Redistribution of uranium and thorium series isotopes during isovolumetric weathering of granite     

Jacqueline Michel 《Geochimica et cosmochimica acta》1984,48(6):1249-1255

Previous studies of the distribution of U and Th in parent versus weathered granites have shown both depletion and enrichment of these elements during weathering. In this study, the distribution of U and Th decay series isotopes was determined in a weathering profile of a granitic saprolite, which showed textural preservation indicating isovolumetric weathering. Two types of dissolution methods were used: a whole-rock dissolution and a sodium-citrate dithionite leach to preferentially attack noncrystalline phases of weathering products. Using volume-based activities, 45–70 percent of the total ²³²Th was gradually removed during weathering. Although the whole-rock ${}^{228}\text{Th}{}^{232}\text{Th}$ activity ratios were in equilibrium, there were large excesses of ²²⁸Th in the leachable fraction of both parent rock (${}^{228}\text{Th}{}^{232}\text{Th}\text{= 2.06}$) and partially weathered saprolite (${}^{228}\text{Th}{}^{232}\text{Th}\text{= 3–6.5}$), due to alpha recoil and release of daughter ²²⁸Th to the weathering rind of the mineral grain. For the most weathered sample, 81 percent of the thorium was in the teachable fraction and ${}^{228}\text{Th}{}^{232}\text{Th}\text{= 1}$, indicating that even the more resistant minerals were attacked.The total U activities showed as much variation in the six parent rock samples as in the weathered profile, and ${}^{234}\text{U}{}^{238}\text{U}$ were in equilibrium in both the whole-rock and leachable fractions. ²³⁰Th was deficient relative to ²³⁴U and ²²⁶Ra in both fractions, suggesting recent addition of U and Ra to the entire profile. The large variation in U was not from absorption onto the intermediate weathering products, because only 11–23 percent of the U was in the leachable fraction.  相似文献   

Comparison of 230Th-238U disequilibrium systematics in lavas from three hot spot regions: Hawaii,Prince Edward and Samoa     

S Newman  R.C Finkel  J.D Macdougall 《Geochimica et cosmochimica acta》1984,48(2):315-324

²³⁰Th-²³⁸U disequilibrium systematics reveal several important characteristics of the mantle source regions and petrogenesis of volcanic rocks in the presumed hot spots of Hawaii, Marion Island (Prince Edward hot spot), and Samoa. The (${}^{230}\text{Th}{}^{232}\text{Th}$) activity ratios of lavas from these three hot spots (1.06 ± 0.07, 1.04 ± 0.08, and 0.81 ± 0.06, respectively) imply that the source regions are each nearly homogeneous with $\text{Th}\text{U}$ weight ratios of 2.9, 3.0, and 3.8. For Marion Island and Mauna Kea, Hawaii, negligible secular variation occurs in the (${}^{230}\text{Th}{}^{232}\text{Th}$) initial ratios. This supports other evidence for very short transfer time between source and surface. Significant residence time at depth prior to eruption cannot be ruled out for the Samoan lavas we have studied; however, the data for one of these flows deviate from the proposed (${}^{230}\text{Th}{}^{232}\text{Th}$)-${}^{87}\text{Sr}{}^{86}\text{Sr}$ correlation (Condomineset al., 1981a) in the opposite sense from that expected for such residence. If it is assumed that the measured (${}^{230}\text{Th}{}^{232}\text{Th}$) ratios of the young lavas reflect $\text{Th}\text{U}$ in their mantle sources, then the observed variations among these three hot spots, combined with those reported by other workers for Iceland, the Azores and Tristan de Cunha, suggest that these sources are characterized by $\text{Th}\text{U}$ ratios ranging from values similar to that of MORB source (~2.5) to values similar to those of bulk earth (~3.8). Mixing of different proportions of depleted and enriched mantle may be responsible for the observed range.  相似文献   

Growth of a manganese nodule from Peru Basin: A radiochemical anatomy     

J.L Reyss  N Lemaitre  T.L Ku  V Marchig  J.R Southon  D.E Nelson  J.S Vogel 《Geochimica et cosmochimica acta》1985,49(11):2401-2408

