Noble gases in E-chondrites     

Jane Crabb  Edward Anders 《Geochimica et cosmochimica acta》1981,45(12):2443-2464

Noble gas data are reported for 12 E-chondrites. Combined with literature data, they show that K-Ar ages are >4 Æ for 14 out of 18 meteorites, yet U, Th-He ages are often shorter, perhaps due to late, mild reheating. Cosmic-ray exposure ages differ systematically between types 4 and 6, with E4's mostly below 16 Myr and E6's above 30 Myr. This may mean that the E-chondrite parent body contains predominantly a single petrologic type on the (~ 1 km) scale of individual impacts, in contrast to the more thoroughly mixed parent bodies of the ordinary chondrites.The heavy noble gases consist of at least two primordial components: the usual planetary component (${}^{36}\text{Ar}{}^{132}\text{Xe}\text{~ 80}$) and a less fractionated, ‘subsolar’ component ($\text{2700 ≤}{}^{36}\text{Ar}{}^{132}\text{Xe}\text{≤ 3800}$). The latter is found in highest concentration in the E4 chondrite South Oman (³⁶Ar = 760 × 10^?8cc/g, ${}^{36}\text{Ar}{}^{132}\text{Xe}\text{= 2700}$). The isotopic compositions of both components are similar to typical planetary values, indicating that some factor other than mass controlled the noble gas elemental ratios. The heavy Xe isotopes occasionally show some of the lowest ${}^{134}\text{Xe}{}^{132}\text{Xe}$ and ${}^{136}\text{Xe}{}^{132}\text{Xe}$ ratios measured in bulk chondrites, suggestive of nearly fission-free Xe (e.g. ${}^{136}\text{Xe}{}^{132}\text{Xe}\text{= 0.3095 ± 0.0020}$). Amounts of planetary gas in E4 E6 chondrites fall in the range for ordinary chondrites of types 4–6, but, in contrast to the ordinary chondrites. fail to correlate with petrologic type or volatile trace element contents. Another unusual feature of E-chondrites is that primordial Ne is present even in most 4's and 5's (²⁰Ne_p ~ 1 to 7 × 10^?8cc/g). with an isotopic composition consistent with planetary Ne.Analyses of mineral separates show that the planetary gases are concentrated in an HF- and HCl-insoluble mineral similar to phase Q, the poorly characterized, HNO₃-soluble carrier of primordial gases in carbonaceous and ordinary chondrites. The subsolar gases, on the other hand, are located in an HCl- and HNO₃-resistant phase, possibly enstatite or a minor phase included in enstatite. Much of the ¹²⁹Xe_r (50% for E4's, > 70% for E6's) is in HCl-resistant but HF-soluble sites, suggestive of a silicate.A similar subsolar component may be responsible for the high ${}^{36}\text{Ar}{}^{132}\text{Xe}$ ratios of some C3's, unequilibrated ordinary chondrites, and the unique aubrite Shallowater. The planet Venus also has a high $\text{Ar}\text{Kr}$ ratio, well above the planetary range, and hence may have acquired its noble gases from an E-chondrite-like material, similar to South Oman.  相似文献   

On the siting of noble gases in E-chondrites     

Jane Crabb  Edward Anders 《Geochimica et cosmochimica acta》1982,46(11):2351-2361

We have investigated the siting of noble gases in 6 E-chondrites, by analyzing fractions separated by density, grain size, and chemical resistance from Qingzhen (E3), Indarch (E4), Abee and Saint Sauveur (E4-5) and Yilmia and North West Forrest (E6).The new “subsolar” (i.e. Ar-rich) component in E6's is concentrated in the main, ensatite-rich fraction of the meteorites, with density 3.06–3.3 g/cm³. It is unaffected by HCl and HNO₃ treatments of such fractions and remains in unchanged concentration when the samples are partially dissolved by HF. These properties suggest that the subsolar component is located in enstatite, or less likely, in a phase closely associated with it. E4-5's have at least half of their subsolar gases in HCl- and HNO₃-resistant sites (enstatite?), but fail to show the increasing gas concentration with decreasing grain size that is characteristic of most other primordial gas carriers. This may mean that the subsolar gases originally were in some other phase, but were then transferred to enstatite by metamorphism.Most of the ¹²⁹Xe_r of E6's is concentrated in the same fractions as the subsolar gases, again suggesting enstatite or an associated phase as the host. Only a few percent of the ¹²⁹Xe_r is contained in fractions enriched in other major and minor minerals. In E4's, on the other hand, ¹²⁹Xe_r is enhanced in finegrained, low density fractions and is also partly associated with chondrules. Perhaps ¹²⁹I was originally contained in fine-grained matrix, but was transferred to enstatite during metamorphism.A carbon-rich fraction of Indarch (E4) is enhanced in Ne-A, CCF-Xe, and L-Xe. Interestingly, both the isotopic composition of Xe and the Ne/CCF-Xe ratios resemble those of C-chondrites, yet these two meteorite classes probably formed rather far apart. Thus, if these components were mixed at a late stage, it must have been in fairly constant ratio over a large scale. Alternatively, they may have been mixed at an earlier stage, into a common carrier that was spread through a significant portion of the solar nebula.The primordial gases of Qingzhen (E3) resemble those of Indarch: they are present in moderate amounts (${}^{20}\text{Ne}{}_{\mathrm{p}}\text{= 1.2 × 10}{}^{\mathrm{?8}}\text{cc/g,}{}^{132}\text{Xe = 10 × 10}{}^{\mathrm{?10}}\text{cc/g)}$, with little or no contribution from the subsolar component. Thus Qingzhen reinforces our earlier finding that E-chondrites show no regular increase in noble gas content with decreasing petrologic type. One notable feature of Qingzhen is its very low ${}^3\text{He}{}^{21}\text{Ne}$ ratio of 1.07, which indicates that ³He has been lost by solar heating. Solar heating may also account for its low, discordant gas retention ages (U,Th-He age = 1.1 AE, KAr age = 3.2AE).  相似文献   

