首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 437 毫秒
1.
Existing experimental evidence suggests that in pure Mg-forsterite with a fixed Mg/Si ratio, vacancies and interstitials on the Mg sublattice are the dominant intrinsic point defects, while O vacancies and conduction-band electrons are the primary oxygen-deficient defects. Orthopyroxene appears to dissolve readily in pure Mg-forsterite at high temperatures, the change in Mg/Si ratio being accommodated by Mg vacancies and Si interstitials; as a consequence, upper-mantle forsterite may be substantially non-stoichiometric. It is postulated that the amount of Fe that can be oxidized in the forsterite phase is directly related to the excess Si present, either incorporated in the structure, or available from another phase richer in SiO2 such as pyroxene.  相似文献   

2.
The diffusivity of18O in forsterite Mg2SiO4 has been measured in the temperature range 1150–1600°C. The activation energy of oxygen self-diffusion in this silicate is found to equal0.32 ± 0.04MJ/mol(77 ± 10kcal/mol), and there is no dependence of the diffusivity upon the oxygen partial pressure surrounding the samples. The diffusion profiles were analysed either with an ion probe or by means of the18O(p, α)15N nuclear reaction. The latter method made use of a resonance in the nuclear cross-section in the case of diffusion profiles shorter than 100 nm (1000Å); for diffusion profiles up to 4 μm the same reaction was used, but in a non-resonant mode. New data on creep in forsterite and natural olivine are also given, including the influence of the oxygen partial pressurepO2 which is zero for forsterite and proportional to(pO2)16 for natural olivine. From this set of data we infer the possible relationship between diffusion and creep for these materials. This relationship may be more complicated than that predicted by simple climb mechanism.  相似文献   

3.
High temperature calorimetric measurements of the enthalpies of solution in molten if2 PbO · B2O3 of α- and γ-Fe2SiO4 and α-, β-, and γ-Co2SiO4 permit the calculation of phase relations at high pressure and temperature. The reported triple point involving α-, β-, and γ-Co2SiO4 is confirmed to represent stable equilibrium. The curvature in the α?β phase boundary in Co2SiO4 and of an α?γ boundary in Fe2SiO4 at high temperature is explained in part by the effects of compressibility and thermal expansion, but better agreement with the observed phase diagram is obtained when one considers the effect of small amounts of cation disorder in the spinel and/or modified spinel phases. The calculated ΔH0 and ΔS0 values for the α?β, α?γ, and β?γ transitions show that enthalpy and en changes both vary strongly in the series Mg, Fe, Co, and Ni, and are of equal importance in determining the stability relations. The disproportionation of Fe2SiO4 and Co2SiO4 spinel to rocksalt plus stishovite is calculated to occur in the 170–190 kbar region; cation disorder and/or changes in wüstite stoichiometry can affect the P?T slope. The calorimetric data for CoSiO3 and FeSiO3 are in good agreement with the observed phase boundary for pyroxene formation from olivine and quartz. The decomposition of pyroxene to spinel and stishovite at pressures near the coesite-stishovite transition is predicted in both iron and cobalt systems. The use of calorimetric data, obtained from small samples of high pressure phases, is very useful in predicting equilibrium phase diagrams in the 50–300 kbar range.  相似文献   

4.
Inclusions of ferropericlase and former (Mg,Fe)(Si,Al)O3 perovskite in diamonds from Kankan, Guinea believed to originate in the lower mantle were studied using Mössbauer spectroscopy to determine Fe3+/ΣFe. Fe3+ concentration in the (Mg,Fe)(Si,Al)O3 inclusion is consistent with empirical relations relating Fe3+/ΣFe to Al concentration, supporting the inference that it crystallised in the perovskite structure at lower mantle conditions. In ferropericlase there is a nearly linear variation of trivalent cation abundance with monovalent cation abundance, suggesting a substitution of the form Na0.5M0.53+O (M=Fe3+, Cr3+, Al3+). Excess positive charge is likely balanced by cation vacancies, where their abundance is observed to increase with increasing iron concentration, consistent with high-pressure experiments. The abundance of cation vacancies is related to oxygen fugacity, where ferropericlase inclusions from Kankan and São Luiz (Brazil) are inferred to have formed at conditions more oxidising than Fe-(Mg,Fe)O equilibrium, but more reducing than Re-ReO2 equilibrium. Fe2+/Mg partition coefficients between (Mg,Fe)(Si,Al)O3 and ferropericlase were calculated for inclusions co-existing in the same diamond using Mössbauer data and empirical relations based on high-pressure experimental work. Most values are consistent with high-pressure experiments, suggesting that these inclusions equilibrated at lower mantle conditions. The measured ferropericlase Fe3+ concentrations are consistent with diamond formation in a region of redox gradients, possibly arising from the subduction of oxidised material into reduced lower mantle. Reduction of carbonate to form ferropericlase and diamond is consistent with a slight shift of Kankan δ13C values to isotopically heavy compositions compared to the worldwide dataset, and could supply the oxygen necessary to satisfy the high Fe3+ concentration in (Mg,Fe)(Si,Al)O3 perovskite, as well as account for the high proportion of ferropericlase in the lower mantle paragenesis. The heterogeneity of lower mantle diamond sources indicates that the composition of lower mantle diamonds do not necessarily reflect those of the bulk mantle.  相似文献   

