Dry Peridotite Solidus Research Articles

Compositions and formation conditions of primitive magmas of the Karymsky volcanic center, Kamchatka: evidence from melt inclusions and trace-element thermobarometry

This paper reports the results of a study of naturally and experimentally quenched melt inclusions in magnesian olivine (Fo77–89) from a basalt sample from the Karymsky volcanic center, which is located in the middle segment of the Eastern Volcanic Front of Kamchatka. The conditions of parental magma formation were estimated using modern methods of trace-element thermometry. Based on direct H2O measurements in inclusions and thermometry of coexisting olivine and spinel, it was shown that the parent melts contained at least 4.5 wt % H2O and crystallized at a temperature of 1114 ± 27°C and an oxygen fugacity of DQFM = 1.5 ± 0.4. The obtained estimates of H2O content and crystallization temperature are among the first and currently most reliable data for the Eastern Volcanic Front of Kamchatka. The primary melt of the Karymsky volcanic center was derived from peridotitic material and could be produced by ~12–17% melting of an enriched MORB source (E-DMM) at ~1230–1250°C and ~1.5 GPa. Our estimates of mantle melting temperature beneath Kamchatka are slightly lower than values reported previously and up to 50°C lower than the dry peridotite solidus, which indicates the influence of a slab-derived hydrous melt. The combined approach to the estimation of the initial H2O content of melt employed in this study can provide a more reliable data in future investigations, and its application will probably to decrease the existing temperature estimates for the mantle wedge above subduction zones.

Compositions and Formation Conditions of Primitive Magmas of the Karymsky Volcanic Center, Kamchatka: Evidence from Melt Inclusions and Trace-Element Thermobarometry

This paper reports the results of a study of naturally and experimentally quenched melt inclusions in magnesian olivine (Fo77–89) from a basalt sample from the Karymsky volcanic center, which is located in the middle segment of the Eastern Volcanic Front of Kamchatka. The conditions of parental magma formation were estimated using modern methods of trace-element thermometry. Based on direct H2O measurements in inclusions and thermometry of coexisting olivine and spinel, it was shown that the parent melts contained at least 4.5 wt % H2O and crystallized at a temperature of 1114 ± 27°C and an oxygen fugacity of ΔQFM = 1.5 ± 0.4. The obtained estimates of H2O content and crystallization temperature are among the first and currently most reliable data for the Eastern Volcanic Front of Kamchatka. The primary melt of the Karymsky volcanic center was derived from peridotitic material and could be produced by ~12–17% melting of an enriched MORB source (E-DMM) at ~1230–1250°C and ~1.5 GPa. Our estimates of mantle melting temperature beneath Kamchatka are slightly lower than values reported previously and up to 50°C lower than the dry peridotite solidus, which indicates the influence of a slab-derived hydrous melt. The combined approach to the estimation of the initial H2O content of melt employed in this study can provide a more reliable data in future investigations, and its application will probably decrease the existing temperature estimates for the mantle wedge above subduction zones.

Composition and conditions of formation of the parental melts of Jurassic dolerites of southwestern Crimea: Evidence from melt inclusions in olivine phenocrysts

This study focuses on Jurassic shallow intrusions and subvolcanic bodies from around Trudolyubovka village in the southwestern Crimea. All the rocks are similar in mineral composition and have similar geochemical features and occur in close spatial and geological association. This allows us to assign the intrusions to a single magmatic series and interpret them as differentiation products of a single parental melt. The investigation of melt inclusions in olivine from the most magnesian sample showed that the composition of igneous melts ranged from basalt to basaltic andesite of a moderately potassic subalkaline affinity. Compared with N-MORB, they are enriched in LILE, but have similar HFSE and REE contents. The early magmatic melts crystallized at temperatures ranging from 1240 to 1125°C, pressures of 6–8 kbar, and an oxygen fugacity of ΔQFM = +0.6; and later melts crystallized at 1090–940°C, ~1.5 kbar, and oxygen fugacity increasing from ΔQFM + 0.9 to ΔQFM + 2.3. The minimum pressure of groundmass crystallization was estimated as 40–60 bar. The primitive melts were formed in a mature island arc or an active continental margin setting by ~13% melting of a DMM-like source. The melting occurred at spinel-facies depths under the influence of a slab-derived fluid at a temperature 25°C below the dry peridotite solidus.

