ABSTRACT

In comparison to the MgO–SiO₂–H₂O (MSH) system, the addition of Al₂O₃ (as required in the pyrope composition) makes the effects of H₂O on the high-pressure phase behavior of pyrope extremely complicated. This is because the MgO–Al₂O₃–SiO₂–H₂O (MASH) system involves more than twice as many crystalline hydrous phases as the MSH system. Experimental investigations of the effect of H₂O on the phase behavior of pyrope have so far revealed only three phase assemblages plus various unidentified assemblages in the pressure range 1.5–3.5 GPa and in the presence of excess H₂O. Thus, a theoretical study based on the ambient densities of various phase assemblages which are relevant to the pyrope-H₂O composition at various H₂O content and at high pressures and temperatures is undertaken in the present work. The results of the present study not only shed light on the unidentified phase assemblages revealed in the experimental studies involving excess H₂O, but also predict many possible phase assemblages at any given amounts of H₂O up to pressures of a few tens of GPa. The high-pressure ilmenite form of pyrope is predicted not to exist when more than 1.1 wt% H₂O is present. For a model mantle which is rich in pyrope and contains less than 1 wt% H₂O, the three polymorphs of Mg₃Al₂Si₃ O₁₂ (pyrope, ilmenite and perovskite) would diversify into more than ten phase assemblages which contain various hydrous phases at various pressure and temperature conditions. The effect of H₂O on the phase behavior of an aluminous enstatite, which is a more likely candidate than both enstatite and pyrope in the Earth's mantle, is closer to pyrope than to enstatite at high pressures and temperatures.