Amir Khan

Last Name

Khan

First Name

Amir

ORCID

0000-0002-7787-4836

Organisational unit

02725 - Institut für Geochemie und Petrologie / Institute of Geochemistry and Petrology

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Publications 1 - 10 of 60

The Composition of Earth's Lower Mantle
Murakami, Motohiko; Khan, Amir; Sossi, Paolo A.; et al. (2024)
Annual Review of Earth and Planetary Sciences
Determining the composition of Earth's lower mantle, which constitutes almost half of its total volume, has been a central goal in the Earth sciences for more than a century given the constraints it places on Earth's origin and evolution. However, whether the major element chemistry of the lower mantle, in the form of, e.g., Mg/Si ratio, is similar to or different from the upper mantle remains debated. Here we use a multidisciplinary approach to address the question of the composition of Earth's lower mantle and, in turn, that of bulk silicate Earth (crust and mantle) by considering the evidence provided by geochemistry, geophysics, mineral physics, and geodynamics. Geochemical and geodynamical evidence largely agrees, indicating a lower-mantle molar Mg/Si of ≥1.12 (≥1.15 for bulk silicate Earth), consistent with the rock record and accumulating evidence for whole-mantle stirring. However, mineral physics–informed profiles of seismic properties, based on a lower mantle made of bridgmanite and ferropericlase, point to Mg/Si ∼ 0.9–1.0 when compared with radial seismic reference models. This highlights the importance of considering the presence of additional minerals (e.g., calcium-perovskite and stishovite) and possibly suggests a lower mantle varying compositionally with depth. In closing, we discuss how we can improve our understanding of lower-mantle and bulk silicate Earth composition, including its impact on the light element budget of the core. ▪ The chemical composition of Earth's lower mantle is indispensable for understanding its origin and evolution. ▪ Earth's lower-mantle composition is reviewed from an integrated mineral physics, geophysical, geochemical, and geodynamical perspective. ▪ A lower-mantle molar Mg/Si of ≥1.12 is favored but not unique. ▪ New experiments investigating compositional effects of bridgmanite and ferropericlase elasticity are needed to further our insight.
Composition, Structure and Origin of the Moon
Sossi, Paolo A.; Nakajima, Miki; Khan, Amir (2024)
arXiv
Here we critically examine the geophysical and geochemical properties of the Moon in order to identify the extent to which dynamical scenarios satisfy these observations. New joint inversions of existing lunar geophysical data (mean mass, moment of inertia, and tidal response) assuming a laterally- and vertically homogeneous lunar mantle show that, in all cases, a core with a radius of 300$\pm$20 km ($\sim$0.8 to 1.5 % the mass of the Moon) is required. However, an Earth-like Mg# (0.89) in the lunar mantle results in core densities (7800$\pm$100 kg/m$^3$) consistent with that of Fe-Ni alloy, whereas FeO-rich compositions (Mg# = 0.80--0.84) require lower densities (6100$\pm$800 kg/m$^3$). Geochemically, we use new data on mare basalts to reassess the bulk composition of the Moon for 70 elements, and show that the lunar core likely formed near 5 GPa, 2100 K and $\sim$1 log unit below the iron-wüstite buffer. Moreover, the Moon is depleted relative to the Earth's mantle in elements with volatilities higher than that of Li, with this volatile loss likely having occurred at low temperatures (1400$\pm$100 K), consistent with mass-dependent stable isotope fractionation of moderately volatile elements (e.g., Zn, K, Rb). The identical nucleosynthetic (O, Cr, Ti) and radiogenic (W) isotope compositions of the lunar and terrestrial mantles, strongly suggest the two bodies were made from the same material, rather than from an Earth-like impactor. Rb-Sr in FANs and Lu-Hf and Pb-Pb zircon ages point Moon formation close to $\sim$4500 Ma. Taken together, there is no unambiguous geochemical or isotopic evidence for the role of an impactor in the formation of the Moon, implying perfect equilibration between the proto-Earth and Moon-forming material or alternative scenarios for its genesis.
Seismic detection of the Martian core by InSight
Stähler, Simon Christian; Khan, Amir; Ceylan, Savas; et al. (2021)
EGUsphere
