Garnet-controlled very low velocities in the lower mantle transition zone at sites of mantle upwelling

Deep mantle plumes and associated increased geotherms are expected to cause an upward deflection of the lower–upper mantle boundary and an overall thinning of the mantle transition zone between about 410 and 660 kilometres depth. We use subsequent forward modelling of mineral assemblages, seismic velocities and receiver functions to explain the common paucity of such observations in receiver function data. In the lower mantle transition zone, large horizontal differences in seismic velocities may result from temperature‐dependent assemblage variations. At this depth, primitive mantle compositions are dominated by majoritic garnet at high temperatures. Associated seismic velocities are expected to be much lower than for ringwoodite‐rich assemblages at undisturbed thermal conditions. Neglecting this ultra‐low‐velocity zone at upwelling sites can cause a miscalculation of the lower–upper mantle boundary on the order of 20 kilometres

Anorogenic plateau formation: The importance of density changes in the lithosphere

International audienceAway from active plate boundaries the relationships between spatiotemporal variations in density and geothermal gradient are important for understanding the evolution of topography in continental interiors. In this context the classic concept of the continental lithosphere as comprising three static layers of different densities (upper crust, lower crust, and upper mantle) is not adequate to assess long-term changes in topography and relief in regions associated with pronounced thermal anomalies in the mantle. We have therefore developed a one-dimensional model, which is based on thermodynamic equilibrium assemblage computations and deliberately excludes the effects of melting processes like intrusion or extrusions. Our model calculates the "metamorphic density" of rocks as a function of pressure, temperature, and chemical composition. It not only provides a useful tool for quantifying the influence of petrologic characteristics on density, but also allows the modeled "metamorphic" density to be adjusted to variable geothermal gradients and applied to different geodynamic environments. We have used this model to simulate a scenario in which the lithosphere-asthenosphere boundary is subjected to continuous heating over a long period of time (130 Ma), and demonstrate how an anorogenic plateau with an elevation of 1400 m can be formed solely as a result of heat transfer within the continental lithosphere. Our results show that, beside dynamic topography (of asthenospheric origin), density changes within the lithosphere have an important impact on the evolution of anorogenic plateaus

Iterative thermodynamic modelling—Part 1: A theoretical scoring technique and a computer program ( Bingo‐Antidote )

This paper introduces the software solution Bingo‐Antidote for thermodynamic calculations at equilibrium based on iterative thermodynamic models. It describes a hybrid strategy combining the strength of Gibbs energy minimization (GEM) and inverse thermobarometry models based on the comparison between the modelled and observed mineral assemblage, modes and compositions. The overall technique relies on quantitative compositional maps acquired by electron probe micro‐analyser for obtaining a mutually consistent set of observed data such as bulk rock and mineral compositions. Thus it offers the opportunity to investigate metamorphic rocks on a microscale. The scoring part Bingo integrates three statistical model quality factors urn:x-wiley:02634929:media:jmg12538:jmg12538-math-0001 for the assemblage, urn:x-wiley:02634929:media:jmg12538:jmg12538-math-0002 for the mineral modes, urn:x-wiley:02634929:media:jmg12538:jmg12538-math-0003 for the mineral compositions combined in a global evaluation criterion urn:x-wiley:02634929:media:jmg12538:jmg12538-math-0004 that quantifies how the model reproduces the observations for the investigated volume. The input parameters of GEM affecting the model quality such as pressure, temperature and eventually some components of the bulk composition (e.g. the molar amount of hydrogen, carbon or oxygen) or activity variables of fluids and gases (e.g. urn:x-wiley:02634929:media:jmg12538:jmg12538-math-0005, urn:x-wiley:02634929:media:jmg12538:jmg12538-math-0006, f(O2)) can be optimized by inversion in Antidote using several mapping stages followed by a direct search optimization. Examples of iterative models based on compositional maps processed with Bingo‐Antidote demonstrate the utility of the program. In contrast to the qualitative interpretation of phase diagrams, the inversion maximizes the benefits of GEM and permits the derivation of statistically ‘optimal’ pressure–temperature conditions for well‐equilibrated samples. In addition, Bingo‐Antidote opens new avenues for petrological investigations such as the generation of chemical potential landscape maps

Modeling Metamorphic Rocks Using Equilibrium Thermodynamics and Internally Consistent Databases: Past Achievements, Problems and Perspectives

The astonishing progress of personal computer technology in the past 30 years as well as the availability of thermodynamic data and modeling programs have revolutionized our ability to investigate and quantify metamorphic processes. Equilibrium thermodynamics has played a central role in this revolution, providing simultaneously a physico-chemical framework and efficient modeling strategies to calculate mineral stability relations in the Earth’s lithosphere (and beyond) as well as thermobarometric results. This Perspectives contribution provides a review of the ingredients and recipes required for constructing models. A fundamental requirement to perform thermodynamic modeling is an internally consistent database containing standard state properties and activity–composition models of pure minerals, solid solutions, and fluids. We demonstrate how important internal consistency is to this database, and show some of the advantages and pitfalls of the two main modeling strategies (inverse and forward modeling). Both techniques are commonly applied to obtain thermobarometric estimates; that is, to derive P–T (pressure–temperature) information to quantify the conditions of metamorphism. In the last section, we describe a new modeling strategy based on iterative thermodynamic models, integrated with quantitative compositional mapping. This technique provides a powerful alternative to traditional modeling tools and permits use of local bulk compositions for testing the assumption of local equilibrium in rocks that were not fully re-equilibrated during their metamorphic history. We argue that this is the case for most natural samples, even at high-temperature conditions, and that this natural complexity must be taken into consideration when applying equilibrium models

