Microscopic mineralogy of zoned pyroxene in NWA 12522: Implications for the crystallization histories of the shergottites

Abstract

Basaltic shergottites are the most abundant rock type of Martian meteorites, and pyroxene grains within shergottites commonly show a zoned structure. Here, the detailed microscopic mineralogical characteristics of patchy zoned pyroxene in basaltic shergottite NWA 12522 were investigated by a combination of scanning electron microscopy, electron microprobe, Raman spectroscopy, and transmission electron microscopy. The results show that the cores of zoned pyroxene in NWA 12522 have a homogeneous Mg# value and consist mainly of augite and pigeonite. By contrast, the rim of zoned pyroxene is extremely ferroan and can be further divided into two regions based on quite distinct mineralogy and textures (i.e., far-core and near-core pyroxene rims). The near-core rim shows narrow exsolution lamellae (~35 nm) that were cross-cut by thin pigeonite veinlets and contain abundant nano-sized particles of metastable pyroxferroite and pigeonite. Only relatively coarse exsolution lamellae (~80 nm) were observed in the far-core pyroxene rim regions. The distinct mineralogical characteristics of the pyroxene rims and cores in NWA 12522 imply different crystallization conditions, and the homogeneous Mg-rich pyroxene cores should have slowly crystallized from magma within a deep-seated chamber, followed by an overgrown evolved melt on these pyroxene cores during their ascent to the Martian surface, and disequilibrium crystallization of nano-sized metastable phase (pyroxferroite) occurred in the near-core region. The abnormally low ΣREE contents and steep REE pattern (high Yb/La ratio) of the pyroxene rims in NWA 12522 imply that merrillite should have crystallized prior to the pyroxene rims, making the residual melt become REE-depleted and HREE-enriched.