共查询到20条相似文献，搜索用时 31 毫秒
1.
Speleothem from West Virginia, ranging in age from 2000 to 200,000 yr B.P. contains uranium with ratios significantly greater than unity. This ratio varies from one speleothem to another, as does average U content. Initial ratios, corrected for age, are remarkably constant for a given speleothem. By contrast, ratios in seepage waters vary significantly from month to month at a given drip site, and their average values differ from that of the speleothem which they are depositing. This discrepancy is attributed either to long-term averaging-out of fluctuations, or fractional precipitation on the speleothem of a chemical species of uranium with a more constant ratio. Constancy of initial ratios from . datable portions of speleothems should permit -dating of older portions of the same speleothem, back to about 106 yr B.P., with estimated precision of ±5 per cent. 相似文献
2.
Matti Asikainen 《Geochimica et cosmochimica acta》1981,45(8):1375-1381
The relative abundance of 226Ra and 228Ra were determined in the groundwater from 125 drilled wells containing from < 0.1 to 51.3 pCi/l of 226Ra. The determination of 228Ra was carried out with a liquid scintillation counter by measuring only the weakly energetic β particles emitted from 228Ra. Thus the interference from the daughter nuclides of 226Ra was avoided, without specific separation of 228Ac. The direct measurement of 228Ra made the method decisively simpler and faster in terms of the chemistry involved.The concentration of 228Ra was found to be independent of the amount of 226Ra present in the samples. The concentrations of 228Ra were nearly the same over the whole range of 226Ra concentrations and the average ratio sharply increased as the 226Ra content of water increased. The ratio in the drilled wells varied from 0.3 to 26. Abnormally high ratios were found in areas with known uranium deposits as well as in several drilled wells at other locations. The abnormally high ratios present in groundwater suggest that the radioactivity anomaly is caused by uranium deposits and not by common rocks. In samples with a low radioactivity level the average ratio was slightly below unity, corresponding to the typical U/Th ratio of granite, the most common kind of rock in the study area. The samples from the rapakivi area proved to be exceptional in that they had a low ratio independent of the concentration of 226Ra. 相似文献
3.
Garrett W. Brass 《Geochimica et cosmochimica acta》1976,40(7):721-730
The ratio in sea water has varied over geologic time due to the addition of strontium to the sea from rocks with a variety of ratios. The measurements by Petermanet al. (Geochim. Cosmochim. Acta34, 105–120, 1970) of the value of the marine 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’ () and ‘granitic’ () 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
4.
KAr and ages have been determined for altered submarine tholeiitic and boninite (high-Mg andesite) lavas from the Dabi Volcanics, Cape Vogel Peninsula, Papua New Guinea. whole rock total fusion and plateau ages identify a Late Paleocene age for the tholeiitic lavas (58.9 ± 1.1 Ma) and also for the boninitic lavas (58.8 ± 0.8 Ma). Apparent KAr ages for the same samples range from 27.2 ± 0.7 to 63.9 ± 4.5 Ma, and young KAr ages for glassy boninites are probably due to variable radiogenic 40Ar (40Ar1) loss. These new ages effectively reconcile previously ambiguous age data for the Dabi Volcanics and indicate contemporaneous tholeiitic and boninitic volcanism occurring in southeast PNG during the Late Paleocene.Smectites, developed as alteration products after glass in oceanic lavas commonly do not retain 39Ar during or subsequent to irradiation, but in some cases may contain 40Ar1. In the absence of other factors modifying K and Ar contents, samples which have not lost 40Ar1 from smectite and suffer 39Ar loss only, are interpreted to have been altered immediately subsequent to the crystallization of the lava; whereas samples which have lost 40Ar1 as well as 39Ar may be the result of either recent alteration, or of continuous 40Ar1 loss since the time of crystallization. 相似文献
5.
M. Gascoyne 《Geochimica et cosmochimica acta》1985,49(5):1165-1171
The 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 and methods. The 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” 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 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
6.
John H. Jones 《Geochimica et cosmochimica acta》1982,46(10):1793-1804
Two very different sets of 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 ratios from other sources are also considered. A low 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
7.
