Mantle dynamics and geodesy

Both completed work and work that is still in progress are presented. The completed work presented includes: (1) core-mantle boundary topography; (2) absolute value for mantle viscosity; (3) code development; (4) lateral heterogeneity of subduction zone rheology; and (5) planning for the Coolfront meeting. The work presented that is still in progress includes: (1) geoid anomalies for a chemically stratified mantle; and (2) geoid anomalies with lateral variations in viscosity

Mantle dynamics and volcanism emplacement in the Azores

The Azores plateau is a triangular shaped topographic feature encompassing the boundary zone where three major tectonic plates (EU, NU and NA) meet. The eastern side of the plateau is delimited by two major tectonic discontinuities: the Mid Atlantic Ridge, and the Terceira Rift, a recently formed ultra-slow-spreading ridge. The origin of the plateau is still under debate. One hypothesis argues that the plateau would have been formed by successive NE jumps of the oblique spreading axis, where the present TR is the latest stage. Other hypotheses invoke the northward jump of the Azores triple junction, during which the Azores region would have been transferred from the Eurasian plate to the Nubian plate. For some authors, the presence of the Azores plume, a low seismic velocity zone in the mantle beneath, is required to explain the observations: the anomalously shallow seafloor depth as well as the geochemistry of the basaltic lavas erupted within the plateau. Here we use a highly resolved tomography model to quantify the influence of this plume and the surrounding mantle.We model the convection pattern, the induced dynamic topography and stresses, and compare them with the surface observations. The dynamic topography shows two maxima: one northwest of St. Miguel, the other encompassing the Terceira, Graciosa, S. Jorge, Faial and Pico islands. Both swells are approximately located on the Terceira Ridge. The convection pattern displays two distinct upwelling towards these two groups of Islands. This may explain the difference in the geochemical signatures, in particular the unique isotopic ratios observed in some lavas from S. Miguel. The stresses induced by the underlying mantle convection are compared with the surface observations (topographic features, seismic and GPS velocities). The modeled and observed tresses fairly correlate west of our study area but their directions depart east

Gravitational field models for study of Earth mantle dynamics

The tectonic forces or stresses due to the small scale mantle flow under the South American plate are detected and determined by utilizing the harmonics of the geopotential field model. The high degree harmonics are assumed to describe the small scale mantle convection patterns. The input data used in the derivation of this model is made up of 840,000 optical, electronic, and laser observations and 1,656 5 deg x 5 deg mean free air anomalies. Although there remain some statistically questionable aspects of the high degree harmonics, it seems appropriate now to explore their implications for the tectonic forces or stress field under the crust

Sr-Nd-Pb-Hf isotope results from ODP Leg 187: Evidence for mantle dynamics of the Australian-Antarctic Discordance and origin of the Indian MORB source

New high precision PIMMS Hf and Pb isotope data for 14–28 Ma basalts recovered during ODP Leg 187 are compared with zero-age dredge samples from the Australian-Antarctic Discordance (AAD). These new data show that combined Nd-Hf isotope systematics can be used as an effective discriminant between Indian and Pacific MORB source mantle domains. In particular, Indian mantle is displaced to lower εNd and higher εHf ratios compared to Pacific mantle. As with Pb isotope plots, there is almost no overlap between the two mantle types in Nd-Hf isotope space. On the basis of our new Nd-Hf isotope data, we demonstrate that Pacific MORB-source mantle was present near the eastern margin of the AAD from as early as 28 Ma, its boundary with Indian MORB-source mantle coinciding with the eastern edge of a basin-wide arcuate depth anomaly that is centered on the AAD. This observation rules out models requiring rapid migration of Pacific MORB mantle into the Indian Ocean basin since separation of Australia from Antarctica. Although temporal variations in isotopic composition can be discerned relative to the fracture zone boundary of the modern AAD at 127°E, the distribution of different compositional groups appears to have remained much the same relative to the position of the residual depth anomaly for the past 30 m.y. Thus significant lateral flow of mantle along the ridge axis toward the interface appears unlikely. Instead, the dynamics that maintain both the residual depth anomaly and the isotopic boundary between Indian and Pacific mantle are due to eastward migration of the Australian and Antarctic plates over a stagnated, but slowly upwelling, slab oriented roughly orthogonal to the ridge axis. Temporal and spatial variations in the compositions of Indian MORB basalts within the AAD can be explained by progressive displacement of shallower Indian MORB-source mantle by deeper mantle having a higher εHf composition ascending ahead of the upwelling slab. Models for the origin of the distinctive composition of the Indian MORB-source based on recycling of a heterogeneous enriched component that consist of ancient altered ocean crust plus<10% pelagic sediment are inconsistent with Nd-Hf isotope systematics. Instead, the data can be explained by a model in which Indian mantle includes a significant proportion of material that was processed in the mantle wedge above a subduction zone and was subsequently mixed back into unprocessed upper mantle

