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Elastic properties of wadsleyite and ringwoodite at hydrous conditions for Mg-Fe mixtures. Left: G0 of wadsleyite, middle: G0 of ringwoodite, right: KT0 of ringwoodite. Red stars indicate individual measurements, blue bands represent our assigned uncertainty bounds. Exact values and associated references are shown in Tables A1–3.
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Although water has a major influence on tectonic and other geodynamic processes, little is known about its quantity and distribution within the deep Earth. In the last few decades, laboratory experiments on nominally anhydrous minerals (NAMs) of the transition zone have shown that these minerals can contain significant amounts of water, up to 3.3 w...
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... 以及迁移过程的关键区域 矿物物理实验表明地幔过渡带中瓦兹利石和林 伍德石的储水能力远大于上地幔的橄榄石和下地幔 的布里奇曼石 (Kohlstedt et al., 1996;Ohtani, 2005;Thio et al., 2016;毛竹和李新阳, 2016). 尽管仍存在 争议，但实际地幔过渡带中总的水量可能是现今地 表海洋总水量的几倍(Bercovici and Karato, 2003;Hirschmann, 2006;Fei et al., 2017;Karato et al., 2020). ...
... For stx08, we cannot simultaneously match MTZ differential travel times and the distance of the minimum in |R S660S | (Fig. 6). A high concentration of volatiles in the MTZ, as some studies suggest (57), increases the discrepancy between thermodynamically predicted wavespeeds for a reasonable T pot and those required to match MTZ travel times because water leads to slower and likely thicker MTZ (58,59). Hence, significant global MTZ hydration is difficult to reconcile with the constraints on absolute wavespeed. ...
... At the same time, wavespeed contrasts at 660, especially for P waves, inferred from reflected or converted seismic phases were previously found to be lower than those expected for PMM or PEA (37,38,41), and our results confirm this. Given the uncertainties of thermodynamic estimates of wavespeed contrasts (55,58), it cannot be ruled out that seismically inferred contrasts could be matched with pyrolitic compositions, but a basalt enrichment would make it easier to reconcile thermodynamic predictions and seismic estimates (Fig. 4) (35). ...
... A water-bearing MTZ relative to mantle above could aid in explaining the smaller seismically observed contrast at 410 than expected for a PMM or PEA composition (58), without the need for a higher basalt content. That is, the basalt fraction at 410 could then be pyrolitic. ...
The mantle's compositional structure reflects the thermochemical evolution of Earth. Yet, even the radial average composition of the mantle remains debated. Here, we analyze a global dataset of shear and compressional waves reflecting off the 410- and 660-km discontinuities that is 10 times larger than any previous studies. Our array analysis retrieves globally averaged amplitude-distance trends in SS and PP precursor reflectivity from which we infer relative wavespeed and density contrasts and associated mantle composition. Our results are best matched by a basalt-enriched mantle transition zone, with higher basalt fractions near 660 (~40%) than 410 (~18-31%). These are consistent with mantle-convection/plate-recycling simulations, which predict that basaltic crust accumulates in the mantle transition zone, with basalt fractions peaking near the 660. Basalt segregation in the mantle transition zone also implies that the overall mantle is more silica enriched than the often-assumed pyrolitic mantle reference composition.
... Considering the reduction of the sound wave velocities of wadsleyite due to the incorporation of water (Mao et al., 2008a(Mao et al., , 2008b, it was proposed that water dissolved in olivine and wadsleyite may reconcile the pyrolite model with seismological observations. Water is preferentially partitioned in wadsleyite (D wad/ol = 6; Thio et al., 2016) and whether the presence of water can resolve seismological observations with pyrolitic mantle olivine contents will depend on both the expected amount of water in olivine and to what (Abramson et al., 1997;Mao et al., 2015;Zha et al., 1998;Zhang and Bass, 2016;Supplementary Variation as a function of pressure of the aggregate compressional and shear wave velocities of anhydrous Fo90 calculated from previous experimental high-pressure elasticity measurements (Abramson et al., 1997;Mao et al., 2015;Zha et al., 1998;Zhang and Bass, 2016;Supplementary Smyth et al. (2006); V 0 anhydrous was calculated using the same expression solved with the data of the hydrous olivine from this study (i.e., V 0 = 291.65(2) Å 3 and ~2000 ppm). ...