Determinations of ages are reported for seven severely shock-heated chondrites. Shaw gives a plateau age of 4.29 Gyr. Louisville, Farmington, and Wickenburg give well-defined intercept ages of 0.5–0.6 Gyr. Orvinio, Arapahoe, and Lubbock show complex release curves, with age minima of 0.7–1.0 Gyr. Degassing times of 0.5–1.0 Gyr are suggested for these meteorites. Most severely shocked chondrites were apparently not totally degassed of 40Ar by the event, but retained from ~ 2 to ~45% of their 40Ar. When calculated values of the diffusion parameter, , for Ar are examined in Arrhenius plots, they show two distinct linear relationships, which apparently correspond to the degassing of different mineral phases with distinct ratios and different average temperatures for Ar release. The experimentally determined values of for the high temperature phase of several severely shocked chondrites are ~10?7 to 10?5sec?1 for their determined shock-heating temperatures of ~950°C to ~ 1200°C. The inferred reheating temperatures, values, and fraction of 40Ar loss during the reheating event for these seven chondrites suggest post-shock cooling rates and burial depth of ~ 10?2 10?4°C/sec and ~0.5–2m, respectively. For three chondrites these cooling rates agree with those determined from Ni diffusion in metal grains: for five chondrites the cooling rates derived from 40Ar and Ni disagree by a factor of ~105. It is suggested that five of these severely shocked chondrites were part of large ejecta blankets containing hot material and cold clasts with a distribution of sizes and that the cooling rate of this ejecta appreciably decreased as a function of time. 相似文献
8.
The ratios measured in 27 Southern Africa diamond stones, four from Premier Mine and the rest of unidentified origin, range from 4.2 × 10?8 to 3.2 × 10?4, with three stones above 1 × 10?4. We conclude that the initial helium isotopic ratio ( in the earth was significantly higher than that of the planetary helium-A (), but close to the solar helium ().The apparent K-Ar ages for the twelve diamonds of unidentified origin show enormously old age, indicating excess argon-40. evolution in diamonds suggests that the diamonds with the high ratio (>2 × 10?4) may be as old as the earth.Noble gas elemental abundance in the diamonds relative to the air noble gas abundance shows monotonie decrease with a decreasing mass number.This paper discusses the implications of these observations on the early solar system and the origin of diamonds. 相似文献
9.
The extent of oxygen isotopic exchange between detrital clay minerals and sea water was investigated by analyzing ratios of separated fine-grained size fractions of deep-sea sediments from three North Pacific ocean cores. Isotopic results were interpreted according to models based on the assumption that the extent of isotopic exchange should increase with decreasing particle size and increasing time of exchange between the sediment and sea water. The data indicate that information concerning the provenance and mode of formation of detrital clay minerals can be obtained from the ratios of the coarser-than-0.1 μm fraction of deep-sea sediments younger than several million years and the finer-than-0.1 μm fraction of deep-sea sediments younger than several tens of thousands of years. Furthermore, if the extent of chemical reaction between detrital clays and sea water is similar to the extent of oxygen isotopic exchange, such reaction may be important in regulating the chemistry of sea water. 相似文献
10.
age spectrum analyses of three microcline separates from the Separation Point Batholith, northwest Nelson, New Zealand, which cooled slowly (~5°C-Ma?1) through the temperature zone of partial radiogenic 40Ar accumulation are characterized by a linear age increase over the first 65 percent of gas release with the lowest ages (~80 Ma) corresponding to the time that the samples cooled below about 100°C. The last 35 percent of 39Ar released from the microclines yields plateau ages (103,99 and 93 Ma) which reflect the different bulk mineral ages, and correspond to cooling temperatures between about 130 to 160°C. Theoretical calculations confirm the likelihood of diffusion gradients in feldspars cooling at rates ≤5°C-Ma?1. Diffusion parameters calculated from the 39Ar release yield an activation energy, E = 28.8 ± 1.9 kcal-mol?1, and a frequency factor/grain size parameter, . This Arrhenius relationship corresponds to a closure temperature of 132 ± 13°C which is very similar to the independently estimated temperature. From the observed diffusion compensation correlation, this implies an average diffusion half-width of about 3 μm, similar to the half-width of the perthite lamellae in the feldspars. The range in microcline K-Ar ages from the Separation Point Batholith is the result of relatively small temperature differences within the pluton during cooling. Comparison of the diffusion laws determined for microcline with those for anorthoclases and other homogeneous K-feldspars (E = 40 to 52 kcal-mol?1) reveals that Ar diffusion is more highly temperature dependent in the disordered structural state than in the ordered structural state. Previously published U-shaped age spectra are probably the result of the superimposition of excess 40Ar upon diffusion profiles of the kind described here. 相似文献
11.
12.
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 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 values indicated that one of the methods (that involving the summation of hydroxide components) was significantly better than all other methods in predicting values of smectites while at the same time requiring no ad hoc thermochemical assumptions. 相似文献
13.
