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 1300–1900°C of the systems pyrope Mg 3 Al 2 Si 3 O 12 ( Prp )-Na 2 MgSi 5 O 12 (Na Grt ) modeling solid solutions of Na-bearing garnets, Prp -jadeite NaAlSi 2 O 6 ( Jd ) in a simplified mode demonstrating melting relations of Na-rich eclogite, and Prp -Na 2 CO 3 are presented. Prp -Na 2 MgSi 5 O 12 join is a pseudobinary that results from the decomposition of Na Grt on to coesite and Na-pyroxene. Synthesized garnets are characterized by Na admixture (>0.32 wt % Na 2 O) and excess Si (3.05–3.15 f.u.). Maximal Na 2 O concentrations (1.5 wt % Na 2 O) 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 Na 2 MgSi 5 O 12 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 -Na Grt -majorite) and (2) pyroxene ( Jd -clinoenstatite-Eskola molecule), and the appearance of kyanite at the solidus of the system, where garnets with the highest Na 2 O 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 % Na 2 O) are formed in a Jd -rich part of the system (20–50 mol % Prp ). In the Prp -Na 2 CO 3 system at 8.5 GPa garnet crystallizes in a wide range of starting compositions as a liquidus mineral containing up to 0.9 wt % Na 2 O. 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 8.5 GPa. 1 This article was translated by the authors.