The effect of Ca-Tschermaks component on trace element partitioning between clinopyroxene and silicate melt   总被引：19，自引：0，他引：19  

Eddy Hill  Bernard J. Wood  Jonathan D. Blundy 《Lithos》2000,53(3-4):203-215

We have studied the influence of Ca-Tschermaks (Calcium Tschermaks or CaTs) content of clinopyroxene on the partitioning of trace elements between this phase and silicate melt at fixed temperature and pressure. Ion probe analyses of experiments carried out in the system Na₂O–CaO–MgO–Al₂O₃–SiO₂, at 0.1 MPa and 1218°C, produced crystal-melt partition coefficients (D) of 36 trace elements (Li, Cl, Sc, Ti, V, Cr, Fe, Co, Ge, Sr, Y, Zr, Nb, Mo, Ru, Rh, In, Sn, Sb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta and W), for clinopyroxene compositions between 10 and 32 mol% CaTs. Partition coefficients for 2+ to 5+ cations show, for each charge, a near parabolic dependence of log D on ionic radius of the substituting cation, for partitioning into both the M1 and M2 sites of clinopyroxene. Fitting the results to the elastic strain model of Blundy and Wood [Blundy, J.D., Wood, B.J., 1994. Prediction of crystal-melt partition coefficients from elastic moduli. Nature 372, 452–454] we obtain results for the strain-free partition coefficients of theoretical cations (D₀), with site radius r₀, and for the site's Young's Modulus (E).

In agreement with earlier data our results show that increasing ^ivAl concentration in cpx is matched by increasing D, E_M1, E_M2 and D₀ for tri-, tetra- and pentavalent cations. The degree of fractionation between chemically similar elements (i.e. Ta/Nb, Zr/Hf) also increases. In contrast, D values for mono-, di- and hexavalent cations decrease with increasing ^ivAl in the cpx. The large suite of trace elements used has allowed us to study the effects of cation charge on D₀, r₀ and E. We have found that D₀ and r₀ decrease with increasing cation charge, e.g. r₀=0.66 Å for 4+ cations and 0.59 Å for 5+ cations substituting into M1. Values of E_M1 and E_M2 increase with cation charge as well as with increasing ^ivAl content. The increase in E_M2 is linear and close to the trend set by Hazen and Finger [Hazen, R.M., Finger, L.W., 1979. Bulk modulus-volume relationship for cation–anion polyhedra. J. Geophys. Res. 84 (10) 6723–6728] for oxides. E_M1 values are much higher and do not fit the trend predicted by the Hazen and Finger relationship.  相似文献   

SIMS determination of trace element partition coefficients between garnet, clinopyroxene and hydrous basaltic liquids at 2–7.5 GPa and 1080–1200°C     

T. H. Green  J. D. Blundy  J. Adam  G. M. Yaxley 《Lithos》2000,53(3-4):165-187

Trace element partition coefficients (D's) for up to 13 REE, Nb, Ta, Zr, Hf, Sr and Y have been determined by SIMS analysis of seven garnets, four clinopyroxenes, one orthopyroxene and one phlogopite crystallized from an undoped basanite and a lightly doped (200 ppm Nb, Ta and Hf) quartz tholeiite. Experiments were conducted at 2–7.5 GPa, achieving near-liquidus crystallization at relatively low temperatures of 1080–1200°C under strongly hydrous conditions (5–27 wt.% added water). Garnet and pyroxene D_REE show a parabolic pattern when plotted against ionic radius, and conform closely to the lattice strain model of Blundy and Wood (Blundy, J.D., Wood, B.J., 1994. Prediction of crystal–melt partition coefficients from elastic moduli. Nature 372, 452–454). Comparison, at constant pressure, between hydrous and anhydrous values of the strain-free partition coefficient (D₀) for the large cation sites of garnet and clinopyroxene reveals the relative importance of temperature and melt water content on partitioning. In the case of garnet, the effect of lower temperature, which serves to increase D₀, and higher water content, which serves to decrease D₀, counteract each other to the extent that water has little effect on garnet–melt D₀ values. In contrast, the effect of water on clinopyroxene–melt D₀ overwhelms the effect of temperature, such that D₀ is significantly lower under hydrous conditions. For both minerals, however, the lower temperature of the hydrous experiments tends to tighten the partitioning parabolas, increasing fractionation of light from heavy REE compared to anhydrous experiments.

Three sets of near-liquidus clinopyroxene–garnet two-mineral D values increase the range of published experimental determinations, but show significant differences from natural two-mineral D's determined for subsolidus mineral pairs. Similar behaviour is observed for the first experimental data for orthopyroxene–clinopyroxene two-mineral D's when compared with natural data. These differences are in large part of a consequence of the subsolidus equilibration temperatures and compositions of natural mineral pairs. Great care should therefore be taken when using natural mineral–mineral partition coefficients to interpret magmatic processes.

The new data for strongly hydrous compositions suggest that fractionation of Zr–Hf–Sm by garnet decreases with increasing depth. Thus, melts leaving a garnet-dominated residuum at depths of about 200 km or greater may preserve source Zr/Hf and Hf/Sm. This contrasts with melting at shallower depths where both garnet and clinopyroxene will cause Zr–Hf–Sm fractionation. Also, at shallower depths, clinopyroxene-dominated fractionation may produce a positive Sr spike in melts from spinel lherzolite, but for garnet lherzolite melting, no Sr spike will result. Conversely, clinopyroxene megacrysts with negative Sr spikes may crystallize from magmas without anomalous Sr contents when plotted on mantle compatibility diagrams. Because the characteristics of strongly hydrous silicate melt and solute-rich aqueous fluid converge at high pressure, the hydrous data presented here are particularly pertinent to modelling processes in subduction zones, where aqueous fluids may have an important metasomatic role.  相似文献   