High-Pressure Melting Experiments on Basalt-Peridotite Layered Source (KLB-1/N-MORB): Implications for Magma Genesis in Hawaii

In order to understand the melting processes that occur within recycled oceanic crust and mantle in a heterogeneous plume (e.g., that beneath the Hawaiian Islands), a series of high-pressure-high-temperature layered experiments were performed at 2.9 GPa, 5 GPa, and 8 GPa, from 1300°C to 1650°C, using a fertile peridotite KLB-1 and N-MORB. Our experiments at conditions below the dry peridotite solidus produced melt compositions that ranged from basaltic andesite to tholeiite. An Opx reaction band formed between eclogite and peridotite layers, likely via chemical reaction between a silica-rich eclogite-derived partial melt and olivine in the peridotite matrix. At temperatures at or above the dry peridotite solidus, substantial melting occurred in both basalt and peridotite layers, and fully molten basalt melt and melt pockets from the peridotite layer combined. In our layered experiments, major and minor element contents in reacted melts closely matched those of Hawaiian tholeiite and picrite, except for Fe. Partial melts of anhydrous run products had ~55 - 57 wt% SiO2 at low temperature (i.e., were andesitic) and had ~50 - 53 wt% SiO2 at high temperatures, slightly below the dry peridotite solidus (i.e., were tholeiitic, and similar to those that occur during the Hawaii shield-building stage). Based on the Fe- and LREE-enriched signature in Hawaiian tholeiites, we propose that recycled components in the Hawaiian plume are not modern N-MORB, but are Fe-rich tholeiite; a lithology that was common in the Archaean and early Proterozoic. We have demonstrated that the entire compositional spectrum of Hawaiian tholeiites (basalt to picrite) can be formed by basalt-peridotite reactive melting near the dry solidus of peridotite. Based on these results, we propose that the potential temperature of the sub-Hawaiian plume may be much lower than previously estimated.

The Composition of Hydrous Partial Melts of Garnet Peridotite at 6 GPa: Implications for the Origin of Group II Kimberlites

Chemical compositions of hydrous melts, compatible with those that would form by incipient melting of upper mantle peridotite at 180 km depth, have been determined using a series of iterative crystallization experiments. Experiments were performed in a multianvil apparatus at 6 GPa and 14008C and a melt was ultimately produced that was saturated in a residual peridotite assemblage (olivineþ clinopyroxeneþgarnet orthopyroxene). The multiply saturated hydrous melts have higher (MgþFe)/Si and Al/Ca compared with hydrous melts produced at lower pressures.The melt compositions are similar to those determined near the dry peridotite solidus at 17008C, when compared on an H2O-free basis. Melt H2O contents were determined to be 11wt % using mass balance, and these estimates were made more accurate by maintaining a large proportion of melt (470 wt %) in each experiment. If the geophysically inferred seismic low-velocity zone is caused by the presence of H2O-rich melt then at the base of this zone, at 220 km, these results imply that the melt must contain 15^16 wt % H2O.The hydrous melt compositions, when compared on a volatile-free basis, are found to be similar to those of group II kimberlites (orangeites). The low FeO and Na2O but enriched K2O concentrations in group II magmas imply their derivation from melt-depleted cratonic lithosphere enriched by the metasomatic addition principally of K2O and H2O. A simple model is proposed in which this enrichment occurs by the addition of phlogopite to the source peridotite. Using determined K2O and H2O partition coefficients and assuming that the ratio of both components in the source is controlled by their ratio in phlogopite, group II kimberlite magmas can be constrained as being the product of 0 2 wt % melting of a garnet peridotite source rock enriched with 1 7 wt % phlogopite, undergoing melting at near-adiabatic temperatures.

Temperature, Pressure, and Composition of the Mantle Source Region of Late Cenozoic Basalts in Hainan Island, SE Asia: a Consequence of a Young Thermal Mantle Plume close to Subduction Zones?

