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![Phase diagram of pyrolite in the upper mantle. Superimposed on our computed phase diagram (light solid lines and shading) are our computed geotherm for 100 Ma oceanic lithosphere and the dry solidus (bold solid lines) taken from McKenzie and Bickle [1988] and Zhang and Herzberg [1994]. In light dashed lines we show our computed fictive supersolidus extension of the spinel phase boundaries to illustrate the proximity of mantle compositions to the spinel-out transition and the high-temperature portion of the opx-out transition, which is not realistic. Subsolidus experimental data are shown in bold dashed lines for plagioclase to spinel-to garnet-facies transitions in the CMAS system [Wood and Holloway, 1984], orthopyroxene to C2/c transition in MgSiO 3 [Pacalo and Gasparik, 1990], and olivine to wadsleyite transition in the olivine system for Fe/(Fe + Mg) = 0.1 [Katsura and Ito, 1989]. Solid circle represents the highest-pressure observation of orthopyroxene, and open circle represents lowest-pressure observation free of orthopyroxene [Irifune and Isshiki, 1998]. Abbreviations for all phases are defined in Table A1.](profile/Carolina-Lithgow-Bertelloni/publication/228631391/figure/fig1/AS:393793254772767@1470899036998/Phase-diagram-of-pyrolite-in-the-upper-mantle-Superimposed-on-our-computed-phase-diagram.png)
Phase diagram of pyrolite in the upper mantle. Superimposed on our computed phase diagram (light solid lines and shading) are our computed geotherm for 100 Ma oceanic lithosphere and the dry solidus (bold solid lines) taken from McKenzie and Bickle [1988] and Zhang and Herzberg [1994]. In light dashed lines we show our computed fictive supersolidus extension of the spinel phase boundaries to illustrate the proximity of mantle compositions to the spinel-out transition and the high-temperature portion of the opx-out transition, which is not realistic. Subsolidus experimental data are shown in bold dashed lines for plagioclase to spinel-to garnet-facies transitions in the CMAS system [Wood and Holloway, 1984], orthopyroxene to C2/c transition in MgSiO 3 [Pacalo and Gasparik, 1990], and olivine to wadsleyite transition in the olivine system for Fe/(Fe + Mg) = 0.1 [Katsura and Ito, 1989]. Solid circle represents the highest-pressure observation of orthopyroxene, and open circle represents lowest-pressure observation free of orthopyroxene [Irifune and Isshiki, 1998]. Abbreviations for all phases are defined in Table A1.
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1] We use a new method to construct an upper mantle model based on self-consistent computation of phase equilibria and physical properties. Computation of the isotropic elastic wave velocities of a pyrolytic bulk composition in thermodynamic equilibrium shows a distinct low-velocity zone with a minimum velocity V S = 4.47 km s À1 along the 100 Ma g...
Citations
... The volume, distribution, and composition of melt may have important implications for the seismic properties of the mantle (Clark and Lesher, 2017). In addition to melt fractions and aspect ratio (Clark and Lesher, 2017;Kendall, 2000;Kendall et al., 2006), the mineralogical variations (Stixrude and Lithgow-Bertelloni, 2005), the presence of volatile species (Karato and Jung, 1998), and the anelasticity enhanced by high temperatures (Faul and Jackson, 2005) may contribute to lower seismic velocities. ...
