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1.
The oxygen fugacity and therefore the iron redox state of a melt is known to have a strong influence on the liquid line of descent of magmas and thus on the composition of the coexisting melts and crystals. We present a new method to estimate this critical parameter from electron probe microanalyses of two of the most common minerals of basaltic series, plagioclase and clinopyroxene. This method is not based on stoichiometric calculations, but on the different partitioning behaviour of Fe3+ and Fe2+ between both minerals and a melt phase: plagioclase can incorporate more Fe3+ than Fe2+, while clinopyroxene can incorporate more Fe2+ than Fe3+. For example, the effect of oxidizing a partly molten basaltic system (Fe3+ is stabilized with respect to Fe2+) results in an increase of FeOtotal in plagioclase, but a decrease in the associated clinopyroxene. We propose an equation, based on published partition coefficients, that allows estimating the redox state of a melt from these considerations. An application to a set of experimental and natural data attests the validity of the proposed model. The associated error can be calculated and is on average < 1 log unit of the prevailing oxygen fugacity.In order to reduce the number of different variables influencing the Fe2+/Fe3+ mineral/melt equilibrium, our model is restricted to basaltic series with SiO2 < 60% that have crystallized at intermediate to low pressure (< 0.5 GPa) and under relatively oxidizing conditions (?FMQ > 0; where FMQ is the fayalite–magnetite–quartz oxygen buffer equilibrium), but it may be parameterized for other conditions. A spreadsheet is provided to assist the use of equations, and to perform the error propagation analysis.  相似文献   

2.
We investigated the concentration and site occupation of ferric iron (Fe3+) in (Mg,Fe)O to understand the influence of point defects on transport properties such as atomic diffusion, electrical conductivity and viscosity. We conducted Mössbauer spectroscopy of (Mg0.8Fe0.2)O single crystals synthesized at temperatures from 1673 to 2273 K and pressures from 5 to 15 GPa with Re–ReO2 and Mo–MoO2 oxygen fugacity buffers. The isomer shift of the Mössbauer spectra suggests that Fe3+ occupies mostly the tetrahedral site at reduced conditions and both the octahedral and tetrahedral sites at oxidized conditions. We formulate a thermodynamic model of point defect dissolution in (Mg,Fe)O which suggests that unassociated tetrahedral Fe3+ is more stable than unassociated octahedral Fe3+ at high-pressure and low oxygen fugacity due to the effect of configurational entropy. The pressure dependence of Fe3+ concentration indicates a change in the dominant site occupancy of Fe3+: (1) Fe3+ in the tetrahedral site, (2) Fe3+ in the octahedral site, and (3) defect clusters of Fe3+ and cation vacancy, in the order of increasing oxygen fugacity and decreasing pressure. This is in reasonable agreement with previously reported experiments on Fe3+ concentration, Mg–Fe interdiffusivity and electrical conductivity. We consider it plausible that (Mg,Fe)O accommodates Fe3+ in the tetrahedral site down to the lower mantle. Based on our results and available experimental data, we discuss the solubility competition between Fe3+ and protons (H+), and its implications for transport properties in the lower mantle.  相似文献   

3.
Aluminum incorporation into the high pressure polymorph of TiO2 with the structure of α-PbO2 has been studied from 10 to 20 GPa and 1300 °C by XRD, high-resolution 27Al MAS-NMR and TEM. Al-doped α-PbO2 type TiO2 can be recovered at atmospheric pressure. Al2O3 solubility in α-PbO2 type TiO2 increases with increasing the synthesis pressure. The α-PbO2 type TiO2 polymorph is able to incorporate up to 35 wt.% Al2O3 at 13.6 GPa and 1300 °C, being the substitution of Ti4+ by Al3+ on normal octahedral sites the mechanism of solubility. The transition to the higher pressure TiO2 polymorph with the ZrO2 baddeleyite structure, Akaogiite, has not been observed in the quenched samples at room pressure. The microstructure of the recovered sample synthesized at 16 GPa and 1300 °C points to the existence of a non-quenchable aluminum titanium oxide phase at these conditions.  相似文献   