Basaltic lavas from Hainan Island near the northern edge of the South China Sea have an age range of between late Miocene (about 13 Ma) and Holocene, with a peak age of late Pliocene to middle Pleistocene. The basaltic province is dominated by tholeiites with subordinate alkali basalts. Most analysed samples display light rare earth element (LREE) enriched REE patterns and ocean island basalt (OIB)-like incompatible element distributions. The basalts contain abundant undeformed high-Mg olivine phenocrysts (up to Fo90·7) that are high in CaO and MnO, indicating high-magnesian parental magmas. Independent barometers indicate that clinopyroxenes in the basalts crystallized over a wide range of pressures of 2–25 kbar (dominantly at 10–15 kbar) and that the melt cooled from about 1350°C to 1100°C during their crystallization. The compositional characteristics of the basalts indicate that their generation most probably involved both low-silica and high-silica melts, as represented by the alkali basalts and tholeiites, respectively. Our results show that the source region for the Hainan basalts is highly heterogeneous. The source for the tholeiites is mainly composed of peridotite and recycled oceanic crust, whereas the source for the bulk of the low-Th alkali basalts consists predominantly of peridotite and low-silica eclogite (garnet pyroxenite). Some high-Th (≥ 4 ppm) alkali basalts may have been produced by partial melting of low-silica garnet pyroxenite (eclogite). We estimated the primary melt compositions for the Hainan basalts using the most forsteritic olivine (Fo90·7) composition and the most primitive bulk-rock samples (MgO > 9·0 wt % and CaO >8·0 wt %), assuming a constant Fe–Mg exchange partition coefficient of KD = 0·31 and Fe3+/FeT = 0·1. The effective melting pressure (Pf) and melting temperature (T) of the primary melts are Pf = 18–32 kbar (weighted average = 23·8 ± 1·8 kbar) and T = 1420–1520°C for the tholeiites, and Pf = 25–32 (weighted average = 28·3 ± 1·4 kbar) and T = 1480–1530°C for the alkali basalts. The Pf –T data form an array that plots systematically above the dry lherzolite solidus but below the base of the lithosphere (∼55 km) and intersects the dry peridotite solidus at a pressure of about 50 kbar. The mantle potential temperature beneath Hainan Island, based on the estimate primary melt compositions, varies from about 1500 to 1580°C with a weighted average of 1541 ± 10°C. The high-magnesian olivine phenocrysts, high mantle potential temperature, and the presence of recycled oceanic crust in the source region provide independent support for the Hainan plume model that has previously been proposed largely based on geophysical observations. The Hainan plume thus provides a rare example of a young mantle plume associated with deep slab subduction.

Constraints on mantle melting and composition and nature of slab components in volcanic arcs from volatiles (H 2O, S, Cl, F) and trace elements in melt inclusions from the Kamchatka Arc

