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1.
We determined the forward rate constant (K+) for the Fe2+–Mg order–disorder between the M2 and M1 sites of orthopyroxene (OPx), which is described by the homogeneous reaction Fe2+ (M2) + Mg(M1) ↔ Mg(M2) + Fe2+ (M1), by both ordering and disordering experiments at isothermal condition and also by continuous cooling experiments. The rate constant was determined as a function of temperature in the range of 550–750°C, oxygen fugacity between quartz–fayalite–iron and Ni–NiO buffers, and at compositions of 16 and 50 mol% ferrosilite component. The K+ value derived from disordering experiment was found to be larger than that derived from ordering experiment at 550°C, while at T>580°C, these two values are essentially the same. The fO2 dependence of the rate constant can be described by the relation K+ α (fO2) n with n=5.5–6.5, which is compatible with the theoretically expected relation. The Arrhenius relation at the WI buffer condition is given by
where C o represents the total number of M2 + M1 sites occupied by Fe2+ and Mg per unit volume of the crystal. The above relation can be used to calculate the cooling rates of natural OPx crystals around the closure temperature (T c) of Fe–Mg ordering, which are usually below 300°C for slowly cooled rocks. We determined the Fe–Mg ordering states of several OPx crystals (∼ Fs50) from the Central Gneissic Complex (Khtada Lake), British Columbia, which yields T c ∼290°C. Numerical simulation of the change of Fe2+-Mg ordering in OPx as a function of temperature using the above expression of rate constant and a non-linear cooling model yields quenched values of ordering states that are in agreement with the observed values for cooling rates of 11–17°C/Myr below 300°C. The inferred cooling rate is in agreement with the available geochronological constraints.  相似文献   

2.
Oxyphlogopite is a new mica-group mineral with the idealized formula K(Mg,Ti,Fe)3[(Si,Al)4O10](O,F)2. The holotype material came from a basalt quarry at Mount Rothenberg near Mendig at the Eifel volcanic complex in Rhineland-Palatinate, Germany. The mineral occurs as crystals up to 4 × 4 × 0.2 mm in size encrusting cavity walls in alkali basalt. The associated minerals are nepheline, plagioclase, sanidine, augite, diopside, and magnetite. Its color is dark brown, its streak is brown, and its luster is vitreous. D meas = 3.06(1) g/cm3 (flotation in heavy liquids), and D calc = 3.086 g/cm3. The IR spectrun does not contain bands of OH groups. Oxyphlogopite is biaxial (negative); α = 1.625(3), β = 1.668(1), and γ = 1.669(1); and 2V meas = 16(2)° and 2V calc = 17°. The dispersion is strong; r < ν. The pleochroism is medium; X > Y > Z (brown to dark brown). The chemical composition is as follows (electron microprobe, mean of 5 point analyses, wt %; the ranges are given in parentheses; the H2O was determined using the Alimarin method; the Fe2+/Fe3+ was determined with X-ray emission spectroscopy): Na2O 0.99 (0.89–1.12), K2O 7.52 (7.44–7.58), MgO 14.65 (14.48–14.80), CaO 0.27 ((0.17–0.51), FeO 4.73, Fe2O3 7.25 (the range of the total iron in the form of FeO is 11.09–11.38), Al2O3 14.32 (14.06–14.64), Cr2O3 0.60 (0.45–0.69), SiO2 34.41 (34.03–34.66), TiO2 12.93 (12.69–13.13), F 3.06 (2.59–3.44), H2O 0.14; O=F2 −1.29; 99/58 in total. The empirical formula is (K0.72Na0.14Ca0.02)(Mg1.64Ti0.73Fe0.302+ Fe0.273+Cr0.04)Σ2.98(Si2.59Al1.27Fe0.143+ O10) O1.20F0.73(OH)0.07. The crystal structure was refined on a single crystal. Oxyphlogopite is monoclinic with space group C2/m; the unit-cell parameters are as follows: a = 5.3165(1), b = 9.2000(2), c = 10.0602(2) ?, β = 100.354(2)°. The presence of Ti results in the strong distortion of octahedron M(2). The strongest lines of the X-ray powder diffraction pattern [d, ? (I, %) [hkl]] are as follows: 9.91(32) [001], 4.53(11) 110], 3.300(100) [003], 3.090(12) [112], 1.895(21) [005], 1.659(12) [−135], 1.527(16) [−206, 060]. The type specimens of oxyphlogopite are deposited at the Fersman Mineralogical Museum in Moscow, Russia; the registration numbers are 3884/2 (holotype) and 3884/1 (cotype).  相似文献   

