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Upper mantle discontinuity structure in the region of the Tonga Subduction Zone
Geophysical Research Letters
Abstract
We study the mantle structure below the southwest Pacific in order to examine the geometry of the Tonga slab at depth and its interaction with the 410- and 660-km discontinuities (hereafter called the 410 and the 660). We utilize data from stations of both the Lau Basin Ocean Bottom Seismogram experiment and island stations of the Southwest Pacific Seismic Experiment. The tectonic complexity of this region, containing both the Tonga subduction zone and the associated Lau back arc spreading center make it an ideal area to investigate the upper mantle discontinuities using a high resolution technique such as common conversion point stacking of receiver functions. We produce a high-resolution image of the upper mantle near the Tonga subduction zone to show the interaction between the discontinuities and the subducting slab. Our results show the 410 uplifted by 30 km near the Tonga slab and the 660 depressed by 20 to 30 km as expected for thermally controlled olivine phase transitions.
... Song et al., 2023). However, several X-discontinuities have been observed beneath subduction zones, which are too deep to be explained by these two transitions (Figure 6b), such as ∼300 km X-discontinuity in the Tonga subduction zone (Gilbert et al., 2001), ∼310 km X-discontinuity in the Kurile subduction zone (Schmerr et al., 2013), and ∼330 km X-discontinuity in northwest Pacific subduction zones (Zhang & Lay, 1993). ...
Anhydrous phase B (anh‐B) is a dense magnesium silicate with the composition Mg14Si5O24 and space group Pmcb. In magnesium‐rich environments, forsterite reacts with periclase to form anh‐B, and the formation of anh‐B was proposed as a plausible mechanism for the origin of the X‐discontinuity. However, the elastic properties of anh‐B, which are critical for evaluating the seismic features associated with its formation, have not been determined. In this study, we investigated the elasticity of anh‐B at high pressure and temperature via first‐principles calculations. Combining with the elasticity of other minerals, we determined the contrasts caused by the formation of anh‐B: ∼3%, ∼7%, and ∼10% jumps for density, VP, and VS, respectively. The 2%–8% impedance contrasts of the X‐discontinuity can be explained by the formation of 15–60 vol% anh‐B, which requires 3–12 vol% MgO as a reactant.
... The receiver function method is one of the most effective tools to study upper mantle discontinuities (e.g., Dueker and Sheehan, 1997;Gao and Liu, 2014a;Gilbert et al., 2001;Langston, 1979;Yuan et al., 1997). The deconvolution technique proposed by Langston (1979) can separate primary converted waves from multiple waves. ...
We determined the topography of the upper mantle 410 km and 660 km discontinuities (‘410’ and ‘660’) beneath the northeastern Tibetan Plateau and surrounding areas using a dense broadband seismic array. We binned the 95474 teleseismic receiver functions into 0.5×0.5° grids; then the P410s and P660s arrival times were selected. Based on receiver function migration using three different regional tomographic models, the average ‘410’ and ‘660’ depths obtained are 413±1 km and 672±2 km, and the mantle transition zone (MTZ) thickness is ∼260 km. Combining these data with the velocity anomalies in the tomographic models, we find that the depressed ‘660’ beneath the northeastern Tibetan Plateau may be affected by cold anomalies delaminated from the thickened lithosphere. The depressed ‘410’ beneath the same area may be mainly affected by upwelling hot materials, which resulted from the delamination process and mixed with the eastward escape flow from the Tibetan Plateau. The slight uplift of ‘410’ is consistent with the extension of the high-velocity anomaly beneath the middle Ordos block. The thinning MTZ and depressed ‘410’ suggest that there are two branches of upwelling hot mantle beneath the northern and southern Ordos blocks. Considering the remote effects of both the subducted Indian Plate and Pacific Plate, we conclude that the dynamic model beneath the northeastern Tibetan Plateau and western North China craton (NCC) is controlled by two different large-scale subduction systems.
