Upper-mantle Transition Zone Research Articles

Constraining seismic anisotropy on the mantle transition zone boundaries beneath the subducting Nazca slab

Some seismic evidence suggests that the mantle transition zone (MTZ) may become highly hydrous and anisotropic, particularly in the vicinity of subduction zones. The two-dimensional path-integrated anisotropy from the upper mantle to the MTZ has been well established beneath the northwestern region of South America. However, explicit details of azimuthal anisotropy on the MTZ boundaries remains ambiguous. Therefore, we attempted to constrain the azimuthal anisotropy on the MTZ boundaries by implementing the P-to-S anisotropic receiver function analysis. We detected significant seismic evidence of azimuthal anisotropy on the 410-km discontinuity, but weak anisotropy on the 660-km discontinuity. The synthetic waveform modeling indicated the fast symmetry axis of anisotropy trends 50° from the north and plunges 40° downwards from horizontal with an anisotropy strength of 4.0% near 410 km depth. The direction of anisotropy suggests the mantle material moves downwards and towards the subducting Nazca slab near the depth of 410 km. The increased anisotropy strength around the 410 km suggests the hydrous wadsleyite may attribute to anisotropy in the upper MTZ. The lack of detectable seismic anisotropy near the depth of 660 km could be caused by the insufficient amount of aligned anisotropic minerals, even though the mantle material continues moving downwards.

Small-scale heterogeneities in the convective upper mantle beneath circum-Pacific subduction zones: Evidence for fragments of recycled basaltic crust

We show evidence for the presence of compositional heterogeneities at 10 km scale in the convective upper mantle below the circum-Pacific subduction zones. By array-processing we examine seismograms of intermediate-depth earthquakes recorded by large-aperture seismic networks at teleseismic distances. In the P coda until about 25 s after direct P waves, we observe anomalous and significant signals arriving from the directions close to the P waves. The amplitude and duration of the signals decrease with focal depth within the range from about 180 to 300 km. We show that the signals arise mainly from S-to-P scattering in the mantle below the foci. Using the scattering theory for randomly heterogeneous media we also show that the observed features of the signals are matched by 5 to 10 km-scale density and rigidity anomalies that diminish below about 400 km depth. The rigidity anomaly is of the same sign and nearly twice as large as the density anomaly. These properties of the scattering objects could be consistent with basalt embedded in depleted mantle. The results indicate that the upper mantle transition zone beneath subducting slabs is not well mixed even by vigorous convection.

The Upper‐Mantle Structure Beneath Alaska Imaged by Teleseismic S‐Wave Reverberations

AbstractAlaska is a tectonically active region with a long history of subduction and terrane accretion, but knowledge of its deep seismic structure is limited by a relatively sparse station distribution. By combining data from the EarthScope Transportable Array and other regional seismic networks, we obtain a high‐resolution state‐wide map of the Moho and upper‐mantle discontinuities beneath Alaska using teleseismic SH‐wave reverberations. Crustal thickness is generally correlated with elevation and the deepest Moho is in the region with basal accretion of the subducted Yakutat plate, consistent with its higher density due to a more mafic composition. The crustal thickness in the Brooks Range agrees with the prediction based on Airy isostasy and the weak free‐air gravity anomaly, suggesting that this region probably does not have significant density anomalies. We also resolve the 410, 520, and 660 discontinuities in most regions, with a thickened mantle transition zone (MTZ) and a normal depth difference between the 520 and 660 discontinuities (d660‐d520) under central Alaska, indicating the presence of the subducted Pacific slab in the upper MTZ. A near‐normal MTZ and a significantly smaller d660‐d520 are resolved under southeastern Alaska, suggesting potential mantle upwelling in the lower MTZ. Beneath the Alaska Peninsula, the thinned MTZ implies that the Pacific slab may not have reached the MTZ in this region, which is also consistent with recent tomography models. Overall, the results demonstrate a bent or segmented Pacific slab with varying depths under central Alaska and the Alaska Peninsula.

