Comendites and pantellerites of Nemrut volcano,eastern Turkey: Genesis and relations between the trachyte-comenditic,comenditic, and pantelleritic melts

The paper reports data on the mineralogy, geochemistry, phase composition of comendites and pantellerites from Nemrut volcano, eastern Turkey; estimates of the crystallization conditions of minerals, composition of matrix glasses and melt inclusions in anorthoclase, fayalite, hedenbergite phenocrysts. LA-ICP-MS was applied to analyze the matrix glasses and phenocryst minerals. The distribution coefficients between phases and glass were calculated for P, B, Li, Rb, Cs, Ba, Sr, Zr, Hf, Ta, Nb, Sc, V, Cr, Ni, Cu, Pb, Th, U, Y, REE. Mass balance simulations of the comenditic and pantelleritic compositions, experimental data, data on melt inclusions are utilized to analyze the processes responsible for the derivation of the magmas, accumulation of components in them and to elucidate genetic links between the trachyte-comenditic, comenditic and pantelleritic melts. The origin of the residual comenditic and pantelleritic melts is explained by variations in the crystallization conditions of anorthoclase (dominant phase), hedenbergite, fayalite, Fe and Ti oxides in the parental trachyte-comenditic magma depending on the pressure and concentration of water dissolved in the melts. The accessory phases (REEand Sr-bearing fluorapatite and zircon) were likely involved in the fractionation of the melts. The following crystallization parameters were obtained by QUILF calculations for the hedenbergite, fayalite, and ilmenite phenocrysts (minimum values for quartz-free melts): 3 kbar, 763°C, ΔFMQ = ?1.27 for the Fe-comendites; 3.3–3.8 kbar, 715°C, ΔFMQ = ?1.8 for the pantellerites; 2.3 kbar, 748°C, ΔFMQ = ?1.16 for the low-Fe comendites. The equilibrium crystallization of anorthoclase phenocrysts in the comenditic melts proceeded at temperature ～750°C. Data on glasses of melt inclusions indicate that the comenditic and pantelleritic melts contained 1–3 wt % H₂O. Analysis of literature data and estimates of the conditions under which the Nemrut magmas were derived suggest that the local chambers with H₂O-undersaturated comenditic and pantelleritic melts could occur at centers of alkaline volcanism at depths within the range of 5 to 10–15 km (lithostatic pressure of 1–4 kbar), at temperatures <750°C and oxygen fugacity below the FMQ buffer.