Partitioning coefficients between olivine and silicate melts   总被引：3，自引：0，他引：3  

J.H. Bdard 《Lithos》2005,83(3-4):394-419

Variation of Nernst partition coefficients (D) between olivine and silicate melts cannot be neglected when modeling partial melting and fractional crystallization. Published natural and experimental ^{olivine/liquid}D data were examined for covariation with pressure, temperature, olivine forsterite content, and melt SiO₂, H₂O, MgO and MgO/MgO + FeO^total. Values of ^{olivine/liquid}D generally increase with decreasing temperature and melt MgO content, and with increasing melt SiO₂ content, but generally show poor correlations with other variables. Multi-element ^{olivine/liquid}D profiles calculated from regressions of D REE–Sc–Y vs. melt MgO content are compared to results of the Lattice Strain Model to link melt MgO and: D₀ (the strain compensated partition coefficient), E_M³⁺ (Young's Modulus), and r₀ (the size of the M site). Ln D₀ varies linearly with Ln MgO in the melt; E_M³⁺ varies linearly with melt MgO, with a dog-leg at ca. 1.5% MgO; and r₀ remains constant at 0.807 Å. These equations are then used to calculate ^{olivine/liquid}D for these elements using the Lattice Strain Model. These empirical parameterizations of ^{olivine/liquid}D variations yield results comparable to experimental or natural partitioning data, and can easily be integrated into existing trace element modeling algorithms. The ^{olivine/liquid}D data suggest that basaltic melts in equilibrium with pure olivine may acquire small negative Ta–Hf–Zr–Ti anomalies, but that negative Nb anomalies are unlikely to develop. Misfits between results of the Lattice Strain Model and most light rare earth and large ion lithophile partitioning data suggest that kinetic effects may limit the lower value of D for extremely incompatible elements in natural situations characterized by high cooling/crystallization rates.  相似文献   

Status report on stability of K-rich phases at mantle conditions   总被引：1，自引：0，他引：1  

George E. Harlow  Rondi Davies 《Lithos》2004,77(1-4):647-653

Experimental research on K-rich phases and observations from diamond inclusions, UHP metamorphic rocks, and xenoliths provide insights about the hosts for potassium at mantle conditions. K-rich clinopyroxene (Kcpx–KM³⁺Si₂O₆) can be an important component in clinopyroxenes at P>4 GPa, dependent upon coexisting K-bearing phases (solid or liquid) but not, apparently, upon temperature. Maximum Kcpx content can reach 25 mol%, with 17 mol% the highest reported in nature. Partitioning ^(K)D_(cpx/liquid) above 7 GPa=0.1–0.2 require ultrapotassic liquids to form highly potassic cpx or critical solid reactions, e.g., between Kspar and Di. Phlogopite can be stable to about 8 GPa at 1250 °C where either amphibole or liquid forms. When fluorine is present, it generally increases in Phl upon increasing P (and probably T) to about 6 GPa, but reactions forming amphibole and/or KMgF₃ limit F content between 6 and 8 GPa. The perovskite KMgF₃ is stable up to 10 GPa and 1400 °C as subsolidus breakdown products of phlogopite upon increasing P. ^(M4)K-substituted potassic richterite (ideally K(KCa)Mg₅Si₈O₂₂(OH,F)₂) is produced in K-rich peridotites above 6 GPa and in Di+Phl from 6 to 13 GPa. K content of amphibole is positively correlated with P; Al and F content decrease with P. In the system 1Kspar+1H₂O K-cymrite (hydrous hexasanidine–KAlSi₃O₈·nH₂O–Kcym) is stable from 2.5 GPa at 400 to 1200 °C and 9 GPa; Kcym can be a supersolidus phase. Formation of Kcym is sensitive to water content, not forming within experiments with H₂O2O>Kspar. Phase X, a potassium di-magnesium acid disilicate ((K_1−x−n)₂(Mg_1−nM_n³⁺)₂Si₂O₇H_2x), forms in mafic compositions at T=1150–1400 °C and P=9–17 GPa and is a potential host for K and H₂O at mantle conditions with a low-T geotherm or in subducting slabs. The composition of phase-X is not fixed but actually represents a solid solution in the stoichiometries □₂Mg₂Si₂O₇H₂–(K□)Mg₂Si₂O₇H–K₂Mg₂Si₂O₇ (□=vacancy), apparently stable only near the central composition. K-hollandite, KAlSi₃O₈, is possibly the most important K-rich phase at very high pressure, as it appears to be stable to conditions near the core–mantle boundary, 95 GPa and 2300 °C. Other K-rich phases are considered.  相似文献   

Strontium diffusion in sanidine and albite, and general comments on strontium diffusion in alkali feldspars     

D.J. Cherniak 《Geochimica et cosmochimica acta》1996,60(24):5037-5043

Strontium chemical diffusion has been measured in albite and sanidine under dry, 1 atm, and QFM buffered conditions. Strontium oxide-aluminosilicate powdered sources were used to introduce the diffusant and Rutherford Backscattering Spectroscopy (RBS) used to measure diffusion profiles. For the 1 atm experiments, the following Arrhenius relations were obtained: The alkali feldspars in this and earlier work display a broad range of activation energies for Sr diffusion, which may be a consequence of the thermodynamic non-ideality of the alkali feldspar system and/or the mixed alkali effect.  相似文献   

Aluminous granulites from the Pulur complex, NE Turkey: a case of partial melting, efficient melt extraction and crystallisation     

Gültekin Topuz  Rainer Altherr  Angelika Kalt  Muharrem Sat&#x;r  Olaf Werner  Winfried H. Schwarz 《Lithos》2004,72(3-4):183-207

