[1] The second critical endpoint in the peridotite-H2O system has been determined using an X-ray radiography technique together with a Kawai-type, double-stage, multianvil system driven by DIA-type cubic press (SPEED-1500) installed at SPring-8, Japan. The pressure of the second critical endpoint was determined by the appearance and disappearance of round shape in the radiographic images with changing the experimental pressure. In the experiments up to 3.6 GPa, two fluid phases (i.e., aqueous fluid and hydrous silicate melt) were observed. At 4.0 GPa, however, we could not distinguish these two phases in the radiographic images. These observations indicate the second critical endpoint occurs at around 3.8 GPa and 1000°C (corresponding to a depth of ∼110 km) in the peridotite-H2O system. Our experimental results suggest that hydrous silicate melt and aqueous fluid in the Earth's mantle become indistinguishable from each other and that melting temperature of hydrous mantle peridotite can no longer be defined beyond this critical condition. This position of the second critical endpoint could explain the previously observed drastic changes in composition and connectivity of aqueous fluid in mantle peridotite at around 3–4 GPa and could play an important role in magmatism and chemical evolution of the Earth's interior.

[1] A recently proposed model links the formation and early evolution of the Tharsis volcanic province on Mars to the preexisting hemispheric dichotomy (Zhong, 2009). A key aspect of this model is the assumption of a deep lithospheric root below the thicker crust of the southern highlands. We implemented a parameterization of partial melting into the 3-D spherical shell mantle convection code CitcomS in order to investigate whether the required lithospheric thickness variation can be generated self-consistently by partial melting when stiffening of the melt residue due to devolatilization is considered. The rate of melt production strongly depends on the mantle temperature, and additional strong coupling between the flow and partial melting is introduced through the stiffening effect on the melt residue. We find that it is possible to generate a lithospheric keel by partial melting above a single upwelling that excites a relative rotation between the one-plate lithosphere and the mantle below while producing the amount of melt distributed in a broad region constrained to one hemisphere that is necessary to form the crustal dichotomy. This scenario thus offers an internal mechanism for the Martian dichotomy formation and validates the hypothesis of Zhong (2009).

We present experimental determinations of the influence of H2O on partial melting of garnet peridotite (+1.5, 2.5, and 5 wt. % added H2O) at 3.5 GPa and 1200–1450°C. Experiments produced complex polyphase regions of quenched melt and equilibrium partial melt compositions were reconstructed by combined EMP and LA‐ICP‐MS analyses. Mass balance‐derived melt fractions (F) range from 0.18 to 0.33 and dissolved water contents range from 4.5 to 23.5 wt. %. One exceptional experiment quenched glass, allowing independent verification of H2O concentration by FTIR. The influence of H2O on melt production is quantified by the temperature difference required to achieve a given F under dry and wet conditions, ΔT, which is controlled by the H2O concentration in partial melts. Melts with 1.5, 5, 10, and 15 wt. % H2O yield ΔT values of 50, 150, 250, and 320°C, respectively, consistent with a cryoscopic parameterization that assumes 3 oxygens per mole of silicate melt. Based on this parameterization, we calculate that beneath oceanic ridges, peridotite H2O storage capacity increases from 0 to 240 ppm from 66 to 110 km depth. For H2O to be solely responsible for melting in the oceanic low velocity zone (LVZ) at least 5.7 wt. % H2O must be dissolved in the melt at 110 km, and considerably more (e.g., 15 wt.% at 220 km) is required for melting throughout the entire observed interval. The addition of H2O results in 3.5 GPa partial melts of garnet peridotite (normalized anhydrous) that are SiO2 and Al2O3 poor (43–50 and 9–11.5 wt. %, respectively), and MgO and CaO rich (18–27 and 7–12 wt. %, respectively) when compared to anhydrous analogues. These effects become highly pronounced deep in the upper mantle, and are opposite to the effect of H2O on melt compositions in the spinel stability field, potentially owing in part to OH−association with network modifying cations in high pressure, depolymerized melts and in part to low‐temperature stabilization of garnet, which enhances CaO/Al2O3.

