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   

Ages of subsurface stratigraphic intervals in the Quaternary of Enewetak Atoll,Marshall Islands     

B.J. Szabo  J.I. Tracey  E.R. Goter 《Quaternary Research》1985,23(1):54-61

Drill cores of Enewetak Atoll, Marshall Islands, reveal six stratigraphic intervals, numbered in downward sequence, which represent vertical coral growth during Quaternary interglaciations. Radiocarbon dates indicate that the Holocene sea transgressed the emergent reef platform by about 8000 yr B.P. The reef grew rapidly upward (about 5 to 10 mm/yr) until about 6500 yr B.P. Afterward vertical growth slowed to about 0.5 mm/yr, then lateral development became dominant during the last several thousand years. The second interval is dated at 131,000 ± 3000 yr B.P. by uranium series. This unit correlates with oxygen-isotope substage 5e and with terrace VIIa of Huon Peninsula, New Guinea, and of Main Reef-2 terrace at Atauro Island. The third interval is not dated because corals were recrystallized and it is tentatively correlated with either oxygen-isotope stages 7 or 9. The age of the fourth interval is estimated at 454,000 ± 100,000 yr B.P. from measured ${}^{234}\text{U}{}^{238}\text{U}$ activity ratios. This unit is correlated with either oxygen-isotope stage 9, 11, or 13.  相似文献   

Half-life of 92Nb     

Tomoko Makino  Masatake Honda 《Geochimica et cosmochimica acta》1977,41(10):1521-1523

The long-lived nuclide ⁹²Nb together with ⁹¹Nb and ⁹⁴Nb were produced from Mo by neutron irradiation. The activities of the nuclides were determined by γ spectroscopy and the atomic abundances were determined by mass spectrometry. Intercomparison of ⁹²Nb and ⁹⁴Nb activities and abundances yielded a value for half-life $\text{(}{}^{92}\text{Nb) = (3.3 ± 0.5) × 10}{}^7$ yr based on a value for half-life $\text{(}{}^{94}\text{Nb) = 2.0 × 10}{}^4$ yr. The same value was obtained within errors from absolute counting data and dilution analysis.  相似文献   

Quaternary sea level fluctuations on a tectonic coast: New ${}^{230}\text{Th}{}^{234}\text{U}$ dates from the Huon Peninsula,New Guinea     

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 ${}^{230}\text{Th}{}^{234}\text{U}$ 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 ${}^{30}\text{Th}{}^{234}\text{U}$ 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   

$\text{U}{}^{234}\text{U}{}^{238}$ Ratios in limestone cave seepage waters and speleothem from West Virginia     

Peter Thompson  Derek C Ford  Henry P Schwarcz 《Geochimica et cosmochimica acta》1975,39(5):661-669

Speleothem from West Virginia, ranging in age from 2000 to 200,000 yr B.P. contains uranium with $\text{U}{}^{234}\text{U}{}^{238}$ 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, $\text{U}{}^{234}\text{U}{}^{238}$ 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 $\text{U}{}^{234}\text{U}{}^{238}$ ratios from $\text{Th}{}^{230}\text{U}{}^{234}$. datable portions of speleothems should permit $\text{U}{}^{234}\text{U}{}^{238}$-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   

Inert gases in a terra sample: measurements in six grain-size fractions and two single particles from Luna 20     

D Heymann  S Lakatos  J.R Walton 《Geochimica et cosmochimica acta》1973,37(4):875-885