Attempts have been made to study the entire growth history of a manganese nodule from the northern part of Peru Basin in the Pacific using radiochemical profiles of ${}^{230}\text{Th}{}^{232}\text{Th}$, ${}^{227}\text{Th}{}^{230}\text{Th}$, and ${}^{10}\text{Be}{}^9\text{Be}$. Combined with the observations on Fe-Mn contents and textural variation, the radiochemical data indicate that the nodule grew more or less concentrically throughout most of its existence since it formed 1.5 my ago, receiving Mn from both bottom water and pore water. This condition appeared to have changed about 180 ky ago when the growth became asymmetric in that the top and bottom sides became fixed in their relative positions on the sea floor. Since then, the bottom side accreted with a fast rate of close to 200 mm/my, apparently fueled by the supply of diagenetically remobilized Mn in pore water from the sediment substrate. In the meantime, the top side accumulated at about 6 mm/my, a value which is in the normal range for deep-sea nodules having their Mn supplied from the hydrogenous source.  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   

Ground water geochemistry of ²²⁸Ra, ²²⁶Ra and ²²²Rn     

Philip T. King  Jacqueline Michel  Willard S. Moore 《Geochimica et cosmochimica acta》1982,46(7):1173-1182

²²⁸Ra, ²²⁶Ra, and ²²²Rn activities were determined on over 150 ground water samples collected from drilled, public water supply wells throughout South Carolina. A wide range of aquifer lithologies were sampled including the crystalline rocks of the Piedmont and sedimentary deposits of the Coastal Plain. A significant linear relationship between log ²²⁸Ra and log ²²⁶Ra (n = 182, r = 0.83) was indistinguishable between Piedmont and Coastal Plain ground water. Median ${}^{228}\text{Ra}{}^{226}\text{Ra}$ activity ratios for the Piedmont, 1.2, and Coastal Plain, 1.3, ground water are close to estimated average crustal ${}^{232}\text{Th}{}^{238}\text{U}$ activity ratios of 1.2 to 1.5 corresponding to Th/U weight ratios of 3.5 to 4.5. A linear correlation was also found between log ²²²Rn and log ²²⁶Ra for Piedmont (n = 68, r = 0.62) and Coastal Plain (n = 89, r = 0.64) ground water. However, the median ${}^{222}\text{Rn}{}^{226}\text{Ra}$ activity ratio for Piedmont ground water, 6100, was much higher than for Coastal Plain ground water, 230. Higher excess ²²²Rn activities may be due to greater retention of ²²⁶Ra by the chemically active Piedmont aquifers compared to the more inert sand aquifers sampled in the Coastal Plain. The relationship between log ²²⁸Ra and log ²²⁶Ra was used to predict total Ra (${}^{228}\text{Ra +}{}^{226}\text{Ra}$) distributions in Appalachian and Atlantic and Gulf Coastal Plain ground water. Predictions estimate that 2.4% of Appalachian and 5.3% of Atlantic and Gulf Coastal Plain ground water supplies contain total Ra activities in excess of the 5 pCi/l limit established by the U.S. Environmental Protection Agency. These predictions also indicate that 40–50% of these ground water wells may be overlooked using the presently suggested screening activity of 3.0 pCi/l of ²²⁶Ra for ²²⁸Ra analysis.  相似文献   

Anomalously high concentrations of uranium,radium and radon in water from drilled wells in the Helsinki region     

M Asikainen  H Kahlos 《Geochimica et cosmochimica acta》1979,43(10):1681-1686

The concentrations of uranium. ²²⁶Ra and ²²²Rn were determined in 308 drilled and 58 dug wells in the Helsinki region. The study area was about 400 km² and geologically highly variable, with granites, amphibolites and migmatites the dominant rocks. The radioactivity of water in the dug wells was on a “normal” level, but in numerous drilled wells it was anomalously high. In 14 drilled wells the concentration of uranium exceeded 1000 μg/l, the highest concentration being 14,870 μg/l. For ²²²Rn the maximum concentration was 880,000 pCi/l. The ${}^{226}\text{Ra}{}^{228}\text{Ra}$ and ${}^{230}\text{Th}{}^{232}\text{Th}$ activity ratios showed the isotopes of the uranium series to be dominant in the study area. A state of disequilibrium between ²³⁸U and ²³⁴U was very common in the samples. The ${}^{234}\text{U}{}^{238}\text{U}$ activity ratios varied in the range 1.0–4.0 regardless of the amount of uranium in the water. The conclusion can be drawn from the isotopic data that the high radioactivity of water is in some cases caused by primary uranium mineralizations, but mostly by uranium deposited in fissures of the bedrock. The paper includes a summary of the results of two studies carried out between 1967 and 1977.  相似文献   

Glacial to interglacial changes in carbonate and clay sedimentation in the Atlantic Ocean estimated from 230Th measurements     