... This conjecture is further supported by the relation where a 1 wt% increase in water content produces a ∼0.7% larger density jump across the 660 (see Text S4 in Supporting Information S1). This establishes an upper bound on the perturbations in density jump due to water as water partitioning between olivine polymorphs and multiple discontinuities would reduce the water effects (Thio et al., 2016). Taken together, the observed lateral variations in density jump are dominated by bulk chemistry, and future constraints on the hydrous behavior of the phase transition would help to isolate water effects from other compositional factors. ...
This study investigates lateral variations in density contrast across the 660‐km discontinuity, which is critical for understanding mantle convection and composition. We produce a global map of density jump, with values ranging from 3.8% to 9.3%, at the mineralogical phase boundary by applying inversions of amplitude variations with offset to underside S reflections. Our observations reveal a global average velocity jump of 4.1% and a density jump of 5.3%, favoring a pyrolitic bulk composition. Near major subduction zones, most notably the western Pacific and South America, we identify reduced density jumps in regions of depressed 660‐km discontinuity, consistent with basalt fractions greater than 30% in a mechanically mixed mantle. The causal association between density jump and compositional changes is further supported by a moderate correlation between lateral variations in density jump and those of water content anomalies.
... A number of studies have been concerned with the potential effects of water on TZ structure or phase transformation kinetics (e.g. Bercovici & Karato 2003;Ohtani et al. 2004;Karato 2011;Thio et al. 2016;Wang et al. 2018). These may, for instance, manifest themselves in variations of extent (or type) of phase stability fields or the strength of observed gradients in seismic velocities. ...
The mantle transition zone (TZ) is expected to influence vertical mass flow between upper and lower mantle as it hosts a complex set of mineral phase transitions and an increase in viscosity with depth. Still, neither its seismic structure nor its dynamic effects have conclusively been constrained. The seismic discontinuities at around 410 and 660 km depth (‘410’ and ‘660’) are classically associated with phase transitions between olivine polymorphs, the pressure of which is modulated by lateral temperature variations. Resulting discontinuity topography is seismically visible and can thus potentially provide insight on temperature and phase composition at depth. Besides the olivine phase changes, the disassociation of garnet may additionally impact the 660 at higher temperatures. However, the volume of material affected by this garnet transition and its dynamic implications have not yet been quantified. This study presents hypothetical realizations of TZ seismic structure and major discontinuities based on the temperature field of a published 3-D mantle circulation model for a range of relevant mineralogies, including pyrolite and mechanical mixtures (MM). Systematic analysis of these models provides a framework for dynamically informed interpretations of seismic observations and gives insights into the potential dynamic behaviour of the TZ. Using our geodynamic-mineralogical approach we can identify which phase transitions induce specific topographic features of 410 and 660 and quantify their relative impact. Areal proportions of the garnet transition at the 660 are ∼3 and ∼1 percent for pyrolite and MM, respectively. This proportion could be significantly higher (up to ∼39 percent) in a hotter mantle for pyrolite, but remains low (< 2 percent) for MM. In pyrolite, both slabs and plumes are found to depress the 660 —with average deflections of 14 and 6 km, respectively— due to the influence of garnet at high temperatures indicating its complex dynamic effects on mantle upwellings. Pronounced differences in model characteristics for pyrolite and MM, particularly their relative garnet proportions and associated topography features, could serve to discriminate between the two scenarios in Earth.
... Hence it is difficult to get a robust water distribution in the MTZ from anisotropy information. Due to large experimental uncertainties (Thio et al., 2016), the depths of the 410-and 660-km discontinuities hardly provide a reliable water content in the MTZ. Since the trade-off between temperature effects and water effects on seismic velocities in the MTZ, only using S wave velocity like the previous studies is not recommended. ...
