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1.
Many ocean island basalts (OIB) that have isotopic ratios indicative of recycled crustal components in their source are silica-undersaturated and unlike silicic liquids produced from partial melting of recycled mid-ocean ridge basalt (MORB). However, experiments on a silica-deficient garnet pyroxenite, MIX1G, at 2.0-2.5 GPa show that some pyroxenite partial melts are strongly silica-undersaturated [M.M. Hirschmann et al., Geology 31 (2003) 481-484]. These low-pressure liquids are plausible parents of alkalic OIB, except that they are too aluminous. We present new partial melting experiments on MIX1G between 3.0 and 7.5 GPa. Partial melts at 5.0 GPa have low SiO2 (<48 wt%), low Al2O3 (<12 wt%) and high CaO (>12 wt%) at moderate MgO (12-16 wt%), and are more similar to primitive OIB compositions than lower-pressure liquids of MIX1G or experimental partial melts of anhydrous or carbonated peridotite. Solidus temperatures at 5.0 and 7.5 GPa are 1625 and 1825°C, respectively, which are less than 50°C cooler than the anhydrous peridotite solidus. The liquidus temperature at 5.0 GPa is 1725°C, indicating a narrow melting interval (∼100°C). These melting relations suggest that OIB magmas can be produced by partial melting of a silica-deficient pyroxenite similar to MIX1G if its melting residue contains significant garnet and lacks olivine. Such silica-deficient pyroxenites could be produced by interaction between recycled subducted oceanic crust and mantle peridotite or could be remnants of ancient oceanic lower crust or delaminated lower continental crust. If such compositions are present in plumes ascending with potential temperatures of 1550°C, they will begin to melt at about 5.0 GPa and produce appropriate partial melts. However, such hot plumes may also generate partial melts of peridotite, which could dilute the pyroxenite-derived partial melts.  相似文献   

2.
The melting curve of perovskite MgSiO3 and the liquidus and solidus curves of the lower mantle were estimated from thermodynamic data and the results of experiments on phase changes and melting in silicates.The initial slope of the melting curve of perovskite MgSiO3 was obtained as dTm/dP?77 KGPa?1 at 23 GPa. The melting curve of perovskite was expressed by the Kraut-Kennedy equation as Tm(K)=917(1+29.6ΔVV0), where Tm?2900 K and P?23 GPa; and by the Simon equation, P(GPa)?23=21.2[(Tm(K)2900)1.75?1].The liquidus curve of the lower mantle was estimated as Tliq ? 0.9 Tm (perovskite) and this gives the liquidus temperature Tliq=7000 ±500 K at the mantle-core boundary. The solidus curve of the lower mantle was also estimated by extrapolating the solidus curve of dry peridotite using the slope of the solidus curve of magnesiowüstite at high pressures. The solidus temperature is ～ 5000 K at the base of the lower mantle. If the temperature distribution of the mantle was 1.5 times higher than that given by the present geotherm in the early stage of the Earth's history, partial melting would have proceeded into the deep interior of the lower mantle.Estimation of the density of melts in the MgOFeOSiO2 system for lower mantle conditions indicates that the initial melt formed by partial fusion of the lower mantle would be denser than the residual solid because of high concentration of iron into the melt. Thus, the melt generated in the lower mantle would tend to move downward toward the mantle-core boundary. This downward transportation of the melt in the lower mantle might have affected the chemistry of the lower mantle, such as in the D″ layer, and the distribution of the radioactive elements between mantle and core.  相似文献   

