Glasses in mantle xenoliths as geochemical indicators of metasomatic agents

摘要

An extensive comparison between world-wide occurrences of mantle glasses in both continental and oceanic settings is presented here. Several localities were chosen on the basis of the available major (and trace) element data, coupled with a clear identification of the nature of the metasomatizing agents. Xenoliths bearing amphibole and/or phlogopite, which can strongly affect the glass chemistry, were not considered. Despite their considerable geochemical variability world-wide, glasses are rather homogeneous within single xenoliths and their main geochemical features are broadly maintained in the same area. Orthopyroxene always plays a major role in glass genesis, as suggested by the relationship between its presence in the primary assemblage and the silica saturation degree, as well as by the FeO vs. SiO_2 decreasing trends. At the same time, the absence of any correlation between mg# and SiO_2 rules out the possibility of glass chemistry resulting from (or being affected by) secondary mineral fractionation. Thus, the geochemical features of mantle glasses are a powerful tool for investigating the nature of the metasomatic melts. According to available data, glasses in mantle xenoliths from Gran Comore, Samoa, Spitsbergen, Mongolia and Hoggar have been related to carbonatite metasomatism, whereas glasses in mantle xenoliths from Mt. Lessini, Hawaii, Cape Verde and Yitong have been related to metasomatism by alkali silicate basic melts. Moreover, the distinctly different alkali contents in Mt. Lessini and Hawaii glasses as compared with those of Cape Verde and Yitong allow a sodic signature to be attributed to the alkali silicate metasomatic melt of the former with respect to a potassic characteristic of the latter. Glasses related to carbonatite metasomatism are characterized by high CaO, Na_2O, and low SiO_2 and K_2O contents, with Na_2O/K_2O ratios usually greater than 2; they tend to have the highest Nb and LREE and the lowest Zr and Ti contents. On the other hand, glasses related to K-alkali silicate metasomatism are mainly characterized by high SiO_2 and K_2O and low CaO and NaO contents, with Na_2O/K_2O ratios less than 1; they generally present higher Rb and lower Nb and REE contents. Glasses related to Na-alkali silicate metasomatism are characterized by SiO_2 contents comparable to those related to K-alkali silicate metasomatism, but present higher Na_2O contents (in any case lower than those of carbonatite metasomatism), with Na_2O/K_2O ratios usually ranging between 1 and 2. On these bases, diagrams for discriminating between carbonatite- and Na-alkali or K-alkali silicate-related metasomatism are presented.