Compilations of continental crustal age distribution show large peaks at 1.1, 1.9-2.1, 2.7 and 3.... more Compilations of continental crustal age distribution show large peaks at 1.1, 1.9-2.1, 2.7 and 3.5Ga. These peaks have been interpreted as evidence of episodicity in the Earth's mantle-lithosphere system. Paleomagnetic evidence suggests periods of rapid plate motions coinciding with the peaks in crustal age distribution. We review a recent model for episodic tectonics in the Precambrian, where higher mantle temperatures result in lower lithospheric stresses, causing rapid pulses of subduction interspersed with periods of relative quiescence. Plate-driven episodicity arises for hotter mantle temperatures of the early Earth and can explain rapid pulses of plate motion and crustal production. Increased internal temperatures within the mantle of a terrestrial planet can also be driven from above by a runaway greenhouse atmosphere and this serves as another trigger for episodic mantle convection. We review a scaling theory and numerical simulations that explore this mechanism and discuss the implications for Earth-Venus differences.
Plate Tectonics or Not: Lithospheric Stress on Terrestrial Planets and Super-Earths
We investigate the role of driving mantle stresses and lithospheric strength in determining wheth... more We investigate the role of driving mantle stresses and lithospheric strength in determining whether or not a planet is capable of lithospheric failure and thus potentially plate tectonics.
Super-continental insulation refers to an increase in the temperature of the mantle below a super... more Super-continental insulation refers to an increase in the temperature of the mantle below a super-continent due to the heat transfer inefficiency of thick, stagnant continental lithosphere relative to thinner, subducting oceanic lithosphere. Recent studies have reached different conclusions as to the physical viability of this hypothesized effect with some groups arguing for a large heat up (100 degrees) and others arguing that any local heating is so small as to be insignificant. We start with the position that both groups are correct, in different limits, and use a combination of thermal network theory, numerical simulations, and laboratory experiments to provide tighter physical insight into the thermal link between continents and the mantle. We isolate two end-member dynamic regimes. In the thermally well mixed regime the insulating effect of continental lithosphere can not cause a localized increase in mantle temperature due to the efficiency of lateral mixing in the mantle. In this regime the potential temperature of the entire mantle is higher than it would be without continents (the magnitude depending on the specific properties of continental and oceanic lithosphere). Thermal mixing can be short circuited if, for example, subduction zones surround a super-coninent or if the convective flow pattern of the mantle becomes spatially fixed relative to a stationary super-continent. This causes a transition to the thermal isolation regime: The potential temperature increases below a super-coninent while the potential temperature below oceanic domains drops such that the volume averaged temperature of the whole mantle remains constant. Transition out of this regime would thus involve the unleashing of a potentially large lateral temperature gradient that would enhance global convective motions. The connection to sub-oceanic domains provides a larger set of predictions that can be compared to the post pangea geologic record to help determine if a hypothesized super-continental thermal effect did or did not occur on our planet.
Tectonic Modes and Atmospheric Argon on Venus and Earth
Differences in tectonic histories of Venus and Earth result from different convective stresses an... more Differences in tectonic histories of Venus and Earth result from different convective stresses and can be understood with melting and degassing models constrained by the atmospheric abundance of radiogenic argon.
The conditions for plate tectonics: what went wrong everywhere else
A necessary precursor for the onset of plate tectonics on a terrestrial planet is that the intrin... more A necessary precursor for the onset of plate tectonics on a terrestrial planet is that the intrinsic system stresses, generally associated with buoyancy anomalies, are great enough to overcome the resistance of the lithosphere to deformation. On Earth, these stresses are generally associated with the subduction of oceanic lithosphere - leading to the notion that the plates drive themselves. On planets without existing plate tectonics, the most significant buoyancy anomalies are associated with the formation and sinking of cold downwelling 'thermals'. The question we address is under what conditions are stresses associated these cold thermals sufficient to initiate failure of the lithosphere? Lithospheric strength is a function of its friction coefficient and elastic thickness (or, equivalently, the depth to the brittle-ductile transition). Both and plate's yield strength, and convective stresses, depend critically on the size and thermal evolution of a planet. We use numerical simulations and scaling theory to identify conditions under which mantle convection generates lithospheric failure, for parameters appropriate to the terrestrial planets. While the Moon and Mercury are predicted to have always been in a stagnant lid regime, Earth is, predictably, in a 'failed-lid' regime. Venus and Io currently fall on the transition between the two regimes. This is consistent with an episodic-style of convection on Venus, and suggests a tectonic component of deformation on Io. Mars is in a stagnant lid regime now, and probably was for most its history; however, early Martian plate tectonics is plausible if the lithosphere was weakened by the presence of surface water during the first 500Myr of its history.
