PIGEONITE MICROSTRUCTURES IN MARTIAN METEORITE EETA79001

Martian meteorite EETA79001, collected in Antarctica, is unique among basaltic shergottite specimens, because it contains two lithologies separated by a linear contact. Lithology A (Pgt, maskelynite, Aug, Ol, Opx) has smaller matrix grains (0.15mm) than Lithology B (0.3mm; Pgt, maskelynite, Aug). The two hypotheses previously posed for the observed contact are 1) Lithology A is an impact melt rock that incorporates Lithology B as a clast, or 2) the contact is a boundary between successive flows.

Pyroxene microstructures are commonly used to interpret magmatic, shock, and annealing processes. Quantitative EBSD orientation maps of pyroxene from Lithologies A and B reveal bands with misorientations of 178±2° about variable axes. Bands in A are well-defined, vary from 3-35µm wide, continue across fractures and melt, and extend to the grain boundaries. Bands in B are patchy, vary from 3-11µm wide, and rarely extend to the grain boundaries. Band presence and position do not correlate to compositional variations in pyroxene. Along with well-defined bands, Lithology A has a high percentage of glass that is reflected in a lower EBSD indexing percentage. In Lithology B, the patchy nature of the bands and their absence near the grain boundaries suggest partial annealing. The EBSD orientation maps also show rotations of 5-10° about [010], possibly due to crystal plastic deformation.

The observed bands do not appear to be exsolution lamellae, because they are much thicker than described pyroxene exsolution lamellae, they occur in a variety of crystallographic orientations, and they do not correspond with compositional differences. The textures suggest that these bands formed before fractures and melt associated with ejection of the meteorite from Mars. They may have formed during igneous crystallization or the early stages of shock induced deformation.