3D visualisation of melts at the conditions of Earth's deep interior
Berg, Madeleine Tamsin Lisa
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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.