SUMMARY
We complete the development and description of a thermodynamic method for the computation of phase equilibria and physical properties of multiphase mantle assemblages. Our previous paper ...focused on the computation of physical properties. In this paper, our focus shifts to the phase equilibria. We further develop our theory to specify the ideal and excess contributions to solution properties and derive properties of multiphase assemblages. We discuss our global inversion strategy for determining the values of the free parameters in our theory and compare inverted parameter values with expectations based on scaling arguments. Comparisons between our method and experimental phase equilibria data encompass the pressure–temperature regime of Earth's mantle. Finally, we present applications of our method that illustrate how it may be used to explore the origins of mantle structure and mantle dynamics. Continuing rapid advances in experimental and theoretical petrology and mineral physics have motivated an expansion of the scope of our model via the addition of several new phases, and of the soda component: an appendix lists all parameters in our model and references to the experimental and theoretical studies that constrain them. Our algorithm for global minimization of the Gibbs free energy is embodied in a code called HeFESTo, and is detailed in a second appendix.
Subduction of bathymetric anomalies (e.g., an active ridge) can alter the morphology of subducted slabs and their coupling to surface processes. A natural laboratory to study these effects is the ...subduction of the Oceanic Chilean Ridge beneath the South American plate, which led to the formation of the Patagonian slab window. Its formation and subsequent northward migration contributed to the regression of Patagoniense sea and exhumation of marine strata to their present elevation. To date, there is no quantitative analysis of the effects on the sediment routing system of the slab window. We modeled the Neogene topographic change and foreland sedimentary evolution from the Andean Cordillera to Atlantic margin. Our results show that subcrustal‐driven subsidence correlated with accelerated subduction of the Nazca plate is required to explain the timing of the Patagonian transgression and thickness and spatial extent of marine beds during the incursion. In other words, traditional mechanisms, such as foreland flexure and global sea‐level rise, are insufficient. The subsequent regression and accumulation of mid‐Miocene alluvial‐fluvial deposits were associated with the growth of the Cordillera and a possible flattening of Nazca subduction in the middle Miocene. Isostatic uplift of ∼1 km due to lithospheric thinning during slab window formation can explain the foreland exhumation, sediment bypass, and increases in the offshore sedimentation rate. However, spatial‐temporal varying dynamic uplift is required to explain the along‐strike variations in foreland sedimentation. Our study provides new insights into the interplay between slab window formation, crustal deformation, and landscape evolution.
Plain Language Summary
The geological evolution of southern Patagonia has been strongly affected by the subduction of Nazca Plate and the subsequent formation of Patagonian slab window. Different uplift and subsidence mechanisms have been proposed to explain its landscape and sedimentary history. However, it remains challenging to elucidate and quantify the impacts of contributing processes. This study focuses on modeling the surface evolution from the Andean Cordillera to the Atlantic margin over the entire Neogene. We propose that the landscape evolution of southern Patagonia involves three stages. Our results not only reveal the substantial contribution of Nazca Plate subduction to foreland sedimentation in early Miocene, but also a potential flattening of the subduction morphology in middle Miocene. Lastly, the isostatic uplift due to lithospheric thinning that accompanies Patagonian slab window formation could explain the foreland exhumation, sediment bypass, and increases in offshore sedimentation rate.
Key Points
We modeled the landscape and sedimentary evolution across southern Patagonia and offshore basins since Neogene
The Nazca plate subduction induced long‐wavelength foreland subsidence; it has likely flattened in mid‐Miocene, generating surface uplift
The Patagonian slab window affected the surface sediment routing system by generating isostatic/dynamic uplift since late Miocene
We present a theory for the computation of phase equilibria and physical properties of multicomponent assemblages relevant to the mantle of the Earth. The theory differs from previous treatments in ...being thermodynamically self-consistent: the theory is based on the concept of fundamental thermodynamic relations appropriately generalized to anisotropic strain and in encompassing elasticity in addition to the usual isotropic thermodynamic properties. In this first paper, we present the development of the theory, discuss its scope, and focus on its application to physical properties of mantle phases at elevated pressure and temperature including the equation of state, thermochemical properties and the elastic wave velocities. We find that the Eulerian finite strain formulation captures the variation of the elastic moduli with compression. The variation of the vibrational frequencies with compression is also cast as a Taylor series expansion in the Eulerian finite strain, the appropriate volume derivative of which leads to an expression for the Grüneisen parameter that agrees well with results from first principles theory. For isotropic materials, the theory contains nine material-specific parameters: the values at ambient conditions of the Helmholtz free energy, volume, bulk and shear moduli, their pressure derivatives, an effective Debye temperature, its first and second logarithmic volume derivatives (γ0, q0), and the shear strain derivative of γ. We present and discuss in some detail the results of a global inversion of a wide variety of experimental data and first principles theoretical results, supplemented by systematic relations, for the values of these parameters for 31 mantle species. Among our findings is that the value of q is likely to be significantly greater than unity for most mantle species. We apply the theory to the computation of the shear wave velocity, and temperature and compositional (Fe content) derivatives at relevant mantle pressure temperature conditions. Among the patterns that emerge is that garnet is anomalous in being remarkably insensitive to iron content or temperature as compared with other mantle phases.
