Low temperature alteration processes affecting ultramafic bodies     

H.Wayne Nesbitt  O.P Bricker 《Geochimica et cosmochimica acta》1978,42(4):403-409

At low temperatures, in the presence of an aqueous solution, olivine and orthopyroxene are not stable relative to the hydrous phases brucite, serpentine and talc. Alteration of dunite and peridotite to serpentine or steatite bodies must therefore proceed via non-equilibrium processes. The compositions of natural solutions emanating from dunites and peridotites demonstrate that the dissolution of forsterite and/or enstatite is rapid compared with the precipitation of the hydrous phases; consequently, dissolution of anhydrous minerals controls the chemistry of such solutions. In the presence of an aqueous phase, precipitation of hydrous minerals is the rate-controlling step.Brucite-bearing and -deficient serpentinites alter at low temperature by non-equilibrium processes, as evidenced by the composition of natural solutions from these bodies. The solutions approach equilibrium with the least stable hydrous phase and, as a consequence, are supersaturated with other hydrous phases. Dissolution of the least stable phase is rapid compared to precipitation of other phases, so that the dissolving mineral controls the solution chemistry.Non-equilibrium alteration of anhydrous ultramafic bodies continues until at least one anhydrous phase equilibrates with brucite, chrysotile or talc. The lowest temperature (at a given pressure) at which this happens is defined by the reaction: (Johannes, 1968, Contrib. Mineral. Petrol. 19, 309–315) so that non-equilibrium alteration may occur well into greenschist facies metamorphic conditions.  相似文献   

Thermodynamic analyses of equilibria involving olivine,orthopyroxene and garnet     

Toshisuke Kawasaki  Yoshito Matsui 《Geochimica et cosmochimica acta》1983,47(10):1661-1679

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Geochemical modelling of basalts from DSDP Leg 65, East Pacific Rise,Gulf of California     

A.M. Kudo  S.E. Barker  K. Keil  Marvin H. Beeson 《Geochimica et cosmochimica acta》1982,46(12):2427-2434

Major and rare earth element (REE) data for basalts from Holes 483, 483B, and 485A of DSDP Leg 65, East Pacific Rise, mouth of the Gulf of California, support a simple fractional crystallization model for the genesis of rocks from this suite. The petrography and mineral chemistry (presented in detail elsewhere) provide no evidence for magma mixing, but rather a simple multistage cooling process. Based on its lowest TiO₂ content (0.88%), $\text{FeO}{}^1\text{MgO}$ ratio (0.95 with total Fe as FeO), and Mg# (100 $\text{Mg}\text{Mg + Fe}\text{″ = 70}$), sample 483-17-2-(78–83) has been selected as the most primitive primary magma of the samples analyzed. This is supported by the REE data which show this sample has the lowest total REE content, a $\text{La}\text{Sm}{}_{\mathrm{cn}}$ (chondrite-normalized) = 0.36, and $\text{Eu}\text{Sm}{}_{\mathrm{cn}}$ = 1.05. Because other samples analyzed have higher SiO₂, lower Mg#, and a negative Eu anomaly ($\text{Eu}\text{Sm}{}_{\mathrm{cn}}$ as low as 0.89), they are most likely derivative magmas. Wright-Doherty and trace element modelling support fractional crystallization of 14.1% plagioclase (An₈₈), 6.7% olivine (Fo₈₆), and 4.7% clinopyroxene (Wo₄₁En₄₉Fs₁₀) from 483-17-2-(78–83) to form the least differentiated sample with Mg# = 63. The $\text{La}\text{Sm}{}_{\mathrm{cn}}$ of this derivative magma is almost identical to the parent magma (0.35 to 0.36), but the other samples have higher $\text{La}\text{Sm}{}_{\mathrm{cn}}$ (0.45 to 0.51), more total REE, and lower Mg# (60 to 56). Both Wright-Doherty and trace element modelling indicate that the primary magma chosen cannot produce these more evolved samples. For the major elements, the TiO₂ and P₂O₅ are too low in the calculated versus the observed (1.38 to 1.90; 0.11 to 0.17, respectively, for example). Rayleigh fractionation calculates a lower $\text{La}\text{Sm}{}_{\mathrm{cn}}$ and requires about 60% crystal removal versus 40% for the Wright-Doherty. These more evolved samples must be derived from a parent magma different from the one selected here and, unfortunately, not sampled in this study. A magma formed by a smaller degree of partial melting with slightly more residual clinopyroxene left in the mantle than for sample 483-17-2-(78–83) is required.  相似文献   

