Rapid Multi-dimensional Seismic Property Mapping of Earth Materials

Key Questions

How do compositional variation, pressure and temperature modify the seismic properties of the main constituents of Earths lower mantle: the silicate (Mg,Fe,Al)(Fe,Al,Si)O3 bridgmanite and the oxide (Mg,Fe)O ferropericlase? How can these changes be probed in an efficient, rapid fashion so as to allow a detailed mapping of seismic property change?

Background

Our planet’s inner dynamics and surface plate tectonics are governed by the properties of and processes in the Earth's lower mantle. A quantitative understanding of the physical and chemical properties of the lower mantle is key to modelling Earth’s dynamic evolution, including the long-term chemical interactions between mantle and atmosphere that are vital to habitability of Earth. The main approach to determining the structure, composition and dynamics of Earth’s inaccessible interior is to compare seismic interior maps of compressional Vp and shear Vs velocities, constructed from the analysis of Earthquake waves, to mineral physics predictions derived from deep Earth models. Unfortunately, this approach has led to highly ambiguous results on the state and composition of the lower mantle. The reason is that a unique interpretation of deep Earth seismic data critically relies on quantitative knowledge of the elastic (seismic) properties of Earth materials at extreme pressures and temperatures characteristic of the Earth’s deep interior. This information is largely incomplete, as existing methods for probing seismic properties are far too slow to provide the required detailed information. 

Aims of the Project

The aim of this project is to develop a new experimental capability, using rapid transient grating spectroscopy (TGS) measurements to probe the elastic, i.e. seismic, properties of Earth materials in a Diamond Anvil Cell (DAC). TGS has the potential to allow measurements up to 4 orders of magnitude faster than currently-used Brillouin scattering methods. To fully exploit this speedup, automated experimental routines must be developed for TGS data collection for multiple sample orientations and changing pressures. This new capability would allow a very high resolution mapping of seismic properties as a function of pressure. By integrating a heating capability into the DAC, this could be further expanded to also allow the mapping of temperature dependence of elastic properties.  

This new tool will then be deployed to map out the pressure and temperature dependent properties of ferropericlase and optionally bridgmanite, the two main constituents of Earth’s lower mantle.

Project Description

Seismic properties of Earth minerals have traditionally been measured by Brillouin spectroscopy (BS), where the frequency shift of a probing laser, caused by inelastic scattering from acoustic phonons, is directly related to seismic velocities. To explore the pressure dependence of seismic velocities, BS has been coupled to diamond-anvil cells (DACs), where high pressures can be achieved by compressing miniaturised mineral samples between the tips of two diamond-anvils. Since the diamond-anvils are transparent to laser light, minerals under high pressures can be probed by BS to map the pressure dependence of elastic properties and seismic velocities, a capability where HM is among the world-leaders. Though the need for elasticity data of lower mantle materials was recognized many years ago, the available data is still extremely limited. The main reason is that BS measurements are very slow: a single measurement take several hours and 20-100 individual measurements are needed to reliably extract the full elastic stiffness tensor for one pressure point. This severely limits the pressure-composition-temperature-range that can be practically probed. 

The aim of this project is to overcome this limitation by using transient grating spectroscopy (TGS) to probe the elastic properties of minerals in-DAC at high pressures. In TGS a brief excitation grating, produced by two spatially overlapping laser pulses, is used to generate phonons with a well-defined wavelength. Following excitation, these are probed in the time domain by diffraction of a probe laser beam from the transient grating produced in the sample by the counter-propagating phonons. In transparent bulk materials, this approach can be used to generate and probe longitudinal phonons, while in opaque materials surface acoustic waves are measured. In both cases the acoustic velocity in a particular direction, determined by the TGS wave vector, is determined. As such, TGS allows the full elastic stiffness tensor to be found from measurement of single crystals with well-known orientation, similar to BS. While TGS experiments are more challenging than BS measurements, they are ~4 orders of magnitude faster. A TGS measurement with excellent signal to noise ratio requires only a few seconds! This opens up an excellent opportunity for mapping out a large processing space, i.e. measurements as a function of temperature, pressure and composition.  

The first part of this project is to establish a new capability on the TGS system in FH’s lab in Oxford to measure samples under pressure in the DAC. This will require the development of a suitable optics setup, integration of the TGS data acquisition with automated measurement of multiple sample orientations, and automated measurement of different pressures. It would also be very interesting to develop an in-situ heating capability and integrate this into the setup to allow the measurement of temperature and pressure dependent behaviour. 

The second part of this project is to deploy this new capability to gain insight into the seismic behaviour of materials in Earth’s lower mantle. Most studies agree that ~95% of Earth’s lower mantle are made up by two mineral phases, namely bridgmanite and the ferropericlase.  

The oxide (Mg,Fe)O ferropericlase is the second most abundant phase in Earth’s lower mantle. Here, a substantial amount of iron is incorporated into the structure of ferropericlase. The iron undergoes a pressure-induced change of electronic spin state that occurs over extended depths, spanning the majority of the lower mantle. Several studies have demonstrated a severe impact of the spin transition on the physical and chemical properties of ferropericlase, including substantial changes to elasticity and hence seismic properties. Unfortunately, using BS, it is not possible to probe ferropericlase with iron content greater than ~20% since it is not sufficiently transparent. TGS measurements, on the other hand, are able to probe both transparent and opaque samples. Thus, this project presents a unique opportunity to study the pressure and composition dependent behaviour of ferropericlase with iron content from 20% to 100%. Since ferropericlase might decompose in the deep mantle into Mg-rich and Fe-rich phases, these data are pivotal to construct reliable mineral-physics based seismic models of Earth’s mantle and possibly explain enigmatic features in the seismic record, such as mid-mantle scatters, large low shear velocity provinces, or ultra-low velocity zones. 

The silicate (Mg,Fe,Al)(Fe,Al,Si)O3 bridgmanite is the most abundant phase in Earth’s lower mantle. Yet at present only one complete data set of its elastic constants at lower mantle pressures has been published (by HM using BS). A very important, largely open, question concerns the effect of chemical variability on the elasticity properties. A quantitative understanding, however, is vital to constrain the chemistry and mineralogy of the lower mantle by comparison to seismic models and to constrain the nature of seismic anomalies (chemical vs. thermal origin). In this optional (i.e. if progress allows) pilot project we will address this question, concentrating on the substitution of Fe and Al (for Mg and Si), using in-DAC TGS to measure a large range of pressure points for different compositions of bridgmanite.  

Methods to be used

This project will concentrate on the use of transient grating spectroscopy, a laser-based method for probing micro-scale elastic and thermal transport properties of transparent and opaque media. FH has developed a TGS setup that uniquely allows the rapid mapping of material properties in extended samples. This setup will be further developed to allow measurements of Earth materials in a diamond anvil cell. Material for this project can either be synthesised in Oxford, produced by collaborators or purchased commercially. Preparation of samples can be carried out in Oxford. HM’s group specialises in the use of DACs for the study of Earth materials at high pressures. Analysis of the experimental data will be performed using Matlab, building on pre-existing analysis codes.

Specialised skills required

An excellent first degree is Engineering, Earth Sciences, Physics or a related discipline is essential. Enthusiasm for experimental work and excellent analytical skills are also vital. Some experience with laser work and data analysis, e.g. in Matlab, would be useful, but is not essential. 

Please contact Felix Hofmann on felix.hofmann@eng.ox.ac.uk  and Hauke Marquardt on hauke.marquardt@earth.ox.ac.uk if you are interested in this project