Geochronologic investigations of the Sudbury Nickel Irruptive and the Superior Province granites north of Sudbury     

R.W. Hurst  J. Farhat 《Geochimica et cosmochimica acta》1977,41(12):1803-1815

RbSr ($\text{λ}\text{Rb}\text{= 1.39 × 10}{}^{\mathrm{?11}}\text{yr}{}^{\mathrm{?1}}\text{)}$ and U-Pb ($\text{λ 238 = 1.54 × 10}{}^{\mathrm{?10}}\text{yr}{}^{\mathrm{?1}}\text{, λ235 = 9.72 × 10}{}^{\mathrm{?10}}\text{yr}{}^{\mathrm{?1}}$) measurements were undertaken in the Sudbury area, Sudbury, Ontario to determine the ages of the Sudbury Nickel Irruptive, Superior Province granites north of Sudbury, Sudbury Breccia and subsequent metamorphism. The Sudbury Nickel Irruptive norite whole rock Rb-Sr data yield an age of $\text{1883 ± 136}\text{Myr}$ ($\text{I.R}\text{. = 0.7071 ± 0.0005}$; all results quoted at 2π level) while the Nickel Irruptive micropegmatite Rb-Sr system has been disturbed and does not yield an isochron. A plagioclase-whole rock pair from the norite near the norite-micropegmatite transition yields an age of 1866 Myr, which when taken in conjunction with field (Stevenson and Colgrove, 1968) and geochemical (Naldrettet al., 1970, 1972) data does not support the conclusion of gibbins and McNurr (1972) that the micropegmatite is a later intrusion rather than a differentiate of the magma which produced the norite. Rb-Sr studies of the Superior Province granites north of Sudbury yield an age of $\text{2698 ± 162}\text{Myr}$ ($\text{I.R.}\text{= 0.7019 ± 0.0012}$). U-Pb zircon studies of these granites and granitic clasts within the Sudbury Breccia yield an age of $\text{2.71 ± 0.05}$ Byr and suggest the breccia granitic clasts were derived from the Superior Province granites. The granitic rocks ~150 km north of Sudbury have been undisturbed for ~ 2.6 Byr based on Rb-Sr mineral studies, whereas the granites and Sudbury Breccia within ~ 15 km of the Nickel Irruptive, as well as the Sudbury norite at the perimeter of the Irruptive have been disturbed by the Penokean Orogeny 1.7–1.75 Byr ago. The Penokean event appears to have overprinted isotopic evidence of the Sudbury impact event at least in the area studied.  相似文献   

Revised values for the Gibbs free energy of formation of [Al(OH)_{4 aq}⁻], diaspore,boehmite and bayerite at 298.15 K and 1 bar,the thermodynamic properties of kaolinite to 800 K and 1 bar,and the heats of solution of several gibbsite samples     

Bruce S. Hemingway  Richard A. Robie  James A. Kittrick 《Geochimica et cosmochimica acta》1978,42(10):1533-1543