5.
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 of18O-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 the 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 in18O relative to the perovskite-structured silicates in the lower mantle but depleted in18O relative to olivines and pyroxenes in the upper mantle. The oxygen isotope layering of the mantle might 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 of18O-enrichment:upper crust>lower crust>upper mantle>transition zone>lower mantle>core. Project supported by the National Natural Science Foundation of China and the Chinese Academy of Sciences.  相似文献   

6.
The oxidation state of iron can significantly influence the physical and chemical properties of lower mantle minerals. To improve methods for estimation of Fe3+/∑Fe, synthetic assemblages of (Mg,Fe)(Si,Al)O3 perovskite and (Mg,Fe)O ferropericlase were synthesised from oxide starting mixtures in Re or Fe capsules at 26 GPa and 1650-1850 °C using a multianvil press. (Mg,Fe)(Si,Al)O3 majorite was also present in some of the run products. Both electron energy loss spectra (EELS) and Mössbauer spectra were measured for each run product, and a robust fitting method was developed for Mössbauer spectra using EELS results as a standard that enabled Fe3+/∑Fe of (Mg,Fe)(Si,Al)O3 perovskite to be determined from Mössbauer spectra of multiphase assemblages. There is a close to linear variation between Fe3+/∑Fe and Al concentration in (Mg,Fe)(Si,Al)O3 perovskite, independent of oxygen fugacity. The concentration of Fe3+ in (Mg,Fe)O increases with increasing iron concentration along curves of constant oxygen fugacity, where higher oxygen fugacity stabilises greater Fe3+ concentrations. Fe2+/Mg partition coefficients calculated from chemical composition data corrected for measured Fe3+/∑Fe showed values nearly identical within experimental error for all samples, and independent of Al concentration and oxygen fugacity. Simple empirical relations were derived to calculate Fe3+/∑Fe in (Mg,Fe)(Si,Al)O3 perovskite and (Mg,Fe)O ferropericlase samples for which no Mössbauer or EELS data were available, and tested by applying them to calculation of Fe2+/Mg partition coefficients from literature data for (Mg,Fe)(Si,Al)O3 perovskite-(Mg,Fe)O assemblages where only total iron concentrations had been measured. Results showed Fe2+/Mg partition coefficients that were equal to existing values within experimental error, hence confirming the validity of the empirical relations.  相似文献   