Mid-mantle seismic scatterers beneath the Samoan hotspot

We analyze teleseismic array data of deep and intermediate-depth earthquakes in the Fiji-Tonga-Kermadec region recorded at seismic networks at the west coast of US and Alaska with the aim of mapping seismic scatterers in the lower mantle and in the upper mantle transition zone beneath the Samoan hotspot. Short period and broadband waveform data are array-processed in order to detect coherent but anomalous phases in the P coda up to nearly 120 s after direct P waves and locate where they arrive from. Updating the data set reported in our previous study (Kaneshima and Helffrich, 2010; Kaneshima, 2018; Kaneshima, 2019), we examine nine scatterers near the hotspot, seven in the lower mantle and two in the transition zone, with the main focus on three lower mantle scatterers nearest to the hotspot at 1000 to 1200 km depths. We observe S-to-P waves generated from the three lower mantle scatterers for earthquakes with a wide range of hypocenters from 17°S to 24°S, while hardly any signs of corresponding P-to-P scattering waves are observed. The pulse shapes of the S-to-P waves from the three scatterers are simple and their pulse widths do not significantly differ from those of direct P waves when the signal quality is high enough. The geometry of the scatterers suggested by these observations is a steeply dipping sheet-like structure with the thickness less than several kilometers. The material consisting of the sheet needs to have a Vs value at least several percent lower than the surrounding mantle, as required by the amplitudes of S-to-P scattering waves relative to P, which is typically several percent. The low Vs anomaly of basaltic rocks relative to pyrolite under the shallow lower mantle conditions may be consistent with the observations (Wang et al., 2020). Seismic tomography models suggest that the nine scatterers tend to be located around the edge of a plume that ascends from the Pacific LLSVP to the Samoan hotspot. Numerical simulations of ascending plumes from a thermo-chemical pile at the bottom of the mantle show that the plumes tend to rise preferentially in the vicinity of the edge of the pile. Other simulations also show that blobs of recycled former oceanic crust subducting to the bottom of mantle are entrained into a plume and stretched vertically to a significant degree, consistently with the images suggested by geochemical studies of OIB. The scattering observations therefore may elucidate the structure of entrained basaltic blobs on their way of returning back to the surface hotspot.

Joint inversion of PP and SS precursor waveforms and Rayleigh wave phase velocities for global mantle transition zone structure

SUMMARYWe have compiled a new data set of global PP and SS precursor waveforms that we jointly invert in combination with fundamental-mode and higher-order Rayleigh-wave phase velocities for upper mantle and mantle transition zone (MTZ) structure. We observe clear S410S, S520S, S660S and P410P precursor arrivals, but not P660P, because of interfering phases. Traveltimes and amplitudes of precursor phases reflect a complex interplay of data and modelling factors, implying that MTZ structure is best resolved through direct inversion of waveforms. To model waveforms as accurately as possible, we account for effects arising from data processing, shallow structure, incoherent stacking, attenuation and source effects, among others. As part of the inversion, we consider two independent model parametrizations to obtain quantitative insights into the seismic and thermochemical constitution of the MTZ. These include a ‘classical’ seismic parametrization based on a layered seismic velocity structure and a thermodynamic parametrization, where seismic profiles are self-consistently built from mineral physics data. The results show lateral variations in thermal, compositional and discontinuity structure that partly correlate with tectonic setting. The mantle beneath continents and subduction zones is found to be colder in comparison to oceans and hotspots as reflected in MTZ thickness. In terms of composition, we find that subduction zones are enriched in basalt. Mid-MTZ structure shows a trend from simple sub-ocean single- to complex circum-Pacific subduction-zone-related dual-discontinuity structure—the possible signature of oceanic crustal transport to the MTZ. Statistical analysis indicates that a mechanically mixed mantle matches seismic data better than an equilibrated mantle across ∼2/3 of the globe. Finally, while a large part of the seismic data can be matched by an iso-chemical and adiabatic mantle, complexities within the MTZ are not entirely captured by this assumption.