... A trend of thinner transition zone thickness is generally observed beneath mid-ocean ridges (e.g., Huang et al., 2019). For example, in the Tonga subduction zone Gilbert et al. (2001) found good correlation of the MTZ thickness with temperature variations inferred from seismic velocity anomalies. Similarly, beneath hot-spots Li et al. (2003) found usually thinner transition zone thickness (smaller time difference of the P-to-s conversions) than expected from the IASP91 global model. ...
... A trend of thinner transition zone thickness is generally observed beneath mid-ocean ridges (e.g., Huang et al., 2019). For example, in the Tonga subduction zone Gilbert et al. (2001) found good correlation of the MTZ thickness with temperature variations inferred from seismic velocity anomalies. Similarly, beneath hot-spots Li et al. (2003) found usually thinner transition zone thickness (smaller time difference of the P-to-s conversions) than expected from the IASP91 global model. ...
The Nazca plate subducting beneath South America makes the 660 km discontinuity deeper and the mantle transition zone (MTZ) thicker under the continent. MTZ variations are often associated with mantle temperature and, therefore, can help confirm the slab position at greater depths. Recent P- and S-wave tomography results show the Nazca plate, near 20° S, being held below the MTZ for longitudes between 70° W and 55° W. We used 63,809 P-wave receiver functions from 1216 stations (using the LQT components of the incident ray system) to image the MTZ in South America. The receiver functions were corrected for move-out, stacked in cells of 3° x 3° degrees every 1° x 1°. We obtained 54,389 RF traces imaging the 410 km and the 660 km discontinuities. The discontinuity times were corrected using the SL2013 global tomography model to obtain depths. A thickened MTZ follows the trend of the Nazca plate beneath the sub-Andes. To the north of 18oS the thickened MTZ is only about 250 km wide; to the south, the thickened zone reaches up to a 1100 km width. This observation clearly indicates that the Nazca slab flattens close to the 660 km discontinuity lowering the mantle temperature and thickening the MTZ. The 660 km discontinuity is more affected than the 410 km, which is consistent with the Nazca slab being held just below the MTZ and not inside the MTZ in most of the region.
... Beneath the Atlantic, 90°E Ridge regions, and southwestern Morocco its depth has been reported to be between 280 and 350 km (Rein et al., 2020;Whitcomb & Anderson, 1970). This reflector has been identified below Tonga, Sumba-Philippines, Tonga-Fiji, and Philippine regions by various researchers at depths ranging from 292 to 343 km (Gilbert et al., 2001;Revenaugh & Jordan, 1991;Zhang & Lay, 1993;Zheng et al., 2007). However, beneath the Northwestern Pacific subduction zone and North Korea, it has been detected with a small percent of impedance contrast (∼4.5%). ...
Beneath the continents and island arcs, a seismic discontinuity is often detected around 300 km depth, referred to as the X‐discontinuity. Various mineralogical and petrological mechanisms have been put forth to explain its occurrence. Because of the large depth variability, it is challenging to explain its origin invoking a single mineralogical phase transition. In order to investigate this discontinuity beneath India, we analyzed 10,216 P wave receiver functions at seismological stations deployed on the Indian shield and the Himalayas. We detect the X‐discontinuity as a sporadic and thin feature, in the depth range of 246–335 km, with a sharp shear velocity jump of 2.5%–3.6%. It neither bears a clear tectonic affinity nor has any correlation with the transition zone discontinuities. Interpreting its origin due to a single mineralogical change warrants a large spatial variation in the mantle temperature. Therefore, we suggest that the observed widespread X‐discontinuity beneath the Indian shield owes its origin to two mechanisms, that is, Orthoenstatite to high pressure Clinoenstatite transformation which shifts to lower pressures (∼2 GPa) due to the presence of water (0.13 wt% H2O) in MgSiO3 and coesite‐stishovite transition occurring at 8–11 GPa due to excess silica in an eclogitic component derived from the Tethys oceanic lithosphere subducted during lower Eocene. The identification of such a discontinuity could allow tracking of subducted material within the upper mantle providing a measure of mantle geochemical heterogeneity.