In the Pulur complex, NE Turkey, a heterogeneous rock sequence ranging from quartz-rich mesocratic gneisses to silica- and alkali-deficient, Fe-, Mg- and Al-rich melanocratic rocks is characterized by granulite-facies assemblages involving garnet, cordierite, sillimanite, ilmenite, ±spinel, ±plagioclase, ±quartz, ±biotite, ±corundum, rutile and monazite. Textural evidence for partial melting in the aluminous granulites, particularly leucosomes, is largely absent or strongly obliterated by a late-stage hydrothermal overprint. However, inclusion relations, high peak P–T conditions, the refractory modes, bulk and biotite compositions of the melanocratic rocks strongly support a model of partial melting. The melt was almost completely removed from the melanocratic rocks and crystallised within the adjacent mesocratic gneisses which are silica-rich, bear evidence of former feldspar and show a large range in major element concentrations as well as a negative correlation of most elements with SiO₂. Peak conditions are estimated to be ≥800 °C and 0.7–0.8 GPa. Subsequent near-isothermal decompression to 0.4–0.5 GPa at 800–730 °C is suggested by the formation of cordierite coronas and cordierite–spinel symplectites around garnet and in the matrix. Sm–Nd, Rb–Sr and ⁴⁰Ar/³⁹Ar isotope data indicate peak conditions at 330 Ma and cooling below 300 °C at 310 Ma.  相似文献   

Experimental synthesis and phase relations of phengitic muscovite from 6.5 to 11 GPa in a calcareous metapelite from the Dabie Mountains, China     

Kenneth J. Domanik  John R. Holloway 《Lithos》2000,52(1-4):51-77

The stability and phase relations of phengitic muscovite in a metapelitic bulk composition containing a mixed H₂O+CO₂ fluid were investigated at 6.5–11 GPa, 750–1050°C in synthesis experiments performed in a multianvil apparatus. Starting material consisted of a natural calcareous metapelite from the coesite zone of the Dabie Mountains, China, ultrahigh-pressure metamorphic complex that had experienced peak metamorphic pressures greater than 3 GPa. The sample contains a total of 2.1 wt.% H₂O and 6.3 wt.% CO₂ bound in hydrous and carbonate minerals. No additional fluid was added to the starting material. Phengite is stable in this bulk composition from 6.5 to 9 GPa at 900°C and coexists with an eclogitic phase assemblage consisting of garnet, omphacite, coesite, rutile, and fluid. Phengite dehydrates to produce K-hollandite between 8 and 11 GPa, 750–900°C. Phengite melting/dissolution occurs between 900°C and 975°C at 6.5–8 GPa and is associated with the appearance of kyanite in the phase assemblage. The formation of K-hollandite is accompanied by the appearance of magnesite and topaz-OH in the phase assemblage as well as by significant increases in the grossular content of garnet (average X_grs=0.52, X_py=0.19) and the jadeite content of omphacite (X_jd=0.92). Mass balance indicates that the volatile content of the fluid phase changes markedly at the phengite/K-hollandite phase boundary. At P≤8 GPa, fluid coexisting with phengite appears to be relatively CO₂-rich (XCO₂/XH₂O=2.2), whereas fluid coexisting with K-hollandite and magnesite at 11 GPa is rich in H₂O (XCO₂/XH₂O=0.2). Analysis of quench material and mass balance calculations indicate that fluids at all pressures and temperatures examined contain an abundance of dissolved solutes (approximately 40 mol% at 8 GPa, 60 mol% at 11 GPa) that act to dilute the volatile content of the fluid phase. The average phengite content of muscovite is positively correlated with pressure and ranges from 3.62 Si per formula unit (pfu) at 6.5 GPa to 3.80 Si pfu at 9 GPa. The extent of the phengite substitution in muscovite in this bulk composition appears to be limited to a maximum of 3.80–3.85 Si pfu at P=9 GPa. These experiments show that phengite should be stable in metasediments in mature subduction zones to depths of up to 300 km even under conditions in which aH₂O1. Other high-pressure hydrous phases such as lawsonite, MgMgAl-pumpellyite, and topaz-OH that may form in subducted sediments do not occur within the phengite stability field in this system, and may require more H₂O-rich fluid compositions in order to form. The wide range of conditions under which phengite occurs and its participation in mixed volatile reactions that may buffer the composition of the fluid phase suggest that phengite may significantly influence the nature of metasomatic fluids released from deeply subducted sediments at depths of up to 300 km at convergent plate boundaries.  相似文献   

Prograde metamorphism and decompression of the Gföhl gneiss, Czech Republic     

J. V. Owen  J. Dostal 《Lithos》1996,38(3-4):259-270

Quartzofeldspathic rocks of the Gföhl gneiss from the Moldanubian of the Czech Republic span amphibolite-to granulite-facies, and are associated with eclogite. Protomylonitic fabrics related to terminal tectonic emplacement and reworking of the gneiss are common. Some non-mylonitic rocks, however, preserve early, prograde features (e.g., Opx-rimmed Hbl in metabasites), whereas others have characteristics generally associated with near-isothermal decompression (e.g., Pl-Opx moats separating Grt and Qtz in metabasites; Crd ± Spl coronas on Grt and aluminosilicates in metapelites); the unequivocal distinction between prograde and decompressional features in these rocks, however, may not be possible or even justified. For example, some metapelites contain growth-zoned (i.e., rimward increase in X_Mg) garnets that also record evidence (i.e., rimward decrease in X_Ca, compensated by the presence of reversely-zoned plagioclase in the same rock) of decompression. In rare instances, eclogitic rocks (P > 11 kbar) interpreted as tectonic enclaves within the gneiss also record mineralogic evidence of decompression (e.g., Crd-Opx-Spr coronas on pyrope). In metapelites, plagioclase-cored coronal garnets with high Prp/Grs ratios (˜ 2.5) record near-isobaric cooling from near the thermal maximum at a relatively shallow but undetermined crustal level.

