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1.
Oxygen isotope fractionations between wollastonite, diopside, jadeite, hedenbergite and water have been experimentally studied at high pressures (1<- PH 2O ≥ 24 kbar) and temperatures (400/ dg ≤ T <- 800/ dgC) using the three-isotope method (Matsuhisa et al., 1978). Initial fractionations were made close to equilibrium and initial ratios were well removed from equilibrium, allowing accurate determinations of the equilibrium fractionations and of the extent of isotopic exchange. Scanning electron microscope and rate studies show that the wollastonite-water and diopside-water exchange reactions occur largely by solution-precipitation (Ostwald Ripening) mechanisms. Equilibrium fractionations between water and the minerals wollastonite, diopside, and hedenbergite are in close agreement with one another, whereas significantly more positive fractionations are found for jadeite-water. These isotopic substitution effects can be ascribed to replacement of SiOM bonds (M is a divalent metal cation in octahedral coordination) by higher frequency SiOAl bonds. The fractionations determined in this study can be combined with quartz- and feldspar-water data of Matsuhisa et al. (1979) and revised magnetite-water data of O'NEIL (1963), to provide a coherent set of mineral-pair fractionations satisfactorily represented by straight lines through the origin on a conventional graph of In /ga versus T?2. Mineral-water data, on the other hand, cannot readily be fitted to the simple relationship suggested by Bottinga and Javoy (1973). Coefficients “ A” for the mineral-pair fractionations 1000 ln α = A × 10 6T?2 are: | Ab | Jd | An | Di | Wo | Mt | Q | 0.50 | 1.09 | 1.59 | 2.08 | 2.20 | 6.11 | Ab | | 0.59 | 1.09 | 1.58 | 1.70 | 5.61 | Jd | | | 0.50 | 0.99 | 1.11 | 5.02 | An | | | | 0.49 | 0.61 | 4.52 | Di | | | | | 0.12 | 4.03 | Wo | | | | | | 3.91 | 相似文献
2.
Self-diffusion of oxygen in adularia, anorthite, albite, oligoclase and labradorite has been measured by isotope exchange of oxygen between natural feldspars and hydrothermal water enriched in 18O. The analysis consisted of measuring the 18O/ 16O gradient inward from the feldspar surface using an ion microprobe, and fitting a solution of the diffusion equation to the data. Depth of the sputtered hole was measured with an optical interferometer. Linear Arrhenius plots were obtained: | | | | Adularia (Or98) | | 25.6 | 350–700 | Albite (Ab97, Ab99) | | 21.3 | 350–800 | Anorthite (An96) | | 26.2 | 350–800 | 相似文献
3.
Intercalibration of one intralaboratory and three interlaboratory standards used in 40Ar- 39Ar dating has been carried out. In order to provide homogeneous values for the standards were prepared by careful handpicking. To control the neutron fluence in the Herald Reactor (A.W.R.E.) 16 aliquots of the standards were arranged along 0.6 × 60 cm of a single silica tube. The corrections for all known interferences from K, Ca, Cl were carefully assessed. Two of the hornblende standards, Hb3gr and MMHb-1 appear homogeneous at the 0.1% level while the other two standards, LP-6 and FY12a are not completely homogeneous. The mean values of when referenced to the previously determined value for Hb3gr (turner et al., 1971) are: Hb3gr | hbld | (±lσ) | 1072. m.y. | MMHb-1 | hbld | | 518.9 m.y. | LP-6 | biot | | 128.5 m.y. | FY12a | hbld | | 435.0 m.y. | 相似文献
4.
A simple differential thermal analysis (DTA) technique has been developed to study phase relations of various chemical systems at elevated pressures and temperatures. The DTA system has been calibrated against known melting temperatures in the system NaCl-KCl. Isobaric sections of the liquidus in the system NaCl-KCl have been determined at pressures of 1 atmosphere and 500, 1000, 1500, and 2000 bars. Using the least-squares method, the following equation was used to fit the experimental data: where T is the liquidus temperature, XKCl is mole fraction of KCl, and ai (listed below) are the derived empirical constants. P (bars) | | | | | | | | 1 atm. | 800.1 | ?334.2 | 781.6 | ?6490.3 | 17553.1 | ?17638.4 | 6098.3 | 500 | 813.5 | ?354.9 | 743.3 | ?6011.7 | 16406.4 | ?16516.3 | 5702.8 | 1000 | 824.5 | ?406.7 | 1446.8 | ?8818.4 | 21253.5 | ?20343.7 | 6839.4 | 1500 | 838.6 | ?418.7 | 1434.7 | ?8819.0 | 21557.9 | ?20908.4 | 7123.1 | 2000 | 848.5 | ?381.5 | 1246.9 | ?8605.0 | 21785.8 | ?21449.1 | 7375.8 | 相似文献
5.
