Bob Myhill

University of Bristol, Department of Earth Sciences, Post-Doc

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I am interested in using seismology and petrology to understand the geochemical and geodynamical evolution of the planets, from the very beginnings of the solar system to the present day.

In February 2016, I joined the Department of Earth Sciences at the University of Bristol to work on NASA's InSight Mission to Mars. I am combining existing geophysical and geochemical information to piece together Mars' history and its place in the solar system. Our intention is to use the seismic data returned from Mars to provide crucial data on the structure and composition of the planet; data for which we've been waiting since the Viking Landers in the 1970s!

Previously, I've worked on understanding the formation of Earth's core and mantle using experimental data and thermodynamics. I've played with squeezing tiny samples of rock to incredible pressures to simulate melting that we believe to be taking place 700 km beneath our feet.

As a student, I was a geologist and seismologist studying subduction through field and petrological studies of ophiolites and geophysical studies of deep focus earthquakes.
Address: University of Bristol
School of Earth Sciences
Wills Memorial Building
Queens Road
Bristol BS8 1RJ

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Papers by Bob Myhill

The Mechanisms of Deep Earthquakes

This dissertation uses a variety of seismological and mathematical tools to investigate the mecha... more This dissertation uses a variety of seismological and mathematical tools to investigate the mechanisms of deep earthquakes, the properties of fault planes and rupture at high pressure within the Earth, and the local conditions where deep earthquakes occur. This work is split into two parts, focusing respectively on the properties and conditions of deep faulting.

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On the P-T-fO2 stability of Fe4O5, Fe5O6 and Fe4O5-rich solid solutions

The high-pressure phases Fe4O5 and Fe5O6 have recently been added to the list of known iron oxide... more The high-pressure phases Fe4O5 and Fe5O6 have recently been added to the list of known iron oxides. As mixed-valence phases, it has been suggested that they could form in the Earth’s mantle once the dominant minerals become saturated in ferric iron. The possibility that Fe4O5 could exist in the mantle is also supported by the fact that it forms extensive solid solutions with both Mg2+ and Cr3+. In this study, we present the results of high-pressure and high-temperature multi-anvil experiments performed between 5 and 24 GPa at 1000–1400 °C aimed at constraining the stability field of the Fe4O5 phase. We combine these results with published phase equilibria, equation of state and Fe–Mg partitioning data to estimate the thermodynamic properties of Fe4O5, Fe5O6 and the (Mg,Fe)2Fe2O5 solid solution. Using our thermodynamic model, the oxygen fugacity at which the high-pressure iron oxides become stable is calculated and the redox stability of (Mg,Fe)2Fe2O5 in an assemblage of olivine and pyroxene is calculated as a function of the bulk Fe/(Fe + Mg) ratio. Fe4O5 and (Mg,Fe)2Fe2O5 are stable at oxygen fugacities higher than the diamond stability field and are, therefore, unlikely to be found as inclusions in diamonds. The stability field of Fe5O6, on the other hand, extends to oxygen fugacities compatible with diamond formation. Using the Mg–Fe solid solution model, we show that Fe4O5-structured phases would be restricted to aluminium-poor environments in the mantle such as dunites or silica–iron oxide-rich sediments transported into the mantle via subduction.

geodynamics/burnman: BurnMan v1.0.1

This patch adds the missing optimize submodule to setup.py.

geodynamics/burnman: BurnMan v0.10.0

This release adds: python 3.5+ compatibility a new Composition class for compositional processing... more This release adds: python 3.5+ compatibility a new Composition class for compositional processing a new CombinedMineral class a new AnisotropicMaterial class new Planet and Layer classes for mass, moment of inertia and seismic velocity calculations new nonlinear fitting routines and a nonlinear solver new static equations of state: Morse potential, Reciprocal K-prime new thermal equations of state: Anderson and Ahrens (metallic iron), de Koker and Stixrude, Ramo and Stixrude (solids and liquids) a large number of new minerals and solid solutions in the databases a large number of new examples updated documentation Online documentation: https://burnman.readthedocs.io/en/latest/

Generation of pressures over 40 GPa using Kawai-type multi-anvil press with tungsten carbide anvils

Review of Scientific Instruments, 2016

We have generated over 40 GPa pressures, namely, 43 and 44 GPa, at ambient temperature and 2000 K... more We have generated over 40 GPa pressures, namely, 43 and 44 GPa, at ambient temperature and 2000 K, respectively, using Kawai-type multi-anvil presses (KMAP) with tungsten carbide anvils for the first time. These high-pressure generations were achieved by combining the following pressure-generation techniques: (1) precisely aligned guide block systems, (2) high hardness of tungsten carbide, (3) tapering of second-stage anvil faces, (4) materials with high bulk modulus in a high-pressure cell, and (5) high heating efficiency.

