Amorphous layers of various thicknesses have been observed at the interfaces of mineral grains in mantle xenoliths from San Carlos, Arizona. The amorphous layers were investigated with transmission electron microscopy (TEM) and analytical electron microscopy (AEM). The studied samples are spinel lherzolites containing olivine (ol), orthopyroxene (opx), clinopyroxene (cpx) and occasionally spinel. In the vicinity of the investigated interfaces there is no spinel. The amorphous state of the layers was confirmed by electron diffraction. The chemical composition was measured using energy dispersive x-ray analysis (EDAX). The following types of grain or phase boundaries have been investigated: olivine-olivine, opx-opx, cpx-cpx grain boundaries and olivine-opx, olivine-cpx phase boundaries. The width of the layers varies from 1-2 nm to 2800 nm.
1. The chemical composition of the glassy layers at the interfaces is inhomogeneous across and along the layers indicating non equilibrium conditions (the range of composition is given in brackets).
2. The chemical composition of the amorphous layers depends on the type of grain or phase boundary. The glassy layers at olivine grain boundaries and olivine - opx phase boundaries also display two different types of chemical composition. At olivine grain boundaries the first type is characterized by high SiO2 (50-65 wt.%), TiO2 (2,5-5 wt.%), CaO (4-9 wt.%) and low FeO (5-11 wt.%)-concentrations. The second type shows lower SiO2 (35-45 wt.%), TiO2 (0-2,3 wt.%) and CaO (0-1,3 wt.%) but higher FeO-concentrations (9-12 wt.%).
A similar distinction in chemical composition is observed at olivine - opx phase boundaries. One type of amorphous layers exhibits high SiO2 (59-70 wt.%) and low FeO concentrations (2-4 wt.%). The other type is characterized by lower SiO2 (29-50 wt.%) and higher FeO concentrations (10-25 wt.%). The difference in the CaO and TiO2 concentrations is not significant.
3. The amorphous layers along olivine-cpx phase boundaries show comparatively low SiO2- (34-35 wt.%) and rather high Al2O3-concentrations (25-33 wt.%). This is also observed at opx grain boundaries and cpx grain boundaries.
4. The adjacent grains at both sides of narrow (< 1000 nm) glassy layers show a transition zone with respect to the chemical composition. In such a transition zone, e.g., in olivine grains, the concentration of MgO decreases towards the amorphous layer, and the concentrations of CaO, TiO2 and Al2O3 increase towards that interface. The width of the transition zone is in the range of several hundred nm and depends on the width of the amorphous layer.
From the chemical composition of the narrow glassy layers and the observed transition zones in adjacent grains it is concluded that the chemical composition of the thin amorphous intergranular layers represents an early stage of partial melting at grain or phase boundaries without melt mixing. Due to the destorted lattice in the interface and an impurity enrichment in the grain or phase boundary, melting will start at the interfaces. Thermodynamical calculations suggest that thin melt films should have a very high viscosity due to the interaction of the melt with the surface of the grains (Hess, 1994). The heterogeneity of the chemical composition is significantly reduced in wider amorphous layers. Mixing of the melts from different sources (grain or phase boundaries) can explain this observation.
Hess, P.C., J. Geophys. Res. 99, 7219-7229 (1994).