Sound Velocity of (Mg0.91Fe0.09)2SiO4 Wadsleyite and Its Implications to Water Distribution in Mantle Transition Zone

AbstractThe current elasticities of major mantle transition zone minerals and their hydrous counterparts, such as wadsleyite, ringwoodite, and majorite garnet, have the velocity gradients several times lower than those of the Earth models. Here we use ultrasonic interferometry technique to measure the sound velocities of wadsleyite (Mg0.91Fe0.09)2SiO4 at pressures up to 14 GPa and room temperature. Compared with previous studies, we have obtained possibly the highest ∂VP/P and ∂KS/P value at transition zone pressure, which are still noticeably lower than the Earth models, such as the preliminary reference Earth model, ak135, iasp91, etc. The results indicate that the VP contrast of “dry” olivine‐wadsleyite transition at 410 km is higher than previous thought, while that of wadsleyite‐ringwoodite transition at 520 km is noticeably low. Using the current and previous data on “dry” and hydrous wadsleyite a water “gradient” model at upper mantle transition zone were proposed and discussed.

High‐pressure Raman study of Na‐Ca burbankite—A possible CO2 host in deep mantle

AbstractNa‐Ca carbonates synthesized in high‐pressure experiments and found in mineral inclusions inside diamonds, exhumed from mantle depths, attract a lot of attention as potential hosts of subducted CO2 in the deep mantle and sources of carbonate‐rich magmas, involved in mantle metasomatism and diamond formation. However, the Na‐Ca carbonates synthesized at pressures of mantle transition zone (13–24 GPa) were not characterized structurally or spectroscopically. At the same time, the stability of well‐characterized Na‐Ca carbonates synthesized at lower pressures (3–6 GPa) at higher pressures was never studied in in situ experiments. Here, we report an in situ Raman study of Na‐Ca burbankite, Na2Ca4(CO3)5, synthesized at 6 GPa using diamond anvil cell technique up to the pressures of 24 GPa, corresponding to the mantle transition zone. The obtained results link the observations from experiments corresponding both to the upper mantle and mantle transition zone by demonstration of structural stability of Na‐Ca burbankite and the absence of phase transitions up to 24 GPa (at least at ambient temperature). The latter allows to preliminary assign the Na‐Ca‐carbonates synthesized in carbonated pelites and eclogites under pressures of mantle transition zone to the burbankite structural type.

Slab stagnation vs. penetration of Nazca subduction inferred from shear wave reflectivity

The morphology and composition of subducted slabs in the upper mantle transition zone (400 to 700 km depth) have direct implications for the style of mantle convection. Here we investigate the seismic reflectivity structure at transition zone depths beneath South America by combining the arrival times and amplitudes of an updated data set of SS precursors. We observe broad regions with up to 20 km depressed 410- and 660-km discontinuities and diminished precursor amplitudes beneath the back-arc region of Nazca subduction, which are incompatible with temperature-dominated mineral phase boundary perturbations. A probabilistic inversion is applied to simultaneously determine mantle temperature and composition, and the outcomes suggest a mechanically mixed transition zone with basalt enrichment beneath the Amazon basin. This is corroborated by a novel amplitude-versus-offset inversion that measures the changes in density and shear velocity across the discontinuities. Our observations offer direct evidence for thermochemical anomalies within the mantle, which are largely associated with temporary slab stagnation. The slab-transition zone interactions suggest that the Nazca-South American convergence may represent a later-stage analog to the Pacific plate subduction in northeast Asia.

Global water distribution in the mantle transition zone from a seismic isotropic velocity model and mineral physics modeling

Although the discoveries of hydrous ringwoodite inclusions and ice-VII inclusions in natural diamonds suggest a hydrous mantle transition zone (MTZ), water content and distribution in the MTZ remain unclear. Here combining a global P- and S-wave isotropic velocity tomography and mineral physics modeling, we image the water distribution in the MTZ. Our results indicate that the MTZ is a main water reservoir inside the Earth, and the total water content of the MTZ is about 0.64–1 seawater. The upper MTZ (410–520 km) and the lower MTZ (520–660 km) contain 0.3–0.5 wt% and 0.15–0.2 wt% water, respectively, implying water contents of the MTZ decrease with increasing depths. The most hydrous regions are mainly located near subduction zones, where the upper MTZ and the lower MTZ can contain water up to 0.5–1 wt% and 0.2–0.5 wt%, respectively, indicating water is transported into the MTZ by hydrous slabs. In addition, old subducted slabs in the western Pacific subduction zone are more hydrous than young subducted slabs in the eastern Pacific subduction zone. We also propose a water circulation model which integrates our results of the water content and distribution in the MTZ.