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Experimental constraints on melting temperatures in the MgO–SiO2 system at lower mantle pressures

Earth and Planetary Science Letters, 2017

Eutectic melting curves in the system MgO-SiO2 have been experimentally determined at lower mantl... more Eutectic melting curves in the system MgO-SiO2 have been experimentally determined at lower mantle pressures using laser-heated diamond anvil cell (LH-DAC) techniques. We investigated eutectic melting of bridgmanite plus periclase in the MgO-MgSiO3 binary, and melting of bridgmanite plus stishovite in the MgSiO3-SiO2 binary, as analogues for natural peridotite and basalt, respectively. The melting curve of model basalt occurs at lower temperatures, has a shallower dT/dP slope and slightly less curvature than the model peridotitic melting curve. Overall, melting temperatures detected in this study are in good agreement with previous experiments and ab initio simulations at ~25 GPa (Liebske and Frost, 2012; de Koker et al., 2013). However, at higher pressures the measured eutectic melting curves are systematically lower in temperature than curves extrapolated on the basis of thermodynamic modelling of low-pressure experimental data, and those calculated from atomistic simulations. We find that our data are inconsistent with previously computed melting temperatures and melt thermodynamic properties of the SiO2 endmember, and indicate a maximum in short-range ordering in MgO-SiO2 melts close to Mg2SiO4 composition. The curvature of the model peridotite eutectic relative to an MgSiO3 melt adiabat indicates that crystallization in a global magma ocean would begin at ~100 GPa rather than at the bottom of the mantle, allowing for an early basal melt layer. The model peridotite melting curve lies ~500 K above the mantle geotherm at the core-mantle boundary, indicating that it will not be molten unless the addition of other components reduces the solidus sufficiently. The model basalt melting curve intersects the geotherm at the base of the mantle, and partial melting of subducted oceanic crust is expected.

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Chemistry of the Lower Mantle

Geophysical Monograph Series, 2016

Earth's lower mantle, extending from 660 to 2891 km depth, contains ~70% of the mass of the mantl... more Earth's lower mantle, extending from 660 to 2891 km depth, contains ~70% of the mass of the mantle and is ~50% of the mass of the entire planet [Dziewonski and Anderson, 1981]. Due to its size, the lower mantle has a large potential to bias the composition of the bulk silicate Earth (BSE), which in major element terms is generally assumed to be the same as the upper mantle [Allegre et al., 1995; McDonough and Sun, 1995; O'Neill and Palme, 1998]. Any chemical differences between the upper and lower mantle would have major implications for our understanding of the scale of mantle convection, the origin of the terrestrial heat flux, and aspects such as the volatile content of the interior. Furthermore, many constraints on the processes involved in the accretion and differentiation of Earth would be lost if element concentrations in the upper mantle could not be reliably assumed to reflect the mantle as a whole. Ultimately the best prospect for determining whether the lower mantle has the same major element composition as the upper mantle is to compare seismic reference models for shear and longitudinal wave velocities in the lower mantle with mineral physical estimates for what these velocities should be if the mantle is isochemical [Birch, 1952;

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$Research paper thumbnail of Semiautomatic fracture zone tracking$