3.
We performed multi-anvil experiments in the system MgO-SiO2 ± H2O at 13.0–13.7 GPa and 1,025–1,300°C and in the system MgO-FeO-SiO2 ± H2O, under reducing conditions, at 11.0–12.7 GPa and 1,200°C, to depict the effect of H2O on the P-T-x coordinates of the 410-km discontinuity, i.e. the olivine–wadsleyite phase boundary. The charges were investigated with Electron Microprobe (EMP), Raman Spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), Secondary Ion Mass Spectrometry (SIMS) and Electron Energy Loss Spectroscopy (EELS). We observe in the MgO-SiO2-H2O system at 1,200°C a 0.6 GPa shift of the phase boundary to lower pressure compared to dry conditions, due to the stronger water fractionation into wadsleyite (wad) rather than in olivine (ol). In the MgO-FeO-SiO2-H2O system, we reproduced the triple point, i.e. observed coexisting hydrous ol, wad and ringwoodite (ring). SIMS H quantifications provided partitioning coefficients for water: D\textwad/ol\textwater D_{\text{wad/ol}}^{\text{water}}  ~ 3.7(5) and D\textring/ol\textwater D_{\text{ring/ol}}^{\text{water}}  ~ 1.5(2) and D\textwad/ring\textwater D_{\text{wad/ring}}^{\text{water}}  ~ 2.5(5). For a bulk composition of x Fe = 0.1, our data indicate only a slight difference in the width of the loop of the two phase field ol–wad under hydrous conditions compared to dry conditions, i.e. no broadening with respect to composition but a shift to lower pressures. For bulk compositions of x Fe > 0.2, i.e. in regions where wad–ring and ol–ring coexist, we observe, however, an unexpected broadening of the loops with a shift to higher iron contents. In total, the stability field of hydrous wad expands in both directions, to lower and higher pressures. Fe3+ concentrations as determined by EELS are very low and are expected to play no role in the broadening of the loops.  相似文献   

4.
We determined experimentally the Nernst distribution coefficient between orthopyroxene and anhydrous silicate melt for trace elements i in the system Na2O–CaO–MgO–Al2O3–SiO2 (NCMAS) along the dry model lherzolite solidus from 1.1 GPa/1,230°C up to 3.2 GPa/1,535°C in a piston cylinder apparatus. Major and trace element composition of melt and orthopyroxene were determined with a combination of electron microprobe and ion probe analyses. We provide partitioning data for trace elements Li, Be, B, K, Sc, Ti, V, Cr, Co, Ni, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Sm, Nd, Yb, Lu, Hf, Ta, Pb, U, and Th. The melts were chosen to be boninitic at 1.1 and 2.0 GPa, picritic at 2.3 GPa and komatiitic at 2.7 and 3.2 GPa. Orthopyroxene is Tschermakitic with 8 mol% Mg-Tschermaks MgAl[AlSiO6] at 1.1 GPa while at higher pressure it has 18–20 mol%. The rare earth elements show a continuous, significant increase in compatibility with decreasing ionic radius from D Laopx−melt ∼ 0.0008 to D Luopx−melt ∼ 0.15. For the high-field-strength elements compatibility increases from D Thopx−melt ∼ 0.001 through D Nbopx−melt ∼ 0.0015, D Uopx−melt ∼ 0.002, D Taopx−melt ∼ 0.005, D Zropx−melt ∼ 0.02 and D Hfopx−melt ∼ 0.04 to D Tiopx−melt ∼ 0.14. From mathematical and graphical fits we determined best-fit values for D 0M1, D 0M2, r 0M1, r 0M2, E 0M1, and E 0M2 for the two different M sites in orthopyroxene according to the lattice strain model and calculated the intracrystalline distribution between M1 and M2. Our data indicate extreme intracrystalline fractionation for most elements in orthopyroxene; for the divalent cations D i M2−M1 varies by three orders of magnitude between D CoM2−M1 = 0.00098–0.00919 and D BaM2−M1 = 2.3–28. Trivalent cations Al and Cr almost exclusively substitute on M1 while the other trivalent cations substitute on M2; D LaM2−M1 reaches extreme values between 6.5 × 107 and 1.4 × 1016. Tetravalent cations Ti, Hf, and Zr almost exclusively substitute on M1 while U and Th exclusively substitute on M2. Our new comprehensive data set can be used for polybaric-polythermal melting models along the Earth’s mantle solidus. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

