Prediction of high-temperature oxygen isotope fractionation factors between mantle minerals

The increment method is adopted to calculate oxygen isotope fractionation factors for mantle minerals, particularly for the polymorphic phases of MgSiO₃ and Mg₂SiO₄. The results predict the following sequence of ¹⁸O-enrichment: pyroxene (Mg,Fe,Ca)₂Si₂O₆>olivine (Mg,Fe)₂SiO₄>spinel (Mg,Fe)₂SiO₄>ilmenite (Mg,Fe, Ca)SiO₃>perovskite (Mg,Fe,Ca)SiO₃. The calculated fractionations for the calcite-perovskite (CaTiO₃) system are in excellent agreement with experimental calibrations. If there would be complete isotopic equilibration in the mantle, the spinel-structured silicates in the transition zone are predicted to be enriched in ¹⁸O relative to the perovskite-structured silicates in the lower mantle but depleted in ¹⁸O relative to olivines and pyroxenes in the upper mantle. The oxygen isotope layering of the mantle would essentially result from differences in the chemical composition and crystal structure of mineral phases at different mantle depths. Assuming isotopic equilibrium on a whole earth scale, the chemical structure of the Earth's interior can be described by the following sequence of ¹⁸O-enrichment: uppr crust>lower crust>upper mantle>transition zone>lower mantle >core.     

Dihedral angle measurements and infiltration property of SiO<Subscript>2</Subscript>-rich melts in mantle peridotite assemblages

Petrological and experimental studies demonstrated that nepheline-normative, SiO₂-rich melts can be present in the upper mantle at pressures 1.5 GPa. To evaluate the role of such melts in mantle processes and magma genesis, we carried out two types of experiments: (1) melt distribution experiments to characterize the grain-scale distribution of a small fraction of typical SiO₂-rich mantle melt (SRMM) in polycrystalline olivine (Ol) at 1,180°C, 1.2 GPa; and (2) an infiltration experiment to test the ability of SRMM to impregnate and metasomatise neighbouring non-molten mantle rocks. The median dihedral angles at Ol-Ol-SRMM contacts are equal to 50°, implying that melt should be interconnected at all melt fractions. Complications arise, however, in the investigated system because Ol–liquid interfacial energy is anisotropic, and we estimate that the connectivity threshold in the SRMM–Ol system is 0.3 vol%. Regarding the very low volume fraction of SRMM in peridotites, we conclude that these melts either occur as isolated pockets or form a network of grain edge channels with a low degree of connectivity due to a large number of dry grain edges. Even in the case where an interconnected network exists, their large viscosities should prohibit the extraction of SRMM from peridotite sources. The infiltration experiment also points to a very reduced mobility of SRMM in the upper mantle. In this experiment, a slice of synthetic dunite was immersed into a magma reservoir made of 60 wt% SRMM+40 wt% Ol, and subjected to 1,180°C-1.2 GPa for 113 h: despite this long duration, the SiO₂-rich liquid was unable to infiltrate measurably the dunite. Our experiments do not support the hypothesis that SRMM represent agents of mantle metasomatism.     

Preferential dissolution of SiO<Subscript>2</Subscript> from enstatite to H<Subscript>2</Subscript> fluid under high pressure and temperature

Stability and phase relations of coexisting enstatite and H₂ fluid were investigated in the pressure and temperature regions of 3.1–13.9 GPa and 1500–2000 K using laser-heated diamond-anvil cells. XRD measurements showed decomposition of enstatite upon heating to form forsterite, periclase, and coesite/stishovite. In the recovered samples, SiO₂ grains were found at the margin of the heating hot spot, suggesting that the SiO₂ component dissolved in the H₂ fluid during heating, then precipitated when its solubility decreased with decreasing temperature. Raman and infrared spectra of the coexisting fluid phase revealed that SiH₄ and H₂O molecules formed through the reaction between dissolved SiO₂ and H₂. In contrast, forsterite and periclase crystals were found within the hot spot, which were assumed to have replaced the initial orthoenstatite crystals without dissolution. Preferential dissolution of SiO₂ components of enstatite in H₂ fluid, as well as that observed in the forsterite H₂ system and the quartz H₂ system, implies that H₂-rich fluid enhances Mg/Si fractionation between the fluid and solid phases of mantle minerals.     