Unlike Gföhl gneisses elsewhere (e.g., in Austria), the rocks described here do not preserve evidence of extreme metamorphic conditions. Texturally stable Grt-Bt pairs in non-mylonitic samples give T_max < 750 °C. P_max is not known, but prograde metamorphism apparently progressed from the kyanite to sillimanite fields, implying P ˜ 8 kbar at the maximum Grt-Bt temperature. At these conditions, dehydration of mafic gneiss occurred in the presence of a CO₂-rich (X_CO2 ˜ 0.85) pore fluid  相似文献   

Reactions and textures in grossular–wollastonite–scapolite calc–silicate granulites from Maligawila, Sri Lanka: evidence for high-temperature isobaric cooling in the meta-sediments of the Highland Complex     

V. Mathavan  G. W. A. R. Fernando   《Lithos》2001,59(4):217-232

Grossular–wollastonite–scapolite calc–silicate granulites from Maligawila in the Buttala klippe, which form part of the overthrusted rocks of the Highland Complex of Sri Lanka, preserve a number of spectacular coronas and replacement textures that could be effectively used to infer their P–T–fluid history. These textures include coronas of garnet, garnet–quartz, and garnet–quartz–calcite at the grain boundaries of wollastonite, scapolite, and calcite as well as calcite–plagioclase and calcite–quartz symplectites or finer grains after scapolite and wollastonite respectively. Other textures include a double rind of coronal scapolite and coronal garnet between matrix garnet and calcite. The reactions that produced these coronas and replacement textures, except those involving clinopyroxene, are modelled in the CaO–Al₂O₃–SiO₂–CO₂ system using the reduced activities. Calculated examples of T–X_CO2 and P–X_CO2 projections indicate that the peak metamorphic temperature of about 900–875 °C at a pressure of 9 kbar and the peak metamorphic fluid composition is constrained to be low in X_CO2 (0.1<X_CO2<0.30). Interpretation of the textural features on the basis of the partial grids revealed that the calc–silicate granulites underwent high-temperature isobaric cooling, from about 900–875 °C to a temperature below 675 °C, following the peak metamorphism. The late-stage cooling was accompanied by an influx of hydrous fluids. The calc–silicate granulites provide evidence for high-temperature isobaric cooling in the meta-sediments of the Highland Complex, earlier considered by some workers to be confined exclusively to the meta-igneous rocks. The coronal scapolite may have formed under open-system metasomatism.  相似文献   

Micro-XANES determination of ferric iron and its application in thermobarometry   总被引：15，自引：0，他引：15  

R. Schmid  M. Wilke  R. Oberhnsli  K. Janssens  G. Falkenberg  L. Franz  A. Gaab 《Lithos》2003,70(3-4):381-392

Micro-X-ray absorption near-edge structure (XANES) analysis was employed to determine the content of ferric iron in minerals formed in ultrahigh-pressure (UHP) eclogites. It is observed that omphacite and phengite contain significant amounts of Fe³⁺/Fe_tot (0.2–0.6), whereas only very low contents are present in garnet (Fe³⁺/Fe_tot=0.0–0.03), the latter being consistent with results from stoichiometric charge-balance calculations. Furthermore, considerable variations in the Fe³⁺/Fe_tot ratios of omphacite and phengite are observed depending on the textural sites and local bulk chemistry (eclogite and calc-silicate matrix) within one thin section. The oxidation state of isofacial minerals is thus likely to depend on the local fluid composition, which, in the studied case, is controlled by calcareous and meta-basic mineral compositions. These first in-situ measurements of ferric iron in an eclogite sample from the Dabie Shan, E China, are used to recalculate geothermobarometric data. Calculations demonstrate that the temperature during UHP metamorphism was as high as 780 °C, about 80–100 °C higher than previously estimated. Temperatures based on charge balance calculations often give erroneous results. Pressure estimates are in good agreement with former results and confirm metamorphism in the stability field of diamond (43.7 kbar at 750 °C). These P–T data result in a geothermal gradient of ca. 6 °C/km during UHP metamorphism in the Dabie Shan. However, accounting for ferric iron contents in geothermobarometry creates new difficulties inasmuch as calibrations of geothermometers may not be correctable for Fe³⁺ and the actual effect on Mg–Fe²⁺ partitioning is unknown. The present study further shows that micro-XANES is a promising technique for the in situ determination of ferric iron contents without destroying the textural context of the sample: a clear advantage compared to bulk methods.  相似文献   

Coesite inclusions and prograde compositional zonation of garnet in whiteschist of the HP-UHPM Kokchetav massif, Kazakhstan: a record of progressive UHP metamorphism   总被引：3，自引：0，他引：3  

Chris D. Parkinson   《Lithos》2000,52(1-4):215-233

Coarse-grained whiteschist, containing the assemblage: garnet+kyanite+phengite+talc+quartz/coesite, is an abundant constituent of the ultrahigh-pressure metamorphic (UHPM) belt in the Kulet region of the Kokchetav massif of Kazakhstan.

Garnet displays prograde compositional zonation, with decreasing spessartine and increasing pyrope components, from core to rim. Cores were recrystallized at T=380°C (inner) to 580°C (outer) at P<10 kbar (garnet–ilmenite geothermometry, margarite+quartz stability), and mantles at T=720–760°C and P_H20=34–36 kbar (coesite+graphite stability, phengite geobarometer, KFMASH system reaction equilibria). Textural evidence indicates that rims grew during decompression and cooling, within the Qtz-stability field.