5.
The bulk (post-eruptive) wt% FeO concentration in each of 11 phenocryst-poor (<5%) andesite and dacite (60–69 wt% SiO2) lavas from different monogenetic vents in the Mexican arc has been measured by titration, in duplicate. The results match, within analytical error, the wt% FeO content of the magmas during phenocryst growth (pre-euptive), which were calculated on the basis of oxygen fugacity and temperature results from Fe–Ti two-oxide oxygen barometry. The average deviation between the pre- and post-eruptive FeO concentrations is ±0.15 wt%. Application of the plagioclase-liquid hygrometer shows that at the time of phenocryst growth, these 11 magmas contained from ~3–8 wt% H2O, which was extensively degassed upon eruption. There is no evidence that degassing of ≤8 wt% H2O changed the oxidation state of these magmas. Calculations of pre-eruptive and post-eruptive oxygen fugacity values relative to the Ni-NiO buffer (in terms of log10 units) for the 11 samples span a similar range; pre-eruptive ∆NNO = −0.9 to +0.7 and post-eruptive ∆NNO = −0.4 to +0.8. The data further show that extensive groundmass (closed-system) crystallization had no affect on bulk Fe3+/Fe2+ ratios. Finally, there is no systematic variation in the range of pre-eruptive Fe3+/FeT values of the samples as a function of SiO2 concentration (i.e., differentiation). Therefore, the results of this study indicate that the elevated Fe3+/FeT ratios of arc andesites and dacites, compared with magmas erupted in other tectonic settings, cannot be attributed to the effects of (1) degassing of H2O, (2) closed-system crystallization, and/or (3) differentiation effects, but instead must be inherited from their parental source rocks (i.e., mantle-derived arc basalts).  相似文献   

6.
Monticellite is a common magmatic mineral in the groundmass of kimberlites. A new oxygen barometer for kimberlite magmas is calibrated based on the Fe content of monticellite, CaMgSiO4, in equilibrium with kimberlite liquids in experiments at 100 kPa from 1,230 to 1,350°C and at logfO2 from NNO-4.1 to NNO+5.3 (where NNO is the nickel–nickel oxide buffer). The XFeMtc/XFeliq was found to decrease with increasing fO2, consistent with only Fe2+ entering the monticellite structure. Although the XFe-in-monticellite varies with temperature and composition, these dependencies are small compared to that with fO2. The experimental data were fitted by weighted least square regression to the following relationship: \Updelta \textNNO = \frac{ log[ 0.858( ±0.021)\fracX\textFe\textLiq X\textFe\textMtc ] - 0.139( ±0.022) }0.193( ±0.004) \Updelta {\text{NNO}} = \frac{{\left\{ {\log \left[ {0.858( \pm 0.021)\frac{{X_{\text{Fe}}^{\text{Liq}} }}{{X_{\text{Fe}}^{\text{Mtc}} }}} \right] - 0.139( \pm 0.022)} \right\}}}{0.193( \pm 0.004)} where ΔNNO is the fO2 relative to that of the Nickel-bunsenite (NNO) buffer and XFeliq/XFeMtc is the ratio of mole fraction of Fe in liquid and Fe-in-monticellite (uncertainties at 2σ). The application of this oxygen barometer to natural kimberlites from both the literature and our own investigations, assuming the bulk rock FeO is that of their liquid FeO, revealed a range in fO2 from NNO-3.5 to NNO+1.7. A range of Mg/(Mg + Fe2+) (Mg#) for kimberlite melts of 0.46–0.88 was derived from the application of the experimentally determined monticellite-liquid Kd Fe2+–Mg to natural monticellites. The range in Mg# is broader and less ultramafic than previous estimates of kimberlites, suggesting an evolution under a wide range of petrologic conditions.  相似文献   

7.
Polarized near-UV spectra have been recorded on 20 μm diameter spots on oriented crystals of microprobe-analyzed olivines from Baikhal Rift, Fa8.8 (I) and Seberget, Fa9.3 (II), which have formed under different fO2 and, therefore, are expected to contain Fe3+-bearing point defects in different concentrations. These should be reflected in the UV-spectra of such minerals (Cemic et al. 1986). The spectra obtained confirm these predictions: The difference in α Y, 26500 in both samples indicates a difference in Fe3+-site fractions of ΔXFe. = 1.78-10−4, which may be related to an fO2 about ten times higher for the formation of olivine II compared to I.  相似文献   