Mineral-solution equilibria—I. An experimental study of complexing and thermodynamic properties of aqueous MgCl₂ in the system MgO-SiO₂-H₂O-HCl     

John D. Frantz  Robert K. Popp 《Geochimica et cosmochimica acta》1979,43(8):1223-1239

Speciation of aqueous magnesium in the system MgO-SiO₂-H₂O-HCl in supercritical aqueous fluids has been investigated using standard rapid-quench hydrothermal techniques and a modification of the Ag + AgCl buffer method (Frantz and Eugster, 1973. Am. J. Sci.267, 268–286). A concentric double-capsule charge was utilized. The outer gold capsule contained the assemblage talc + quartz + Ag + AgCl + H₂O-MgCl₂ fluid; the inner platinum capsule, Ag + AgCl + H₂O-HCl fluid. During the experiments, and thus $\text{?}{}_{\mathrm{HCl}}$ equilibrated between the two capsules. After quenching, measurement of the chloride concentration in the fluid in the inner capsule and total magnesium in the fluid in the outer capsule defines the concentrations of HCl and Mg that coexist with talc + quartz in the outer capsule. Changes in the measured molality of HCl as a function of the total magnesium concentration at constant P and T were used to identify the predominant species of magnesium in the hydrothermal fluid. Experimental results showed that at 2000 bar, MgCl°₂ is the predominant species above 550°C and Mg²⁺, below 400°C. Data at intermediate temperatures when combined with the dissociation constant for HCl were used to obtain the dissociation constant for MgCl°₂. The results of these experiments were combined with results from experiments using Ag + AgCl in conjunction with the oxygen buffer, hematite-magnetite, to obtain the equilibrium constant for the reaction $\text{1}\text{3}\text{Talc + 2HC1° H}{}_2\text{O MgCl°}{}_2\text{+}\text{4}\text{3}\text{Quartz +}\text{4}\text{3}\text{H}{}_2\text{O}$ from which the difference in Gibbs free energy of MgCl°₂ and HC1° was obtained as a function of temperature at 1000, 1500 and 2000 bar pressure, Solubility constants for brucite. forsterite, chrysotile, and talc were calculated.  相似文献   

Granulite-facies metabasite ejecta in the Laacher See area,Eifel, West Germany     

Martin Okrusch  Bernd Schröder  Achim Schnutgen 《Lithos》1979,12(4):251-270

Near the village of Engeln, Laacher See area, garnet-bearing pyriclasite and pyribolite ejecta were recognized as constituents of alkaline basaltic tuffs; they are interpreted as fragments of the lowermost crust. During the first main stage of granulite facies metamorphism, assemblages with garnet (Alm₄₇Pyr₃₄Spess₂Gross + Andr₁₇), clinopyroxene (Wo₃₇En₃₅Fs₁₅Ts_8.5Jd_4.5), orthopyroxene I (En₃₄Fs₃₈Ts₄Jd₂), and plagioclase I (An₄₀-An₆₀) were formed in a temperature range of 730–850°C and rock pressures somewhere between 6.5 and 12 kb, $\text{P}{}_{\mathrm{tot}}\text{>}\text{P}{}_{\mathrm{<mtext>H</mtext>}}{}_2\text{O}\text{> 0}$. The rare sulfate-rich meionite, and at least a part of the ubiquitous brown hornblende were presumably also formed during this stage. A retrograde metamorphic event under slightly lower pressures and temperatures led to the breakdown of the assemblage garnet + clinopyroxene thereby forming coronas of plagioclase II $\text{(}\text{An}{}_{75}\text{) +}\text{orthopyroxene}\text{II +}\text{Ti-magnetite}\text{±}\text{brown hornblende}$.  相似文献   

