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1.
High-pressure and temperature experiments (28–62 GPa, and 1,490–2,000 K, corresponding to approximately 770–1,500 km depth in the mantle) have been conducted on a MgCO3 + SiO2 mixture using a laser-heated diamond anvil cell combined with analytical transmission electron microscope observation of the product phases to constrain the fate of carbonates carried on the subducting basalt into the lower mantle. At these conditions, the decarbonation reaction MgCO3 (magnesite) + SiO2 (stishovite) → MgSiO3 (perovskite) + CO2 (solid) has been recognized. This indicates that above reaction takes place as a candidate for decarbonation of the carbonated subducting mid ocean ridge basalts in the Earth’s lower mantle.  相似文献   

2.
Carbonation and decarbonation of eclogites: the role of garnet   总被引：3，自引：0，他引：3  
Carbonates are potentially significant hosts for primordial and subducted carbon in the Earth's mantle. In addition, the coexistence of carbonate with silicates and reduced carbon (diamond or graphite), allows constraints to be placed on the oxidation state of the mantle. Carbonate-silicate-vapor reactions control how carbonate + silicate assemblages may form from carbon-bearing vapor + silicate assemblages with increasing pressure. In olivine-bearing rocks such as peridotite, considered the dominant rock type in the upper mantle, the lowest-pressure carbonate-forming reactions involve olivine (±clinopyroxene) reacting with CO2 (e.g., Wyllie et al. 1983). In eclogitic rocks, the essential mineral assemblage is omphacitic clinopyroxene + garnet, without olivine. Therefore, alternative carbonate-forming reactions must be sought. The carbonation of clinopyroxene via the reaction dolomite + 2 coesite = diopside + 2 CO2 was studied experimentally by Luth (1995). The alternative possibility that garnet reacts with CO2 is explored here by determining the location of the reaction 3 magnesite + kyanite + 2 coesite = pyrope + 3 CO2 between 5 and 11 GPa in multi-anvil apparatus. At the temperatures ≥1200 °C, carbonation of eclogitic rocks with increasing pressure will proceed initially by reaction with clinopyroxene, because the pyrope-carbonation reaction lies at higher pressures for a given temperature than does the diopside-carbonation reaction. Diluting the pyrope component of garnet and the diopside component of clinopyroxene to levels appropriate for mantle eclogites does not change this conclusion. At lower temperatures, appropriate for “cold” slabs, it is possible that the converse situation will hold, with initial carbonation proceeding via reaction with garnet, but this possibility awaits experimental confirmation. Decarbonation of an eclogite under “normal mantle” geothermal conditions by a decrease in pressure, as in an ascending limb of a mantle convection cell, would be governed by the formation of clinopyroxene + CO2. At higher pressure than this reaction, any CO2 produced by the breakdown of magnesite reacting with kyanite and coesite would react with clinopyroxene to produce dolomite + coesite. Release of CO2 from eclogite into mantle peridotite would form carbonate at sub-solidus conditions and produce a dolomitic carbonate melt if temperatures are above the peridotite-CO2 solidus. Received: 4 May 1998 / Accepted: 23 December 1998  相似文献   

3.
Subduction carries atmospheric and crustal carbon hosted in the altered oceanic crystalline basement and in pelagic sediments back into the mantle. Reactions involving complex carbonate solid solutions(s) lead to the transfer of carbon into the mantle, where it may be stored as graphite/diamond, in fluids or melts, or in carbonates. To constrain the thermodynamics and thus reactions of the ternary Ca–Mg–Fe carbonate solid solution, piston cylinder experiments have been performed in the system CaCO3–MgCO3–FeCO3 at a pressure of 3.5 GPa and temperatures of 900–1,100°C. At 900°C, the system has two miscibility gaps: the solvus dolomite–calcite, which closes at X MgCO3 ~0.7, and the solvus dolomite–magnesite, which ranges from the Mg to the Fe side of the ternary. With increasing temperature, the two miscibility gaps become narrower until complete solid solutions between CaCO3–Ca0.5Mg0.5CO3 is reached at 1,100°C and between CaCO3–FeCO3 at 1,000°C. The solvi are characterized by strong compositional asymmetry and by an order–disorder mechanism. To deal with these features, a solid solution model based on the van Laar macroscopic formalism has been calculated for ternary carbonates. This thermodynamic solid solution model is able to reproduce the experimentally constrained phase relations in the system CaCO3–MgCO3–FeCO3 in a broad P–T range. To test our model, calculated phase equilibria were compared with experiments performed in carbonated mafic protolithes, demonstrating the reliability of our solid solution model at pressures up to 6 GPa in complex systems.  相似文献   

