Paul Tackley

Last Name

Tackley

First Name

Paul

ORCID

0000-0003-4878-621X

Organisational unit

03698 - Tackley, Paul / Tackley, Paul

Show all metadata

Search Results

Publications 1 - 10 of 71

Narrow, Fast, and "Cool" Mantle Plumes Caused by Strain-Weakening Rheology in Earth's Lower Mantle
Gülcher, Anna; Golabek, Gregor J.; Thielmann, Marcel; et al. (2022)
Geochemistry, Geophysics, Geosystems
The rheological properties of Earth's lower mantle are key for mantle dynamics and planetary evolution. The main rock-forming minerals in the lower mantle are bridgmanite (Br) and smaller amounts of ferropericlase (Fp). Previous work has suggested that the large differences in viscosity between these minerals greatly affect the bulk rock rheology. The resulting effective rheology becomes highly strain-dependent as weaker Fp minerals become elongated and eventually interconnected. This implies that strain localization may occur in Earth's lower mantle. So far, there have been no studies on global-scale mantle convection in the presence of such strain-weakening (SW) rheology. Here, we present 2D numerical models of thermo-chemical convection in spherical annulus geometry including a new strain-dependent rheology formulation for lower mantle materials, combining rheological weakening and healing terms. We find that SW rheology has several direct and indirect effects on mantle convection. The most notable direct effect is the changing dynamics of weakened plume channels as well as the formation of larger thermochemical piles at the base of the mantle. The weakened plume conduits act as lubrication channels in the mantle and exhibit a lower thermal anomaly. SW rheology also reduces the overall viscosity, notable in terms of increasing convective vigor and core-mantle boundary heat flux. Finally, we put our results into context with existing hypotheses on the style of mantle convection and mixing. Most importantly, we suggest that the new kind of plume dynamics may explain the discrepancy between expected and observed thermal anomalies of deep-seated mantle plumes on Earth.
Self‐consistent generation of tectonic plates in time‐dependent, three‐dimensional mantle convection simulations
Tackley, Paul (2000)
Geochemistry, Geophysics, Geosystems
Presented here are self-consistent, three-dimensional simulations of mantle convection, some of which display an approximation of plate tectonic behavior that is continuous in space and time. Plate behavior arises through a reasonable material description of silicate deformation, with a simple yield stress being sufficient to give first-order plate-like behavior; however, the required yield strength or fault frictional coefficient is much less than experimentally determined values. Toroidal:poloidal ratios are within geologically observed limits. The sensitivity of the system to yield strength and the form of strength envelope is systematically investigated. Optimum plate character is obtained in a narrow range of yield strength, below which diffuse boundaries, and above which episodic behavior, and eventually a rigid lid, are observed. Models with mobile lids develop very long wavelength horizontal structure, the longest wavelength possible in the domain. Two-dimensional models display much greater time dependence than three-dimensional models.
Mantle Convection and Plate Tectonics: Toward an Integrated Physical and Chemical Theory
Tackley, Paul (2000)
Science
Plate tectonics and convection of the solid, rocky mantle are responsible for transporting heat out of Earth. However, the physics of plate tectonics is poorly understood; other planets do not exhibit it. Recent seismic evidence for convection and mixing throughout the mantle seems at odds with the chemical composition of erupted magmas requiring the presence of several chemically distinct reservoirs within the mantle. There has been rapid progress on these two problems, with the emergence of the first self-consistent models of plate tectonics and mantle convection, along with new geochemical models that may be consistent with seismic and dynamical constraints on mantle structure.
Interior dynamics of super-Earth 55 Cancri e
Meier, Tobias G.; Bower, Dan J.; Lichtenberg, Tim; et al. (2023)
Astronomy & Astrophysics
The ultra-short-period super-Earth 55 Cancri e has a measured radius of 1.88 Earth radii. Previous thermal phase curve observations suggest a strong temperature contrast between the dayside and nightside of around 1000 K; the hottest point is shifted 41 ± 12 degrees east from the substellar point, indicating some degree of heat circulation. The dayside (and potentially even the nightside) is hot enough to harbour a magma ocean. We used results from general circulation models (GCMs) of atmospheres to constrain the surface temperature contrasts. There is still a large uncertainty on the vigour and style of mantle convection in super-Earths, especially those that experience stellar irradiation high enough to harbour a magma ocean. In this work our aim is to constrain the mantle dynamics of the tidally locked lava world 55 Cancri e. Using the surface temperature contrasts as a boundary condition, we model the mantle flow of 55 Cancri e using 2D mantle convection simulations, and investigate how the convection regimes are affected by the different climate models. We find that large super-plumes form on the dayside if that hemisphere is covered by a magma ocean and the nightside remains solid or only partially molten. Cold material descends into the deep interior on the nightside, but no strong downwellings form. In some cases the super-plume also moves several tens of degrees towards the terminator. A convective regime where the upwelling is preferentially on the dayside might lead to preferential outgassing on that hemisphere which could lead to the build-up of atmospheric species that could be chemically distinct from the nightside.
