Further ${}^{40}\text{A}\text{r}{}^{39}\text{Ar}$ evidence for the multi-collisional heating of the Kirin chondrite     

T.Mark Harrison  Wang Sungshan 《Geochimica et cosmochimica acta》1981,45(12):2513-2517

Results of an ${}^{40}\text{Ar}{}^{39}\text{Ar}$Ar age spectrum obtained on a sample of the Kirin chondrite (K-5-13) show a similar character to previous published analyses of Kirin samples K-1 and K-2. The K-5-13 age spectrum shows clear evidence of having been substantially outgassed during a presumed collisional event about 0.5 Ga, ago, in good agreement with the estimate obtained from K-2, The differing amounts of ⁴⁰Ar loss registered by K-2 and K-5-13 during the 0.5 Ga event of about 60 and 50%, respectively, allows calculation of their vertical separation in the parent body at about 10cm.  相似文献   

Astronomical tuning of the Tortonian ⁸⁷Sr/⁸⁶Sr curve in the Mediterranean basin     

Mario Sprovieri  Mario Barbieri  Adriana Bellanca  Rodolfo Neri 《地学学报》2003,15(1):29-35

ABSTRACT This work presents a detailed ⁸⁷Sr/⁸⁶Sr isotope curve for the interval 7.5–9.7 Ma obtained by a high-resolution analysis (sampling spacing of about 40 kyr) of an astronomically calibrated land-based sedimentary sequence exposed in the central Mediterranean area (Gibliscemi section, southern Sicily). The main aim is to verify a synchronous response of the Mediterranean seawater Sr isotope record to the oceanic forcing on the basis of multiple comparisons of the Gibliscemi record with published coeval ⁸⁷Sr/⁸⁶Sr curves. A good correlation with the ⁸⁷Sr/⁸⁶Sr data from the ODP site 926 (equatorial Atlantic ocean), considered to be the Sr chemostratigraphic reference section for the Late Miocene, and from the Pacific DSDP site 590B was registered. Conversely, the comparison of the Gibliscemi Sr isotope data with ⁸⁷Sr/⁸⁶Sr ratios from the coeval segment of the land-based Sardella section (eastern Mediterranean) shows important differences highlighting a local control on the seawater Sr isotope changes in semi-isolated subbasins within the Late Miocene Mediterranean.  相似文献   

The geochemical coherence of Pu and Nd and the ${}^{244}\text{Pu}{}^{238}\text{U}$ ratio of the early solar system     

John H. Jones 《Geochimica et cosmochimica acta》1982,46(10):1793-1804

Two very different sets of ${}^{244}\text{Pu}{}^{238}\text{U}$ ratios have been reported for early solar system materials. One group of samples yields high (0.015–0.016) ratios (Podosek, 1970a, 1972; Drozd et al., 1977) and calculations based on another group of analyses yield low ratios (~0.004) (Marti et al., 1977). Recently measured partition coefficients for Pu and Sm are used to evaluate the data of Marti et al. and $\text{sol}{}^{244}\text{Pu}{}^{238}\text{U}$ ratios from other sources are also considered. A low $\text{sol}{}^{244}\text{Pu}{}^{238}\text{U}$ ratio (~0.005) is favored, and some implications of this low ratio to galactic nucleosynthesis and meteorite age dating are briefly discussed.  相似文献   

⁴⁰Ar/³⁹Ar Constraints on the tectonic history and architecture of the ultrahigh-pressure Sulu orogen     