7.
To calculate accurately the pressure interval and mineral proportions (i.e. yields) across the olivine to wadsleyite and wadsleyite to ringwoodite transformations requires a detailed knowledge of the non-ideality of Fe-Mg mixing in these (Mg,Fe)2SiO4 solid solutions. In order to constrain the activity-composition relations that describe non-ideal mixing, Fe-Mg partitioning experiments have been conducted between magnesiowüstite and (Mg,Fe)2SiO4 olivine, wadsleyite and ringwoodite as a function of pressure at 1400°C. Using known activity-composition relations for magnesiowüstite the corresponding relations for the three polymorphs were determined from the partitioning data. In all experiments the presence of metallic iron ensured redox conditions compatible with the Earth’s transition zone. The non-ideality of the (Mg,Fe)2SiO4 solid solutions was found to decrease in the order WwadsleyiteFeMg>WringwooditeFeMg>WolivineFeMg. These partitioning data were used, along with published phase equilibria measurements for the Mg2SiO4 and Fe2SiO4 end-member transformations, to produce an internally consistent thermodynamic model for the Mg2SiO4-Fe2SiO4 system at 1400°C. Using this model the pressure interval of the olivine to wadsleyite transformation is calculated to be significantly smaller than previous determinations. By combining these results with Fe-Mg partitioning data for garnet, the widths of transition zone phase transformations in a peridotite composition were calculated. The olivine to wadsleyite transformation at 1400°C in dry peridotite was found to occur over a pressure interval equivalent to approximately 6 km depth and the mineral yields were found to vary almost linearly with depth across the transformation. This transformation is likely to be even sharper at higher temperatures or could be significantly broader in wet mantle or in regions with a significant vertical component of mantle flow. The entire range of estimated widths for the 410 km discontinuity (4-35 km) could, therefore, be explained by the olivine to wadsleyite transformation in a peridotite composition over a range of quite plausible mantle temperatures and H2O contents. The wadsleyite to ringwoodite transformation in peridotite mantle was calculated to take place over an interval of 20 km at 1400°C. This transformation yield was also found to be near linear.  相似文献   

8.
In order to understand the origin of iron-rich olivine in the matrices of type 3 ordinary chondrites, the reaction of metallic iron and enstatite, with and without forsterite and SiO2, has been experimentally reproduced at temperatures between 1150° and 800°C and PO2 between 10−11 and 10−16 atm (between the IQF and MW buffers). The olivine produced ranges from Fo58 to Fo34 and this composition does not change significantly with temperature and time of the runs. The magnesian olivine which forms does become more magnesian with increasing forsterite/enstatite ratio of the starting materials. Iron-rich olivine (Fo< 35) cannot be formed by the reaction of enstatite and metallic iron, with or without forsterite as starting materials but it can be formed in the presence of free silica. The composition of olivine becomes more iron-rich with increasing silica/enstatite ratio. The compositional range of olivine formed from each mixture is 25–30 mole% Fo regardless of the temperature, composition, mineral assemblage, and run duration.From these experimental results, two possibilities suggested for the origin of the iron-rich olivine in the matrices of type 3 ordinary chondrites: (1) free silica must have been present if the iron-rich olivine was formed by solid-state reactions under oxidizing condition in the solar nebula; (2) reaction of silicon-rich gas with metallic iron took place under oxidizing condition in the solar nebula. Though it is difficult to define which alternative was dominant, the formation of free silica or silicon-rich gas may be a result of fractional condensation. This is possible if there is a reaction relation between forsterite and gas to produce enstatite. The suggested fractional condensation is supported by the fact that the compositions of the fine-grained matrices of type 3 ordinary chondrites are more silica-rich than the bulk compositions of the chondrites. Though it is not known whether such conditions were established all over the nebula or locally in the nebula, both fractionation and more oxidizing conditions than the average solar nebula are required for the formation of matrix olivine.  相似文献   

9.
Ferromagnesian silicate olivines, pyroxenes and garnets with Mg/(Mg + Fe)?0.3 (molar) have been found to transform to high-pressure phases characterized by the orthorhombic perovskite structure when compressed to pressures above 250 kbar in a diamond-anvil press and heated to temperatures above 1,000°C with a YAG laser. The zero-pressure density of the perovskite phase of (Mg,Fe)SiO3 is about 3–4% greater than that of the close-packed oxides, rocksalt plus stishovite. For (Mg,Fe)2SiO4 compounds, the perovskite plus rocksalt phase assemblage is 2–3% denser than the mixed oxides. The experimental synthesis of such high-density perovskite phases in olivine, pyroxene and garnet compounds suggests that (Mg,Fe)SiO3-perovskite is the dominant mineral phase in the earth's lower mantle.  相似文献   

10.
Shock recovery experiments on olivine single crystals (mineralogical composition: forsterite～80) have been carried out in a pressure range from～200up to～ 575kbar. Infrared spectroscopic investigations indicate a transformation of the mineral within a pressure interval from～420to～575kbar. Exposure to dynamic pressures of～575kbar yielded complete decomposition of the former olivine into an X-ray amorphous material upon pressure release. The infrared investigations suggest the post-shock products to consist essentially out of short-range-ordered (Mg,Fe)O and SiO2.  相似文献   