Introduction: A plethora of geophysical, geo-chemical, and geodynamical observations indicate that the terrestrial planets have differentiated into silicate crusts and mantles that surround a dense core. The latter consists primarily of Fe and some lighter alloying elements (e.g., S, Si, C, O, and H). There is strong evidence from measurements of the tidal deformation of the planet that the core of Mars is presently liquid. The InSight mission aims at constraining these numbers via the RISE radio tracking experiment, and the SEIS seismic package. We used data recorded by SEIS for high SNR marsquakes between March 2019 and July 2020. The InSight Marsquake Service located these events in the distance range 27-40 degrees, based on identification of P- and S-body waves. Later studies identified a number of secondary, surface-reflected phases, which were used to constrain the upper mantle. We build upon the velocity models derived from these phase picks to constrain the time window in which to look for shear waves reflected from the core mantle boundary. Since shear waves cannot propagate in a fluid medium, the core mantle boundary (CMB) acts as a polarization filter, which fully reflects horizontally polarized shear waves back into the mantle. Shear waves reflected from the CMB, called ScS, are therefore expected to have a predominantly horizontal polarization at the receiver, with an azimuth orthogonal to the source direction. In this distance range, ScS is separated in time from any other body wave phase and therefore well-observable. Methods: We follow a two-step approach: 1. Confirm seismic arrivals as ScS, based on existing mantle velocity models. 2. Pick precise arrival times and invert those for mantle profiles and core size, constrained by mineralogy, moment of inertia and average density of the planet. Results: The inversion of travel times constrains the core radius to the upper end of pre-mission geophysics-based estimates. This value is compatible with estimates from the geodetic experiment RISE onboard and implies that a lower mantle is unlikely to be present. Moreover, a large core has important implications for core composition. Average retrieved core density is 6 g/cm^3, which implies that for a (Fe-Ni)-S composition, a sulfur content in excess of 18% is required. This is above the eutectic composition observed experimentally with potentially profound implications for the future crystallization of the Martian core, subject to further laboratory research of Fe-S data under core conditions. All ScS candidate phases that were observed show significant seismic energy and a relatively flat spectrum above 0.1 Hz, which implies a low seismic attenuation throughout the mantle. The spectral character of direct S-phases for the distant-most events is consistent with that of the ScS-phases for more nearby events, which supports the identification of the arrivals as core-reflected.
Constraining marsquake focal mechanisms and martian crustal structure using InSight's seismic data
Brinkman, Nienke; Stähler, Simon Christian; Schmelzbach, Cédric; et al. (2020)
AGU Fall Meeting Abstracts
Magnitude Scales for Marsquakes Calibrated from InSight Data
Böse, Maren; Stähler, Simon Christian; Deichmann, Nicholas; et al. (2021)
Bulletin of the Seismological Society of America
Introduction to special issue
Khan, Amir (2022)
Advances in Geophysics ~ Geophysical Exploration of the Solar System
The collection of topical reviews presented herein provides a comprehensive overview of geophysical exploration of the solar system from the point of view of seismology, geodynamics, and geodesy and tides.
Observation of a Core-Diffracted P-Wave From a Farside Impact With Implications for the Lower-Mantle Structure of Mars
Duran, Andrea Cecilia; Khan, Amir; Ceylan, Savas; et al. (2022)
Geophysical Research Letters
We report on the observation of a diffracted P-wave (Pdiff) along the core of Mars from a distant impact that has been recorded by the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission. The identification of Pdiff allows us to sample the P-wave velocity structure of the lower mantle that hitherto could not be constrained because of lack of lower-mantle-traversing P-waves. In addition to Pdiff, we are able to pick PP-, PPP-, and SS-wave arrivals and locate the event to the farside of Mars in the vicinity of Tharsis, in agreement with the imaged location of the impact. This indicates that our joint single-station seismic event-location and structure-inversion scheme is both robust and accurate. Based on inversion of the body-wave arrival time picks made here, we find lower P-wave velocities in the deep mantle relative to predictions based on thermochemically homogeneous models.
Joint inversion of PP and SS precursor waveforms and Rayleigh wave phase velocities for global mantle transition zone structure
Bissig, Felix; Khan, Amir; Giardini, Domenico (2023)
Geophysical Journal International
We have compiled a new data set of global PP and SS precursor waveforms that we jointly invert in combination with fundamental-mode and higher-order Rayleigh-wave phase velocities for upper mantle and mantle transition zone (MTZ) structure. We observe clear S410S, S520S, S660S and P410P precursor arrivals, but not P660P, because of interfering phases. Traveltimes and amplitudes of precursor phases reflect a complex interplay of data and modelling factors, implying that MTZ structure is best resolved through direct inversion of waveforms. To model waveforms as accurately as possible, we account for effects arising from data processing, shallow structure, incoherent stacking, attenuation and source effects, among others. As part of the inversion, we consider two independent model parametrizations to