B. R. HACKER  S. R. WALLIS  M. O. MCWILLIAMS  P. B. GANS 《Journal of Metamorphic Geology》2009,27(9):827-844

New ⁴⁰Ar/³⁹Ar ages are presented from the giant Sulu ultrahigh-pressure (UHP) terrane and surrounding areas. Combined with U-Pb ages, Sm-Nd ages, Rb-Sr ages, inclusion relationships, and geological relationships, they help define the orogenic events before, during and after the Triassic collision between the Sino–Korean and Yangtze Cratons. In the Qinling microcontinent, tectonism occurred between 2.0 and 1.4 Ga. The UHP metamorphism occurred in the Yangtze Craton between 240 and 222 Ma; its thermal effect on the Qinling microcontinent was limited to partial resetting of K-feldspar ⁴⁰Ar/³⁹Ar ages. Subsequent unroofing at rates of 5–25 km Myr⁻¹ brought the UHP terrane to crustal levels where it underwent a relatively short amphibolite facies metamorphism. The end of that metamorphism is marked by ⁴⁰Ar/³⁹Ar ages in the 219–210 Ma range, implying cooling at crustal depths at rates of 50–200 °C Myr⁻¹. Ages in the 210–170 Ma range may reflect protracted cooling or partial resetting by Jurassic or Cretaceous magmatism. Jurassic 166–149 Ma plutonism was followed by cooling at rates of c. 15 °C Myr⁻¹, suggesting relatively deep crustal conditions, whereas Cretaceous 129–118 Ma plutonism was succeeded by cooling at rates of c. 50 C Myr⁻¹, suggesting relatively shallow crustal depths.  相似文献   

H T -XRD study of synthetic ferrian magnesian spodumene: the effect of site dimension on the P 2₁/ c → C 2/ c phase transition     

F. Cámara  G. Iezzi  R. Oberti 《Physics and Chemistry of Minerals》2003,30(1):20-30

 Ferrian magnesian spodumene was synthesized in the MLFSH system at P=0.4 GPa, T=700 °C, fO₂=NNO+2.3. The space group at room T is P2₁/c [a=9.638(3) ?, b=8.709(2) ?, c=5.258(2) ?, β=109.83(3)^∘, V=415.2 ?³]. The structure is topologically equivalent to that of ferrian spodumene, LiFeSi₂O₆, and has two symmetrically independent tetrahedral chains, A and B, and two independent octahedral sites, M1 and M2. The crystal-chemical composition was determined combining EMP, SIMS and single-crystal XRD analysis, yielding ^M2(Li_0.85Mg_0.09Fe²⁺ _0.06) ^M1(Fe³⁺ _0.85Mg_0.15)Si₂O₆. Li is ordered at the M2 site and Fe³⁺ is ordered at the M1 site, whereas Mg (and Fe²⁺) distribute over both octahedral sites. Structure refinements done at different temperatures (25, 70, 95, 125, 150 and 200 °C) allowed characterization of a reversible displacive P2₁/c→C2/c transition at 106 °C. Previous HT-XRD studies of Li-clinopyroxenes had shown that the transition temperature is inversely related to the size of the M1 cation. For the crystal of this work, the aggregate ionic radius at M1 is longer than that of ferrian spodumene, for which the transition temperature is −44 °C. The higher transition temperature observed can only be explained on the basis of the shorter aggregate radius at the M2 site (due to the presence of Mg substituting after Li), in keeping with the results obtained for ferromagnesian P2₁/c pyroxenes. The effects of all the chemical substitutions must be considered when modelling transition temperatures and thermodynamic behaviour in clinopyroxenes. Received: 7 May 2002 / Accepted: 23 October 2002  相似文献   

The estimation of the pK¹_HA of acids in seawater using the Pitzer equations     

Frank J. Millero 《Geochimica et cosmochimica acta》1983,47(12):2121-2129

The equations of Pitzer have been used to calculate the stoichiometric ionization constants, pK¹_HA, for acids in NaCl media at 25°C. The calculated results for the ionization of HAc, H₂O, B(OH)₃, H₂CO^?₃, H₃PO₄, H₂PO^?₄, HPO^2?₄, H₃AsO₄, H₂AsO^?₄ and HAsO^2?₄ are in good agreement with the measured values, providing higher order interaction terms (θ and ψ) are used. The pK¹_HA measurements of these acids in NaCl media containing Mg²⁺ and Ca²⁺ were used to determine Pitzer specific interaction parameters at I = 0.7. With these Pitzer coefficients, it was possible to make reliable estimates for the activity coefficients of anions in seawater (S = 35) that form strong interactions with Mg²⁺ and Ca²⁺. The calculated activity coefficients yield reliable estimates for the pK¹_HA of acids in seawater.  相似文献   

