NWA 2724 Eucrite

The assembly of polymict eucrites is neither gentle nor precise. It involves magma, impact excavation and cycles of rock crushing, mixing, lithification and more crushing. Drs. Wittke and Bunch tell us that NWA 2724 contains both subophitic and cumulate textured rock and is therefore polymict. Here is a thin section that shows this nicely.

The top left quarter features the prominent plagioclase needles of the subophitic lithology. On the bottom edge toward the right and also rounding the bottom right corner are small clasts of cumulate rock. In between is pulverized rock from both lithologies. Much of this pulverized material had been lithified and broken again into clasts that were reassembled and lithified into the meteorite we have here. The brown rust stains are limited in extent indicating little terrestrial weathering. NWA 2724 eucrite thin section in PPL (plane polarized light) above and in XPL (cross-polarized light) below. Sample is about 25mm long.

NWA 2724 eucrite thin section in XPL. Sample is about 25mm long.

Ophitic and subophitic are rock textures where laths of plagioclase are surrounded (more or less) by pyroxene. Above and below, the plagioclase stands out simply by shape. In XPL, below, plagioclase’s tell-tale twinning appears as dark lines or as dark/light areas, usually longitudinally, in the long white, gray, black crystals. NWA 2724 eucrite thin section in plane polarized light. FOV=3.1mm.

NWA 2724 eucrite thin section in XPL. FOV=3.1mm.

Looking closer, we still see the light colored needles of plagioclase among the pyroxene. In XPL, below, the dark line runs along the length of each of the two parallel laths. In both PPL and XPL we see the fine exsolution lamellae in the pyroxene running SSW to NNE. NWA 2724 eucrite thin section in PPL. FOV=0.3mm.

NWA 2724 eucrite thin section in XPL. FOV=0.3mm.

Cumulate rock consists of mineral grains that crystallized, settled and aCUMULATEd from a magmatic melt. These pyroxene grains have little other material between them. Sometimes the settling will result in crystals assuming a preferred alignment but none is apparent here. Mineral grains in this clast show the effects of shock on their crystal lattice as patches of darkness and squared-off areas of darkening – undulatory extinction and mosaicism – in the XPL view below. When the section is rotated relative to the polarizing filters these effects are animated and hence more prominent. In XPL we see evidence of twinning in the pyroxene in the color banding within individual crystals. NWA 2724 eucrite thin section in PPL. FOV=3.1mm.

NWA 2724 eucrite thin section in XPL. FOV=3.1mm.

Clasts of formerly pulverized rock packed up against the subophitic clast at top left and each other. Some clasts are made of very fine mineral grains. Note the small subophitic clast within a clast – far left center. NWA 2724 eucrite thin section in partially cross-polarized light. FOV=8mm.

Clasts separated by black impact shock melt. NWA 2724 eucrite thin section in partially cross-polarized light. FOV=9mm.

Angular clasts. Minor rust halos. NWA 2724 eucrite thin section in partially cross-polarized light. FOV=4.4mm.

John Kashuba

John is a natural history enthusiast living in Oregon.