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Dr Oliver T LordMSc, PhD

Royal Society University Research Fellow and Proleptic Lecturer

Oliver Lord

Dr Oliver T LordMSc, PhD

Royal Society University Research Fellow and Proleptic Lecturer

Member of

Research interests

My research aims to shed light on the structure and evolution of the Earth and other planets by studying the structure and properties of matter at extreme pressures and temperatures. To do this I combine micro-fabrication, laser-heated diamond anvil cell experiments, synchrotron based micro- and nano-scale analytical techniques and ab initio computations. Beyond the geosciences, I collaborate with physicists to study the fundamental properties of matter at extreme conditions and with engineers to study the mechanical behaviour of industiral materials. Current projects include:

 

The Chemical Architecture of the Deep Earth The character of our planet was defined by its earliest experiences. The kinetic energy from giant impacts as it accreted by collision with smaller planetesimals, combined with the heat from radioactive decay caused repeated, wholesale melting of the Earth, the segregation of the metallic core from the silicate mantle, and ultimately, the formation of the moon1. The ensuing magma oceans, which may have extended all the way to the core mantle boundary, would have solidified within a few tens of millions of years at most2. Despite being completed within the first few per cent of Earth’s lifespan, these processes had effects on the geodynamics and habitability of Earth that were profound3 and long lasting: after 4.567 billion years of vigorous convection, driving plate tectonics, volcanism and the continual renewal of Earth’s surface, not all traces of this period have been obscured4. Evidence from the fields of geochemistry, geophysics and geodynamics strongly suggest that its signature has been written, indelibly, into the chemical architecture of the deep Earth. The overarching goal of this proposal is to harness technological advances in high-pressure mineral physics, both experimental and computational, to unpick this short but critical period in Earth’s history. 

 

Modelling Mantle Mineralogy A Novel Experimental Approach Our understanding of the internal structure of the Earth is based on measurements of the speed with which seismic waves pass from sources (earthquakes) to receivers (seismometers). Using this method, seismologists have discovered two vast, dense structures in the lowermost mantle called large low shear-wave velocity provinces (LLSVPs), one below the Pacific and one below Africa, as well as smaller, even denser patches right at the boundary between the core and the mantle called ultra-low velocity zones. It has been suggested that these structures may have formed as a result of the crystallization of magma oceans present during the earliest stages of Earth's evolution (a proposition that I plan to test as part of my URF research project). Alternatively, they may represent piles of oceanic crust that have accumulated as a result of subduction at convergent plate margins over geological time. 

To test these hypotheses (and others) and thus determine what these structures are made of, we need to know two pieces of information about the minerals that we expect to be present to a very high precision: their density and the velocity at which sound passes through them at the conditions of the deep lower mantle, where pressures approach 1.35 million atmospheres and temperatures reach over 3000 K. 

The problem is that current density and sound velocity data for the major lower mantle minerals is not sufficiently accurate to choose between the various hypotheses. I will solve this problem by improving the data we have through the application of a novel methodology that relies on heating samples in a high pressure device called a diamond anvil cell (DAC) using electrical resistance (akin to a lightbulb) rather than the typical laser heating approach. This will reduce temperature gradients in the sample and uncertainties in our measurements. The sample will also be entirely encapsulated in metal so that it can't react chemically with its environment, removing another source of error. While held at lowermost mantle pressures and temperatures, the density and sound velocity of the samples will be determined to very high precision and accuracy using micron- scale X-ray beams generated at a synchrotron or a visible light laser. These results will be used to create new, more accurate mineralogical models of the LLSVPs, discriminate between the competing hypotheses and aid those who rely on such data to model how these structures have evolved through geological time. 

 

The influence of volatiles on lower mantle phase relations Earth is the only terrestrial planet to host an exosphere rich in volatile elements (e.g. H, C, N) and thus the only habitable planet in our solar system. These volatile elements are cycled between surface reservoirs including the atmosphere, oceans and biosphere on geologically short timescales. However, it is the establishment of deep volatile cycles through volcanic degassing coupled with ingassing through subduction that has modulated our climate and maintained habitable surface conditions for the majority of Earth’s lifespan, in spite of external forcing induced by changing orbital mechanics and variations in solar luminosity1. Indeed, due to its vast volume and mass, the Earth’s lower mantle has the potential to be the largest reservoir for volatiles in Earth2, but whether it fulfils this potential depends on the answers to key questions including Q1: how much water and CO2 can be stored in the major lower mantle phases including bridgmanite, Ca-perovskite (Ca-PV), ferropericlase and stishovite? and Q2: if this solubility limit is exceeded, are accessory hydrous and carbonated solid phases stable at reasonable lower mantle compositions, pressures and temperatures and what is their identity and structure?