K and Rb distributions between aqueous alkali chloride vapour phase (0.7 molar) and coexisting phlogopites and sanidines have been investigated in the range 500 to 800°C at 2000 kg/cm 2 total pressure.Complete solid solution of RbMg 3AlSi 3O 10(OH) 2 in KMg 3AlSi 3O 10(OH) 2 exists at and above 700°C. At 500°C a possible miscibility gap between approximately 0.2 and 0.6 mole fraction of the Rb end-member is indicated.Only limited solid solution of Rb AlSi 3O 8 in KAlSi 3O 8 has been found at all temperatures investigated.Distribution coefficients, expressed as in solid/(Rb/K) in vapour, are appreciably temperature-dependent but at each temperature are independent of composition for low Rb end-member mole fractions in the solids. The determined values and their approximate Rb end-member mole fraction ( ) ranges of constancy are summarized as follows: (°C) (°C)T | | | | | 500 | | 0–0.2 | | 0–0.07 | 700 | | 0–0.2 | | 0–0.1 | 800 | | 0–0.2 | | 0–0.1 | 相似文献
6.
Rb/Sr geochronology on a folded greenstone-granitoid complex in the Agnew area, Western Australia, yields four distinct ages of igneous activity that conform with stratigraphic and intrusive relationships. They are (using , NBS 70A = 522 ppm Rb and 65.3 ppm Sr): | Ma | 87Sr/86Sr initial (IR) | (1) Differentiated gabbro-granophyre from a stratigraphically old (Kathleen Valley) greenstone sequence | | | (2) Voluminous tonalite, the Lawlers Tonalite | | | | | | (3) A less voluminous leucogranite, and a large complex pegmatite cutting the Perseverance nickel orebody | | | (4) Aplitic leucotonalite (very minor volumes but widespread) | | | 相似文献
7.
Oxygen and carbon isotope analyses of samples from three mines in the Krivoy Rog iron formation, Ukranian SSR, are reported here. Maximum and minimum quartz-magnetite fractionation values ( ) and inferred temperature range in degrees centrigrade for each mine are: Mine | | Corresponding temperature | Sevgok | 9.4 to 14.2 | 475° to 320°C | Ugok | 10.0 to 12.7 | 450° to 355°C | Annovsky | 10.5 to 12.6 | 430° to 360°C | 相似文献
8.
A differential rate equation for silica-water reactions from 0–300°C has been derived based on stoichiometry and activities of the reactants in the reaction SiO 2(s) + 2H 2O(l) = H 4SiO 4(aq) where ( ) = (the relative interfacial area between the solid and aqueous phases/the relative mass of water in the system), and k+ and k? are the rate constants for, respectively, dissolution and precipitation. The rate constant for precipitation of all silica phases is and Eact for this reaction is 49.8 kJ mol ?1. Corresponding equilibrium constants for this reaction with quartz, cristobalite, or amorphous silica were expressed as . Using , k was expressed as and a corresponding activation energy calculated: | a | b | c | Eact(kJ mol -1) | Quarts | 1.174 | -2.028 x 103 | -4158 | 67.4–76.6 | α-Cristobalite | -0.739 | 0 | -3586 | 68.7 | β-Cristobalite | -0.936 | 0 | -3392 | 65.0 | Amorphous silica | -0.369 | -7.890 x 10-4 | 3438 | 60.9–64.9 | 相似文献
10.
Delayed neutron measurements of U and Th in three meteorites yield the following values: Bruderheim | U (ppb) | Th(ppb) | Bruderheim | | | Peace River | | | Stannern | | | 相似文献
11.
The Vermilion district of northerneastern Minnesota is a classic example of a lower Precambrian greenstone-granite terrane. It is a complex volcanic-sedimentary pile, characterized by repeated periods of volcanism and the presence of intercalated pyroclastic, volcanoclastic and epiclastic rocks. The volcanic-sedimentary pile is surrounded and intruded by contemporaneous granitic batholiths. Several rock units from the district have been dated by the whole-rock Rb-Sr method. The isochron ages and the corresponding initial Sr 87/Sr 86 ratios (= I) are: | | | Ely Greenstone | | | Newton Lake Formation | | | Granitic pebbles | | | 相似文献
12.