The inert gases have been measured in six size fractions covering the range below 500 μm, in a single feldspathic fragment weighing 523 μg, and in an agglutinate particle weighing 465 μg. The two size fractions between 125 and 250 μm as well as 250 and 500 μm were separated into magnetic and non-magnetic portions, which were measured separately. Like the Apollo and Luna 16 fines, the terra fines represented by Luna 20 are very rich in trapped solar-wind gases, but they contain relatively less He⁴ and Ne²⁰, which is revealed by their average $\text{He}{}^4\text{Ne}{}^{20}$ ratio of 35 and $\text{Ne}{}^{20}\text{Ar}{}^{36}$ ratio of 2.9. Obviously the terra materials are less retentive for solar-wind He and Ne than typical mare fines such as 10084. Whether this is due to the relatively small TiO₂ or the relatively large plagioclase content of the former is not resolved. ($\text{Ar}{}^{36}\text{Kr}{}^{84}\text{)}{}_{\mathrm{trapped}}$ and ($\text{Ar}{}^{36}\text{Xe}{}^{132}\text{)}{}_{\mathrm{trapped}}$ ratios are relatively large; the average values are 2800 and 14400, respectively. The apparent Ne²¹ radiation ages of all the size fractions are in the range 209–286 × 10⁶ yr; the average is 260 × 10⁶ yr. This is in the range of values known for the Apollo and Luna 16 fines. The feldspathic fragment has a much greater apparent Ne_c²¹ age of 780 × 10⁶ yr. The Ar⁴⁰-Ar³⁶ systematic reveals the presence of two Ar⁴⁰ components, because Ar⁴⁰ = (1.41 ± 0.076)Ar³⁶ + (0.490 ± 0.130) × 10^?4 (cm³ STP/g). The $\text{Ar}{}^{40}\text{Ar}{}^{36}$ slope of 1.41 is not inconsistent with an origin of the sample from a relatively old terra region.  相似文献   

Oxygen and sulfur isotope equilibria in the BaSO4HSO4−H2O system from 110 to 350°C and applications     

Minoru Kusakabe  Brian W. Robinson 《Geochimica et cosmochimica acta》1977,41(8):1033-1040

Oxygen isotope exchange between BaSO₄ and H₂O from 110 to 350°C was studied using 1 m H₂SO₄-1 m NaCl and 1 m NaCl solutions to recrystallize the barite. The slow exchange rate (only 7% exchange after 1 yr at 110°C and 91% exchange after 22 days at 350°C in 1 m NaCl solution) prompted the use of the partial equilibrium technique. However, runs at 300 and 350°C were checked by complete exchange experiments. The temperature calibration curve for the isotope exchange is calculated giving most weight to the high temperature runs where the partial equilibrium technique can be tested. Oxygen isotope fractionation factors (α) in 1 m NaCl solution (110–350°C), assuming a value of 1.0407 for α_CO2H2O at 25°C, are: .These data, when corrected for ion hydration effects in solution (Truesdell, 1974), give the fractionation factors in pure water: .In the 1 m H₂SO₄-1 m NaCl runs, sulfur isotope fractionation between HSO^?₄ and BaSO₄ is less than the detection limit of 0.4%. A barite-sulfide geothermometer is obtained by combining HSO^?₄H₂S and sulfide-H₂S calibration data.Barite in the Derbyshire ore field, U.K., appears to have precipitated in isotopic equilibrium with water and sulfur in the ore fluid at temperatures less than 150°C. At the Tui Mine, New Zealand, the barite-water geothermometer indicates temperatures of late stage mineralization in the range 100–200°C. A temperature of 350 ± 20°C is obtained from the barite-pyrite geothermometer at the Yauricocha copper deposit, Peru, and oxygen isotope analyses of the barite are consistent with a magmatic origin for the ore fluids.  相似文献   

The geochemical evolution of the Pleistocene Lake Lisan-Dead Sea system     

Amitai Katz  Yehoshua Kolodny  Arie Nissenbaum 《Geochimica et cosmochimica acta》1977,41(11):1609-1626