Silica inclusions (quartz and/or coesite) of various textural types within garnets display a systematic zonal distribution. Cores contain abundant inclusions of euhedral quartz (type 1 inclusions). Inner mantle regions contain inclusions of polycrystalline quartz pseudomorphs after coesite (type 2), with minute dusty micro-inclusions of chlorite, and more rarely, talc and kyanite in their cores; intense radial and concentric fractures are well developed in the garnet. Intermediate mantle regions contain bimineralic inclusions with coesite cores and palisade quartz rims (type 3), which are also surrounded by radial fractures. Subhedral inclusions of pure coesite without quartz overgrowths or radial fractures (type 4) occur in the outer part of the mantle. Garnet rims are silica-inclusion-free.

Type 1 inclusions in garnet cores represent the low-P, low-T precursor stage to UHPM recrystallization, and attest to the persistence of low-P assemblages in the coesite-stability field. Coesites in inclusion types 2, 3, and 4 are interpreted to have sequentially crystallized by net transfer reaction (kyanite+talc=garnet+coesite+H₂O), and were sequestered within the garnet with progressively decreasing amounts of intragranular aqueous fluid.

During the retrograde evolution of the rock, all three inclusion types diverged from the host garnet P–T path at the coesite–quartz equilibrium, and followed a trajectory parallel to the equilibrium boundary resulting in inclusion overpressure. Coesite in type 2 inclusions suffered rapid intragranular H₂O-catalysed transformation to quartz, and ruptured the host garnet at about 600°C (when inclusion P27 kbar, garnet host P9 kbar). Instantaneous decompression to the host garnet P–T path, passed through the kyanite+talc=chlorite+quartz reaction equilibrium, resulting in the dusty micro-assemblage in inclusion cores. Type 3 inclusions suffered a lower volumetric proportion transformation to quartz at the coesite–quartz equilibrium, and finally underwent rupture and decompression when T<400°C, facilitating coesite preservation. Type 4 coesite inclusions are interpreted to have suffered minimal transformation to quartz and proceeded to surface temperature conditions along or near the coesite–quartz equilibrium boundary.  相似文献   

Basalt magma genesis and fractionation in collision- and extension-related provinces: A comparison between eastern, central and western Anatolia   总被引：1，自引：0，他引：1  

Hakan oban 《Earth》2007,80(3-4):219-238

Experimental studies of synthetic and natural basalt systems suggest that conditions of magma genesis and fractionation depend fundamentally on mantle temperatures and lithospheric stress fields. In general, compressional settings are more conducive to polybaric fractionation than extensional settings and in this regard, the Anatolian magmatic province offers a natural laboratory for comparing near-coeval basalt eruptions as a function of regional tectonics — compressional (collision-related) régimes dominating in eastern Anatolia and extensional tectonics characterizing a western province related to Aegean Sea opening. Projection of Plio-Quaternary basalt normative compositions from the Western Anatolia Extensional Province (WAEP), the Central Anatolian ‘Ova’ Province (CAOP), and Eastern Anatolia Compressional Province (EACP) are projected onto Ol–Ne–Cpx and Pl–Cpx–Ol planes in the simplified basalt system (Ne–Cpx–Ol–Qz), each showing distinctive liquid lines of descent. WAEP basalts are mostly constrained by low-pressure (< 0.5 GPa) cotectics while CAOP and EACP compositions conform to moderate and/or high-pressure (0.8–3.0 GPa) cotectics. Overall, a quasi-linear shift from moderate and/or high-pressure to low-pressure equilibria matches the westward transgression from compressional east Anatolia to the extensional west Anatolian–Aegean region. Comparison of their respective primary (mantle-equilibrated) magmas–simulated by normalizing their compositions to MgO = 15 wt.% (Mg-15)–with parameterized anhydrous and H₂O-undersaturated experimental melts suggests they segregated from spinel- to garnet-lherzolite mantle facies at pressures between c. 2 and 3 GPa (c. 70–100 km depth) under H₂O-undersaturated conditions. Interpolated potential temperatures (T_p) and lithospheric stretching factors (β) range as follows: (1) eastern Anatolian basalts associated with the Arabian foreland show T_p varying between 1250 and 1400 °C (except for the Karacalidag alkali basalts, south of the Bitlis–Zagros fracture zone, for which T_p ranges up to 1450 °C), for β values of 1.2–1.8. T_p values for central Anatolia (e.g. Sivas) range between 1300 and 1375 °C (except for Karapinar, Egrikuyu and Hasandag, which show < 1150 °C), and β values of 1.3–1.4. For western Anatolian basalts, T_p range mostly between 1250 and 1330 °C, except for a single value for Canakkale of 1400 °C and Kula sample showing T_p < 1200 °C, and β values of 1.3–2.0. Variation of these conditions is as great or greater than that between provinces, although there are clearly significant constraints on the inferred polybaric to low-pressure isobaric fractionation régimes. Covariation of total FeO, TiO₂, La/Yb, Ce/Sm, Zr/Y and Zr/Nb reflects small but significant differences in bulk composition and ambient melt fraction while the covariance of Ce/Sm and Sm/Yb is consistent with the segregation of primitive melts at the spinel- to garnet-lherzolite transition.  相似文献   

Ultra high temperature-metamorphism and its significance in the Central Indian Tectonic Zone     