New and published data on the composition of melt inclusions in olivine (Fo 73–91) from volcanoes of the Kamchatka and northern Kurile Arc are used 1) to evaluate the combined systematics of volatiles (H 2O, S, Cl, F) and incompatible trace elements in their parental magmas and mantle sources, 2) to constrain thermal conditions of mantle melting, and 3) to estimate the composition of slab-derived components. We demonstrate that typical Kamchatkan arc-type magmas originate through 5–14% melting of sources similar or slightly more depleted in HFSE (with up to ∼ 1 wt.% previous melt extraction) compared to MORB-source mantle, but strongly enriched in H 2O, B, Be, Li, Cl, F, LILE, LREE, Th and U. Mean H 2O in parental melts (1.8–2.6 wt.%) decreases with increasing depth to the subducting slab and correlates negatively with both ‘fluid-immobile’ (e.g. Ti, Na, LREE) and most ‘fluid-mobile’ (e.g. LILE, S, Cl, F) incompatible elements, implying that solubility in hydrous fluids or amount of water does not directly control the abundance of ‘fluid-mobile’ incompatible elements. Strong correlation is observed between H 2O/Ce and B/Zr (or B/LREE) ratios. Both, calculated H 2O in mantle sources (0.1–0.4%) and degrees of melting (5–14%) decrease with increasing depth to the slab indicating that the ultimate source of water in the sub-arc mantle is the subducting oceanic plate and that water flux (together with mantle temperature) governs the extent of mantle melting beneath Kamchatka. A parameterized hydrous melting model [Katz et al. 2003, G 3, 4(9), 1073] is utilized to estimate that mantle melting beneath Kamchatka occurs at or below the dry peridotite solidus (1245–1330 °C at 1.5–2.0 GPa). Relatively high mantle temperatures (yet lower than beneath back-arc basins and ocean ridges) suggest substantial corner flow driven mantle upwelling beneath Kamchatka in agreement with numerical models implying non-isoviscous mantle wedge rheology. Data from Kamchatka, Mexico and Central America indicate that < 5% melting would take place beneath continental arcs without water flux from the subducting slab. A broad negative correlation appears to exist between crustal thickness and the temperature of magma generation beneath volcanic arcs with larger amounts of decompression melting occurring beneath thinner arc crust (lithosphere). In agreement with the high mantle temperatures, we observe a systematic change in the composition of slab components with increasing slab depth from solute-poor hydrous fluid beneath the volcanic front to solute-rich hydrous melt or supercritical liquid at deeper depths beneath the rear arc. The solute-rich slab component dominates the budget of LILE, LREE, Th and U in the magmas and originates through wet-melting of subducted sediments and/or altered oceanic crust at ≥ 120 km depth. Melting of the upper parts of subducting plates under water flux from deeper lithosphere (e.g. serpentinites), combined with high temperatures in the mantle wedge, may be a more common process beneath volcanic arcs than has been previously recognized.

Mantle solidus: Experimental constraints and the effects of peridotite composition

A review of experiments on natural peridotites allows improved constraints on the location of the mantle solidus. Available constraints on the location of the nominally dry peridotite solidus show considerable scatter, owing to interlaboratory uncertainties and the effects of bulk composition. When experiments on enriched peridotite are filtered from the database, the best fit to the solidus between 0 and 10 GPa is given by T(°C) = aP2 + bP + c where a = −5.104, b = 132.899, and c = 1120.661 and P is in gigapascals. Compared to previous models, the solidus in this parameterization is at lower temperature between 2 and 6 GPa, with the largest differences near 4 GPa, where it is 30°–60°C cooler. Consideration of experimental constraints on the peridotite solidus and of a theoretical model of melting in a simple analogue system suggests that a key variable affecting peridotite solidus temperature is the near‐solidus liquid alkali concentration. The effect of alkalis on the solidus increases with bulk concentration in the peridotite but decreases with bulk partition coefficient. Thus small bulk concentrations of K can have a significant influence on the peridotite solidus, and the effect of Na diminishes with increasing pressure, as it becomes more compatible in the solidus residua. Mg # [=100 × MgO/(MgO + FeO)] variations are subordinate to alkali variations in controlling solidus temperature at lower pressures but may increase in relative importance as alkalis become more compatible in peridotite residua with increasing pressure. Increased clinopyroxene mode has the effect of making Na more compatible in residual solids and so diminishes the solidus‐lowering tendencies of alkalis. As a consequence, experiments performed on a range of peridotite compositions may not reflect the likely effect of variable mantle composition on solidus temperature if they do not match the appropriate correlation between alkali content and clinopyroxene mode.