We present a local earthquake tomography to illuminate the crustal and uppermost mantle structure beneath the southern Puna plateau and to test the delamination hypothesis. Vp and Vp/Vs ratios were obtained using travel time variations recorded by 75 temporary seismic stations between 2007 and 2009. In the upper crust, prominent low Vp anomalies are found beneath the main volcanic centers, indicating the presence of magma and melt beneath the southern Puna plateau. Beneath the Moho at around 90‐km depth, a strong high Vp anomaly is detected just west of the giant backarc Cerro Galan ignimbrite caldera. This high Vp anomaly is only resolved if earthquakes with an azimuthal gap up to 300° are included in the inversion. However, we show through data subset and synthetic tests that the anomaly is robust due to our specific station‐event geometry and interpret it as a delaminated block of lower crust and uppermost mantle lithosphere under the southern Puna plateau. The low velocities in the crust are interpreted as a product of the delamination event that triggered the rise of fluids and melts into the crust and induced the high topography in this part of the plateau. The tomography also reveals the existence of low‐velocity anomalies that link arc magmatism at the Ojos del Salado volcanic center with slab seismicity clusters at depths of about 100 and 150 km and support fluid transport in the mantle wedge due to dehydration reaction within the subducted slab.

Two models, a simple cooling model and the plate model, have been advanced to account for the variation in depth and heat flow with increasing age of the ocean floor. The simple cooling model predicts a linear relation between depth and t½, and heat flow and 1/t½, where t is the age of the ocean floor. We show that the same t½ dependence is implicit in the solutions for the plate model for sufficiently young ocean floor. For larger ages these relations break down, and depth and heat flow decay exponentially to constant values. The two forms of the solution are developed to provide a simple method of inverting the data to give the model parameters. The empirical depth versus age relation for the North Pacific and North Atlantic has been extended out to 160 m.y. B.P. The depth initially increases as t½, but between 60 and 80 m.y. B.P. the variation of depth with age departs from this simple relation. For older ocean floor the depth decays exponentially with age toward a constant asymptotic value. Such characteristics would be produced by a thermal structure close to that of the plate model. Inverting the data gives a plate thickness of 125±10 km, a bottom boundary temperature of 1350°±275°C, and a thermal expansion coefficient of (3.2±1.1) × 10−5°C−1. Between 0 and 70 m.y. B.P. the depth can be represented by the relation d(t) = 2500 + 350t½ m, with t in m.y. B.P., and for regions older than 20 m.y. B.P. by the relation d(t) = 6400 - 3200 exp (−t/62.8) m. The heat flow data were treated in a similar, but less extensive manner. Although the data are compatible with the same model that accounts for the topography, their scatter prevents their use in the same quantitative fashion. Our analysis shows that the heat flow only responds to the bottom boundary at approximately twice the age at which the depth does. Within the scatter of the data, from 0 to 120 m.y. B.P., the heat flow pan be represented by the relation q(t) = 11.3/t½ μcal cm−2s−1. The previously accepted view that the heat flow observations approach a constant asymptotic value in the old ocean basins needs to be tested more stringently. The above results imply that a mechanism is required to supply heat at the base of the plate.