High-pressure Raman studies and heat capacity measurements on the MgSiO<Subscript>3</Subscript> analogue CaIr<Subscript>0.5</Subscript>Pt<Subscript>0.5</Subscript>O<Subscript>3</Subscript>

Raman spectroscopy and heat capacity measurements have been used to study the post-perovskite phase of CaIr_0.5Pt_0.5O₃, recovered from synthesis at a pressure of 15 GPa. Laser heating CaIr_0.5Pt_0.5O₃ to 1,900 K at 60 GPa produces a new perovskite phase which is not recoverable and reverts to the post-perovskite polymorph between 20 and 9 GPa on decompression. This implies that Pt-rich CaIr_1−xPt_xO₃ perovskites including the end member CaPtO₃ cannot easily be recovered to ambient pressure from high P–T synthesis. We estimate an increase in the thermodynamic Grüneisen parameter across the post-perovskite to perovskite transition of 34%, of similar magnitude to those for (Mg,Fe)SiO₃ and MgGeO₃, suggesting that CaIr_0.5Pt_0.5O₃ is a promising analogue for experimental studies of the competition in energetics between perovskite and post-perovskite phases of magnesium silicates in Earth’s lowermost mantle. Low-temperature heat capacity measurements show that CaIrO₃ has a significant Sommerfeld coefficient of 11.7 mJ/mol K² and an entropy change of only 1.1% of Rln2 at the 108 K Curie transition, consistent with the near-itinerant electron magnetism. Heat capacity results for post-perovskite CaIr_0.5Rh_0.5O₃ are also reported.     

Calorimetric study on high-pressure transitions in KAlSi<Subscript>3</Subscript>O<Subscript>8</Subscript>

KAlSi₃O₈ sanidine dissociates into a mixture of K₂Si₄O₉ wadeite, Al₂SiO₅ kyanite and SiO₂ coesite, which further recombine into KAlSi₃O₈ hollandite with increasing pressure. Enthalpies of KAlSi₃O₈ sanidine and hollandite, K₂Si₄O₉ wadeite and Al₂SiO₅ kyanite were measured by high-temperature solution calorimetry. Using the data, enthalpies of transitions at 298 K were obtained as 65.1 ± 7.4 kJ mol^–1 for sanidine wadeite + kyanite + coesite and 99.3 ± 3.6 kJ mol^–1 for wadeite + kyanite + coesite hollandite. The isobaric heat capacity of KAlSi₃O₈ hollandite was measured at 160–700 K by differential scanning calorimetry, and was also calculated using the Kieffer model. Combination of both the results yielded a heat-capacity equation of KAlSi₃O₈ hollandite above 298 K as C_p=3.896 × 10²–1.823 × 10³T^–0.5–1.293 × 10⁷T^–2+1.631 × 10⁹T^–3 (C_p in J mol^–1 K^–1, T in K). The equilibrium transition boundaries were calculated using these new data on the transition enthalpies and heat capacity. The calculated transition boundaries are in general agreement with the phase relations experimentally determined previously. The calculated boundary for wadeite + kyanite + coesite hollandite intersects with the coesite–stishovite transition boundary, resulting in a stability field of the assemblage of wadeite + kyanite + stishovite below about 1273 K at about 8 GPa. Some phase–equilibrium experiments in the present study confirmed that sanidine transforms directly to wadeite + kyanite + coesite at 1373 K at about 6.3 GPa, without an intervening stability field of KAlSiO₄ kalsilite + coesite which was previously suggested. The transition boundaries in KAlSi₃O₈ determined in this study put some constraints on the stability range of KAlSi₃O₈ hollandite in the mantle and that of sanidine inclusions in kimberlitic diamonds.     