Plausible constraints on the range of bulk terrestrial exoplanet compositions in the Solar neighbourhood
Spaargaren, Rob J.; Wang, Haiyang; Mojzsis, Stephen; et al. (2022)
arXiv
Rocky planet compositions regulate planetary evolution by affecting core sizes, mantle properties, and melting behaviours. Yet, quantitative treatments of this aspect of exoplanet studies remain generally under-explored. We attempt to constrain the range of potential bulk terrestrial exoplanet compositions in the solar neighbourhood (<200 pc). We circumscribe probable rocky exoplanet compositions based on a population analysis of stellar chemical abundances from the Hypatia and GALAH catalogues. We apply a devolatilization model to simulate compositions of hypothetical, terrestrial-type exoplanets in the habitable zones around Sun-like stars, considering elements O, S, Na, Si, Mg, Fe, Ni, Ca, and Al. We further apply core-mantle differentiation by assuming constant oxygen fugacity, and model the consequent mantle mineralogy with a Gibbs energy minimisation algorithm. We report statistics on several compositional parameters and propose a reference set of (21) representative planet compositions for using as end-member compositions in imminent modelling and experimental studies. We find a strong correlation between stellar Fe/Mg and metallic core sizes, which can vary from 18 to 35 wt%. Furthermore, stellar Mg/Si gives a first-order indication of mantle mineralogy, with high-Mg/Si stars leading to weaker, ferropericlase-rich mantles, and low-Mg/Si stars leading to mechanically stronger mantles. The element Na, which modulates crustal buoyancy and mantle clinopyroxene fraction, is affected by devolatilization the most. While we find that planetary mantles mostly consist of Fe/Mg-silicates, core sizes and relative abundances of common minerals can nevertheless vary significantly among exoplanets. These differences likely lead to different evolutionary pathways among rocky exoplanets in the solar neighbourhood.
Buoyant melting instabilities beneath extending lithosphere: 2. Linear analysis
Hernlund, John W.; Stevenson, David J.; Tackley, Paul (2007)
Journal of Geophysical Research: Solid Earth
In a companion paper, numerical models reveal that buoyant melting instabilities can occur beneath extending lithosphere for a sufficiently small mantle viscosity, extension rate, and rate of melt percolation. However, in some cases, instabilities do not develop during extension but only occur after extension slows or stops. These results are suggestive of a critical behavior in the onset of these kinds of instabilities and motivate a linear analysis to study the onset of instability in a partially melting, passively upwelling plane layer of mantle beneath extending lithosphere. The model we employ includes the effects of buoyancy arising from thermal expansion, the presence of a retained fraction of partial melt, and depletion of the solid by melt extraction. We find a critical behavior in the onset of instability controlled by melt retention buoyancy that is characterized by a “Rayleigh” number M, such that M must exceed some critical value Mcrit which depends on the efficiency of Stokes rise of a partially molten body relative to the rate of background percolation. Comparison of this theory to the numerical results in the companion paper yields a close quantitative agreement. We also find that solid depletion buoyancy can either stabilize or destabilize a partially melting layer, depending upon both the distribution of preexisting depletion and the magnitude of density changes with depth. This theory is compared with previous studies of buoyant melting instabilities beneath mid-ocean ridges where similar behavior was reported, and it suggests that the stability of passively upwelling, partially melting mantle underlying both narrow and wide rift settings is controlled by similar processes.
Contrasts in 2-D and 3-D system behaviour in the modelling of compositionally originating LLSVPs and a mantle featuring dynamically obtained plates
Langemeyer, Sean M.; Lowman, Julian P.; Tackley, Paul (2022)
Geophysical Journal International