Solution calorimetric measurements compared with solubility determinations from the literature for the same samples of gibbsite have provided a direct thermochemical cycle through which the Gibbs free energy of formation of [Al(OH)_{4 aq}^?] can be determined. The Gibbs free energy of formation of [Al(OH)_{4 aq}^?] at 298.15 K is ?1305 ± 1 kJ/mol. These heat-of-solution results show no significant difference in the thermodynamic properties of gibbsite particles in the range from 50 to 0.05 μm.The Gibbs free energies of formation at 298.15 K and 1 bar pressure of diaspore, boehmite and bayerite are ?9210 ± 5.0, ?918.4 ± 2.1 and ?1153 ± 2 kJ/mol based upon the Gibbs free energy of [A1(OH)_{4 aq}^?] calculated in this paper and the acceptance of ?1582.2 ± 1.3 and ?1154.9 ± 1.2 kJ/mol for the Gibbs free energy of formation of corundum and gibbsite, respectively.Values for the Gibbs free energy formation of [Al(OH)_{2 aq}⁺] and [AlO_{2 aq}^?] were also calculated as ?914.2 ± 2.1 and ?830.9 ± 2.1 kJ/mol, respectively. The use of [AlC_{2 aq}^?] as a chemical species is discouraged.A revised Gibbs free energy of formation for [H₄SiO_4aq⁰] was recalculated from calorimetric data yielding a value of ?1307.5 ± 1.7 kJ/mol which is in good agreement with the results obtained from several solubility studies.Smoothed values for the thermodynamic functions C_P⁰, ($\text{H}{}_{\mathrm{T}}{}^0\text{- H}{}_{298}{}^0\text{)}\text{T}$, $\text{(}\text{G}{}_{\mathrm{T}}{}^0\text{- H}{}_{298}{}^0\text{)}\text{T}$, S_T⁰ - S₀⁰, $\text{ΔH}{}_{\mathrm{?,298}}{}^0$ kaolinite are listed at integral temperatures between 298.15 and 800 K. The heat capacity of kaolinite at temperatures between 250 and 800 K may be calculated from the following equation: C_P⁰ = 1430.26 ? 0.78850 T + 3.0340 × 10^?4T² ?1.85158 × 10^?4T²$\text{1}\text{2}\text{+ 8.3341 × 10}{}^6\text{T}{}^{\mathrm{?2}}$.The thermodynamic properties of most of the geologically important Al-bearing phases have been referenced to the same reference state for Al, namely gibbsite.  相似文献   

Precise age determinations and petrogenetic studies using the KCa method     

B.D. Marshall  D.J. DePaolo 《Geochimica et cosmochimica acta》1982,46(12):2537-2545

High precision mass spectrometric determination of calcium isotope ratios allows the ⁴⁰K → ⁴⁰Ca radioactive decay to be used for dating a much broader range of geologic materials than is suggested by previous work. ${}^{40}\text{Ca}{}^{42}\text{Ca}$ is used to monitor enrichments in ⁴⁰Ca and can be measured to ±0.01% (2σ) using an exponential mass discrimination correction (Russell et al., 1978) and large ion currents. The earth's mantle has such a low $\text{K}\text{Ca}$ (～0.01) that it has retained “primordial” ${}^{40}\text{Ca}{}^{42}\text{Ca}\text{= 151.016}$ ± 0.011 (normalized to ${}^{42}\text{Ca}{}^{44}\text{Ca}\text{= 0.31221}$), as determined by measurements on two meteorites, pyroxene from an ultramafic nodule, metabasalt, and carbonatite. ${}^{40}\text{Ca}{}^{42}\text{Ca}$ ratios can be conveniently expressed relative to this value as ?_Ca in units of 10^?4. To test the method for age dating, a mineral isochron has been obtained on a sample of Pikes Peak granite, which has been shown to have concordant KAr, RbSr, and UPb ages. Plagioclase, K-feldspar, biotite, and whole rock yield an age of 1041 ± 32 m.y. (2σ) in agreement with previous age determinations (λ_K = 0.5543 b.y.^?1, $\text{λ}{}_{\mathrm{\beta ?}}\text{λ}{}_{\mathrm{<mtext>K</mtext>}}\text{= 0.8952}$, ⁴⁰K = 0.01167%). The initial ${}^{40}\text{Ca}{}^{42}\text{Ca}$ of 151.024 ± 0.016 (?_Ca = +0.5 ± 1.0), indicates that assimilation of high K/Ca crust was insufficient to affect calcium isotopes. Measurements on two-mica granite from eastern Nevada indicate that the magma sources had K/Ca ≈ 1, similar to intermediate-composition crustal rocks. These results show that the KCa system can be used as a precise geochromometer for common felsic igneous and metamorphic rocks, and may prove applicable to sedimentary rocks containing authigenic K minerals. The relatively short half-life of ⁴⁰K, the non-volatile daughter, and the fact that potassium and calcium are stoichiometric constituents of many minerals, make the KCa system complementary to other dating methods, and potentially applicable to a variety of geologic problems.  相似文献   

On the observation of 92Nb and 94Nb in nature     

K.E Apt  J.D Knight  D.C Camp  E.W Perkins 《Geochimica et cosmochimica acta》1974,38(9):1485-1488