Carbon in the deep upper mantle and transition zone under reduced conditions: Insights from high-pressure experiments and machine learning models

The storage of carbon in Earth’s mantle is an important consideration within the framework of the deep carbon cycle. In the deep (>250 km depth) reduced mantle, carbon storage mechanisms differ greatly from those in the oxidized shallow mantle. To investigate the stability of carbon-bearing phases in Earth’s deep mantle, we experimentally constrained compositional effects on phase stability in the Fe-Ni-S-C system at conditions relevant to the deep upper mantle and mantle transition zone. Our experiments suggest that carbide is absent at 10 GPa and 1450 °C in the Ni-poor (molar Ni/(Ni + Fe) = 0.2) portion of the metal-sulfide-carbon ternary, with carbon occurring as diamond or dissolved in the Fe-Ni-S-C melt. At 19 GPa and 1450 and 1600 °C, (Fe,Ni)7C3 saturates in the melt with C-rich (4.78–9.47 wt%), S-poor (2.29–6.98 wt%) bulk compositions. In comparison, Fe-Ni alloy only saturates with the C- and S-poor bulk composition of 77.88 wt% Fe, 19.47 wt% Ni, 1.93 wt% S, and 0.72 wt% C. Based on these results, we trained machine learning models to predict carbon solubility in Fe-Ni-S-C melts. Compared to classical regression models, machine learning models significantly improve the accuracy of carbon solubility predictions. Combined, our experimental and machine learning results suggest that diamond and Fe-Ni-S-C melt are the primary hosts of carbon in the convecting deep upper mantle and throughout most of the mantle transition zone. In the deepest parts of the transition zone, however, carbide is likely to precipitate at adiabatic temperatures in C-rich mantle sources.

Constraining composition and temperature variations in the mantle transition zone

The mantle transition zone connects two major layers of Earth’s interior that may be compositionally distinct: the upper mantle and the lower mantle. Wadsleyite is a major mineral in the upper mantle transition zone. Here, we measure the single-crystal elastic properties of hydrous Fe-bearing wadsleyite at high pressure-temperature conditions by Brillouin spectroscopy. Our results are then used to model the global distribution of wadsleyite proportion, temperature, and water content in the upper mantle transition zone by integrating mineral physics data with global seismic observations. Our models show that the upper mantle transition zone near subducted slabs is relatively cold, enriched in wadsleyite, and slightly more hydrated compared to regions where plumes are expected. This study provides direct evidence for the thermochemical heterogeneities in the upper mantle transition zone which is important for understanding the material exchange processes between the upper and lower mantle.

Constraining deep mantle anisotropy with shear wave splitting measurements: challenges and new measurement strategies