The geochemical history of Lake Lisan, the Pleistocene precursor of the Dead Sea, has been studied by geological, chemical and isotopic methods.Aragonite laminae from the Lisan Formation yielded (equivalent) Sr/Ca ratios in the range 0.5 × 10^?2?1 × 10^?2, Na/Ca ratios from 3.6 × 10^?3 to 9.2 × 10^?3, $\text{δ}{}^{18}\text{O}{}_{\mathrm{PDB}}$ values between 1.5 and 7%. and $\text{δ}{}^{13}\text{C}{}_{\mathrm{PDB}}$ from ?7.7 to 3.4%..The distribution coefficient of Na⁺ between aragonite and aqueous solutions, $\text{λ}{}^{\mathrm{A}}{}_{\mathrm{Na}}$, is experimentally shown to be very sensitive to salinity and nearly temperature independent. Thus, Na/Ca in aragonite serves as a paleosalinity indicator.Sr/Ca ratios and $\text{δ}{}^{18}\text{O}$ values in aragonite provide good long-term monitors of a lake's evolution. They show Lake Lisan to be well mixed, highly evaporated and saline. Except for a diluted surface layer, the salinity of the lake was half that of the present Dead Sea (15 vs 31%).Lake Lisan evolved from a small, yet deep, hypersaline Dead Sea-like, water body. This initial lake was rapidly filled-up to its highest stand by fresh waters and existed for about 40,000 yr before shrinking back to the present Dead Sea. The chemistry of Lake Lisan at its stable stand represented a material balance between a Jordan-like input, an original large mass of salts and a chemical removal of aragonite. The weighted average depth of Lake Lisan is calculated, on a geochemical basis, to have been at least 400, preferably 600 m.The oxygen isotopic composition of Lake Lisan water, which was higher by at least 3%. than that of the Dead Sea, was probably dictated by a higher rate of evaporation.Na/Ca ratios in aragonite, which correlate well with $\text{δ}{}^{13}\text{C}$ values, but change frequently in time, reflect the existence of a short lived upper water layer of varying salinity in Lake Lisan.  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 marine oxygen isotope record in Pleistocene coral,Barbados, West Indies     

Richard G. Fairbanks  R.K. Matthews 《Quaternary Research》1978,10(2):181-196

The reef-crest coral Acropora palmata from late Pleistocene reefs on Barbados has recorded the same global variations in oxygen isotopes as planktonic and benthonic foraminifera. Although the record of oxygen isotopes in Acropora palmata is discontinuous, it offers several advantages over the isotope records from deep-sea sediments: (1) the coral grows at water depths of less than 5 m; (2) the samples are unmixed; (3) specimens may be sampled from various elevations of paleo-sea level; and (4) aragonitic corals are suitable for ${}^{230}\text{Th}{}^{234}\text{U}$ and $\text{He}\text{U}$ dating techniques. The latter advantage means that direct dating of the marine oxygen isotope record is possible. Oxygen isotope stage 5e corresponds to Barbados III, dated at 125,000 ± 6000 yr BP. Petrographic and geochemical evidence from five boreholes drilled into the south coast of Barbados indicates a major eustatic lowering (greater than 100 m below present sea level) occurred between 180,000 and 125,000 yr BP. The age and isotopic data suggest correlation of this change in sea level to Emiliani's oxygen isotope stage 6. Acropora palmata deposited at various elevations of sea level during oxygen isotope stage 6 vary by 0.11 ‰ δ¹⁸O for each 10 m of change in sea level. We further hypothesize a minimum drop of 2°C in the average temperature occurred during the regressive phase of oxygen isotope stage 6. These data indicate that temperature lowering of surface water near Barbados lagged behind a major glacial buildup during this time period. Using the δ¹⁸O vs sea level calibration herein derived, we estimate the relative height of sea stands responsible for Barbados coral reef terraces in the time range 80,000 to 220,000 yr BP.  相似文献   

Isotopic anomalies of noble gases in meteorites and their origins—VI. Presolar components in the Murchison C2 chondrite     

Leo Alaerts  Roy S. Lewis  Jun-ichi Matsuda  Edward Anders 《Geochimica et cosmochimica acta》1980,44(2):189-209

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Variability of the Al²⁶ production rate in ordinary chondrites     

Gregory F Herzog  Philip J Cressy 《Geochimica et cosmochimica acta》1974,38(12):1827-1841