Mountain building and mantle dynamics

International audienceMountain building at convergent margins requires tectonic forces that can overcome frictional resistance along large-scale thrust faults and support the gravitational potential energy stored within the thickened crust of the orogen. A general, dynamic model for this process is still lacking. Here we propose that mountain belts can be classified between two end-members. First, those of "slab pull" type, where subduction is mainly confined to the upper mantle, and rollback trench motion lead to moderately thick crustal stacks, such as in the Mediterranean. Second, those of "slab suction" type, where whole-mantle convection cells ("conveyor belts") lead to the more extreme expressions of orogeny, such as the largely thickened crust and high plateaus of present-day Tibet and the Altiplano. For the slab suction type, deep mantle convection produces the unique conditions to drag plates toward each other, irrespective of their nature and other boundary conditions. We support this hypothesis by analyzing the orogenic, volcanic, and convective history associated with the Tertiary formation of the Andes after ~40 Ma and Himalayas after collision at ~55 Ma. Based on mantle circulation modeling and tectonic reconstructions, we surmise that the forces necessary to sustain slab-suction mountain building in those orogens derive, after transient slab ponding, from the mantle drag induced upon slab penetration into the lower mantle, and from an associated surge of mantle upwelling beneath Africa. This process started at ~65-55 Ma for Tibet-Himalaya, when the Tethyan slab penetrated into the lower mantle, and ~10 Myr later in the Andes, when the Nazca slab did. This surge of mantle convection drags plates against each other, generating the necessary compressional forces to create and sustain these two orogenic belts. If our model is correct, the available geological records of orogeny can be used to decipher time-dependent mantle convection, with implications for the supercontinental cycle