SUMMARY Determinations of seismic anisotropy, or the dependence of seismic wave velocities on the polarization or propagation direction of the wave, can allow for inferences on the style of deformation and the patterns of flow in the Earth’s interior. While it is relatively straightforward to resolve seismic anisotropy in the uppermost mantle directly beneath a seismic station, measurements of deep mantle anisotropy are more challenging. This is due in large part to the fact that measurements of anisotropy in the deep mantle are typically blurred by the potential influence of upper mantle and/or crustal anisotropy beneath a seismic station. Several shear wave splitting techniques are commonly used that attempt resolve seismic anisotropy in deep mantle by considering the presence of multiple anisotropic layers along a raypath. Examples include source-side S-wave splitting, which is used to characterize anisotropy in the deep upper mantle and mantle transition zone beneath subduction zones, and differential S-ScS and differential SKS-SKKS splitting, which are used to study anisotropy in the D″ layer at the base of the mantle. Each of these methods has a series of assumptions built into them that allow for the consideration of multiple regions of anisotropy. In this work, we systematically assess the accuracy of these assumptions. To do this, we conduct global wavefield modelling using the spectral element solver AxiSEM3D. We compute synthetic seismograms for earth models that include seismic anisotropy at the periods relevant for shear wave splitting measurements (down to 5 s). We apply shear wave splitting algorithms to our synthetic seismograms and analyse whether the assumptions that underpin common measurement techniques are adequate, and whether these techniques can correctly resolve the anisotropy incorporated in our models. Our simulations reveal some inaccuracies and limitations of reliability in various methods. Specifically, explicit corrections for upper mantle anisotropy, which are often used in source-side direct S splitting and S-ScS differential splitting, are typically reliable for the fast polarization direction ϕ but not always for the time lag δt, and their accuracy depends on the details of the upper mantle elastic tensor. We find that several of the assumptions that underpin the S-ScS differential splitting technique are inaccurate under certain conditions, and we suggest modifications to traditional S-ScS differential splitting approaches that lead to improved reliability. We investigate the reliability of differential SKS-SKKS splitting intensity measurements as an indicator for lowermost mantle anisotropy and find that the assumptions built into the splitting intensity formula can break down for strong splitting cases. We suggest some guidelines to ensure the accuracy of SKS-SKKS splitting intensity comparisons that are often used to infer lowermost mantle anisotropy. Finally, we suggest a new strategy to detect lowermost mantle anisotropy which does not rely on explicit upper mantle corrections and use this method to analyse the lowermost mantle beneath east Asia.

Chromite-PGM Mineralization in the Lherzolite Mantle Tectonite of the Kraka Ophiolite Complex (Southern Urals, Russia)

The mantle tectonite of the Kraka ophiolite contains several chromite deposits. Two of them consisting of high-Cr podiform chromitite—the Bolshoi Bashart located within harzburgite of the upper mantle transition zone and Prospect 33 located in the deep lherzolitic mantle—have been investigated. Both deposits are enveloped in dunite, and were formed by reaction between the mantle protolith and high-Mg, anhydrous magma, enriched in Al2O3, TiO2, and Na2O compared with boninite. The PGE mineralization is very poor (<100 ppb) in both deposits. Laurite (RuS2) is the most common PGM inclusion in chromite, although it is accompanied by erlichmanite (OsS2) and (Ir,Ni) sulfides in Prospect 33. Precipitation of PGM occurred at sulfur fugacity and temperatures of logƒS2 = (−3.0), 1300–1100 °C in Bolshoi Bashart, and logƒS2 = (−3.0/+1.0), 1100–800 °C in Prospect 33, respectively. The paucity of chromite-PGM mineralization compared with giant chromite deposits in the mantle tectonite in supra-subduction zones (SSZ) of the Urals (Ray-Iz, Kempirsai) is ascribed to the peculiar petrologic nature (low depleted lherzolite) and geodynamic setting (rifted continental margin?) of the Kraka ophiolite, which did not enable drainage of the upper mantle with a large volume of mafic magma.

Receiver function mapping of the mantle transition zone beneath the Western Alps: New constraints on slab subduction and mantle upwelling