钙长辉长无球粒陨石NWA 12274/12275/12276/12277/12278稀有气体及年代学研究     

张川统  缪秉魁  贺怀宇  苏菲  解庆林 《矿物岩石地球化学通报》2021,40(5):1143-1153

岩石矿物学研究发现HED (Howardite-Eucrite-Diogenite)族陨石NWA 12274/12275/12276/12277/12278与NWA 11586可能为成对陨石,但需要稀有气体等证据支持.本次工作对这5块陨石开展了稀有气体、宇宙暴露(CRE)年龄和气体保存年龄研究.结果 发现它们具有相一致的稀有气体同位素比值和CRE年龄,表明NWA系列陨石互为降落成对陨石;陨石CRE年龄[(36.2±3.7)Ma]与灶神星主要溅射事件/时期年龄(35.0～42.5 Ma)一致,表明它们可能在约36 Ma前从灶神星或Ⅴ型小行星溅离;陨石的U、Th-4 He(T4)及40K-40Ar(T40)气体保存年龄分别为0.3～1.3 Ga和2.8～3.4 Ga,显示这些陨石不同程度受到了后期热事件的影响;该流星体进入大气层的初始半径约为12.2 em,在穿越大气层期间质量损失了约56％.  相似文献   

BVI_c photometry of classical cepheids from the ASAS-3 catalog     

L. N. Berdnikov  A. Yu. Kniazev  R. Sefako  V. V. Kravtsov  E. N. Pastukhova  S. V. Zhuiko 《Astronomy Reports》2011,55(9):816-823

In 2008–2010, we obtained 6095 CCD frames in the BV I _c photometric systemfor 49 classical Cepheids discovered in the ASAS-3 project. The observations were obtained on the 76-cm telescope of the South African Astronomical Observatory (SAAO, RSA) and the 40-cm telescope of the Cerro Armazones Observatory of the Catholic University of the North (OCA, Chile), with a SBIGCCD ST-10XME camera. We present tabulated observations, light curves, and revised elements for the brightness variations.  相似文献   

Geochronological Constraints Using ⁴⁰Ar/³⁹Ar Dating on the Mineralization of the Hishikari Epithermal Gold Deposit, Japan     

Kenzo Sanematsu    Robert Duncan  Akira Imai  Koichiro Watanabe 《Resource Geology》2005,55(3):249-266

Abstract. Ages for thirty adularia samples collected from various veins were in the Hishikari gold deposit determined by ⁴⁰Ar/³⁹Ar dating to constrain the timing of adularia‐quartz vein formation and to determine the temporal change in temperature of hydrothermal fluid. Plateau ages were obtained from all adularia samples, and significant excess ⁴⁰Ar is not recognized from inverse isochrones. The duration of mineralization within individual veins was determined by adularia ages from the early and late stages of mineralization within the same vein. The durations of mineralization in the Daisen‐1, Daisen‐3, Hosen‐2 and Keisen‐3 veins in the Honko‐Sanjin zone were 7,000, 140,000, 160,000 and 170,000 years, respectively. The durations of mineralization in the Seisen‐2 and Yusen‐1–2 veins in the Yamada zones were 360,000 and 320,000 years, respectively. Mineralization lasted for a relatively longer period in individual veins at the Yamada zone. Mineralization ages from the Honko‐Sanjin zone range from 1.04 to 0.75 Ma, and most mineralization ages are concentrated in a short period from 1.01 to 0.88 Ma. In contrast, mineralization ages for the Yamada zone range from 1.21 to 0.64 Ma. These results indicate that fracturing and subsequent vein formation lasted for a longer period in the Yamada zone (about 570,000 years) compared with those events in the Honko‐Sanjin zone (about 290,000 years). The homogenization temperatures of liquid‐rich fluid inclusions in columnar adularia used for age determination were determined to be 223°C on average, and most of these temperatures range from 180 to 258d?C. No significant temporal change in homogenization temperature is recognized in this study. However, adularia in the Keisen veins indicated higher homogenization temperatures compared with elsewhere in the deposit, suggesting that the principal ascent of mineralizing hydrothermal fluid was via the Keisen veins.  相似文献   

U/Pb and Sm/Nd geochronologic studies of the eastern Borborema Province, Northeastern Brazil: initial conclusions     