This investigation tests the mixed-oxide model against the elastic and density requirements of the mantle within the region immediately below the 650 km discontinuity (approximately 650 to 1200 km). Mixed-oxide compositions containing combinations of (Mg.Fe)O and SiO/sub 2/ which correspond to dunite-peridotite and pyroxenite were emphasized in the modelling procedure. Models in the compositional range dunite-peridotite were found to satisfy the P and S-wave velocities, as well as the density requirements below the 650 km discontinuity; satisfactory solutions were provided by such models when Fe/(Mg + Fe) = 0.15 +- 0.05 along an adiabatic temperature gradient of 1300/sup 0/C at zero pressure. Models of pyroxenite composition do not satisfy the physical properties of the mantle in the region of interest. The successful dunite-peridotite mixed-oxide compositional models provide an estimate of the geotherm in the 650 to 1200 km region of the mantle. Since the physical properties within the region of interest are satisfied along an adiabat, the problem is reduced to calculating the increase of temperature with depth for the preferred model along the adiabatic gradient (initial temperature 1300/sup 0/C). This procedure yields a geotherm which is consistent with several independent upper mantle temperature estimates. The present results indicate amore » temperature of 1600 +- 400/sup 0/C at 671 km; the primary source of uncertainty is the accuracy of the seismic model interpretations.« less

The solidus comprises three curves, corresponding to subsolidus mineral assemblages with cusps at about 11 and 26 kbar. A thin layer of basalt was sandwiched between compressed blocks of powdered peridotite minerals and then was equilibrated with its host at melting temperatures. The basalt melt, was completely homogenized with the partial melt in the peridotite matrix within 24 hours. The role of K 2 O in the melting was investigated. Hypothesis of shallow-depth origin for MORBS is supported.--Modified journal abstract.

The existence of the low-velocity layer (or layers) in the uppermost parts of the mantle, at least in oceanic and tectogenic regions, is now accepted without any doubt. Different explanations for the origin of the low-velocity layers have been suggested. One suggestion has been that partial melting is the principal cause; however, this suggestion presents two problems: (1) poor knowledge of the temperature at each respective depth, and (2) discovery in some areas of high-velocity layers that are situated just above the low-velocity zones. These two problems are discussed in this paper.

Recently developed ideas of global tectonics haye provided a new framework within which to consider the origin of alpine-type peridotites. In plate theory, compressional zones associated with island arcs are considered to represent plate boundaries where oceanic lithosphere is subducted. The subduction zones are characterized by lithospheric underthrusting, andesitic volcanoes, and deep seismic activity that generally dips under the continental edge (the Benioff zone). The presence of large oceanic-mantle crustal slabs thrust over or into continental edges contemporaneously with blueschist metamorphism in New Caledonia and New Guinea establishes an important variant of plate tectonics in the zones of compression. The ‘obduction’ zones are characterized by a complete lack of volcanic activity and by high-pressure metamorphism. During formation, they can be represented by shallow seismic zones dipping oceanward. The common association of peridotites and blueschists in these orogenic belts may result from the initial stage of compressional impact (or orogeny) between an oceanic and a continental lithospheric plate. Disturbed zones combined with a lack of high-temperature contacts at boundaries between cold mantle-peridotite slabs and trench sediments provide geologic evidence of emplacement by obduction (tectonic overriding). Internal subsolidus plastic deformation of these peridotites can be attributed to deep-seated strain within the upper mantle during spreading. Serpentinites represent alteration developed during tectonic emplacement into wet sediments of the continental plate, which produces a less dense and plastic envelope that facilitates further tectonic movement in these compressional zones. Recognition of these peridotite-serpentinite-blueschist belts within exhumed subduction or obduction zones will allow delineation of ancient compressional impacts between moving lithospheric plates.

Mechanisms permitting the steady state deformation of crystalline solids are critically reviewed, and an approximate constitutive relationship is derived for fluid phase transport in a partial melt. (Fluid phase transport has a linear stress dependence and an inverse squared grain size dependence.) A set of rheologic material constants for olivine (Fo85-Fo95) are derived from a combination of experimental data and empirical generalizations. Our preferred power law exponent n is 4.2, and the associated Dorn parameter is 1.2 × 104. Dislocation creep below the high-stress breakdown is the dominant deformation mechanism in the upper mantle. The possibility exists that the bottom of the upper mantle is not deforming at a significant rate (≥10−14/sec) if the activation volume for diffusion is greater than 40 cm³/mole.