We use mantle xenoliths brought to the surface by alkaline lavas to determine the chemical and physical properties of the metasomatized lithospheric mantle that contribute to the earliest rifting stage in East Africa. Our results help to interpret the seismic tomographic images in terms of vein and inclusions proportions in the lithospheric mantle. We focus on mantle xenoliths from the in-rift Pello Hill volcano in the North Tanzanian Divergence (NTD). These xenoliths reveal the presence of refractory mantle harzburgites and dunites with coarse granular to porphyroclastic textures and 6–80 % of diopside, phlogopite and amphibole-bearing veins and phlogopite-rich hornblendite xenolith. The presence of calc-potassic and FeO, TiO2-rich veins, and mineral equilibria of olivine and pyroxenes indicate that fluid/melt-rock interactions occurred at depth from 40 km to 80–90 km, and indicate the presence of a high-temperature isotherm beneath the NTD (T = 1040–1200�C). We computed the seismic properties of the mantle xenoliths with different proportions, compositions, and geometric distributions of crystallized and fluid-filled veins. Compared to vein-free peridotites, for crystallized vein-bearing xenoliths, the velocity is lowered by 2–4 % to 28–37 % for Vp and by 2–3 % to 25–29 % for Vs for 6 % to 60 % veins, respectively. For fluid-filled inclusions, hydrous melt lens-shape inclusions are the most effective parameter to reduce P velocity, compared to dry or 2.5 %–CO2 peridotitic melt. A comparison with seismic tomography velocities allows us to discuss the current state of the lithospheric mantle. The best agreement obtained between P teleseismic tomography (Vp anomalies between �9 % and �15 %) and vein-bearing peridotites (depth 40–90 km) corresponds to 12–25 % of crystallized veins or 8–15 % for fluid filled-veins for a vertical foliation and transtensional strain regime in the mantle lithosphere beneath the NTD.
... 14 SEISMICA | volume 2.2 | 2023 Figure 13 Tests of the hypothesis that the upper mantle is melt-free. A) Shear-wave velocity below the NVG is contoured and identical to Fig 9c, 95% 2.2 (km/s)/km x 10 -3 (Gaherty et al., 1996;Stixrude and Lithgow-Bertelloni, 2005;Tan and Helmberger, 2007), including melt fractions just below the NVG from 0 to 1.5% that linearly taper to 0% over 50 km depth can explain the full range of Vs and the vertical gradient in Vs to within error (Fig 13c). This distribution is qualitatively consistent with melt production in the 120-150 km depth range in a hydrated (Katz et al., 2003) and/or carbonated asthenosphere, accompanied by an upward migration and systematic accumulation of melt between the initiation depth and the base of the thermally controlled lithosphere. ...
Quantitative evaluation of the physical state of the upper mantle, including mapping temperature variations and the possible distribution of partial melt, requires accurately characterizing absolute seismic velocities near seismic discontinuities. We present a joint inversion for absolute but discontinuous models of shear-wave velocity (Vs) using 4 types of data: Rayleigh wave phases velocities, P-to-s receiver functions, S-to-p receiver functions, and Pn velocities. Application to the western United States clarifies where upper mantle discontinuities are lithosphere-asthenosphere boundaries (LAB) or mid-lithospheric discontinuities (MLD). Values of Vs below 4 km/s are observed below the LAB over much of the Basin and Range and below the edges of the Colorado Plateau; the current generation of experimentally based models for shear-wave velocity in the mantle cannot explain such low Vs without invoking the presence of melt. Large gradients of Vs below the LAB also require a gradient in melt-fraction. Nearly all volcanism of Pleistocene or younger age occurred where we infer the presence of melt below the LAB. Only the ultrapotassic Leucite Hills in the Wyoming Craton lie above an MLD. Here, the seismic constraints allow for the melting of phlogopite below the MLD.
... Addition of both melt and water would likely lead to reduction of the effective viscosity and enhanced dissipation (e.g., Faul et al. 2004;McCarthy & Takei 2011;Yamauchi & Takei 2016;Takei 2017;Havlin et al. 2021). While partial melting has often been cited as a possible physical mechanism for the low-velocity, highattenuation zone in the upper mantle (Karato 2014), the latter can be explained by purely solid state mechanisms, such as anelastic relaxation (Jackson et al. 2005), without the presence of melt or fluids (see also Stixrude & Lithgow-Bertelloni 2005b). In this context, the laboratory-based visco-elastic model represents a limitation inasmuch as it has been calibrated at conditions relevant to the upper mantle. ...