... The receiver function method is one of the most effective tools to study upper mantle discontinuities (e.g., Dueker and Sheehan, 1997;Gao and Liu, 2014a;Gilbert et al., 2001;Langston, 1979;Yuan et al., 1997). The deconvolution technique proposed by Langston (1979) can separate primary converted waves from multiple waves. ...
... The standard explanation of the 520 is that it is due to a solid-solid passage from β-spinel (wadsleite) to γ-spinel (ringwoodite; e.g., Shearer, 1990). In cold subduction zones (e.g., Gilbert et al., 2001), the wadsleite to ringwoodite transition might occur in a broader depth range making the converted signal too small to be observed. In this case, hydration of the overlying mantle by a stagnating slab in the transition zone has been proposed as, for example, for the Mediterranean region ( . ...
We investigate the upper mantle discontinuities in the central Mediterranean region by applying the P and S receiver function techniques on waveforms recorded at broadband stations located around the Tyrrhenian basin. P and S wave velocity profiles (down to 300‐km depth) are calculated with joint inversion of P and S receiver functions. We could identify the Moho, lithosphere‐asthenosphere boundary, and an underlying low‐velocity layer between ~60‐ and ~200‐km depth. The low‐velocity layer is interpreted as asthenospheric material, and its lower boundary is identified below the western Ionian and Tyrrhenian basins as a sharp Lehmann discontinuity. Although the stations are located on different lithospheric domains we find a strong correlation between Moho and the lithosphere‐asthenosphere boundary depths, which suggests ubiquitous coupling of the crust and lithospheric mantle, consistently with the southward opening of the Tyrrhenian basin. The Tyrrhenian and western Ionian basins present thinning of the transition zone of ~14 km, as inferred from a reduced P660s‐P410s differential time. Below the southern Apennines we observe a standard differential time that implies an average mantle transition zone thickness. We explain these mantle transition zone thickness variations as due to temperature heterogeneity linked to the area's subduction history. Finally, under central Europe (the location of the deep S‐to‐P conversion points) two strong signals from nonstandard discontinuities within the mantle transition zone are observed. These signals can be explained as being generated at the boundaries of high seismic velocity layers that are spatially correlated with stagnant slabs in the transition zone detected by seismic tomography.
... The 520 has been found to occur over a diffuse depth range and therefore might not be sharp enough to observe seismically and observations remain controversial (Bock 1994). The 520 might be observable in subduction zones (Gilbert et al. 2001) where the wadsleyite-ringwoodite phase transition may occur over a small pressure interval due to variations in olivine content (Gu et al. 1998), hydrated mantle or slab material (Inoue et al. 1998). ...
The interaction of subducted oceanic lithosphere with the discontinuities of the mantle transition zone (MTZ) provides insight into the composition and temperature of the subducted slab as well as potential melting of the slab or the surrounding mantle and loss of volatiles from the slab. Detailed mapping of the structure of the MTZ will help to better understand how slabs transport material and volatiles into the mantle and how phase transitions affect the slab dynamics. Here we use a dense network of seismic stations in northern Anatolia to image the structure of the MTZ discontinuities in detail using P-wave receiver functions. With a station spacing of about 7 km and a surface footprint of ~35 km by ~70 km, analysing receiver functions calculated from teleseismic earthquakes that occurred during an ~18 month deployment produced clear images of where the mantle transition zone interacts with the Tethys/Cyprus slabs that either lie flat on the 660-km discontinuity or pass into the lower mantle. We observe an undulating 660-km discontinuity depressed by up to 30 km and a slightly depressed (1 – 2 km) 410-km discontinuity, apparently undisturbed by the slab. The MTZ is thickened to ~270 km as result of the cool slab in the MTZ influencing the 660-km discontinuity and includes an arrival at ~520-km depth likely from the top of a flat lying slab or a discontinuity related to a solid-solid phase transition in the olivine component of the mantle. We find evidence for low-velocity zones both above and below the 410-km discontinuity and above the 660-km discontinuity. The low velocity zones around the 410-km discontinuity might be the result of hydration of the MTZ from the slab and upward convection of MTZ material into the upper mantle. The origin of the low velocity zone around the 660-km discontinuity is less clear and could be related to sedimentation of subducted mid-ocean ridge basalts. The small footprint of the seismic array provides accurate information on the structure of the MTZ in an area influenced by subduction and shows small-scale changes in MTZ structure that might be lost in studies covering larger areas with sparser sampling.