implication for lunar mantle dynamics

학위논문(석사) -- 서울대학교대학원 : 자연과학대학 지구환경과학부, 2022. 8. 이성근.The evolution of the lunar mantle melts since the moon-forming giant impact event remains poorly understood. The mantle melts contain substantially more Ti compared to melts found on Earth. Therefore, evolution of the Moon might be affected by its Ti-rich mantle, as melt properties like viscosity and density are most likely influenced by these compositional changes. By incorporating Ti in simple Na trisilicate glasses and sodium aluminosilicate glasses, structural changes in this glass can be observed. This allows us to provide more information about the possible influence of Ti on the physical properties of the lunar melts. To obtain structural information from the Ti-bearing Na trisilicate glass, 29Si- 17O- and 27Al high resolution Nuclear Magnetic Resonance(NMR) spectroscopy were used. The 29Si NMR technique reveals detailed structures around the Si environment, particularly Qn species information where n is the number of Bridging Oxygen(BO) of a single Si tetrahedron. The 17O NMR technique provides detailed information about the O-bonds within the glass samples. In general terms, Ti(network former) replaces the amount of Na(network modifier) in the glass while the amount of Si remains unchanged. For the endmember Na trisilicate(x=0), at -90 ppm and -104 ppm, two peaks appear on the 29Si NMR spectra that represent Q3 and Q4 species respectively where the Q3 peak at -90 ppm is more dominant. This Q3 peak has a Q fraction of around 0.6 while the peak representing Q4 makes up the remaining fraction. Overall, the Q4 peak increases at the expense of the Q3 peak with increasing the Ti content. When 18.75 mol% Ti is incorporated in the glass, the Q species fractionation has completely changed where the Q4 species clearly became the dominant fraction. This suggests that the number of Si surrounded by only BO would increase resulting in an increase of the network polymerization of the melt. From the 17O data a similar observation can be made. The Na-O-Si bonds are gradually replaced by Si-O-Si and Ti-O-Si bonds with increasing Ti content. Also, some fraction of Ti-O-Ti forms. For the aluminosilicate glasses, no Ti-O-Al bond was observed indicating a clear titanium and aluminium avoidance. The obtained structural knowledge regarding the incorporation of Ti on the network polymerization of the silicate glasses, allows us to make a direct link between the Ti content and the viscosity of the melt. Less upper mantle mixing could affect the previously believed timespan of magma ocean cooling to be much longer as heat is meanly being transferred with conduction. Because of the Lunar mantle overturn model driven by the higher density of the IBC layer, Ti coordination is believed to change from [4]Ti to [6]Ti. The Ti becomes a network modifier, lowering the viscosity of the melt with increased mantle mixing in the lower mantle. The simple melt used in this study lays a solid foundation for further research regarding lunar melts with a more realistic melt composition or environmental conditions and could therefore help to constrain the possibilities of lunar mantle dynamics and lunar magma ocean evolution.Abstract 2 1. Introduction 10 1.1. Understanding the lunar interior 10 1.2. Effects of Ti on the structure of glasses 19 2. Experimental methods 24 2.1. Sample composition 24 2.2. Heating and quenching 26 2.3. XRD analyses 28 2.4. NMR Spectroscopy 30 3. Results 31 3.1. Ti-bearing sodium trisilicate glasses 31 3.2. Ti-bearing sodium aluminosilicate glasses 38 4. Discussion 47 4.1. Q species fitting 47 4.2. Glass network structure 49 4.3. Geological implications 52 5. Conclusion 55 5.1. The effect of Ti on sodium tri- and aluminosilicate glasses. 55 5.2. Lunar mantle dynamics implications 55 6. Appendix section 57 References 63석

Evolution of attached and detached slabs and their associated mantle dynamics

Over the two years of the NASA grant, this project has produced a significant amount of research results related to the plate subduction process and the surface crustal deformation at convergent boundaries (i.e., above subduction zones). While some research objectives are completely accomplished, other research tasks remain active and continue to be investigated at present. A steady state analytic thermal model for subducting slabs was used to examine the torques acting on a descending slab. It is found that gravitational torque vanishes when a slab is dipping either vertically or horizontally, unlike previous studies indicating that the magnitude of gravitational torque decreases as dip angle increases. Subsequently, a new time-dependent, analytic thermal model for a subducting slab was developed. The new model enables us to study transient phenomena associated with plate subduction analytically. On the basis of this model, the nature of slab dip angles was evaluated. Slab dip angles are found to be transient features. As they penetrate into the mantle and increase their lengths, the associated gravitational torque also increases resulting in a downward pulling of the slab to the steeper dip angle. This is especially true once a slab penetrates the olivine-spinel phase boundary at about 400 km depth. However, if the phase transformation does not follow the equilibrium condition, the gravitational torque may have a different behavior. This problem was investigated. Except for fast descending slabs, non-equilibrium phase transformation can only slow down the transient increase of slab dip angles discussed earlier. Its effect is not sufficiently strong to reverse the downward pulling for most of the slabs. However, when slabs subducting at 10 cm/yr or faster, a sufficient amount of metastable olivine can exist beneath 400 km. Because of its low density compared with the surrounding spinel, an upward buoyancy is produced resulting in an upward bending of the slab and possibly an upward rotation of the slab such that smaller dip angles are formed. Seismic studies of the Japanese Slab seem to support this interpretation. The development of oroclinal geometries at convergent boundaries was also examined to study plate obduction which is an important ingredient to the initiation of plate subduction. Although the study suggests that surface features are better modeled by block models, the large scale deformation can be adequately studied by viscous models. Such a model is now under development to complete our original objective to study the initiation of plate subduction. Finally, a three-dimensional, finite element, spherical convective model is developed to study dynamic plate subductions. The model development is now complete and it is being tested to ensure its proper operation. The model is able to generate convection results with a viscosity contrast of about 100. Our research continues to push the viscosity contrast to a level that is appropriate for a subducting slab

