[Paper Review] Large density deficit of Earth's core revealed by a multi-megabar primary pressure scale

Daijo Ikuta, Eiji Ohtani+4 more|arXiv (Cornell University)|Apr 5, 2021

High-pressure geophysics and materials33 references4 citations

TL;DR

This study establishes a primary pressure scale up to multi-megabar pressures using inelastic X-ray scattering on rhenium in a diamond anvil cell, revealing that previous pressure scales overestimated laboratory pressures at core conditions by at least 20%. The corrected scale implies a 9% density deficit in Earth's inner core, doubling the inferred light-element content compared to prior estimates.

ABSTRACT

Precise information about the composition of the Earth's core is critical to understand planetary evolution and for discussing current hot topics in geodynamic behavior, such as core-mantle boundary heat flow. However, samples from deep in the Earth's interior are not available, so our knowledge is based on comparison of laboratory measurements with seismological observations, informed by meteorite composition, and indications of the Earth's core temperature. One of the most interesting results of such work has been the suggestion that Earth's inner core must contain light elements because the density of the core, as determined from seismological measurements, is lower than the density of pure iron, its main constituent, as determined from laboratory measurements and/or theoretical work: the density deficit is now considered to be ~4%. However, this conclusion relies critically on having an accurate pressure scale to relate lab generated pressures to geological pressures. Establishing such a scale has been the subject of intensive research but still involves significant extrapolation and approximations, especially at higher pressures. Further, a pressure scale to the multi-megabar pressures is indispensable for discussing super-Earth planets. Here we establish the first primary pressure scale extending to the multi-megabar pressures of Earth's core by measuring acoustic phonon velocities using inelastic scattering from a rhenium sample in a diamond anvil cell. Our new pressure scale agrees with previous primary scales at lower pressures and also shock compression experiments, but is significantly different from previous secondary and theoretical scales at Earth's core pressures: previous scales have overestimated, by at least 20%, laboratory pressures at 230 gigapascals. Our new scale suggests the density deficit of the inner core is ~9%, doubling the light-element content of the core.

Motivation & Objective

• To establish an accurate, primary pressure scale extending to multi-megabar pressures relevant for Earth's core and super-Earth planets.
• To resolve uncertainties in core composition by improving the accuracy of laboratory pressure calibration at extreme conditions.
• To address the long-standing discrepancy between seismological density measurements and pure iron's theoretical density, known as the 'density deficit'.

Proposed method

• Measuring acoustic phonon velocities via inelastic X-ray scattering in a rhenium sample compressed in a diamond anvil cell.
• Using the measured sound velocities to derive a pressure scale based on first-principles equations of state.
• Validating the new scale against shock compression experiments and lower-pressure primary standards.
• Extending the pressure calibration to 230 gigapascals (GPa), corresponding to Earth's inner core conditions.
• Comparing the new scale with secondary and theoretical pressure scales to identify systematic overestimations in prior calibrations.

Experimental results

Research questions

• RQ1 What is the true pressure at which rhenium reaches 230 GPa in a diamond anvil cell, as measured by inelastic scattering of phonons?
• RQ2 How does the new primary pressure scale compare to existing secondary and theoretical pressure scales at multi-megabar pressures?
• RQ3 What is the revised density deficit of Earth's inner core when using the new pressure calibration?
• RQ4 To what extent does the corrected pressure scale alter estimates of light-element content in Earth's core?

Key findings

• The new primary pressure scale shows that previous secondary and theoretical pressure scales overestimated laboratory pressures at 230 GPa by at least 20%.
• The corrected pressure scale leads to a revised inner core density deficit of approximately 9%, up from the previously accepted ~4%.
• The new scale is consistent with lower-pressure primary standards and shock compression experiments, validating its accuracy at the extremes.
• The increased density deficit implies a significantly higher light-element content in Earth's core than previously estimated.
• The study provides the first primary pressure calibration extending to multi-megabar pressures, enabling more accurate modeling of planetary interiors, including super-Earths.