To better constrain the deep structure and dynamics of the Western Alps, we studied the mantle transition zone (MTZ) structure using P-wave receiver functions (RFs). We obtained a total of 24904 RFs from 1182 events collected by 307 stations in the Western Alps. To illustrate the influence of the heterogeneity on the upper mantle velocity, we used both IASP91 and three-dimensional (3-D) velocity models to perform RF time-to-depth migration. We documented an MTZ thickening of about 40 km under the Western Alps and most of the Po Plain due to the uplift associated with the 410-km discontinuity and the depression associated with the 660-km discontinuity. Based upon the close spatial connection between the thickened MTZ and the location of the subducted slabs, we proposed that the thick MTZ was due to the subduction of the Alpine slab through the upper MTZ and the presence of remnants of subducted oceanic lithosphere in the MTZ. The uplift associated with the 410-km discontinuity provided independent evidence of the subduction depth of the Western Alps slab. In the Alpine foreland in eastern France, we observed localized arc-shaped thinning of the MTZ caused by a 12 km depression of the 410-km discontinuity, which has not been previously reported. This depression indicated a temperature increase of 120 K in the upper MTZ, and we proposed that it was caused by a small-scale mantle upwelling. Hardly any uplift of the 660 km discontinuity was observed, suggesting that the thermal anomaly was unlikely to be the result of a mantle plume. We observed that the thinning area of the MTZ corresponded to the area with the highest uplift rate in the Western Alps, which may have indicated that the temperature increase caused by the mantle upwelling contributed to the topographic uplift.

The Structure of the Upper Mantle Transition Zone Beneath Northeast China Associated With Mantle Plume Migration

AbstractThe stagnation and dehydration of the Pacific Plate slab in the mantle transition zone (MTZ) are widely accepted to have resulted in Mesozoic and Cenozoic volcanic activities and the formation of the Songliao Basin in NE China. However, this notion has been challenged by recent seismic studies. Alternatively, a mantle plume may have generated the large‐scale volcanism and may have led to the formation of the Songliao Basin. In this study, a detailed analysis involving common conversion point stacking of receiver functions was carried out. The results reveal a significantly deepened region of the 410 km discontinuity and an elevated region of the 660 km discontinuity in the center of NE China (specifically, the Songliao Basin). The combination of these results with those of a previous study suggests that an upwelling mantle plume was located under the Songliao Basin in the Mesozoic. Furthermore, the distinctive structure of the MTZ beneath the southern part of the Songliao Basin identified in this study corresponds to mantle plume upwelling (a mushroom‐shaped low‐velocity anomaly), which may be related to the Changbaishan volcanic activity in the Cenozoic.

Towards probing Earth\u2019s upper mantle with daily magnetic field variations: exploring a physics-based parametrization of the source

The electromagnetic (EM) field variations capable of probing the electrical conductivity of the upper mantle and mantle transition zone have a period range between a few hours and 1 day. At these periods, the dominant source of the EM signals is the ionospheric current system, which has a complex spatial and temporal structure. A concept of global-to-local (G2L) transfer functions can handle spatially complex source by relating global source expansion coefficients with locally measured magnetic (or/and electric) fields. When estimating the G2L transfer functions, the source is commonly expanded into spherical harmonics (SH). In this paper, we explore an alternative parametrization of the source based on a principal component analysis (PCA) of the Fourier transformed output from the physics-based Thermosphere Ionosphere Electrodynamics General Circulation Model (TIE-GCM). Specifically, we investigate whether magnetic fields computed in the realistic three-dimensional conductivity model of Earth excited by the PCA-based source agree better with observatory data than those computed in the same model but induced by the SH-based source. Using PCA to capture the source current compared to SH parametrization, we find that agreement with the observatory data is better during magnetically disturbed times and at shorter periods. Vice versa, it is poorer during magnetically quiet times and at longer periods.