In this study we inverted a large set of normal-mode centre frequencies and quality (attenuation) factors, including geodetic data (mass, moment of inertia and tidal response), using self-consistently-built models of the radial elastic and anelastic seismic structure of the Earth. The mantle models are constructed using petrologic phase equilibria in combination with a laboratory-based visco-elastic model that connects dissipation from seismic to tidal periods, whereas seismic properties for a well-mixed and homogeneous core are computed using equations-of-state. Relative to the preliminary seismic reference model (PREM), we find that for the models to fit the observations, mantle P- and S-wave velocities have to be slightly faster and slower, respectively, while outer-core P-wave velocity is slower on account of a different velocity gradient, whereas inner-core velocity structure is similar, within the uncertainties of the inferred model parameters. In terms of density, we find that the mantle is less dense and the outer core more dense than PREM, while the inner core is similar to PREM. These changes are driven in part by the astronomic-geodetic data. The laboratory-based visco-elastic model considered here resolves the anelastic response of Earth’s mantle from long-period seismic (∼100 s) to tidal (18.6 yrs) periods, accounting for both normal-mode and tidal dissipation measurements. To study the impact of the inferred mantle seismic velocity structure, we computed P- and S-wave travel times and compared these to the observations of globally-averaged P- and S-wave travel times from the reprocessed ISC catalog that resulted in an excellent match. In an attempt to further refine the seismic P-wave velocity structure of the outer core, we also considered multiple core-mantle-boundary underside-reflected body wave travel time data. While the match to the underside reflections clearly improves as a result of a steeper velocity gradient in the outer core relative to the normal-modes- and astronomic-geodetic-data-only case, subtle differences nevertheless persist that appear to support a change in velocity gradient in the outermost core, evocative of a stably stratified layer. Finally, as a potential means of refining core composition, we considered the density contrast across the inner-core boundary (ICB) based on our inverted models. The most probable ICB density difference found here is 0.3–0.45 g/cm3, which is in the lower range of earlier body-wave- and normal-mode-based predictions. This suggests that the compositional heterogeneity associated with light-element partitioning, which is considered the principal driving mechanism for the compositional convection that powers the geodynamo, may be less effective than previously thought, calling for exsolution of solids from the liquid outer core as a possible additional source of energy. This would also help address the problem of a young inner core.
... Yet, markedly different views remain prevalent on the origin of the LVZ. These include the effect of incipient melt [1,2], weak asthenosphere dictated by hydrous olivine [3], solid-state rheology change in response to optimal pressure and temperature conditions in the upper mantle [4], and mantle temperature variation [5]. Such debate is common and healthy in the Earth science research towards unifying solutions, but can often cause confusions in the community that some scientific questions such as the origin of the LVZ may be forever debatable with no prospect for any consensus in the foreseeable future. ...
... Such debate is common and healthy in the Earth science research towards unifying solutions, but can often cause confusions in the community that some scientific questions such as the origin of the LVZ may be forever debatable with no prospect for any consensus in the foreseeable future. The latter perception is understandable because the LVZ debate has been continuing for over half a century [1][2][3][4][5]. While a comprehensive review with a lengthy list of ''pros" and ''cons" is welcome, such a review may, on the other hand, make the above no-solution perception more persistent because reviews are commonly considered as summaries of state-of-the-art research progress without solid conclusions. ...