4.
The melting behaviour of three carbonated pelites containing 0–1 wt% water was studied at 8 and 13 GPa, 900–1,850°C to define conditions of melting, melt compositions and melting reactions. At 8 GPa, the fluid-absent and dry carbonated pelite solidi locate at 950 and 1,075°C, respectively; >100°C lower than in carbonated basalts and 150–300°C lower than the mantle adiabat. From 8 to 13 GPa, the fluid-present and dry solidi temperatures then increase to 1,150 and 1,325°C for the 1.1 wt% H2O and the dry composition, respectively. The melting behaviour in the 1.1 wt% H2O composition changes from fluid-absent at 8 GPa to fluid-present at 13 GPa with the pressure breakdown of phengite and the absence of other hydrous minerals. Melting reactions are controlled by carbonates, and the potassium and hydrous phases present in the subsolidus. The first melts, which composition has been determined by reverse sandwich experiments, are potassium-rich Ca–Fe–Mg-carbonatites, with extreme K2O/Na2O wt ratios of up to 42 at 8 GPa. Na is compatible in clinopyroxene with D\textNa\textcpx/\textcarbonatite = 10-18 D_{\text{Na}}^{{{\text{cpx}}/{\text{carbonatite}}}} = 10{-}18 at the solidus at 8 GPa. The melt K2O/Na2O slightly decreases with increasing temperature and degree of melting but strongly decreases from 8 to 13 GPa when K-hollandite extends its stability field to 200°C above the solidus. The compositional array of the sediment-derived carbonatites is congruent with alkali- and CO2-rich melt or fluid inclusions found in diamonds. The fluid-absent melting of carbonated pelites at 8 GPa contrasts that at ≤5 GPa where silicate melts form at lower temperatures than carbonatites. Comparison of our melting temperatures with typical subduction and mantle geotherms shows that melting of carbonated pelites to 400-km depth is only feasible for extremely hot subduction. Nevertheless, melting may occur when subduction slows down or stops and thermal relaxation sets in. Our experiments show that CO2-metasomatism originating from subducted crust is intimately linked with K-metasomatism at depth of >200 km. As long as the mantle remains adiabatic, low-viscosity carbonatites will rise into the mantle and percolate upwards. In cold subcontinental lithospheric mantle keels, the potassic Ca–Fe–Mg-carbonatites may freeze when reacting with the surrounding mantle leading to potassium-, carbonate/diamond- and incompatible element enriched metasomatized zones, which are most likely at the origin of ultrapotassic magmas such as group II kimberlites.  相似文献   