Imaging passive margins: a petrological perspective

International audienceWhile several tectonic models (e.g., simple shear, detachment or mantle exhumation) are proposed to explain theformation of extensive basins and passive margins; a single thermal model (McKenzie, 1978), as a kind of dogma,is used to model the formation and evolution of sedimentary basins. The thermal evolution of such basins, coupledwith other tectonic models, have been scarcely studied in detail. For instances, petrological changes (i.e. phasetranisitions), related to temperature changes, affect rock density and thus influence the subsidence history of thebasin. Recent studies of continental passive margins collectively describe a great number of processes accountingfor the extreme thinning of the continental crust. Among all the parameters that may act during crustal stretching,the thermal state of the system and the temporal evolution of the heat distribution during thinning appear of majorimportance.We explore the effect of different thermal evolution models on petrological changes and their consequences on thegeophysical signature of rifted zones.We will present computed geodynamic models quantifying mineralogical and physical changes in the lithosphereduring rifting processes and early margin formation. In the light of these high temperature evolution modelssupported by new field data from the north Pyrenean basins, we discuss the effect on subsidence as well as ongravimetric and seismic velocities signatures of passive margins.Consequently, we are able to distinguish two types of margins according to their thermal evolution:- An Alpine-type basin in which the temperature increase is 50 to 100 Ma older than the tectonic extension,leading to the "cold" opening of the ocean.- A Pyrenean type basin where heating is coincident with basin formation, leading to a crustal boudinage andformation of an “anomalous” geophysical layer at the OC

Thermal evolution of early passive marginsformation and consequences on theirgeophysical signature

National audienceMany large-scale dynamic processes, from continental riftingto plate subduction, are intimately linked to metamorphic reactions.This close relation between geodynamic processes andmetamorphic reactions is, in spite of appearances, yet poorly understood.For example, during extension processes, rocks will beexposed to important temperature, pressures and stress changes.Meanwhile less attention has been paid to other important aspectsof the metamorphic processes. When reacting rocks expand andcontract, density and volume changes will set up in the surroundingmaterial.While several tectonic models are proposed to explain the formationof extensive basins and passive margins ( simple sheardetachment mantle exhumation .... ) a single thermal model(McKenzie, 1978), as a kind of dogma, is used to understandingand modeling the formation and evolution of sedimentary basins.The study of the thermal evolution, coupled with other tectonicmodels, and its consequences have never been studied in detail,although the differences may be significant. And it is clear thatthe petrological changes associated with changes in temperatureconditions, influence changes reliefs.Constrained by the new field data of north Pyrenean basins onthermal evolution of pre-rift and syn-rift sediments, we explorethe petrological changes associated to different thermal evolutionand the consequences on the subsidence of the basins. We willalso present numerical models quantifying mineralogical and physicalchanges inside the whole lithosphere during rifting processes.In the light of these models, we discuss the consequences of differentthermal evolution on the subsidence processes as well as ongravimetry and seismic velocities signature of passive margins

Constraining the pressure-temperature evolution and geodynamic setting of UHT granulites and migmatitic paragneisses of the Gruf Complex, Central Alps

Thermodynamic modeling of compositionally mapped microdomains and whole-rock compositions is used to constrain the pressure–temperature (P–T) evolution of sapphirine granulites and migmatitic paragneisses from the Gruf Complex of the Central Alps. The P–T paths and conditions estimated from granulite microdomains and whole-rock compositions are consistent with one another, indicating that the estimates from both types of compositions are accurate. The sapphirine granulites were heated to ultra-high temperature conditions of 900–1000 °C and 7.0–9.5 kbar as they were decompressed from ca. 800 °C and 9–12 kbar, resulting in garnet breakdown. In a subsequent step, nearly isothermal decompression led to the development of cordierite-bearing coronae and symplectites. By ca. 27 Ma, the sapphirine granulites had been exhumed to the midcrustal level of the migmatitic paragneisses, which were undergoing peak metamorphism at ca. 675–750 °C and 5–7 kbar. These results are consistent with a geodynamic model that invokes heat advection to the lower crust closely following the continental-subduction (ultra-high pressure) stage of the Alpine orogeny. The most plausible geodynamic model consistent with the results of this study is breakoff of a southward subducting lithospheric slab, resulting in asthenospheric mantle flow.This research is supported by the American National Science Foundation under Grant no. EAR 0911633 to A. Möller

