Melt/Diffusion in Globule-bearing Alkaline Magmas:
Origin of the Alkaline Series?

U. Schreiber Geologisches Institut der Universität Bonn, Nußallee 8, D-53115 Bonn, Germany

D. Anders Geologisches Institut der Universität Bonn, Nußallee 8, D-53115 Bonn, Germany

J. Koppen Geologisches Institut der Universität Bonn, Nußallee 8, D-53115 Bonn, Germany

Special textures in alkaline volcanics generated by melt/melt intrusions can often be observed in the Tertiary volcanic province of the Westerwald (Rhenish Massif, Germany). Due to differences in temperature and chemistry, the intruding melt is dispersed as globules with diameters from less than a few millimetres to several centimetres. Globules can ideally be studied on the latite/trachyte intrusion of the Bittersberg subvolcanic complex where latite is dispersed as globules in a volumetrically dominant trachytic host. The dark grey, primary latitic globules have a diameter from 3 millimetres to several centimetres. The globule matrix consisting of feldspars and brownish glass does not show any flow regulation. The matrix minerals of the trachytic host rock are strongly flow regulated around the globules. The results of microprobe matrix measurements on globule-bearing trachytes show a stepwise enrichment of Na (±K) in the globules while the concentrations of Fe, Ca, Mg, and Ti are lowered. The cause of the element transfer are diffusion processes on the interface between the globules and the host melt. Dispersion as globules leads to an extreme enlargement of the surface of contact between the two melts. Therefore, the
diffusive exchange of elements is highly effective. Surprisingly, the element distribution shows a signature of the Soret-effect. The physical parameters of the globules change during diffusion of elements. Due to Na-enrichment, the specific weight of the globules is lowered. Subsequent aggregation of globules after Na-enrichment leads to the development of an independent "extraction melt". In an extreme case, a phonolitic composition can be achieved. This is documented by phonolites which totally consist of globules. In addition to the occurrence in intermediate volcanics, globules can also be observed in basaltic rocks of the Westerwald. In these globules trends of element distribution are similar to the intermediate globules: Na (±K) is enriched
in the matrix of the globules while the contents of Ca, Mg, Fe, and Ti are decreased relatively to the host rock. However,
chemistry of minerals (olivine, clinopyroxene, plagioclase) within the globules and the host rock does not show any differences. Changing of melt composition and extraction of a new, alkali-enriched melt possibly took place in the same way. The melt/melt-diffusion process so leads to new aspects for the discussion of the generation of alkali basaltic magmas: e.g. low grade partial anatexis of a metasomatically enriched mantle source.