Phase relations and formation of sodium-rich majoritic garnet in the system Mg3Al2Si3O12–Na2MgSi5O12 at 7.0 and 8.5 GPa |
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| Authors: | A V Bobrov Yu A Litvin L Bindi A M Dymshits |
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| Institution: | (1) Department of Petrology, Geological Faculty, Moscow State University, Leninskie Gory, Moscow, 119991, Russia;(2) Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka, Moscow District, 142432, Russia;(3) Museo di Storia Naturale, Sezione di Mineralogia, University of Firenze, Via La Pira 4, 50121 Firenze, Italy |
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| Abstract: | The pseudo-binary system Mg3Al2Si3O12–Na2MgSi5O12 modelling the sodium-bearing garnet solid solutions has been studied at 7 and 8.5 GPa and 1,500–1,950°C. The Na-bearing garnet
is a liquidus phase of the system up to 60 mol% Na2MgSi5O12 (NaGrt). At higher content of NaGrt in the system, enstatite (up to ∼80 mol%) and then coesite are observed as liquidus phases.
Our experiments provided evidence for a stable sodium incorporation in garnet (0.3–0.6 wt% Na2O) and its control by temperature and pressure. The highest sodium contents were obtained in experiments at P = 8.5 GPa. Near the liquidus (T = 1,840°C), the equilibrium concentration of Na2O in garnet is 0.7–0.8 wt% (∼6 mol% Na2MgSi5O12). With the temperature decrease, Na concentration in Grt increases, and the maximal Na2MgSi5O12 content of ∼12 mol% (1.52 wt% Na2O) is gained at the solidus of the system (T = 1,760°С). The data obtained show that most of natural diamonds, with inclusions of Na-bearing garnets usually containing
<0.4 wt% Na2O, could be formed from sodium-rich melts at pressures lower than 7 GPa. Majoritic garnets with higher sodium concentrations
(>1 wt% Na2O) may crystallize at a pressure range of 7.0–8.5 GPa. However the upper pressure limit for the formation of naturally occurring
Na-bearing garnets is restricted by the eclogite/garnetite bulk composition. |
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| Keywords: | Na-bearing majoritic garnet Eclogite High-P– T experiments Phase relations |
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