Experimental Modeling of the Interaction Between Sulfur-Rich Fluids and Garnets of Eclogitic and Lherzolitic Paragenesis at Lithospheric Mantle P – T Conditions

Experimental studies modeling the interaction of garnets from eclogitic and lherzolitic parageneses with reduced and oxidized sulfur-rich fluids were conducted using a split sphere multianvil high-pressure apparatus (BARS). The experiments were performed in garnet–sulfur–carbon and garnet–anhydrite–carbon systems at a pressure of 6.3 GPa and temperatures of 1150, 1300, and 1450 °C, using graphite capsules as well as platinum capsules with a buffering hematite container. We demonstrated that interaction of both reduced and oxidized sulfur-rich fluids with garnets of mantle paragenesis leads to their partial dissolution and recrystallization, resulting in the formation of zoned crystals with reduced iron concentrations in the rims. In addition, this metasomatic interaction results in (1) garnet carbonation, accompanied by the formation of a magnesite ± dolomite + kyanite + coesite assemblage or a carbonate–silicate melt, and (2) garnet sulfidation, with the formation of pyrite (1150–1300 °C) or a sulfide melt (1450 °C) as inclusions within garnet and in the interstitial space. The experiments yielded indicator characteristics for mantle–paragenesis garnets that have been affected by sulfur-bearing fluids. These characteristics include decreased FeO contents in the peripheral zones of the crystals relative to their cores, as well as the formation of sulfide inclusions that trace the boundaries between zones. Here we present the first study of diamond dissolution features under lithospheric mantle P–T conditions in a silicate-sulfur-carbon system that models mantle metasomatic alteration of garnet-bearing rocks by sulfur-rich fluids. We conclude that, under these conditions, diamond crystals undergo partial dissolution, producing characteristic etch pits in the form of negative trigons on {111} and rectangles on {100}.