Diopside-jadeite join at 16–22 GPa

Abstract

Phase relations on the diopside-jadeite join were experimentally determined at 16–22 GPa pressures and temperatures in the vicinity of 1500 °C under hydrous and 2100 °C under anhydrous conditions, using a split-sphere anvil apparatus (USSA-2000). Starting compositions on the diopside-jadeite join produced assemblages containing CaSiO₃ perovskite. This assured that the coexisting garnet with compositions in the ternary system Mg₂Si₂O₆(En)-CaMgSi₂O₆(Di)-NaAlSi₂ O₆(Jd) had the maximum Ca content possible under the given conditions. Garnet reached its maximum Ca content at 17 GPa, and exsolved CaSiO₃ perovskite at higher pressures. The garnet composition closest to the join, En₅Di_47.5Jd_47.5 (mol%), was reached at 18–19 GPa and 2100 °C. The maximum Na content of garnet limited by the coexisting pyroxene did not exceed 51 mol% jadeite at 22 GPa and 2100 °C. At 22 GPa, pyroxene was replaced with NaAlSiO₄ (calcium ferrite structure) and stishovite under anhydrous conditions, while in the presence of H₂O a new hydrous Na-bearing phase with the ideal composition Na₇(Ca, Mg)₃AlSi₅O₉(OH)₁₈ was synthesized instead. Garnet coexisting with CaSiO₃ perovskite and MgSiO₃ ilmenite at 22 GPa and 1400 °C was En₅₁Di₉Jd₄₀, coincidentally identical to the first garnet forming in the ternary system at 13 GPa. The new data are applicable to the Earth's transition zone (400–670 km depths) and suggest that the transformation from eclogite to garnetite would occur primarily over a limited depth interval from 400 to 500 km. Gaps in the observed garnet compositions suggest immiscibility, which could potentially cause a sharp 400 km discontinuity in an eclogitic mantle.