Michael P. Bacon 《Chemical Geology》1984,46(2):97-111

The cause of the climatically controlled fluctuations in the carbonate content of deep-sea sediments remains the subject of uncertainty and debate. Three variables are involved: supply of biogenic carbonate, loss by dissolution, and dilution by non-carbonate phases. It is suggested that ²³⁰Th, which is produced in the ocean at a constant rate provides a reliable reference for measuring variations in rate of sedimentation on a regional scale. Results of a preliminary analysis based on published data indicate that, for depths at and above the lysocline, the carbonate fluctuations observed in cores from the North Atlantic Ocean are due primarily to variations in the terrigenous clay input, which was 2–5 times higher during glacials than during interglacials. Carbonate deposition appears to have been somewhat reduced during glacials, but probably not by more than a factor of 2. From published ${}^{230}\text{Th}{}^{232}\text{Th}$ profiles it appears that the South Atlantic Ocean also received increased inputs of terrigenous clay during glacial periods.  相似文献   

The relative mobility of U,Th and Ra isotopes in the weathered zones of the Eye-Dashwa Lakes granite pluton,northwestern Ontario,Canada     

《Geochimica et cosmochimica acta》1987,51(10):2787-2793

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Radiochemical observations on manganese nodules from three sedimentary environments in the north Pacific     

Hun Chih-An  Ku Teh-Lung 《Geochimica et cosmochimica acta》1984,48(5):951-963

Decay-series isotopes were measured on manganese nodules from three sedimentary environments, characterized by substrates of red clay (R), siliceous ooze (S) and hemipelagic clay (H). Growth rates of nodules are shown to be site dependent: 1–2 mm/Myr at site R, 3–8 mm/Myr at sites S, and 20–50 mm/ Myr at site H. Correlation between growth rate and the parameter Mn/Fe² suggests that regional diagenetic variations control the compositions and growth rates of the nodules. The frequency of nodule turnover and the period of their growth are assessed from the “top” vs. “bottom” distributions of several nuclides. Based on the ²³⁰Th, ²³¹Pa and ²²⁶Ra data, turnover times of 10³ to 10⁵ years are estimated and they vary with relative size and shape of the specimens at a given site. The presence of unsupported ²¹⁰Pb and ²²⁸Th in the top surfaces but not in the bottom surfaces of the surface nodules suggests active growth during the last decades or years, despite their old ages. The data also indicate that turnover rates are not more frequent than once every several years or decades.The ²³⁰Th-²²⁶Ra, ²²⁶Ra-²¹⁰Pb and ²³²Th-²²⁸Th disequilibrium relationships in the nodules allow the migratory behavior of ²²⁶Ra, ²²²Rn and ²²⁸Ra and their fluxes to be deduced: thus radium leaves the top sides but enters the bottom sides of the surface nodules at sites R and S. At site R there is a net loss of radium to the sea; the opposite is true at site S. Surface nodules at site H trap radium from both sides, probably due to more intense diagenetic input of radium from sediment pore water. The effective diffusivities of ²²⁶Ra in nodules vary from $\text{3 × 10}{}^{\mathrm{?12}}$ to $\text{8 × 10}{}^{\mathrm{?14}}$ cm²/s, dependent on the textural variation of the nodule material, which crudely reflects the growth rate and hence sedimentary environment. ²²²Rn diffuses out of nodules from all sides, with an effective diffusivity of ca. $\text{(2–6) × 10}{}^{\mathrm{?8}}\text{cm}{}^2\text{/s}$. At all sites nodules serve as a more effective source of ²²²Rn to sea water than their adjacent sediments. The outward flux of ²²²Rn from nodules relative to that from adjacent sediments tends to be higher in more reduced environments, an effect caused by the fact that Mn-rich nodules from more reduced environment act as a more efficient trap for ²²⁶Ra.  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   

Magmatic evolution of a volcano studied by ²³⁰Th-²³⁸U disequilibrium and trace elements systematics: The Etna case     

M Condomines  J.C Tanguy  G Kieffer  C.J Allègre 《Geochimica et cosmochimica acta》1982,46(8):1397-1416