4.
Experiments were performed to determine concentration-dependent diffusion coefficients of Cr3+ and Ga3+ in periclase at temperatures of 1563–2273 K. Diffusion profiles measured in the quenched samples are consistent with a theoretical model in which the mobile species is a bound M3+-vacancy pair, and each profile was fitted to determine the binding energy and diffusion coefficient of the pair. Trivalent chromium-vacancy pairs diffuse more slowly than Ga3+-vacancy pairs, and with higher migration energy, 237 kJ/mol vs. 190 kJ/mol. Cation vacancies also bind less tightly to Cr3+ than to Ga3+, with average binding free energies of ?22 and ?83 kJ/mol, respectively. At all concentrations and temperatures, Cr3+ diffuses much more slowly than Ga3+, by up to two orders of magnitude. The differences between Cr3+ and Ga3+ cannot be explained by differences in ionic radius or dipole polarizability, but are consistent with the influence of the crystal field on the partially occupied 3d orbitals of Cr3+. The crystal field splitting stabilizes Cr3+ on the octahedral cation site, increasing the energy required for Cr3+ to exchange positions with an adjacent vacancy. It also makes Cr3+-vacancy pairs less favorable, with the presence of a nearest-neighbor vacancy disrupting the symmetry of the octahedral site, thus diminishing the crystal field stabilization. Trends in the diffusion of first-row divalent transition metals in periclase can also be explained by the crystal field effect. High-spin to low-spin transitions in Fe2+, Co2+ or Mn2+ would significantly enhance their crystal field stabilization in periclase, and if such spin transitions occur in the deep mantle, they would be expected to slow the diffusivity of these ions significantly, perhaps by several orders of magnitude.  相似文献   

5.
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6.
In the central Western Alps, a combined structural, petrological and 40Ar–39Ar geochronological study of the Modane-Aussois and Southern Vanoise units yields important constraints on the timing of deformation and exhumation of the Briançonnais zone. These data help to decipher the respective roles of oceanic subduction, continental subduction and collision in the burial and exhumation of the main units through time. In the Modane-Aussois unit top to the NW thrusting (D1) was followed by top to the east shearing (D2) interpreted by some as normal faulting and by others as backthrusting. Pseudosection calculations imply that D1 deformation occurred at 1.0 ± 0.1 GPa and 350 ± 30 °C. Analysis of chlorite–phengite pairs yield P–T estimates between 0.15 and 0.65 GPa and between 220 and 350 °C for the D2 event. Phengites along the D1 schistosity (sample M80) yields an 40Ar–39Ar age of 37.12 ± 0.39 Ma, while D2 phengites yield ages of 35.42 ± 0.38 (sample M173) and 31.60 ± 0.33 Ma (sample M196). It was not possible to test whether these ages are altered by excess argon or not. Our interpretation is that the D1/D2 transition occurred at ∼37 Ma at the beginning of decompression, and that D2 lasted until at least ∼32 Ma. Pseudosection calculation suggests that the Southern Vanoise unit was buried at 1.6 ± 0.2 GPa and 500–540 °C. D1 deformation occurred during exhumation until 0.7–10.5 GPa and 370 ± 30 °C. Published ages suggest that D1 deformation possibly started at ∼50 Ma and lasted until ∼37 Ma. D2 deformations started at P–T conditions close to that recorded in Modane-Aussois unit and lasted until 0.2 ± 0.1 GPa and 280 ± 30 °C at ∼28 Ma. The gap of 0.6 ± 0.3 GPa and 150 ± 130 °C between peak metamorphic conditions in the two units was concealed by thrusting of the South Vanoise unit on top of the Modane-Aussois unit during D1 Deformation. Top to the east deformation (D2) affects both units and is interpreted as backthrusting.Based on these data, we propose a geodynamic reconstruction where the oceanic subduction of the Piedmont unit until ∼50 Ma, is followed by its exhumation at the time of continental subduction of the continental Southern Vanoise unit until ∼45 Ma. The Southern Vanoise is in turn underthrusted by the Modane-Aussois unit until ∼37 Ma (D1). Between 37 and 31 Ma the Modane-Aussois and Southern Vanoise units exhume together during backthrusting to the east (D2). This corresponds to the collision stage and to the activation of the Penninic Thrust. In the ∼50 Ma to ∼31 Ma time period the main thrusts propagated westward as the tectonic context switched from oceanic to continental subduction and finally to collision. During each stage, external units are buried while internal ones are exhumed.  相似文献   