An automated technique for detailed ?FTIR mapping of diamond and spectral deconvolution
Since the original classification of diamonds based upon their absorption in the one-phonon regio... more Since the original classification of diamonds based upon their absorption in the one-phonon region of the mid-infrared (IR) range was first introduced, a vast amount of research has been carried out in this field. The result today is that IR analysis has become the principle tool for classifying diamonds based upon the concentration and aggregation state of nitrogen, the most common impurity found within their crystal lattice. These studies have shown that diamonds can contain a large range of nitrogen, from nominally nitrogen free i.e. below detection limits (termed Type II) to nitrogen rich (termed Type I) with up to 5000 ppm. It has also been shown that the nitrogen concentration, aggregation and distribution in an individual stone can be either homogeneous or heterogeneous. Nitrogen has been shown to reside within diamond in three different aggregation states. The first is in the form of single substitutional nitrogen atoms, known as C centres. Diamonds that contain nitrogen only in this form are termed Type Ib. The second aggregation state is pairs of nitrogen atoms forming A centres (termed Type IaA diamonds) and the final state is four nitrogen atoms tetrahedrally arrange around a vacancy, forming a B centre (termed Type IaB). The sequence of aggregation has been shown to progress from C centres to A centres to B centres and is a function of time and temperature. As such it is a commonly used tool in the geological study of diamonds to gauge their mantle residence time / temperature history. The first step in the sequence is thought to occur relatively quickly in geological terms; the vast age of most diamonds therefore makes Type Ib samples rare in cratonic diamond deposits. The second step takes considerably more time, meaning that the A to B centre conversion may not always continue through to completion. So diamonds containing a mixture of both A and B centres are commonly termed Type IaAB. IR analysis of diamond also has the capability of identifying other commonly found defects and impurities. Whether these are intrinsic defects like platelets, extrinsic defects like hydrogen or boron atoms, or inclusions of minerals or fluids. Recent technological developments in the field of spectroscopy allow detailed μ-FTIR analysis to be performed rapidly in an automated fashion. The Nicolet iN10 microscope has an integrated design that maximises signal throughput and allows spectra to be collected with greater efficiency than is possible with conventional μ-FTIR spectrometer-microscope systems. Combining this with a computer controlled x-y stage allows for the automated measuring of several thousand spectra in only a few hours. This affords us the ability to record 2D IR maps of diamond plates with minimal effort, but has created the need for an automated technique to process the large quantities of IR spectra and obtain quantitative data from them. We will present new software routines that can process large batches of IR spectra, including baselining, conversion to absorption coefficient, and deconvolution to identify and quantify the various nitrogen components. Possible sources of error in each step of the process will be highlighted so that the data produced can be critically assessed. The end result will be the production of various false colour 2D maps that show the distribution of nitrogen concentrations and aggregation states, as well as other identifiable components.
