Thin Amorphous Intergranular Layers at Mineral Interfaces in Xenoliths: the Early Stage of Melting

Abstract

The presence of intergranular glassy layers and pockets along mineral interfaces, on microfractures and as inclusions in minerals in mantle peridotite xenoliths from different locations is revealed by optical microscopy and transmission electron microscopy (TEM). All these glasses represent former melts, that is confirmed by electron diffraction as well as their typical geochemical signatures. Often very thin glass layers are present on grain boundaries and show characteristic chemical compositions that strongly depend on the adjacent minerals. The composition of these layers differs distinctly from the bulk melt composition of partial melt experiments or from the compositions of wider melt pools of glasses observed in other xenoliths. Furthermore, a relation of these glasses to the adjacent host basalt can be excluded by the distinctly different geochemistry of the melts. The chemical composition of the melt changes with increasing thickness of the glass layers, which is due to mixing processes of the different types of glasses in the xenoliths. Wider melt films (>1 µm) are more similar to glasses observed in large melt pools and veins given in the literature as well as partial melting experiments. Thus, the chemical composition varies from that of the very first melt at different interfaces to the bulk composition of partial melts created by experiments depending on the melt film thickness. Melts are probably formed by grain boundary melting due to lattice mismatch and impurity segregation in the xenolith triggered by decompression processes during the uplift of the xenolith. This point is consistent with the corrosion textures and the absence of chemical equilibrium between melt and adjacent olivine crystals. Chemical equilibrium is only found for very few melt films along olivine boundaries and melt inclusions in olivine neoblasts. These early melts were generated during thermal overprint and dynamic recrystallisation of the xenolith in the mantle. The occurrence of melt on grain boundaries has important geological and petrological implications. Intergranular layers give an insight into the very first melting processes and the development of melt composition with time and degree of partial melting. Furthermore, melt films on interfaces are suggested to have an important significance for the rheology of the mantle by distinctly increasing the creep rate of the rock. Finally, diffusion processes may be distinctly enhanced by the presence of melt and may give way for a very fast reequilibration of the mineral chemistry.