Semiautomatic fracture zone tracking

Geochemistry, Geophysics, Geosystems, 2015

Oceanic fracture zone traces are widely used in studies of seafloor morphology and plate kinemati... more Oceanic fracture zone traces are widely used in studies of seafloor morphology and plate kinematics. Satellite altimetry missions have resulted in high-resolution gravity maps in which all major fracture zones and other tectonic fabric can be identified, and numerous scientists have digitized such lineaments. We have initiated a community effort to maintain low-cost infrastructure that allows seafloor fabric lineaments to be stored, accessed, and updated. A key improvement over past efforts is our processing software (released as a GMT5 supplement) that allows for semiautomatic corrections to previously digitized fracture zone traces given improved gridded data sets. Here we report on our seafloor fabric processing tools, which complement our database of seafloor fabric lineations, magnetic anomaly identifications, and plate kinematic models. However, the usefulness of specific FZ traces decay with time as new data become available. For instance, the efforts of Gahagan et al. [1988] and Cande et al. [1989] relied on early processing of widely spaced, stacked 1-D SEASAT and GEOSAT profiles. Today, we have extensive 2-D coverage of higher accuracy that allows ever subtler fracture zone trends to be found [Sandwell et al., 2014]. Furthermore, digitized FZ traces are usually not updated, or perhaps only some FZs are redigitized, leading to a heterogeneous, partly global data set reflecting different eras of data qualities and personal styles and procedures for digitizing (i.e., did Key Points: Infrastructure to facilitate research in plate tectonic reconstructions Refines location of digitized fracture zone traces Analyzes fracture zones for alongtrace changes

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Lower-mantle water reservoir implied by the extreme stability of a hydrous aluminosilicate

Nature Geoscience, 2014

The source rocks for basaltic lavas that form ocean islands are often inferred to have risen as p... more The source rocks for basaltic lavas that form ocean islands are often inferred to have risen as part of a thermal plume from the lower mantle. These rocks are water-rich compared with average upper-mantle rocks. However, experiments indicate that the solubility of water in the dominant lower-mantle phases is very low, prompting suggestions that plumes may be sourced from as-yet unidentified reservoirs of water-rich primordial material in the deep mantle. Here we perform high-pressure experiments to show that Al 2 SiO 4 (OH) 2-the aluminium-rich endmember of dense, hydrous magnesium silicate phase Dis stable at temperatures extending to over 2,000 • C at 26 GPa. We find that under these conditions, Al-rich phase D is stable within mafic rocks, which implies that subducted oceanic crust could be a significant long-term water reservoir in the convecting lower mantle. We suggest that melts formed in the lower mantle by the dehydration of hydrous minerals in dense ultramafic rocks will migrate into mafic lithologies and crystallize to form Al-rich phase D. When mantle rocks upwell, water will be locally redistributed into nominally anhydrous minerals. This upwelling material provides a potential source for ocean-island basalts without requiring reservoirs of water-rich primordial material in the deep mantle.

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Clustering of deep focus earthquakes in the southwest Pacific

The Mechanisms of Deep Earthquakes

This thesis (such as it is) would not have been possible without the assistance and encouragement... more This thesis (such as it is) would not have been possible without the assistance and encouragement of many people over the last three years. A huge thank you to Dan McKenzie, for providing many fascinating conversations on diverse aspects of geology, from impact craters to pseudotachylites, earthquakes to metamorphism, the mantle to the moon. Thanks also for the awe-inspiring trip to Iceland, which gave me memories I shall never forget. Heartfelt thanks to Keith Priestley, my primary source of knowledge for all things seismological, his wisdom in finding out just what is tractable, which results are trustworthy, and for teaching me that many things are possible with good stations and arrays. Pecuniarily, thank you to NERC and to Magdalene College. My bye-fellowship at Magdalene was both terrifying and extremely enjoyable (occasionally at the same time), and gave me a fantastic insight into how the other half lives. I am grateful to the then-President and chaplain, and to Mike Carpenter and all the other fellows who made my time with them so pleasant. Thanks to Gill and Barbara for always being willing to go the extra mile, for keeping the Bullard functioning, and for helping make Bullard the wonderful place that it is. To Joyce, for our many conversations and for reminding me that my home town (Lowestoft) isn't all bad. To Dave Lyness for installing more and more obscure software, working on Sundays to restore access to the computer network, and for many amusing and interesting lunchtime discussions. In the Downing Site, thank you to Sarah Humbert, for her friendship, not to mention her skill in finding obscure references, acquiring books, and providing much-needed coffees between supervisions. The Bullard would not have been the second home that it has been without my friends here. Thanks to my compatriots Al, Lauren, Ross and Steve, with whom I have shared many trials and tribulations. To Jess and Lizzie for guiding me over many of the hurdles required to succeed in academia, and for teaching me that presenting at conferences is not the same as taking a holiday. Thanks also to Tim and Paula for providing code, calm and a sounding board off which to bounce my ideas. Downtown, I am hugely grateful to Rob Sparkes for his ever-constant friendship, not to mention proofreading skills and giving me a chance to savour the delights of fieldwork in Northern Spain. Without taking up several more pages, I'll never be able to adequately thank everyone in the department and outside; suffice it to say that there are many more postgrad and undergrad students without whom my PhD would have been much less enjoyable. Annie, Adamos and Mike Rassios (and Terry the dog); Dina Ghikas and Babi; and Anna Mpatsi, an enormous thank you for your hospitality during my many trips to Greece. To Annie and Dina, thank you for showing me the geo-wonders that are your home. I'll always remember the perfect days we've shared in the Pindos, Vourinos and Olympos, and hope for many more. These acknowledgements could not be complete without thanking my family for their genuine interest during my scientific education, and for buying me books, rocks, fossils, magazines, microscopes and storage for my samples. To my parents, thank you for always encouraging me whatever my new interest, and giving me the gift of wonder in the natural world. Finally, thank you to my sister Anna, for her honesty, patience and kindness at times when the world felt lacking in all three.