5.
We have experimentally investigated melting phase relation of a nominally anhydrous, carbonated pelitic eclogite (HPLC1) at 2.5 and 3.0 GPa at 900–1,350°C in order to constrain the cycling of sedimentary carbon in subduction zones. The starting composition HPLC1 (with 5 wt% bulk CO2) is a model composition, on a water-free basis, and is aimed to represent a mixture of 10 wt% pelagic carbonate unit and 90 wt% hemipelagic mud unit that enter the Central American trench. Sub-solidus assemblage comprises clinopyroxene + garnet + K-feldspar + quartz/coesite + rutile + calcio-ankerite/ankeritess. Solidus temperature is at 900–950°C at 2.5 GPa and at 900–1,000°C at 3.0 GPa, and the near-solidus melt is K-rich granitic. Crystalline carbonates persist only 50–100°C above the solidus and at temperatures above carbonate breakdown, carbon exists in the form of dissolved CO2 in silica-rich melts and as a vapor phase. The rhyodacitic to dacitic partial melt evolves from a K-rich composition at near-solidus condition to K-poor, and Na- and Ca-rich composition with increasing temperature. The low breakdown temperatures of crystalline carbonate in our study compared to those of recent studies on carbonated basaltic eclogite and peridotite owes to Fe-enrichment of carbonates in pelitic lithologies. However, the conditions of carbonate release in our study still remain higher than the modern depth-temperature trajectories of slab-mantle interface at sub-arc depths, suggesting that the release of sedimentary carbonates is unlikely in modern subduction zones. One possible scenario of carbonate release in modern subduction zones is the detachment and advection of sedimentary piles to hotter mantle wedge and consequent dissolution of carbonate in rhyodacitic partial melt. In the Paleo-NeoProterozoic Earth, on the other hand, the hotter slab-surface temperatures at subduction zones likely caused efficient liberation of carbon from subducting sedimentary carbonates. Deeply subducted carbonated sediments, similar to HPLC1, upon encountering a hotter mantle geotherm in the oceanic province can release carbon-bearing melts with high K2O, K2O/TiO2, and high silica, and can contribute to EM2-type ocean island basalts. Generation of EM2-type mantle end-member may also occur through metasomatism of mantle wedge by carbonated metapelite plume-derived partial melts.  相似文献   

6.
Phase relations of phlogopite with magnesite from 4 to 8 GPa   总被引：2，自引：2，他引：0  
To evaluate the stability of phlogopite in the presence of carbonate in the Earth’s mantle, we conducted a series of experiments in the KMAS–H2O–CO2 system. A mixture consisting of synthetic phlogopite (phl) and natural magnesite (mag) was prepared (phl90-mag10; wt%) and run at pressures from 4 to 8 GPa at temperatures ranging from 1,150 to 1,550°C. We bracketed the solidus between 1,200 and 1,250°C at pressures of 4, 5 and 6 GPa and between 1,150 and 1,200°C at a pressure of 7 GPa. Below the solidus, phlogopite coexists with magnesite, pyrope and a fluid. At the solidus, magnesite is the first phase to react out, and enstatite and olivine appear. Phlogopite melts over a temperature range of ~150°C. The amount of garnet increases above solidus from ~10 to ~30 modal% to higher pressures and temperatures. A dramatic change in the composition of quench phlogopite is observed with increasing pressure from similar to primary phlogopite at 4 GPa to hypersilicic at pressures ≥5 GPa. Relative to CO2-free systems, the solidus is lowered such, that, if carbonation reactions and phlogopite metasomatism take place above a subducting slab in a very hot (Cascadia-type) subduction environment, phlogopite will melt at a pressure of ~7.5 GPa. In a cold (40 mWm−2) subcontinental lithospheric mantle, phlogopite is stable to a depth of 200 km in the presence of carbonate and can coexist with a fluid that becomes Si-rich with increasing pressure. Ascending kimberlitic melts that are produced at greater depths could react with peridotite at the base of the subcontinental lithospheric mantle, crystallizing phlogopite and carbonate at a depth of 180–200 km.  相似文献   

7.
Diamond crystallization has been studied in the SiO2–H2O–С, Mg2SiO4–H2O–С and H2O–С subsystems at 7.5 GPa and 1,600°C. We found that dissolution of initial graphite is followed by spontaneous nucleation of diamond and growth of diamond on seed crystals. In 15-h runs, the degree of graphite to diamond transformation [α = MDm/(MDm + MGr)100, where MDm is mass of obtained diamond and MGr mass of residual graphite] reached 100% in H2O-rich fluids but was only 35–50% in water-saturated silicate melts. In 40-h runs, an abrupt decrease of α has been established at the weight ratio H2O/(H2O + SiO2) ≤ 0.16 or H2O/(H2O + Mg2SiO4) ≤ 0.15. Our results indicate that α is a function of the concentration of water, which controls both the kinetics of diamond nucleation and the intensity of carbon mass transfer in the systems. The most favorable conditions for diamond crystallization in the mantle silicate environment at reliable PT-parameters occur in the fluid phase with low concentration of silicates solute. In H2O-poor silicate melts diamond formation is questionable.  相似文献   

