Major, volatile, ore, and trace elements in magmatic melts from main geodynamic settings. II. Similarity and differences

Abstract

Based on the mean contents of elements and their standard deviations estimated in the first part of this research project (Naumov et al, 2022), we compared in detail the distinguished geodynamic settings. In order to compare the compositions of mafic melts, a correction to take into account changes related to the fractionation of main minerals was introduced. The use of numerical criteria made it possible to determine the sequence of elements by the degree of coherence during melting and crystallization of the main magmatic melts. Within this sequence, a regular variation in elemental contents normalized to the average composition of oceanic island melts was established. The melts of mid-ocean ridges show a monotonous increase in normalized contents from the most incompatible (Cs, Ba, U, La, etc.) to compatible elements (Sc, Ni, Cr). The settings of convergent plate boundaries show relative enrichment in the most incompatible elements and significant negative Ta-Nb anomalies relative to neighboring elements. The magmas of continental rifts show the highest enrichment in the most incompatible elements, as well as Pb, Li, and some other elements. Indicator element ratios showing significant variations between the settings were distinguished for mafic melts. Some element ratios are almost identical (within observed variations) in mafic melts from all the settings. The mean element ratios in mafic, intermediate, and silicic magmas show three types of behavior. Some ratios (including the canonic ratios Nb/Ta, Zr/Hf, etc.) in intermediate and silicic magmas are inherited from the composition of mafic melts. Some ratios show irregular changes from mafic to silicic melts (Sr/Cr, F/Th, etc.). There are ratios that changes monotonously and significantly in the sequence from mafic to silicic melts (Ni/Yb, Lu/P, etc.). The variations of element ratios are related to the crystallization differentiation of melts and contributions of geochemically contrasting reservoirs.