11.
Two synthetic pyroxenes (FeSiO3, MgSiO3) and five natural pyroxenes with compositions of about Fs80En20, Fs60En40, Fs50En50, Fs40En60, and Fs20En80 have been subjected to pressures up to250 ± 50kbars at a temperature of about1500 ± 200°C in a diamond anvil cell heated by an infrared laser beam. After quenching and unloading X-ray data analysis indicates that (1) those with Mg less than 50% undergo the following reactions: 2(Mg,Fe)SiO3 (pyroxene) → (Mg,Fe)2SiO4 (spinel) + SiO2 (stishovite) → 2(Mg,Fe)O (magnesiowu¨stite) + SiO2 (stishovite) with increase of pressure, and (2) those with Mg higher than 60%, undergo the following reactions: 2(Mg,Fe)SiO3 (pyroxene) → (Mg,Fe)2SiO4 (spinel) + SiO2 (stishovite) → 2(Mg,Fe)SiO3 (hexagonal phase) → 2(Mg,Fe)O (magnesiowu¨stite) + SiO2 (stishovite) with increase of pressure.  相似文献   

12.
The textures of chondrules have been reproduced by crystallizing melts of three different compositions at 1 atm with cooling rates ranging from 400 to 20°C/min under 10?9 to 10?12 atmPO2. A porphyritic olivine texture has been formed from a melt of olivine-rich composition (SiO2 = 45 wt.%), a barred-olivine texture from melt of intermediate composition (SiO2 = 47 wt.%), and radial-olivine texture from melt of pyroxene-rich composition (SiO2 = 57 wt.%). The cooling rate for producing barred olivine is most restricted; the rate ranges from 120 to 50°C/min. Other textures can be formed with wider ranges of cooling rate. The results of the experiments indicate that some of the major types of textures of chondrules can be formed with cooling rate of about 100°C/min. With this cooling rate, the texture varies depending on the composition of melt.  相似文献   

13.
The elastic moduli of a synthetic single crystal of pyrope (Mg3Al2Si3O12) have been determined using a technique based on Brillouin scattering. These results are used in an evaluation of the effect of composition on the elastic properties of silicate garnet solid solution series (Mg, Fe, Mn, Ca)3 (Al, Fe, Cr)2 Si3O12. In the pyralspites (Mg FeMn aluminum garnets), for which a large amount of data is available, this analysis indicates that the bulk modulus K is independent of the Fe2+/Mg2+ ratio, which is similar to the behavior observed in olivines and pyroxenes. However, the shear modulus μ of the garnets increases by 10% from the Mg to the Fe end member, in contrast to the decrease of μ with Fe content which is observed in olivines and pyroxenes. This contrasting behavior is most probably related to the oxygen coordination of the cation site occupied by Mg2+ and Fe2+ in these different minerals.  相似文献   

14.
The dislocation annihilation rate in experimentally deformed olivine single crystals was measured as a function of oxygen partial pressure (PO2). It was shown that the dislocation annihilation rate decreased with increasing PO2. This result is inconsistent with the reported PO2 dependence of creep rate (～ PO2+16) in single olivine crystals, thus indicating that the creep in single olivine crystals is not rate-controlled by recovery, under the experimentally investigated conditions.  相似文献   