Santanu Kumar Bhowmik   《Lithos》2006,92(3-4):484-505

In the present study from the southern margin of the Central Indian Tectonic Zone, it is demonstrated how the metamorphic P–T path of ultrahigh-temperature granulite terranes can be reconstructed using the metamorphic transition in corundum granulites from early biotite melting to later FMAS solid–solid reaction. The extreme metamorphism in these rocks caused two-stage biotite melting, resulting in initial porphyroblastic garnet₁ and later sapphirine–spinel₁ incongruent solid mineral assemblages. During this process, the leucocratic and melanocratic layers in the corundum granulites evolved from an initial silica-oversaturated to a later silica-undersaturated domain. In the melanocratic layer, this allowed localized concentration of sapphirine-spinel₁ and residual sillimanite₁, producing an extremely restitic assemblage, at the culmination of peak metamorphism, BM₁. BM₁ is constrained at  1000 °C at relatively deep crustal levels (P  9 kbar) from the stability of ferroaugite in a co-metamorphosed Iron Formation granulite. During subsequent metamorphism (BM₂), the reaction path and history in the corundum granulites shifted to the restitic domain allowing reacting sapphirine, spinel₁ and sillimanite to produce coronal garnet₂–corundum assemblage via a FMAS univariant reaction. In the final stages of reaction history, biotite₂–sillimanite₂–spinel₂ assemblage was produced after garnet₂–corundum due to localized melt–crystal interaction. The metamorphic sequence, when interpreted with the help of a newly constructed, qualitative KFMASH petrogenetic grid, reveals successive stages of heating, increasing pressure and cooling around the KFMASH invariant point, [Opx,Crd], which is consistent with a counterclockwise metamorphic P–T path. The near isobaric nature of post-peak cooling (ΔT  250–300 °C) is also evident from multistage pyroxene exsolution and by the appearance of lamellar and coronal garnets in the Iron Formation granulites. This study provides the first tight constraint for ultrahigh-T metamorphism along a counter clockwise P–T trajectory in the Central Indian Tectonic zone, and has important bearing for terrane correlations in this part of East Gondwanaland. In addition, the new KFMASH grid allows evaluation of metamorphic phase relations in ultrahigh-T, corundum-bearing and corundum-absent aluminous granulites.  相似文献   

Distribution of Fe and Mg between coexisting garnet and hornblende in synthetic and natural systems: an empirical calibration of the garnet–hornblende Fe–Mg geothermometer     

Erling Krogh Ravna 《Lithos》2000,53(3-4):265-277

Multiple regression analysis of a compilation of the Fe²⁺–Mg distribution between garnet and hornblende from experimental runs on basaltic to intermediate compositions (n=22) and coexisting garnet–clinopyroxene–hornblende from natural (intermediate to basaltic) rocks (n=43) has been performed to define ln K_{D(Fe²⁺/Mg)}^Grt–Hbl as a function of temperature and garnet composition. The regression of data covering a large span in pressure (5–16 kbar), temperature (515–1025°C) and composition yields the ln K_{D(Fe²⁺/Mg)}^Grt–Hbl–P–T compositional relationship (r²=0.93):where

Application of this expression to natural garnet–hornblende pairs in intermediate to basaltic and semipelitic rock types from various settings gives temperatures that are consistent with other methods.  相似文献   

Marcasite precipitation from hydrothermal solutions   总被引：3，自引：0，他引：3  

James B. Murowchick  H. L. Barnes 《Geochimica et cosmochimica acta》1986,50(12):2615-2629

Pyrite and marcasite were precipitated by both slow addition of aqueous Fe²⁺ and SiO₃²⁻ to an H₂S solution and by mixing aqueous Fe²⁺ and Na₂S₄ solutions at 75°C. H₂S₂ or HS₂⁻ and H₂S₄ or HS₄⁻ were formed in the S₂O₃²⁻ and Na₂S₄ experiments, respectively. Marcasite formed at pH < pK₁ of the polysulfide species present (for H₂S₂, pK₁ = 5.0; for H₂S₄, pK₁ = 3.8 at 25°C). Marcasite forms when the neutral sulfane is the dominant polysulfide, whereas pyrite forms when mono-or divalent polysulfides are dominant. In natural solutions where H₂S₂ and HS₂ are likely to be the dominant polysulfides, marcasite will form only below pH 5 at all temperatures.

The pH-dependent precipitation of pyrite and marcasite may be caused by electrostatic interactions between polysulfide species and pyrite or marcasite growth surfaces: the protonated ends of H₂S₂ and HS₂ are repelled from pyrite growth sites but not from marcasite growth sites. The negative ions HS₂ and S₂²⁻ are strongly attracted to the positive pyrite growth sites. Masking of 1π_g^* electrons in the S₂ group by the protons makes HS₂ and H₂S₂ isoelectronic with AsS²⁻ and As₂²⁻, respectively ( et al., 1981). Thus, the loellingitederivative structure (marcasite) results when both ends of the polysulfide are protonated.

Marcasite occurs abundantly only for conditions below pH 5 and where H₂S₂ was formed near the site of deposition by either partial oxidation of aqueous H₂S by O₂ or by the reaction of higher oxidation state sulfur species that are reactive with H₂S at the conditions of formation e.g., S₂O₃²⁻ but not SO₄²⁻. The temperature of formation of natural marcasite may be as high as 240°C ( and , 1985), but preservation on a multimillion-year scale seems to require post-depositional temperatures of below about 160°C ( , 1973; and , 1985).  相似文献   

Solubility of silica polymorphs in electrolyte solutions, I. Activity coefficient of aqueous silica from 25° to 250°C, Pitzer''s parameterisation     