7.
We report observations of seasonal and local time variation of the averaged electron and iron concentrations, as well as simultaneous measurements of the two species, above the Arecibo Observatory (18.35°N, 66.75°N), Puerto Rico. The average Fe profile between 21:00 and 24:00 LT has a single peak at about 85 km with the exception of the summer when an additional peak exists at about 95 km. The higher Fe peak in the summer is correlated with higher electron concentrations in this season. The three nights of simultaneous measurements of electron and iron concentrations show that narrow layers of Fe and electrons are well correlated. Comparison of the climatological and simultaneous Fe and electron data suggests that recombination of Fe+ plays an important role in determining the Fe profile in the upper part of the Fe layer. Above 93 km, the Fe concentration appears to increase after sunset if the electron concentration exceeds about 4000 electrons cm−3. The average rate of Fe production is about 0.1 atom cm−3 s−1 for all seasons at 100 km in the early evening hours. A chemical model reveals that the concentration of Fe+ must be 50–80% of the total ionization over Arecibo for typical equinox conditions to explain the observed rate of Fe production. These high relative Fe+ concentrations are consistent with in situ observations that Fe+ is usually the dominant ion in sporadic E layers in the nighttime lower E region. This suggests that the source of Fe+ is provided by sporadic E layers descending over Arecibo after sunset. The Fe density between 80 and 85 km decreases during the night, for all seasons. This is attributed to the formation of stable molecular Fe species, such as FeOH, due to the increase in O3 and decrease in atomic O and H during the night at these altitudes.  相似文献   

8.
We have determined the post-perovskite phase transition boundary in MgSiO3 in a wide temperature range from 1640 to 4380 K at 119–171 GPa on the basis of synchrotron X-ray diffraction measurements in-situ at high-pressure and -temperature in a laser-heated diamond-anvil cell (LHDAC). The results show a considerably high positive Clapeyron slope of + 13.3 ± 1.0 MPa/K and a transition temperature of about 3520 ± 70 K at the core–mantle boundary (CMB) pressure. The thermal structure in D″ layer can be tightly constrained from precisely determined post-perovskite phase transition boundary and the depths of paired seismic discontinuities. These suggest that temperature at the CMB may be around 3700 K, somewhat lower than previously thought. A minimum bound on the global heat flow from the core is estimated to be 6.6 ± 0.5 TW.  相似文献   

9.
The melting curve of iron monosilicide, FeSi, has been determined in a laser-heated diamond anvil cell from 6 up to 70 GPa by direct visual observation of the continuous laser-speckle motion in the liquid state. At 12 GPa and 1700 K, a discontinuous change in the slope of the melting curve indicates the first-order phase transition between the ?-FeSi (B20) and the CsCl-type FeSi structures (B2). During the phase transition the coordination number of both, Fe and Si atoms, increases from 7 to 8. Above this pressure, the melting curve rises steeply but shows significant flattening at higher pressures. A comparison with the melting curve of Fe shows that both curves cross at 32 ± 3 GPa, FeSi having higher melting temperatures (about 100 K) at high pressures.  相似文献   

10.
We present ab initio calculations of the zero-temperature iron high- to low-spin crossover in (Mg1 ? xFex)SiO3 perovskite at pressures relevant to Earth's lower mantle. Equations of state are fit for a range of compositions and used to predict the Fe spin transition pressure and associated changes in volume and bulk modulus. We predict a dramatic decrease in transition pressure as Fe concentration increases. This trend is contrary to that seen in ferropericlase, and suggests the energetics for spin crossover is highly dependent on the structural environment of Fe. Both Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA) exchange-correlation methods are used, and both methods reproduce the same compositional trends. However, GGA gives a significantly higher transition pressure than LDA. The spin transition is made easier by the decreasing spin-flip energy with pressure but is also driven by the change in volume from high to low spin. Volume trends show that high-spin Fe2+ is larger than Mg2+ even under pressure, but low-spin Fe2+ is smaller at ambient conditions and approximately the same size as Mg2+ under high pressure, indicating that low-spin Fe2+ is less compressible than high-spin Fe2+. We find large changes between high- and low-spin in the slope of volume with Fe concentration. Although these changes are small in absolute magnitude for small Fe content, they are still important when measured per Fe and could be relevant for calculating partitioning coefficients in the lower mantle.  相似文献   