8.
The solid‐state reaction magnesite (MgCO3) + calcite (aragonite) (CaCO3) = dolomite (CaMg(CO3)2) has been identified in metapelites from western Tianshan, China. Petrological studies show that two metamorphic stages are recorded in the metapelites: (1) the peak mineral assemblage of magnesite and calcite pseudomorphs after aragonite which is only preserved as inclusions within dolomite; and (2) the retrograde glaucophane‐chloritoid facies mineral assemblage of glaucophane, chloritoid, dolomite, garnet, paragonite, chlorite and quartz. The peak metamorphic temperatures and pressures are calculated to be 560–600 °C, 4.95–5.07 GPa based on the calcite–dolomite geothermometer and the equilibrium calculation of the reaction dolomite = magnesite + aragonite, respectively. These give direct evidence in UHP metamorphic rocks from Tianshan, China, that carbonate sediments were subducted to greater than 150 km depth. This UHP metamorphism represents a geotherm lower than any previously estimated for subduction metamorphism (< 3.7 °C km?1) and is within what was previously considered a ‘forbidden’ condition within Earth. In terms of the carbon cycle, this demonstrates that carbonate sediments can be subducted to at least 150 km depth without releasing significant CO2 to the overlying mantle wedge.  相似文献   

9.
Bowen's petrogenetic grid was based initially on a series of decarbonation reactions in the system CaO-MgO-SiO2-CO2 with starting assemblages including calcite, dolomite, magnesite and quartz, and products including enstatite, forsterite, diopside and wollastonite. We review the positions of 14 decarbonation reactions, experimentally determined or estimated, extending the grid to mantle pressures to evaluate the effect of CO2 on model mantle peridotite composed of forsterite(Fo)+orthopyroxene(Opx)+clinopyroxene(Cpx). Each reaction terminates at an invariant point involving a liquid, CO2, carbonates, and silicates. The fusion curves for the mantle mineral assemblages in the presence of excess CO2 also terminate at these invariant points. The points are connected by a series of reactions involving liquidus relationships among the carbonates and mantle silicates, at temperatures lower (1,100–1,300° C) than the silicate-CO2 melting reactions (1,400–1,600° C). Review of experimental data in the bounding ternary systems together with preliminary data for the system CaO-MgO-SiO2-CO2 permits construction of a partly schematic framework for decarbonation and melting reactions at upper mantle pressures. The key to several problems in the peridotite-CO2 subsystem is the intersection of a subsolidus carbonation reaction with a melting reaction at an invariant point near 24 kb and 1,200°C. There is an intricate series of reactions between 25 kb and 35 kb involving changes in silicate and carbonate phase fields on the CO2-saturated liquidus surfaces. Conclusions include the following: (1) Peridotite Fo+Opx+Cpx can be carbonated with increasing pressure, or decreasing temperature, to yield Fo+Opx+Cpx+Cd (Cd=calcic dolomite), Fo+Opx+Cd, Fo+Opx+Cm (Cm=calcic magnesite), and finally Qz+Cm. (2) Free CO2 cannot exist in subsolidus mantle peridotite with normal temperature distributions; it is stored as carbonate, Cd. (3) The CO2 bubbles in peridotite nodules do not represent free CO2 in mantle peridotite along normal geotherms. (4) CO2 is as effective as H2O in causing incipient melting, our preferred explanation for the low-velocity zone. (5) Fusion of peridotite with CO2 at depths shallower than 80 km produces basic magmas, becoming more SiO2-undersaturated with depth. (6) The solubility of CO2 in mantle magmas is less than about 5 wt% at depths to 80 km, increasing abruptly to about 40 wt% at 80 km and deeper. (7) Deeper than 80 km, the first liquids produced are carbonatitic, changing towards kimberlitic and eventually, at considerably higher temperatures, to basic magmas. (8) Kimberlite and carbonatite magmas rising from the asthenosphere must evolve CO2 at depths 100-80 km, which contributes to their explosive emplacement. (9) Fractional crystallization of CO2-bearing SiO2-undersaturated basic magmas at most pressures can yield residual kimberlite and carbonatite magmas.  相似文献   