Radioactivity measurements have shown evidence for long-lived ⁹²Nb and $\text{2.03 × 10}{}^4\text{yr}{}^{94}\text{Nb}$ in natural niobium. The specific activity of ⁹⁴Nb was observed to be 0.32 ± 0.03 dis/min. kg Nb and that of ⁹²Nb to be 0.058 ± 0.035 dis/min. kg Nb. With $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ taken as ≈ 1.7 × 10⁸yr, the isotopic abundance of ⁹⁸Nb is 1.2 × 10^?10 per cent.  相似文献   

The ²¹Ne production rate in stony meteorites estimated from ¹⁰Be and other radionuclides     

R.K Moniot  T.H Kruse  C Tuniz  W Savin  G.S Hall  T Milazzo  D Pal  G.F Herzog 《Geochimica et cosmochimica acta》1983,47(11):1887-1895

The ¹⁰Be contents of 28 stony meteorites with known ²¹Ne contents range from 0.97 to 23 dpm/kg and give an average ²¹Ne production rate (P₂₁) of (0.28 ± 0.02) × 10⁸ cm³ STP/g-Myr for shielding conditions corresponding to ${}^{22}\text{Ne}{}^{21}\text{Ne}\text{= 1.114}$ in an H-chondrite. Our $\text{P}{}_{21}\text{(}{}^{10}\text{Be)}$ agrees with others' P₂₁ based on ²²Na, ⁸¹Kr and ⁵³Mn but not on ²⁶A1. Temporal variations in the cosmic ray flux do not explain the disagreement satisfactorily; major errors in the radionuclide half-lives are not indicated. The discrepancy seems rooted in the data selection and the difficulties of making accurate corrections for shielding, chemical composition and other sources of variability.  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

  相似文献   

Constantes de formation des complexes hydroxydés de l'aluminium en solution aqueuse de 20 a 70°C     

Yves Couturier  Gil Michard  Gérard Sarazin 《Geochimica et cosmochimica acta》1984,48(4):649-659

Stability constants of hydroxocomplexes of Al(III):Al(OH)²⁺ and A1(OH)₄^? have been measured in the 20–70°C temperature range by reactions involving only dissolved species. The stability constant ${}^1\text{K}{}_1$ of the first complex ion is studied by measuring pH of solutions of aluminium salts at several concentrations. ${}^1\text{β}{}_4$ of aluminate ion is deduced from equilibrium constants of the reaction between the trioxalato aluminium (III) complex ion and Al³⁺ in acid medium, and between the same complex ion and A1(OH)₄^? in alkaline medium. The K values and the associated ΔH are ${}^1\text{K}{}_1\text{= 10}{}^{\mathrm{?5.00}}$ and ΔH₁ = 11.8 Kcal; ${}^1\text{β}{}_4\text{= 10}{}^{\mathrm{?22.20}}$ and ΔH₄ = 42.45 Kcal. These last results are not in agreement with the values of recent tables for $\text{ΔG}{}^0\text{?}$ and $\text{ΔH}{}^0\text{?}$ of Al³⁺ and Al(OH)₄^?. We suggest a consistent set of data for dissolved and solid Al species and for some aluminosilicates.  相似文献   

Strontium isotope geology of the South Mountain batholith,Nova Scotia     

D.B. Clarke  A.N. Halliday 《Geochimica et cosmochimica acta》1980,44(8):1045-1058

The South Mountain batholith of southwestern Nova Scotia is a large, peraluminous, granodiorite-granite complex which intrudes mainly greenschist facies metasediments of the Cambro-Ordovician Meguma Group. Using Rb-Sr isochrons constructed from whole rocks and mineral separates, the present study shows a variation in age and initial ratios of the intrusive phases of the batholith as follows: biotite granodiorite (371.8 ± 2.2 Ma, (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{\mathrm{i}}$ ranges from 0.7076 ± 0.0003 to 0.7090 ± 0.0003, with the average = 0.7081); adamellite (364.3 ± 1.3 Ma, $\text{(}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{\mathrm{i}}\text{= 0.70942 ± 35}$); porphyry (361.2 ± 1.4 Ma, $\text{(}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{\mathrm{i}}\text{= 0.71021 ± 119}$); using λ⁸⁷Rb = 1.42 × 10^?11yr^?1.A suite of Meguma country rock samples showed a variation of ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.7113?0.7177}$ at the time of intrusion of the batholith. A number of xenoliths of this material occurring in the marginal granodiorite had partially equilibrated isotopically with the granodiorite at a higher ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio than elsewhere in the granodiorites. This evidence demonstrates that isotopic (and probably some accompanying bulk chemical) contamination by the Meguma rocks has been an important factor in determining the ultimate chemical composition and mineralogy of the South Mountain batholith.The $\text{(}{}^{87}\text{Sr}{}^{86}\text{Sr}\text{)}{}_{372}\text{= 0.7081}$ of the early granodiorites indicates that the parent magma of the South Mountain batholith was derived from a source unlike the Meguma Group. The precise nature of the source region cannot be determined by Rb-Sr work unless the degree of contamination with Megumalike material is known.  相似文献   