11.
The South Sandwich volcanic arc is sited on a young oceanic crust, erupts low-K tholeiitic rocks, is characterized by unexotic pelagic and volcanogenic sediments on the down-going slab, and simple tectonic setting, and is ideal for assessing element transport through subduction zones. As a means of quantifying processes attending transfer of subduction-related fluids from the slab to the mantle wedge, boron concentrations and isotopic compositions were determined for representative lavas from along the arc. The samples show variable fluid-mobile/fluid-immobile element ratios and high enrichments of B/Nb (2.7 to 55) and B/Zr (0.12 to 0.57), similar to those observed in western Pacific arcs. δ11B values are among the highest so far reported for mantle-derived lavas; these are highest in the central part of the arc (+ 15 to + 18‰) and decrease toward the southern and northern ends (+ 12 to + 14‰). δ11B is roughly positively correlated with B concentrations and with 87Sr/86Sr ratios, but poorly coupled with other fluid-mobile elements such as Rb, Ba, Sr and U. Peridotites dredged from the forearc trench also have high δ11B (ca. + 10‰) and elevated B contents (38–140 ppm). Incoming pelagic sediments sampled at ODP Site 701 display a wide range in δ11B (+ 5 to ? 13‰; average = ? 4.1‰), with negative values most common. The unusually high δ11B values inferred for the South Sandwich mantle wedge cannot easily be attributed to direct incorporation of subducting slab materials or fluids derived directly therefrom. Rather, the heavy B isotopic signature of the magma sources is more plausibly explained by ingress of fluids derived from subduction erosion of altered frontal arc mantle wedge materials similar to those in the Marianas forearc. We propose that multi-stage recycling of high-δ11B and high-B serpentinite (possibly embellished by arc crust and volcaniclastic sediments) can produce extremely 11B-rich fluids at slab depths beneath the volcanic arc. Infiltration of such fluids into the mantle wedge likely accounts for the unusual magma sources inferred for this arc.  相似文献   

12.
Patterns in fault slip rates through time and space are examined across the transition from the Sierra Nevada to the Eastern California Shear Zone–Walker Lane belt. At each of four sites along the eastern Sierra Nevada frontal fault zone between 38 and 39° N latitude, geomorphic markers, such as glacial moraines and outwash terraces, are displaced by a suite of range-front normal faults. Using geomorphic mapping, surveying, and 10Be surface exposure dating, mean fault slip rates are defined, and by utilizing markers of different ages (generally, ~ 20 ka and ~ 150 ka), rates through time and interactions among multiple faults are examined over 104–105 year timescales.At each site for which data are available for the last ~ 150 ky, mean slip rates across the Sierra Nevada frontal fault zone have probably not varied by more than a factor of two over time spans equal to half of the total time interval (~ 20 ky and ~ 150 ky timescales): 0.3 ± 0.1 mm year? 1 (mode and 95% CI) at both Buckeye Creek in the Bridgeport basin and Sonora Junction; and 0.4 + 0.3/?0.1 mm year? 1 along the West Fork of the Carson River at Woodfords. Data permit rates that are relatively constant over the time scales examined. In contrast, slip rates are highly variable in space over the last ~ 20 ky. Slip rates decrease by a factor of 3–5 northward over a distance of ~ 20 km between the northern Mono Basin (1.3 + 0.6/?0.3 mm year? 1 at Lundy Canyon site) to the Bridgeport Basin (0.3 ± 0.1 mm year? 1). The 3-fold decrease in the slip rate on the Sierra Nevada frontal fault zone northward from Mono Basin is indicative of a change in the character of faulting north of the Mina Deflection as extension is transferred eastward onto normal faults between the Sierra Nevada and Walker Lane belt.A compilation of regional deformation rates reveals that the spatial pattern of extension rates changes along strike of the Eastern California Shear Zone-Walker Lane belt. South of the Mina Deflection, extension is accommodated within a diffuse zone of normal and oblique faults, with extension rates increasing northward on the Fish Lake Valley fault. Where faults of the Eastern California Shear Zone terminate northward into the Mina Deflection, extension rates increase northward along the Sierra Nevada frontal fault zone to ~ 0.7 mm year? 1 in northern Mono Basin. This spatial pattern suggests that extension is transferred from more easterly fault systems, e.g., Fish Lake Valley fault, and localized on the Sierra Nevada frontal fault zone as the Eastern California Shear Zone–Walker Lane belt faulting is transferred through the Mina Deflection.  相似文献   

