Conditions of magmatic crystallization of Na-bearing majoritic garnets in the earth mantle: Evidence from experimental and natural data |
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| Authors: | A V Bobrov A M Dymshits Yu A Litvin |
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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: | Results of experimental study at 7.0–8.5 GPa and 1300–1900°C of the systems pyrope Mg3Al2Si3O12 (Prp)-Na2MgSi5O12 (NaGrt) modeling solid solutions of Na-bearing garnets, Prp-jadeite NaAlSi2O6 (Jd) in a simplified mode demonstrating melting relations of Na-rich eclogite, and Prp-Na2CO3 are presented. Prp-Na2MgSi5O12 join is a pseudobinary that results from the decomposition of NaGrt on to coesite and Na-pyroxene. Synthesized garnets are characterized by Na admixture (>0.32 wt % Na2O) and excess Si (3.05–3.15 f.u.). Maximal Na2O concentrations (1.5 wt % Na2O) are reached on the solidus of the system at 8.5 GPa. Clear correlation between Na and Si was established in synthesized
garnets; this provides evidence for heterovalent isomorphism of the Mg + Al → Na + Si type with the appearance of Na2MgSi5O12 component as a mechanism of such garnet formation. The Prp-Jd join is also pseudobinary that results from the formation of two series of solid solutions: (1) garnet (Prp-NaGrt-majorite) and (2) pyroxene (Jd-clinoenstatite-Eskola molecule), and the appearance of kyanite at the solidus of the system, where garnets with the highest
Na2O contents (>0.8 wt %) are formed. In spite of quite a wide field of garnet crystallization (20–100 mol % Prp), garnets with significant sodium concentration (>0.3 wt % Na2O) are formed in a Jd-rich part of the system (20–50 mol % Prp). In the Prp-Na2CO3 system at 8.5 GPa garnet crystallizes in a wide range of starting compositions as a liquidus mineral containing up to 0.9
wt % Na2O. Our experiments demonstrate that melt alkalinity, as well as PT-parameters control the crystallization of Na-bearing majoritic garnets. The results obtained provide evidence for the fact
that the majority of natural diamonds with inclusions of Na-bearing majoritic garnets containing <0.4 wt % Na2O were formed in alkaline silicate (carbonate-silicate) melts at a pressure of <7 GPa. Only a small portion of garnets with
higher sodium concentrations (>1 wt % Na2O) could be formed at a pressure of >8.5 GPa. 1 This article was translated by the authors. |
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