8.
Sekaninaite (XFe > 0.5)-bearing paralava and clinker are the products of ancient combustion metamorphism in the western part of the Kuznetsk coal basin, Siberia. The combustion metamorphic rocks typically occur as clinker beds and breccias consisting of vitrified sandstone–siltstone clinker fragments cemented by paralava, resulting from hanging-wall collapse above burning coal seams and quenching. Sekaninaite–Fe-cordierite (XFe = 95–45) is associated with tridymite, fayalite, magnetite, ± clinoferrosilite and ±mullite in paralava and with tridymite and mullite in clinker. Unmelted grains of detrital quartz occur in both rocks (<3 vol% in paralavas and up to 30 vol% in some clinkers). Compositionally variable siliceous, K-rich peraluminous glass is <30% in paralavas and up to 85% in clinkers. The paralavas resulted from extensive fusion of sandstone–siltstone (clinker), and sideritic/Fe-hydroxide material contained within them, with the proportion of clastic sediments ≫ ferruginous component. Calculated dry liquidus temperatures of the paralavas are 1,120–1,050°C and 920–1,050°C for clinkers, with calculated viscosities at liquidus temperatures of 101.6–7.0 and 107.0–9.8 Pa s, respectively. Dry liquidus temperatures of glass compositions range between 920 and 1,120°C (paralava) and 920–960°C (clinker), and viscosities at these temperatures are 109.7–5.5 and 108.8–9.7 Pa s, respectively. Compared with worldwide occurrences of cordierite–sekaninaite in pyrometamorphic rocks, sekaninaite occurs in rocks with XFe (mol% FeO/(FeO + MgO)) > 0.8; sekaninaite and Fe-cordierite occur in rocks with XFe 0.6–0.8, and cordierite (XFe < 0.5) is restricted to rocks with XFe < 0.6. The crystal-chemical formula of an anhydrous sekaninaite based on the refined structure is | \textK0.02 |(\textFe1.542 + \textMg0.40 \textMn0.06 )\Upsigma 2.00M [(\textAl1.98 \textFe0.022 + \textSi1.00 )\Upsigma 3.00T1 (\textSi3.94 \textAl2.04 \textFe0.022 + )\Upsigma 6.00T2 \textO18 ]. \left| {{\text{K}}_{0.02} } \right|({\text{Fe}}_{1.54}^{2 + } {\text{Mg}}_{0.40} {\text{Mn}}_{0.06} )_{\Upsigma 2.00}^{M} [({\text{Al}}_{1.98} {\text{Fe}}_{0.02}^{2 + } {\text{Si}}_{1.00} )_{\Upsigma 3.00}^{T1} ({\text{Si}}_{3.94} {\text{Al}}_{2.04} {\text{Fe}}_{0.02}^{2 + } )_{\Upsigma 6.00}^{T2} {\text{O}}_{18} ].  相似文献   

9.
Plagioclase-melt partition coefficients (D) for 34 trace elements at natural concentration levels were determined experimentally in a natural MORB composition at atmospheric pressure using thin Pt-wire loops. Experiments were carried out at three temperatures (1,220, 1,200, and 1,180°C), and at three different oxygen fugacities (fO2 = IW, QFM, air) in order to assess the effect of fO2 on the partitioning of elements with multiple valence (Fe, Eu, Cr). Run products were analyzed by laser-ablation ICP-MS. Most trace element Ds increase slightly as temperature decreases, except for D Zr, D Fe, D Eu and D Cr that vary systematically with fO2. Applying the Lattice Strain Model to our data suggests the presence of Fe2+ entirely in the octahedral site at highly to moderate reducing conditions, while Fe3+ was assigned wholly to the tetrahedral site of the plagioclase structure. Furthermore, we provide a new quantitative framework for understanding the partitioning behaviour of Eu, which occurs as both 2+ and 3+ cations, depending on fO2and confirm the greater compatibility of Eu2+, which has an ionic radius similar to Sr, relative to Eu3+ in plagioclase and the higher Eu2+/ Eu3+ under reducing conditions. For petrogenetic basaltic processes, a combined fractionation of Eu2+–Sr and Fe–Mg by plagioclase has considerable potential as an oxybarometer for natural magmatic rocks.  相似文献   

10.
The H2O content of wadsleyite were measured in a wide pressure (13–20 GPa) and temperature range (1,200–1,900°C) using FTIR method. We confirmed significant decrease of the H2O content of wadsleyite with increasing temperature and reported first systematic data for temperature interval of 1,400–1,900°C. Wadsleyite contains 0.37–0.55 wt% H2O at 1,600°C, which may be close to its water storage capacity along average mantle geotherm in the transition zone. Accordingly, water storage capacity of the average mantle in the transition zone may be estimated as 0.2–0.3 wt% H2O. The H2O contents of wadsleyite at 1,800–1,900°C are 0.22–0.39 wt%, indicating that it can store significant amount of water even under the hot mantle environments. Temperature dependence of the H2O content of wadsleyite can be described by exponential equation C\textH2 \textO = 6 3 7.0 7 \texte - 0.00 4 8T , C_{{{\text{H}}_{2} {\text{O}}}} = 6 3 7.0 7 {\text{e}}^{ - 0.00 4 8T} , where T is in °C. This equation is valid for temperature range 1,200–2,100°C with the coefficient of determination R 2 = 0.954. Temperature dependence of H2O partition coefficient between wadsleyite and forsterite (D wd/fo) is complex. According to our data apparent Dwd/fo decreases with increasing temperature from D wd/fo = 4–5 at 1,200°C, reaches a minimum of D wd/fo = 2.0 at 1,400–1,500°C, and then again increases to D wd/fo = 4–6 at 1,700–1,900°C.  相似文献   

