TY - JOUR
T1 - Na-bearing majoritic garnet in the Na2MgSi5O12-Mg3Al2Si3O12 join at 11-20GPa
T2 - Phase relations, structural peculiarities and solid solutions
AU - Dymshits, Anna M.
AU - Bobrov, Andrey V.
AU - Bindi, Luca
AU - Litvin, Yuriy A.
AU - Litasov, Konstantin D.
AU - Shatskiy, Anton F.
AU - Ohtani, Eiji
N1 - Funding Information:
The constructive reviews by Ken Collerson, Oleg Safonov, and anonymous referee were very helpful for improving the quality of the manuscript. This study was supported by the Russian Foundation for Basic Research (projects 12-05-00426, 12-05-31351, 12-05-33044, and 11-05-00401), by the Foundation of the President of Russian Federation (grant MD-534.2011.5 for Young Doctors of Sciences and grant NSh-5877.2012.5 for Leading Scientific Schools), and Integration project No. 97 (Siberian Branch, Russian Academy of Sciences). The experiments were conducted as a part of the Tohoku University Global Center-of-Excellence program, Japan “Global Education and Research Center for Earth and Planetary Dynamics”.
PY - 2013/3/5
Y1 - 2013/3/5
N2 - 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 11-20GPa 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 Mg3Al2Si3O12 (Prp)-Na2MgSi5O12 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+NaAlSiO4 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 Na2O increased from 1.52 to 5.71wt.%, which correspond to a NaMaj content of ∼40mol.%. The highest Na2O 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.% 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 (>1wt.% Na2O) allow us to consider Na-bearing majoritic garnet as an important concentrator of sodium in the deep upper mantle and transition zone.
AB - 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 11-20GPa 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 Mg3Al2Si3O12 (Prp)-Na2MgSi5O12 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+NaAlSiO4 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 Na2O increased from 1.52 to 5.71wt.%, which correspond to a NaMaj content of ∼40mol.%. The highest Na2O 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.% 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 (>1wt.% Na2O) allow us to consider Na-bearing majoritic garnet as an important concentrator of sodium in the deep upper mantle and transition zone.
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U2 - 10.1016/j.gca.2012.11.032
DO - 10.1016/j.gca.2012.11.032
M3 - Article
AN - SCOPUS:84872303348
VL - 105
SP - 1
EP - 13
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
SN - 0016-7037
ER -