Phase relations in the system (Mg,Ca)3Al2Si3O12-Na2MgSi5O12 at 7.0 and 8.5 GPa and 1400–1900°C
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The CaO-MgO-Al2O3-SiO2-Na2O multicomponent system was experimentally studied at 7.0 and 8.5 GPa using an anvil-with-hole toroidal high-pressure apparatus to examine two binary joins: pyropegrossular and grossular-Na-majorite. These and literature data were employed to simulate the liquidus surface of the pyrope-grossular-Na-majorite system. The liquidus surface of garnet of predominantly pyrope composition is dominant in the diagram, and the garnet contains much of the Na2MgSi5O12 end member. Melting was observed in this region at temperatures above 1900°C, and the solidus of the system occurs at temperatures below 1550°C. The pyrope-grossular system shows a miscibility gap at 50–65 mol % of the pyrope component and two series of garnet solid solutions. The dominant phase at grossular and Na-majorite mixing is pyroxene, and garnet crystallizes within a fairly narrow field in the grossular-rich region. All garnets synthesized in the systems have elevated Si and Na concentrations and belong to the majorite series, for which a uniform mechanism of isomorphism (Mg, Ca) + Al = Si + Na was proved.
Keywordspyrope sodic majorite grossular experiment phase relations mantle
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