3.
High pressure experiments using the sink/float method have bracketed the density of hydrous iron-rich ultrabasic silicate melt from 1.35 to 10.0 GPa at temperatures from 1400 to 1860 °C. The silicate melt composition was a 50–50 mixture of natural komatiite and synthetic fayalite. Water was added in the form of brucite Mg(OH)2 and was present in the experimental run products at 2 wt.% and 5 wt.% levels as confirmed by microprobe analyses of total oxygen. Buoyancy marker spheres were olivines and garnets of known composition and density. The density of the silicate melt with 5 wt.% water at 2 GPa and 1500 °C is 0.192 g cm? 3 less than the anhydrous form of this melt at the same P and T. This density difference gives a partial molar volume of water in silicate melt of ～ 7 cm3 mol? 1, which is similar to previous studies at high pressure. The komatiite–fayalite liquids with 0 and 2 wt.% H2O, have extrapolated density crossovers with equilibrium liquidus olivine at 8 and 9 GPa respectively, but there is no crossover for the liquid with 5 wt.% H2O. These results are consistent with the hypothesis that dense hydrous melts could be gravitationally stable atop the 410 km discontinuity in the Earth. The results also support the notion that equilibrium liquidus olivine could float in an FeO-rich hydrous martian magma ocean. Extrapolation of the data suggests that FeO-rich hydrous melt could be negatively buoyant in the Earth's D″-region or atop the core–mantle-boundary (CMB), although experiments at higher pressure are needed to confirm this prediction.  相似文献   

4.
We report new metal-silicate partition coefficients for Ni, Co and P at 7.0 GPa (1650–1750°C), and Ni, Co, Mo, W and P at 0.8, 1.0 and 1.5 GPa (1300–1400°C). Guided by thermodynamics, all available metal-silicate partition coefficients, D(i), where i is Ni, Co, P, Mo and W, are regressed against 1/T, P/T, lnf(O2), ln(1 − Xs) (XS is mole fraction of S in metallic liquid) and nbo/t (non-bridging oxygen/tetrahedral cation ratio, a silicate melt compositional-structural parameter) to derive equations of the following form: ln D(i) = aln f(O2) + (b/T) + (cP/T) + d(nbo/t) + eln(1 − XS) + f. Expressions for solid metal-liquid silicate and liquid metal-liquid silicate partition coefficients are derived for S-free and S-bearing systems.

We investigate whether Earth's upper-mantle siderophile element abundances can be reconciled with simple metal-silicate equilibrium. Sulfur-free metallic compositions do not allow a good fit. However, Ni, Co, Mo, W and P abundances in the upper mantle are consistent with simple metal-silicate equilibrium at mantle pressures and temperatures (27 GPa, 2200 K, ΔIW(iron-wüstite) = −0.15, nbo/t = 2.7; XS = 0.15). Although these conditions are near the anhydrous peridotite solidus, they are well above the hydrous solidus and probably closer to the liquidus. A hydrous magma ocean and early mantle are consistent with predicted planetary accretion models. These results suggest that siderophile element abundances in Earth's upper mantle were established by liquid metal-liquid silicate equilibrium near the upper-mantle-lower-mantle boundary.  相似文献   


5.
Garnets crystallized experimentally from within the anhydrous melting ranges of an olivine tholeiite, a tholeiitic andesite and an augite leucitite at pressures between 18 and 45 kbars contain up to 0.4% Na2O and 0.6% P2O5. The Na and P are thought to form a substitution couple, replacing Ca and Si in the garnet structure; representing limited solid solution between grossular (Ca3Al2Si3O12) and the phosphate Na3Al2P3O12. This substitution is enhanced by increasing pressure and by falling temperature (increasing degree of crystallization) at constant pressure.Current knowledge of the crystalline site of P in the upper mantle is hampered by lack of data on the stability of apatite and other phosphates at appropriate pressures and temperatures. If all samples of garnetiferous upper mantle brought to the surface by magmatic processes have been depleted to some extent by previous escape of a partial-melt fraction, P2O5 concentrations below 0.1% in their garnets could nevertheless signify that this phase was the sole predepletion host for P in the upper mantle, at the depths from which such inclusions are derived. If garnet and apatite are the principal minerals containing P in the upper mantle, it may be possible to use covariances between P and rare-earth elements in mafic liquids to detect which of these phases was the dominant host for P at the site of magma genesis. This approach confirms the widely-held opinion that strongly alkalic mafic magmas are products of upper-mantle partial fusion in the presence of residual garnet. It also leads to a contrasting proposal that mid-ocean ridge basalts may be generated by upper-mantle partial fusion at comparatively small depths, in the presence of residual apatite.  相似文献   