Pyroxene geothermometry and geobarometry: experimental and thermodynamic evaluation of some subsolidus phase relations involving pyroxenes in the system CaO-MgO-Al₂O₃-SiO₂     

Claude T. Herzberg 《Geochimica et cosmochimica acta》1978,42(7):945-957

One petrogenetic grid for plagioclase-, spinel- and garnet-lherzolite analogues in the system CaO-MgO-Al₂O₃-SiO₂ is presented from 1 bar to 30 kbar and 400 to 1500°C. Another grid for olivine-gabbro, spinel-gabbro and garnet-pyroxenite analogues in the same system is presented from 1 bar to 25 kbar and 500 to 1500°C. Both grids show the distribution of the mineral assemblages and the variations in the composition of clinopyroxene with temperature and pressure. They were developed by applying simple thermodynamic mixing models of clinopyroxene to experimentally determined clino-pyroxene compositions.Calcium tschermak's pyroxene (CaAl₂SiO₆) in complex CaMgSi₂O₆-CaAl₂SiO₆-Mg₂Si₂O₆ clinopyroxenes is best represented by a local charge balance mixing model where Enthalpy and entropy changes of subsolidus reactions involving variations in the CaAl₂SiO₆ and Mg₂Si₂O₆ content of clinopyroxene are interdependent due to nonideal mixing of these two end-members. CaAl₂SiO₆ can strongly reduce the mutual solubility of clinopyroxene and orthopyroxene at moderate pressures and high temperatures. Failure to take this into account can result in temperature underestimates (up to 150°C) of spinel-lherzolites, garnet-pyroxenites, low pressure garnet-lherzolites, spinel-gabbros, and high pressure plagioclase-lherzolites and olivine-gabbros. However, at temperatures and pressures where the Al₂O₃ content of clinopyroxene is low (e.g. garnet-lherzolite nodules in kimberlite), the mutual solubility is adequantely represented by experimental results in the system CaO-MgO-SiO₂.  相似文献   

Mineral chemistry and crystallization sequences in kimberlite and lamproite dikes from the Sisimiut area,central West Greenland     

《Lithos》1987,20(5):391-417

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Complex formation in the ternary system Mg(II)-CO₂-H₂O     

Walter Riesen  Heinz Gamsjäger  Paul W. Schindler 《Geochimica et cosmochimica acta》1977,41(9):1193-1200

The carbonato and hydrogencarbonato complexes of Mg²⁺ were investigated at 25 and 50° in solutions of the constant ClO₄^? molality (3 M) consisting preponderantly of NaClO₄. The experimental data could be explained assuming the following equilibria: Mg²⁺ + CO_2B + H₂O ag MgHCO⁺₃ + H⁺, $\text{log}{}^1\text{β}{}_1\text{= ?7.64}{}_4\text{± 0.01}{}_7\text{(25°), ?7.46}{}_2\text{± 0.01}{}_1\text{(50°)}$, Mg²⁺ + 2 CO_2g + 2 H₂Oag Mg(HCO₃)⁰₂ ± 2 H⁺, $\text{log}{}^1\text{β}{}_2\text{= ?15.00 ± 0.14 (25°)}$, ?15.37 ± 0.39 (50°), Mg²⁺ + CO_2g + H₂Oag MgCO⁰₃ + 2 H⁺, $\text{log}{}^1\text{k}{}_1\text{= ?15.64 ± 0.06 (25°)}$,?15.23 ± 0.02 (50°), with the assumption γ_MgCO3⁰ = γ_Mg(HCO3)⁰2, ΔG⁰(I = 0) for the reaction MgCO⁰₃ + CO_2g + H₂O = Mg(HCO₃)⁰₂ was estimated to be ?3.9₁ ± 0.8₆ and 0.6 ± 2.4 kJ/mol at 25 and 50°C, respectively. The abundance of carbonate linked Mg(II) species in fresh water systems is discussed.  相似文献   