11.
H2O-undersaturated melting experiments of synthesized basalt (SiO2 = 50.7 wt.%, MgO = 8.3 wt.%, Mg# = 60) were conducted at fO2 corresponding to NNO+1 and NNO−1 to clarify the effects of pressure (2–7 kbar) and H2O on fractional crystallization in island arcs. H2O content was ranged from nominally anhydrous to 4.4 wt.%. Differentiation trends, namely the liquid lines of descent, change sensitively according to pressure-H2O relations. Tholeiitic differentiation trends are reproduced with H2O ≤ ∼2 wt.% in primary magma. With such quantities of H2O, fractional crystallization is controlled by olivine + plagioclase at 2 kbar. Increasing the pressure from 2 to ≥4 kbar induces early crystallization of orthopyroxene instead of olivine and therefore SiO2 enrichment in the residual melts is suppressed. Increasing H2O (≥ ∼2 wt.% in primary magma) stabilizes clinopyroxene relative to orthopyroxene and/or magnetite. Although the phase relations and proportions strongly depend on fO2 and H2O content, differentiation trends are always calc-alkaline.  相似文献   

12.
Sogdianite, a double-ring silicate of composition ( \textZr0. 7 6 \textTi0. 3 84 + \textFe0. 7 33 + \textAl0.13 )\Upsigma = 2 ( \square 1. 1 5 \textNa0. 8 5 )\Upsigma = 2 \textK[\textLi 3 \textSi 1 2 \textO 30 ] ( {\text{Zr}}_{0. 7 6} {\text{Ti}}_{0. 3 8}^{4 + } {\text{Fe}}_{0. 7 3}^{3 + } {\text{Al}}_{0.13} )_{\Upsigma = 2} \left( {\square_{ 1. 1 5} {\text{Na}}_{0. 8 5} } \right)_{\Upsigma = 2} {\text{K}}[{\text{Li}}_{ 3} {\text{Si}}_{ 1 2} {\text{O}}_{ 30} ] from Dara-i-Pioz, Tadjikistan, was studied by the combined application of 57Fe M?ssbauer spectroscopy and electronic structure calculations. The M?ssbauer spectrum confirms published microprobe and X-ray single-crystal diffraction results that indicate that Fe3+ is located at the octahedral A-site and that no Fe2+ is present. Both the measured and calculated quadrupole splitting, ΔE Q, for Fe3+ are virtually 0 mm s−1. Such a value is unusually small for a silicate and it is the same as the ΔE Q value for Fe3+ in structurally related sugilite. This result is traced back to the nearly regular octahedral coordination geometry corresponding to a very symmetric electric field gradient around Fe3+. A crystal chemical interpretation for the regular octahedral geometry and the resulting low ΔE Q value for Fe3+ in the M?ssbauer spectrum of sogdianite is that structural strain is largely “taken up” by weak Li–O bonds permitting highly distorted LiO4 tetrahedra. Weak Li–O bonding allows the edge-shared more strongly bonded Fe3+O6 octahedra to remain regular in geometry. This may be a typical property for all double-ring silicates with tetrahedrally coordinated Li.  相似文献   

13.
The dependence of water concentration in synthetic (Mg, Fe2+)-cordierite on the composition of the solid solution was examined in experiments that lasted for 10 days at = 200–230 MPa, t = 600–700°C, and oxygen fugacity corresponding to the Fe-FeO buffer. Mass spectrometric data indicate that the dependence of water concentration in cordierite on its Fe mole fraction Fe2+/(Fe2+ + Mg) has maxima at compositions with F = 0.2–0.3. IR diffuse reflectance spectroscopic data and data on the structural setting of H2O molecules in the structural channels of alkali-free (Mg, Fe2+)-cordierite indicate that the H-H vector of some H2O molecules (H2O-II) is perpendicular to [001] of the crystal. The dependence of the magnetic properties of synthetic (Mg, Fe2+)-cordierite was studied by static magnetization technique at 5–300 K in an external magnetic field up to 20 kOe in strength.  相似文献   