6.
Mafic and ultramafic xenoliths are well represented within a large basaltic lava field of Stromboli. These basalts, known as San Bartolo lavas, show a high-K calc-alkaline (HKCA) affinity and were erupted <5 ka BP. Xenoliths consist of olivin-gabbro, gabbronorite, anorthosite, dunite, wehrlite and clinopyroxenite. Thermobarometric estimates for the crystallization of gabbroic materials show minima equilibration pressures of 0.17–0.24 GPa, at temperatures ranging from 940 to 1,030°C. These materials interacted with hydrous ascending HKCA basaltic magmas (with temperatures of 1,050–1,100°C) at pressures of about 0.2–0.4 GPa. These pressure regimes are nearly identical to those found for the crystallization of phenocrystic phases within HKCA basaltic lavas. Gabbroic inclusions are regarded as cumulates and represent crystallized portions of earlier HKCA Strombolian basalts.Dunite and wehrlite show porphyroclastic-heterogranular textures, whereas the clinopyroxenite exhibit a mosaic-equigranular texture typical of mantle peridotites. These ultramafic materials are in equilibrium with more primitive basaltic magmas (under moderately hydrous and anhydrous conditions) at pressures of 0.8–1.2 GPa, which is below the crust-mantle transition, located at about 20 km depth under Stromboli.Major and trace element distributions indicate comagmatism between the host basaltic lava and the mafic and ultramafic inclusions. REE patterns for mafic nodules are relatively regular and overlap the field of basaltic lavas (HKCA). They show moderate to high LREE enrichments and moderate enrichments in HREE relative to chonrites. Spider diagrams also show significant similarities between the lavas and the mafic-ultramafic xenoliths as well.During their ascent, primitive Strombolian magmas may be stored in upper-mantle regions where they interact with peridotitic materials and partly differentiate (to give dunite and wehrlite) before migrating to upper crustal levels. In this region, hydrous basaltic magmas (with estimated water contents of 2–3.5 wt%) are stored in the subvolcanic environment, and are allowed to crystallize the gabbroic materials before reaching the surface under nearly anhydrous conditions.An erratum to this article can be found at  相似文献   

7.
Phase equilibrium experiments were performed on typical ‘oceanic’ and ‘cratonic’ peridotite compositions and a Ca, Al-rich orthopyroxene composition, to test the proposal that garnet lherzolites exsolved from high-temperature harzburgites, and to further our understanding of the origin of ancient cratonic lithospheres. ‘Oceanic’ peridotites crystallize a garnet harzburgite assemblage at pressures above 5 GPa in the temperature range 1450–1600°C, but at 5 GPa and temperatures less than 1450°C, crystallize clinopyroxene to become true lherzolites. ‘Cratonic’ peridotites crystallize a garnet harzburgite assemblage at pressures above 5 GPa in the temperature range 1300–1600°C. Garnet-free harzburgite crystallizes from both ‘cratonic’ and ‘oceanic’ peridotite at temperatures above 1450°C and pressures below 4.5–5 GPa. Phase relations for the high Ca, Al-rich orthopyroxene composition essentially mirror those for ‘oceanic’ peridotite.The complete solution of garnet and clinopyroxene into orthopyroxene observed in all three starting compositions at temperatures near or above the mantle solidus at pressures less than 6 GPa supports the hypothesis that garnet lherzolite could have exsolved from harzburgite. The inferred cooling path for the original high-temperature harzburgite protoliths of garnet lherzolites differs depending on bulk composition. The precursor harzburgite protoliths of garnet lherzolites and harzburgites with ‘cratonic’ bulk compositions apparently experienced simple isobaric cooling from formation temperatures near the peridotite solidus to those at which most of these peridotites were sampled in the mantle (< 1200°C). The cooling histories for harzburgite protoliths of sheared garnet lherzolites with ‘oceanic’ compositional affinity are speculated to have involved convective circulation of mantle material to depths deeper than those at which it was originally formed.Phase equilibria and compositional relationships for orthopyroxenes produced in phase equilibrium experiments on peridotite and komatiite are consistent with an origin for ‘cratonic’ peridotite as a residue of Archean komatiite extraction, which has since cooled and exsolved clinopyroxene and garnet to become the common low-temperature, coarse-grained peridotite thought to comprise the bulk of the mantle lithosphere beneath the Archean Kaapvaal craton.  相似文献   