Rates of reaction of pyrite and marcasite with ferric iron at pH 2     

C.L Wiersma  J.D Rimstidt 《Geochimica et cosmochimica acta》1984,48(1):85-92

The relative reactivities of pulverized samples (100–200 mesh) of 3 marcasite and 7 pyrite specimens from various sources were determined at 25°C and pH 2.0 in ferric chloride solutions with initial ferric iron concentrations of 10^?3 molal. The rate of the reaction: was determined by calculating the rate of reduction of aqueous ferric ion from measured oxidation-reduction potentials. The reaction follows the rate law: where m_Fe³⁺ is the molal concentration of uncomplexed ferric iron, k is the rate constant and $\text{A}\text{M}$ is the surface area of reacting solid to mass of solution ratio. The measured rate constants, k, range from 1.0 × 10^?4 to 2.7 × 10^?4 sec^?1 ± 5%, with lower-temperature/early diagenetic pyrite having the smallest rate constants, marcasite intermediate, and pyrite of higher-temperature hydrothermal and metamorphic origin having the greatest rate constants. Geologically, these small relative differences between the rate constants are not significant, so the fundamental reactivities of marcasite and pyrite are not appreciably different.The activation energy of the reaction for a hydrothermal pyrite in the temperature interval of 25 to 50°C is 92 kJ mol^?1. This relatively high activation energy indicates that a surface reaction controls the rate over this temperature range. The BET-measured specific surface area for lower-temperature/early diagenetic pyrite is an order of magnitude greater than that for pyrite of higher-temperature origin. Consequently, since the lower-temperature types have a much greater $\text{A}\text{M}$ ratio, they appear to be more reactive per unit mass than the higher temperature types.  相似文献   

Influences of kinetics and mechanism in metamorphism: a study of albite crystallization     

Alan Matthews 《Geochimica et cosmochimica acta》1980,44(3):387-402

  相似文献   

The ionization of acids in estuarine waters     

Frank J Millero 《Geochimica et cosmochimica acta》1981,45(11):2085-2089

The stoichiometric, $\text{K}{}_{\mathrm{HA}}{}^1$, and apparent, K'_HA, constants for the ionization of a number of weak acids (NH₄⁺, HSO₄^?, HF, H₂O, B(OH)₃, H₂CO₃, HCO₃^?, H₃PO₄, H₂PO₄^?, HPO₄², H₃AsO₄ H₂AsO₄^? and HAsO₄^2?) in seawater at 25°C diluted with water have been fitted to equations of the form (Millero, 1979). In $\text{K}{}_{\mathrm{HA}}{}^1\text{= In K}{}_{\mathrm{HA}}\text{+ AS}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{+ BS}$ where In K_HA is the thermodynamic constant in water, S is the salinity, A and B are adjustable parameters. The validity of this equation in estuarine waters has been examined by using an ion pairing model (Millero and Schreiber, 1981). The calculated values of $\text{K}{}_{\mathrm{HA}}{}^1$ and K'_HA at S = 35%. are in good agreement with the measured values for all the systems examined. The equation used to extrapolate the measured values to pure water K_HA predicted values that agreed with those determined by using the ion pairing model. The exception was the ionization of HPO₄^2? due to the strong interactions of Ca²⁺ and Mg²⁺ with PO₄^3?. The differences in the predicted values of $\text{K}{}_{\mathrm{HA}}{}^1$ in seawater diluted with pure water and average river water were very small for all the acids except HPO₄^2? (the maximum ΔpK = 0.96 in average river water). The larger difference in the $\text{K}{}_{\mathrm{HA}}{}^1$ for HPO₄^2? in river waters is due to the strong interactions of Ca²⁺ and PO₄^3?.  相似文献   