Tertiary-Quaternary intra-plate magmatism in Europe and its relationship to mantle dynamics

Anorogenic intra-plate magmatism was widespread in Europe from early Tertiary to Recent times, extending west to east from Spain to Bulgaria, and south to north from Sicily to northern Germany. Magmatism is spatially and temporally associated with Alpine-Pyrenean collisional tectonics, the development of an extensive lithospheric rift system in the northern foreland of the Alps, and, locally, with uplift of Variscan basement massifs (Massif Central, Rhenish Massif, Bohemian Massif). The volcanic regions vary in volume from large central volcanoes (e.g. Cantal, Massif Central;Vogelsberg, northern Germany), to small isolated plugs (e.g. Urach and Hegau provinces in southern Germany). Within the Mediterranean region, the Dinarides, the Pannonian Basin and Bulgaria, anorogenic volcanism locally post-dates an earlier phase of subduction-related magmatism. The major and trace element and Sr-Nd-Pb isotope characteristics of the most primitive mafic magmatic rocks (MgO > 6 wt %) provide important constraints on the nature of the mantle source and the conditions of partial melting.. These are predominantly sodic (melilitites, nephelinites, basanites and alkali olivine basalts); however, locally, potassic magma types (olivine leucitites, leucite nephelinites) also occur. In several localities (e.g., Sicily; Vogelsberg and the Rhine Graben, Germany; Calatrava, central Spain) olivine- and quartz-tholeiites form a significant component of the magmatism. The sodic magmas were derived by variable degrees of partial melting (~ 0.5 - 5 %) within a transitional zone between garnet-peridotite and spinel-peridotite mantle facies, close to the base of the lithosphere; the potassic magma types are interpreted as partial melts of enriched domains within the lithospheric mantle. Mantle partial melting was induced by adiabatic decompression of the asthenosphere, locally in small-scale, plume-like, diapirs which appear to upwell from ~ 400 km depth

Deformation-Induced Mechanical Instabilities at the Core-Mantle Boundary

Post-Perovskite: The Last Mantle Phase Transition Our understanding of the core-mantle boundary (CMB) region has improved significantly over the past several years due, in part, to the discovery of the post-perovskite phase. Sesimic data suggest that the CMB region is highly heterogeneous, possibly reflecting chemical and physical interaction between outer core material and the lowermost mantle. In this contribution we present the results of a new mechanism of mass transfer across the CMB and comment on possible repercussions that include the initiation of deep, siderophile-enriched mantle plumes. We view the nature of core-mantle interaction, and the geodynamic and geochemical ramifications, as multiscale processes, both spatially and temporally. Three lengthscales are defined. On the microscale (1-50 km), we describe the effect of loading and subsequent shearing of the CMB region and show how this may drive local flow of outer core fluid upwards into D". We propose that larger scale processes operating on a mesoscale (50-300 km) and macroscale regimes (> 300 km) are linked to the microscale, and suggest ways in which these processes may impact on global mantle dynamics