In the study of subduction zone processes, there has been much debate on slab dehydration against slab melting, as well as on flux melting of the mantle wedge against metasomatism and a prolonged period of storage and heating before mantle melting. H2O‐saturated solidus of rock is critical to the debate. Unfortunately, there is a significant discrepancy in H2O‐saturated solidus between different experimental studies, mainly due to the difficulty in distinguishing hydrous melt from aqueous fluid in quenched experimental products. In this study, we developed a new approach to locating H2O‐saturated solidus of rock systems by monitoring the electrical conductivity of the system in situ. A mixture of albite and >10 wt% H2O was heated stepwise in the range of 0.35–1.7 GPa in a piston cylinder apparatus. A jump in electrical conductivity (up to 1.8 log units) was observed over a small temperature interval (10–20°C). The temperature corresponding to the electrical jump decreased from 790 ± 10°C at 0.35 GPa to 640 ± 10°C at 1.7 GPa, in good agreement with previously reported H2O‐saturated solidus of albite. Examination of the experimental products quenched immediately after observation of the electrical conductivity jump confirmed the occurrence of albite melting. Our experimental results suggest that the electrical conductivity jump serves as a good indicator for rock melting under H2O‐saturated conditions, and this in situ approach can overcome the ambiguity in phase identification in quench experiments. This new approach has the potential to solve the discrepancy over H2O‐saturated solidus of a variety of rock systems and shed light on subduction zone processes.

Fully two-dimensional analytic boundary layer solutions are used to model the thermomechanical structure of the oceanic upper mantle when a shallow horizontal return flow helps balance the lithospheric transport of mass from ridge to trench. The following are all incorporated in the solutions: horizontal and vertical advection of heat, vertical heat conduction, viscous dissipation, adiabatic heating and cooling, buoyancy, and the pressure- and temperature-dependent nonlinear rheology of olivine. Depth profiles of horizontal and vertical velocities, temperature, and shear stress are calculated for several ages of the ocean floor. Such solutions are used to construct accurate isotherm and streamline patterns within the rigid lithosphere and high-shear, return flow asthenosphere of the oceanic upper mantle boundary layer. Ocean floor topography is inferred from the thermal contraction of the cooling lithosphere and asthenosphere and from the adverse horizontal pressure gradient required by the dynamics to drive the shallow return flow. The flattening of the topography of the old ocean floor can be attributed to the retardation of boundary layer cooling by shear heating and/or to the adverse pressure gradient of a shallow return flow. The latter effect would be dominant if a shallow return flow did occur in the earth's upper mantle. Solutions which provide adequate fits to the ocean floor bathymetry data for the Pacific plate can be found if the activation volume for the creep of olivine is small, i.e., about 11 cm3/mol, and if the deep mantle temperature T is high, i.e., if T is about 1500°C at depths of 100–200 km (depending on age) for dry olivine or about 1400°C at similar depths for wet olivine. The upper mantle temperatures required for a shallow return flow to be compatible with bathymetry data imply partial melting in the lower lithosphere and upper asthenosphere from the ridge axis to ages of about 10 m.y. Shear stresses at the base of the rigid lithosphere generally range from about 1 to several tens of bars, and horizontal adverse pressure gradients at great depth vary from about 10 to 100 mbar/km for wide variations in the extent of return flow and deep mantle temperature. For a relatively cold deep mantle, shear stresses in the lower lithosphere and asthenosphere decrease almost linearly with depth, implying horizontal pressure gradients essentially constant with depth. For a hot deep mantle, shear stress versus depth profiles show curvature in the lower lithosphere and upper asthenosphere and a linearly decreasing pattern at greater depths. Under such circumstances, horizontal pressure gradients exhibit considerable depth variation including the possibility of a reversal in sign between the top and the bottom of the asthenosphere.