Chemical characteristics of island-arc basalts: Implications for mantle sources   总被引：3，自引：0，他引：3  

M.R. Perfit  D.A. Gust  A.E. Bence  R.J. Arculus  S.R. Taylor 《Chemical Geology》1980,30(3):227-256

Major-element, trace-element and isotopic compositions of approximately 1200 basalts (< 53 wt. % SiO₂) from intra-oceanic island arcs have been compiled to assess the nature and possible sources of primitive island-arc basalts (IAB). The chemical characteristics of IAB are examined with reference to those of mid-ocean ridge basalts (MORB) and intraplate oceanic basalts (IPB). Major-element compositions of primitive [$\text{Mg}\text{(}\text{Mg +Fe}{}^{\mathrm{2+}}\text{)}\text{> 65}$] IAB and MORB are similar, but differ significantly from IPB. In general, IAB do not have higher Al₂O₃, lower TiO₂ or a lack of Fe enrichment compared to primitive MORB but many do have greater K₂O contents. Differences in major- and minor-element contents between more evolved IAB and MORB result from the dominance of plagioclase + olivine crystal fractionation in MORB magmas vs. clinopyroxene + olivine controlled fractionation in IAB suites. This difference in crystallization history may be related to the higher P_H2O or greater depth of crystallization of IAB magmas compared to those inferred for MORB.IAB are characteristically enriched in large-ion-lithophile (LIL) elements and depleted in high-field-strength ions (e.g., Zr, Nb and Hf) relative to normal MORB (N-type) and IPB. The enrichment of some LIL elements (e.g., Sr, Rb, Ba and Pb) relative to the rare-earth elements in IAB is difficult to explain by simple partial melting alone and suggests a multistage petrogenesis involving an LIL-enriched component. Low abundances of high-field-strength ions in evolved IAB are explicable in terms of fractional crystallization, but the cause for consistently low abundances in primitive IAB remains problematic.Island-arc lavas contain greater concentrations of volatiles and have higher $\text{CO}{}_2\text{H}{}_2\text{O}$ and Cl/F ratios than either MORB or IPB, suggesting involvement of a slab-derived volatile component. However, this is not consistent with ${}^3\text{He}{}^4\text{He}$ data which indicate that only near-trench volcanics have been significantly affected by dehydration of the oceanic crust.Sr-, Nd-, Pb- and O-isotopic data, in conjunction with the trace-element data, clearly indicate that IAB are derived from heterogeneous, LIL-depleted mantle sources most similar to those which give rise to enriched MORB (E-type). The marked shift towards higher ${}^{87}\text{Sr}{}^{86}\text{Sr}$ in IAB compared to oceanic lavas with similar ${}^{143}\text{Nd}{}^{144}\text{Nd}$ values cannot be explained simply by the addition of radiogenic Sr from the slab. Variable degrees of contamination from a crustally-derived sedimentary component is consistent with the isotopic and trace-element data from a number of arcs. However, the lack of correlation between LIL/REE ratios and more radiogenic isotopic ratios suggests that this enrichment/contamination process is complex. A multi-stage petrogenetic model involving subducted oceanic crust (± sediments), dehydration/volatile transfer, and partial melting of metasomatized mantle beneath island arcs is considered the most reasonable, although least constrained, method to generate a variety of primitive IAB.  相似文献   

Olivine/silicate melt partitioning of germanium: an example of a nearly constant partition coefficient     