H<Subscript>2</Subscript>O solubility in basalt at upper mantle conditions

This study presents a new experimental approach for determining H₂O solubility in basaltic melt at upper mantle conditions. Traditional solubility experiments are limited to pressures of ~600 MPa or less because it is difficult to reliably quench silicate melts containing greater than ~10 wt% dissolved H₂O. To overcome this limitation, our approach relies on the use of secondary ion mass spectrometry to measure the concentration of H dissolved in olivine and on using the measured H in olivine as a proxy for the concentration of H₂O in the co-existing basaltic melt. The solubility of H₂O in the melt is determined by performing a series of experiments at a single pressure and temperature with increasing amounts of liquid H₂O added to each charge. The point at which the concentration of H in the olivine first becomes independent of the amount of initial H₂O content of the charge (added + adsorbed H₂O) indicates its solubility in the melt. Experiments were conducted by packing basalt powder into a capsule fabricated from San Carlos olivine, which was then pressure-sealed inside a Ni outer capsule. Our experimental results indicate that at 1000 MPa and 1200 °C, the solubility of H₂O in basaltic melt is 20.6 ± 0.9 wt% (2 × standard deviation). This concentration is considerably higher than predicted by most solubility models but defines a linear relationship between H₂O fugacity and the square of molar H₂O solubility when combined with solubility data from lower pressure experiments. Further, our solubility determination agrees with melting point depression determined experimentally by Grove et al. (2006) for the H₂O-saturated peridotite solidus at 1000 MPa. Melting point depression calculations were used to estimate H₂O solubility in basalt along the experimentally determined H₂O-saturated peridotite solidus. The results suggest that a linear relationship between H₂O fugacity and the square of molar solubility exists up to ~1300 MPa, where there is an inflection point and solubility begins to increase less strongly with increasing H₂O fugacity.     

Phase transformations in descending plates and implications for mantle dynamics

The role of phase transformations in a mantle of pyrolite composition is reviewed in the light of recent experimental data. The pyroxene component of pyrolite transforms to the garnet structure at 300–350 km whilst olivine transforms to beta-Mg₂SiO₄ near 400 km. Between about 500 and 550 km, beta-Mg₂SiO₄ probably transforms to a partially inverse spinel structure whilst the CaSiO₃ component of the complex garnet solid solution exsolves and transforms to the perovskite structure. The major seismic discontinuity near 650–700 km is probably caused by disproportionation of Mg₂SiO₄ spinel into periclase plus stishovite. At a slightly greater depth, the remaining magnesian garnet transforms to the corundum or ilmenite structure. Finally, at a depth probably in the vicinity of 800–1000 km, the (Mg,Fe)SiO₃ component of the ilmenite phase transforms to a perovskite structure whilst stishovite and some of the periclase recombine to form perovskite also. The mineral assemblage so formed is about 4% denser than mixed oxides (MgO + FeO + A1₂O₃ + CaO + stishovite) isochemical with pyrolite. The above sequence of phase transformations in pyrolite provides a satisfactory general explanation of the elastic properties and density distribution in the mantle. In particular, there is no evidence requiring an increase of FeO/(FeO + MgO) ratio with depth.The depths at which major phase transformations occur in subducted lithosphere differ from those in ‘normal’ mantle. These differences are caused by two factors: (1) Temperatures within sinking plates are much lower than in surrounding mantle to depths of 700 km or more. (2) Irreversible chemical differentiation of pyrolite occurs at oceanic ridges. Lithosphere plates so formed consist of a layer of basaltic rocks underlain successively by layers of harzburgite, lherzolite, and pyrolite slightly depleted in highly incompatible elements (e.g. La, Ba, Rb, U). The phase-transformation behaviour of the first three of these layers differs from that of pyrolite.The effects of these and other factors connected with phase transformations on the dynamics of plate subsidence are discussed. It appears quite likely that plates penetrate the 650–700 km discontinuity, largely because the slope of the spinel disproportionation is probably positive, not negative as generally supposed. The former basaltic oceanic crust probably sinks deeply into the lower mantle, whilst the former harzburgite component of the plate may collect above the perovskite transition boundary. Phase transformations may thus serve as a kind of filter, leading to increased and irreversible mantle heterogeneity with time.The possible roles of phase transformations in causing deep-focus earthquakes and introducing water into the mantle in subduction zones are also briefly discussed.     

