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
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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
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