We study the mantle transition zone discontinuities beneath the Andaman Subduction Zone using receiver functions generated from broadband seismic stations. The receiver functions were geographically binned using the Common Conversion Point (CCP) technique. Moveout corrected receiver functions shows that P-SV phase conversions from 410 km discontinuity arrives around ~5 to –7 s earlier than expected (~44 s). Migrating and projecting these receiver functions along a depth profile shows that the mean apparent depth of 410 and 660 km discontinuities is 357 ± 5 km and 660 ± 7 km, respectively. This translates to around ~53 ± 6 km elevation in 410 km discontinuity. We attribute this elevation to the perturbation in isotherm caused by the young and steep subducting slab, which results in phase transformation of Olivine (α- phase) to wadsleyite (β-phase) at much earlier depth. There is no substantial topographical variation in depth of discontinuity at 660 km. We infer this flat topography of the 660 km discontinuity due to the termination of the descending slab, as evidenced in recent tomographic studies. The apparent mean MTZ thickness beneath the Andaman Subduction Zone (ASZ) is 296 ± 6 km.
Analysis of a deployment of broadband sensors along a 500-km-long line crossing the Yellowstone hotspot track (YHT) has provided 423 in-plane receiver functions with which to image lateral variations in mantle discontinuity structure. Imaging is accomplished by performing the converted wave equivalent of a common midpoint stack, which significantly improves resolution of mantle discontinuity structure with respect to single-station stacks. Timing corrections are calculated from locally derived tomographic P and S wave velocity images and applied to the Pds (where dis the depth of the conversion) ray set in order to isolate true discontinuity topography. Using the one-dimensional TNA velocity model and a V P/V S ratio of 1.82 to map our Pds times to depth, the average depths of the 410- and 660-km discontinuities are 423 and 664 km, respectively, giving an average transition zone thickness of 241 km. Our most robust observation is provided by comparing the stack of all NW back-azimuth arrivals versus all SE back-azimuth arrivals. This shows that the transition zone thickness varies between 261 and 232 km, between the NW and SE portions of our line. More spatially resolved images show that this transition zone thickness variation results from the occurrence of 20-30 km of topography over 200-300 lateral scale lengths on the 410- and 660-km discontinuities. The topography on the 410- and 660-km discontinuities is not correlated either positively or negatively beneath the 600-km-long transect, albeit correlation could be present for wavelengths larger than the length of our transect. If this discontinuity topography is controlled exclusively by thermal effects, then uncorrelated 250° lateral temperature variations are required at the 410- and 660-km discontinuities. However, other sources of discontinuity topography such as the effects of garnet-pyroxene phase transformations, chemical layering, or variations in mantle hydration may contribute. The most obvious correlation between the discontinuity structure and the track of the Yellowstone hotspot is the downward dip of the 410-km discontinuity from 415 km beneath the NW margin of the YHT to 435 km beneath the easternmost extent of Basin and Range faulting. Assuming this topography is thermally controlled, the warmest mantle resides not beneath the Yellowstone hotspot track, but 150 km to the SE along the easternmost edge of the active Basin and Range faulting.
