Evaporation Features of the Melts of Ca–Al–Inclusions in Chondrites: Experimental Data and Their Implications

The paper presents experimental results on the evaporation of the main types (A and B) of Ca–Al inclusions of chondrites in a Knudsen tungsten cell in the temperature range of 1000–2400°C. A peculiarity of the evaporation of the inclusions is that the silicon component is preserved in the residual melt to high temperatures exceeding 2200°C. At the same time, magnesium oxide completely evaporates from the melt, which leads to a decrease in the spinel content and even its disappearance during the crystallization of the residual melt at 2000°C. Magnesium evaporation leads, in turn, to an increase in the Al₂O₃ content in the melt and, accordingly, to an increase in the content of gehlenite in melilite during its crystallization, as is observed in the high-temperature rims of subtype B1 inclusions. Experiments have also shown that the high-temperature evaporation of Ca–Al inclusions occurs in an oxygen-rich atmosphere compared to carbonaceous chondrite matter. In this regard, it is assumed that the condensation of the first mineral products of the inclusions occurred in the oxygen-rich atmosphere of the star (O-rich AGB) outside the protosolar system.