Metasomatism of the Continental Lithosphere: Simulation of Isotope and Element Abundance Behaviour and Case Studies

Abstract

A simple quantitative model of isotope and element exchange between a solid and an infiltrating, metasomatising fluid is developed (Model I). This model is then used as a building block to calculate isotope and relative element abundances in melts derived by fusion of a metasomatically veined source (Model II). The influence of each parameter is evaluated. Important results among others are that melt isotopic compositions are predicted to correlate with depths of magma sources and to approach initial solid isotopic compositions in magmas which originate from the least depths; initial random heterogeneities in the solid are erased and a discontinuity exists between fluid and magma isotopic compositions if pristine matrix contributed to the melts.

Applying Model II to the Italian and Virunga potassic alkaline volcanic rocks demonstrates that characteristic features in their large-scale isotope systematics can be accounted for. Identification of fluid and solid end-members suggest for both volcanic provinces that fluids which presumably had their origin in the asthenosphere invaded, penetrated and metasomatised the lithospheric mantle and crustal material, which may be previously subducted or which may be the lower crust itself. Later, these metasomatised domains fused to become the sources for potassic basic to felsic alkaline magmas. Hence potassic magmas, at least for these two occurences, come from sources which themselve are mixtures between mantle- and crustal-derived material. Whether these magma sources lie now in the upper mantle or in the lower crust cannot be decided on geochemical arguments but only from wider tectonic considerations.