M. Azaroual  C. Fouillac  J. M. Matray 《Chemical Geology》1997,140(3-4):155-165

Within the framework of Pitzer's specific interaction model, interaction parameters for aqueous silica in concentrated electrolyte solutions have been derived from Marshall and co-authors amorphous silica solubility measurements. The values, at 25°C, of the Pitzer interaction parameter (λ_SiO2(aq)−i) determined in this study are the following: 0.092 (i = Na⁺), 0.032 (K⁺), 0.165 (Li⁺), 0.292 (Ca²⁺, Mg²⁺), −0.139 (SO₄²⁻), and −0.009 (NO₃⁻). A set of polynomial equations has been derived which can be used to calculate λ_SiO2(aq)−i for these ions at any temperature up to 250°C. A linear relationship between the aqueous silica-ion interaction parameters (λ_SiO2(aq)−i) and the surface electrostatic field (Z_i/r_e,i) of ions was obtained. This empirical equation can be used to estimate, in first approximation, λ_SiO2(aq)−i if no measurements are available. From this parameterisation, the calculated activity coefficient of aqueous silica is 2.52 at 25°C and 1.45 at 250°C in 5 m NaCl solution. At lower concentrations, e.g. 2 m NaCl, the activity coefficient of silica is 1.45 at 25°C and 1.2 at 250°C. Hence, in practice, it is necessary to take into account the activity coefficient of aqueous silica (λ_SiO2(aq)≠1) in hydrothermal solutions and basinal brines where the ionic strength exceeds 1. A comparison of measured [Marshall, W.L., Chen, C.-T.A., 1982. Amorphous silica solubilities, V. Prediction of solubility behaviour in aqueous mixed electrolyte solutions to 300°C. Geochim. Cosmochim. Acta 46, 289–291.] and computed amorphous silica solubility, using this parameterisation, shows a good agreement. Because the effect of individual ions on silicate and silica polymorph solubilities are additive, the present study has permitted to derive Pitzer interaction parameters that allow a precise computation of γ_SiO2(aq) in the Na---K---Ca---Mg---Cl---SO₄---HCO₃---SiO₂---H₂O system, over a large range of salt concentrations and up to temperatures of 250°C.  相似文献   

An empirical sapphirine-spinel Mg---Fe exchange thermometer and its application to high grade xenoliths in the Popes Harbour dyke, Nova Scotia, Canada     

J. Victor Owen  John D. Greenough 《Lithos》1991,26(3-4):317-332

Spinel-sapphirine-corundum-rutile parageneses in metapelitic xenoliths from the lamprophyric Popes Harbour dyke are enclosed by feldspathic (±rare quartz) haloes that embay aluminosilicates and biotite. These feldspathic haloes contain plagioclase (An_20–40) and/or an alkali or ternary (hypersolvus) feldspar, and show a variety of igneous and devitrification textures, suggesting an anatectic origin. The spinel-bearing parageneses are interpreted as the refractory residue formed by the incongruent melting of biotite, aluminosilicates and associated phases.

Equilibration temperatures of these assemblages are estimated from an empirical sapphirine-spinel Mg---Fe exchange thermometer derived from literature data on both silica-saturated and undersaturated sapphirine granulites. Linear regression (R=0.81) of the calibrant data yields the expression T(°C) = [800 + (228^*InKd)] − 273 where

Precision is estimated at ± 100°C, but will likely be less for highly oxidized sapphirine granulites owing to (1) errors in the stoichiometric estimation of X_Fe³⁺ from microprobe data and (2) the formation of magnetite at the expense of spinel or sapphirine, leading to an increase in X_Mg in either or both phases during cooling. Application of this expression to the reduced (graphite-bearing) Popes Harbour xenoliths yields T of 725–795°C. Anatexis is attributed to thermal metamorphism by the lamprophyric magma prior to and/or during entrainment of xenolith material in the dyke. Higher-T assemblages were quenched before the xenoliths attained thermal equilibrium with the magma, consequently prograde reaction textures and compositional zoning patterns are preserved.  相似文献   

The Homestead kimberlite, central Montana, USA: mineralogy, xenocrysts, and upper-mantle xenoliths     

B. Carter Hearn Jr.   《Lithos》2004,77(1-4):473-491

The Homestead kimberlite was emplaced in lower Cretaceous marine shale and siltstone in the Grassrange area of central Montana. The Grassrange area includes aillikite, alnoite, carbonatite, kimberlite, and monchiquite and is situated within the Archean Wyoming craton. The kimberlite contains 25–30 modal% olivine as xenocrysts and phenocrysts in a matrix of phlogopite, monticellite, diopside, serpentine, chlorite, hydrous Ca–Al–Na silicates, perovskite, and spinel. The rock is kimberlite based on mineralogy, the presence of atoll-textured groundmass spinels, and kimberlitic core-rim zoning of groundmass spinels and groundmass phlogopites.

Garnet xenocrysts are mainly Cr-pyropes, of which 2–12% are G10 compositions, crustal almandines are rare and eclogitic garnets are absent. Spinel xenocrysts have MgO and Cr₂O₃ contents ranging into the diamond inclusion field. Mg-ilmenite xenocrysts contain 7–11 wt.% MgO and 0.8–1.9 wt.% Cr₂O₃, with (Fe⁺³/Fe^tot) from 0.17–0.31. Olivine is the only obvious megacryst mineral present. One microdiamond was recovered from caustic fusion of a 45-kg sample.

Upper-mantle xenoliths up to 70 cm size are abundant and are some of the largest known garnet peridotite xenoliths in North America. The xenolith suite is dominated by dunites, and harzburgites containing garnet and/or spinel. Granulites are rare and eclogites are absent. Among 153 xenoliths, 7% are lherzolites, 61% are harzburgites, 31% are dunites, and 1% are orthopyroxenites. Three of 30 peridotite xenoliths that were analysed are low-Ca garnet–spinel harzburgites containing G10 garnets. Xenolith textures are mainly coarse granular, and only 5% are porphyroclastic.

Xenolith modal mineralogy and mineral compositions indicate ancient major-element depletion as observed in other Wyoming craton xenolith assemblages, followed by younger enrichment events evidenced by tectonized or undeformed veins of orthopyroxenite, clinopyroxenite, websterite, and the presence of phlogopite-bearing veins and disseminated phlogopite. Phlogopite-bearing veins may represent kimberlite-related addition and/or earlier K-metasomatism.