We studied trapping of noble-gases by chromite and carbon: two putative carriers of primordial noble gases in meteorites. Nineteen samples were synthesized in a Ne-Ar-Kr-Xe atmosphere at 440 K to 720 K, by the following reactions: Fe,Cr + 4H 2O → (Fe,Cr) 3O 4 + 4H 2 (1) or Fe,Cr + 4CO → (Fe,Cr) 3O 4 + 4C + carbides (2)The reactant metal films were prepared either by vacuum evaporation of alloy or by thermal decomposition of Fe- and Cr-carbonyls. The products—including Fe 3O 4, Cr 2O 3, carbides, and unreacted metal—were partially separated by selective solvents, such as HCl, H 2SO 4?H 3PO 4, or HClO 4. Samples were characterized by XRD, SEM, and atomic absorption; noble gases were measured by mass spectrometry. Surface areas, as measured by the BET method, were 2 to 100 m 2/g.All samples are dominated by an adsorbed noble gas component that is largely released upon heating at ?400°C or slight etching. Elemental abundance patterns show that this component is derived from the highest-pressure noble gas reservoir seen by the sample—atmosphere or synthesis vessel—indicating that desorption or exchange rates at room T are slow on the time scale of our experiments (up to 1 year). Adsorptive capacity is reduced by up to 2 orders of magnitude upon light etching with HClO 4 (though the surface area actually doubles in this treatment) and, less drastically, by heating. Apparently some active adsorption sites are destroyed by these treatments. A trapped component (typically 30% of the total) is readily detectable only in samples synthesized at partial pressures close to or greater than atmospheric.Noble gas contents roughly obey Henry's Law, but show only slight, if any, correlations with composition, surface area, or adsorption temperature. (Geometric) mean distribution coefficients for bulk samples and HCl-residues are, in 10 ?3 cc STP/g atm: Xe (100), Kr (15), Ar (3.5), Ne (0.62). Elemental fractionations are large and variable, but are essentially similar for the adsorbed and trapped components, or for chromite and carbon. They bracket the values for the corresponding meteoritic minerals. | | | | Geom. mean | 0.006 | 0.035 | 0.15 | Range | 0.0004-0.03 | 0.01-0.2 | 0.06-0.4 | 相似文献
13.
Using the model of fictive ideal components, Gibbs free energies of formation of pyrope and Al 2O 3-enstatite have been determined from the experimental data on coexisting garnet and orthopyroxene and orthopyroxene and spinel in the temperature range of 1200–1600 K. The negative free energies in kJ/mol are: TK | 1200 | 1300 | 1400 | 1500 | 1600 | Pyrope | 4869.92 | 4747.05 | 4614.26 | 4462.63 | 4311.00 | Al2O3-enstatite | 1257.25 | 1244.28 | 1191.93 | 1158.67 | 1125.64 | 相似文献
14.
Concentrations of Au, As, Co, Ga, Ge, Ir, Ni and W were determined in the metal of 28 different pallasites plus 6 which are probably paired, to help elucidate their origin. Most divide into two clusters: | No. | Ni (%) | Ga (μg/g) | Ge (μg/g) | Au (μg/g) | Fa (mole %) | Main group | 19 | 7.8–11.7 | 16–26 | 29–65 | 1.7–3.0 | 11–13 | Eagle Station trio | 3 | 14–16 | 4.5–6 | 75–120 | 0.8–1.0 | 19–20 | 相似文献
15.
Concentrations of Ni, Ga, Ge and Ir in 106 iron meteorites are reported. Three new groups are defined: IC, IIE and IIIF containing 10, 12 and 5 members, respectively, raising the number of independent groups to 12. Group IC is a cohenite-rich group distantly related to IA. Group IIE consists of those irons previously designated Weekeroo Station type and five others having similar compositions though diverse structures. The IIE irons are compositionally similar to the mesosiderites and pallasites, and the three groups probably formed at similar heliocentric distances. The mixing of the globular IIE silicates with the metal probably occurred during shock events. Group IIIF is a well-defined group of low-Ni and low-Ge irons. The compositions of these groups are summarized as follows: Group | Ni (%) | Ga (ppm) | Ge (ppm) | Ir (ppm) | IC | 6.1–6.8 | 42–54 | 85–250 | 0.07–10 | IIE | 7.5–9.7 | 21–28 | 62–75 | 0.5–8 | IIIF | 6.8–7.8 | 6.3–7.2 | 0.7–1.1 | 1.3–7.9 | 相似文献
16.