C.J Capobianco  E.B Watson 《Geochimica et cosmochimica acta》1982,46(2):235-240

The partitioning of germanium between forsterite (Fo) and liquids in the diopside-anorthiteforsterite join was investigated by electron microprobe analysis of Ge-doped samples equilibrated at 1300°–1450°C. Germanium is somewhat incompatible in Fo relative to the haplobasaltic melts, with a grand mean for all simple partition coefficients (D_Fo-l^Ge) of 0.68 ± 0.06. For the melt composition range studied, D_Fo-l^Ge is virtually constant in isothermal series of experiments, and shows only minor overall temperature dependence. The exchange reaction partition coefficient $\text{K}{}_{\mathrm{D}}\text{= (}\text{Mg}{}_2\text{GeO}{}_4\text{)}{}_{\mathrm{Fo}}\text{(SiO}{}_2\text{)}{}_{\mathrm{l}}\text{(Mg}{}_2\text{SiO}{}_4\text{)}{}_{\mathrm{Fo}}\text{(GeO}{}_2\text{)}{}_{\mathrm{l}}$] is near unity in all cases, with a grand mean of 0.93 ± 0.11. One exploratory run at 20 kbar yielded a distinctly lower partition coefficient (D_Fo-l^Ge = 0.54 ± 0.04), which confirms the negative pressure dependence predicted by the thermodynamics of Ge ai Si exchange.These new data indicate that absolute Ge enrichment must occur in terrestrial magmas undergoing olivine fractionation, while $\text{Ge}\text{Si}$ remains nearly constant.  相似文献   

Intrinsic oxygen fugacity measurements: techniques and results for spinels from upper mantle peridotites and megacryst assemblages     

Richard J. Arculus  John W. Delano 《Geochimica et cosmochimica acta》1981,45(6):899-913

A new technique for the determination of intrinsic oxygen fugacities () of single and polyphase geological samples with solid ZrO₂, oxygen-specific electrolytes is described. Essentially the procedure involves isolating the emf signal from the sample from that unavoidably imposed by the residual atmosphere inside the sample-bearing sensor. By varying the of the residual atmosphere, it is possible to determine a ‘plateau’ value of constant recorded from the sensor which represents a reversed intrinsic measurement for the sample alone, and where the extent of the plateau reflects the innate buffering capability of the sample. A measure of the precision and accuracy of the data obtained is the fact that identical values are obtained whether on a heating or cooling cycle of the sample + compatible atmosphere system.These techniques have been applied to measurements of the intrinsic of spinels from peridotites and megacryst assemblages from Australia, West Germany and the U.S.A. Oxidation states range from ～- 0.25 log₁₀ units more oxidized to 1 log₁₀ unit more reduced than the iron-wüstite (IW) buffer. The overall reduced nature of the spinels and the range of 's obtained are striking features of the data. One implication of the results is that the majority of mantle-derived magmas are initially highly reduced, and the relatively oxidized values observed at surface (～- 4–5 log₁₀ orders more oxidized than IW) reflect late-stage alteration, perhaps by H₂ loss (Sato, 1978).  相似文献   

Titan-aegirine from early Tertiary ash layers in northern Denmark     

J.G Rønsbo  A.K Pedersen  J Engell 《Lithos》1977,10(3):193-204

Microprobe analyses on a xenocrystic suite of salites, aegirine-augites, aegirines, titan-aegirines and acmites from a lower Tertiary ash layer in northern Denmark are presented. The sodic pyroxenes show an unusual titan-enrichment and up to 42 mol.% of the component $\text{NaTi}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}{}^{\mathrm{4+}}\text{M}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}{}^{\mathrm{2+}}\text{Si}{}_3\text{O}{}_6\text{(}\text{M = Fe}{}^{\mathrm{2+}}\text{,}\text{Mn or Mg}\text{)}$, is estimated. Optical absorption measurements show no evidence for Ti³⁺. The titan-aegirines were formed during late to post-magmatic crystallization in a system with a high Ti⁴⁺/Fe²⁺ ratio and were followed by acmite showing enrichment in jadeite. Comparison with experimentally investigated titan-aegirine indicates crystallization far below the Mn₂O₃Mn₃O₄f₀₂ buffer.  相似文献   

Medium pressure crystallization of a monchiquitic magma — evidence from megacrysts of Drever''s block, Ubekendt Ejland, West Greenland     