... Since the trade-off between temperature effects and water effects on seismic velocities in the MTZ, only using S wave velocity like the previous studies is not recommended. Combined studies of P and S wave velocities may be more informative (Thio et al., 2016). Furthermore, the previous studies tended to assume a linear relationship between seismic velocity and water content, which is inaccurate because of the complicated phase transformations and anelastic effect of water in the MTZ. ...
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.
... The topography of the d410 and d660 seismic discontinuities can reveal the presence of fluid in the MTZ and the effects of in situ temperature (Ringwood 1975;Thio et al. 2016). The d410 represents the phase transition from olivine to wadsleyite, which has a positive Clapeyron slope, such that a low-temperature zone corresponds to an uplift of the d410 and vice versa. ...
... For instance, under wet conditions at 1473 K, the phase transformation of the d410 shifts to lower pressures, while that of the d660 shifts to higher pressures, uplifting the d410 and depressing the d660, respectively (Litasov et al. 2005). However, it is difficult to distinguish thermal and water effects due to the average velocities and densities variation with water content and temperature in the MTZ, thus the conventional observation of low-wave speed and thicker MTZ for water and low-wave speed and thin MTZ for high temperature should be treated with caution (Thio et al. 2016). Controversy exists regarding the actual magnitude of the Clapeyron slopes, especially for the d660 (Ghosh et al. 2013). ...
Cratons are generally defined as stable continental blocks with a strong cratonic root that typically is at least ∼200 km thick. Many cratons have undergone little internal tectonism and destruction since their formation, but some of them, such as the eastern part of the North China Craton (NCC), the Dharwar Craton, and the Wyoming Craton, have lost their thick cratonic root and become reactivated in recent geological history, leading to widespread Meso-Cenozoic volcanisms. The mechanisms responsible for such decratonization remain debated. To provide new constraints on models leading to decratonization, in this study we stack 612,854 source-normalized P-to-S conversions (receiver functions or RFs) from the 410 and 660 km discontinuities (d410 and d660 respectively) bordering the mantle transition zone (MTZ) recorded at 1,986 stations in the NCC. Both the number of RFs and the number of stations are unprecedented in the study area. The average apparent depths of the d410 and d660 and the thickness of the MTZ are 413 ± 6 km, 669 ± 8 km, and 255 ± 6 km, respectively. A depression of up to 37 km and mean 11 km of the d660 are clearly observed beneath the eastern NCC, mainly caused by the possible existence of a relatively large amount of water in the MTZ. Our study provides strong observational evidence for geodynamic modeling that suggests water in the MTZ can be driven out into the upper mantle by poloidal mantle flow induced by the subduction and retreat of subducted oceanic slabs. The results are consistent with the weakening of the lithosphere beneath the eastern NCC by the release of water (in the form of structurally-bound H/OH) brought down to the MTZ by subduction of the Pacific slab. Continuous slab dehydration and the ascent of fluids would have triggered intraplate volcanism and mantle upwelling in the eastern NCC, as evidenced by the spatial correspondence among the lower-than-normal upper-mantle seismic velocities, unusually large depressions of the d660, Cenozoic basaltic volcanism, and thinning of the cratonic lithosphere.
... We estimate only a 6 K temperature difference for a 12% increase in f. Currently, the lack of thermodynamic data precludes considerations on the effects of minor components, such as volatiles or ferric iron 43,44 . ...
