Conditions of magmatic crystallization of Na-bearing majoritic garnets in the earth mantle: Evidence from experimental and natural data

Results of experimental study at 7.0-8.5 GPa and 1300-1900A degrees 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.