Mean Concentrations of Volatile Components and of Major and Trace Elements in Magmatic Melts of the Dominant Geodynamic Settings of the Earth. II. Silicic Melts

Аннотация

Continuing our earlier publication, this paper presents estimates (based on our database, which included more than 1 500 000 analyses of melt inclusions and chilled glasses of rocks for 75 elements, as of 2017) of the mean concentrations of volatile, major, and trace elements in silicic magmatic rocks (SiO₂ > 66 wt %) in the Earth’s dominant geodynamic setting. Three settings were examined for silicic compositions: III and IV are settings related to subduction processes (III are zones of island-arc magmatism on the oceanic crust; and IV are zones of magmatism in active continental margins, with magma-generating regions involving continental crust), and V is continental rifts and continental hotspots. For each geodynamic setting, mean concentrations of the elements and confidence levels are first calculated in three variants: for mineral-hosted melt inclusions, for chilled glasses in rocks, and for all available data. Systematic differences are determined for the mean compositions of melt inclusions and glasses in rocks for these geodynamic settings. Patterns of primitive mantle-normalized mean concentrations of elements in magmatic melts are constructed for each of the settings. The ratios of some of the elements and volatile compounds (H₂O/Ce, K₂O/Cl, La/Yb, Nb/U, Ba/Rb, Ce/Pb, etc.) are compared in silicic and mafic melts. Variations in the ratios of the concentrations of Th, one of the most incompatible elements, to other elements in silicic and mafic rocks are discussed.