We use a new method to construct an upper mantle model based on self‐consistent computation of phase equilibria and physical properties. Computation of the isotropic elastic wave velocities of a ...pyrolytic bulk composition in thermodynamic equilibrium shows a distinct low‐velocity zone with a minimum velocity VS = 4.47 km s−1 along the 100 Ma geotherm. In the vicinity of the low‐velocity zone the velocity of this null hypothesis is approximated along oceanic geotherms by VS = 4.77 + 0.0380(P, z/29.80) − 0.000378(T − 300), with pressure P in GPa, depth z in km, temperature T in K, and velocity VS in km s−1. The null hypothesis predicts a minimum VS 0.1–0.2 km s−1 higher than that in seismological models of 100 Ma Pacific. We find that dispersion, estimated solely on the basis of seismological attenuation models, can account for this residual velocity deficit. Except in the immediate vicinity of the ridge (t < 5 Ma), a solid‐state low‐velocity zone provides a satisfactory quantitative explanation of seismic observations. We do not find a satisfactory explanation for the magnitude of the Gutenberg discontinuity or for the high shear wave velocity gradient zone.
The shear tractions that mantle flow exerts on the base of Earth's lithosphere contribute to plate‐driving forces and lithospheric stresses. We investigate the sensitivity of these tractions to ...sub‐lithospheric viscosity variations by comparing shear tractions computed from a mantle flow model featuring laterally‐varying lithosphere and asthenosphere viscosity with those from a model with layered viscosity. Lateral viscosity variations generally do not change the direction of shear tractions, but deeply penetrating continental roots increase traction magnitudes by a factor of 2–5 compared to 100 km thick lithosphere. A low‐viscosity asthenosphere decreases traction magnitudes by a smaller amount, and is important only if >100 km thick. Increased plate‐mantle coupling beneath thick continental lithosphere may increase plate‐driving forces, surface deformation, and mantle‐derived lithospheric stresses in these regions. By contrast, a low‐viscosity asthenosphere does not decouple the lithosphere from mantle flow, highlighting the geological importance of mantle tractions on the lithosphere.
The water content in Earth's mantle today remains poorly constrained, but the bulk water storage capacity in the solid mantle can be quantified based on experimental data and may amount to a few ...times the modern surface ocean mass (OM). An appreciation of the mantle water storage capacity is indispensable to our understanding of how water may have cycled between the surface and mantle reservoirs and changed the volume of the oceans through time. In this study, we parameterized high pressure‐temperature experimental data on water storage capacities in major rock‐forming minerals to track the bulk water storage capacity in Earth's solid mantle as a function of temperature. We find that the mantle water storage capacity decreases as mantle potential temperature (Tp) increases, and its estimated value depends on the water storage capacity of bridgmanite in the lower mantle: 1.86–4.41 OM with a median of 2.29 OM for today (Tp = 1600 K), and 0.52–1.69 OM with a median of 0.72 OM for the early Earth's solid mantle (for a Tp that was 300 K higher). An increase in Tp by 200–300 K results in a decrease in the mantle water storage capacity by 1.19−0.16+0.9 –1.56−0.22+1.1 OM. We explored how the volume of early oceans may have controlled sea level during the early Archean (4–3.2 Ga) with some additional assumptions about early continents. We found that more voluminous surface oceans might have existed if the actual mantle water content today is > 0.3–0.8 OM and the early Archean Tp was ≥1900 K.
Plain Language Summary
At the Earth's surface, the majority of water resides in the oceans, while in the interior, major rock‐forming minerals can incorporate significant amounts of water as hydroxyl groups (OH), likely forming another reservoir of water inside the planet. The amount of water that can be dissolved in Earth's mantle minerals, called its water storage capacity, generally decreases at higher temperatures. Over billion‐year timescales, the exchange of water between Earth's interior and surface may control the surface oceans' volume change. Here, we calculated the water storage capacity in Earth's solid mantle as a function of mantle temperature. We find that water storage capacity in a hot, early mantle may have been smaller than the amount of water Earth's mantle currently holds, so the additional water in the mantle today would have resided on the surface of the early Earth and formed bigger oceans. Our results suggest that the long‐held assumption that the surface oceans' volume remained nearly constant through geologic time may need to be reassessed.