It is reasonable to state that if there were no seismic low velocity zone (LVZ) beneath ocean basins, there would be no seafloor spreading and plate tectonics. Over the past 50 years, plate tecton-ics has been developed into a powerful theory with unquestionable and irrefutable lines of evidence and predictive efficacies. Yet, markedly different views remain prevalent on the origin of the LVZ. These include the effect of incipient melt [1,2], weak astheno-sphere dictated by hydrous olivine [3], solid-state rheology change in response to optimal pressure and temperature conditions in the upper mantle [4], and mantle temperature variation [5]. Such debate is common and healthy in the Earth science research towards unifying solutions, but can often cause confusions in the community that some scientific questions such as the origin of the LVZ may be forever debatable with no prospect for any consensus in the foreseeable future. The latter perception is understandable because the LVZ debate has been continuing for over half a century [1-5]. While a comprehensive review with a lengthy list of ''pros" and ''cons" is welcome, such a review may, on the other hand, make the above no-solution perception more persistent because reviews are commonly considered as summaries of state-of-the-art research progress without solid conclusions. My 30-year dedicated research on mantle melting and basalt petrogenesis convinces me that it is time to inform the community that a clear picture on the LVZ origin has long been established based on geological and geophysical observations that are fully consistent with experimental petrology under upper mantle conditions [1,2] (Supplementary materials online). The news & views offers a prime forum to discuss this unifying understanding on the LVZ origin in simplest clarity and transparency (vs. lengthy and ambiguous reviews), which is summarized below, followed by elaborations. It is the water that causes incipient melting beneath oceanic lithosphere that gives rise to the seismic energy attenuation, reduced S-wave velocity, and enhanced electrical conductivity, forming the LVZ as simulated experimentally [1,2] and inferred petrologically [6] (Supplementary materials online). The lack of LVZ beneath continental shield and cratonic regions is consistent with the construct of compositionally depleted and physically buoyant Archean mantle lithosphere of up to 300 km thick [7,8]. The presence of the LVZ beneath eastern China [9], especially beneath the North China Craton (NCC), is thus unexpected, but is readily understood as a straightforward consequence of basal hydration weakening that converted the cratonic basal lithosphere into the asthenosphere [9] with LVZ properties [10] (Supplemen-tary materials online). That is, the eastern China lithosphere thinning [9] or NCC destruction [11,12] since the Mesozoic is the very process of LVZ formation [9], confirming geologically in simple clarity that the LVZ results from mantle hydration and water-effected incipient melting (Supplementary materials online). The LVZ was first recognized in 1959 [13] to show unusually low seismic velocities, especially for S waves, in the upper mantle beneath a high velocity lid, which is known as the lithosphere [8]. Many studies equate the LVZ with asthenosphere, but they are not the same. The asthenosphere is globally continuous, but the LVZ is prominent beneath ocean basins and less obvious or absent beneath continents [8]. Hence, the LVZ is the upper portion of the asthenosphere beneath oceanic lithosphere. The lithosphere-asthenosphere boundary (LAB) is thus referred to the boundary at the base of the lithosphere with the subjacent LVZ. The thickness of the LVZ varies, defined by the LAB at the top and its base that is on average $220 ± 30 km called Lehmann Discontinuity [5,8]. Hence, the thickness of the LVZ is largely determined by the depth of the LAB [6]. The origin of the LVZ can be approached by discussing the origin of the LAB [9]. The origin of the LAB beneath ocean basins had been debated for decades in the geophysical community with varying thermal models based on globally valid first-order observations (see Ref. [6] for details). The oceanic lithosphere thickens with age by accreting asthenosphere material from below and reaches its full thickness (L) of $90 km at the age (t) of $70 Ma. This lithospheric thickening fits the relation L / t 1/2 , consistent with conductive cooling to the seafloor. However, despite continued conductive cooling, the oceanic lithosphere does not grow any thicker than $90 km when t > 70 Ma. Small scale convection has been invoked to explain this puzzle, but why such convection keeps constant L $90 km only after t > 70 Ma has not been elaborated. A thorough analysis of the results of many years of experimental petrology [1,2] and petrological studies of oceanic basalts (Supplementary materials online) has led Niu and Green [6] to conclude that the LAB is a petrological phase boundary resulting from the
... We first established a prior model that represents pre-existing knowledge of, or hypotheses for, the possible probabilities of the free state variables (temperature, melt fraction, and grain size). For the prior model we assumed that temperature is the single most important parameter for governing the seismic properties and viscoelastic moduli, assuming the pressure is known (Cammarano et al., 2003;Stixrude & Lithgow-Bertelloni, 2005). We therefore created an initial ("first guess") temperature distribution on the simplified premise that all the spatial variation in Vs and Q −1 is linked to temperature. ...
