3D visualisation of melts at the conditions of Earth's deep interior
Date
27/06/2016Author
Berg, Madeleine Tamsin Lisa
Metadata
Abstract
Constraining the behaviour of small fractions of partial melt in a solid silicate
matrix has been the focus of numerous experimental petrology studies over several
decades, and is an important factor in constraining upper mantle rheology, melt
extraction at mid-ocean ridges and mechanisms of core formation in the early solar
system. Deformation of partially molten rock has been observed to change melt
geometry, and may enhance permeability and interconnectivity of melt otherwise
trapped in a solid silicate matrix, although it is uncertain how applicable results of high
strain-rate laboratory experiments are to the real Earth. The addition of deformation
precludes attainment of textural equilibrium, complicating textural analysis, which
has previously relied on extrapolation of 3D textures from quenched and polished 2D
sections for hydrostatically annealed samples. X-ray computed tomography gives the
potential to visualise sample textures directly in three dimensions, and is becoming
popular as a complementary technique for textural analysis in petrologic studies. The
aim of this project has been to develop techniques to improve visualisation of small
fractions of partial melt within a solid silicate matrix using X-ray CT, to examine
textures of various partially molten systems at high PT in hydrostatic, and dynamically
deforming systems.
Experiments carried out in the FeS-melt, solid olivine system have examined
the potential for deformation-enhanced percolation of core forming melts before the
onset of silicate melting. Access to the newly designed rotational Paris-Edinburgh
Cell (roPEC/rotoPEC) equipment has allowed us to carry out controlled, torsional
deformation experiments under PT conditions applicable to planetary interiors.
Experiments conducted at lower strain-rates over longer duration than in previously
published studies show that deformation enhances connectivity at low melt fractions, at
strain-rates down to 10-6s-1. This is in contrast to earlier work suggesting melt textures
are unaffected at strain-rates below 10-5s-1. Quenched melt networks have been fully
characterised in 3D using multi-scale CT, with voxel sizes down to 70nm for small
sample sub-volumes. Results suggest segregation of metallic melt below the silicate
solidus could be an efficient process, and should be taken into account in geochemical
models of planetary evolution.
Experiments on basaltic melt in a solid silicate matrix were conducted in
application to upper mantle melting. A heavy element, hafnium, was added
to the basaltic glass starting composition to enhance contrast between the basalt
and olivine phases during CT scans. In-house micro-CT equipment was used to
visualise post-quench run products of hydrostatic and deformation experiments. The
doping technique was successful for long-duration, high temperature hydrostatic
experiments. Some issues with undissolved / re-precipitated HfO¬2 crystals
complicated tomographic imaging of partial melt textures in a number of experiments,
particularly those carried out on the rotoPEC equipment, limiting comparison between
samples. The doping technique requires further adjustment, but is shown to be a viable
way to improve visibility of basaltic melt without significantly affecting melt texture. The X-ray transparent design and fully rotating top and bottom anvils of the
rotoPEC allow X-ray tomography to be carried out in-situ while experiments are
in progress, enabling collection of 4D datasets. During this project, the rotoPEC
equipment was incorporated into two different synchrotron beamlines, to carry out
time-resolved studies of textural development within samples of varying composition.
The migration of gold melt along fractures with a BN matrix was imaged using 2D
radiography, in combination with repeated 3D tomography to fully characterise the
3D fracture geometry. This allowed melt migration velocity to be estimated directly
from in-situ observations. These techniques could be developed further to constrain
melt migration processes quantitatively for a number of geological systems in the near
future.
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