W. R. Van Schmus  B. B. de Brito Neves  P. Hackspacher  M. Babinski   《Journal of South American Earth Sciences》1995,8(3-4)

The Borborema Province of NE Brasil comprises the central part of a wide Pan-African-Brasiliano orogenetic belt that formed as a consequence of late Neoproterozoic convergence and collision of the São Luis-West Africa craton and the São Francisco-Congo-Kasai cratons. New Sm/Nd and U/Pb results from the eastern part of this province help to define the basic internal architecture and pre-collisional history of this province, with particular emphasis on delineating older cratonic terranes, their fragmentation during the Mesoproterozoic, and their assembly into West Gondwana during the Pan African-Brasiliano orogeny at ca. 600 Ma.The region can be divided into three major geotectonic domains: a) Rio Piranhas-Caldas Brandão massif, with overlying Paleoproterozoic to Neoproterozoic supracrustal rocks, north of the Patos Lineament; b) the Archean to Paleoproterozoic São Francisco craton (SFC) to the south; and c) a complex domain of Paleoproterozoic to Archean basement blocks with several intervening Mesoproterozoic to Neoproterozoic fold belts in the center (south of Patos Lineament and north of SFC). The northern and central domains comprise the Borborema Province.Archean basement gneiss and Transamazonian granulite of northern SFC are exposed in the southern part of the central domain, underlying southern parts of the Sergipano fold belt. Basement in the Rio Piranhas massif appears to consist mostly of Transamazonian (2.1 to 2.2 Ga) gneissic rocks; Nd model ages (T_DM) of ca. 2.6 Ga for 2.15 Ga gneisses indicate a substantial Archean component in the protoliths to these gneisses. The Caldas Brandão massif to the east yields both Transamazonian and Archean U/Pb zircon and Nd (T_DM) ages, indicating a complex architecture. Metasedimentary rocks of the Jucurutu Formation yield detrital zircons with original crystallization ages as young as 1.8 Ga, indicating that these rocks may be late Paleoproterozoic and correlate with other ca. 1.8 Ga cratonic supracrustal rocks in Brazil such as the Roraima Group and Espinhaço Group.Most metavolcanic and pre-Brasiliano granitic units of the Sergipano (SDS), Pajeú-Paraíba (SPP), Riacho Pontal (SRP), and Piancó-Alto Brígida (SPAB) fold belts in the central domain formed ˜ 1.0 ± 0.1 Ga, based on U/Pb ages of zircons. Nd model ages (T_DM) for these same rocks, as well as Brasiliano granites intruded into them and large parts of the Pernambuco-Alagoas massif, are commonly 1.3–1.7 Ga, indicating that rocks of the fold belts were not wholly derived from either older (> 2.1 Ga) or juvenile (ca. 1.0 Ga) crust, but include mixtures of both components. A simple interpretation of Brasiliano granite genesis and the Nd data implies that there is no Transamazonian or Archean basement underlying large parts of these fold belts or of the Pernambuco-Alagoas massif. An exception is a belt of syenitic Brasiliano plutons (Syenitoid Line) and host gneisses between SPAB and SPP that clearly has a Transamazonian (or older) source. In addition, there are several smaller blocks of Archean to Transamazonian gneiss that can be defined within and among these fold belts. These blocks do not appear to constitute a continuous basement complex, but appear to be isolated older crustal fragments.Our data support a model in which ca. 1.0 Ga rifting was an important tectonic and crust-forming event along the northern edge of the São Francisco craton. Our data also show that significant parts of the Borborema Province are not remobilized Transamazonian to Archean crust, but that Mesoproterozoic crust is a major feature of the Province. There are several small remnants of older crust within the area dominated by Mesoproterozoic crust, suggesting that the rifting event created several small continental fragments that were later incorporated into the Brasiliano collisional orogen. We cannot at present determine if the Rio Piranhas-Caldas Brandão massifs and the older crustal blocks of the central domain were originally part of the São Francisco craton or whether some (or all) of them came from more exotic parts of the Proterozoic Earth. Finally, our data have not yet revealed any juvenile terranes of either Transamazonian or Brasiliano age.  相似文献   