The Rameka Gabbro, emplaced 367 Ma ago, experienced a well documented reheating on intrusion of the Separation Point Batholith 114 Ma ago. age spectrum analyses of hornblende from the Rameka Gabbro show diffusion gradients which provide information on the 40Ar boundary concentration during reheating.Three samples of hornblende exhibit age spectra that conform to a model of 40Ar loss by diffusion, implying a zero 40Ar boundary concentration during heating. The calculated 40Ar loss from these samples, together with a model of heat flow in the aureole, provide estimates of diffusion coefficients of 40Ar in Mg-rich hornblende which correspond to an activation energy, E, of ~60 kcal-mol?1 and a frequency factor. D0, of ~ 10?3 cm2-sec?1. When combined with laboratory diffusion results, these data yield a well defined diffusion law (E = 63.3 ± 1.7 kcal-mol?1, ).The age spectra of the eight other samples record steep gradients of excess 40Ar over the first few percent of gas release. Although this effect causes high apparent conventional K-Ar ages, the plateau segments of many sampes still record the crystallization age of 367 ± 5 Ma. These measurements show that the excess 40Ar phase developed locally in the intergranular regions of the gabbro, following intrusion of the batholith. on time scales that varied from 104 to 106years. The minimum average ratio of this component was found to be 1300 ± 400. The partial pressure of Ar was at least 10?2 bars in some places.A single age spectrum analysis of plagioclase reveals a ‘saddle-shaped” release pattern with a minimum at 140 Ma.In conjunction with theoretical diffusion models and a diffusion law, age spectrum analysis of hornblende that has experienced a post-crystallization heating can provide close estimates of the maximum temperature of the thermal event as well as both age of crystallization and reheating. 相似文献
14.
incremental-release ages have been determined for 15 hornblende and 20 biotite concentrates separated from rocks collected across the garnet and kyanite zones of Grenvillian metamorphism in southwestern Labrador. Most hornblende spectra from the kyanite zone are slightly discordant, with low-temperature increments yielding ages older than the ca 1000 Ma date suggested for culmination of Grenvillian metamorphism in the area. However, all the hornblende concentrates record well-defined plateau ages. These range from 968 to 905 Ma across the kyanite zone and date times of diachronous post-metamorphic cooling. The discordant spectra are interpreted to result from low-temperature liberation of excess 40Ar components from grain margins. Two hornblende concentrates from the garnet zone display very discordant spectra (total-gas ages of 2100 and 3017 Ma) in which incremental dates systematically decrease during analysis. This pattern of discordance suggests that excess argon components are inhomogeneously distributed throughout these hornblende grains.Most biotites from the garnet and kyanite zones record total-gas or plateau ages in excess of 1000 Ma (2066-857 Ma), reflecting the widespread presence of excess argon components. Because most of the age spectra are internally concordant, the ratios of excess 40Ar relative to radiogenic 40Ar must have been uniform in the various gas fractions liberated from each sample. This is also reflected in the inability of isotope correlation diagrams to differentiate between excess, radiogenic, and atmospheric argon components. The biotite total-gas or plateau dates show marked local variation. This is interpreted to indicate that the biotite grains were in contact with a post-metamorphic intergranular vapor phase that was characterized by large and variable ratios. Such ratios most likely resulted from widespread diffusion of the argon liberated from adjacent Archean basement gneisses during the Grenvillian metamorphic overprint. 相似文献
15.
Results of an 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
16.
Ion microprobe analyses of returned lunar material have helped to demonstrate that U, Th and radiogenic Pb are concentrated in small accessory mineral phases. It is possible to measure the isotopic composition of this Pb and obtain a radiometric age for the mineral. The ages so derived compare favorably with crystallization ages determined by conventional methods. A grain mount () of Luna 20 material was searched for such accessory mineral phases and two were found. One of these phases gives an age of 4.12 ± 0.04 b.y. and the other an age of 4.42 ± 0.11 b.y. Ages of minerals dated by the ion probe in Apollo samples 14310 and 15555 are given for comparison. Data on the upper limit for Pb concentration in the outermost surface layers of free lunar soil particles are also given. 相似文献
17.