15 ordinary chondrites for which unusually high spallogenic $\text{Ne}{}^{22}\text{Ne}{}^{21}$ or $\text{He}{}^3\text{Ne}{}^{21}$ ratios had been reported and one meteorite with marked shock characteristics were selected in order to investigate the relations between $\text{Ne}{}^{22}\text{Ne}{}^{21}$ ratios, Al²⁶ contents and depth. We report Al²⁶ and K contents of 13 samples from 11 of these and-noble gas contents of 30 samples from all of these stones.A decrease in the Al²⁶ production rate accompanies the increase of $\text{Ne}{}^{22}\text{Ne}{}^{21}$ towards the pre-atmospheric surface: $\text{Al}{}_{\mathrm{obs}}{}^{26}\text{Al}{}_{\mathrm{calc}}{}^{26}\text{= 3.2?2.0}\text{Ne}{}^{22}\text{Ne}{}^{21}$ for $\text{1.08 ≤}\text{Ne}{}^{22}\text{Ne}{}^{21}\text{≤ 1.2}$. Large deviations from this relationship may indicate that a meteorite experienced an abnormal flux of cosmic rays.For $\text{Ne}{}^2\text{Ne}{}^{21}\text{> 1.2}$ this trend continues but the data scatter more, probably because of the steadily increasing influence of pre-atmospheric size. $\text{Ne}{}^{22}\text{Ne}{}^{21}$ ratios increase most rapidly in the outermost few centimeters according both to a plot of $\text{Ne}{}^{22}\text{Ne}{}^{21}$ vs (recovered mass)^{$\text{1}\text{3}$} and to track studies. The increase seems to derive from the enhanced importance of nuclear reactions on Si.$\text{Ne}{}^{22}\text{Ne}{}^{21}\text{< 1.08}$ defines a region where the Al²⁶ production rates are less sensitive to depth and vanish in the limit of large shielding; the weak correlation between $\text{Ne}{}^{22}\text{Ne}{}^{21}$ and Al²⁶ in this region rules out the use of the $\text{Ne}{}^{22}\text{Ne}{}^{21}$ ratio as a basis for a shielding correction to Al²⁶.  相似文献   

Noble gas concentrations and cosmic ray exposure ages of eight recently fallen chondrites     

D.D Bogard  M.A Reynolds  L.A Simms 《Geochimica et cosmochimica acta》1973,37(11):2417-2433

Abundances and isotopic compositions of He, Ne, Ar, and Xe have been measured in eight recently fallen chondrites. Ratios of concentrations of cosmic ray-produced ³He, ²¹Ne, ²²Ne and ³⁸Ar indicate that all eight samples experienced less than average cosmic ray shielding. ³He and ²¹Ne exposure ages were calculated using shielding corrected chondritic production rates and the measured ${}^{22}\text{Ne}{}^{21}\text{Ne}$. Exposure ages calculated from ${}^{22}\text{Na}{}^{22}\text{Ne}$ and ${}^{26}\text{Al}{}^{21}\text{Ne}$ ratios and constant relative production rates show a bias between the two ages due to variations in ${}^{22}\text{Na}{}^{26}\text{Al}$. Arguments are presented that this bias is due to irradiation hardness differences, and therefore the use of constant values for both the ${}^{22}\text{Na}{}^{22}\text{Ne}$ and ${}^{26}\text{Al}{}^{21}\text{Ne}$ production ratios is not permitted. Dwaleni, Swaziland, was found to be an unusual gas-rich chondrite with high concentrations of solar-derived He and Ne and planetary-type Xe.  相似文献   

Uranium-series dating of lacustrine limestones from pan deposits with final Acheulian assemblage at Rooidam,Kimberley district,South Africa     