Age determinations of several lava flows from Etna through ²³⁰Th-²³⁸U disequilibrium (internal isochrons) yield a precise chronology of the volcano's history for the last 200,000 years, and emphasize the main episodes in the formation of this huge complex strato-volcano. Study of $\text{(}{}^{230}\text{Th}{}^{232}\text{Th}\text{)}{}_0$ initial ratios of lavas together with their trace-element compositions yields a consistent model of magmatic evolution implying the existence, for 200,000 years, of a deep reservoir of alkalic magma periodically mixed with magmas of tholeiitic affinity. These short periods of mixing appear to be related to the formation of the large calderas of Etna. In addition to these processes affecting the deep reservoir, fractional crystallization also occurred in more superficial levels of the volcanic edifice, thereby yielding several series of differentiation of relatively short duration. As for its geochemistry, Etna's volcanism is of oceanic type but with its own characteristics and in a peculiar geodynamic context, at the edge of the African Plate.  相似文献   

Radium isotopes in saline seepages,south-western Yilgarn,Western Australia     

B.L Dickson 《Geochimica et cosmochimica acta》1985,49(2):361-368

Water samples from saline seepages in the south-western Yilgarn Block of Western Australia contain high activities of the four naturally-occurring radium isotopes. Activities of up to 310 $\text{pCi}\text{l}$ for ²²⁶Ra and 1720 $\text{pCi}\text{l}$ for ²²⁸Ra were measured and the ${}^{228}\text{Ra}{}^{226}\text{Ra}$ ratio averaged 6.1. Activities of the two short-lived radium isotopes were also high. ²²³Ra activities of up to 94 $\text{pCi}\text{l}$ were found with an average ${}^{226}\text{Ra}{}^{223}\text{Ra}$ ratio of 3.3, considerably lower than the natural abundance ratio of 21.4. Activities of up to 23 $\text{pCi}\text{l}$²²⁷Ac, the long-lived ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 22 years}$) grandparent of ²²³Ra, were also measured. The analysis of surface granite samples, the probable source rocks of the radium, gave $\text{Th}\text{U}$ activity ratios of around 1.5. The higher ${}^{228}\text{Ra}{}^{226}\text{Ra}$ ratios of the waters were attributed to readily leached ²²⁸Ra in the weathered granites as a result of thorium remaining after weathering. Leach experiments on U-Th ore by NaCl solutions showed that all four radium isotopes were equally leached. Sulphate anions reduced the ²²⁶Ra and ²²⁸Ra leaching to a greater extent than for ²²³Ra and ²²⁴Ra, suggesting that the latter isotopes were being supported in solution by parent isotopes. In particular this suggested ²²⁷Ac was leached into the sulphate solution but this does not fully account for the amount of ²²⁷Ac seen in the seepage waters.  相似文献   

Uranium-thorium series radionuclides in brines and reservoir rocks from two deep geothermal boreholes in the Salton Sea Geothermal Field,southeastern California     

《Geochimica et cosmochimica acta》1987,51(10):2719-2731

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$\text{U}{}^{234}\text{U}{}^{238}$ 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   

The effect of bioturbation and adsorption gradients on solid and dissolved radium profiles in sediments from the eastern equatorial Pacific     

《Geochimica et cosmochimica acta》1987,51(6):1613-1623

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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   

The distribution and behaviour of 230Th and 231Pa at an ocean margin,Baja California,Mexico     

《Geochimica et cosmochimica acta》1986,50(11):2499-2507

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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   

Helium-uranium dating of corals     

Michael L. Bender 《Geochimica et cosmochimica acta》1973,37(5):1229-1247

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Unscrambling the lead model ages     

Francis Albarede 《Geochimica et cosmochimica acta》1984,48(1):207-212

A linear relation is derived for the secular evolution of ${}^{206}\text{Pb}{}^{204}\text{Pb}$, ${}^{207}\text{Pb}{}^{204}\text{Pb}$ and ${}^{208}\text{Pb}{}^{204}\text{Pb}$ ratios, that permits tests to be made for open system evolution on each system independently. Application of the method to conformable ore bodies of various geological age indicates that the available data do not demand an open system evolution for the last 3.8 b.y. ${}^{238}\text{U}{}^{204}\text{Pb}$ and ${}^{232}\text{Th}{}^{204}\text{Pb}$ of 9.66 ± 0.15 and 37.65 ± 1.14 respectively fit best the data for this time interval.A single stage evolution since 4.5 b.y. is unlikely, however, and the major events of continent formation postdate the Earth accretion by at least 400 m.y. The larger scatter of ${}^{207}\text{Pb}{}^{204}\text{Pb}$ data about the evolution line relative to the other isotopic ratios is also interpreted as resulting from a series of continental differentiation events taking place at 3.85 ± 0.15 b.y.  相似文献