Experimental paleotemperature equation for planktonic foraminifera     

Jonathan Erez  Boaz Luz 《Geochimica et cosmochimica acta》1983,47(6):1025-1031

Small live individuals of Globigerinoides sacculifer which were cultured in the laboratory reached maturity and produced garnets. Fifty to ninety percent of their skeleton weight was deposited under controlled water temperature (14° to 30°C) and water isotopic composition, and a correction was made to account for the isotopic composition of the original skeleton using control groups.Comparison of. the actual growth temperatures with the calculated temperature based on paleotemperature equations for inorganic CaCO₃ indicate that the foraminifera precipitate their CaCO₃ in isotopic equilibrium. Comparison with equations developed for biogenic calcite give a similarly good fit. Linear regression with Craig's (1965) equation yields: $\text{t = ?0.07 + 1.01}\text{t}\text{?}\text{(r= 0.95)}$ where t is the actual growth temperature and $\text{t}\text{?}$ Is the calculated paleotemperature. The intercept and the slope of this linear equation show that the familiar paleotemperature equation developed originally for mollusca carbonate, is equally applicable for the planktonic foraminifer G. sacculifer.Second order regression of the culture temperature and the delta difference (δ¹⁸Oc ? δ¹⁸Ow) yield a correlation coefficient of r = 0.95: $\text{t}\text{?}\text{= 17.0 ? 4.52(δ}{}^{18}\text{Oc ? δ}{}^{18}\text{Ow) + 0.03(δ}{}^{18}\text{Oc ? δ}{}^{18}\text{Ow)}{}^2$$\text{t}\text{?}\text{, δ}{}^{18}\text{Oc}$ and δ¹⁸Ow are the estimated temperature, the isotopic composition of the shell carbonate and the sea water respectively.A possible cause for nonequilibnum isotopic compositions reported earlier for living planktonic foraminifera is the improper combustion of the organic matter.  相似文献   

I-Xe and ⁴⁰Ar-³⁹Ar dating of silicate from Weekeroo Station and Netschaëvo IIE iron meteorites     

Sidney Niemeyer 《Geochimica et cosmochimica acta》1980,44(1):33-44

Silicate inclusions from two IIE iron meteorites were dated by the I-Xe and ⁴⁰Ar-³⁹Ar techniques. Weekeroo Station, a ‘normal’ IIE iron, shows no loss of radiogenic ⁴⁰Ar at low temperature, and the well-defined ⁴⁰Ar-³⁹Ar plateau yields an age of 4.54 ± 0.03 Byr. The xenon data define a good I-Xe correlation with an age of +10.9 ± 0.5 Myr relative to Bjurböle [the monitor error (±2.5 Myr) is not included].^Despite its relatively young age, Weekeroo Station's $\text{(}{}^{129}\text{Xe}{}^{132}\text{Xe)}{}_{\mathrm{trappad}}$ ratio (= 0.84 ± 0.05) lies significantly below the solar value. Netschaëvo silicate has a chondritic composition, unlike ‘normal’ IIE silicate which is more differentiated. Nevertheless Netschaëvo gives a ⁴⁰Ar-³⁹Ar plateau-age of only 3.79 ± 0.03 Byr, with the xenon data failing to define an I-Xe isochron.Only irons from the IAB and IIE groups contain silicate inclusions, but these two groups differ in many other respects, mostly suggesting that IAB meteorites are more primitive. The I-Xe chronology supports this suggestion inasmuch as Weekeroo Station formed well after (8–15 Myr) IAB silicates. In terms of Scott and Wasson's (1976) model, ages for Weekeroo Station date the shock event which formed ‘normal’ IIE irons by mixing the low-melting fraction of the parent silicate with shock-liquefied metal. Scott and Wasson's suggestion that Netschaëvo represents IIE parent material, however, is contradicted by Netschaëvo's 3.8 Byr age.The four silicate-bearing IIE irons which have now been dated can be subdivided into distinct pairs: Weekeroo Station and Colomera formed near the beginning of the solar system, while Netschaëvo and Kodaikanal both formed only 3.8 Byr ago. A review of other properties of these meteorites generally support this subdivision.This work underscores the complexity of the history of IIE meteorites; in particular, an adequate model must account for the formation of two IIE irons at 3.8 Byr without disturbing rare gases in Weekeroo Station. All formation models are quite speculative, but the one which seems best to fit the available evidence postulates two parent bodies: the 3.8 Byr old silicate formed on one parent body, all other IIE material resided in a separate body, and subsequent collision(s) mixed the young silicate with IIE metal.  相似文献   

Thermochemical redox equilibria of ZoBell's solution     

Darrell Kirk Nordstrom 《Geochimica et cosmochimica acta》1977,41(12):1835-1841

The redox potential of ZoBell's solution, consisting of $\text{3.33 × 10}{}^{\mathrm{?3}}$ molar K₄Fe(CN)₆, $\text{3.33 × 10}{}^{\mathrm{?3}}$ molar K₃Fe(CN)₆ and 0.10 molar KCl, has been measured by a polished platinum electrode vs a saturated KCl, Ag/AgCl reference electrode. Measurements in the temperature range 8–85°C fit the equation $\text{E(}\text{volts}\text{) = 0.23145 ? 1.5220 × 10}{}^{\mathrm{?3}}\text{(t ? 25) ? 2.2449 × 10}{}^{\mathrm{?6}}\text{(t ? 25)}{}^2$ where $\text{t}$ is in degrees Celsius. Evaluation of literature data was necessary to obtain a reliable value for the Ag/AgCl half-cell reference potential as a function of temperature. Combining the measurements from this study with the literature evaluation of the Ag/AgCl reference potential yields the temperature dependent potential for ZoBell's solution: $\text{E(}\text{volts}\text{) = 0.43028 ? 2.5157 × 10}{}^{\mathrm{?3}}\text{(t ? 25) ? 3.7979 × 10}{}^{\mathrm{?6}}\text{(t ? 25)}{}^2$ relative to the standard hydrogen potential. From these data the enthalpy, entropy, free energy and heat capacity for the ferro-ferricyanide redox couple have been calculated. The temperature equation for the potential of ZoBell's solution may be used for checking potentiometric equipment in the determination of the redox potential of natural waters.  相似文献   