We stack long-period, transverse-component seismograms recorded by the Global Digital Seismograph Network (GDSN) (1976-1996), Incorporated Research Institutions for Seismology-International Deployment of Accelerometers (IRIS-IDA) (1988-1996), and Geoscope (1988-1996) networks to map large-scale topography on the 410- and 660-km seismic velocity discontinuities. Underside reflections from these discontinuities arrive as precursors to the SS phase, and their timing can be used to obtain global variations of the depth to the reflectors. We analyze over 13,000 records from events mb>5.5, focal depth < 75 km, and range 110° to 180° by picking and aligning on SS, then stacking the records along the theoretical travel time curves for the discontinuity reflections. Separate stacks are obtained for 416 equally spaced caps of 10° radius; clear 410- and 660-km reflections are visible for almost all of the caps while 520-km reflections are seen in about half of the caps. The differential travel times between the precursors and the SS arrival are measured on each stack, with uncertainty estimates obtained using a bootstrap resampling method. We then compute discontinuity depths relative to the isotropic Preliminary Reference Earth Model (PREM) at 40-s period, correcting for surface topography and crustal thickness variations using the CRUST5.0 model of Mooney et al. [1995], and for upper mantle S velocity heterogeneity using model S16B30 of Masters et al. [1996]. The resulting maps of discontinuity topography have more complete coverage than previous studies; observed depths are highly correlated between adjacent caps and appear dominated by large-scale topography variations. The 660-km discontinuity exhibits peak-to-peak topography of about 38 km, with regional depressions that correlate with areas of current and past subduction around the Pacific Ocean. Large-scale topography on the 410-km discontinuity is lower in amplitude and largely uncorrelated with the topography on the 660-km interface. The width of the transition zone, WTZ, as measured by the separation between the 410- and 660-km discontinuities, appears thickest in areas of active subduction (e.g., Kurils, Philippines, and Tonga) and thins beneath Antarctica and much of the central Pacific Ocean. Spatial variations in WTZ appear unrelated to ocean-continent differences but do roughly correlate with the S16B30 velocities in the transition zone, consistent with a common thermal origin for both patterns. The lower-amplitude 520-km reflector is more difficult to resolve but appears to be a global feature as it is observed preferentially for those bounce point caps with the most data.
We experiment with backprojection migration processing of teleseismic receiver functions from the Snake River Plain (SRP) broadband seismic experiment. Previous analyses of data from this experiment have used a common midpoint (CMP) stacking approach, a method widely applied for analysis of P-SV converted phases (receiver functions) to obtain high-resolution imaging of upper mantle discontinuities. The CMP technique assumes that all P-SV conversions are produced by flat-lying structures and may not properly image dipping, curved, or laterally discontinuous interfaces. In this paper we adopt a backprojection migration scheme to solve for an array of point scatterers that best produces the large suite of observed receiver functions. We first perform synthetic experiments that illustrate the potential improvement of migration processing over CMP stacks. Application of the migration processing to the SRP data set shows most of the major features as in the original CMP work, but with a weaker 410-km discontinuity and a more intermittent discontinuity at 250 km apparent depth. Random resampling tests are also performed to assess the robustness of subtle features in our discontinuity images. These tests show that a 20-km elevation of the 660-km discontinuity directly beneath the Snake River Plain is robust, but that the variations in 410-km discontinuity topography that we observe are not stable upon resampling. ``Bright spots'' near 250 km apparent depth are robust upon resampling, but interpretation of these features is complicated by possible sidelobe artifacts from topside Moho reverberations.