 

Deep Water: Hydrous Silicate Melts and the Transition Zone Water Filter Water-rich partial melts of the mantle have been postulated to exist both above and below the mantle transition zone (e.g. at ~ 400 and 700 km depths), creating a ‘transition zone water filter’. This model, if correct, hugely impacts our understanding of the deep water cycle, which strongly influences the chemistry and dynamics of the mantle at local to global scales, and across geological time. Testing this model and understanding its implications requires detailed information about the composition and physical properties of hydrous silicate melts, basic knowledge that is woefully incomplete. 

The aim of this project is to provide the fundamental data essential for understanding the chemical and dynamical behavior of hydrous silicate melts in the deep mantle, and thus to explicitly test the transition zone water filter model. To achieve this, we will determine the composition and thermophysical properties of hydrous silicate melts in a simplified chemical system at high pressures and temperatures.
 High P-T experiments and first principles molecular dynamics simulations will be used to determine ternary liquidus phase relations in the system MgO-SiO2-H2O, along with a full set of thermophysical properties (density, elasticity, viscosity, wetting properties) for key hydrous melt compositions at conditions applicable to the deep upper mantle, transition zone and upper lower mantle. This will enable us to model in detail the chemistry, dynamics and seismic detectability of hydrous silicate melts at these conditions, and to test models for their presence and influence on the mantle. 

 

The Volatile Legacy of the Early Earth The Earth is unique among its terrestrial planetary neighbours in having a volatile element rich exosphere (e.g. H, C, N, S) that has been habitable for several billions of years. Answering the key question ‘Why did Earth develop a habitable exosphere?’ requires an understanding of the origin of Earth’s volatile elements, knowledge of volatile behavior as the planet cooled and differentiated, and integrated models for the role of the deep Earth in retaining volatiles and communicating with the surface. 

The bulk Earth has refractory element abundances proportional to those in primitive, chondritic meteorites. However, this is clearly not the case for the volatile elements, which have a depletion trend that correlates with nebular condensation temperature. Yet Earth is not volatile free as might be expected from some accretion models, as evidenced by its habitable atmosphere and hydrosphere which have a combined mass of ~1.5x1021 kg, constituting ~0.025% of its total mass, with the potential for many times this amount stored in internal reservoirs. The origin of Earth’s volatile inventory is a long-standing and enigmatic issue, and currently there is little consensus (see e.g. here and here), leading us to ask the following questions:

(1) Can planets retain volatiles during impact accretion? We will use hydrodynamic modelling to trace the fate of volatiles using planetesimal impact simulations that include the effects of collisional erosion. 

(2) 
Did Earth accrete dry with volatiles brought in by a post-accretion addition of material? We will use a new isotopic tracer, Se, to explicitly test this longstanding but unproven hypothesis. 

(3) 
Did segregation of Earth’s core perturb isotopic tracers? We will use partitioning and isotopic fractionation experiments between molten metal and volatile-rich silicate at high P and T to test for the effects of core formation on key isotopic systems. 

(4) 
Do the Earth and the Moon share the same volatile history? We will determine H partitioning as a function of oxygen activity to constrain the Moon’s water content as a proxy for the early Earth-Moon system.

During the early stages of accretion the planet went through one or more magma ocean stages related to melting from large impacts. Early magma ocean degassing likely set the composition of the early atmosphere and the volatile inventory of the mantle. Core formation, magma ocean crystallization and fractional degassing, mantle overturn and subduction can all lead to volatiles reaching and potentially residing in the deep mantle and core which begs the following additional questions:

(1) 
How did early magma ocean degassing control atmosphere and mantle volatile reservoirs? We will use novel experimental techniques to measure volatile element solubilities and compositions in magma ocean melts as a function of P, T, and X. 

(2) 
In what phases will volatile elements be stored in the deep mantle? We will use high P-T experiments combined with in situ synchrotron X-ray techniques to determine phase equilibria and physical properties in volatile-bearing systems at lower mantle P-T conditions. 

(3) 
Can we detect volatile-rich regions in the lower mantle today? We will use seismological and geodynamic modelling to investigate whether observational evidence exists for a modern deep mantle volatile-rich reservoir. 

(4) 
Did core formation sequester H, C and N and oxidize the mantle? We will use high P-T experiments to determine partitioning between molten metal and hydrous silicate at core forming conditions.

Direct evidence for deep mantle cycling and volatile feedback is difficult to obtain for lack of samples. Diamonds and their mineral inclusions are unique space-time capsules that can record deep mantle volatile evolution. A cohort of consortium studentships will investigate rare diamonds and their inclusions to make direct observation on volatile geodynamics. 

This project is a consortium effort led by Bristol and including researchers at a range of other institutions. It is one of three consortia that make up the NERC Deep Volatiles programme.

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Postal address:
Wills Memorial Building
Queens Road
Clifton
Bristol
United Kingdom