Jrgen Gutzon Larsen 《Lithos》1981,14(4):241-262

Megacrysts and polymineralic fragments of extraordinary diversity from a Tertiary monchiquitic dyke of Ubekendt Ejland comprise three groups: (1) Cr-diopside-fassaitic diopside + olivine, Fo_90.5?81.5 + CrAl spinels. (II) Fassaitic salite-ferrisalite + KTi-pargasite-ferropargasite + apatite + AlTi-magnetite, (III) Scapolite + hyalophane + potassium feldspar + nepheline + analcime. By comparison with mineralogy and phase relations in the host rock and experimental data from alkaline rocks the megacrysts are related to a sequence of crystallization from primitive monchiquitic to potassic phonolitic magmas rich in H₂O and CO₂ at 5–11 kb. Group I megacrysts formed at temperatures of 1300-1150°C and group II between ? bar at the latter temperature. High P_co2 may have stabilized the scapolite in the more evolved liquid and K-feldspar and nepheline began to crystallize at ca. 800°C possibly together with the ferrisalite.  相似文献   

Evaluation of deep temperatures of hydrothermal systems by a new gas geothermometer     

Franco D&#x;Amore  Costanzo Panichi 《Geochimica et cosmochimica acta》1980,44(3):549-556

The chemical composition of gas mixtures emerging in thermal areas can be used to evaluate the deep thermal temperatures. Chemical analyses of the gas compositions for 34 thermal systems were considered and an empirical relationship developed between the relative concentrations of H₂S, H₂, CH₄ and CO₂ and the reservoir temperature. The evaluated temperatures can be expressed by: $\text{t°C =}\text{24775}\text{α + β + 36.05}\text{?273}$ where $\text{α = 2 log}\text{CH}{}_4\text{CO}{}_2\text{?log}\text{H}{}_2\text{CO}{}_2\text{?3 log}\text{H}{}_2\text{S}\text{CO}{}_2$ (concentrations in % by volume) and β = 7 logP_co2  相似文献   

Density calculations for silicate liquids. I. Revised method for aluminosilicate compositions     

Y Bottinga  D Weill  P Richet 《Geochimica et cosmochimica acta》1982,46(6):909-919

Equations are developed for calculating the density of aluminosilicate liquids as a function of composition and temperature. The mean molar volume at reference temperature T_r, is given by $\text{V}{}_{\mathrm{r}}\text{= ∑X}{}_{\mathrm{i}}\text{V}\text{?}{}^{\mathrm{o}}{}_{\mathrm{i}}\text{+ X}{}_{\mathrm{A}}\text{V}\text{?}{}^{\mathrm{o}}{}_{\mathrm{A}}$, where the summation is taken over all oxide components except A1₂O₃, X stands for mole fraction, terms are constants derived independently from an analysis of volume-composition relations in alumina-free silicate liquids, and $\text{V}\text{?}{}^{\mathrm{o}}{}_{\mathrm{A}}$ is the composition-dependent apparent partial molar volume of Al₂O₃. The thermal expansion coefficient of aluminosilicate liquids is given by $\text{α = ∑X}{}_{\mathrm{i}}\text{}\text{?}\text{ga}{}_{\mathrm{i}}{}^{\mathrm{o}}\text{+ X}{}_{\mathrm{A}}\text{}\text{?}\text{ga}{}_{\mathrm{A}}{}^{\mathrm{o}}$, where $\text{}\text{?}\text{ga}{}_{\mathrm{i}}{}^{\mathrm{o}}$ terms are constants independent of temperature and composition, and $\text{}\text{?}\text{ga}{}^{\mathrm{o}}{}_{\mathrm{A}}$ is a composition-dependent term representing the effect of Al₂O₃ on the thermal expansion. Parameters necessary to calculate the volume of silicate liquids at any temperature T according to V(T) = V_rexp[α(T-T_r)], where T_r = 1400°C have been evaluated by least-square analysis of selected density measurements in aluminosilicate melts. Mean molar volumes of aluminosilicate liquids calculated according to the model equation conform to experimentally measured volumes with a root mean square difference of 0.28 $\text{cc}\text{mole}$ and an average absolute difference of 0.90% for 248 experimental observations. The compositional dependence of $\text{V}\text{?}{}^{\mathrm{o}}{}_{\mathrm{A}}$ is discussed in terms of several possible interpretations of the structural role of Al³⁺ in aluminosilicate melts.  相似文献   

Geochemistry and petrogenesis of lamproites,late cretaceous age,Woodson County,Kansas, U.S.A.     