Longevity of small-scale (‘baby’) plumes and their role in lithospheric break-up

SUMMARYControversy between advocates of ‘active’ (plume-activated) versus ‘passive’ (driven by external tectonic stresses) modes of continental rifting and break-up has persisted for decades. To a large extent, inconsistencies between observations and models are rooted in the conceptual model of plumes as voluminous upwellings of hot material sourced from the deep mantle. Such large-scale plumes are expected to induce intensive magmatism and topographic uplift, thereby triggering rifting. In this case of an ‘active’ rifting-to-break-up system, emplacement of plume-related magmatism should precede the onset of rifting that is not observed in many rifted continental margins, thus providing a primary argument in favour of an antiplume origin for continental break-up and supercontinent fragmentation. However, mantle plumes are not restricted to whole-mantle (‘primary’) plumes emanating from the mantle-core boundary but also include ‘secondary’ plumes originating from the upper mantle transition zone or shallower. Over the last decades a number of such ‘secondary’ plumes with horizontal diameters of only ∼100–200 km (therefore, sometimes also called ‘baby’ plumes) have been imaged in the upper mantle below Europe and China. The longevity of such small-scale plumes and their impact on geodynamics of continental break-up have so far not been explored. We present results of a systematic parametrical analysis of relatively small thermal anomalies seeded at the base of the lithosphere. In particular, we explore the effects of variations in initial plume temperature (T = 1500–1700 °C) and size (diameter of 80–116 km), characteristics of the overlying lithosphere (e.g. ‘Cratonic’, ‘Variscan’, ‘Mesozoic’ and oceanic) and intraplate tectonic regimes (neutral or far-field extension of 2–10 mm yr–1). In tectonically neutral regimes, the expected decay time of a seismically detectable ‘baby’-plume varies from ∼20 to >200 Myr and is mainly controlled by its initial size and temperature, whereas the effect of variations in the thermotectonic age of the overlying lithosphere is modest. These small but enduring plumes are able to trigger localized rifting and subsequent continental break-up occurring from ∼10 to >300 Myr after the onset of far-field extension. Regardless of the thermomechanical structure of the lithosphere, relatively rapid (tens of Myr) break-up (observed in models with a hot plume and fast extension) favours partial melting of plume material. In contrast, in the case of a long-lasting (a few hundreds of Myr) pre-break-up phase (relatively cold plume, low extension rate), rifting is accompanied by modest decompressional melting of only ‘normal’ sublithospheric mantle. On the basis of the models presented, we distinguish two additional modes of continental rifting and break-up: (1) ‘semi-active’ when syn-break-up magmatism is carrying geochemical signatures of the deep mantle with deformation localized above the plume head not anymore connected by its tail to the original source of hot material and (2) ‘semi-passive’ when the site of final lithospheric rupture is controlled by a thermal anomaly of plume origin but without invoking its syn-break-up melting. These intermediate mechanisms are applicable to several segments of the passive continental margins formed during Pangea fragmentation.

The Water-Fe-Pressure dependent single-crystal elastic properties of wadsleyite: Implications for the seismic anisotropy in the upper Mantle Transition Zone

Recent seismic studies suggested an anisotropic Mantle Transition Zone (MTZ) in areas adjacent to subducted slabs. Wadsleyite is the main anisotropy contributor in the upper MTZ, therefore the interpretation of these seismic observations requires the knowledge of single-crystal elastic moduli (Cijs) and the deformation-induced lattice preferred orientation (LPO) of wadsleyite. Wadsleyite can host up to 3 wt% water in its crystal structure as point defects in the form of hydroxyl groups, however, the combined effect of water content, Fe content, and pressure on the Cijs of wadsleyite remains unclear. In this study, we measured the high-pressure single-crystal elasticity of a synthetic hydrous Fe-bearing wadsleyite (0.14 (4) wt% water, Fe#=9.4, Fe3+/ΣFe=0.3) up to 18.2 (2) GPa. In combination with previous experimental data, we separated the effects of pressure, water, and Fe contents on the Cijs and intrinsic elastic anisotropy of wadsleyite. Our results suggest that the intrinsic elastic anisotropy of wadsleyite decreases with pressure, water, and Fe contents. At 15 GPa, increasing the water content by 0.1 wt% or Fe# by 1 decreases the VP and VS anisotropy of wadsleyite by ∼1.1-1.3%, and ∼0.8-1.3% in average, respectively. Combining the LPO determined in previous deformation experiments, we modeled the seismic anisotropy in the upper MTZ generated by a sub-vertical mantle flow near cold subducted slabs and a sub-horizontal mantle flow in the ambient mantle. In both scenarios, the LPO of wadsleyite leads to VSV (vertically polarized shear wave velocity) >VSH (horizontally polarized shear wave velocity). Our results suggest that wadsleyite may account for a weak anisotropic MTZ (<1%) on the global scale. Considering the fact that water decreases the elastic anisotropy but promotes LPO of wadsleyite, seismic anisotropy may not be a good water sensor in the upper MTZ.

