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- Author or Editor: Elias Chatzitheodoridis x
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The study of shock-metamorphic features of the Zagami meteorite revealed pseudotachylite-like melt veins with inhomogeneous chemistry and schlieren structure of silica-glass and alkali feldspar melt glass. The feldspar occurs as diaplectic glass in the interstitial area indicating short-time (few seconds) quenching of shock pressure during the impact event, with post-shock annealing. At several locations, apatite needles were identified, which are formed by fluids (cold water with dissolved ions) after the crystallization of cumulate magmatic minerals. Phosphates also could form in impact melts due to circulation of fluids after the impact event. The other signature for the high shock temperature is the presence of Ca–Ti-rich pyroxenes and titanomagnetite, which indicate temperature above 1,200 °C. The formation of silica-rich melt in interstitial area has two scenarios: (a) fractional melting of the Martian crust or (b) formation by pseudotachylite-like impact melting. According to textural observations (schlieren pattern), we propose an impact origin of the large amount of silica-rich melt in this meteorite. Pseudotachylite-like textures were mentioned earlier in terrestrial impact craters; however, we first propose them to form in a Martian meteorite based on their similarity of texture with terrestrial pseudotachylites.
Shock-driven annealing of pyroxene and shock deformation of olivine were analyzed in a recently found H chondrite called Csátalja. The most characteristic infrared (IR) spectral shape of shock-annealed sub-grained pyroxene was identified: the strongest peak occurs at 860 cm−1 with a smaller shoulder at 837−840 cm−1, and small bands are at 686, 635−638, and 1,044−1,050 cm−1. The appearance of forbidden bands in pyroxene and shift of band positions to a lower wave number in olivines clearly demonstrate the crystal lattice disordering due to shock metamorphism. The shock annealing produced mixed dark melt along fractures, which consists of feldspar−pyroxene and olivine−pyroxene melt. The dark shock melt at sub-grain boundaries of shocked pyroxenes and along fracture of pyroxenes is characterized by elevated Ca, Na, and Al content relative to its environment, detected by element mapping. So far, shock deformation of pyroxene and olivine was not studied by IR spectroscopy; this method has turned out to be a powerful tool in identifying the mixed composition of shock melt minerals. Further study of shock annealing of minerals, together with the context of shock melting at sub-grain boundaries, will provide a better understanding of the formation of high P–T minerals.