Decarbonation reaction of magnesite in subducting slabs at the lower mantle

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.     

High-pressure phase transitions of anorthosite crust in the Earth's deep mantle

We investigated phase relations, mineral chemistry, and density of lunar highland anorthosite at conditions up to 125 GPa and 2000 K. We used a multi-anvil apparatus and a laser-heated diamond-anvil cell for this purpose. In-situ X-ray diffraction measurements at high pressures and composition analysis of recovered samples using an analytical transmission electron microscope showed that anorthosite consists of garnet, CaAl₄Si₂O₁₁-rich phase (CAS phase), and SiO₂ phases in the upper mantle and the mantle transition zone. Under lower mantle conditions, these minerals transform to the assemblage of bridgmanite, Ca-perovskite, corundum, stishovite, and calcium ferrite-type aluminous phase through the decomposition of garnet and CAS phase at around 700 km depth. Anorthosite has a higher density than PREM and pyrolite in the upper mantle, while its density becomes comparable or lower under lower mantle conditions. Our results suggest that ancient anorthosite crust subducted down to the deep mantle was likely to have accumulated at 660–720 km in depth without coming back to the Earth's surface. Some portions of the anorthosite crust might have circulated continuously in the Earth's deep interior by mantle convection and potentially subducted to the bottom of the lower mantle when carried within layers of dense basaltic rocks.     

Compositions of aqueous fluids in equilibrium with pyroxenes and olivines at mantle pressures and temperatures

Solubility experiments were performed at 30 kbars in the system Mg₂SiO₄-SiO₂-H₂O, and at 20 and 30 kbars on omphacitic pyroxene-water mixtures. They confirm that the solubility of the forsterite component in aqueous fluids remains rather low (up to 5 wt.%), whereas the solubility of the SiO₂ component from solids of appropriate SiO₂-rich compositions in the system Mg₂SiO₄-SiO₂-H₂O increases with temperature up to some 75% at 1,100° C. At this temperature a simplified harzburgite consisting of forsterite and enstatite coexists with a fluid containing about 35% (MgO+SiO₂). Hydrous fluids coexisting with omphacitic clinopyroxenes leach sodium silicate component from the solid leaving less jadeitic pyroxenes behind. Most interestingly, the amount of sodium leached at constant temperature increases with decreasing pressure.Comparison of the results with previous solubility studies in the system K₂O-MgO-Al₂O₃-SiO₂-H₂O indicates that hydrous fluids in the mantle must be alkaline rather than silicanormative. Alkali metasomatism caused by such fluids would lead to potassium enrichment in deeper portions of the upper mantle and to sodium enrichment at shallower levels, where amphiboles become stable. This K/Na fractionation in the upper mantle may explain the generation of K-rich or of Na-rich magmas through partial melting at different depths.     

The influence of water on melting of mantle peridotite

This experimental study examines the effects of variable concentrations of dissolved H₂O on the compositions of silicate melts and their coexisting mineral assemblage of olivine + orthopyroxene ± clinopyroxene ± spinel ± garnet. Experiments were performed at pressures of 1.2 to 2.0 GPa and temperatures of 1100 to 1345 °C, with up to ∼12 wt% H₂O dissolved in the liquid. The effects of increasing the concentration of dissolved H₂O on the major element compositions of melts in equilibrium with a spinel lherzolite mineral assemblage are to decrease the concentrations of SiO₂, FeO, MgO, and CaO. The concentration of Al₂O₃ is unaffected. The lower SiO₂ contents of the hydrous melts result from an increase in the activity coefficient for SiO₂ with increasing dissolved H₂O. The lower concentrations of FeO and MgO result from the lower temperatures at which H₂O-bearing melts coexist with mantle minerals as compared to anhydrous melts. These compositional changes produce an elevated SiO₂/(MgO + FeO) ratio in hydrous peridotite partial melts, making them relatively SiO₂ rich when compared to anhydrous melts on a volatile-free basis. Hydrous peridotite melting reactions are affected primarily by the lowered mantle solidus. Temperature-induced compositional variations in coexisting pyroxenes lower the proportion of clinopyroxene entering the melt relative to orthopyroxene. Isobaric batch melting calculations indicate that fluid-undersaturated peridotite melting is characterized by significantly lower melt productivity than anhydrous peridotite melting, and that the peridotite melting process in subduction zones is strongly influenced by the composition of the H₂O-rich component introduced into the mantle wedge from the subducted slab. Received: 7 April 1997 / Accepted: 9 January 1998     