13.
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   

14.
Volume measurements for magnesiowüstite (Mg0.6Fe0.4)O, were carried out up to pressures of 10.1 GPa in the temperature range 300–1273 K, using energy-dispersive synchrotron X-ray diffraction. These data allow reliable determination of the temperature dependence of the bulk modulus and good constraint on the thermal expansitivity at ambient pressure which was previously not known for magnesiowüstite. From these data, thermal and elastic parameters were derived from various approaches based on the Birch–Murnaghan equation of state (EOS) and on the relevant thermodynamic relations. The results from three different equations of state are remarkably consistent. With (∂KT/∂P)T fixed at 4, we obtained K0=158(2) GPa, (∂KT/∂T)P=−0.029(3) GPa K−1, (∂KT/∂T)V=−3.9(±2.3)×10−3 GPa K−1, and αT=3.45(18)×10−5+1.14(28)×10−8T. The K0, (∂KT/∂T)P, and (∂KT/∂T)V values are in agreement with those of Fei et al. (1992) and are similar to previously determined values for MgO. The zero pressure thermal expansitivity of (Mg0.6Fe0.4)O is found to be similar to that for MgO (Suzuki, 1975). These results indicate that, for the compositional range x=0–0.4 in (Mg1−xFex)O, the thermal and elastic properties of magnesiowüstite exhibit a dependence on the iron content that is negligibly small, within uncertainties of the experiments. They are consequently insensitive to the Fe–Mg partitioning between (Mg, Fe)SiO3 perovskite and magnesiowüstite when applied to compositional models of the lower mantle. With the assumption that (Mg0.6Fe0.4)O is a Debye-like solid, a modified equation of heat capacity at constant pressure is proposed and thermodynamic properties of geophysically importance are calculated and tabulated at high temperatures.  相似文献   

15.
Melting in the Fe–FeO system was investigated at pressures up to 93 GPa using synchrotron X-ray diffraction (XRD) and a laser heated diamond anvil cell (DAC). The criteria for melting were the disappearance of reflections associated with one of the end-member phases upon raising the temperature above the eutectic and the reappearance of those reflections on dropping the temperature below the eutectic. The Fe–FeO system is a simple eutectic at 50 GPa and remains eutectic to at least 93 GPa. The eutectic temperature was bound at several pressure points between 19 and 93 GPa, and in some cases the liquidus temperature was also determined. The eutectic temperature rises rapidly with pressure closely following the melting curve of pure Fe. A detailed phase diagram at 50 GPa is presented; the eutectic temperature is 2500 ± 150 K and the eutectic composition is bound between 7.6 ± 1.0 and 9.5 ± 1.0 wt.% O. The coefficient of thermal expansion of FeO is a strong function of volume and decreases with pressure according to a simple power law.  相似文献   

16.
In-situ cosmogenic 3He exposure ages of pyroxene phenocrysts from basalts from the Upper Neostromboli formation in southwest Stromboli date its eruption at 7.0 ± 0.3 ka (1σ, n = 3, Ginostra site) and 6.8 ± 0.2 ka (1σ, n = 10, Timpone del Fuoco site) respectively. Correlation of our new data to previous K/Ar and palaeomagnetic ages from the northwestern Neostromboli phase suggests that it erupted within a confined period between roughly 6 and 14 ka. The low uncertainty on the 3Hecos ages as well as on individual exposure ages (4.4–8.7%) demonstrates that 3Hecos exposure dating is a viable tool for dating Holocene basalt lavas. The ages compare favourably to uncertainties obtained for radiocarbon dating of similar rocks.  相似文献   

17.
The elastic moduli of polycrystalline ringwoodite, (Mg0.91Fe0.09)2SiO4, were measured up to 470 K by means of the resonant sphere technique. The adiabatic bulk (KS) and shear (μ) moduli were found to be 185.1(2) and 118.22(6) GPa at room temperature, and the average slopes of dKS/dT and dμ/dT in the temperature range of the study were determined to be −0.0193(9) and −0.0148(3) GPa/K, respectively. Using these results, we estimate seismic wave velocity jumps for a pure olivine mantle model at 520 km depth. We find that the jump for the S-wave velocity is about 1.5 times larger than that for the P-wave velocity at this depth. This suggests that velocity jumps at the 520 km discontinuity are easier to detect using S-waves than P-waves.  相似文献   

