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Experimental modeling of decarbonation reactions with the formation of Mg,Fe-garnets and CO2 fluid during mantle-crustinteractions was carried out in a wide range of the upper-mantle pressures and temperatures. Experimental studies were performed in the MgCO3-Al2O3-SiO2 and (Mg,Fe)CO3-Al2O3-SiO2 systems in the pressure range 3.0-7.5 GPa and temperature range 950-1450 °C (t = 10-60 h), using a multianvil high-pressure apparatus of the "split-sphere" type (BARS). Experiments were carried out with a specially designed high-pressure buffered cell with a hematite container that prevents the diffusion of hydrogen into a Pt-capsule with a sample. It has been experimentally established that in the MgCO3-Al2O3-SiO2 system decarbonation occurs by the schematic reaction MgCO3 + SiO2 + Al2O3 Mg3Al2Si3O12 + CO2 at 1100 ± 20 °c (3.0 GPa), 1150 ± 20 °C (6.3 GPa), and 1400 ± 20 °C (7.5 GPa) and in the (Mg,Fe)CO3-Al2O3- SiO2 system, by the reaction (Mg,Fe)CO3 + SiO2 + Al2O3 (Mg,Fe)3Al2Si3O12 + CO2 at 1000 ± 20 °C (3.0 GPa), 1150 ± 20 °C (6.3 GPa), and 1400 ± 20 °C (7.5 GPa). Based on Raman spectroscopic characterization of the synthesized garnets, the position of the main modes R, ?2, and ?1 in the pyrope has been determined to be 364, 562, and 924-925 cm-1, respectively, and that in pyrope-almandine, 350-351, 556-558, and 918-919 cm-1. The effectiveness of the hematite container was demonstrated by means of mass spectrometry analysis. It has been found that the fluid composition corresponded to pure CO2 in all experiments. The P,T-positions of decarbonation curves leading to the formation of a CO2 fluid in assemblage with pyrope and pyrope-almandine have been experimentally reconstructed and compared with the previous calculation and experimental data. It has been established that the experimentally reproduced reaction lines with the formation of pyrope + CO2 or pyrope-almandine + CO2 assemblages are shifted to lower temperatures by 50-150 °? relative to the calculated ones. When considering the obtained results with regard to the stability of natural carbonates of various compositions in subduction settings, it has been found that at depths of ~90-190 km Mg,Fe-carbonates react with oxides in the temperature range 1000-1250 °C, and at depths of ~225 km, at 1400 °C. [ABSTRACT FROM AUTHOR]

Data on the carbon and oxygen isotopic composition of carbonates from the rocks of the Giyani greenstone belt, Kaapvaal craton, South Africa are presented. This belt is immediately adjacent to the Hout River Shear Zone, along which the Southern Marginal Zone (SMZ) of the Limpopo granulite complex is overthrusted onto the Kaapvaal craton. The δ13C values are –2.1 and –2.2‰ for carbonates from two metapelite samples, –5.1‰ for a meta ultrabasite sample, and –7.1 and –7.7‰ for two metabasite samples, respectively. The δ18О values are 17.1 and 17.5‰ for carbonates from the metapelites, 14.3‰ for carbonates from the metaultrabasite, and 12.9 and 13.0‰ for carbonates from the metabasites. The comparison of the data obtained to the published data on the isotopic composition of carbonates from the rocks of the different greenstone belts indicates both hydrothermal (in the metabasites and the metaultrabasite) and sedimentary (in the metapelites) origins of the carbonate material in the rocks studied. Based on the comparison of these data to δ13C of magnesite that formed in ultrabasic granulites during the interaction of the Limpopo complex with the craton, the graphite and fluid inclusions in leucocratic garnet-bearing granitoids that transported fluids, and the carbon isotopic composition of graphite from metapelites of the SMZ, it is concluded that the carbon source was located outside the SMZ and represented a heterogeneous carbonate-bearing lithologies. It could be carbonate-bearing volcanogenic–sedimentary sequences of the greenstone belts of the Kaapvaal craton, which underwent prograde metamorphism during the interaction with the SMZ granulites. [ABSTRACT FROM AUTHOR]