14.
The crystal structure of Bi2Al4−x Fe x O9 compounds (x = 0–4) has striking similarities with the crystal structure of mullite. A complete substitution of Al by Fe3+ in both octahedral and tetrahedral sites is a particular structural feature. The infrared (IR) spectra of the Bi2M4O9 compounds (M = Al, Fe3+) are characterised by three band groups with band maxima in the 900–800, 800–600 and 600–400 cm−1 region. Based on the spectroscopic results obtained from mullite-type phases, the present study focuses on the composition-dependent analysis of the 900–800 cm−1 band group, which is assigned to Al(Fe3+)–O stretching vibrations of the corner-sharing MO4 tetrahedra. The Bi2Al4O9 and Bi2Fe4O9 endmembers display single bands with maxima centred at 922 and 812 cm−1, respectively. Intermediate Bi2Al4−x Fe x O9 compounds exhibit a distinct splitting into three relatively sharp bands, which is interpreted in terms of ordering effects within the tetrahedral pairs. Thereby the high-energy component band of the band triplet relates to Al–O–Al conjunctions and the low-energy component band to Fe–O–Fe conjunctions. The intermediate band is assigned to stretching vibrations of Al–O–Fe or Fe–O–Al configurations of the corner-sharing tetrahedral pairs. Bands in the 800–600 cm−1 range are assigned to low-energy stretching vibrations of the MO4 tetrahedra and to M–O–M bending vibrations of the tetrahedral pairs. Absorptions in the 600–400 cm−1 range are essentially determined by M–O stretching modes of the M cations in octahedral coordination.  相似文献   

15.
To get deeper insight into the phase relations in the end-member system Fe2SiO4 and in the system (Fe, Mg)2SiO4 experiments were performed in a multi-anvil apparatus at 7 and 13 GPa and 1,000–1,200°C as a function of oxygen fugacity. The oxygen fugacity was varied using the solid oxygen buffer systems Fe/FeO, quartz–fayalite–magnetite, MtW and Ni/NiO. The run products were characterized by electron microprobe, Raman- and FTIR-spectroscopy, X-ray powder diffraction and transmission electron microscopy. At fO2 corresponding to Ni/NiO Fe-ringwoodite transforms to ferrosilite and spinelloid according to the reaction: 9 Fe2SiO4 + O2 = 6 FeSiO3 + 5 Fe2.40Si0.60O4. Refinement of site occupancies in combination with stoichiometric Fe3+ calculations show that 32% of the total Fe is incorporated as Fe3+ according to From the Rietveld refinement we identified spl as spinelloid III (isostructural with wadsleyite) and/or spinelloid V. As we used water in excess in the experiments the run products were also analyzed for structural water incorporation. Adding Mg to the system increases the stability field of ringwoodite to higher oxygen fugacity and the spinel structure seems to accept higher Fe3+ but also water concentrations that may be linked. At oxygen fugacity corresponding to MtW conditions similar phase relations in respect to the breakdown reaction in the Fe-end-member system were observed but with a strong fractionation of Fe into spl and Mg into coexisting cpx. Thus, through this strong fractionation it is possible to stabilize very Fe-rich wadsleyite with considerable Fe3+ concentrations even at an intermediate Fe–Mg bulk composition: assuming constant K D independent on composition and a bulk composition of x Fe = 0.44 this fractionation would stabilize spl with x Fe = 0.72. Thus, spl could be a potential Fe3+ bearing phase at P–T conditions of the transition zone but because of the oxidizing conditions and the Fe-rich bulk composition needed one would expect it more in subduction zone environments than in the transition zone in senso stricto.
M. Koch-MüllerEmail:
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16.
We have determined mineral-melt partition coefficients (D values) for 20 trace elements in garnet-pyroxenite run products, generated in 3 to 7 GPa, 1,425–1,750°C experiments on a high-Fe mantle melt (97SB68) from the Paraná-Etendeka continental-flood-basalt (CFB) province. D values for both garnet (∼Py63Al25Gr12) and clinopyroxene (∼Ca0.2Mg0.6Fe0.2Si2O6) show a large variation with temperature but are less dependent on pressure. At 3 GPa, D cpx/liq values for pyroxenes in garnet-pyroxenite run products are generally lower than those reported from Ca-rich pyroxenes generated in melting experiments on eclogites and basalts (∼Ca0.3–0.5Mg0.3–0.6Fe0.07–0.2Si2O6) but higher than those for Ca-poor pyroxenes from peridotites (∼Ca0.2Mg0.7Fe0.1Si2O6). D grt/liq values for light and heavy rare-earth elements are ≤0.07 and >0.8, respectively, and are similar to those for peridotitic garnets that have comparable grossular but higher pyrope contents (Py70–88All7–20Gr8–14). 97SB68 D LREEgrt/liq values are higher and D HREEgrt/liq values lower than those for eclogitic garnets which generally have higher grossular contents but lower pyrope contents (Py20–70Al10–50Gr10–55). D values agree with those predicted by lattice strain modelling and suggest that equilibrium was closely approached for all of our experimental runs. Correlations of D values with lattice-strain parameters and major-element contents suggest that the wollastonite component and pyrope:grossular ratio exert major controls on 97SB68 clinopyroxene and garnet partitioning, respectively. These are controlled by the prevailing pressure and temperature conditions for a given bulk-composition. The composition of co-existing melt was found to have a relatively minor effect on 97SB68 D values. The variations in D values displayed by different mantle lithologies are subtle and our study confirms previous investigations which have suggested that the modal proportions of garnet and clinopyroxene are by far the most influential factor in determining incompatible trace-element concentrations in mantle melts. The trace-element partition coefficients we have determined may be used to place high-pressure constraints on garnet-pyroxenite melting models.  相似文献   