15.
Previous hypotheses for mare basalt petrogenesis involving either the remelting or assimilation of 4.6-4.4-b.y. cumulates cannot satisfactorily account for certain key geochemical similarities between high-Ti and low-Ti primitive basalts, e.g. Mg/(Mg + Fe) ratios, Cr2O3 contents, eruption temperatures, and high-pressure liquidus phase relations. In addition, many thermal problems remain unanswered. A new petrogenetic hypothesis which appears to satisfy these constraints, is outlined below.The 4.6-4.4-b.y. melting event affected only the outer few hundred kilometers of the moon, leaving the undifferentiated primordial interior with its full complement of radiogenic elements. The differentiated lithosphere consisted of the crust and an underlying sequence of Mg-rich olivine + pyroxene cumulates and refractory residua. The late-stage differentiates were sandwiched between the crust and the cumulate sequence in large (perhaps 5–20 km diameter) segregations. The plagioclase-depleted late-stage residual system itself differentiated to form dense, Fe, Ti-rich, pyroxene + ilmenite±olivine cumulates, overlain by a complementary, solidified, residual liquid component, rich in incompatible elements. The dynamical consequence of this gravitationally unstable situation was the sinking of the dense (about 3.8 g/cm3) ilmenite-bearing cumulate pods into the lunar interior (density about 3.35 g/cm3). Mean-while the primordial mantle was approaching solidus temperatures due to radiogenic heating. The arrival of the sinking Fe, Ti-rich pods into this region initiated partial melting and complex assimilative interactions. High-Ti hybrid liquids were produced under equilibrium conditions, with olivine-pyroxenite remaining as the refractory residuum. Incompatible elements including Ti, U, and REE (depleted in Eu) derived from the sinking cumulates were strongly partitioned into the liquid, thereby accounting for the high TiO2 contents, rare earth element characteristics, and two-stage isotopic record displayed by high-Ti mare basalts. Equilibrium between high-Ti hybrid liquids and the olivine-pyroxenite residuum was also responsible for controlling and buffering Mg/(Mg + Fe) and Cr distributions, thereby producing the relatively high Mg and Cr contents of primitive high-Ti magmas.By about 3.3 b.y., the primordial mantle immediately beneath the differentiated lithosphere had experienced partial melting due to radiogenic heating. Smaller and less-efficiently differentiated pods of 4.6-4.4-b.y. late-stage assemblages, sinking more slowly, contaminated the partially molten region and produced low-Ti hybrid liquids in equilibrium with residual olivine-pyroxenite. These hybrid magmas, although predominantly primordial in character, acquired incompatible-element characteristics from the sinking cumulates, and thereby obtained the Eu anomaly in their REE patterns, and their two-stage isotope record.The above model, invoking hybridization at depth accompanied by equilibrium between the hybrid liquids and the local olivine-pyroxenite residuum, accounts for the similar Mg/(Mg + Fe) ratios and Cr2O3 contents observed in primitive high-Ti and low-Ti basalts.  相似文献   

16.
The β-phase, spinelloid polymorph of (Mg, Fe)2SiO4 makes up a major part of the transition of the Earth's mantle. Naturally occurring β-(Mg, Fe)2SiO4 wadsleyite, from the Peace River meteorite was found to carry a variety of stacking faults, the nature of which have been studied using high resolution transmission electron microscopy. The faults lie on (010) and are generally of a complex nature, best described in terms of various stacking sequences of the component spinelloid units. The stacking faults locally transform the cation distribution, so that in the plane of the fault the structure is that of spinel. The development of such stacking faults is consequently a significant feature of the martensitic transformation of spinel to β-phase. The possible occurrence of transformation enhanced plasticity associated with this inversion is discussed, and the probable deformation mechanisms of β-(Mg, Fe)2SiO4 are outlined.  相似文献   

17.
Meteoric smoke forms in the mesosphere from the recondensation of the metallic species and silica produced by meteoric ablation. A photochemical flow reactor was used to generate meteoric smoke mimics using appropriate photolytic precursors of Fe and Si atoms in an excess of oxidant. The following systems were studied: (i) Fe+O3/O2, (ii) Fe+O3/O2+H2O, (iii) Fe+Si/SiO+O3/O2 and (iv) Si/SiO+O3/O2. The resulting nano-particles were captured for imaging by transmission electron microscopy, combined with elemental analysis using X-ray (EDX) and electron energy loss (EELS) techniques. These systems generated particle compositions consistent with: (i) Fe2O3 (hematite), (ii) FeOOH (goethite), (iii) Fe2SiO4 (fayalite) and (iv) SiO2 (silica). Electron diffraction revealed that the Fe-containing particles were entirely amorphous, while the SiO2 particles displayed some degree of crystallinity. The Fe-containing particles formed fractal aggregates with chain-like morphologies, whereas the SiO2 particles were predominantly spherical and compact in appearance. The optical extinction spectra of the Fe-containing particles were measured from 300 nm<λ<650 nm. Excellent agreement was found with the extinction calculated from Mie theory using the refractive indices for the bulk compounds, and assuming that the fractal aggregates are composed of poly-disperse distributions of constituent particles with radii ranging from 5 to 100 nm. These sizes were confirmed from measurements of the particle size distributions and microscopic imaging. Finally, the particle growth kinetics of the Fe-containing systems exhibit unexpectedly rapid agglomerative coagulation. This was modelled by assuming an initial period of coalescent particle growth resulting from diffusional (Brownian) coagulation to form primary particles; further growth of these particles is then dominated by long-range magnetic dipole–dipole interactions, leading to the fractal aggregates observed. The atmospheric implications of this work are then discussed.  相似文献   