Experiments to determine silicate structural species in silicate‐saturated aqueous fluids in equilibrium with silica polymorphs (quartz and coesite), enstatite, and enstatite + forsterite in the SiO2–H2O and MgO–SiO2–H2O systems have been carried out in situ in the 0.4–5.4 GPa and 700–900°C pressure and temperature ranges, respectively. MicroRaman spectroscopy was the structural probe. In the SiO2–H2O system (1.6–5.4 GPa/700–900°C), the detected silicate species are Q0 (SiO44−), Q1 (0.5 Si2O76−), and Q2 (SiO32−). The expression 2Q1 ⇌ Q0 + Q2 describes the equilibrium among these species with ∆H and ∆V values from the isochoric temperature and isothermal pressure dependence of its equilibrium constant, K = XQ0·XQ2/(XQ1)2, range from −23 to −69 kJ/mol and −1 to −2 cm3/mol, respectively. In the system MgO–SiO2–H2O the calculated silica solubility, using literature algorithms, is approximately 50% of that in the SiO2–H2O system at similar temperature and pressure. Only Q1 and Q0 species were detected in the MgO–SiO2–H2O fluids, whether in equilibrium with enstatite + forsterite (P < 3 GPa) or enstatite only (P > 3 GPa). The temperature and pressure dependence of the equilibrium constant, K = XQ1/XQ0, for this system yields average values of ∆H = 40 ± 5 kJ/mol and ∆V = −2.3 ± 0.4 cm3/mol. The speciation of silicate in aqueous fluids resembles that in hydrous melts as a function of temperature and pressure at deep crustal and upper mantle temperature and pressure conditions, and they become increasingly similar with depth. As the silicate speciation and solubility in the aqueous fluid depend on silicate composition, the pressure and temperature at which complete miscibility occurs will also vary with silicate composition. The structural similarity between fluids and melts will also lead to fluid/melt element partition coefficients trending toward 1 and mineral/fluid partition coefficients trending toward mineral/melt values in the upper mantle as the silicate‐H2O systems approach complete miscibility with increasing temperature and pressure.

Dislocation densities of olivine grains in peridotite nodules from Ichinomegata (25 samples), Sannomegata (18 samples), Oki-Dogo (13 samples), Hamada (3 samples) and Takashima (10 samples) were measured for estimating differential stress in the upper mantle. The density was in the range of 106-107cm-2. The differential stress is estimated as 100-300 bars using KOHLSTEDT and GOETZE'S empirical relationship (1974) between the dislocation density and the differential stress. Strain rate is inferred from the geotherm of the island arc and the flow law of olivine single crystal proposed by KOHLSTEDT and GOETZE (1974) and DURHAM and GOETZE (1977). Strain rate in the upper mantle from 30 to 100km depth is less than 10-15 sec-1, but it is nearly constant around 10-13-10-12sec-1 from 100 to 200km depth.

Crystalline pargasite-rich spinel peridotite mylonite from St. Paul's Rocks containing 5.7% water bound in hydrous minerals was reacted in sealed platinum capsules in a piston-cylinder apparatus from 10 - 30 kb. At 10 kb the subsolidus assemblage is amphibole, olivine, orthopyroxene, clinopyroxene and spinel, an amphibole lherzolite; with increasing pressure garnet appears at 18 kb, spinel and amphibole disappear at about 25 kb; the resulting high pressure assemblage is that of a garnet lherzolite. The solidus was located in the presence of water-rich vapor, but vapor dissolves completely in the liquid at higher temperatures, and the liquid becomes water-undersaturated. Stability limits in the melting interval were determined for amphibole, garnet, spinel, clinopyroxene, orthopyroxene, and olivine (the liquidus mineral). The results are consistent with a published conclusion that St. Paul's Rocks is a diapiric, solid-state mantle intrusion initially mobilized at a depth between 45 km and 70 km near 1,000 - 1,050°C. An estimate of the solidus of peridotite with 0.2% water is presented and compared with other studies. At intermediate pressures this solidus is determined by the breakdown of amphibole. Discrepancies among results of the various studies probably arise at least in part from experimental problems involved in the complex systems.