Pressure-induced structural evolution of pyrite-type SiO<Subscript>2</Subscript>

The equation of state and pressure-induced structural evolution of pyrite-type SiO₂ have been investigated based on synchrotron X-ray diffraction measurements in a diamond-anvil cell. The Rietveld refinement revealed that the oxygen coordinate x of pyrite-type SiO₂ increases with increasing pressure. The SiO₆ coordination polyhedra of pyrite-type SiO₂ is less compressible than the unit cell, and the increase in x induces a rotation of the SiO₆ coordination polyhedra to fill the blank space in the unit cell. Thus, the volume reduction in pyrite-type SiO₂ is achieved mainly by the rotation of the SiO₆ polyhedra, rather than by the compression of the SiO₆ polyhedra. In addition, the increase in x with increasing pressure enhances a distortion of the coordination polyhedra of pyrite-type SiO₂, implying that pyrite-type SiO₂ is not likely to transform into a fluorite-type structure at higher pressures.     

Low-temperature heat capacities,entropies and enthalpies of Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript> polymorphs,and α−β−γ and post-spinel phase relations at high pressure

The low-temperature isobaric heat capacities (C _p) of β- and γ-Mg₂SiO₄ were measured at the range of 1.8–304.7 K with a thermal relaxation method using the Physical Property Measurement System. The obtained standard entropies (S°₂₉₈) of β- and γ-Mg₂SiO₄ are 86.4 ± 0.4 and 82.7 ± 0.5 J/mol K, respectively. Enthalpies of transitions among α-, β- and γ-Mg₂SiO₄ were measured by high-temperature drop-solution calorimetry with gas-bubbling technique. The enthalpies of the α−β and β−γ transitions at 298 K (ΔH°₂₉₈) in Mg₂SiO₄ are 27.2 ± 3.6 and 12.9 ± 3.3 kJ/mol, respectively. Calculated α−β and β−γ transition boundaries were generally consistent with those determined by high-pressure experiments within the errors. Combining the measured ΔH°₂₉₈ and ΔS°₂₉₈ with selected data of in situ X-ray diffraction experiments at high pressure, the ΔH°₂₉₈ and ΔS°₂₉₈ of the α−β and β−γ transitions were optimized. Calculation using the optimized data tightly constrained the α−β and β−γ transition boundaries in the P, T space. The slope of α−β transition boundary is 3.1 MPa/K at 13.4 GPa and 1,400 K, and that of β−γ boundary 5.2 MPa/K at 18.7 GPa and 1,600 K. The post-spinel transition boundary of γ-Mg₂SiO₄ to MgSiO₃ perovskite plus MgO was also calculated, using the optimized data on γ-Mg₂SiO₄ and available enthalpy and entropy data on MgSiO₃ perovskite and MgO. The calculated post-spinel boundary with a Clapeyron slope of −2.6 ± 0.2 MPa/K is located at pressure consistent with the 660 km discontinuity, considering the error of the thermodynamic data.     

Stability of various hydrous phases in CMAS pyrolite-H<Subscript>2</Subscript>O system up to 25 GPa

 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.     