obtain quantitative insights into the seismic and thermochemical constitution of the MTZ. These include a 'classical' seismic parametrization based on a layered seismic velocity structure and a thermodynamic parametrization, where seismic profiles are self-consistently built from mineral physics data. The results show lateral variations in thermal, compositional and discontinuity structure that partly correlate with tectonic setting. The mantle beneath continents and subduction zones is found to be colder in comparison to oceans and hotspots as reflected in MTZ thickness. In terms of composition, we find that subduction zones are enriched in basalt. Mid-MTZ structure shows a trend from simple sub-ocean single- to complex circum-Pacific subduction-zone-related dual-discontinuity structure-the possible signature of oceanic crustal transport to the MTZ. Statistical analysis indicates that a mechanically mixed mantle matches seismic data better than an equilibrated mantle across similar to 2/3 of the globe. Finally, while a large part of the seismic data can be matched by an iso-chemical and adiabatic mantle, complexities within the MTZ are not entirely captured by this assumption.
On Earth’s Mantle Constitution and Structure from Joint Analysis of Geophysical and Laboratory-Based Data: An Example
Khan, Amir (2016)
Surveys in Geophysics
Determining Earth’s structure is a fundamental goal of Earth science, and geophysical methods play a prominent role in investigating Earth’s interior. Geochemical, cosmochemical, and petrological analyses of terrestrial samples and meteoritic material provide equally important insights. Complementary information comes from high-pressure mineral physics and chemistry, i.e., use of sophisticated experimental techniques and numerical methods that are capable of attaining or simulating physical properties at very high pressures and temperatures, thereby allowing recovered samples from Earth’s crust and mantle to be analyzed in the laboratory or simulated computationally at the conditions that prevail in Earth’s mantle and core. This is particularly important given that the vast bulk of Earth’s interior is geochemically unsampled. This paper describes a quantitative approach that combines data and results from mineral physics, petrological analyses of mantle minerals, and geophysical inverse calculations, in order to map geophysical data directly for mantle composition (major element chemistry and water content) and thermal state. We illustrate the methodology by inverting a set of long-period electromagnetic response functions beneath six geomagnetic stations that cover a range of geological settings for major element chemistry, water content, and thermal state of the mantle. The results indicate that interior structure and constitution of the mantle can be well-retrieved given a specific set of measurements describing (1) the conductivity of mantle minerals, (2) the partitioning behavior of water between major upper mantle and transition-zone minerals, and (3) the ability of nominally anhydrous minerals to store water in their crystal structures. Specifically, upper mantle water contents determined here bracket the ranges obtained from analyses of natural samples, whereas transition-zone water concentration is an order-of-magnitude greater than that of the upper mantle and appears to vary laterally underneath the investigated locations.
Evidence for a liquid silicate layer atop the Martian core
Khan, Amir; Huang, Dongyang; Duran, Andrea Cecilia; et al. (2023)
Nature
Seismic recordings made during the InSight mission suggested that Mars’s liquid core would need to be approximately 27% lighter than pure liquid iron, implying a considerable complement of light elements. Core compositions based on seismic and bulk geophysical constraints, however, require larger quantities of the volatile elements hydrogen, carbon and sulfur than those that were cosmochemically available in the likely building blocks of Mars. Here we show that multiply diffracted P waves along a stratified core–mantle boundary region of Mars in combination with first-principles computations of the thermoelastic properties of liquid iron-rich alloys require the presence of a fully molten silicate layer overlying a smaller, denser liquid core. Inverting differential body wave travel time data with particular sensitivity to the core–mantle boundary region suggests a decreased core radius of 1,675 ± 30 km associated with an increased density of 6.65 ± 0.1 g cm⁻³, relative to previous models, while the thickness and density of the molten silicate layer are 150 ± 15 km and 4.05 ± 0.05 g cm⁻³, respectively. The core properties inferred here reconcile bulk geophysical and cosmochemical requirements, consistent with a core containing 85–91 wt% iron–nickel and 9–15 wt% light elements, chiefly sulfur, carbon, oxygen and hydrogen. The chemical characteristics of a molten silicate layer above the core may be revealed by products of Martian magmatism.

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