Tectonic controls on magmatism associated with continental break-up: an example from the Paraná–Etendeka Province

The high- and low-Ti basalts of the Paraná–Etendeka province were primarily derived from old, trace element-enriched source regions in the lithospheric mantle, and they are associated with dyke swarms of different orientations. These swarms appear to reflect different amounts of extension, and it is inferred that the high- and low-Ti magma types were characterised by different melt generation rates of ∼0.15 km 3 yr −1 and ∼0.4 km 3 yr −1, respectively [Stewart et al. Earth Planet. Sci. Lett. 143 (1996) 95–109]. There is probably a gap of ∼2 Myr between the end of the main phase of CFB magmatism and the oldest rocks on the adjacent ocean floor. A simple numerical model has been used to constrain the amounts and rates of melt generated from the continental lithosphere and asthenosphere under finite duration extension. Melting in the mantle is assumed to be controlled by the dry peridotite solidus in the asthenosphere and the hydrous (0.2% H 2O) peridotite solidus in the lithosphere. For a maximum β of 4 and a duration of extension of 10 Myr, the derivation of melt from the asthenosphere by dry peridotite melting depends primarily on potential temperature ( T p) and is relatively insensitive to the thickness of the MBL, while the converse is the case for melt derived from the lithosphere by hydrous peridotite melting. For a T p of 1450±50°C inferred from the crustal thickness estimates along the Rio Grande Rise and Walvis Ridge, the model successfully generates 2–4 km of lithosphere-derived melt before producing significant volumes of asthenosphere-derived melt. It is concluded that increases of melt volume with time can be generated by decompression melting of the mantle lithosphere. Critically, in areas of significant melt generation within the mantle lithosphere during extension and break-up, there is likely to be a gap in the volcanic record between the end of melt generation in the lithosphere and the onset of melting in the underlying asthenosphere. No such gap is present if all melts are generated within the mantle plume, and thus these models may in principle be tested in the geologic record.

Upper mantle temperatures and lithosphere-asthenosphere system beneath the French Massif Central constrained by seismic, gravity, petrologic and thermal observations

The new tomographic image of the mantle beneath the French Massif Central reaching a depth of 270 km is interpreted in terms of mantle temperature, considering effects of anharmonicity and anelasticity on seismic velocities as well as effects of mineral reactions, composition and partial melt. For every block of the tomographic model we calculate the absolute temperature required to fit the observed velocity perturbation, the average temperature of the tomographic layer being constrained by P-T estimates from mantle xenoliths and by surface heat flow. From the 3-D temperature distribution we estimate the topography of the thermal lithosphere-asthenosphere boundary as well as 3-D distributions of density, absolute P- and S-velocities and seismic attenuation.The observed velocity perturbations in the mantle beneath the Massif Central can be explained nearly entirely by temperature variations. Temperatures approach the dry peridotite solidus in the depth range from 50 to 90 km just below Cenozoic volcanic areas, but no large-scale partial melting is required to fit the seismic observations. Model temperatures agree well with P-T estimates from mantle xenoliths and measured surface heat flow. Model-predicted seismic velocities, seismic attenuation and density fit well the observations from seismic refractions, surface waves and gravity. The model predicts a broad uplift of the thermal lithosphere-asthenosphere boundary to a depth of 65–70 km with a 50–70 km wide band of stronger lithospheric thinning which crosses the main volcanic fields and strikes parallel to the direction of maximal compression in the crust. The Limagne Graben, which is the major rift structure of the Massif Central, has no clear expression in the topography of the lithosphere-asthenosphere boundary.Our interpretation suggests a mantle plume below the central and southern part of the Massif Central with a potential temperature which is about 150–200°C higher than the average potential temperature of the upper mantle. The structure of the lithosphere-asthenosphere boundary provides evidence for a possible thinning of the mantle part of the lithosphere beneath the volcanic fields parallel to the direction of minimal horizontal compression in the crust.

The generation of small melt-fractions in truncated melt columns: constraints from magmas erupted above slab windows and implications for MORB genesis