8.
Biotite granite from the Sierra Nevada batholith was reacted, with known water contents in sealed platinum capsules, in a piston-cylinder apparatus between 10 and 35 kb. With the liquid just over-saturated with respect to water, temperatures for solidus and liquidus (quartz/coesite-out curve), respectively, are: 2 kb, 680°C, 715°C; 10 kb, 620°C, 725°C; 25 kb, 655°C, 800°C; 35 kb, 700°C, 850°C. The temperature interval is 35°C at 2 kb, 105°C at 10 kb, and 150°C at 35 kb, indicating that granite departs from a eutectic composition at depths greater than about 40–50 km. We conclude that crystal-liquid equilibria are not likely to yield primary rhyolite or granite magmas by partial fusion of oceanic crust in subduction zones. The solubility of water in granite liquids, in wt%, is 22.5 ± 2.5 at 25 kb and 810°C and 27 ± 2.5 at 35 kb and 850°C. These results indicate that a miscibility gap persists between water-saturated silicate magmas and aqueous vapor phase at least to pressures corresponding to 100 km depth in the mantle. The formation of kyanite near the liquidus of water over-saturated granite indicates that the aqueous vapor phase is enriched in alkalis and possibly silica, relative to the condensed phases.  相似文献   

9.
Abstract Melting experiments have been carried out on an olivine andesite of Mt Yakushi-Yama from the Miocene Setouchi volcanic belt in northeastern Shikoku, Japan. This andesite has been characterized by a low ratio of FeO*/Mg° (= 0.78). Phase relations have been determined within the pressure range of 2.8 to 19.3 kbar at 1000-1300°C under anhydrous and water-saturated conditions. At pressures less than 8.8 kbar, olivine is a liquidus phase. Orthopyroxene appears on the liquidus at 9.3 kbar under the anhydrous conditions. The multiple saturation point rises up to 17.5 kbar for water-saturated experiments. The andesite melt coexists with olivine and orthopyroxene just below the liquidus at 8.8–9.3 kbar and 1230°C for dry conditions, and at 17.5 kbar and 1060°C under water-saturated conditions. These experimental results indicate that the Yakushi-Yama olivine andesite magma could coexist with a harzburgitic mantle at depths between about 30 and 60 km, and at temperatures between 1060 and 1230°C. Experimental data also suggest a possibility that a high magnesian andesite magma would be generated by a direct partial melting of the uppermost harzburgitic mantle under anhydrous conditions.  相似文献   

10.
Experimental determinations of the dry liquidus temperatures of two pantellerite, and two pantelleritic trachyte glasses in the pressure range 0–2 kilobars, show minima in the liquidus curves between 0.1 and 0.2 kb. The pantellerite minima are 830°–850° C; the trachyte minima are 920°–940° C. At pressures below the minima a separate vapour phase co-exists with liquid, at higher pressures the intrinsic volatiles are completely soluble in the liquid and the liquidus curves have a positivedT/dP. Similar results have been obtained from a range of other pantelleritic glasses, and together with consistent alkali feldspar compositions (from a wide range of experimental conditions) are indicative of a close approach to equilibrium. The form of the liquidus curves above the minima, if rellecting natural conditions, offers a ready explanation of the near-or super-liquidus aspect of many peralkaline lavas. The temperatures in these anhydrous experiments are 100°–150° C higher than those for similar compositions in the presence of excess water. (Also, in the presence of excess water, the crystallization sequences in the natural glasses are profoundly modified, with pyroxene appearing on the liquidus). At lower pressures, feldspar is the liquidus phase in the dry pantellerites, but is joined by quartz around 1 kb, and superseded by quartz at higher pressures. As pantellerites with quartz phenocrysts are uncommon, low pressure equilibration is perhaps normal in these magmas. Feidspar is the usual liquidus phase in the trachytes, except at very low pressures where it is preceded by iron oxide. Preliminary studies at 5 kb indicate that the pantelleritic and trachytic liquidus curves are converging (in the range 950°–1000° C). Crystallization sequences, and the forms and positions of the solidus curves are therefore of vital importance. These, together with the vapour-present/vapour-absent conditions, are currently under investigation.  相似文献   