Key Points
We developed a temperature‐dependent model to estimate the bulk water storage capacity in Earth's solid mantle
The solid mantle water storage capacity decreases as mantle potential temperature (Tp) increases
If the mantle today holds >0.3–0.8 ocean mass water, larger surface oceans might have existed during the early Archean (Tp ≥ 1900 K)
Mantle Phase Changes Detected From Stochastic Tomography Cormier, Vernon F.; Lithgow‐Bertelloni, Carolina; Stixrude, Lars ...
Journal of geophysical research. Solid earth,
February 2023, 2023-02-00, 20230201, Letnik:
128, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Stochastic tomography, made possible by dense deployments of seismic sensors, is used to identify phase changes in Earth's mantle that occur over depth intervals. This technique inverts spatial ...coherences of amplitudes and travel times of body waves to determine the depth and dependence of the spatial spectrum of seismic velocity. This spectrum is interpreted using the predicted thermodynamic stability of mineral composition and phase as a function of temperature and pressure, in which the metamorphic temperature derivative of seismic velocities is used as a proxy for the effects of heterogeneity induced in a region undergoing a phase change. Peaks in the temperature derivative of seismic velocity closely match those found from applying stochastic tomography to elements of Earthscope and Flex arrays. Within ±12 km, peaks in the fluctuation of P velocity at 425, 500, and 600 km depth beneath the western US agree with those predicted by a mechanical mixture of harzburgite and basalt, 180 K cooler than a 1600 K adiabat in the mantle transition zone. A broad peak at 250 km depth may be associated with chemical heterogeneity induced by dehydration of subducted oceanic sediments, and a peak at 775 km depth with a phase change in subducted basalt. Non‐detection of predicted phase changes less than 10 km in width is consistent with the resolution possible with the seismic arrays used in the inversion, including the sharp endothermic phase change near 660 km. These interpretations are consistent with the known history of plate subduction beneath North America.
Plain Language Summary
The speed of seismic waves in a rock depends on the chemistry and arrangement of the atoms making up the minerals in the rock. These atoms are arranged in different ways, depending on temperature and pressure. Increasing pressure with depth in Earth squeezes the atoms of minerals into denser arrangements. An abrupt change with depth in the arrangement of mineral atoms, termed a phase change, can speed up, slow down, and scatter earthquake waves in different directions, changing their arrival times and amplitudes. This study describes a way in which the fluctuations in the arrival times and amplitudes of seismic waves, recorded by dense collections of seismometers, can be used to locate, detect, and image the depth and thickness of mineral phase changes in Earth's mantle. These phase changes are found to control much of the observed scattering of seismic waves in Earth's mantle from distant earthquakes and agree with the history of plate tectonics of western North America.
Key Points
Stochastic tomography can capture higher order phase and chemical changes undetectable from reflection imaging
The heterogeneity spectrum of the upper mantle is marked by peaks associated with depth regions undergoing phase or compositional change
The upper mantle beneath the western US is consistent with a history of subduction and slab stagnation in the transition zone
Although mantle slabs ultimately drive plate motions, the mechanism by which they do so remains unclear. A detached slab descending through the mantle will excite mantle flow that exerts shear ...tractions on the base of the surface plates. This “slab suction” force drives subducting and overriding plates symmetrically toward subduction zones. Alternatively, cold, strong slabs may effectively transmit stresses to subducting surface plates, exerting a direct “slab pull” force on these plates, drawing them rapidly toward subduction zones. This motion induces mantle flow that pushes overriding plates away from subduction zones. We constrain the relative importance of slab suction and slab pull by comparing Cenozoic plate motions to model predictions that include viscous mantle flow and a proxy for slab strength. We find that slab pull from upper mantle slabs combined with slab suction from lower mantle slabs explains the observation that subducting plates currently move ∼4 times faster than nonsubducting plates. This implies that upper mantle slabs are strong enough to support their own weight. Slab suction and slab pull presently account for about 40 and 60% of the forces on plates, but slab suction only ∼30% if a low‐viscosity asthenosphere decouples plates from mantle flow. The importance slab pull has been increasing steadily through the Cenozoic because the mass and length of upper mantle slabs has been increasing. This causes subducting plates to double their speed relative to nonsubducting plates during this time period. Our model explains this temporal evolution of plate motions for the first time.
Thank You to Our 2019 Reviewers Faccenna, Claudio; Becker, Thorsten; Behr, Whitney ...
Geochemistry, geophysics, geosystems : G3,
March 2020, Letnik:
21, Številka:
3
Journal Article
Recenzirano
Odprti dostop
The publishing process relies on the work of volunteer reviewers, and evaluating the interdisciplinary papers published in G‐Cubed can be challenging. As Editors and Associate Editors, we would like ...to give our appreciation to all reviewers and would like to acknowledge them in this editorial. G‐Cubed published 326 manuscripts out of 650 submissions in 2019, thanks on the efforts of 860 dedicated reviewers. Their names are listed below, and in italics are those who provided three or more reviews. A big thank you from the G‐Cubed team!
Key Point
The editors thank the 2019 peer reviewers
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