Analysis of a deployment of broadband sensors along a 500-km-long line crossing the Yellowstone hotspot track (YHT) has provided 423 in-plane receiver functions with which to image lateral variations in mantle discontinuity structure. Imaging is accomplished by performing the converted wave equivalent of a common midpoint stack, which significantly improves resolution of mantle discontinuity structure with respect to single-station stacks. Timing corrections are calculated from locally derived tomographic P and S wave velocity images and applied to the Pds (where d is the depth of the conversion) ray set in order to isolate true discontinuity topography. Using the one-dimensional TNA velocity model and a Vp/Vs ratio of 1.82 to map our Pds times to depth, the average depths of the 410- and 660-km discontinuities are 423 and 664 km, respectively, giving an average transition zone thickness of 241 km. Our most robust observation is provided by comparing the stack of all NW back-azimuth arrivals versus all SE back-azimuth arrivals. This shows that the transition zone thickness varies between 261 and 232 km, between the NW and SE portions of our line. More spatially resolved images show that this transition zone thickness variation results from the occurrence of 20-30 km of topography over 200-300 lateral scale lengths on the 410- and 660-km discontinuities. The topography on the 410- and 660-km discontinuities is not correlated either positively or negatively beneath the 600-km-long transect, albeit correlation could be present for wavelengths larger than the length of our transect. If this discontinuity topography is controlled exclusively by thermal effects, then uncorrelated 250° lateral temperature variations are required at the 410- and 660-km discontinuities. However, other sources of discontinuity topography such as the effects of garnet-pyroxene phase transformations, chemical layering, or variations in mantle hydration may contribute. The most obvious correlation between the discontinuity structure and the track of the Yellowstone hotspot is the downward dip of the 410-km discontinuity from 415 km beneath the NW margin of the YHT to 435 km beneath the easternmost extent of Basin and Range faulting. Assuming this topography is thermally controlled, the warmest mantle resides not beneath the Yellowstone hotspot track, but 150 km to the SE along the easternmost edge of the active Basin and Range faulting.
We stack the teleseismic depth phases sS, sP, and pP produced by deep focus earthquakes to image precursory arrivals that result from near-source, underside reflections off the 410-km seismic velocity discontinuity (hereinafter referred to as the 410) and use differential time measurements between these phases and their precursors to compute discontinuity depths near seven subduction zones around the Pacific Ocean margin. We begin by selecting seismograms with high-quality depth phase arrivals recorded by several long-period networks between the years 1976 and 1996. Filtering the waveforms and stacking them along theoretical travel-time curves reveals clear precursors which vary in shape and timing. We compute confidence levels to evaluate the reliability of the observed precursory features using a bootstrap method that randomly resamples the seismograms prior to stacking. We measure the differential travel time between the reference pulse and the precursor using a cross-correlation technique and convert this time to an apparent discontinuity depth using the isotropic Preliminary Reference Earth Model (PREM) at 25-s period, corrected to an oceanic crustal thickness. The lateral resolution of our long-period stacks for 410 topography is limited compared to that sometimes achieved in short-period analyses but is much higher than that obtained from global SS precursor studies. For most subduction zones the results indicate little change in the average depth to the 410-km discontinuity in the local areas sampled by the precursor bounce points compared to broad regional depths inferred from SS precursor results. This implies that any large variations in depth to the 410-km discontinuity near subduction zones are limited to a narrow zone within the slab itself where they may be difficult to resolve with long-period data. Coverage for the Tonga and Peru-Chile subduction zones is sufficiently dense that we can observe lateral variations in 410 depths. In Tonga the results suggest depth variations perpendicular to the slab of up to 33 km, after correcting for probable lateral heterogeneity in velocity above 400 km depth, and variations parallel to the slab orientation as large as 13 km. The cross-slab variation is consistent with the elevation of olivine phase transformations in cold regions; the variation along strike suggests a more complex thermal heterogeneity that may be related to the subduction history of this region. We see evidence for additional reflectors above the 410 in some of the waveform stacks, but the inconsistency and weak amplitude of these features preclude definitive interpretations.
The anelastic structure of the region surrounding the Tonga slab and Lau back arc spreading center in the southwest Pacific is studied using data from 12 broadband island stations and 30 ocean bottom seismographs. Two differential attenuation methods determine deltat* over the frequency band 0.1 to 3.5 Hz for earthquakes in the Tonga slab. The S-P method measures the difference in spectral decay between P and S waves arriving at the same station. The P-P method measures the difference in spectral decay for P waves with different paths through the upper mantle. Eight hundred sixty phase pairs are used to invert for two-dimensional 1/Qalpha structure using a nonnegative least squares algorithm. A grid search method determines the Qalpha/Qbeta ratio most compatible with both the S-P and P-P differential measurements. The highest attenuation (Qalpha=90) is found within the upper 100 km beneath the active portions of the Lau Basin extending westward to the Lau Ridge. These regions probably delineate the source region for the back arc spreading center magmas, expected to be within the upper 100 km based on petrological considerations. The high attenuation regions also correlate well with zones of low P wave velocity determined by regional velocity tomography. Somewhat lower attenuation is found beneath the Fiji Plateau than beneath the Lau Basin. The entire back arc is characterized by a gradual decrease in attenuation to a depth of 300 to 400 km. The slab is imaged as a region of low attenuation (Qalpha>900) material. A Qalpha/Qbeta ratio of 1.75 provides the best fit between the S-P and P-P data sets upon inversion. Spectral stacking shows no frequency dependence within the frequency band analyzed.