Thermodynamics of brine-salt equilibria — II. The system NaCl-KCl-H₂O from 0 to 200°C     

James R Wood 《Geochimica et cosmochimica acta》1976,40(10):1211-1220

Thermodynamic functions describing salt solubilities and activities in the system NaCl-KCl-H₂O have been obtained over the temperature range 0–200°C using existing isopiestic and solubility data. The two-parameter equation used previously at 25°C was sufficient to characterize the activity coefficient of the aqueous binary systems from infinite dilution to saturation, and the mixing behavior could be described by Harned's Rule over this temperature range. Activity coefficients have been calculated for NaCl-KCl-H₂O mixtures as a function of temperature using the expression: $\text{log γ}{}_{\mathrm{\pm A}}\text{= ? Aγ√I/(1+}\text{a}\text{?}\text{Bγ√I) + B}{}^{\mathrm{.}}\text{I + α}{}_{\mathrm{AB}}\text{I}$. Calculated solubility curves are in good agreement with the experimental data up to 200°C and estimates of ΔH⁰_(sat) and ΔC⁰_p(sat) calculated from the first and second temperature derivatives of log K are consistent with measured standard state enthalpies and heat capacities for both NaCl and KCl. The absolute value of the mixing parameters, α_AB, decrease with increasing temperature and approach zero for both salts in the vicinity of 200°C. The excess free energy of mixing thus approaches zero and may be due to the transition from a dissociated electrolyte solution to one in which associated ions or molecular species are beginning to dominate over ‘free’ ions. The mixing parameter decreases more rapidly for KCl than for NaCl and suggests that KCl-rich systems become associated at lower temperatures than NaCl-rich systems.  相似文献   

Rare gases and ³⁶Cl in stony-iron meteorites: cosmogenic elemental production rates,exposure ages,diffusion losses and thermal histories     

F Begemann  H.W Weber  E Vilcsek  H Hintenberger 《Geochimica et cosmochimica acta》1976,40(3):353-368

Metal and silicate portions from 13 mesosiderites, one pallasite, Bencubbin (“unique”) and Udei Station (‘iron with silicate inclusions’) have been analysed for their content of He, Ne and Ar; in most cases ³⁶Cl could be determined as well. ³⁶Cl-³⁶Ar cosmic ray exposure ages fall between 10 and 160 Myr. Half of the metal samples show a deficit of spallogenic ³He (up to 30%) which we ascribe to a loss of tritium. The observed depletion of ³He in the silicates is correlated with their mineralogical composition: feldspar has lost its ³He in all cases, pyroxene definitely in one and possibly in five others, while olivine has been affected in only two meteorites. The thermal histories during their exposure to the cosmic radiation have been different for different meteoroids. Nevertheless, with the exception of Veramin, the data are compatible with the assumption of a continuous diffusion loss during a considerable fraction of the exposure era. For Veramin, however, an episodic event late in the exposure history is required. The exceptionally high ${}^{39}\text{Ar}{}^{36}\text{Cl}$ ratio in the metal, which is due to a high ³⁹Ar activity, indicates that the event occurred during the last 500,000 years or so and resulted in an extremely excentric orbit (large aphelion).Production rates of ^38,39Ar from Ca and ^21,22Ne from Mg are given. The ratio $\text{P}{}^{38}{}_{\mathrm{Ca}}\text{P}{}^{21}{}_{\mathrm{Mg}}$ is close to unity. The ratios $\text{P}{}^{38}{}_{\mathrm{Ca}}\text{P}{}^{38}{}_{\mathrm{Fe}}$ vary between 20 and 50, and are not correlated with the absolute production rate of ³⁸Ar from metal. The ${}^{22}\text{Ne}{}^{21}\text{Ne}$ production ratio from Mg is found to be close to but below unity.Of the mesosiderites only Veramin shows unambiguous evidence for primordial rare gases with larger amounts and a higher ${}^{20}\text{Ne}{}^{36}\text{Ar}$ ratio in the olivine, suggesting in situ fractionation to have at least been partly responsible for the abundance pattern found. Bencubbin contains large amounts of strongly fractionated primordial gases, but again part of the fractionation may have occurred in situ. Udei Station shows an excess of (3.5 ± 0.6) × 10^?10 cm³ STP ¹²⁹Xe/g in the non-magnetic portion.  相似文献   