18.
IRSL single-aliquot dating of three feldspar fractions from a Howieson's Poort (SA) industry site has been investigated. In the case of the plagioclase fraction (ρ > 2.62 g cm?3) equivalent dose is similar to that of the other fractions (i.e. ρ < 2.58 g cm?3, high-K-feldspar; and 2.58 < ρ < 2.62 g cm?3; low-K-feldspar). However, the g values measured for this fraction is such that the fading-corrected age is overestimated. The K content of each fraction was estimated by SEM. The DRC-corrected ages for the two lighter feldspar fractions are consistent with the archaeological interpretation. This study shows that reliable single-aliquot luminescence ages can be obtained using K-feldspar and Na-plagioclase.  相似文献   

19.
SHRIMP zircon U–Pb dating, mineral chemical, element geochemical and Sr–Nd–Pb–Hf isotopic data have been determined for the Yulong monzogranite-porphyry in the eastern Tibet, China. The Yulong porphyry was emplaced into Triassic strata at about 39 Ma. The rocks are weakly peraluminous and show shoshonitic affinity, i.e., alkalis-rich, high K2O contents with high K2O / Na2O ratios, enrichment in LREE and LILE. They also show some affinities with the adakite, e.g., high SiO2 and Al2O3, and low MgO contents, depleted in Y and Yb, and enrichment in Sr with high Sr / Y and La / Yb ratios, and no Eu anomalies. The Yulong porphyry has radiogenic 87Sr / 86Sr (0.7063–0.7070) and unradiogenic 143Nd / 144Nd (εNd = − 2.0 to − 3.0) ratios. The Pb isotopic compositions of feldspar phenocrysts separated from the Yulong porphyry show a narrow range of 206Pb / 204Pb ratios (18.71–18.82) and unusually radiogenic 207Pb / 204Pb (15.65–15.67) and 208Pb / 204Pb (38.87–39.00) ratios. In situ Hf isotopic composition of zircons that have been SHRIMP U–Pb dated is characterized by clearly positive initial εHf values, ranging from + 3.1 to + 5.9, most between + 4 and + 5. Phenocryst clinopyroxene geothermometry of the Yulong porphyry indicates that the primary magmas had anomalously high temperature (> 1200 °C). The source depth for the Yulong porphyry is at least 100 km inferred by the metasomatic volatile phase (phlogopite–carbonate) relations. Detailed geochemical and Sr–Nd–Pb–Hf isotopic compositions not only rule out fractional crystallization or assimilation-fractional crystallization processes, but also deny the possibility of partial melting of subducted oceanic crust or basaltic lower crust. Instead, low degree (1–5%) partial melting of a metasomatized lithosphere (phlogopite–garnet clinopyroxenite) is compatible with the data. This example gives a case study that granite can be derived directly by partial melting of an enriched lithospheric mantle, which is important to understand the source and origin of diverse granites.  相似文献   

20.
The evolution with pressure of the unit-cell parameters brownmillerite (Ca2Fe2O5), a stoichiometric defect perovskite structure, has been determined to a maximum pressure of 9.46 GPa, by single-crystal X-ray diffraction measurements at room temperature. Brownmillerite does not exhibit any phase transitions in this pressure range. A fit of a third-order Birch–Murnaghan equation-of-state to the P–V data yields values of KT0=127.0(5) GPa and K0′=5.99(13). Analysis of the unit-cell parameter data shows that the structure compresses anisotropically. Compressional moduli for the axes are Ka0=141(1) GPa, Kb0=118(3) GPa and Kc0=122.2(2) GPa, with Ka0′=8.9(3), Kb0′=6.2(6) and Kc0′=4. The stiffest direction (i.e. along a) coincides with the direction of the FeO4 tetrahedral chains. Comparison of these data with the elasticity systematics of Ca-perovskites shows that the presence of oxygen vacancies in the brownmillerite structure softens the structure by ∼25% and that the ordering of vacancies in the perovskite structure increases the anisotropy of compression.  相似文献   

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