The high-pressure transformation in MgSiO3 and those in the
spinel phases of compositions from Mg2SiO4 to
(Mg0.5Fe0.5)2SiO4 in the
Mg2SiO4-Fe2SiO4 system were
investigated using a uniaxial split-sphere apparatus. The phase
boundaries between ilmenite-perovskite in MgSiO3 and between
Mg2SiO4 spinel and the assemblage of
MgSiO3 perovskite and MgO periclase were determined to be
P(GPa) = 26.8-0.0025T(°C) and P(GPa) = 27.6-0.0028T(°C),
respectively, in the temperature range 1000-1600°C. The pseudobinary
diagrams for the system
Mg2SiO4-Fe2SiO4 were
determined at temperatures of 1100°C and 1600°C. It was
demonstrated that the magnesian spinel (with Fe/Mg + Fe < 0.22 at
1100°C and <0.26 at 1600°C) dissociates into perovskite and
magnesiowüstite within an extremely narrow pressure interval
(<0.15 GPa at 1600°C). The dissociation pressure was found to be
almost independent of iron content and to coincide to that at 670 km
depth within experimental uncertainties. These experimental results
indicate that the sharpness of the 670-km discontinuity may indeed be
due to this dissociation in a peridotitic or pyrolitic mantle. The
current status of our understanding of deep mantle mineralogy and
chemistry is discussed based on recent high-pressure and
high-temperature experiments.
The sequence of high-pressure phase transitions alpha->beta->gamma in olivine is traditionally used as a model for seismic velocity variations in the 200- to 650-km-depth interval in a mantle of peridotitic bulk composition. It has been proposed that the observed seismic velocity increase at 400-km depth is too sharp and of too small a magnitude to be attributable to the alpha->beta phase change and that the upper mantle must thus be chemically stratified, with the 400-km discontinuity being due either to a combination of phase changes in a layer of pyroxene/garnet-rich ``piclogite'' composition or to a chemical boundary between such a piclogite layer and an overlying peridotitic layer. Using available calorimetric, thermoelastic, and synthesis data (and their associated experimental uncertainties), we have derived internally consistent high-pressure phase relations for the Mg2SiO4-Fe2SiO4 join. We find that the divariant transition alpha->alpha+beta->beta, which is generally regarded as occurring over a broad depth interval for mantle olivine compositions, is, in fact, extremely sharp. The seismic discontinuity corresponding to the alpha->alpha+beta->beta transition in (Mg0.9Fe0.1)2SiO4 should occur over a depth interval (isothermal) of about 6 km at a depth of approximately 400 km; the sharpness of this transition is quite insensitive to uncertainties in the constraining calorimetric, thermoelastic, and synthesis data. In addition, we have computed seismic velocity profiles for a model mantle consisting of pure olivine of (Mg0.9Fe0.1)2SiO4 composition. Comparison of these computed profiles to those derived from recent seismic studies indicates that the magnitude of the observed velocity increase at 400-km depth is consistent with a mantle transition zone composed of about 60-70% olivine. We conclude that there is no need to infer the existence of pyroxene/garnet-rich compositions, such as eclogite or ``piclogite,'' in the transition zone, since an upper mantle of homogeneous olivine-rich peridotitic composition is consistent with the available seismic velocity data.