10.
The pressure–temperature conditions of the reactions of the double carbonates CaM(CO3)2, where M = Mg (dolomite), Fe (ankerite) and Mn (kutnohorite), to MCO3 plus CaCO3 (aragonite) have been investigated at 5–8 GPa, 600–1,100°C, using multi-anvil apparatus. The reaction dolomite = magnesite + aragonite is in good agreement with the results of Sato and Katsura (Earth Planet Sci 184:529–534, 2001), but in poor agreement with the results of Luth (Contrib Mineral Petrol 141:222–232, 2001). The dolomite is partially disordered at 620°C, and fully disordered at 1,100°C. All ankerite and kutnohorite samples, including the synthetic starting materials, are disordered. The P–T slopes of the three reactions increase in the order M = Mg, Fe, Mn. The shallower slope for the reaction involving magnesite is due partly to its having a higher compressibility than expected from unit-cell volume considerations. At low pressures there is a preference for partitioning into the double carbonate of Mg > Fe > Mn. At high pressures the partitioning preference is reversed. Using the measured reaction positions, the P–T conditions at which dolomite solid solutions will break down on increasing P and T in subduction zones can be estimated.  相似文献   

11.
Diamond crystallization in multicomponent melts of variable composition is studied. The melt carbonates are K2CO3, CaCO3?MgCO3, and K-Na-Ca-Mg-Fe-carbonatites, and the melt silicates are model peridotite (60 wt.% olivine, 16 wt.% orthopyroxene, 12 wt.% clinopyroxene, and 12 wt.% garnet) and eclogite (50 wt.% garnet and 50 wt.% clinopyroxene). In the experiments carried out under the PT-conditions of diamond stability, the carbonate-silicate melts behave like completely miscible liquid phases. The concentration barriers of diamond nucleation (CBDN) in the melts with variable proportions of silicates and carbonates have been determined at 8.5 GPa. In the system peridotite–K2CO3–CaCO3?MgCO3–carbonatite they correspond to 30, 25, and 30 wt.% silicates, respectively, and in the analogous eclogite–carbonate system, 45, 30, and 35 wt.%. In the silicate-carbonate melts with higher silicate contents seed diamond growth occurs, which is accompanied by the crystallization of thermodynamically unstable graphite phase. In the experiments with melts compositionally corresponding to the CBDN at 7.0 GPa and 1200–1700 °C, a full set of silicate minerals of peridotite (olivine, orthopyroxene, clinopyroxene, garnet) and eclogite (garnet, clinopyroxene) parageneses was obtained. The minerals occur as syngenetic inclusions in natural diamonds; moreover, the garnets contain an impurity of Na, and the pyroxenes, K. The experimental data indicate that peridotite-carbonate and eclogite-carbonate melts are highly effective for the formation of diamond (or unstable graphite) together with syngenetic minerals and melts, which agrees with the carbonate-silicate (carbonatite) model for the mantle diamond formation.  相似文献   