Oxygen diffusion in amphiboles     

John R. Farver  Bruno J. Giletti 《Geochimica et cosmochimica acta》1985,49(6):1403-1411

The kinetics of oxygen isotope self-diffusion in natural samples of hornblende, tremolite, and richterite have been measured. Samples were run under hydrothermal conditions using ¹⁸O enriched water. Profiles of ${}^{18}\text{O}\text{(}{}^{16}\text{O +}{}^{18}\text{O)}$vs depth into the crystal were obtained using an ion microprobe; the depths of sputtered holes were measured using an optical interferometer. At 1000 bars (100 MPa) water pressure, the activation energies (Q) and pre-exponential factors (D₀) for diffusion parallel to c are: D₀(cm²/sec) Q (kcal/gm-atom) T (°C) Hornblende 1⁺²⁰_?1 × 10^?741 ± 6 650–800 Tremolite 2⁺³⁰_?2× 10^?8 39 ± 5 650–800 Richterite 3⁺⁵_?2 × 10^?4 57 ± 2 650–800The diffusion coefficient (D) for hornblende at 800°C and 1000 bars water pressure measured parallel to the c crystallographic direction is at least ten times greater than that parallel to the a or b directions. An increase in water pressure from 200 to 2000 bars increases D by a factor of 2.7 for diffusion parallel to c at 800°C. The D value for hornblende at 800°C is about 0.01 that for quartz and 0.001 that for anorthite. As a result, closure temperatures for oxygen exchange in natural primary amphiboles are significantly higher than for quartz or feldspars. It is unlikely that amphiboles will exchange oxygen isotopes by diffusion under most crustal conditions.  相似文献   

The solubility of noble gases in water and in NaCl brine     

S.P. Smith  B.M. Kennedy 《Geochimica et cosmochimica acta》1983,47(3):503-515

A direct-sampling, mass-spectrometric technique has been used to measure simultaneously the solubilities of He, Ne, Ar, Kr, and Xe in fresh water and NaCl brine (0 to 5.2 molar) from 0° to 65 °C, and at 1 atm total pressure of moist air. The argon solubility in the most concentrated brines is 4 to 7 times less than in fresh water at 65 °C and 0°C, respectively. The salt effect is parameterized using the Setschenow equation. where M is NaCl moiarity, βⁱ_o(T) and βⁱ(T) the Bunsen solubility coefficients for gas i in fresh water and brine, and Kⁱ_M(T) the empirical salting coefficient. Values of Kⁱ_M(T) are calculated using volumetric concentration units for noble gas and NaCl content and are independent of NaCl molarity. Below about 40°C, temperature coefficients of all Kⁱ_M are negative. The value of K^He_M is a minimum at 40°C. K^Ar_M decreases from about 0.40 at 0°C to 0.28 at 65 °C. The absolute magnitudes of the differences in salting coefficients (relative to K^Ar_M) decrease from 0° to 65°C. Over the range of conditions studied, all noble gases are salted out, and K^He_M ? K^Ne_M < K^Ar_M < K^Kr_M < K^Xe_M.From the solubility data, we calculated $\text{Δ}\text{G}{}^0{}_{\mathrm{tr}}$, $\text{Δ}\text{S}{}^0{}_{\mathrm{tr}}$, $\text{Δ}\text{H}{}^0{}_{\mathrm{tr}}$ and $\text{Δ}\text{C}{}^{\mathrm{O}}{}_{\mathrm{p,tr}}$ for the transfer of noble gases from fresh water to 1 molar NaCl solutions. At low temperatures $\text{Δ}\text{S}{}^0{}_{\mathrm{tr}}$, is positive, but decreases and becomes negative at temperatures ranging from about 25°C for He to 45°C for Xe. At low temperatures, the dissolved electrolyte apparently interferes with the formation of a cage of solvent molecules about the noble gas atom. At higher temperatures, the local environment of the gas atom in the brine appears to be slightly more ordered than in pure water, possibly reflecting the longer effective range of the ionic fields at higher temperature.The measured solubilities can be used to model noble gas partitioning in two-phase geothermal systems at low temperatures. The data can also be used to estimate the temperature and concentration dependence of the salt effect for other alkali halides. Extrapolation of the measured data is not possible due to the incompletely-characterized minima in the temperature dependence of the salting coefficients. The regularities in the data observed at low temperatures suggest relatively few high-temperature data will be required to model the behavior of noble gases in high-temperature geothermal brines.  相似文献