Magnesian andesite and dacite lavas from Mt. Shasta,northern California: products of fractional crystallization of H<Subscript>2</Subscript>O-rich mantle melts

Mt. Shasta andesite and dacite lavas contain high MgO (3.5–5 wt.%), very low FeO*/MgO (1–1.5) and 60–66 wt.% SiO₂. The range of major and trace element compositions of the Shasta lavas can be explained through fractional crystallization (~50–60 wt.%) with subsequent magma mixing of a parent magma that had the major element composition of an H₂O-rich primitive magnesian andesite (PMA). Isotopic and trace element characteristics of the Mt. Shasta stratocone lavas are highly variable and span the same range of compositions that is found in the parental basaltic andesite and PMA lavas. This variability is inherited from compositional variations in the input contributed from melting of mantle wedge peridotite that was fluxed by a slab-derived, fluid-rich component. Evidence preserved in phenocryst assemblages indicates mixing of magmas that experienced variable amounts of fractional crystallization over a range of crustal depths from ~25 to ~4 km beneath Mt. Shasta. Major and trace element evidence is also consistent with magma mixing. Pre-eruptive crystallization extended from shallow crustal levels under degassed conditions (~4 wt.% H₂O) to lower crustal depths with magmatic H₂O contents of ~10–15 wt.%. Oxygen fugacity varied over 2 log units from one above to one below the Nickel-Nickel Oxide buffer. The input of buoyant H₂O-rich magmas containing 10–15 wt.% H₂O may have triggered magma mixing and facilitated eruption. Alternatively, vesiculation of oversaturated H₂O-rich melts could also play an important role in mixing and eruption.     

Crystal chemistry of ferric iron in (Mg,Fe)(Si,Al)O<Subscript>3</Subscript> majorite with implications for the transition zone

Fifteen samples of (Mg,Fe)SiO₃ majorite with varying Fe/Mg composition and one sample of (Mg,Fe)(Si,Al)O₃ majorite were synthesized at high pressure and temperature under different conditions of oxygen fugacity using a multianvil press, and examined ex situ using X-ray diffraction and Mössbauer and optical absorption spectroscopy. The relative concentration of Fe³⁺ increases both with total iron content and increasing oxygen fugacity, but not with Al concentration. Optical absorption spectra indicate the presence of Fe²⁺–Fe³⁺ charge transfer, where band intensity increases with increasing Fe³⁺ concentration. Mössbauer data were used in conjunction with electron microprobe analyses to determine the site distribution of all cations. Both Al and Fe³⁺ substitute on the octahedral site, and charge balance occurs through the removal of Si. The degree of Mg/Si ordering on the octahedral sites in (Mg,Fe)SiO₃ majorite, which affects both the c/a ratio and the unit cell volume, is influenced by the thermal history of the sample. The Fe³⁺ concentration of (Mg,Fe)(Si,Al)O₃ majorite in the mantle will reflect prevailing redox conditions, which are believed to be relatively reducing in the transition zone. Exchange of material across the transition boundary to (Mg,Fe) (Si,Al)O₃ perovskite would then require a mechanism to oxidize sufficient iron to satisfy crystal-chemical requirements of the lower-mantle perovskite phase.     

Kankan diamonds (Guinea) II: lower mantle inclusion parageneses

Frequent inclusions of ferropericlase, some coexisting with phases of MgSiO₃, CaSiO₃ and SiO₂ composition, suggest that a large proportion of diamonds from Guinea are derived from the lower mantle. Low aluminium contents in MgSiO₃ inclusions indicate derivation from the uppermost lower mantle, where Al solubility in perovskite is low. Trace element analyses (SIMS) of CaSiO₃ inclusions reveal extreme degrees of LREE (200–2000 times chondritic) and Sr enrichment (70–1000 times chondritic) together with negative and positive Eu anomalies. This implies a highly enriched lower mantle source, possibly a product of a subducted oceanic slab. A number of phases that are only stable in the upper mantle are found to coexist with lower mantle phases and thereby indicate retrograde equilibration during slow exhumation within a rising plume or convection cell. In one case, however, an inclusion paragenesis of ferropericlase and olivine can be shown to have formed within the upper mantle, indicating that the occurrence of ferropericlase inclusions alone is an unreliable indicator of lower mantle origin. Received: 26 January 2000 / Accepted: 18 May 2000     