Integrating the Motion of the Kerguelen Hotspot With Plate Reconstruction Models
The fixed hotspot hypothesis provides a useful reference frame for constraining absolute plate mo... more The fixed hotspot hypothesis provides a useful reference frame for constraining absolute plate motions. However, mounting observational evidence together with physical arguments is challenging the notion of the fixity of hotspots in a convecting mantle. It has been postulated that the Kerguelen hotspot was responsible for the formation of the Raj Mahal traps. However, existing plate reconstructions, and the assumed fixity of the hotspot, place Kerguelen around 1000km south of the Raj Mahal traps at the time of their formation. Recent numerical experiments (Steinberger and O'Connell, 1998) have predicted a southward motion of the Kerguelen hotspot of more than 5 degrees since the formation of the Raj Mahal traps. This is consistent with recent paleomagnetic evidence (Antretter et al., 2000) which suggests a southward drift of the Kerguelen hotspot. Here, we use a variety of tomographic models, and advect the density anomalies back through time (Steinberger and O'Connell, 1998) to calculate the motion of hotspots in a convecting mantle. The predicted motion of the Kerguelen hotspot, conservatively estimated at over 5 degrees in the past 100Ma, can in itself explain a large amount of the discrepancy in existing plate reconstruction models and provides evidence for a genetic link between the Kerguelen plume and the Raj Mahal traps. We also present updated plate reconstructions based on the predicted motion of African hotspots. A combined model including the revised plate reconstructions and the motion of the Kerguelen hotspot can constrain, within a reasonable uncertainty, the position of the Kerguelen hotspot as coincident with the Raj Mahal traps at the time of their formation. References. Antretter, M.B., Steinberger, B., Heider, F., Soffel, H., A stationary hotspot or hotspot motion? - A contribution from the Kerguelen Plateau, in EGS annual conference, Nice, 2000. Steinberger, B., O'Connell, R.J., Advection of plumes in mantle flow: implications for hotspot motion, mantle viscosity, and plume distributions, Geophys. J. Int., 132, 412-434, 1998.
1] It is widely accepted that substantial relative motion has occurred between the Indo-Atlantic ... more 1] It is widely accepted that substantial relative motion has occurred between the Indo-Atlantic and Pacific hot spots since the Late Cretaceous. At the same time, a fixed Indo-Atlantic hot spot reference frame has been argued for and used since the advent of plate tectonics, implying relatively little motion between the hot spots in this domain since about 130 Ma. Most plumes purported to have caused these hot spots, while being advected in the global-scale mantle flow field, are assumed to move an order of magnitude more slowly than plates. However, the lifetime of a plume may be over $100 Myr, and the integrated motion of a plume is expected to be significant over these times. The uncertainties inherent in hot spot reconstructions are of a magnitude similar to the expected plume motion, and so any differences between a fixed and moving frame of reference must be discernible beyond the level of these uncertainties. We present a method for constraining hot spot reconstruction uncertainties, similar to that in use for relative plate motion. We use a modified Hellinger criterion of fit for the hot spot problem, using track geometries and radiometric dating, and derive covariance matrices for our Indo-Atlantic rotations for the last 120 Myr. However, any given mantle convection model introduces additional uncertainties into such models, based on its model parameters and starting conditions (e.g., choice of global tomography model, viscosity profile, nature of mantle phase transitions). We use an interactive evolutionary approach, where we constrain the hot spot motion resulting from convection models to fit paleomagnetic constraints, and converge on an acceptable motion solution by varying unknowns over several generations of simulations. Our hot spot motion model shows large motion (5-10°) of the Indo-Atlantic hot spots for times >80 Ma, consistent with available paleomagnetic constraints. The differences between the fixed and moving hot spot reference frames are not discernible over the level of uncertainty in such rotations for times <80 Ma.
Stopping the Palaeoproterozoic plate tectonic machine: effects on melt production from 3D mantle convection simulations (Invited
Rapid pulses of geological activity at ~2.7Ga and 2.1-1.9Ga have been attributed to episodic mant... more Rapid pulses of geological activity at ~2.7Ga and 2.1-1.9Ga have been attributed to episodic mantle dynamic effects in the deep Precambrian, and the geological age gap between ~2.45-2.2Ga ascribed to an associated period of quiescence. Here we simulate episodic Precambrian tectonics in 3D spherical convection simulations to understand the effects of this regime on global observables such as melt production rate, mantle degassing, and ocean basin deepening. Periods of tectonic quiescence, which represents lulls in plate tectonic activity, result in significantly depressed global mantle melt production rates, and correspondingly diminished mantle outgassing rates. This lull in tectonic activity also has important effects on atmospheric CO2 sinks via decreased erosion/weathering of emergent mountain belts or volcanic arcs. The net effect is a significant coupling of the atmospheric system with the global tectonic regime, suggesting that the fundamental atmospheric changes between 2.5-2.0Ga may have an underlying tectonic contribution.