Robert L. Cullers  Subramanian Ramakrishnan  Pieter Berendsen  Tom Griffin 《Geochimica et cosmochimica acta》1985,49(6):1383-1402

Lamproite sills and their associated sedimentary and contact metamorphic rocks from Woodson County, Kansas have been analyzed for major elements, selected trace elements, and strontium isotopic composition. These lamproites, like lamproites elsewhere, are alkalic (molecular $\text{K}{}_2\text{O + Na}{}_2\text{O}\text{Al}{}_2\text{O}{}_3\text{= 1.6–2.6}$), are ultrapotassic $\text{(}\text{K}{}_2\text{O}\text{Na}{}_2\text{O}\text{= 9.6–150)}$, are enriched in incompatible elements (LREE or light rare-earth elements, Ba, Th, Hf, Ta, Sr, Rb), and have moderate to high initial strontium isotopic compositions (0.7042 and 0.7102). The silica-saturated magma (olivine-hypersthene normative) of the Silver City lamproite could have formed by about 2 percent melting of a phlogopite-garnet lherzolite under high $\text{H}{}_2\text{O}\text{CO}{}_2$ ratios in which the Iherzolite was enriched before melting in the incompatible elements by metasomatism. The Rose Dome lamproite probably formed in a similar fashion although the extreme alteration due to addition of carbonate presumably from the underlying limestone makes its origin less certain. Significant fractional crystallization of phases that occur as phenocrysts (diopside, olivine, K-richterite, and phlogopite) in the Silver City magma and that concentrate Co, Cr, and Sc are precluded as the magma moved from the source toward the surface due to the high abundances of Co, Cr, and Sc in the magma similar to that predicted by direct melting of the metasomatized Iherzolite.Ba and, to a lesser extent, K and Rb and have been transported from the intrusions at shallow depth into the surrounding contact metamorphic zone. The Silver City lamproite has vertical fractionation of some elements due either to volatile transport or to variations in the abundance of phenocrysts relative to groundmass most probably due to flow differentiation although multiple injection or fractional crystallization cannot be conclusively rejected.  相似文献   

Use of Pitzer's equations to determine the media effect on the formation of lead chloro complexes     

Frank.J. Millero  Robert H. Byrne 《Geochimica et cosmochimica acta》1984,48(5):1145-1150

The spectrophotometric measurements of chloro complexes of lead in aqueous HCl, NaCl, MgCl₂ and CaCl₂ solutions at 25°C have been analyzed using Pitzer's specific interaction equations. Parameters for activity coefficients of the complexes PbCl⁺, PbCl₂⁰ and PbCl₃^? have been determined for the various media. Values of $\text{K}{}_1\text{= 30.0 ± 0.6}$, $\text{K}{}_2\text{= 106.7 ± 2.1}$ and $\text{K}{}_3\text{= 73.0 ± 1.5}$ were obtained for the cumulative formation constants. [$\text{Pb}{}^{\mathrm{2+}}\text{+ nCl}{}^{\mathrm{?}}\text{→ PbCl}{}_{\mathrm{n}}{}^{\mathrm{2?n}}\text{)}$]. These values are in reasonable agreement with literature data. The Pitzer parameters for the PbCl ion pairs in various media were used to calculate the speciation of Pb²⁺ in an artificial seawater solution.  相似文献   

