Shock Formation of Kaersutite in Martian Meteorites: an Experimental Study

Introduction: Magmatic inclusions in Martian meteorites contain some hydrous minerals such as kaersutites [e.g., 1-2]. The characteristic of Martian kaersutites is their water-poor [3] and Ti-rich (~10 wt%) compositions. The presence of kaersutite gives evidence that Martian parent magma contained water. In fact, several attempts have been performed to estimate the water content in Martian magmas and mantle [e.g., 4]. Thus, it is important to reveal the crystallization process of kaersutite for understanding the Martian magma water content. However, no one has been successful in explaining crystallization process of Martian kaersutites because of difficulty in explaining crystallization of Ti-rich kaersutite. Johnson et al. [2] studied a partially crystallized magmatic inclusion in Chassigny and suggested that low TiO 2 content in synthetic kaersutite was due to ilmenite crystallization prior to kaersutite in the experiments. We suggested that Ti-rich kaersutites in Martian meteorites is a shock origin rather than a primary phase crystallized at depth [5]. In order to explore this hypothesis, we have performed shock experiments on analogous mixture of magmatic inclusions in Martian meteorites. Observation of magmatic inclusion: We performed a detailed observation of kaersutite-bearing magmatic inclusions in Zagami (Fig. 1) and LEW 88516 (lherzolite) Martian meteorites. The size of magmatic inclusions is usually 100-200 µm in diameter, but their shapes are irregular. They are surrounded by radiation cracks in many cases. Kaersutite-bearing magmatic inclusions in Zagami and LEW 88516 usually occur only within cumulus pyroxene. Kaersutite coexists with Al-Ti diopside, Si-rich glass, and hercynite spinel. One of the most important observations is that kaersutite almost always has contact with both Al-Ti diopside and Si-rich glass. The size of kaersutites is up to 20 µm long. Olivine in LEW88516 also conrains a similar inclusion, but they do not contain kaersutites. The magmatic inclusions within olivine instead contain ilmenite and Fe-Ti oxides. Their bulk compositions are similar to those of magmatic inclusions containing kaersutite. Shock experiment: The shock-recovery experiments were performed using a single stage 30 mm-bore propellant gun to generate shock waves by hypervelocity impact at the National Institute for Materials Science. The detailed experimental procedure is similar to Sekine et al. [6]. Starting