Abee meteorite, Canada
Abee is an enstatite chondrite breccia dominantly composed of kamacite,
enstatite, silica, plagioclase, troilite and niningerite. Clasts are up to
220 mm long and vary in shape from angular to rounded. Some clasts are zoned
with kamacite-enriched rims that follow the edge of the clast. Spatial
compositional variations were examined in a small block to find out more
about the petrological processes that produced this rock, particularly the
relationship between the clasts, the matrix and the cores/rims of the zoned
clasts. Compositional maps produced using a focussed-beam XRF were segmented
into clasts and matrix, and rims and cores where possible. Compositions of
most clasts, matrix and rim/cores define a simple, linear trend on simple
variation diagrams. If it is assumed that all components were derived from
an original homogeneous composition then the variation can be explained
either by addition of kamacite or by loss of all other phases. Within this
overall compositional variation the kamacite content generally increases as
follows: matrix < large homogeneous clasts ≈ zoned clast cores < small
homogeneous clasts ≈ zoned clast rims. Production of diversity by addition
of kamacite to clasts and rim seems to require a complex history as the
source cannot have been the current matrix. It is also difficult to produce
the observed chemical variations and zoning by partial melting. However,
differentiation by removal of all non-metallic phases may result from
repeated impacts: Shock waves would deform kamacite whilst fracturing all
other phases. The broken grains would then migrate towards the surface of
the clasts where they would spall off into the matrix. This process would
also lead to the observed rounding of some clasts. We propose that this
shock-differentiation process be called ‘smithing’, as it resembles the
ancient process of iron refining.
Higgins, M.D. and
Martin, P.-E.M.C., 2018. Chemical variations within and between the
clasts, and the matrix of the Abee enstatite chondrite suggest an
impact-based differentiation mechanism. Geochim. Cosmochim. Acta, 220:
71-81. doi:10.1016/j.gca.2017.09.047