Phase relations on the diopside–jadeite–hedenbergite join up to 24 GPa and stability of Na-bearing majoritic garnet

Abstract

Phase relations on the diopside (Di)–hedenbergite (Hd)–jadeite (Jd) system modeling mineral associations of natural eclogites were studied for the compositions (mol %) Di 70Jd 30, Di 50Jd 50, Di 30Jd 70, Di 20Hd 80, and Di 40Hd 10Jd 50 using a toroidal anvil-with-hole (7 GPa) and a Kawai-type 6–8 multianvil apparatus (12–24 GPa). We established that Di, Hd, and Jd form complete series of solid solutions at 7 GPa, and melting temperatures of pure Di (1980 °C) and Jd (1870 °C) for that pressure were estimated experimentally. The melting temperature for the Di 50Jd 50 composition at 15.5 GPa is 2270 °C. The appearance of garnet is clearly dependent on initial clinopyroxene composition: at 1600 °C the first garnet crystals are observed at 13.5 GPa in the jadeite-rich part of the system (Di 30Jd 70), whereas diopside-rich starting material (Di 70Jd 30) produces garnet only above 17 GPa. The proportion of garnet increases rapidly above 18 GPa as pyroxene dissolves in the garnet structure and pyroxene-free garnetites are produced from diopside-rich starting materials. In all experiments, garnet coexists with stishovite (St). At a pressure above 18 GPa, pyroxene is completely replaced by an assemblage of majorite (Maj) + St + CaSiO 3–perovskite (Ca-Pv) in Ca-rich systems, whereas Maj is associated with almost pure Jd up to a pressure of 21.5 GPa. Above ∼22 GPa, Maj, and St are associated with NaAlSiO 4 with calcium ferrite structure (Cf). We established that an Hd component also spreads the range of pyroxene stability up to 20 GPa. In the Di 70Jd 30 system at 24 GPa an assemblage of Maj + Ca-Pv + MgSiO 3 with ilmenite structure (Mg-Il) was obtained. The experimentally established correlation between Na, Si, and Al contents in Maj and pressure in Grt(Maj)–pyroxene assemblages, may be the basis for a “majorite” geobarometer. The results of our experiments are applicable to the upper mantle and the transition zone of the Earth (400–670 km), and demonstrate a wide range of transformations from eclogite to perovskite-bearing garnetite. In addition, the mineral associations obtained from the experiments allowed us to simulate parageneses of inclusions in diamonds formed under the conditions of the transition zone and the lower mantle.