William E. Leonhard Professor of Mineral Physics
Seismological Laboratory


Research Topics & Highlights

We combine results from experimental mineral physics with geophysical observations and geodynamic modeling to better understand multi-scale structures in Earth's interior: Caltech's Journey to the Center of the Earth

A Pair of Techniques Explores Silicon’s Effect on Earth’s Core Temperature (APS Highlight) (PDF)

New reults on melting temperatures of hcp-structured Fe0.8Ni0.1Si0.1 and Fe0.9Ni0.1 represent a significant step toward reasonable constraints on the temperature of the Earth's core and core-mantle boundary (Dobrosavljevic et al. 2022)(Zhang et al. 2016). We developed a novel method for detecting the solid-liquid phase boundary of compressed iron-rich alloys in-situ at high temperatures that now employs a pair of synchrotron techniques, synchrotron Mössbauer spectroscopy (SMS) (Jackson et al. 2013) (Zhang et al. 2015) and x-ray diffraction (Dobrosavljevic et al. 2022). Our approach is unique because the dynamics of the iron atoms are monitored using SMS. This process is described by the Lamb-Mössbauer factor, which is related to the mean-square displacement of the iron atoms. Focused synchrotron radiation with 1 meV bandwidth passes through a laser-heated 57Fe sample inside a diamond-anvil cell, and the characteristic SMS time signature drops significantly when melting occurs. When compared with previously reported melting points for iron using static compression methods, our melting trends for iron and Fe0.9Ni0.1 define a trend inbetween that of the two extreme melting curve reports, as a function of pressure. Utilizing x-ray diffraction together with SMS permits long-range atomic ordering to be monitored, and the novel use of these two techniques has now constrained the high-pressure melting curve of Fe0.8Ni0.1Si0.1 (Dobrosavljevic et al. 2022).

PhysicsBuzz podcast on Earth's core

Iron-rich (Mg,Fe)O at the CMB: Very low wave speeds & viscosity, and highly anisotropic!
(Dobrosavljevic et al. 2019) (Reali et al. 2019) (Finkelstein et al. 2018)

Developing a Picture of Earth's Mantle: Bridgmanite Provinces

Mystery "earmuffs" (blobs!) sit deep inside Earth

Seismic observations reveal that three distinct structures make up the Earth's core-mantle boundary, where the earth's metallic core meets the overlying silicate mantle at a depth of about 2900 kilometers, an area whose composition is key to understanding the evolution and dynamics of our planet. These structures include remnants of subducted plates that originated near the earth's surface, ultra-low velocity zones believed to be enriched in iron, and large dense provinces of unknown composition and mineralogy. We have placed tight constraints on the composition of these provinces, which exhibit a degree-2 pattern, and show that extrenal forces are likely needed to shape their topography (Wolf et al. 2015).

Sound Velocities of Enstatite at High-Pressures

Comparison of calculated shear wave velocities from candidate upper mantle petrological models (1400 degrees C adiabat) with seismic profiles. At P > 10 GPa: assuming C2/c transition (dashed curve) [Kung et al. 2005]; speculating that the P21/c transition occurs (dotted curve) (this study). Seismic X discontinuity observed (shaded region) [Revenaugh and Jordan 1991; Bagley and Revenaugh 2008]. Global seismic models: PREM (230-390 km in depth, 7.5-13.0 GPs in pressure, interpolated between reported values) [Dziewonski and Anderson, 1981] and AK135 [Kennett et al. 1995]. Regional seismic models: SNA [Grand and Helmberger 1984], TNA [Grand and Helmberger 1984], ATL [Grand and Helmberger 1984], and PAC06 (interpolated between reported values) [Tan and Helmberger 2007]. Shear velocity jump required for a seismic reflection with 1.5% reflection coefficient in a pyrolytic mantle (blue scale bar at 300 km) [Bagley and Revenaugh 2008]. Figure taken from Zhang et al. (2013).

Effects of the spin crossover on mantle dynamics

Combining the known effect of pressure and temperature on the density of iron-poor (Mg,Fe)O through its spin crossover, one gains an understanding of the spin buoyancy contribution to mantle dynamics (Bower et al. 2009). We determined the spin population as a function of pressure and temperature (Sturhahn et al. 2005), which requires input from the pressure-volume equation of state. See Chen et al., JGR 2012 for a new spin crossover equation of state that can be applied to measured PV data.

