Vol 57, No 10 (2019)

The peritectic mechanisms controlling the ultramafic–mafic evolution of magmatism during fractional crystallization and the genesis of peridotite–pyroxenite–eclogite rock series of the garnet–peridotite facies of the upper mantle were substantiated experimentally. The melting phase relations of differentiated mantle material in the olivine–clinopyroxene/omphacite–corundum–coesite multicomponent system were studied by the polythermal sections method; their boundary phases reproduced the compositions of peridotitic and eclogitic minerals. The peritectic reaction between orthopyroxene and melt with the formation of clinopyroxene proceeds at the liquidus of the olivine–orthopyroxene–clinopyroxene–garnet system as the mechanism for “clinopyroxenization of orthopyroxene,” which yields the regressive olivine + clinopyroxene + garnet + melt monovariant cotectic reaction. The further evolution of magmatism was studied experimentally at 6 GPa in the ultramafic–mafic olivine–diopside–jadeite–garnet system with a variable composition of the diopside–jadeite solid solutions (clinopyroxene ↔ omphacite). The peritectic reaction between olivine and melt with the formation of garnet was detected on the liquidus of the triple system olivine–diopside–jadeite as the of olivine garnetization mechanism yielding the omphacite + garnet + melt cotectic reaction with the formation of bimineral eclogite. The structure of the liquidus of the olivine–diopside–jadeite–garnet system was defined, as well as its critical role as the “physicochemical bridge” between the ultramafic olivine-bearing peridotite–pyroxenite composition and mafic silica-saturated eclogitic composition of matter within the garnet–peridotite facies. The experimental physicochemical results illustrate the genetic links between ultramafic and mafic rocks and the mechanisms of continuous fractional magmatic evolution and petrogenesis from olivine-bearing peridotite–pyroxenite rocks to silica-saturated eclogite–grospydite rocks. This explains the complete petrochemical trends for the rock-forming components in clinopyroxenes and garnets from differentiated rocks of the garnet–peridotite facies.

Strontium adsorption was studied by acid–base potentiometric titration at various pH, ionic strength, the sorbate/sorbent ratio, and temperatures (at 25, 50, and 75°C). The experimental data were interpreted using two models of surface complexation with two different electrostatic models of the interface: the constant capacitance (CCM) and triple-layer (TLM) models. Although both models are able to take into account acid–base reactions and surface complexation of Sr on birnessite, we believe that TLM is more suitable for the description of the H⁺–>MnOH–Sr²⁺ heterogeneous system. At a low ionic strength and negatively charged surface, Sr²⁺ ions compete with electrolyte ions and form both inner- and outer-sphere complexes. Although the application of CCM in describing Sr adsorption may be mathematically satisfactory, it has little physical sense. We suggest a model that involves both inner-sphere (>MnOHSr²⁺, >MnOSr⁺, >MnOSrOH⁰) and outer-sphere ([>MnO–Sr²⁺]⁺) surface complexes. The corresponding constants of formation of these surface complexes were calculated at 25, 50, and 75°C.