Interactions between basalts and oil source rocks in rift basins： CO2 generation

Basalts interbedded with oil source rocks are discovered frequently in rift basins of eastern China, where CO2 is found in reservoirs around or within basalts, for example in the Binnan reservoir of the Dongying Depression. In the reservoirs, CO2 with heavy carbon isotopic composition (δ13C>-10‰ PDB) is in most cases accounts for 40% of the total gas reserve, and is believed to have resulted from degassing of basaltic magma from the mantle. In their investigations of the Binnan reservoir, the authors suggested that the CO2 would result from interactions between the source rocks and basalts. As the source rocks around basalts are rich in carbonate minerals, volcanic minerals, transition metals and organic matter, during their burial history some of the transition metals were catalyzed on the thermal degradation of organic matter into hydrocarbons and on the decomposition of carbonate minerals into CO2, which was reproduced in thermal simulations of the source rocks with the transition metals (Ni and Co). This kind of CO2 accounts for 55%-85% of the total gas reserve generated in the process of thermal simulation, and its δ13C values range from -11‰- -7.2‰ PDB, which are very similar to those of CO2 found in the Binnan reservoir. The co-generation of CO2 and hydrocarbon gases makes it possible their accumulation together in one trap. In other words, if the CO2 resulted directly from degassing of basaltic magma or was derived from the mantle, it could not be accumulated with hydrocarbon gases because it came into the basin much earlier than hydrocarbon generation and much earlier than trap formation. Therefore, the source rocks around basalts generated hydrocarbons and CO2 simultaneously through catalysis of Co and Ni transition metals, which is useful for the explanation of co-accumulation of hydrocarbon gases and CO2 in rift basins in eastern China.     

Solid solution behaviour of CaSiO<Subscript>3</Subscript> and MgSiO<Subscript>3</Subscript> perovskites

Using density functional simulations within the generalized gradient approximation and projector-augmented wave method together with thermodynamic modelling, the reciprocal solubilities of MgSiO₃ and CaSiO₃ perovskites were calculated for pressures and temperatures of the Earth’s lower mantle from 25 to 100 GPa and 0 to 6,000 K, respectively. The solubility of Ca in MgSiO₃ at conditions along a mantle adiabat is found to be less than 0.02 atoms per formula unit. The solubility of Mg in CaSiO₃ is even lower, and most important, the extent of solid solution decreases with pressure. To dissolve CaSiO₃ perovskite completely in MgSiO₃ perovskite, a solubility of 7.8 or 2.3 mol% would be necessary for a fertile pyrolytic or depleted harzburgitic mantle, respectively. Thus, for any reasonable geotherm, two separate perovskites will be present in fertile mantle, suggesting that Ca-perovskite will be residual to low degree melting throughout the entire mantle. At the solidus, CaSiO₃ perovskite might completely dissolve in MgSiO₃ perovskite only in a depleted mantle with <1.25 wt% CaO. These implications may be modified if Ca solubility in MgSiO₃ is increased by other major mantle constituents such as Fe and Al.     

Water transportation from the subducting slab into the mantle transition zone

Using a recently developed petrogenetic grid for MORB + H₂O, we propose a new model for the transportation of water from the subducting slab into the mantle transition zone. Depending on the geothermal gradient, two contrasting water-transportation mechanisms operate at depth in a subduction zone. If the geothermal gradient is low, lawsonite carries H₂O into mantle depths of 300 km; with further subduction down to the mantle transition depth (approximately 400 km) lawsonite is no longer stable and thereafter H₂O is once migrated upward to the mantle wedge then again carried down to the transition zone due to the induced convection. At this depth, hydrous β-phase olivine is stable and plays a role as a huge water reservoir. In contrast, if the geothermal gradient is high, the subducted slab may melt at 700–900 °C at depths shallower than 80 km to form felsic melt, into which water is dissolved. In this case, H₂O cannot be transported into the mantle below 80 km. Between these two end-member mechanisms, two intermediate types are present. In the high-pressure intermediate type, the hydrous phase A plays an important role to carry water into the mantle transition zone. Water liberated by the lawsonite-consuming continuous reaction moves upward to form hydrous phase A in the hanging wall, which transports water into deeper mantle. This is due to a unique character of the reaction, because Phase A can become stable through the hydration reaction of olivine. In the case of low-pressure intermediate type, the presence of a dry mantle wedge below 100 km acts as a barrier to prevent H₂O from entering into deeper mantle.