Ironing out the details of Earth's Core

Vibrational spectrum of hexagonal close-packed iron. From high-statistical measurements of the volume dependence of the complete phonon density of states of hcp-Fe that a generalized scaling law applies. We determined the Grüneisen parameter of hcp-Fe up to 171 GPa (density of 12.43(3) g/cm3).
(Murphy et al., GRL 2011)
(Press Release; APS Science Highlight, direct APS link)
The vibrational pressure of hcp-Fe to 151 GPa, constrained by the volume dependence of its phonon density of states. We compute the total thermal pressure (which includes an electronic term) to explore the implications for Earth's inner-core density deficit.
(Murphy et al., PEPI 2011)

Deep Mantle Ultralow Velocity Zones

Wave speeds of iron-rich (Mg,Fe)O as a function of pressure and compared with the preliminary reference Earth model (PREM). We measured the wave speeds in iron-rich (Mg,Fe)O using nuclear resonant inelastic x-ray scattering to 120 GPa at 300 K and find them to be extremely low. Mixing a small amount of this oxide with either iron-poor or iron-rich silicates can reproduce the seismic wave speed reductions observed for "ultralow velocity zones".
(Wicks et al. 2010) (Press Release)
Modeling solid-state chemically-distinct ultralow velocity regions. In this figure, a small amount (~4% by volume) of iron-rich (Mg,Fe)O is mixed into iron-bearing silicates at the pressure-temperature conditions of the core-mantle boundary. (Bower et al. 2011)

Geometry and seismic properties of the subducting Cocos platein central Mexico

We combine receiver function data with mineral physics to understand the composition of subducting flat slabs (Kim et al. 2010, Kim et al. 2012). Left: Map showing the region of the study and stations in the MASE array indicated as red triangles. Right: Calulated Vp/Vs for candidate hydrated phases and rock types at 35 km depth and a range of 500-800 degrees C. Our seismic observations are plotted as depth-dependent colored symbols.

Yellowstone magma chamber

Complete Bouguer gravity anomaly map of the Yellowstone area showing structural, hydrothermal,volcanic, geophysical and seismological features (Phillips et al., 1993). The solid thick contour marks the boundary of theYellowstone National Park. The dashed line denotes the boundary of the Yellowstone Caldera generated by the eruption 0.64 Ma ago.
Red stars are locations of post-caldera volcanic vents and yellow stars stand for mapped hydrothermalfeatures. The interpreted magma migration paths are shown by arrows. Thin lines are Quaternary faults. Earthquakes with ML 1.0 between 2000 and 2008 are shown by gray circles whose size represents magnitude. Broadband seismic stations include H17A (red squares), LKWY (red star), and Y100,Y102, and Y103 (red triangles). The yellow contour indicates the position of Yellowstone Lake. (see Chu et al. 2009 for details and complete references).

Terrestrial Planet Interiors: Exoplanets

Comparison between temperature profiles and melting curves of H2O and iron alloys (see Sotin, Jackson, & Seager 2010 in Exoplanets for details and references). (a) Earth-like planets: Thermal profiles for 1, 5, and 10 Earth-masses are above the melting curve of the FeS component in the core (P,T) domain. (b) Ocean-planets: The transition from liquid to ice VII occurs at low pressure (between 1 and 2 GPa depending on the surface temperature).

For more details on links between geophysics and planetary sciences, please check out the Planetary Sciences program website.

Research support

Jackson's research is supported by grants through:
NSF early CAREER award (EAR-0956166), NSF Geophysics (EAR-0711542,1316362),
NSF Research Experience for Undergraduates,
NSF Collaborative Studies on Earth's Deep Interior (CSEDI EAR-0855815,1161046),
Argonne National Laboratory Visiting Graduate Student Program,
Keck Institute for Space Studies,
Tectonics Observatory through the Gordon & Betty Moore Foundation, and the
California Institute of Technology.

Jackson is also involved in COMPRES Infrastructure & Development Projects
(COMPRES NSF-EAR-06-49658).

Mineral Physics Research at Caltech