11.
The phase relations in hydrous and anhydrous mid-ocean ridge basalt were determined at pressures of 18-28 GPa. Liquidus phase relations in hydrous and anhydrous MORB are different. Garnet is the liquidus phase at pressures below 21 GPa, Ca-Al (CAS) phase and stishovite are the liquidus phases at pressures of 22-27 GPa, and stishovite and Ca-perovskite are the liquidus phases above 27 GPa, whereas Ca-perovskite is a liquidus phase of anhydrous MORB at pressures above 23 GPa. Under subsolidus conditions, we have found that in the hydrous MORB system the stability fields of Al-bearing perovskite and Na-Al (NAL) phase might shift to lower pressure by about 1.5 GPa compared to the dry MORB system. This shift could be explained by oxidation of a garnet-bearing assemblage by hydrous fluid and formation of Fe3+-bearing aluminous perovskite at lower pressures relative to the anhydrous system and/or differences in water solubility of the phases existing in perovskite-bearing assemblages. Our data indicate that hydrous basaltic crust remains denser than peridotite along the geotherm of a subducting slab, i.e. there is no density crossover between peridotite and basalt. Therefore, in slabs going through the 660 km discontinuity, basalt would gravitationally sink into the lower mantle under relatively hydrous conditions. The delamination of former basaltic crust near the 660 km discontinuity might be possible under relatively dry conditions of subduction. There are no stable highly hydrous phases in MORB above 10 GPa even at lower temperatures corresponding to subducting slabs. Therefore, MORB cannot be an important carrier of water to the deep Earth interior. However, it can be constantly supplied by water-bearing fluid from the underlying peridotite part of the descending slab. Thus, it is plausible that water can control subduction of the oceanic crust into the lower mantle.  相似文献   

12.
Synthesis of pyrope-knorringite solid solution series   总被引：1，自引：0，他引：1  
The garnet solid solution series between pyrope Mg3Al2Si3O12 and knorringite Mg3Cr2Si3O12 has been synthesized from oxide mixtures at pressures of 60–80 kbars and 1400–1500°C. Lattice parameters and refractive indices of solid solutions vary linearly with (molecular) composition within the limits of measurement. The lattice parameter of pure knorringite is 11.600Åand its refractive index is 1.83. The genetic significance of mineral inclusions in natural diamonds is discussed, particularly in the light of the very high knorringite contents often found in garnet inclusions. It is suggested that the most common mineral assemblage occurring as inclusions in diamonds (olivine + knorringite-rich garnet + enstatite) might be explained in terms of subduction into the mantle of olivine + chrome-spinel + enstatite cumulates originally formed by crystallization of mafic magmas within the oceanic crust. The cumulate assemblage experienced alteration by circulating hydrothermal solutions, resulting in the introduction of some carbonate and serpentine minerals. During subduction, this assemblage was partially melted at depth below 150 km, accompanied by reduction of carbonate, to form a reconstituted assemblage consisting of olivine + knorringite-rich garnet + enstatite ± diamond.  相似文献   