Coupling the volcanic and atmospheric evolution of Earth and Venus to their long-term tectonic state
Despite their superficial similarities, Venus and Earth's atmospheric evolution have diverged sig... more Despite their superficial similarities, Venus and Earth's atmospheric evolution have diverged significantly. Without significant CO2 sinks, ongoing volcanism has resulted in the build up of extremely high CO2 concentrations, which have contributed to the dehydration of Venus's surface, and perhaps also the cessation of plate tectonics on Venus, either due to dry faults or surface temperatures. Its degassing history is to some extant recorded in its atmospheric argon signatures. Nonradiogenic Argon-36 is ~80 times that off Earth. Given most 36-Ar is primordial, this suggests very different initial atmospheric conditions for the two planets, with Venus retaining most of its initial atmosphere due to its fortuitous impact history. On the other hand, the deficit of radiogenic Argon-40 (~24 percent escaped from the mantle, compared with ~52 percent for Earth), hints at a very different volcanic and tectonic history, particularly in its deepest past. Recent convection modelling has shown that plate tectonic regimes break down under hot mantle conditions, due to a partial decoupling of stress from the less viscous mantle to the plates, resulting in insufficient stress for plate boundary deformation. This sends the system into an "episodic overturn" regime - similar to that which has been proposed for Venus today - where long periods of stagnant lid convection are interrupted by periods of massive lid recycling and overturn. Conversely, a planet in an episodic regime may transit into a stagnant lid regime for hotter mantle conditions. We couple a model for production and degassing of radiogenic Ar-40 from the mantle, and couple it with evolutionary models for stagnant, episodic and mobile lid tectonics. Earth's deficit in radiogenic Ar-40 may in some part be due to the different degassing efficiency of episodic convection in a hot early Earth. If Venus was stagnant for a large portion of its early evolution, the cumulative degassing efficiency of Ar-40 would be much lower than Earth, providing an explanation for Venus's very low atmospheric Ar-40 concentrations.
The tectonic quiescence of cratons on a tectonically active planet has been attributed to their p... more The tectonic quiescence of cratons on a tectonically active planet has been attributed to their physical properties such as buoyancy, viscosity, and yield strength. Previous modelling has shown the conditions under which cratons may be stable for the present, but cast doubt on how they survived in a more energetic mantle of the past. Here we incorporate an endothermic phase change at 670 km, and a depth-dependent viscosity structure consistent with post-glacial rebound and geoid modelling, to simulate the dynamics of cratons in an "Earth-like" convecting system. We find that cratons are unconditionally stable in such systems for plausible ranges of viscosity ratios between the root and asthenosphere (50-150) and the root/oceanic lithosphere yield strength ratio (5-30). Realistic mantle viscosity structures have limited effect on the average background cratonic stress state, but do buffer cratons from extreme stress excursions. An endothermic phase change at 670 km introduces an additional time-dependence into the system, with slab breakthrough into the lower mantle associated with 2-3 fold stress increases at the surface. Under Precambrian mantle conditions, however, the dominant effect is not more violent mantle avalanches, or faster mantle/plate velocities, but rather the drastic viscosity drop which results from hotter mantle conditions in the past. This results in a large decrease in the cratonic stress field, and promotes craton survival under the evolving mantle conditions of the early Earth.