12.
Based on experimental and mineralogical data, the model of mantle carbonate-silicate (carbonatite) melts as dominating parental media for natural diamonds was substantiated. It was demonstrated that the compositions of silicate constituents of parental melts were variable and saturated with respect to mantle rocks, namely pyrope peridotite, garnet pyroxenite, and eclogite. Based on concentration contributions and role in diamond genesis, major (carbonate and silicate) and minor (admixture) components were distinguished. The latter components may be both soluble (oxides, phosphates, chlorides, carbon dioxide, and water) and insoluble (sulfides, metals, and carbides) in silicate-carbonate melts. This paper presents the results of a study of diamond crystallization in multicomponent melts of variable composition with carbonate components (K2CO3, CaCO3 · MgCO3, and K-Na-Ca-Mg-Fe carbonatite) and silicate components represented by model peridotite (60 wt % olivine, 16 wt % orthopyroxene, 12 wt % clinopyroxene, and 12 wt % garnet) and eclogite (50 wt % garnet and 50 wt % clinopyroxene). Carbonate-silicate melts behave like completely miscible liquid phases in experiments performed under the P-T conditions of diamond stability. The concentration barriers of diamond nucleation (CBDN) in melts with variable proportions of silicates and carbonates were determined at 8.5 GPa. In the peridotite system with K2CO3, CaCO3 · MgCO3, and carbonatite, they correspond to 30, 25, and 30 wt % silicates, respectively, and in the eclogite system, the CBDN is shifted to 45, 30, and 35 wt % silicates. In the silicate-carbonate melts with higher silicate contents, diamond grows on seeds, which is accompanied by the crystallization of thermodynamically unstable graphite. At P = 7.0 GPa and T = 1200−1800°C, we studied and constructed phase diagrams for the multicomponent peridotite-carbonate and eclogite-carbonate systems as a physicochemical basis for revealing the syngenetic relationships between diamond and its silicate (olivine, ortho- and clinopyroxene, and garnet) and carbonate (aragonite and magnesite) inclusions depending on the physicochemical conditions of growth media. The results obtained allowed us to reconstruct the evolution of diamond-forming systems. The experiments revealed similarity between the compositions of synthetic silicate minerals and inclusions in natural diamonds (high concentrations of Na in garnets and K in clinopyroxenes). It was experimentally demonstrated that the formation of Na-bearing majoritic garnets is controlled by the P-T parameters and melt alkalinity. Diamonds with inclusions of such garnets can be formed in alkalic carbonate-silicate (aluminosilicate) melts. A mechanism was suggested for sodic end-member dissolution in majoritic garnets, and garnet with the composition Na2MgSi5O12 and tetragonal symmetry was synthesized for the first time.  相似文献   

13.
 We carried out a series of melting experiments with hydrous primitive mantle compositions to determine the stability of dense hydrous phases under high pressures. Phase relations in the CaO–MgO–Al2O3–SiO2 pyrolite with ˜2 wt% of water have been determined in the pressure range of 10–25 GPa and in the temperature range between 800 and 1400 °C. We have found that phase E coexisting with olivine is stable at 10–12 GPa and below 1050 °C. Phase E coexisting with wadsleyite is stable at 14–16 GPa and below 900 °C. A superhydrous phase B is stable in pyrolite below 1100 °C at 18.5 GPa and below 1300 °C at 25 GPa. No hydrous phases other than wadsleyite are stable in pyrolite at 14–17 GPa and 900–1100 °C, suggesting a gap in the stability of dense hydrous magnesium silicates (DHMS). We detected an expansion in the stability field of wadsleyite to lower pressures (12 GPa and 1000 °C). The H2O content of wadsleyite was found to decrease not only with increasing temperature but also with increasing pressure. The DHMS phases could exist in a pyrolitic composition only under the conditions present in the subducting slabs descending into the lower mantle. Under the normal mantle and hot plume conditions, wadsleyite and ringwoodite are the major H2O-bearing phases. The top of the transition zone could be enriched in H2O in accordance with the observed increase in water solubility in wadsleyite with decreasing pressure. As a consequence of the thermal equilibration between the subducting slabs and the ambient mantle, the uppermost lower mantle could be an important zone of dehydration, providing fluid for the rising plumes. Received: 9 September 2002 / Accepted: 11 January 2003 Acknowledgements The authors are thankful to Y. Ito for the assistance with the EPMA measurement, A. Suzuki, T. Kubo and T. Kondo for technical help with the high-pressure experiments and Raman and X-ray diffraction measurements and C.R. Menako for technical support. K. Litasov thanks H. Taniguchi for his continuous encouragement and the Center for Northeast Asian Studies of Tohoku University and the Japanese Society for the Promotion of Science for the research fellowships. This work was partially supported by the Grant-in-Aid of Scientific Research of the Priority Area (B) of the Ministry of Education, Science, Sport, and Culture of the Japanese government (no. 12126201) to E. Ohtani.  相似文献   