13.
The rheological properties of upper mantle rocks play an important role in controlling the dynamics of the lithosphere and mantle convection. Experimental studies and microstructures in naturally deformed mantle rocks usually imply that olivine controls the upper mantle rheology. Here we show for the first time evidence from the geometry of folded compositional layers in mantle rocks from Western Norway that garnet-rich rocks can have lower solid-state viscosities than olivine-rich rocks. Modeling of melt-free and dry rheology of garnet and olivine confirms that the reversed viscosity contrast between garnet-rich and olivine-rich layers for this folding event can be achieved over a relatively wide range of temperatures at low stress conditions when the fine-grained garnet deforms by diffusion creep while the coarse-grained olivine deforms by dislocation creep and/or diffusion creep.In general, modeling of the fold viscosity contrast shows that in the stable subcontinental lithospheric mantle or convecting mantle such a reversed viscosity contrast can be formed due to diffusion creep processes in fine-grained garnets in a dry mantle environment or at conditions where the garnet-pyroxene layer is partially molten, i.e. close to solidus–liquidus conditions in the upper mantle. Alternatively in cold plate tectonic settings, e.g. in subduction zones, some water-weakening is a feasible mechanism to create the reversed viscosity contrast between garnet and olivine.  相似文献   

14.
High-pressure and high temperature experiments at 20 GPa on (Mg,Fe)SiO3 have revealed stability fields of two types of aluminium-free ferromagnesian garnets; non-cubic garnet and cubic garnet (majorite). Majorite garnet is stable only within a limited compositional variation, 0.2 < Fe/(Mg + Fe)< 0.4, and in the narrow temperature interval of 200°C around 2000°C, while the stability of non-cubic garnet with more iron-deficient compositions persists up to higher temperatures. These two garnets show fractional melting into iron-deficient garnet and iron-rich liquid, and the crystallization field of cubic garnet extends over Fe/(Mg + Fe)= 0.5. The assemblage silicate spinel and stishovite is a low-temperature phase, which also occurs in the iron-rich portion of the MgSiO3—FeSiO3 system. The sequence as given by the Fe/(Mg + Fe) value for the coexisting phases with the two garnets at 2000°C and 20 GPa is: silicate modified spinel aluminium-free garnets silicate spinel.Natural majorite in shock-metamorphosed chondrites is clarified to be produced at pressures above 20 GPa and temperatures around 2000°C. Similar shock events may cause the occurrence of non-cubic garnet in iron-deficient meteorites. Non-cubic garnet could be a stable phase in the Earth's mantle if a sufficiently low concentration of aluminium is present in the layer corresponding to the stable pressure range of non-cubic garnet. The chemical differentiation by melting in the deep mantle is also discussed on the basis of the present experimental results and the observed coexistence of majorite garnet with magnesiowüstite in chondrites.  相似文献   

15.
This paper provides new constraints on the crystallization conditions of the 3.49 Ga Barberton komatiites. The compositional evidence from igneous pyroxene in the olivine spinifex komatiite units indicates that the magma contained significant quantities of dissolved H2O. Estimates are made from comparisons of the compositions of pyroxene preserved in Barberton komatiites with pyroxene produced in laboratory experiments at 0.1 MPa (1 bar) under anhydrous conditions and at 100 and 200 MPa (1 and 2 kbar) under H2O-saturated conditions on an analog Barberton composition. Pyroxene thermobarometry on high-Ca clinopyroxene compositions from ten samples requires a range of minimum magmatic water contents of 6 wt.% or greater at the time of pyroxene crystallization and minimum emplacement pressures of 190 MPa (6 km depth). Since high-Ca pyroxene appears after 30% crystallization of olivine and spinel, the liquidus H2O contents could be 4 to 6 wt.% H2O. The liquidus temperature of the Barberton komatiite composition studied is between 1370 and 1400°C at 200 MPa under H2O-saturated conditions. When compared to the temperature-depth regime of modern melt generation environments, the komatiite mantle source temperatures are 200°C higher than the hydrous mantle melting temperatures inferred in modern subduction zone environments and 100°C higher than mean mantle melting temperatures estimated at mid-ocean ridges. When compared to previous estimates of komatiite liquidus temperatures, melting under hydrous conditions occurs at temperatures that are 250°C lower than previous estimates for anhydrous komatiite. Mantle melting by near-fractional, adiabatic decompression takes place in a melting column that spans 38 km depth range under hydrous conditions. This depth interval for melting is only slightly greater than that observed in modern mid-ocean ridge environments. In contrast, anhydrous fractional melting models of komatiite occur over a larger depth range ( 130 km) and place the base of the melting column into the transition zone.  相似文献   