More than two decades of systematic investigation has made steady progress towards generating plate-like surface behaviour in models of vigorous mantle convection. Accordingly, properties required to obtain dynamic plates from mantle convection have become widely recognized and used in both 2-D and 3-D geometries. Improving our understanding of the properties required to obtain durable (or replenishable) deep mantle features with LLSVP-like characteristics has received interest fora period with similar longevity. Investigation ultimately focuses on discovering the properties able to produce the presence of a detached pair of 3-D features, distinct from the ambient mantle. here, we assume the large low shear-wave velocity provinces (LLSVPs) have a chemical origin by incorporating a compositionally anomalous and intrinsically dense (CAID) mantle component comprising 2-3.5 per cent of the total mantle volume. The feedback between plate formation and the presence of a CAID mantle component is investigated in both 2-D and 3-D spherical geometries. We explore the impact of both an intrinsic contrast in density and viscosity for the CAID component, with the objective of finding system parameter values that encourage the formation of a pair of LLSVP-like assemblages and a surface that exhibits the principle features of terrestrial plate tectonics; including recognizable and narrowly focused divergent, convergent and (in 3-D) transform plate boundaries that separate 8-16 distinct plate interiors. We present the results of nine 2-D and 11 3-D calculations and show that for some of the cases examined, a pair of CAID material provinces can be freely obtained in 2-D cases while maintaining a surface characterized by plate-like behaviour. However, specifying the same system parameters in the 3-D model does not readily yield a pair of enduring provinces for any values of the parameters investigated. Moreover, the inclusion of the CAID component in the mantle can affect the global geotherm so that in comparison to the surface behaviour obtained for the initial condition isochemical model, the surface behaviour of the cases incorporating the dense component are less exemplary of plate tectonics. In general, CAID material components that are 3.75-5 per cent denser than the surrounding mantle (at surface temperatures), and up to a factor of 100 times greater in intrinsic viscosity, form layers populated by voids, or nodes connected by tendril-like ridges that reach across the core-mantle boundary (CMB), rather than distinct piles resembling LLSVPs. Due to its inherently heavy and stiff character, in equilibrated systems, we find the CAID material becomes especially hot so that the temperature-dependence of its density and viscosity results in reduced distinction between the intrinsically dense assemblages and the ambient mantle. Accordingly, the CAID material forms masses on the CMB that are relatively less dense (0.625-1.5 per cent) and viscous than the adjacent mantle material, in comparison to the percentage differences obtained at common temperatures. We find that by adjusting our yield stress model to account for the influence of the CAID material on the geotherm, a highly satisfactory plate-like surface can be re-attained, however, the formation of a pair of LLSVP-shaped masses remains elusive.
Earth Sphericity Effects on Subduction Morphology
Morra, G.; Chatelain, Philippe; Tackley, Paul; et al. (2007)
EOS
The tracer nudging method for correcting and preventing uneven tracer distributions in geodynamical models
Tackley, Paul (2025)
Geoscientific Model Development
Tracers/markers/particles are commonly used in geodynamical models to track composition and sometimes other quantities throughout the domain. A common problem is that over time, gaps in the tracer distribution can develop, often resulting in cells with no tracers as well as bunching of tracers. These arise when tracer advection does not perfectly respect the mass conservation equation, so here this equation is used to derive a correction method that perturbs or "nudges"the positions of tracers in such a way as to close gaps and eliminate bunching. Test results show that this tracer nudging method is highly effective. Starting from an extremely heterogeneous tracer distribution with large regions of the domain devoid of tracers, it can produce an even distribution in only a few nudge iterations. In a time-stepping situation with a nudge every time-step, the amplitudes of the nudges are small yet sufficient to prevent gaps and bunches, allowing a low-order tracer advection method to be used while maintaining a tracer distribution that is more even than that obtained using higher-order advection methods alone. The computational cost is small-slightly larger than that of a first-order tracer advection step alone-because the method simply requires solving a Poisson equation. If an accuracy threshold is applied, a nudge correction may be necessary in only a fraction of time steps, with tests indicating that it is fastest to use low order advection with more frequent nudges than high order advection with less frequent nudges.
Construction of semi-dynamic model of subduction zone with given plate kinematics in 3D sphere
Morishige, Manabu; Honda, Satoru; Tackley, Paul (2010)
Earth, Planets and Space

Publications 1 - 10 of 71

Paul Tackley

Last Name

First Name

ORCID

Organisational unit

Filters

Search Results