The abrupt changes in mineralogical properties across the Earth’s mantle transition zone substantially impact convection and thermochemical fluxes between the upper and lower mantle. While the 410-km discontinuity at the top of the mantle transition zone is detected with all types of seismic waves, the 660-km boundary is mostly invisible to underside P-wave reflections (P660P). The cause for this observation is debated. The dissociation of ringwoodite and garnet into lower-mantle minerals both contribute to the ‘660’ visibility; only the garnet reaction favours material exchanges across the discontinuity. Here, we combine large datasets of SS and PP precursors, mineralogical modelling and data-mining techniques to obtain a global thermal map of the mantle transition zone, and explain the lack of P660P visibility. We find that its prevalent absence requires a chemically unequilibrated mantle, and its visibility in few locations is associated with potential temperatures greater than 1,800 K. Such temperatures occur in approximately 0.6% of Earth, indicating that the 660 is dominated by ringwoodite decomposition, which tends to impede mantle flow. We find broad regions with elevated temperatures beneath the Pacific surrounded by major volcanic hotspots, indicating plume retention and ponding of hot materials in the mantle transition zone. The mantle transition zone is poorly, mechanically mixed, and acts to impede mantle flow, according to seismic observations integrated with detailed mineral-physics models.
... The effects of water on seismic characteristics of the MTZ are poorly constrained (Thio et al., 2016). Initial studies (at relatively low temperatures) suggested that the presence of water reduces wave speeds and increases the thickness of the MTZ (e.g. ...
The plume model has been the leading hypothesis for hotspot volcanism for five decades. Whereas experiments have clarified how plumes form and rise, the onus has been put on seismologists to find plumes in Earth. Seismic evidence has almost always been contentious because plumes perturb waves slightly and interpretations are inherently nonunique. Using simulations of convection and experimental constraints, we show that traveltime delays of teleseismic shear waves are less than a second and that it is difficult to resolve the plume conduit in the lower mantle using teleseismic traveltime tomography. We discuss the effects of plumes on the transition zone and review analyses of wave diffraction around plumes in the deep mantle, which indicate that plume stems may leave a detectable imprint on the seismic wavefield. We anticipate significant progress in the imaging of deep mantle plumes if seismologists continue their commitment to the deployment of seismic instrumentation in the oceans. Seismic studies will continue to benefit from three‐dimensional waveform modeling techniques and advances in adjoint tomography. It is essential to compare seismic modeling results with the seismic signatures predicted by dynamic models and to test them against other geophysical and geological observations.
... This implies that the water concentration in the MTZ may decrease with depth globally, from ∼0.5 wt% in the upper part to ∼0.15-0.2 wt% in the lower part, which may partly contribute to the observed velocity gradients in the MTZ (Thio et al., 2016). Accordingly, the MTZ contains about one ocean mass equivalent water. ...
The mantle transition zone (MTZ) is potentially a geochemical water reservoir because of the high H2O solubility in its dominant minerals, wadsleyite and ringwoodite. Whether the MTZ is wet or dry fundamentally impacts our understanding of the deep-water distribution, geochemical recycling, and the pattern of mantle convection. However, the water content in the MTZ inferred from previous studies remains disputed. Seismic observations such as velocity anomalies were used to evaluate the water content in the MTZ, but the hydration effect on the velocities of MTZ minerals under appropriate pressure (P) and temperature (T) conditions is poorly constrained. Here we investigated the elastic properties and velocities of hydrous ringwoodite at high P-T conditions using first-principles calculations. Our results show that the hydration effects on elastic moduli and velocities of ringwoodite are significantly reduced by pressure but strongly enhanced by temperature. The incorporation of 1.0 wt% water into ringwoodite decreases the compressional and shear velocities of the pyrolitic mantle by −1.0% and −1.4% at the conditions of MTZ, respectively. Using results from seismic tomography and together with the topography of the 660-km discontinuity, we evaluate the global distribution of water in the lower MTZ. We find that about 80% of the MTZ can be explained by varying water content and temperature, however, the remaining 20% requires the presence of high-velocity heterogeneities such as harzburgite. Our models suggest an average water concentration of ∼0.2 wt% in the lower MTZ, with an interregional variation from 0 to 0.9 wt%. Together with our previous work, we conclude that the water concentration in the MTZ likely decreases with depth globally and the whole MTZ contains the equivalent of about one ocean mass of water.