Siderite, dolomite and ankerite were reacted with “>103°” phosphoric acid at temperatures up to 150°C with >99° yields achieved in less than two hours, using a modification of the McCrea (1950) technique. The oxygen fractionation factors, α, between the δ 18O of the carbonate and that of the acid-extracted CO 2 are: | Siderite | Dolomite | Ankerite | 100°C | 1.00881 | 1.00913 | 1.00901 | 150°C | 1.00771 | - | - | 相似文献
17.
We report Ni, Ga, Ge and Ir concentrations for 193 irons. The compositional trends in groups IIIA and IIIB are redefined, and the suggestion by Wasson and Kimberlin that they represent a single fractionation sequence (group IIIAB) is confirmed. A new group, HIE, is similar in its properties to group IIIA but distinguished by lower Ga/Ni and Ge/Ni ratios, larger bandwidths and the formation of haxonite (Fe, Ni) 23C 6 in each of its members. A sixth member, Hassi-Jekna, has been added to group IIIC, extending its Ge range up to 70 ppm. The characteristics of these groups can be summarized as follows: Group | Structure | Ni% | Ga(ppm) | Ge(ppm) | Ir(ppm) | IIIA | Om | 7.1–9.3 | 7–23 | 32–47 | 0.17–19 | IIIB | Om | 8.4–10.5 | 16–21 | 27–46 | 0.014–0.17 | IIIC | Off-Of | 10.5–13.0 | 11–27 | 8.6–70 | 0.08–0.6 | IIIE | Og | 8.2–8.9 | 17–19 | 34–37 | 0.05–0.6 The Ge-Ni correlation is positive in IIIA, negative in IIIB and IIIC, and there is no significant correlation in IIIE. San Cristobal is identified as a member of group IAB, thereby extending the Ge and Ni range of this group to 25 ppm and 25 per cent, respectively. Previous reports of wide cooling-rate variations in group IIIAB are not substantiated, and current evidence favors a core over a raisin-bread model for this group. There appears to be no genetic relationship between group IIIAB and either the pallasites or the mesosiderites | 相似文献
18.
The heat capacities of synthetic pyrope (Mg 3Al 2Si 2O 12), grossular (Ca 3Al 2Si 3O 12) and a solid solution pyrope 60grossular 40 (Mg 1.8Ca 1.2Al 2Si 3O 12) have been measured by adiabatic calorimetry in the temperature range 10–350 K. The samples were crystallized from glasses in a conventional piston-cylinder apparatus.The molar thermophysical properties at 298.15 K (J mol ?1 K ?1) are: | Cop | So298?So0 | Ho298?Ho0/T | Pyrope | 325.31 | 266.27 | 47852 | Grossular | 333.17 | 260.12 | 47660 | Py60Gr40 | 328.32 | 268.32 | 47990 | 相似文献
20.
Fractionation factors between epidote minerals and water, and between the AlO(OH) dimorphs boehmite and diaspore and water, have been determined between 150 and 650°C. Small water mineral ratios were used to minimise the effect of incongruent dissolution of epidote minerals. Waters were extracted and analysed directly by puncturing capsules under vacuum. Hydrogen diffusion effects were eliminated by using thick-walled capsules. Exchange rates are very fast between epidote and water (and between boehmite and water), complete exchange taking only minutes above 450°C but several months at 250°C. Exchange between zoisite and water (and between diaspore and water) is very much slower, and an interpolation method was necessary to determine fractionation factors at 450 and below.For the temperature range 300–650°C, the equilibrium fractionation factor ( αe) between epidote and water is independent of temperature and Fe content of the epidote, and is given by 1000 In αepidote-H2Oe = ?35.9 ± 2.5, while below 300°C 1000 In , with a ‘cross-over’ estimated to occur at around 185°C. By contrast, zoisite-water fractionations fit the relationship 1000 In .All studied minerals have hydrogen bonding. Fractionations are consistent with the general relationship: the shorter the O-H -- O bridge, the more depleted is the mineral in D.On account of rapid exchange rates, natural epidotes probably acquired their H-isotope compositions at or below 200°C, where fractionations are near or above 0%.; this is in accord with the observation that natural epidotes tend to concentrate D relative to other coexisting hydrous minerals. 相似文献
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