Na-bearing majoritic garnet in the Na_2MgSi_5O_(12)-Mg_3Al_2Si_3O_(12) join at 11-20GPa: Phase relations, structural peculiarities and solid solutions

摘要

Na-majorite Na_2MgSi_5O_(12) (NaMaj), an end-member of sodium-rich majoritic garnet, was synthesized and garnet/pyroxene transition was studied in Kawai-type multi-anvil experiments at 11-20GPa and 1500-2300°C. Na-majorite was obtained at 16GPa and 1500°C; its stability spreads to the high-temperature region with pressure (1900°C at 17GPa and 2100°C at 19.5GPa). Experimental study of the Mg_3Al_2Si_3O_(12) (Prp)-Na_2MgSi_5O_(12) join at 11-20GPa 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+NaAlSiO_4 with calcium ferrite-type structure+stishovite. For a given bulk composition (Prp50NaMaj50), a regular increase of both Na and Si concentrations and consequently, NaMaj mole portion in garnets was observed. From 8.5 to 20GPa, the concentration of Na_2O increased from 1.52 to 5.71wt.%, which correspond to a NaMaj content of ～40mol.%. The highest Na_2O content (12.24wt.%) was registered at 18.5GPa and 1600°C for the Prp20NaMaj80 starting composition. Experiments at 18.5GPa and 1600°C on the pyrope-Na-majorite join allowed us to study mixing peculiarities for sodium-rich majoritic garnet. The transition from cubic (Ia3d) to tetragonal (I41/acd) symmetry was observed for the composition with ～80mol.% Na_2MgSi_5O_(12), which is consistent with the similar change of the structure in the pyrope-majorite (Mg_4Si_4O_(12)) system. Significant Na-majorite solubility in pyrope, as well as findings of natural garnets with high Na concentrations (>1wt.% Na_2O) allow us to consider Na-bearing majoritic garnet as an important concentrator of sodium in the deep upper mantle and transition zone.