17.
The Tongshan copper deposit at Guichi can be considered as a skarn-type copper deposit. Multi-stage carbonatization was well developed in the skarn zone and copper-bearing orebodies. Mineralogy, mineral chemistry and stable isotope data allow it to be divided into five stages: (1) carbonatization of single calcite crystals in the skarn stage; (2) calcite carbonatization in the oxide stage; (3) carbonatization in the early sulfide stage; (4) carbonatization in the late sulfide stage; and (5) carbonatization in the post-sulfide stage. Carbonatization in the early sulfide stage is, among other things, closely related to copper mineralization and is one of the alteration indicators of copper mineralization of this type. C. O, Rb and Sr isotopic studies indicate that the calcites of skarn and oxide stages were formed from hydrothermal solutions predominated by magmatic water, and those of sulfide stage were formed from hydrothermal solutions mainly involving heated meteoric water. The former was formed in the environment wherefO 2 (fO 2 < 10−33 and 10−33< fO2>10−36)(pH = 7–8) is high as compared with the latter (10−35< fO2< 10−38; pH = 5–7).  相似文献   

18.
In order to explore possible quantitative relations between crystal field stabilization energy, CFSE, and partitioning behaviour of the 3d 6-configured Fe2+ ion, a suite of 29 paragenetic rock-forming minerals from 12 high-grade metamorphic rock samples of the Ukrainian shield, including the parageneses garnet/orthopyroxene/clinopyroxene (2x), orthopyroxene/clinopyroxene, garnet/clinopyroxene, garnet/orthopyroxene/biotite, garnet/biotite, garnet/cordierite, garnet/cordierite/biotite, garnet/orthopyroxene/clinopyroxene/Ca-amphibole, Ca-amphibole/biotite (retrograde), was studied by electron microprobe analysis to obtain the respective K D Fe2+ (Ph1/Ph2) values and by polarized single crystal electronic absorption spectroscopy to evaluate the respective CFSEFe2+ values. Other than in the case of Cr3+, a clear quantitative relation between K D (Ph1/Ph2) and the ΔCFSE(Ph1/Ph2) was only observed when geometrical factors, mainly the volume of crystallographic sites and ionic radii of ions competing in the partitioning process, are similar in the respective two paragenetic phases to within 15–20%. In such cases, the ΔCFSEFe2+ contribution to K D (Ph1/Ph2) amounts to 0.1 to 0.2 log K D per 100 cm−1ΔCFSE. The conclusion is that ΔCFSEFe2+ plays only a secondary role after geometrical factors, in the partitioning behaviour of Fe2+. The reason for this is seen in the facts that, compared to the 3d  3-configured Cr3+ ion, CFSE of the 3d 6-configured Fe2+ amounts only to 20–25%, and that the former ion enters only octahedral sites with similar geometrical properties in the paragenetic mineral phases. Received: 17 November 1998 / Accepted: 28 June 1999  相似文献   