AbstractAt a number of locations along the Pacific margin of the Americas and west Antarctica, small volumes of alkalic basalts were erupted following successive ridge crest-trench collisions. The basalts were generated as a result of upwelling of asthenosphere through windows in the subducted plate. There is no evidence for local high temperature mantle plumes or significant lithospheric extension associated with these basalts. For the best sampled area, the Antarctic Peninsula, mean values for fractionation-corrected iron content (FeO*) vary from 6.9 to 10.6 wt.%, and for Na2O (Na8.0) from 3.25 to 4.6 wt.%, implying generation of small melt-fractions at variable mean pressures. The results of rare earth element inversion modelling yield a melt generation interval of 100 to 52 km, with a maximum melt fraction of c. 7% generated from a MORB-like source atTp1300°C. Trace element and isotope systematics are also consistent with the generation of the basalts from a MORB-like source.Mean pressures of melt generation increase with increasing distance from the original trench, but trace element and Na8.0data suggest that there is no systematic variation in extent of melting with distance from the trench. The data are consistent with a model whereby a MORB melting column, initially intersecting the peridotite solidus at between 15 and 30 kbar, is truncated by a lithospheric cap which thickens fromc. 15–20 km (≈ 5–8 kbar) up to a maximum of c. 50 km (≈ 15 kbar), such that the mean pressures of melting increase with increasing distance from the palaeo-trench. The MORB-like major element geochemistry of basalt samples closest to the ancestral trench are consistent with initial intersection of the peridotite solidus at low pressures (c. 15 kbar). For areas of thickest lithosphere, mean pressures of melt generation are higher, but extents of melting are lower than for a MORB melting column with a similar initial pressure of intersection of the peridotite solidus. All these basalts therefore potentially represent analogues for the small melt-fraction precursors to the generation of MOR tholeiites.Thermal constraints suggest that these low volume, small melt-fractions, were generated with CO2on the solidus, because mean pressures of melt generation are greater than the pressure of intersection of theTp1300°C mantle adiabat and the dry peridotite solidus. Potentially, all MORB may be generated initially with CO2on the solidus, and if this is correct, it does not require thermal anomalies to generate large extents of melting at high mean pressures.

Mesozoic magmatic activity in the North Sea Basin: implications for stretching history

Abstract Recent developments in the theory underlying magma generation allow the volumes and compositions of melt generated during extension of the lithosphere to be used to make quantitative statements about basin evolution. In the Mesozoic North Sea Basin there is a well defined relationship between magma chemistry and the degree of lithospheric attenuation. The largest extents of melting (&lt;2%) resulted in the alkali basalts of the Forties province which occur in an area where β (the stretching factor) may be as high as 2, and which erupted within 30 million years of the onset of extension. In off-axis areas of the rift the degrees of melting are smaller and melt compositions are more undersaturated and extreme (nephelinites and ultrapotassic rocks). In the Forties province the observed melt compositions may be reconciled with melting on the dry peridotite solidus at a normal (1280°C) potential temperature ( T p ) assuming a relatively thin mechanical boundary layer (MBL) of 70 km prior to rifting. Alternatively, a T p of 1380°C will produce the alkali basalts from dry periodotite at the observed extension factors with an initial MBL thickness of 100 km. A T p as high as 1480°C, which would give rise to regional pre- and syn-rift uplift, can be ruled out because the observed extent of melting is too small, the amounts of extension too large, and the rift episode too short. Off-axis magmatism may be explained by melting within a MBL with an initial thickness of ∼100 km or more at a T p of 1280°C. Models for extension of the lithosphere by simple shear along an initially planar detachment fault fail to account for either the existence or the location of North Sea magmatism, with or without elevated potential temperatures if the dry solidus governs melting. This conclusion is independent of the initial dip of the detachment and also holds when the initial dip in the lithospheric mantle is double that in the crust. The timing, nature, and location of magmatic activity can be used to provide information about the onset of rifting, the rates of extension, the amount of strain, and the thermal history of a basin. When linked with studies of faulting and subsidence this approach may be of some importance for hydrocarbon exploration.

The use of laboratory velocity data for estimating temperature and partial melt fraction in the low‐velocity zone: Comparison with heat flow and electrical conductivity studies