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The distribution of ferric iron within the Earth's interior and evolution in the mantle's redox state

Constraints on the evolution of the Mesohellenic Ophiolite from subophiolitic metamorphic rocks

Mélanges: Processes of Formation and Societal Significance, 2011

Narrow, discontinuous bands of high-grade subophiolitic metamorphic rocks, comprising predominant... more Narrow, discontinuous bands of high-grade subophiolitic metamorphic rocks, comprising predominantly amphibolite facies metabasites with rare metasediments, are observed at the contact between the complexes and subjacent melanges of the Mesohellenic Ophiolite exposed in northwestern Greece. Both conventional and pseudosection thermobarometry have been used to yield estimated peak pressure-temperature (P-T) conditions of these tectonic sheets. Toward the leading edge of the ophiolite, subophiolitic rocks of the Vourinos Complex record peak metamorphic temperatures of 770 ± 100 °C. Pressures of 4 ± 1 kbar beneath the Vourinos are estimated on the basis of hornblende composition and are similar to the expected pressures from ophiolitic overburden. Beneath the exposed Dramala Complex, at the trailing edge of the ophiolitic body southwest of the Vourinos, estimated temperatures reached 800 ± 40 °C and 12.00 ± 1.27 kbar at the top of an apparent inverted metamorphic gradient imposed by discrete phases of accretion. High pressure assemblages beneath ophiolitic bodies imply exhumation relative to the overlying ophiolite. Estimated homologous temperatures in the upper plate are similar to those inferred for channeled exhumation during continental collision. Mineral assemblages lower in the Dramala sole indicate reduced temperatures and peak pressures. Similar pressures obtained within lower temperature sole rocks beneath Vourinos and Pindos suggest that a shallowly dipping thrust may have been responsible for obduction. Peak temperatures and pressures are in agreement with those estimated for secondary thrust propagation beneath a proto-arc after subduction in an intra-oceanic setting.

Fault plane orientations of deep earthquakes in the Izu-Bonin-Marianas subduction zone

Journal of Geophysical Research: Solid Earth, 2012

[1] We make use of the effects of rupture directivity on waveform shape to analyze the rupture pr... more [1] We make use of the effects of rupture directivity on waveform shape to analyze the rupture processes of 58 large, deep earthquakes in the Izu-Bonin-Marianas subduction zone. For more than half of the analyzed earthquakes, we determine a best-fitting unilateral rupture direction using teleseismic data. For 20 of these earthquakes, the constraints on the rupture direction allow the fault plane to be identified. Where the subducting slab dips at a moderate angle, near-horizontal fault planes dominate at all studied depths (50–600 km). Within more steeply dipping slabs, fault planes tend to dip toward the south and west. Rotated into the plane of the slab, the poles of the definitively identified faults form a single tight cluster pointing up and toward the surface of the slab. Identified ruptures have a tendency to propagate away from the top surface of the slab between 100 and 300 km depth, but appear to be randomly oriented at greater depths. The occurrence of predominantly near-horizontal faults at intermediate depths agrees with previous observations in several other subduction zones. However, at depths ≥300 km, the results from the Izu-Bonin-Marianas system differ from those previously obtained for Tonga-Kermadec, where both horizontal and vertical faults were identified. We consider various physical mechanisms to explain our observations and conclude that, while pre-existing slab structures may be reactivated if they are favorably oriented, the observed asymmetry in deep fault orientations may instead result from external forces acting on the slab, and resulting changes in slab morphology.