19.
 Complete chemical analyses, including ferric and ferrous iron, H2O contents and δD values for 16 phlogopite and biotite and 2 hornblende separates are presented. Samples were obtained from volcanic rocks from four localities: (1) phlogopite phenocrysts from minette lavas from the western Mexico continental arc, (2) biotite and hornblende phenocrysts from andesite lavas from Mono Basin, California, (3) phlogopite and biotite from clinopyroxenite nodules entrained in potassic lavas from the East African Rift, Uganda, and (4) phlogopite phenocrysts from a wyomingite lava in the Leucite Hills, Wyoming. The Fe2O3 contents in the micas range from 0.8 to 10.5 wt%, corresponding to 0.09 to 1.15 Fe3+ per formula unit (pfu). Water contents vary from 1.6 to 3.0 wt%, corresponding to 1.58 to 3.04 OH pfu, significantly less than would be expected for a site fully occupied by hydroxyl. Cation- and anion-based normalization procedures provide accurate mineral formulae with respect to most cations and anions, but are unable to generate accurate estimates of Fe3+/FeT, and overestimate OH at the expense of O on the hydroxyl site. These inaccuracies are present despite acceptable adjusted totals and stoichiometric calculated site occupancies. The phlogopite and biotite phenocrysts in arc-related lavas from western Mexico and eastern California have the highest Fe3+/FeT ratios (56–87%), reflecting high magmatic oxygen fugacities (ΔNNO = +2 to +5), in contrast to those from Uganda (25–40%) and the Leucite Hills (23%). There is no correlation between the OH content and the Fe3+/FeT ratio in the micas. Values of KMg/Fe2+D (± 2σ errors) were calculated for three phlogopite-olivine pairs (0.12 ± 0.12, 0.26 ± 0.14, 0.09 ± 0.12), two biotite-hornblende pairs (0.73 ± 0.08 and 1.22 ± 0.10) and a single phlogopite-augite pair (1.15 ± 0.12). Values of KF/OHD for two biotite and hornblende pairs could not be determined without significant error because of the extremely low F contents (< 0.2 wt%) of the four phases. The δD values obtained in this study encompass a large range (−137 to −43‰). The phlogopite and biotite separates from Uganda have δD values of −70 to −49‰, which overlap those believed to represent “primary” mantle. There is a larger range in δD values (−137 to −43‰) for phlogopite phenocrysts from western Mexico minette lavas, although their range in δ18O values (5.2–6.2‰) is consistent with “normal” mantle. It is unlikely, therefore, that the variable δD values reflect heterogeneity in the mantle source region of the minette magmas. Nor can the extremely low δD values reflect degassing of H2 or H2O since almost 100% loss of dissolved water in the magma is required, an unrealistic scenario given the stability of the hydrous phenocrysts. The very low δD values of the Mascota minette phlogopites require that the hydrogen be introduced from an external source (e.g., meteoric water). Whatever the process responsible for the observed hydrogen isotope composition, it had no effect on the δ18O value, f O 2, a H 2O or bulk composition of the host magmas. Received: 5 January 1995 / Accepted: 19 March 1996  相似文献   

20.
Phase relations of basalts from the Kerguelen large igneous province have been investigated experimentally to understand the effect of temperature, fO2, and fugacity of volatiles (e.g., H2O and CO2) on the differentiation path of LIP basalts. The starting rock samples were a tholeiitic basalt from the Northern Kerguelen Plateau (ODP Leg 183 Site 1140) and mildly alkalic basalt evolved from the Kerguelen Archipelago (Mt. Crozier on the Courbet Peninsula), representing different differentiation stages of basalts related to the Kerguelen mantle plume. The influence of temperature, water and oxygen fugacity on phase stability and composition was investigated at 500 MPa and all experiments were fluid-saturated. Crystallization experiments were performed at temperatures between 900 and 1,160°C under oxidizing (log fO2 ~ ΔQFM + 4) and reducing conditions (log fO2 ~ QFM) in an internally heated gas-pressure vessel equipped with a rapid quench device and a Pt-Membrane for monitoring the fH2. In all experiments, a significant influence of the fO2 on the composition and stability of the Mg/Fe-bearing mineral phases could be observed. Under reducing conditions, the residual melts follow a tholeiitic differentiation trend. In contrast, melts have high Mg# [Mg2+/(Mg2+ + Fe2+)] and follow a calk-alkalic differentiation trend at oxidizing conditions. The comparison of the natural phenocryst assemblages with the experimental products allows us to constrain the differentiation and pre-eruptive conditions of these magmas. The pre-eruptive temperature of the alkalic basalt was about 950–1,050°C. The water content of the melt was below 2.5 wt% H2O and strongly oxidizing conditions (log fO2 ~ ΔQFM + 2) were prevailing in the magma chamber prior to eruption. The temperature of the tholeiitic melt was above 1,060°C, with a water content below 2 wt% H2O and a log fO2 ~ ΔQFM + 1. Early fractionation of clinopyroxene is a crucial step resulting in the generation of silica-poor and alkali-rich residual melts (e.g., alkali basalt). The enrichment of alkalis in residual melts is enhanced at high fO2 and low aH2O.  相似文献   

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