Geochemistry and growth morphology of alkali feldspar crystals from an IAB iron meteorite : insight into possible hypotheses of their crystallization

Alkali feldspar crystals have been recognized in the troilite-graphite nodules of the Morasko IAB iron meteorite. Their chemical, microtextural and structural properties were studied using electron microprobe analysis (EMPA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), transmission electron microscopy (TEM) and Raman spectroscopy. The feldspars occur as perthitic or antiperthitic intergrowths, whereas the albite lamellae are perfectly twinned. The structural properties reveal intergrown phases with fairly disordered patterns. The electron microprobe analyses demonstrate that the intergrown phases are mainly rich in sodium or potassium, resulting in compositions that are close to those of albite or orthoclase. The compositions, calculated on the basis of a segmented perthite-antiperthite image, showed that the Or-to-Ab proportions in the homogenized crystals were almost 0.3:0.7, thus indicating that the anorthoclase crystallized under high-temperature conditions. Two hypotheses of crystal formation could account for these characteristics: crystallization from a melt or from a metasomatic solution. Relics with evidence of metasomatic replacement of former minerals were not found. Accordingly, this work focuses on arguments that support the other hypothesis. Large ion lithophile elements (LILEs, e.g., Ba, Sr, Rb, LREE, Pb, and Ga) were used to track the origin of the crystals. Their concentrations indicate crystallization from a parent melt strongly depleted in LILEs. Alkali feldspar is commonly a product of a highly differentiated melt. However, highly differentiated melts are typically enriched in LILEs, which here is not the case. The melt that crystallized the feldspar cannot be related to impact-induced partial melting of the chondritic material alone. The derived melt probably was contaminated by silica-rich target material during interaction between the IAB projectile and the target material and was accompanied by metal and sulphide melts that were both immiscible with a silicate melt.