Slab buckling and its effect on the distributions and focal mechanisms of deep-focus earthquakes

Geophysical Journal International, 2012

This integrated study of deep earthquake locations and focal mechanisms investigates the relation... more This integrated study of deep earthquake locations and focal mechanisms investigates the relationship between the shape of subducting slabs and seismic behaviour in Wadati-Benioff zones. High quality earthquake locations are used to map the shapes of subducting slabs which have reached the upper-lower mantle boundary. The resulting slab models reveal the presence of large slab folds in the mantle transition zone. The distributions and focal mechanisms of deep earthquakes are analysed to determine whether these folds have a role in governing Wadati-Benioff zone seismicity. Bands or lineations of dense seismicity are associated with the hinge zones of identified folds. The focal mechanisms of earthquakes within these bands reveal that the mapped fold hinges are commonly perpendicular to the directions of maximum coseismic extension and compression. The hinges plunge at a variety of angles, resulting in systematic deviations from the downdip stress field expected within planar slabs. Slab synforms are typified by earthquake focal mechanisms indicating in-plane compression (e.g. Izu-Bonin, Tonga), while antiforms have earthquake focal mechanisms indicating in-plane extension (e.g. Solomons) or a mixture of in-plane compression and extension (e.g. Tonga). Slab buckling explains both the clustering of earthquakes and the observed focal mechanism orientations within fold hinges. The localization of strain within buckle zones results in several of the peaks observed in regional earthquake depth distributions. During buckling, the directions of maximum shortening and extension are expected to be perpendicular to the fold hinges, in agreement with deep earthquake moment tensors. Displacement of the minimumstrain surface away from the centre of each seismogenic zone can explain the predominance of in-plane compression within synforms and in-plane extension within antiforms. More complex local variation in focal mechanism orientations in the Tonga slab can be explained by a superposition of in-plane compression and bending strain. Buckling appears to be a common mechanism facilitating convergence between subducting slabs and the lower mantle. The consequent rotation and translation of fold limbs may explain the discrepancy between estimates of convergence based on subduction velocities and longterm coseismic strain.

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The distribution of earthquake multiplets beneath the southwest Pacific

Earth and Planetary Science Letters, 2011

Earthquakes beneath the southwest Pacific occur from the surface down to 700 km depth. Teleseismi... more Earthquakes beneath the southwest Pacific occur from the surface down to 700 km depth. Teleseismic waveforms created by some of these earthquakes are almost identical. We investigate Tonga-Kermadec and Vanuatu subduction zone earthquake P-coda waveforms using a cross-correlation technique and hierarchical clustering algorithm in order to determine the origin of waveform similarity and the distribution of earthquakes producing similar waveforms. We show that scatterers forming the majority of power in the P-wave coda are localised around the receiver. As a result, waveform similarity provides a much weaker constraint on source separation than in local studies. Waveform similarity can provide stronger constraints on focal mechanism. Most earthquake multiplets within the Tonga-Fiji-Kermadec Wadati-Benioff zone are found at depths between 0-60 km and 520-620 km. A significant proportion of all deep-focus events in south Pacific subduction zones have waveforms similar to those of at least one other event. Relative relocation of events within the largest identified multiplet reveals a planar zone of seismicity sub-parallel to the nodal plane of a related centroid moment tensor solution. Groups of earthquakes with similar waveforms remain active on at least the 14-year recording timescale. We equate this to repeated rupture on single or closely related shear systems within the subducting slabs.

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geodynamics/burnman: BurnMan v1.0

This release adds: python 3.7+ compatibility major restructuring of source directory documentatio... more This release adds: python 3.7+ compatibility major restructuring of source directory documentation overhaul a new equilibrate function for chemical equilibration of Composite materials. new composition fitting functions (solution and bulk composition) new MaterialPolytope class implementation of melt phases from the the Holland and Powell (2011) and Holland et al. (2018) datasets ternary terms added to the subregular solution model

Earthquake distributions and their relationship with structural, chemical and thermobaric heterogeneities in subducting slabs

Slab bending and its effect on the distributions and focal mechanisms of deep-focus earthquakes

On the P–T–fO2 stability of Fe4O5, Fe5O6 and Fe4O5-rich solid solutions

by Luca Ziberna and Bob Myhill

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