Contemporary crustal uplift and relative sea level (RSL) change in Greenland is caused by the response of the solid Earth to ongoing and historical ice mass change. Glacial isostatic adjustment (GIA) models, which seek to match patterns of land surface displacement and RSL change, typically employ a linear Maxwell viscoelastic model for the Earth's mantle. In Greenland, however, upper mantle viscosities inferred from ice load changes and other geophysical phenomena occurring over a range of timescales vary by up to two orders of magnitude. Here, we use full‐spectrum rheological models to examine the influence of transient deformation within the Greenland upper mantle, which may account for these differing viscosity estimates. We use observations of shear wave velocity combined with constitutive rheological models to self‐consistently calculate mechanical properties including the apparent upper mantle viscosity and lithosphere thickness across a broad spectrum of frequencies. We find that the contribution of transient behavior is most significant over loading timescales of 10²–10³ years, which corresponds to the timeframe of ice mass loss over recent centuries. Predicted apparent lithosphere thicknesses are also in good agreement with inferences made across seismic, GIA, and flexural timescales. Our results indicate that full‐spectrum constitutive models that more fully capture broadband mantle relaxation provide a means of reconciling seemingly contradictory estimates of Greenland's upper mantle viscosity and lithosphere thickness made from observations spanning a range of timescales.
... For example, Becker (2006), Conrad and Lithgow-Bertelloni (2006), and Conrad and Behn (2010) use a conversion factor between shear velocity anomalies and density anomalies of 0.15, while Ghosh et al. (2010Ghosh et al. ( , 2017 and Wang et al. (2015) assume a conversion factor of 0.25. Some studies consider the mineral physics approach of Stixrude and Lithgow-Bertelloni (2005a, 2005b to derive depth-dependent values for the conversion factor typically varying between 0.2 and 0.4 (e.g., Steinberger and Calderwood, 2006;Adam et al., 2021). Other studies determine the conversion factor through laboratory experiments to range between 0.2 and 0.4 (Karato and Wu, 1993;Steinberger and Calderwood, 2006;Karato, 2008). ...
... 5.1 Velocity-density conversion factor: Implications for thermal versus compositional effects on the density structure Studies suggest that S-wave velocity perturbations in the upper mantle relate more to temperature variations (e.g., Goes and Van der Lee, 2002;Stixrude and Lithgow-Bertelloni, 2005a) than compositional variations. We examine the assumption that S-wave velocity perturbations in the upper mantle are controlled only by temperature effects (e.g., Goes and Van der Lee, 2002;Stixrude and Lithgow-Bertelloni, 2005b) and that the S-wave velocity anomalies can be converted to density anomalies through a constant conversion factor of 0.15 (e.g., Becker, 2006;Conrad and Lithgow-Bertelloni, 2006;Conrad et al., 2007;Conrad and Behn, 2010). With these assumptions, the calculated density anomalies (dρ) beneath the RVP and surrounding region are given by (Eq. ...
Density perturbations in the subsurface are the main driver of mantle convection and can contribute to lithospheric deformation. However, in many places the density structure in the subsurface is poorly constrained. Most geodynamic models rely on simplified equations of state or use linear seismic velocity perturbations to density conversions. In this study, we investigate the density structure beneath the Rungwe Volcanic Province (RVP), which is the southernmost volcanic center in the Western Branch of the East African Rift (EAR). We use shear-wave velocity perturbations ( d l n v s ) as a reference model to perform constrained inversions of satellite gravity data centered on the RVP. We use the code jif3D with a d l n v s -density coupling criterion based on mutual information to generate a 3D density model beneath the RVP up to a depth of 660 km. Our results reveal a conspicuous negative density anomaly (∼−200 kg/m³) in the sublithospheric mantle (at depths ranging from ∼100 km to ∼250 km) beneath the central part of the Malawi Rift extending to the west, beneath the Niassa Craton, coincident with locations with positive shear-wave velocity perturbations (+7%). We calculate a 3D model of the velocity-to-density conversion factor (f) and find negative f-values beneath the Niassa Craton which suggests the observed negative density anomaly is mostly due to compositional variations. Apart from the Niassa Craton, there are generally positive f-values in the study area, which suggest dominance of temperature control on the density structure. Although the RVP generally shows negative density anomalies and positive f-values, at shallow depths (<120 km), f ≈ 0, which suggests important contributions of both temperature and composition on the density structure possibly due to the presence of plume material. The negative buoyancy of the Niassa Craton contributes to its long stability, while constituting a barrier to the southward flow of plume material, thus restricting the southward continuation of magmatism in the Western Branch of the EAR. The presence of a negative-density anomaly where d l n v s are positive is incompatible with models based on the use of simple d l n v s to density conversion factors. These results have implications on how d l n v s models are converted to density perturbations.