Peridotite is widely considered to be the dominant component of the upper most mantle. Seismic velocities of a dry peridotite determined in the laboratory at high pressure and hypersolidus temperature show a rapid decrease with increasing temperature. The strong temperature dependence of velocities can be used to estimate temperature and melt fraction in the low‐velocity zone of the Earth. The laboratory results show that the pressure dependence of both velocity and melt fraction appears to be well accounted for by the pressure dependence of the solidus temperature of the peridotite, i.e., homologous temperature dependence. This observation allows us to extrapolate laboratory results to higher pressures (greater depths) with some confidence, requiring only a knowledge of the solidus as a function of pressure. Using the ratios of lithospheric to asthenospheric velocities, temperature and melt fraction in the asthenosphere (the low‐velocity zone) can be determined from laboratory velocity data. Examples of thermal structures of the upper mantle beneath oceanic plates are presented. We choose three locations of geophysical interest: Iceland Plateau, Pacific Ocean, and Philippine Sea, where reliable seismic velocities have been determined in the upper mantle from surface wave studies. A large amount of partial melt (≥5 vol %) and a higher temperature than the dry peridotite solidus are inferred in 0–5 m.y. asthenosphere under the Iceland Plateau and the active marginal basin of the east Philippine Sea. The partial melt zone appears to extend deeper (&gt;100 km) and to greater ages (&gt;20 m.y.) in the Pacific Ocean region than under the slowly spreading Iceland Plateau. Partial melting is not expected in the asthenosphere older than 5 m.y. under the Iceland Plateau. Even though the volume fraction of partial melt is not so large (≤3 vol %) beneath the Pacific plate, the melting zone may extend to a distance of above 2000 km from the ridge. Many seamounts observed in the Pacific Ocean may result from this vast region of melting under the plate. We suggest a linear relation between the width of melting zone and the plate velocity. The laboratory results are also applied to other low‐velocity zones. It is suggested that a velocity drop up to about 6% in the asthenosphere does not necessarily require the existence of partial melt but can be explained by high subsolidus temperature. This implies that partial melt may not exist generally but only in limited areas in the low‐velocity zone. Comparison of seismic results with theoretical models may overestimate the temperature and melt fraction, if the velocity drop at subsolidus temperature is neglected. Temperature and melt fraction obtained in this study are discussed together with results from heat flow and electrical studies. Almost the same temperatures as inferred from seismic velocity and a dry peridotite solidus are calculated from heat flow data, indicating that the upper mantle under mid‐ocean ridges may be fairly dry. Even a water‐ or CO2‐undersaturated solidus (0.1 wt %) does not give a temperature consistent with the lithospheric geotherm and laboratory velocity results. It is noted that electrical conductivity studies generally yield lower temperatures and larger melt fractions than this study. Unknown pressure, frequency, and impurity effects on laboratory conductivities of partially molten mantle rocks are suggested as possible reasons for this discrepancy.

Anelasticity and thermal structure of the oceanic upper mantle: Temperature calibration with heat flow data

The thermal structure of the oceanic upper mantle is examined on the basis of seismic attenuation, surface heat flow, and mid‐ocean ridge basalt (MORB) composition. The comparison of seismic Q with laboratory Q determined at sufficiently high pressure and temperature allows us to constrain temperature in the Earth. Since the inferred absolute temperature relies on knowledge of the frequency dependence of Q (if any), we calibrate an unknown frequency effect on Q by comparing the derived temperatures with geotherms calculated from surface heat flow and MORB composition. This comparison yields a correction factor (found to be small) for the geotherm derived from laboratory (ultrasonic) Q. Examples of thermal structure are presented for the Iceland Plateau and the Pacific Ocean. Under the Iceland Plateau, we expect partial melt for the asthenosphere only in the vicinity of the spreading ridge. In asthenosphere older than ∼5 m.y. of the Iceland Plateau and older than ∼40 m.y. of the Pacific Ocean, temperatures are below the dry peridotite solidus, and no partial melt is expected. These results are consistent with the thermal structure derived from a comparison of seismic and laboratory velocities. It is shown that there is a good agreement between temperatures determined from seismic attenuation, heat flow, and MORB chemistry. Since the laboratory measurements were made on a dry peridotite and we employ a dry peridotite solidus for determining the geotherm, the consistency in temperatures derived from three independent data strongly suggests that the upper mantle (remote from subduction zones) is essentially dry. It appears that the spreading process in mid‐ocean ridge causes asthenosphere material to ascend from greater depths where it was at higher temperature. This gives rise to pressure release melting.