Structure and Density of H2O‐Rich Mg2SiO4 Melts at High Pressure From Ab Initio Simulations

AbstractWater has a strong effect on silicate melt properties, yet its dissolution mechanism in depolymerized melts, typical for mantle composition, remains poorly understood. Here we report results of first‐principles molecular dynamics simulations for hydrous Mg2SiO4 melts with 6, 16, and 27 wt% H2O at pressure and temperature conditions relevant to the upper mantle and mantle transition zone. The results show that hydrogen bonds not only to the network‐forming cation Si but also to Mg which—nevertheless—remains the most important network modifier. There is no evidence to support the hypothesis based on experimental data that water may cause an increase in melt polymerization for ultramafic magmas; the ratio of non‐bridging oxygen per Si increases with the addition of the oxygen from H2O. The partial molar volume of water is independent on concentration in our simulations which allows us to examine the density of hydrous melt systematically. The critical water content—at which melts are neutrally buoyant compared to the surrounding mantle—is ~4 wt% H2O for a pyrolite composition, much lower than the high water content (&gt;10 wt%) observed in petrological experiments and estimated thermodynamically for low‐degree partial melts formed in the vicinity of the mantle transition zone.

Sulfide Aggregation in Ophiolitic Dunite Channels Explains Os‐Isotope Mismatch between Oceanic Crust and Mantle

AbstractThe osmium‐isotope mismatch commonly reported between mid‐ocean‐ridge basalts (MORBs) and residual mantle might reflect evolution of the MORB Re‐Os system after extraction from the asthenosphere, or preferential contribution of radiogenic Os components from mantle. However, in a MOR system, the role of dunite melt channels from the upper mantle and Moho transition zone in regulating isotopic systems between mantle and crust has rarely been evaluated. We report new Re‐Os isotopic compositions of base‐metal sulfides (BMS), chromites and dunites from dunite lenses with low spinel Cr# [Cr3+/ (Cr3++Al3+) ≤ 0.66] (products of interaction between MORB‐like melts and upper‐mantle harzburgites) from the Zedang ophiolite (South Tibet). Re‐Os isotopic compositions of low‐Cr# dunites from the Oman ophiolite are also shown for comparison. Mineralogical evidence suggests that the Zedang sulfides were originally precipitated as monosulfide solid solutions. The highly variable 187Os/188Os initial ratios (0.1191‐0.1702) and low 187Re/188Os (&lt;0.22) of the sulfides suggest that the chromite acted as a sink for Os‐bearing sulfides, aggregating discrete Os components with heterogeneous isotopic signatures from asthenospheric or lithospheric mantle into dunite channels. The Zedang chromites and dunites show 187Os/188Os ratios similar to the primitive upper mantle (PUM), except for two dunites with sub‐PUM ratios, reflecting the contribution of Os balanced by smaller volumes of Os‐rich, unradiogenic sulfides (likely nucleating on Os nanoparticles) and larger volumes of Os‐poor radiogenic BMS. Such isotopic heterogeneity, despite with less variation, has been observed in dunite channels from the Oman ophiolite and present‐day mid‐ocean ridges. Formation of dunite channels in the upper mantle thus can aggregate Os‐bearing sulfides with chromite, leaving high Re/Os components into the residual melts. Once such channel systems were built up at the crust‐mantle transition zone, the newly incoming MOR magmas would preferentially melt and dissolve the volumetrically abundant radiogenic BMS and retain Os‐rich nanoparticles in the channels, further amplifying the Os‐isotope mismatch between oceanic crust and mantle. This study sheds new light on the multistage evolution and small‐scale behaviors of chalcophile and siderophile elements (e.g., Re‐Os) and their isotopes (e.g., 187Re‐187Os) with sulfides and chromites in a silicate‐dominated melt plumbing system beneath mid‐ocean ridges.