B.J. Szabo  K.W. Butzer 《Quaternary Research》1979,11(2):257-260

Lacustrine limestone samples from sedimentary pan deposits at Rooidam, near Kimberley, South Africa, that contain late Acheulian (Fauresmith) artifacts have been dated by ${}^{230}\text{Th}{}^{234}\text{U}$ and ${}^{231}\text{Pa}{}^{235}\text{U}$ methods. Results indicate a minimum age of about 200,000 yr B.P. for the terminal Acheulian in the interior of South Africa.  相似文献   

Late Pleistocene sea level history of Bermuda     

Russell S. Harmon  Henry P. Schwarcz  Derek C. Ford 《Quaternary Research》1978,9(2):205-218

The timing of eustatic sea level fluctuations over the vertical range + 15 to ?11 m has been deduced from ${}^{230}\text{Th}{}^{234}\text{U}$ dating of Bermudan corals and speleothems. On this tectonically stable carbonate island, interglacial periods are characterized by platform submergence, development of patch reefs, and the deposition of littoral and eolian carbonates, whereas glacial periods are times of platform emergence, carbonate diagenesis, soil development, and the deposition of speleothems in caves extending below present sea level. Interglacial periods are observed at about 200,000, 130,000 to 90,000, and 10,000 yr BP to present. The sea level history of the last interglacial period (130,000 to 90,000 yr BP) is complex, consisting of at least two short, distinct episodes of high sea stand (at 125,000 and 97,000 yr BP) superimposed on a longer period of general platform submergence. The sea level data derived from this study are compatible with those from other stable areas such as the Bahamas, but in addition suggest that eustatic sea level changes can be rapid, on the order of 5 to 10 m/1000 yr.  相似文献   

Vredefort core: a cross-section of the upper crust?     

W.F Slawson 《Geochimica et cosmochimica acta》1976,40(1):117-121

Sixty-three samples from the granitic core of the Vredefort ring structure have been analyzed for K, Rb, and Sr. The $\text{Sr}{}^{87}\text{Sr}{}^{86}$ ratios were determined for thirteen of these. An isochron yielding an age of 2.85 × 10⁹ yr was obtained. The $\text{Rb}\text{Sr}$ and $\text{K}\text{Rb}$ ratios exhibit a ‘bulls-eye’ pattern in the granite suggesting the central portion has been extruded from depth. Diapiric flow and subsequent erosion has exposed a section of the continental crust within the Vredefort core.  相似文献   

Radium content and the ${}^{226}\text{Ra}{}^{228}\text{Ra}$ activity ratio in groundwater from bedrock     

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 $\text{sol}{}^{226}\text{Ra}{}^{228}\text{Ra}$ ratio sharply increased as the ²²⁶Ra content of water increased. The ${}^{226}\text{Ra}{}^{228}\text{Ra}$ ratio in the drilled wells varied from 0.3 to 26. Abnormally high ${}^{226}\text{Ra}{}^{228}\text{Ra}$ ratios were found in areas with known uranium deposits as well as in several drilled wells at other locations. The abnormally high ${}^{226}\text{Ra}{}^{228}\text{Ra}$ 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 ${}^{226}\text{Ra}{}^{228}\text{Ra}$ 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 ${}^{226}\text{Ra}{}^{228}\text{Ra}$ ratio independent of the concentration of ²²⁶Ra.  相似文献   

Une dépression piézométrique naturelle en hausse au Sahel (Sud-Ouest du Niger)A rising piezometric depression in the Sahel (southwestern Niger)     

Guillaume Favreau  Christian Leduc  Christelle Marlin  Abdou Guéro 《Comptes Rendus Geoscience》2002,334(6):395-401

In southwest Niger, the Continental Terminal water table displays a natural hollow shape about 10 m in depth over an area of 4000 km². A 10-year survey of this hollow aquifer has shown that current recharge is above $\text{20 }\text{mm}\text{yr}{}^{\mathrm{?1}}$. The water table has risen continuously since the 1950–1960s as a result of land clearance. This shows a disequilibrium in the aquifer balance. The long-term recharge rate is estimated by radioisotopes to be around $\text{1 }\text{mm}\text{yr}{}^{\mathrm{?1}}$. This figure fits with the only possible origin of the piezometric depression, i.e. evapotranspiration losses in its centre. To cite this article: G. Favreau et al., C. R. Geoscience 334 (2002) 395–401.  相似文献   