14.
Diamond crystallization from carbon solutions in compositionally variable melts of model eclogite with dolomite [CaMg(CO3)2], potassium carbonate (K2CO3), and multicomponent K-Na-Ca-Mg-Fe carbonates was studied at 7.0–8.5 GPa. Concentration barriers for the nucleation of the diamond were determined at a standard pressure of 8.5 GPa for variable proportions of silicate and carbonate components in the growth solutions. They correspond to 35, 65, and 40 wt % of silicate components for systems with dolomite, K2CO3, and carbonatites, respectively. At higher contents of silicates in silicate-carbonate melts, the nucleation of diamond phase ceases, but diamond crystallization on seed crystals continues and is accompanied by the spontaneous crystallization of thermodynamically unstable graphite. In melts of the albite (NaAlSi3O8)-K2CO3-C compositions, the concentration barrier of diamond nucleation at 8.5 GPa is up to 90–92 wt % of the albite component, and diamond growth on seeds was observed in albite-carbon melts. Using mineralogical and experimental data, we developed a model of mantle carbonate-silicate (carbonatite) melts as the main parental media for natural diamonds; it was shown that the composition of the silicate constituent of such parental melts is variable and corresponds to the mantle ultrabasic-basic series. With respect to concentration contributions and dominant role in the genesis of diamond in the Earth’s mantle, major (carbonate and silicate) and minor or admixture components were distinguished. The latter include both soluble in carbonate-silicate melts (oxides, phosphates, chlorides, carbon dioxide, and water) and insoluble components (sulfides, metals, and carbides). Both major and minor components may affect the position of the concentration barriers of diamond nucleation in natural parent media.  相似文献   

15.
The high-pressure stability limit of calcium aluminosilicate (CAS) phase has been examined in its end-member CaAl4Si2O11 composition at 18–39 GPa and 1,670–2,300 K in a laser-heated diamond-anvil cell (LHDAC). The in-situ synchrotron X-ray diffraction measurements revealed that the CAS phase decomposes into three-phase assemblage of cubic Al-bearing CaSiO3 perovskite, Al2O3 corundum, and SiO2 stishovite above 30 GPa and 2,000 K with a positive pressure–temperature slope. Present results have important implications for the subsolidus mineral assemblage of subducted sediment and the melting phase relation of basalt in the lower mantle.  相似文献   

16.
Genesis of diamonds in the lower mantle   总被引：3，自引：0，他引：3  
The “forbidden” assemblage (ferropericlase + enstatite) as inclusions in diamonds has been taken as evidence to imply that these inclusions and their host diamonds formed initially in the lower mantle. Magnesite is probably the only stable carbonate at depths greater than ∼220 km. Like dehydration reactions, the reaction boundary for the decarbonation of magnesite has a positive dT/dP slope at lower pressures, which becomes negative at higher pressures, if no other phase intervenes. This reaction boundary probably intersects the geotherm between ∼900 and ∼1100 km, below which magnesite decomposes into an assemblage periclase + diamond + oxygen. Thus, ferropericlase is the most likely inclusion in diamond formed in the lower mantle. The high frequency of sole occurrence of ferropericlase in diamonds from Sao Luiz, Brazil seems to substantiate the present speculation. Received: 8 June 1998 / Accepted: 28 September 1998  相似文献   