... The isothermal contour (1100 • C to 1200 • C) of a reference model shows the potential thickness of the lithosphere for all the ocean floor. Barruol et al. (2019) observed the potential lithospheric thickness at 1200 • C isotherm (Stixrude and Lithgow-Bertelloni, 2005) in the Indian Ocean. This suggests that the base of a conductive lithosphere should lie between 80 and 100 km, however, our result shows a depth between c.a.53 km and 79 km, indicating a thin lithosphere. ...
... For all other profiles, the velocity of the LVZ is c.a.4.33 km/s and lies at different depths ( Figs. 9 and 10). The depth interval of the upper boundary of the LVZ follows the northward dipping trend from c.a.75 km at P1 to c.a.99 km at P6. Stixrude and Lithgow-Bertelloni (2005) have observed that the lowest velocity for the age of 50 Ma should not be less than ~4.42 km/s (at 110 km depth). The age of the oceanic crust around the IOGL region is >65 Ma (Fig. 9). ...
... Within the depth interval of the asthenosphere, rapid increase in velocity from the point of minimum velocity (Gu et al., 2005;Maggi et al., 2006;Nishimura and Forsyth, 1989;Weeraratne et al., 2007) may correspond to its solidus nature (Dunn, 2015). A suggested discontinuity near 285 km depth is due to phase transformation from orthopyroxene to Mg-rich pyroxene, where its depth primarily depends upon the temperature (Stixrude and Lithgow-Bertelloni, 2005). ...
... Therefore, our results suggest that the stability of CaCO 3 -IIIb and CaCO 3 -III phase in limestone also behaves differently from that of single-crystals, and the coexistence of CaCO 3 -IIIb and CaCO 3 -III in limestone is from 2.5 to 5 GPa. Based on the elasticity in previous work, the velocities of major minerals in the ocean crust and mantle are calculated and compared with that of limestone, the effect of temperature has not been taken into account for comparison [17,[58][59][60][61][62][63]. Except for the albite, limestone has the lowest velocities, the V P /V S ratio of limestone also contrasts with those minerals (Figure 4). ...
... The velocities of limestone at high pressure were measured using the ultrasonic interferometry method. The result shows that the velocity softening of limestone induced by [57]; An (anorthite) [58]; Arag (aragonite) [17]; Cpx (diopside) [59]; Gt [60]; Ol (olivine) [61]; Opx (orthopyroxene) [62]. ...
Limestone mainly consisting of CaCO3 is one of the most abundant carbonates on Earth's surface. The sound velocities of limestone at high pressure were determined at room temperature using ultrasonic interferometry in a multi-anvil apparatus. Softening and discontinuities in compressional (P) and shear (S) wave velocities have been observed at around 1.45 GPa due to the phase transition of CaCO3-I to CaCO3-II, and the coexistence of CaCO3-IIIb and CaCO3-III stay up to ∼5 GPa. Limestone under CaCO3-I and CaCO3-III phase has much lower velocities and higher VP/VS ratio than most crustal and mantle minerals and the PREM model. Phase transitions from CaCO3-I to CaCO3-II cause the abrupt reduction of VP, VS, and VP/VS. The low velocities and high/low VP/VS ratio are well consistent with the seismology observed in the mantle wedge. The result suggests that the subduction of limestone into Earth's interior would cause low-velocity anomalies in mantle wedges.
... It could be slightly larger due to higher pressure effects on the crystallizing gabbroic mineralogy beneath the normal oceanic Moho. While the normal uppermost mantle velocity for the 70-100 Ma oceanic lithosphere is typically thought to be ∼4.6 km/s (Stixrude & Lithgow-Bertelloni, 2005), the uppermost mantle beneath the Hawaiian Swell may be materially involved in active upwelling with temperature elevated by 135 K (Agius et al., 2017). Furthermore, this lithospheric mantle could contain frozen melts generated during lithosphere formation (Ohira et al., 2017). ...