State of disequilibrium between ²³⁸U, ²³⁴U, ²²⁶Ra and ²²²Rn in groundwater from bedrock     

Matti Asikainen 《Geochimica et cosmochimica acta》1981,45(2):201-206

Approximately one thousand drilled wells were investigated for their natural radioactivity. The determinations of ²³⁸U, ²³⁴U, ²²⁶Ra and ²²²Rn from 310 samples showed a high state of radioactive disequilibrium between the members of the uranium series present in water. The ${}^{238}\text{U}{}^{226}\text{Ra}$ activity ratio usually fell in the range 1–20 and the ${}^{238}\text{U}{}^{222}\text{Rn}$ activity ratio in the range 1–20 × 10^?4, the highest activity ratios being from samples with an elevated uranium content. The ${}^{234}\text{U}{}^{238}\text{U}$ activity ratio varied between 0.76 and 4.67, the most frequent values showing a 60% excess of ²³⁴U in the samples. Most of the ${}^{234}\text{U}{}^{238}\text{U}$ activity ratios near unity were found in samples with a high uranium content. Several drilled wells with anomalously high uranium contents were found in southern Finland. The average ²²⁶Ra and ²²²Rn contents of these wells were not exceptionally high, which suggests high mobility of uranium in groundwater from the areas involved.  相似文献   

FUN with PANURGE: High mass resolution ion microprobe measurements of Mg in Allende inclusions     

J.C Huneke  J.T Armstrong  G.J Wasserburg 《Geochimica et cosmochimica acta》1983,47(9):1635-1650

The performance characteristics of PANURGE, a modified CAMECA IMS3F ion microprobe, have been studied at a mass resolving power of 5000 for the purpose of determining isotopic ratios at a precision level approaching that of counting statistics using beam switching. The techniques used for this type of measurement are described. Using this approach, the isotopic composition of Mg and Si and the atomic ratio of $\text{Al}\text{Mg}$ in minerals from the Allende inclusion WA and the Allende FUN inclusion Cl have been measured with the ion microprobe at high mass resolving power. Enrichments in ²⁶Mg of up to 260%. have been found. Mg and $\text{Al}\text{Mg}$ measurements on cogenetic spinel inclusions and host plagioclase crystals yield Mg-Al isochrons in excellent agreement with precise mineral isochrons determined by thermal emission mass spectrometry. The measurements confirm the presence of substantial excess ²⁶Mg in WA (${}^{26}\text{Mg}{}^1{}^{27}\text{Al = 5 × 10}{}^{\mathrm{?5}}$) and its near absence in Cl (${}^{26}\text{Mg}{}^1{}^{27}\text{Al}\text{< 4 × 10}{}^{\mathrm{?6}}$). In WA plagioclase, data for which ${}^{27}\text{Al}{}^{24}\text{Mg}\text{= 300 to 1000}$ define a linear array with ${}^{26}\text{Mg}{}^1{}^{27}\text{Al}\text{= 3 × 10}{}^5$ and with initial ${}^{26}\text{Mg}{}^{24}\text{Mg}$ composition 30%. greater than in high Mg phases. This suggests a metamorphic reequilibration of Mg in Allende plagioclase at least 0.6 my after WA formation. There were no variations in detected ${}^{26}\text{Mg}{}^1{}^{27}\text{Al}$ in WA plagioclase associated with concentration of ²⁶Mg¹ into isolated clusters. We have confirmed by ion probe measurements that the Mg composition in Allende Cl is highly fractionated and is uniform among pyroxene, melilite, plagioclase, spinel crystals and spinel included in melilite and plagioclase crystals. Likewise, the Si composition is mass fractionated and is the same in pyroxene, melilite and plagioclase.  相似文献