Deformation mechanisms and melt nano-structures in experimentally deformed olivine-orthopyroxene rocks with low melt fractions

Abstract

The major part of the Earth’s upper mantle is thought to be solid, with some regions in the mantle where the rocks contain a small melt fraction These partially molten rocks are associated with important geological processes such as magma production beneath mid-oceanic ridges and may also play an important role in the motion of lithospheric plates For a better understanding of the physical properties of these rocks, it is essential to characterise the melt distribution and the effect of a partial melt on deformation for melt fractions representative for the upper mantle In this thesis, the distribution of the melt phase and its influence on the mechanical properties of olivine and olivine-orthopyroxene rocks with ~1 vol% melt deforming in the dislocation creep field are investigated The melt phase in the studied rocks originated from in situ melting in the samples The effects of the melt phase on rheology were examined using two sets of olivine and olivine-orthopyroxene materials that showed a different response to an applied stress at the experimental conditions (P=300 Pa, T=1473-1573 K), with some samples being significantly weaker than others The melt and deformation microstructures in selected samples have been investigated in detail using scanning and transmission electron microscopy in order to determine the mechanisms that govern deformation in these materials The melt distribution in partially molten rocks, as established in earlier studies, mainly consists of melt tubes along grain edges, larger melt bodies that occur between several grains and layers or ellipsoidal inclusions along grain interfaces In this study, two additional types of nanometer-scale melt occurrences that are present along grain interfaces are reported Continuous 1 to 3 nanometer thin films of amorphous material with a melt-like composition were detected along olivine grain interfaces in both the olivine and olivine-orthopyroxene samples using high-resolution chemical analysis and imaging in the TE These thin films are relatively Si-rich and were characterised by the presence of Al and Ca Amorphous films were only present along some boundaries in the hot-pressed starting material After deformation and after long-term annealing tests, amorphous films were present along all boundaries investigated, suggesting that the films are stable features of the melt microstructure The second type of sub-micrometer melt occurrence along grain interfaces consisted of tubes with triangular cross-section and typical dimensions of 100 x 500 nm (height x width) These tubes were associated with subgrain boundaries and the tube morphology was determined by the subgrain boundary misorientation and the composition of the melt phase The aspect ratio (height/width) of the subgrain melt tubes increased with increasing subgrain misorientation Ultimately, melt tubes similar to those along grain edges formed along intersections of subgrains with misorientations exceeding 4°, thereby allowing melt to penetrate into the rim of deforming crystals The ubiquitous occurrence of both ultrathin amorphous films and subgrain melt tubes in all studied samples indicates that such melt occurrences may be a uniform, but unrecognised, feature in many olivine-bearing materials studied at elevated temperature and pressure As many samples from experimental deformation studies may contain amorphous films and subgrain melt tubes, the magnitude of melt-related weakening in rocks containing a small melt fraction (<1%) with respect to melt-free materials remains unknown A detailed microstructural characterisation of specimens from two sets of olivine-orthopyroxene samples showed minor variations in (local) melt content and in the distributions of melt pocket size, grain size, and grain boundary misorientation None of these microstructural features, however, produced a consistent weakening in all the samples and a correlation between these features and the differences in mechanical properties could not be demonstrated As the samples have been deformed in the dislocation creep field, it is feasible that strength differences were related to differences in activated dislocation slip systems, with intergranular misfits being accommodated by melt-enhanced grain boundary processes This hypothesis was not supported by the dislocation microstructures in two studied samples Dislocation densities were highly variable between individual grains and no dominance for dislocations with either Burgers vector b=[a] or b=[c] was observed The variability in dislocation substructure was also present within individual grains, where regions could be identified in which different dislocation slip systems had been active The occurrence of localised activation of dislocation slip systems inside crystals is interpreted to result from variations in local stresses due to interaction with adjacent grains The main difference in dislocation microstructure was indicated by the orientation distribution of the rotation axes that describe the crystallographic misorientations accommodated by subgrain boundaries This distribution, however, indicated a more abundant activation of the weakest olivine slip system in the stronger sample and is apparently not related to the observed weakening The characterisation of the melt and deformation microstructures was inconclusive in the identification of the (melt-related) mechanisms that caused the differences in rheological behaviour between the studied sample sets Similarly, the differences in mechanical properties could not be attributed to the nanometer-scale melt bodies, as these were present in all samples It is concluded that the observed rheological differences between the studied samples are most likely related to minor variations in the local melt distribution and differences in melt composition, which can influence the rheology through effects on diffusion kinetics and the grain boundary film thickness A b s t r a c t