Uplift rates of the Loyalty Islands as determined by ${}^{230}\text{Th}{}^{234}\text{U}$ dating of raised coral terraces     

John F. Marshall  Jean Launay 《Quaternary Research》1978,9(2):186-192

Uplift rates of the Loyalty Islands (S.W. Pacific) have been determined from ${}^{230}\text{Th}{}^{234}\text{U}$ dating of raised coral terraces standing 2 to 7.5 m above sea level. The ages of the terraces on Ouvéa and Lifou correspond closely to previously documented periods of high sea levels at 120,000 and 180,000 yr BP. A +2-m terrace on Beautemps-Beaupré is considered to be beyond the range of the dating technique. The uplift rates of the various islands show a decrease toward the west. This lends support to the hypothesis of a lithospheric bulge of the leading edge of the Australian plate prior to its subduction at the level of the New Hebrides trench.  相似文献   

Eclogite facies metarodingites – phase relations in the system SiO₂-Al₂O₃-Fe₂O₃-FeO-MgO-CaO-CO₂-H₂O: an example from the Zermatt-Saas ophiolite     

X-P. LI  M. RAHN  K. BUCHER 《Journal of Metamorphic Geology》2008,26(3):347-364

Eclogite facies metarodingites occur as deformed dykes in serpentinites of the Zermatt‐Saas ophiolite (Western Alps). They formed during the subduction of the Tethys oceanic lithosphere in the Early Tertiary. The metarodingites developed as a consequence of serpentinization of the oceanic mantle. Three major types of metarodingites (R1, R2 & R3) can be distinguished on the basis of their mineralogical composition. All metarodingites contain vesuvianite, chlorite and hydrogrossular in high modal amounts. In addition they contain: R1 – diopside, tremolite, clinozoisite, calcite; R2 – hydroandradite, diopside, epidote, calcite; and R3 – hydroandradite. Both garnets contain a small but persistent amount of hydrogarnet component. The different metarodingites reflect different original dyke rocks in the mantle. In each group of metarodingite, textural relations suggest that reactions adjusted the assemblages along the P–T path travelled by the ophiolite during subduction and exhumation. Reactions and phase relations derived from local textures in metarodingite can be modelled in the eight‐component system: SiO₂‐Al₂O₃‐Fe₂O₃‐FeO‐MgO‐CaO‐CO₂‐H₂O. This permits the analysis of redox reactions in the presence of andradite garnet and epidote in many of the rocks. Within this system, the phase relations in eclogite facies metarodingites have been explored in terms of T–X_CO2, T–μ(SiO₂), μ(Cal)–μ(SiO₂) and P–T sections. It was found that rodingite assemblages are characterized by low μ(SiO₂) and low X_CO2 conditions. The low SiO₂ potential is externally imposed onto the rodingites by the large volume of antigorite‐forsterite serpentinites enclosing them. Moreover, μ(SiO₂) decreases consistently from metarodingite R1 to R3. The low μ(SiO₂) enforced by the serpentinites favours the formation of hydrogarnet and vesuvianite. Rodingite formation is commonly associated with hydrothermal alteration of oceanic lithosphere at the ocean floor, in particular to ocean floor serpentinization. Our analysis, however, suggests that the metarodingite assemblages may have formed at high‐pressure conditions in the subduction zone as a result of serpentinization of oceanic mantle by subduction zone fluids.  相似文献   

A 3.1 billion year old marble and the ⁸⁷Sr/⁸⁶Sr of late-Archean seawater     

Zachariah 《地学学报》1998,10(6):312-316

A marble band in the ≈ 2.75 Gyr old Ramagiri schist belt in the Dharwar craton of south India gave a Pb–Pb age of 3.075 ± 0.095 Gyr. The geochemical data, including high Sr and low Ba and Mn indicate seawater origin for the parent rock, and that there was insignificant geochemical exchange between the marble and the surrounding rocks. The calculated initial Nd isotopic composition and μ₁ indicate an older continental crustal source for the Nd and Pb. The initial ⁸⁷Sr/⁸⁶Sr of the marble is 0.70128, which is higher than the calculated mantle value at ≈ 3 Ga. Although pre-3 Gyr old marine carbonate rocks are thought to be buffered by mantle Sr, the Ramagiri marble contains evolved, crustal Sr. Despite this, the marble has the lowest measured ⁸⁷Sr/⁸⁶Sr among carbonates and represents one of the least radiogenic periods in seawater Sr isotope composition.  相似文献   