CO 2 can play an important role in eruptive processes; in particular, it has the potential to reach saturation at lower concentrations than H 2 O and initiate degassing. The effect of such CO 2 loss on magma viscosity is not well constrained, especially compared to the established effects of H 2 O loss. In terms of understanding the CO 2 solubility mechanism, recent spectroscopic studies have shown that CO 2 speciation is strongly temperature dependent and that CO 2 speciation preserved in quenched glasses below T g is different from the true CO 2 speciation observed in the melts. However, the effect of CO 2 on the glass transition temperature, and by inference the viscosity, has not been previously established. In this study, calorimetric measurements were conducted on synthetic H 2 O- and CO 2 -bearing phonolite and jadeite glasses in order to investigate the volatile’s effect on the glass transition interval, by defining a single glass transition temperature ( T g onset ). The samples were synthesised in a piston-cylinder apparatus between 1300 and 1550 °C, at 1.0 to 2.5 GPa, and contained up to 2.29 wt.% CO 2 and up to 5.49 wt.% H 2 O. For both compositions, H 2 O has a large effect in reducing T g onset , but CO 2 appears to have little or no effect. For the entire range of H 2 O contents, T g onset decreases exponentially with H 2 O content from 870 to 523 K and 1036 to 636 K for phonolite and jadeite, respectively, regardless of the CO 2 content. No measurable effect of CO 2 on T g onset was observed. These results suggest that compared to H 2 O, CO 2 contributes little to changes in the physical properties of the melt. They also provide strong evidence for the decoupling of CO 2 speciation from the bulk silicate melt structural relaxation process at T g .

An interpretation of the geomagnetic inductive response function, C( W, 0), observed at Kiruna in northern Sweden, is herein undertaken. The bounds of acceptable solutions are initially discovered by a Monte-Carlo random search procedure, and the best-fitting solutions are examined by the application of linear theory to the problem. The data are shown to have a higher degree of internal consistency than that described by the estimated variances of each datum. A further Monte-Carlo inversion of the variancereduced data set gives solutions with well defined model parameters. The two major features of the models are: (1) a small, or non-existent, electrical conductivity variation across the seismic Moho boundary, and (2) the unequivocal existence of an electrical asthenosphere, under the Fennoscandian shield, beginning at a depth of between 155-185 km, and of 60 km minimum thickness. Both of these observations have seismic counterparts. Finally, possible mantle temperature profiles are deduced which depend on the assumptions and laboratory data employed.

Abstract Natural diamonds from a variety of African localities, Venezuela and Thailand contain abundant monomineralic inclusions with a grain size that is generally in the range 10–200 μm. Many of these were incorporated in diamond at the time of crystallization of the host and have been insulated from further reaction with their surroundings. Inclusions of olivine, garnet, chromite, enstatite and Ca-rich pyroxene have been studied by electron probe and X-ray techniques. The chemical compositions of many inclusions resemble those of similar minerals that occur as xenocrysts and in peridotite xenoliths in kimberlite, but there are a number of consistent differences. A minority of the garnet and pyroxene inclusions show some resemblances to minerals from eclogites and grospydites. There are few experimental data that can be used to interpret the differences between the inclusions and their counterparts in peridotite xenoliths. However, it can be hypothesized that the diamonds formed in igneous events and that the inclusions they contain crystallized in equilibrium with liquid. The minerals in the peridotite xenoliths, in contrast, may represent assemblages formed after cooling and equilibration at a lower temperature, perhaps in a volatile-rich liquid or perhaps in the solid state. We suggest that the inclusions have not participated in lower temperature equilibration because they were armored by diamond.