Aragonite from deep sea ultramafic rocks     

E. Bonatti  J.R. Lawrence  P.R. Hamlyn  D. Breger 《Geochimica et cosmochimica acta》1980,44(8):1207-1214

Aragonite mineralization was observed in serpentinized peridotites from the Romanche and Vema Fracture Zones in the Atlantic and the Owen Fracture Zone in the Indian Ocean, either in veins or as radial aggregates in cavities within the serpentinites. Evidence of incipient dissolution of the aragonite crystals was observed in one case. The aragonites tend to have lower Mg content (< 0.03%) and higher Sr content (> 0.95%) relative to other marine aragonites. Their ${}^{18}\text{O}{}^{16}\text{O}$, ${}^{13}\text{C}{}^{12}\text{C}$ and ${}^{87}\text{Sr}{}^{86}\text{Sr}$ isotopic ratios suggest the aragonite was deposited at ocean floor temperatures from solutions derived from sea water circulating in fissures and fractures within the ultramafic rocks. The ${}^{18}\text{O}{}^{16}\text{O}$ ratios of the serpentines indicate serpentinization occurred at higher temperatures, probably deeper in the crust. Low-T reactions between circulating seawater and Mg-silicates (primarily serpentine and pyroxenes) caused high pH and enrichment of Mg and Ca in the solution, conditions favoring carbonate precipitation. Aragonite was formed rather than calcite presumably because the high Mg²⁺ concentration in the solution inhibited calcite precipitation. The high Sr content of the aragonites is probably related, at least in part, to their low temperature of formation. Opaque mineral grains containing over 8% NiO and over 40% MnO were observed concentrated along the margins of some of the aragonite veins, suggesting that Ni is one of the elements mobilized during reactions between ultramafic rocks and circulating seawater.  相似文献   

Etude des inclusions fluides du système N₂-CO₂ de dolomites et de quartz de Tunisie septentrionale. Données de la microcryoscopie et de l'analyse à la microsonde à effet Raman     

N Guilhaumou  P Dhamelincourt  J-C Touray  J Touret 《Geochimica et cosmochimica acta》1981,45(5):657-673

Optical and analytical studies were performed on 400 N₂ + CO₂ gas bearing inclusions in dolomites and quartz from Triassic outcrops in northern Tunisia. Other fluids present include brines (NaCl and KCl bearing inclusions) and rare liquid hydrocarbons. At the time of trapping, such fluids were heterogeneous gas + brine mixtures. In hydrocarbon free inclusions the $\text{N}{}_2\text{(N}{}_2\text{+ CO}{}_2\text{)}$ mole ratio was determined using two different non-destructive and punctual techniques: Raman microprobe analysis, and optical estimation of the volume ratios of the different phases selected at low temperatures. In the observed range of compositions, the two methods agree reasonably well.The N₂ + CO₂ inclusions are divided into three classes of composition: (a) $\text{N}{}_2\text{(N}{}_2\text{+ CO}{}_2\text{)}\text{> 0,57}$: Liquid nitrogen is always visible at very low temperature and homogenisation occurs in the range ?151°C to ? 147°C (nitrogen critical temperature) dry ice (solid CO₂) sublimates between ?75°C and ?60°C; (b) $\text{0,20 <}\text{N}{}_2\text{(N}{}_2\text{+ CO}{}_2\text{)}\text{? 0,57}$: liquid nitrogen is visible at very low temperature but dry ice melts on heating; liquid and gas CO₂ homogenise to liquid phase between ?51°C to ?22°C; (c) $\text{N}{}_2\text{(N}{}_2\text{+ CO}{}_2\text{)}\text{? 0,20}$: liquid nitrogen is not visible even at very low temperature (?195°C) and liquid and gas CO₂ homogenise to liquid phase between ?22°C and ?15°C. The observed phases changes are used to propose a preliminary phase diagram for the system CO₂-N₂ at low temperatures.Assuming additivity of partial pressures, isochores for the CO₂-N₂ inclusions have been computed. The intersection of these isochores with those for brine inclusions in the same samples may give the P and T of trapping of the fluids.  相似文献   

Densities of Na2O-K2O-CaO-MgO-FeO-Fe2O3-Al2O3-TiO2-SiO2 liquids: New measurements and derived partial molar properties     

《Geochimica et cosmochimica acta》1987,51(11):2931-2946

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