17.
Resulting from static experiments performed to study the phase state of CaCO3, it was found that its melting is congruent at 20–22 GPa and 3500 K. The obtained experiment data show that the field of congruent melting of calcium carbonate is rather broad (form 2300 to 3500–3800 K at 20–22 GPa). However, the potential presence of a high-temperature phase boundary at which CaCO3 is decomposed into CaO and CO2 is not ruled out. The existence of a wide area of congruent melting of calcium carbonate (a common primary inclusion in diamonds of the transition zone and lower mantle of the Earth) allow one to consider deep-seated melts as potential parental media for ultradeep diamonds.  相似文献   

18.
 Ultramafic xenoliths are found in Kishyuku Lava, Fukue-jima, Southwest Japan. These include spinel lherzolite, harzburgite and dunite, as well as pyroxenite. The compositions of the constituent minerals of the peridotite xenoliths are in the range of upper mantle peridotites. Variable Cr/(Cr+Al) ratios (0.1–0.5) of spinel, together with a limited range in olivine composition (Fo90–Fo92), indicate that the xenoliths are derived from slightly to highly depleted residual mantle. The combination of previously published clinopyroxene-olivine geothermobarometry and clinopyroxene-orthopyroxene geothermometry applied to the xenoliths yields a high geotherm of 1070° C at 1.0 GPa up to 1200° C at 2.2 GPa. Existence of such depleted upper mantle is compatible with the existing model of asthenospheric injection during the rifting of the Northeast China and the Japan Sea. The high geotherm is caused by thermal perturbation due to the injection of the hot asthenosphere and/or post-rifting uprise of mantle diapirs since 11 Ma. Received: 15 May 1995 / Accepted: 3 January 1996  相似文献   

19.
This study demonstrates that a hydrous, halide bearing silicate melt is a viable medium for diamond growth. Experiments were conducted in the MgO–SiO2–H2O–C ± KCl ± NaCl system, which was used as a model for harzburgitic mantle. In no case did we observe crystals that could be interpreted as spontaneously nucleated, but growth of diamond on seed crystals at 1,400–1,600°C and 7 GPa in experiments of 4 h duration was observed. The addition of KCl to the system produced crystallization of diamond at temperatures as low as 1,400°C. At higher temperatures, larger growth features were produced than those that seen in the KCl-free system at the same conditions. The NaCl-bearing system is different; in these experiments, the diamond seed crystals show evidence of possible dissolution and layer growth, albeit more subdued growth than in the KCl system. Therefore, NaCl may be an inhibitor of diamond growth in a hydrous silicate melt. Based on these results, hydrous silicate melts could play a role in formation of diamond in either deep subduction zones, or above slabs imbricated against a lithospheric ‘root’ in the sub-continental lithospheric mantle. The water and halide necessary for their formation could be transported into the mantle in hydrous phases such as serpentine in subducting lithospheric slabs. Dehydration of serpentine at >200 km depth would release hydrous, halide-bearing fluids into the overlying mantle wedge or lithospheric root, triggering melting at conditions similar to those of the formation of natural diamond.  相似文献   

20.
Experimental studies in the system Fe,Ni–olivine–carbonate–S (P = 6.3 GPa, T = 1050–1550°C, t = 40–60 h) aimed at modeling of the interaction of subducted carbonates and sulfur with rocks of the silicate mantle and at investigation of the likely mechanism of the formation of mantle sulfides were performed. It is shown that an association of olivine + orthopyroxene + magnesite + pyrite coexisting with a sulfur melt/fluid with dissolved Fe, Ni, and O is formed at T ≤ 1250°C. An association of low-Fe olivine, orthopyroxene, and magnesite and two immiscible melts of the carbonate and S–Fe–Ni–O compositions are formed at T ≥ 1350°C. It is shown that the reduced S-bearing fluids may transform silicates and carbonates, extract metals from the solid-phase matrix, and provide conditions for generation of sulfide melts.  相似文献   

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