Metamorphic density changes as key process to form anorogenic plateaus

International audienceAnorogenic plateaus are those topographic barriers that reach medium elevations of approximately 1500 m, e.g. South African-, East African- or Mongolian Plateau. They are inferred to be closely link to mantle plumes away from plate boundaries. Actually, plateau formation processes in geodynamic settings outside of orogens have not been unambiguously established. Recently, Wichura et al. [1] have clearly shown a pre-rift uplift of the East African plateau. They suggested pre-rift topographic variations by lithospheric thermal expansion, due to mantle plume-lithosphere heat interactions. Following their assumption, we developed an one-dimensional model, which calculates density as a function of pressure, temperature, and chemical composition, based on the fact that heat variations in the continental lithosphere and crust influences metamorphic density. Thus, we present a new petrologic aspect for plateau uplift [2], because models on plateau uplift generally do not take into account the effects of metamorphic phase transitions and ignore the fact that chemical reactions influence both the stability of mineral assemblages and rock density. Our model underscores how metamorphic density of the lithosphere varies with depth and reveals how combination of chemical composition of rocks, mineralogy, and geothermal gradient all have significant effects on the density distribution within the lithosphere and ultimately the evolution of anorogenic plateaus. Thus, we show that metamorphic phase transitions in crust and lithospheric mantle due to heating at the lithosphere-asthenosphere boundary by a mantle plume are key processes that drive significantly uplift and the generation of long-wavelength topography. Furthermore, in order to better understand the temporal characteristics of mantle plume related topography we calculated the timing to generate significant topographic uplift. Our results are very instructive and suggest considerable primary thermal uplift of approximately 1400 m as a viable mechanism for anorogenic plateau formation. In this way, our model may help to explain pre-rift topography of the East-African Plateau, related to heat generated and transferred by the activity of a mantle plume. In addition, we show that density-change models that ignore metamorphic processes and/or mineral reactions will result in a reduced amount of uplift or may require inadequate temperatures to explain uplift scenarios

The role of excess oxygen for modeling high-Mn, low-Ca garnets in metapelites from the northern Central Metasedimentary Belt of the Grenville Province, Ontario, Canada

The Bancroft terrane and the associated Central Metasedimentary Belt boundary thrust zone represent the northern part of the Central Metasedimentary Belt (CMB) of the Canadian Grenville Province. Only a few direct pressure and temperature calculations based on phase equilibrium petrology methods exist in the central Bancroft terrane, and this study applies thermodynamic approaches such as garnet isopleth geothermobarometry to fill this gap and investigate the metamorphic history of the northern CMB. Four metapelitic rock samples were collected in the vicinity of the enigmatic Bancroft shear zone, which approximates the border between the Bancroft terrane and the Elzevir terrane to the south. Garnet isopleths for these samples only intersect if a certain amount of excess oxygen is added to the bulk rock composition corresponding to a Fe3+/Fetot ratio of 0.33–0.38. The northernmost sample records metamorphic peak conditions of approximately 1 GPa and 780 °C, whereas the southernmost sample, which is located in the Elzevir terrane, records a peak metamorphic pressure of approximately 0.9 GPa at a temperature of 520 °C. The latter result contradicts previous pressure estimates of the region and the proposed metamorphic field gradient but is based on a poorly constrained sample in terms of thermodynamic modeling. Hence, we conclude that the metamorphic field gradient in the northern CMB conceals two different P–T trajectories. Such a scenario is commonly observed in crustal thickening models and suggests that the cold upper plate (Elzevir terrane) was thrust over the warm lower plate (Bancroft terrane) in a northwesterly direction.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Discrepancies of mineral volumes predicted by thermodynamic databases

International audienceThermodynamic databases are an essential tool to predict complex equilibrium mineral assemblages and mineral properties like mineral volumes. They consist of numerous thermodynamic data of various minerals, extracted from experiments. Each database follows its own methodology in calculating chemical and physical properties. Therefore a direct comparision between different database predictions was avoided, due to the contrasting methodolgies and philosophy. Here, we present a direct comparison between the databases of Berman [1] and Holland & Powell [2][3], focusing on mineral volumes [4]. For this propose, a reevaluation of the equation of states was nesscary. In this context, we identify an error also implemented in common thermodynamic softwares, concerning the calculation of excess volume. Even after treating the excess energy correctly, volumes show significant discrepancies between the different database predictions. These discrepancies impact geodynamic interpretations and geothermobarometrical estimations, due to the fact that the Gibbs free energy and rock density depends on mineral volumes. The imagination that pressure can vary by 4 kbar, temperature by 150 C or rock-density up to 30 %, by changing the thermodynamic database is dramatic. These enormous differences must be considered keeping in mind that calculations were done for well studied minerals (e.g. quartz and forsterite). The results play an important role for studies of geodynamic interpretations extracted from thermobarometric software packages like Perple_X, Theriak-Domino or Thermocalc. It is important to estimate the influence of the thermodynamic database on Gibbs free energy, volume and rock density. Summarizing, more experimental data will lead to a better comprehension of these discrepancies