Complex brecciation and shock effects in the Buck Mountain Wash (H3–5) chondrite

Abstract

Buck Mountain Wash (BMW) is a new genomict breccia (H35) found in the Franconia (H5) strewn field in Arizona that shows complex brecciation and shock effects. It contains three distinct chondritic lithologies in sharp contact: a) a main lithology that consists primarily of petrographic type 5 material but which has finely intermixed type 3 and 4 material, b) a shockblackened (shock stage S5) type 3 lithology (lithology A), and degrees C) a shock-blackened type 3/4 lithology(lithology B). Buck Mountain Wash was lithified after impact-mixing and impact-melting of weakly and strongly metamorphosed materials, possibly at depth in the regolith of the parent body. Shock effects included brecciation on a fine scale, localized impact-melting of silicates, partial melting, and mobilization of metal-sulfide, and chemical fractionations that produced non-H-group composition kamacite by two disequilibrium mechanisms. Shock heating did not cause significant thermal metamorphism in the shock-blackened lithologies of BMW, except possibly in areas adjacent to whole-rock shock melt. During lithification, cooling must have been rapid at high temperatures to preserve glass and inhomogeneous silicate compositions, but not so fast at lower temperatures as to produce dendritic metal-sulfide globules or martensite.