Na-bearing majoritic garnet in the system Na2MgSi5O12–Mg3Al2Si3O12 at 11–20 GPa: phase relations, structural peculiarities and solid solutions

Na-majorite Na2MgSi5O12 (NaMaj), an end-member of sodium-rich majoritic garnet, was synthesized and garnet/pyroxene transition was studied in Kawai-type multi-anvil experiments at 11–20 GPa and 1500–2300°C. Na-majorite was obtained at 16 GPa and 1500°C; its stability spreads to the high-temperature region with pressure (1900°C at 17 GPa and 2100°C at 19.5 GPa). Experimental study of the Mg3Al2Si3O12 (Prp)–Na2MgSi5O12 system at 11–20 GPa demonstrated the following phase assemblages changing with pressure: garnet + two pyroxenes (enstatite-rich and Na-pyroxene); garnet + pyroxene (enstatite–jadeite–Na-pyroxene) + stishovite; garnet + stishovite; garnet + NaAlSiO4 with calcium ferrite-type structure + stishovite. For a given bulk composition (Prp50NaMaj50), regular increase of both Na and Si concentrations and, consequently, NaMaj mole portion in garnets was observed. From 8.5 to 20 GPa, the concentration of Na2O increased from 1.52 to 5.71 wt %, which corresponded to a NaMaj content of ~40 mol %. The highest Na2O content (12.24 wt %) was registered at 18.5 GPa and 1600°C for the Prp20NaMaj80 starting composition. Experiments at 18.5 GPa and 1600°C on the pyrope–Na-majorite join allowed us to study mixing peculiarities for sodium-rich majoritic garnet. The transition from cubic to tetragonal symmetry (space group I41/acd) was observed for the starting composition with ~80 mol % Na2MgSi5O12, which is consistent with the similar change of the structure in the pyrope–majorite (Mg4Si4O12) system. Significant Na-majorite solubility in pyrope, as well as findings of natural garnets with high Na concentrations (>1 wt % Na2O) allow us to consider Na-bearing majoritic garnet as a concentrator of sodium in the deep upper mantle and transition zone.