1] It is widely accepted that substantial relative motion has occurred between the Indo-Atlantic ... more 1] It is widely accepted that substantial relative motion has occurred between the Indo-Atlantic and Pacific hot spots since the Late Cretaceous. At the same time, a fixed Indo-Atlantic hot spot reference frame has been argued for and used since the advent of plate tectonics, implying relatively little motion between the hot spots in this domain since about 130 Ma. Most plumes purported to have caused these hot spots, while being advected in the global-scale mantle flow field, are assumed to move an order of magnitude more slowly than plates. However, the lifetime of a plume may be over $100 Myr, and the integrated motion of a plume is expected to be significant over these times. The uncertainties inherent in hot spot reconstructions are of a magnitude similar to the expected plume motion, and so any differences between a fixed and moving frame of reference must be discernible beyond the level of these uncertainties. We present a method for constraining hot spot reconstruction uncertainties, similar to that in use for relative plate motion. We use a modified Hellinger criterion of fit for the hot spot problem, using track geometries and radiometric dating, and derive covariance matrices for our Indo-Atlantic rotations for the last 120 Myr. However, any given mantle convection model introduces additional uncertainties into such models, based on its model parameters and starting conditions (e.g., choice of global tomography model, viscosity profile, nature of mantle phase transitions). We use an interactive evolutionary approach, where we constrain the hot spot motion resulting from convection models to fit paleomagnetic constraints, and converge on an acceptable motion solution by varying unknowns over several generations of simulations. Our hot spot motion model shows large motion (5-10°) of the Indo-Atlantic hot spots for times >80 Ma, consistent with available paleomagnetic constraints. The differences between the fixed and moving hot spot reference frames are not discernible over the level of uncertainty in such rotations for times <80 Ma.
Revised Indo-Atlantic absolute plate rotations and their uncertainties based on moving hotspots
Inconsistencies between the paleomagnetic and hotspot reference frames have been ascribed to the ... more Inconsistencies between the paleomagnetic and hotspot reference frames have been ascribed to the effects of hotspot motion and true polar wander (TPW). While revised models based on moving hotspots have been shown to improve a measure of fit; namely the difference between the predicted and observed hotspot tracks, the real uncertainties in such models can be quite large. We present a method for constraining the uncertainties in absolute plate motion models using a modified version of the Hellinger criterion. We use the dated tracks of four of the best studied hotspots in the Indo-Atlantic hemisphere; namely, Kerguelen, Reunion, Tristan de Cunha and the New England hotspots, together with their present positions to obtain finite rotations for the African plate. We also use models of hotspot motion to obtain analogous rotations for a `moving hotspot' reference frame. The motion is consistent with available paleomagnetic constraints, and results in a significantly improved fit to the hotspot tracks. The uncertainty regions are less for the moving hotspot model due to the improved fit, and the covariance matrices obtained are equivalent to published relative plate rotation covariance matrices, and allow a combination of these uncertainties.
Reconciling hotspot and TPW reference frames within the uncertainties by evolutionary computation of hotspot motion
A significant discrepancy exists between TPW and fixed hotspot reference frames in the time perio... more A significant discrepancy exists between TPW and fixed hotspot reference frames in the time period between 90-120Ma. A number of Indo-Atlantic hotspots, including Tristan da Cunha, Great Meteor and Kerguelen, display a significant paleolatitude drift at this time, which cannot be explained by polar wander. However, the degree of motion implied is of a similar magnitude to the uncertainties inherent in the hotspot reconstructions. We present a method for contraining the uncertainties in hotspot reconstructions, and present a model for the motion of the Indo-Atlantic hotspot group based on an interactive evolutionary approach to the inverse mantle convection problem. The hotspot motion is consistent with available paleolatitudinal constraints, and calculated plume conduit tilts are similar to tomographically imaged conduits. We show that fixed and moving hotspot reference frames are not significantly different until 80Ma, and the discrepancy between TPW and hotspot reference can be greatly alleviated for moving hotspots.
Three-dimensional continental lithosphere extension modeling via particle-in-cell finite element analysis
We extended the two-dimensional particle-in-cell (PIC) finite element code Ellipsis to three-dime... more We extended the two-dimensional particle-in-cell (PIC) finite element code Ellipsis to three-dimensions for application to problems in geodynamics. The particle in cell scheme is a hybrid method which combines a fixed mesh of computational points and a dense arrangement of mobile material points. The fixed Eulerian mesh allows very fast computation (performed in Ellipsis via a multigrid iteration method) whereas the Lagrangian particle reference frame allows the tracking of material interfaces and history dependent properties such as strain history for strain-softening materials. The PIC method is exceptionally useful in very large deformation analyses where purely Lagrangian approaches would be severely hampered by the need for remeshing to minimize element distortion. We apply the Ellipsis particle-in-cell finite element code to a series of problems involving extension of the continental lithosphere. This constitutes one of the first applications of 3D particle-in-cell technology to a problem of geodynamic importance. Specifically, we model a 3 layer lithosphere (with upper crust, lower crust, and upper mantle components), and incorporate phase changes via decompression melting. The extension proceeds in multiple steps, including orientations normal to and oblique to predefined weak zones that serve to aid the onset of localization. The rheological model is viscoplastic (work is continuing on the 3D code s elastic aspect), where the plastic part of the rheology is set to mimic brittle deformation and the viscosity is temperature-dependent. We focus on the distribution of brittle deformation patterns in the upper crust with variable loading schemes, as well as the distribution of melt in the system for realistic parameter values.