16.
Mineral assemblages displayed by MORB and alkali-poor olivine tholeiites have been investigated over the pressure interval 4.6–18 GPa at 1200°C. Both compositions crystallize to form normal eclogites between 4.6 and 10 GPa and there is little change in the relative proportions of garnet and pyroxene over this range. However, the proportion of garnet increases rapidly above 10 GPa as pyroxene dissolves in the garnet structure and pyroxene-free garnetites (±stishovite) are produced by 14–15 GPa, dependent upon composition. The garnetite facies for both compositions possess zero-pressure densities of 3.75 g/cm3, implying that subducted oceanic crust remains appreciably denser than surrounding mantle to depths exceeding 600 km. It is demonstrated that the seismic velocity distributions in the mantle between 400 and 650 km are inconsistent with Anderson's hypothesis that this region is of eclogitic composition.  相似文献   

17.
High-temperature and high-pressure recovery experiments were made on experimentally deformed olivines at temperatures of 1613–1788 K and pressures of 0.1 MPa to 2.0 GPa. In the high-pressure experiments, a piston cylinder apparatus was used with BN and NaCl powder as the pressure medium, and the hydrostatic condition of the pressure was checked by test runs with low dislocation density samples. No dislocation multiplication was observed. The kinetics of the dislocation annihilation process were examined by different initial dislocation density runs and shown to be of second order, i.e.
dρdt= ?p2K0exp[?(E1+PV1RT]
where ρ is the dislocation density, k0 is a constant, E1and V1 are the activation energy and volume respectively, and P, R and T are pressure, gas constant and temperature, respectively. Activation energy and volume were estimated from the temperature and pressure dependence of the dislocation annihilation rate as E1=389±59 kJ mol?1 and V1=14±2 cm3mol?1, respectively.The diffusion constants relevant to the dislocation annihilation process were estimated from a theoretical relation k=αD where k=k0 exp[?(E1 + PV1)/RT], D is the diffusion constant and α is a non-dimensional constant of ca. 300. The results agree well with the self-diffusion constant of oxygen in olivine. This suggests that the dislocation annihilation is rate-controlled by the (oxygen) diffusion-controlled dislocation climb.The mechanisms of creep in olivine and dry dunite are examined by using the experimental data of static recovery. It is suggested that the creep of dry dunite is rate-controlled by recovery at cell walls or at grain boundaries which is rate-controlled by oxygen diffusion. Creep activation volume is estimated to be 16±3 cm3 mol?1.  相似文献   

18.
Tholeiitic basalt glasses from the FAMOUS area of the Mid-Atlantic Ridge are among the most primitive basaltic liquids reported from the ocean basins. One of the more primitive of these[Mg/(Mg+Fe2+) = 0.68;Ni= 232ppm;TiO2 = 0.61] glasses (572-1-1) was selected for an experimental investigation. This study found olivine to be the liquidus phase from 1 atm to 10.5 kbar where it is replaced by clinopyroxene. The sequence of appearance of phases at 1 atm pressure is olivine (1268°C), plagioclase (1235°C) and clinopyroxene (1135°C). The sample is multiply saturated at 10.5 kbar with olivine (Fo88), clinopyroxene (Wo32En60Fs9), and orthopyroxene (Wo5En83Fs12). From the 1-atm data we have measured (FeO/MgO) olivine/(FeO*/MgO) liquid (K′D) for olivine-melt pairs equilibrated at 12 temperatures in the range 1268–1205°C.K′D varies from 0.30 at 1205°C to 0.27 at 1268°C. Analysis of high-pressure olivine melt pairs indicates a systematic increase inK′D with pressure.Evaluation of the 1-atm experiments reveals that fractionation of olivine followed by olivine + plagioclase can generate much of the variation in major element chemistry observed in the FAMOUS basalt glasses. However, it cannot account for the entire spectrum of glass compositions — particularly with respect to TiO2 and Na2O. The variations in these components are such as to require different primary liquids.Comparison of clinopyroxene microphenocrysts/xenocrysts found in oceanic tholeiites with experimental clinopyroxenes reveal that the majority of those in the tholeiites may have crystallized from the magma at pressures greater than ～ 10 kbar and are not accidental xenocrysts. Clinopyroxene fractionation at high pressures may be a viable mechanism for fractionating basaltic magmas.The major and minor element mineral/meltK′d's from our experiments have been used to model the source region residual mineralogy for given percentages of partial melting. These data suggest that ～20% partial melting of a lherzolite source containing 0–10% clinopyroxene can generate the major and minor element concentrations in the parental magmas of the Project FAMOUS basalt glasses.  相似文献   