We infer the lithospheric structure beneath the Hawaiian Swell based on a joint inversion of ambient noise and teleseismic Rayleigh waves collected during the PLUME experiment. These combined datasets let us use Rayleigh waves with a period range of 8–50 s that provides imaging resolution for the shallow lithosphere and constrains interactions between the upwelling plume and migrating plate. We find an elongated low‐velocity anomaly beneath the lithosphere along the island chain that connects to the plume conduit, consistent with the melting region associated with a restite hotspot swell root that has mechanically eroded the base of its overlying lithosphere. It could also be consistent with a plume refracted by the overriding plate motion if the plume could manage to deeply erode the lithospheric base, as only a more viscous restite root is seen to do in thermomechanical experiments. There is also a low‐velocity body beneath the North Arch that coincides with the location of recent off‐chain volcanic fields discovered there. Its location relative to the Molokai Fracture Zone (MFZ) supports the concept that swell‐root material has preferentially spread beneath the younger side of the MFZ. We also find a clear low‐velocity anomaly associated with the uppermost ∼50 km of the fracture zone lithosphere. The relatively well‐resolved shallow lithospheric structure determined here allows us to estimate the volumes of underplating and the total flux of the rising plume material that is added beneath the Hawaiian lithosphere, which we constrain to be in the range of 0.5–0.6 km³/yr.
... These are difficult to estimate and propagate because inter-laboratory comparisons due to pressure and temperature calibration are difficult. Given technological advances these uncertainties are likely to be smaller than those estimated by Stixrude and Lithgow-Bertelloni (2005a), which are on the order of 0.06 km/s. The spread due to the averaging scheme (Voigt-Reuss-Hill) are smaller than the experimental uncertainties Lithgow-Bertelloni, 2005a, 2012). ...
... In addition, the different mixing models (e.g., Archie's Law) employed in averaging electrical conductivities have their inherent limitations and assumptions that can also add uncertainties into the inferred temperature and water content. Averaging schemes for seismic velocities (Voigt-Reuss-Hill bounds) are unlikely to add significant uncertainty as their effects are smaller than experimental uncertainties (Stixrude & Lithgow-Bertelloni, 2005a, 2012. Where multiple experimental constraints exist, we have made conservative choices, including those used to calculate bulk conductivity (Table A1, Appendix) within MATE (Özaydin & Selway, 2020) and seismic velocities via Hacker and Abers (2004) and Lithgow-Bertelloni (2005b, 2011). ...
Mantle viscosity controls a variety of geodynamic processes such as glacial isostatic adjustment (GIA), but it is poorly constrained because it cannot be measured directly from geophysical measurements. Here we develop a method that calculates viscosity using empirical viscosity flow laws coupled with mantle parameters (temperature and water content) inferred from seismic and magnetotelluric (MT) observations. We find that combining geophysical constraints allows us to place significantly tighter bounds on viscosity estimates compared to using seismic or MT observations alone. In particular, electrical conductivity inferred from MT data can determine whether upper mantle minerals are hydrated, which is important for viscosity reduction. Additionally, we show that rock composition should be considered when estimating viscosity from geophysical data because composition directly affects seismic velocity and electrical conductivity. Therefore, unknown composition increases uncertainty in temperature and water content, and makes viscosity more uncertain. Furthermore, calculations that assume pure thermal control of seismic velocity may misinterpret compositional variations as temperature, producing erroneous interpretations of mantle temperature and viscosity. Stress and grain size also affect the viscosity and its associated uncertainty, particularly via their controls on deformation regime. Dislocation creep is associated with larger viscosity uncertainties than diffusion creep. Overall, mantle viscosity can be estimated best when both seismic and MT data are available and the mantle composition, grain size and stress can be estimated. Collecting additional MT data probably offers the greatest opportunity to improve geodynamic or GIA models that rely on viscosity estimates.