Xenolith thermobarometry using published two-pyroxene and Al-in-opx methods suggest that garnet–spinel peridotites are derived from 1180 to 1390 °C and 3.6 to 4.7 GPa, close to the diamond–graphite boundary and above a 38 mW/m² shield geotherm. Low-Ca garnet–spinel harzburgites with G10 garnets fall in about the same T and P range. Most spinel peridotites with assumed 2.0 GPa pressure are in the same T range, possibly indicating heating of the shallow mantle. Four of 79 Cr diopside xenocrysts have P–T estimates in the diamond stability field using published single-pyroxene P–T calculation methods.  相似文献   

Geochemical evolution of peraluminous plutons in southern Nova Scotia, Canada—a pegmatite-poor suite     

K. L. Currie  J. B. Whalen  W. J. Davis  F. J. Longstaffe  B. L. Cousens   《Lithos》1998,44(3-4):117-140

Three southern Nova Scotia plutons crystallized rapidly at 375 Ma from magma containing both mantle and crustal components. Isotopic and chemical data suggest that the crustal contribution included both lower crustal material and Cambro-Ordovician turbidites of the host Meguma Group. Despite local evidence of mixing and mingling of magmas, the bulk of the plutons evolved by assimilation and fractional crystallisation. Evolved portions of the plutons have compositions appropriate for development of rare-metal pegmatite fields, but pegmatites are relatively rare and little differentiated. Like parental plutons, pegmatites fall into biotite+plagioclase and muscovite+potassium feldspar assemblages. The latter locally contain Mn-rich garnet+biotite, giving calculated P–T conditions of pegmatite crystallisation of 620°C, 0.44 GPa under water-saturated conditions. Host rocks at the time of emplacement experienced P–T conditions varying from <500° to 620°C at 0.44 GPa. Beryl-bearing pegmatites occur only where host rocks were below sillimanite-grade, and pegmatites of any kind are scarce in sillimanite-grade host rocks. Rarity of pegmatites can be ascribed to a combination of insufficient amounts of F, Li, and B in the magma with rapid cooling which prevented extensive fractionation and undercooling. Our data suggest rather that peraluminous suites, produced by rapid heating due to incursions of lithospheric mantle into supracrustal rocks, and typified by biotite-rich, cordierite-bearing plutons, are not favorable locales for major rare-metal-enriched pegmatite fields. Such fields appear to require relatively prolonged anatexis resulting from crustal thickening, or an areally extensive mafic underplate,  相似文献   

Prograde garnet-bearing ultramafic rocks from the Tromsø Nappe, northern Scandinavian Caledonides     

Erling J.K. Ravna  Kre Kullerud  Edel Ellingsen 《Lithos》2006,92(3-4):336-356

Garnet-bearing peridotitic rocks closely associated with eclogite within the Tromsø Nappe of the northern Scandinavian Caledonides show good evidence for prograde metamorphism. Early stages are recognized as inclusions of hornblende and chlorite in the cores of large garnet poikiloblasts. Closer to the garnet rim, clinopyroxene and Cr-poor spinel appear as additional inclusion phases. Four suites of spinel inclusions can be distinguished based on optical properties and chemical composition. The innermost suite (suite 1) has the lowest Cr# and highest Mg#. Further rimward, the spinel inclusions gradually change in composition, with increasing Cr# and decreasing Mg#. Spinel is rare in the matrix, but locally chromitic spinel occurs as larger grains. Garnet poikiloblasts are rimmed by a kelyphite zone consisting of Hbl + Cr-poor Spl or Opx ± Cpx + Cr-poor Spl, and locally an inner zone of Na-rich Hbl + Chl. Matrix assemblage in the garnet-bearing peridotitic rocks is Hbl + Chl + Cpx + Ol ± Cr-rich spinel, defining a strong foliation wrapping around garnets and associated kelyphites. Thin layers of garnet-orthopyroxenite and garnet–hornblende–zoisite–chlorite rocks are presumably coeval with the matrix foliation of the peridotitic rocks.

In dunitic to harzburgitic compositions large undulatory grains of Ol + Opx ± Chl + Spl apparently define the maximum-P conditions. This assemblage is succeeded by a recrystallized assemblage of Ol ± Tlc ± Mgs, which in turn is overgrown by strain-free poikiloblasts of orthopyroxene, indicating a temperature increase. This is postdated by Tlc + Ath ± Mgs, and finally serpentine.

P–T estimates for the inclusion suites of clinopyroxene and spinel in garnet clearly indicate garnet growth and spinel consumption in a regime of increasing P. The inner suite (suite 1) apparently was in equilibrium with garnet, clinopyroxene and olivine at 1.40 GPa, 675 °C, whereas included spinel with maximum Cr# (suite 4) indicate 2.40 GPa at 740 °C. Grt + Opx from garnet-orthopyroxenite give 1.5–1.9 GPa at 740–770 °C, and Grt + Hbl + Zo + Chl from a zoisite-rich rock give 1.75 ± 0.25 GPa at 740 ± 30 °C, interpreted to represent recrystallization during uplift. In dunitic to harzburgitic compositions, early Ol + Opx ± Chl + Spl is succeeded by Ol ± Tlc ± Mgs, which in turn is overgrown by neoblasts of strain-free orthopyroxene, indicating temperature increase. This is postdated by Tlc + Ath ± Mgs, and finally serpentine.

The ultramafic rocks in the Tromsø Nappe were locally strongly hydrated before subduction along with associated eclogites and metasedimentary rocks during the early (Ordovician) stages of the Caledonian orogeny.  相似文献