⁸⁷Sr/⁸⁶Sr composition of seawater during the Phanerozoic     

Ján Veizer  William Compston 《Geochimica et cosmochimica acta》1974,38(9):1461-1484

${}^{87}\text{Sr}{}^{86}\text{Sr}$ measurements of 108 sedimentary carbonate rocks have been used to trace variations in the strontium isotopic composition of seawater during the Phanerozoic. The lowest ⁸⁷Sr/⁸⁶Sr observed for any suite of carbonates is taken as the best approximation to the value in well-mixed contemporary seawater. Our data support the existence of low ${}^{87}\text{Sr}{}^{86}\text{Sr}$ in the Cretaceous and Late Jurassic but they do not support further structure beyond a general trend through the Phanerozoic, which may correlate with the continental denudation rate.  相似文献   

Reconsideration of the age of blueschist facies metamorphism on the Seward Peninsula, Alaska, based on phengite ⁴⁰Ar/³⁹Ar results     

K. A. HANNULA  M. O. McWILLIAMS 《Journal of Metamorphic Geology》1995,13(1):125-139

Abstract ⁴⁰Ar/³⁹Ar step-heating and single-grain laser fusion ages from phengites from the polydeformed and polymetamorphosed blueschist-greenschist facies Nome Group fall into two groups. Samples from the upper part of the structural section that have experienced a relatively weak metamorphic and deformational post-blueschist facies overprint and one sample from the Cape Nome orthogneiss yield plateau ages of 116-125 Ma. More intensely overprinted samples yield hump-shaped spectra with minimum ages of 123 Ma and maximum ages of 334 Ma. Samples with hump-shaped spectra are derived from a greater structural depth than most samples with plateau ages. Unreasonably old maximum ages from some of the disturbed spectra suggest that the hump-shaped spectra result from the incorporation of excess ⁴⁰Ar. This interpretation conflicts with previous interpretations of similarly disturbed spectra from the Brooks Range, which have been argued to provide minimum ages for blueschist facies metamorphism. Since the maximum temperatures achieved by all samples were probably above the blocking temperature of Ar in phengite, the 116-125 Ma plateau ages are a minimum age for blueschist facies metamorphism on the Seward Peninsula, Alaska.  相似文献   

Thermochemical study of glasses in the system NaAlSi₃O₈-KAlSi₃O₈-Si₄O₈ and the join Na_1.6Al_1.6Si_2.4O₈-K_1.6Al_1.6Si_2.4O₈     

Richard L. Hervig  Alexandra Navrotsky 《Geochimica et cosmochimica acta》1984,48(3):513-522

Enthalpies of solution in 2PbO · B₂O₃ at 981 K have been measured for glasses in the system albite-orthoclase-silica and along the join Na_1.6Al_1.6Si_2.4O₈-K_1.6Al_1.6Si_2.4O₈. The join KAlSi₃O₈-Si₄O₈ shows zero heat of mixing similar to that found previously for NaAlSi₃O₈-Si₄O₈ glasses. Albite-orthoclase glasses show negative heats of mixing symmetric about Ab₅₀Or₅₀ (W_n = ? 2.4 ± 0.8 kcal). Negative heats of (Na, K) mixing are also found at $\text{Si}\text{(Si + Al)}\text{= 0.6.}$ Ternary excess enthalpies of mixing in the glassy system Ab-Or-4Q are positive but rarely exceed 1 kcal mol^?1.Using earlier studies of the thermodynamic properties of the crystals, the present calorimetric data and the “two-lattice” entropy model, the albite-orthoclase phase diagram is calculated in good agreement with experimental data. Attempts to calculate albite-silica and orthoclase-silica phase diagrams reveal complexities probably related to significant (but unknown) mutual solid solubility between cristobalite and alkali feldspar and to the very small heat and entropy of fusion of SiO₂.  相似文献   