R.D Dallmeyer 《Geochimica et cosmochimica acta》1975,39(12):1655-1669
Biotite and hornblende from a portion of the Blue Ridge Precambrian basement terrane that was progressively retrograded during Paleozoic metamorphism have been analyzed by the dating technique to determine if incremental release spectra can distinguish thermally altered samples. Where not severely overprinted by Paleozoic metamorphism, both minerals show generally undisturbed age spectra with plateau ages similar to those of hornblende and biotite from non-retrograded portions of the Grenville terrane elsewhere in the Appalachians (hornblende ~1000 m.y.; biotite ~ 790 m.y.). The age spectra show a progressive disturbance which is correlated with increasing intensity of Paleozoic metamorphism. Modification of the hornblende spectra is that expected of diffusive argon loss during geologic reheating (incremental ages become older from low to high release temperatures). Disturbed biotite spectra do not show this type of modification, but develop increasingly broader low-age ‘saddles’ with increasing retrograde intensity. Eventually, Paleozoic metamorphism effected total retrograde alteration of the Grenville minerals and new generations of chemically distinct biotite and hornblende occur. Release spectra of these phases generally define plateaus although they are of different ages (biotite ~310–340 m.y.; hornblende ~355–460 m.y.). This discordancy is similar to that reported for other recrystallized portions of the Appalachian Grenville terrane and suggests that the ages represent times of argon retention following a 480 m.y. Paleozoic metamorphism.The data suggest that age spectra can distinguish thermally altered samples. 相似文献
18.
Uplift rates of the Loyalty Islands (S.W. Pacific) have been determined from 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
19.
Initial isotopic ratios of strontium have been determined in some calc-alkali rocks of Sardinia. The values of these ratios are from 0.7044 and 0.7047 in two basalts, and from 0.7063 to 0.7081 in five andesites. A dacite and a rhyolite have intermediate values of 0.7056 and 0.7058.It is suggested that increase of radiogenic strontium in andesite is most likely related to a contamination process involving water expelled from an underthrusting plate. This water also contributes to the increase in the potassium and related trace element contents in andesites.In spite of theSr difference between basalt and andesite, a cogenetic relationship between these two kinds of rocks cannot be precluded 相似文献
20.
A.L. Bloom W.S. Broecker J.M.A. Chappell R.K. Matthews K.J. Mesolella 《Quaternary Research》1974,4(2):185-205
Emerged coral reef terraces on the Huon Peninsula in New Guinea were reported in a reconnaissance dating study by Veeh and Chappell 1970. Age definition achieved was not good for several important terraces, and we report here a series of new dates, which further clarify the history of late Quaternary eustatic sea level fluctuations. More than 20 reef complexes are present, ranging well beyond 250,000 yr old: we are concerned with the seven lowest complexes. Major reef-building episodes dated by are reef complex I at 5–9 ka (kilo anno = 1000 yr), r.c. IIIb at 41 ka (four dates), r.c. IV at 61 ka (four dates), r.c. V at 85 ka (two dates), r.c. VI at 107 ka (two dates), and r.c. VII at 118–142 ka. Complex II was previously dated by 14C at 29 ka: this age has not yet been confirmed, and may be only a lower limit. The reef crests were built during or immediately before intervals of sea level maxima, when rates of rising sea level and tectonic uplift briefly coincided. The culmination of each reef-building episode was only a few thousand years in duration, and multiple dates from the same reef complex generally group within the statistical errors of the individual dates.Several methods can be used to estimate the altitude of each sea level maximum relative to present sea level. The least complicated is to calculate mean tectonic uplift rate for each profile of the terraces, and use the mean rate to calculate the tectonic displacement of each dated reef complex on that profile. The difference between the present altitude of a reef complex and its calculated tectonic uplift gives the paleosea level at the time the reef grew. We estimate uplift rates for six surveyed sections by calibrating against published paleosea level estimates from Barbados and elsewhere, viz 125 ka, paleosea at +6 m; 103 ka, ?15 m; 82 ka, ?13 m. For each section the individual uplift rates for reefs V, VI, and VIIb are within 5% of their section means. Using the mean rates. paleosea level estimates for reef crests II, IIIB, and IV are made for each section. Consistency of estimates between sections is good, giving ?28 m for the 60 ka paleosea level, around ?38 m for the 42 ka level and ?41 m for the 28 ka level (if the age is older the paleosea level would be lower. Using the mean uplift rates, the 82 ka and 103 ka paleosea levels are also estimated for each section: all individual estimates are plotted graphically, and a sea level curve drawn. The reef stratigraphy indicates sea level lowerings between each dated reef crest: the crests probably represent the interstadials of the Wisconsin (Würm, Weichsel) Glaciation, and intervening lower levels correspond to stadials. Since the last time of eustatic sea level higher than the present (about 125 ka), five sea level maxima occurred at roughly 20-ka intervals, none being as high as the present. 相似文献