Glasses in mantle xenoliths as geochemical indicators of metasomatic agents

An extensive comparison between world-wide occurrences of mantle glasses in both continental and oceanic settings is presented here. Several localitie s were chosen on the basis of the available major (and trace) element data, coupled with a clear identification of the nature of the metasomatizing ag ents. Xenoliths bearing amphibole and/or phlogopite, which can strongly aff ect the glass chemistry, were not considered. Despite their considerable ge ochemical variability world-wide, glasses are rather homogeneous within sin gle xenoliths and their main geochemical features are broadly maintained in the same area. Orthopyroxene always plays a major role in glass genesis, a s suggested by the relationship between its presence in the primary assembl age and the silica saturation degree, as well as by the FeO vs. SiO2 decrea sing trends. At the same time, the absence of any correlation between mg# a nd SiO2 rules out the possibility of glass chemistry resulting from (or bei ng affected by) secondary mineral fractionation. Thus, the geochemical feat ures of mantle glasses are a powerful tool for investigating the nature of the metasomatic melts. According to available data, glasses in mantle xenol iths from Gran Comore, Samoa, Spitsbergen, Mongolia and Hoggar have been re lated to carbonatite metasomatism, whereas glasses in mantle xenoliths from Mt. Lessini, Hawaii, Cape Verde and Yitong have been related to metasomati sm by alkali silicate basic melts. Moreover, the distinctly different alkal i 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 si licate metasomatic melt of the former with respect to a potassic characteri stic of the latter. Glasses related to carbonatite metasomatism are charact erized by high CaO, Na2O and low SiO2 and K2O contents, with Na2O/K2O ratio s 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 SiO2 and K2O and low CaO and NaO contents, with Na2O/K2O ratios less than 1; they generally pre sent higher Rb and lower Nb and REE contents. Glasses related to Na-alkali silicate metasomatism are characterized by SiO2 contents comparable to thos e related to K-alkali silicate metasomatism, but present higher Na2O conten ts (in any case lower than those of carbonatite metasomatism), with Na2O/K2 O ratios usually ranging between 1 and 2. On these bases, diagrams for disc riminating between carbonatite- and Na-alkali or K-alkali silicate-related metasomatism are presented. (C) 2000 Elsevier Science B.V. All rights reser ved.