The geodynamics of super-sized Earths
The habitability of a planet is strongly contingent on its tectonic regime. Recently, the discove... more The habitability of a planet is strongly contingent on its tectonic regime. Recently, the discovery of giant Earth-like planets has led to speculation on their surface conditions and dynamics. Here we explore the tectonic regimes of super-sized Earths, which are a function of the balance between driving forces, and the resistive strength of the lithosphere. We use mantle convection simulations to show that simply increasing planetary radius acts to decrease the ratio of driving to resisting stresses, and thus super-sized Earths are likely to be in an episodic or stagnant lid regime. This effect is robust when associated increases in gravity are included, as the more dominant effect is increased fault strength rather than greater buoyancy forces. The thermo-tectonic evolution of large terrestrial planets is more complex than often assumed, and this has implications for the surface and conditions habitability of such worlds.
As a result of our discussions (most recently our telecon on 05/15/06), I here summarize the curr... more As a result of our discussions (most recently our telecon on 05/15/06), I here summarize the current status and work plan to develop a modular, shared, and well documented global, Hager & O'Connell (1981) type, spectral mantle flow tool. Please keep sending me comments on this document, also if you are not on our mailing list but wish to be kept up to date about progress.
Keywords: zircon ages plate tectonics crustal growth episodicity mantle evolution magmatic age ga... more Keywords: zircon ages plate tectonics crustal growth episodicity mantle evolution magmatic age gap Analysis of the global distribution of U/Pb ages of both subduction-related granitoids and of detrital zircons suggests that a widespread reduction in magmatic activity on Earth beginning about 2.45 Ga and lasting for 200-250 My. There are no arc-type greenstones or tonalite-trondhjemite-granodiorite (TTG) suites and only one large igneous province (LIP) reported in this time window. There is little Nd or Hf isotopic evidence to support significant additions to the continental crust at convergent plate margins between 2.45 and 2.2 Ga. Also during this time, there are major unconformities on most cratons and a gap in deposition of banded iron formation (BIF), both consistent with a major drop in sea level. Oxygenation of the atmosphere at 2.4 Ga followed by widespread glaciation at 2.4-2.3 Ga also may be related to the initiation of the global magmatic lull. We suggest that an episodic mantle thermal regime, during which a large part of the plate circuit effectively stagnates, may explain the 250-My magmatic age gap on Earth and a remarkable feature of the Paleoproterozoic record.
Of the many changes that have been proposed to occur in the late Archean centered around 2.7 Ga, ... more Of the many changes that have been proposed to occur in the late Archean centered around 2.7 Ga, only seven are well documented: 1) a decrease in the Eu anomaly in hydrothermal cherts and BIF; 2) a large decrease in the proportion of komatiites in greenstone belts and in the ...
Enriched, insulating crustal units and runaway crustal growth on Mars
Crustal growth can either cool or heat the mantle, as the crust not only depletes the mantle of h... more Crustal growth can either cool or heat the mantle, as the crust not only depletes the mantle of heat producing elements, but thermally insulates it. We employ mantle convection models including thickened crustal units of variable heat production to show that for increasing extents of thickened crust, mantle temperatures and melt production can either increase of decrease, depending on the degree of enrichment of the crust with respect to the mantle, the total heat production, and the Rayleigh number. The formation of the continents on Earth would have efficiently cooled the upper mantle, resulting in lower subsequent rates of melt production and continental formation. In contrast, the growth of the Martian highlands would have raised the temperatures of the Martian mantle, increasing rates of melt production, and leading to runaway crustal growth. This would have continued as long as the lithosphere of Mars was mobile.
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