the matrix, as well as chondrules and larger (50-100 micrometers) single mineral grains (mainly olivine and orthopyroxene). Composite (polyphase) clasts can be serveral millimeters in length. Numerous examples of post-brecciation and post-annealing sheraring and displacement at the micron to millimeter scale occur in the form of shock veins. Benton is a shock stage S3 chondrite, which experienced shock pressures on the order of 15-20 GPa, with an estimated post-shock temperature increase of 100-150 degrees C. Benton's history comprises a sequence of events as follows: 1) chondrule formation and initial assembly; 2) brecciation; 3) thermal metemorphism; and 4) shock veining. Events (2) and (4) can be equated with distinct impact events, the former representing bombardment of target material that remained in situ or collisionally fragmented during metamorphism, and then gravitationally reassembled, the latter probably with release from the source body to yield a meteorite. Thermal metamorphism post-dates brecciation. The mean equilibration temperature recorded in the Benton LL6 chondrite is 890 degrees C, obtained using the two pyroxene geothermometer.