Compositional Controls on the Formation of Kaersutite Amphibole in Shergottite Meteorites

Introduction: The shergottite basalts, meteorites of Martian origin, contain rare small grains (~10-100 μm diam.) of kaersutite, a Ca-amphibole rich in Al and Ti [1-3]. Kaersutites have been used to estimate the water content of shergottites and the Martian mantle [4]; however, questions remain about the original water content of the amphiboles [5] and if they formed from magma [6]. We investigated the petrographic settings of amphiboles in two shergottites and confirm that these amphiboles occur only in multiphase inclusions in pyroxene. In fact, kaersutite is found only in pigeonite. This suggests that the occurrence of amphibole is controlled in part by the composition of its host phase. Crystallization of host (cognate) pigeonite from a magmatic inclusion will enrich the remaining melt in Ca, Al, and Ti, supporting formation of kaersutite. Samples and Methods: We identified kaersutite grains in thin sections of the shergottites Zagami (UNM 999) and EETA79001 (,443). Zagami and EETA79001 include several lithologies: UNM 999 is fine-grained [2], and EETA79001,443 represents the A lithology (relatively fine-grained and magnesian) [7]. Both samples contain ~70 % pyroxene grains. Zagami contains nearly equal proportions of augite (high-Ca) and pigeonite (low-Ca) [2], while EETA79001A has three times more pigeonite than augite [8]. In both shergottites, the pyroxene grains are larger (up to 0.5 mm across and 1 mm long) than plagioclase (maskelynite), oxides, sulfides, or Ca-phosphate grains.