18.
Some of the factors which control the solubility of ferrous iron and olivine in basaltic liquids have been determined from experiments conducted in equilibrium with metallic iron at temperatures of 1150° to 1306°C and oxygen fugacities of log?O2 = ?12.5to?14.5atm. In order to insure that the samples were in equilibrium with metallic iron and the gas atmosphere during the experiment, the samples were continuously mixed (60 r.p.m.) for at least 90 hours with a metallic iron mixing rod.The solubility of ferrous iron and olivine in basaltic liquid was found to decrease as the alkali and alumina content of the liquid increased. The activity coefficient of ferrous iron in the liquid was calculated from the oxygen fugacity and mole fraction of ferrous oxide in the liquid and varied from 0.79 to 2.00. The ferrous iron/magnesium ratio in the liquid relative to olivine was insensitive to the changing alkali content as measured by the constant distribution coefficient of0.30 ± 0.03 (SD). The calculated silica activity of these liquids varied from 0.11 to 0.50 and the distribution coefficient of ferrous iron-manganese between olivine and liquid was found to be1.4 ± 0.2 (SD). It was found that the major element content of olivine does not exert a strong structural control on the manganese content of olivine.The magnitude of depletion in the liquid of the metal oxides by the crystallization of olivine is in the order Ni > Mg > Co > Fe > Mn, which reflects the relative octahedral site preference of these elements. The composition of the crystallizing olivine reflects the ratio of the above elements in the liquid and is relatively insensitive to temperature, variation of the other elements in the liquid or to the changing magnesium/ferrous iron ratio in the olivine.  相似文献   

19.
A microprobe study of olivine found in allivalite nodules from lavas discharged by Malyi Semyachik Volcano and of olivine phenocrysts from basalts discharged by Klyuchevskoi Volcano revealed the presence of rhonite as a daughter mineral, for the first time ever. Rhonite was found in small (10–50 μm) grains that are xenomorphic in intergrowths with other minerals and have regular crystallographic outlines in contact with glass. We also found high-alumina clinopyroxene, chromium-free spinel, and hornblende. Residual glass is distinguished by its higher concentrations of SiO2, Al2O3, alkalies, and by lower concentrations of FeO, MgO, and CaO. The chemical composition of the rhonite we studied is characterized by limited variations of the major components and fits the formula (Si,Al)6(Ti,Al,Fe+3,Fe+2,Mn,Mg)6(Ca,Na)2O20 well. The data points in the composition of the rhonites we have studied lie in the overall field of much more variable compositions to be found in the field. Unlike the previous findings in alkaline and subalkaline rocks, the rhonite we describe in the present report was found in rocks of the tholeiitic and calc-alkaline series.  相似文献   

20.
A ternary diagram using MnO, TiO2, P2O5 can discriminate between five petrotectonic environments of basaltic rocks (45–54% SiO2). Fields for mid-ocean ridge, island arc tholeiite, island arc calc-alkaline, ocean island tholeiite, and ocean island alkalic rocks were distinguished on the basis of 507 analyses from well-defined environments. Boninites plot within island arc fields. Continental tholeiites, such as the Columbia River basalts, are high in P2O5 relative to MnO and TiO2, and overlap portions of all five oceanic fields.MnO is depleted relative to TiO2 in mid-ocean ridge analyses and may be controlled by early fractionation of olivine and/or clinopyroxene under conditions of lowfO2. In island arc rocks, MnO is enriched relative to TiO2 due to early crystallization of titanomagnetite in a high-fO2 environment. Primitive mid-ocean ridge and arc tholeiites have similar MnO/TiO2/P2O5 ratios which indicate a grossly similar parent magma. Increasingly differentiated basaltic rocks are more easily classified by the diagram. High relative abundances of TiO2 and P2O5 in ocean island rocks are consistent with their derivation from a separate source.Despite the purported high mobility of MnO, the MnO/TiO2/P2O5 discriminant diagram may be applied to unspilitized and moderately spilitized zeolite to greenschist facies greenstones with good agreement between the environment determined by MnO/TiO2/P2O5 and by other means such as trace elements, REE, or field relations.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号