19.
The mineralogy adopted by a depleted harzburgite composition has been studied over the pressure interval 5–26 GPa at temperatures of 1300–1400°C. The pyroxene-garnet component of the harzburgite composition (harzburgite minus 82 wt.% olivine) transforms to majorite garnet by 18–19 GPa, and further disproportionates to the assemblage of garnet + stishovite + Mg2SiO4 spinel above 20 GPa. At still higher pressures, first ilmenite (22–24 GPa) and then perovskite MgSiO3 (24–26 GPa) are found to coexist with garnet. Garnet disappears at 26 GPa and almost complete transition to perovskite is achieved at this pressure. The mineral proportions and density profiles in the subducting oceanic lithosphere, modelled by a combination of 80% harzburgite + 20% primitive MORB compositions are calculated as a function of depth under conditions isothermal with surrounding pyrolite mantle, and also for a temperature distribution in which the slab is substantially cooler than surrounding mantle to below 700 km. Under isothermal conditions, the slab has a density similar to surrounding mantle to a depth of 600 km. However, between 600 and 700 km, the slab is up to 0.08 g/cm3 denser than surrounding mantle. This is caused primarily by the higher alumina content in pyrolite as compared to harzburgite, which causes the transition to perovskite in pyrolite to occur at substantially higher pressures than in harzburgite. The presence of alumina also smears out the garnet-perovskite transition in pyrolite over a depth interval of 50 km, whereas this transformation is much sharper in the harzburgite composition. Calculations based on the observed phase equilibria also show that a subducted cool slab remains much denser (by 0.1–0.3 g/cm3) than surrounding mantle to a depth of 700 km but possesses a density similar to surrounding mantle below this depth. These results have important implications for the dynamical behaviour of slabs possessing different thermal regimes when they encounter the 670 km discontinuity and also for the nature of this discontinuity.  相似文献   

20.
The densities of silicate liquids with basic, picritic, and ultrabasic compositions have been estimated from the melting curves of minerals at high pressures. Silicate liquids generated by partial melting of the upper mantle are denser than olivine and pyroxenes at pressures higher than 70 kbar, and garnet is the only phase which is denser than the liquid at pressures from 70 kbar to at least 170 kbar. In this pressure range, garnet and some fraction of liquid separate from ascending partially molten diapirs. It is therefore suggested that aluminium-depleted komatiite with a high Ca/OAl2O3 ratio may be derived from diapirs which originated in the deep upper mantle at pressures from 70 kbar to at least 140 kbar (200–400 km in depth), where selective separation of pyropic garnet occurs effectively. On the other hand, aluminium-undepleted komatiite is probably derived from diapirs originating at shallower depths (< 200 km). Enrichment of pyropic garnet is expected at depths greater than 200 km by selective separation of garnet from ascending diapirs. The 200-km discontinuity in the seismic wave velocity profile may be explained by a relatively high concentration of pyropic garnet at depths greater than 200 km.  相似文献   

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