Abstract Hydrogen-rich and water-rich fluids exert different control on dissolution mechanism of oxide and silicate minerals in the Earth’s interior. With Mgsilicate- H2 fluids, dissolution tends to be incongruent with the (Mg/Si)fluid < (Mg/Si)Mg-silicate with formation of SiOH and SiH4 complexes in the fluid (Shinozaki et al. 2013, 2014). In contrast, in Mg-silicate-H2O systems, Mg-silicate minerals in the mantle (pyroxene and forsterite) dissolve stoichiometrically (congruently) in aqueous fluids to at least 10 GPa pressure. Metasomatic alteration by H2-rich fluids enriches, therefore, the mantle in SiO2 compared with alteration by H2O fluid. This difference becomes increasingly important with mantle depth because the environment becomes more reducing, which results in an increase of H2/H2O fluids (Shinozaki et al. 2014, this issue). Chemical gradients with depth of the Earth could be affected by increased H2/H2O of mantle fluids whereby Mg/Si ratios, for example, will become variable. Silicate-H2 alteration processes likely also played major roles during the early, core-forming stages of the Earth. Such a process could be responsible for Mg/Si changes in the early silicate Earth.

Abstract The rate of hydrogen diffusion in clinopyroxene is relevant to interpreting hydrogen (“water”) concentrations in xenoliths, phenocrysts, and clinopyroxene-hosted melt inclusions to provide insight into the deep-earth water cycle and volcanic explosivity. Here, we determine bulk and site-specific hydrogen diffusivities in two diopsides and an augite by heating initially homogeneous water-bearing samples in a 1-atm CO/CO2 gas-mixing furnace at 800–1000 °C and oxygen fugacity at the quartz–fayalite–magnetite buffer and observing H-loss profiles. The O–H stretching range between wavenumbers 3000 and 4000 cm−1 in FTIR spectra is resolved into 4–6 peaks, each of which is assumed to represent a distinct defect site for the hydrogen, to determine peak-specific diffusivities using our previously published whole-block method. For the diopside from the Kunlun Mts. in China, Arrhenius relations are reported for peaks at 3645, 3617, 3540, 3443, and 3355 cm−1 based on measurements at 816, 904, and 1000 °C. Bulk and site-specific diffusivities are determined for the same set of peaks at 904 °C for the second diopside (Jaipur). The augite (PMR-53) was a triangular thin slab, and hydrogen diffusivities were determined for bulk hydrogen and peaks at 3620, 3550, 3460, and 3355 cm−1 in the thickness direction at 800 °C. Bulk hydrogen diffusivity in the Jaipur diopside is consistent with previous work, and hydrogen diffusivity in augite PMR-53 is slightly lower than the fast direction diffusivities measured || [100] and [001]* in Jaipur diopside. Both diopsides show 1–2 orders of magnitude differences in the peaks-specific diffusivities, with the fastest diffusivities at 3450 cm−1 and the slowest at 3645 cm−1. However, the hydrogen diffusivities in Jaipur diopside are 2–4 orders of magnitude higher than those in Kunlun diopside for bulk hydrogen and all peaks. Thus, peak-specific differences cannot by themselves adequately explain the 5 orders of magnitude range in hydrogen diffusivities observed experimentally in different diopsides. The results are broadly consistent with a previously proposed increase in hydrogen diffusivity in diopside with Fe up to 0.6–0.8 a.p.f.u., although there may be an opposing relationship with Al(IV). The results of this study and others predict high water diffusivities in Fe-bearing mantle xenolith clinopyroxene, on the order of ~10−9 to 10−11 m2/s at 1000 °C. The common observation of hydrogen zonation in mantle xenolith olivine, but not in clinopyroxene implies that hydrogen diffusion is much faster in olivine than in pyroxene, which then requires the operation of the fastest diffusion mechanism quantified in olivine and diffusivities in clinopyroxene at the lower end of this 2 orders of magnitude range. Such high diffusivities strongly suggest that water in mantle xenoliths has at least partially equilibrated with the host magma, and that the diffusion profiles observed in mantle xenolith olivine reflect only the final stage of ascent after water begins to exsolve.