Improved method for estimating the standard free energies of formation ($\text{ΔG}{}_{\mathrm{f,298.15}}{}^0$) of smectites     

Shas V. Mattigod  Garrison Sposito 《Geochimica et cosmochimica acta》1978,42(12):1753-1762

Two alternative empirical methods were developed for estimating the standard free energies of formation of layer silicate minerals with cation exchange capacity due to isomorphous substitutions. These methods involve the summation of either hydroxide or oxide components to form the desired smectite mineral similarly to a method developed by Nriagu, but differ in that (1) no ad hoc ‘calibrations’ are necessary for the $\text{ΔG}{}_{\mathrm{f,298.15}}{}^0$ values of alkali hydroxides and (2) a three-parameter equation is used for the correction factor, with the charge on the mineral due to isomorphous substitutions and the ionic radius and charge on the exchangeable cation taken as independent variables.A comparison between experimental and estimated $\text{ΔG}{}_{\mathrm{f,298.15}}{}^0$ values indicated that one of the methods (that involving the summation of hydroxide components) was significantly better than all other methods in predicting $\text{ΔG}{}_{\mathrm{f,298.15}}{}^0$ values of smectites while at the same time requiring no ad hoc thermochemical assumptions.  相似文献   

Some limitations of the R_f/φ technique of strain analysis — Discussion     

A.W.B. Siddans 《Tectonophysics》1981,72(1-2)

A recent note by De Paor (1980) suggests that finite strain cannot be decomposed into tectonic and compaction components on the basis of asymmetry of the R_fφ data about the mean φ trace. This is true if only the shape of the R_fφ data field is considered, but shown to be incorrect when the distribution of data within this field is considered.  相似文献   

Application of the ${}^{227}\text{Th}{}^{230}\text{Th}$ method to dating Pleistocene carbonates and comparison with other dating methods     

M. Gascoyne 《Geochimica et cosmochimica acta》1985,49(5):1165-1171

The ${}^{227}\text{Th}{}^{230}\text{Th}$ dating method is described in detail and its usefulness investigated by comparing ages of sixteen Pleistocene carbonates (mainly cave deposits) with those determined by the ${}^{231}\text{Pa}{}^{235}\text{U}$ and ${}^{230}\text{Th}{}^{234}\text{U}$ methods. The ${}^{227}\text{Th}{}^{230}\text{Th}$ ages are found to be critically dependent on corrections for decay of ²²⁷Th prior to alpha counting and ingrowth of daughter isotopes of ²³²Th derived from clastic detritus. Of nineteen sets of ages determined for the sixteen samples, good agreement is found for only seven sets. Differences are attributed to low U content of some samples and the possibility of excess ²²⁷Th in the calcite of samples younger than ~50 ky, possibly due to the coprecipitation of ²³¹Pa during formation. Calculated “negative” ${}^{227}\text{Th}{}^{230}\text{Th}$ ages may be a direct result of this process and the fact that, unlike the other methods, the activity ratio is non-zero at zero age. Nevertheless, the ${}^{227}\text{Th}{}^{230}\text{Th}$ is found to be a useful alternative dating technique for carbonates which are between ~50 and 300 ky, because no spiking is required. It also serves as a check for partial concordancy with ages dated by the other methods.  相似文献   

The variation of the marine ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio during Phanerozonic time: interpretation using a flux model     

Garrett W. Brass 《Geochimica et cosmochimica acta》1976,40(7):721-730

The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio in sea water has varied over geologic time due to the addition of strontium to the sea from rocks with a variety of ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios. The measurements by Petermanet al. (Geochim. Cosmochim. Acta34, 105–120, 1970) of the value of the marine ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio have been confirmed by several other workers and by some new measurements on JOIDES samples. They form the basis of a model calculation of the relative proportions of ‘basaltic’ (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.704}$) and ‘granitic’ (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.718}$) strontium being supplied to the sea. For the last 200 million years, the proportions of these two sources appear to reflect the history of global tectonics; ‘basaltic’ during rifting and increasingly ‘granitic’ during the present episodes of uplift and continental collision  相似文献