Thorium and radium isotopic relationships in manganese nodules and sediments at MANOP Site S

Relationships among Th and Ra isotopes in nodule, sediment and water phases at MANOP Site S establish the most likely source for Th in the nodules, the frequency of nodule turning, and the similarity of micro and macro nodules. Manganese nodules and bottom waters have ${}^{230}\text{Th}{}^{232}\text{Th}$ activity ratios considerably higher than other phases at this site suggesting that sea water is the likely source of Th for the nodules. Similar ${}^{230}\text{Th}{}^{232}\text{Th}$ activity ratios in nodule tops and bottoms and in certain cases departure from expected ${}^{226}\text{Ra}{}^{230}\text{Th}$ activity ratios in nodule tops and bottoms indicate that the nodules rotate every one to ten thousand years. The micro nodules have diffusion coefficients of Ra similar to macro nodule bottoms. I suggest that they may act as a carrier phase for transporting metals through oxic sediments to nodules.     

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

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.     

Application of the 227Th230Th method to dating Pleistocene carbonates and comparison with other dating methods

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.     

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

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.     

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

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.     

U234U238 Ratios in limestone cave seepage waters and speleothem from West Virginia

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.     

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

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.     

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

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.     

Neodymium isotopic composition of Quaternary island arc lavas from Indonesia

${}^{143}\text{Nd}{}^{144}\text{Nd}$ ratios measured in Quaternary lavas from Java and the Banda arc of Indonesia range from 0.51242 to 0.51280 and exhibit an inverse correlation with ${}^{87}\text{Sr}{}^{86}\text{Sr}$. Isotopically, the Indonesian samples resemble Andean rather than island arc lavas. The samples from Java plot either within, or adjacent to the mantle array, towards higher ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios. Samples from the Banda arc and the anomalous calc-alkaline volcano Papandajan are characterized by relatively low ${}^{143}\text{Nd}{}^{144}\text{Nd}$ and high ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios. These characteristics are consistent with the interpretation that subducted terrigenous material was involved in the genesis of these lavas. Furthermore the Banda arc samples appear to lie on a mixing line between isotopic compositions characteristic of the mantle and upper continental crust. A high-K trachyte from the alkaline volcano Muriah, Java, has isotopic characteristics of the mantle (${}^{143}\text{Nd}{}^{144}\text{Nd}\text{= 0.51270}$, ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.70424}$), which implies that the extreme enrichment in large-ion-lithophile elements in its source must have occurred only shortly before its formation. The inferred ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ratio of the unmodified mantle beneath Java and the Banda arc is lower than that observed in mid-ocean ridge basalt, which may have important implications for a better understanding of the geochemical structure of the mantle.     

Chemical and isotopic evidence for mixing between depleted and enriched mantle,northwestern U.S.A.

Combined elemental and Sr, Nd, Pb and O isotopic data for late Cenozoic olivine tholeiite lavas from the northwestern Great Basin indicate derivation from at least two chemically and isotopically distinct mantle source regions with no significant modification by interaction with continental crust. The lack of crustal involvement is a direct reflection of the extensional tectonic environment which favors rapid ascent of magmas, minimal residence time in crustal magma chambers and scattered fissure eruptions.The observed chemical and isotopic variations in the tholeiite suite are attributed to mixing between depleted oceanic type mantle (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{~ 0.7030}$ and ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{~ 0.51305}$) and old, chemically heterogeneous, isotopically enriched subcontinental mantle (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{~ 0.7078}$ and ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{~ 0.51233}$). Model incompatible element concentrations suggest strong similarities between the depleted mantle and the mantles beneath normal oceanic ridge segments and back-arc basins and between the enriched mantle and the mantle beneath enriched oceanic ridge segments such as the Azores. Superimposed upon the characteristics derived from the two component mixing model may be the effects of a third mantle source which is identifiable only by its apparent radiogenic ${}^{206}\text{Pb}{}^{204}\text{Pb}$ ratios. If present, this third source may reflect a component derived from the downgoing slab of an ancient subduction zone.     

Extreme fractionation of 234U238U and 230Th234U in spring waters,sediments, and fossils at the Pomme de Terre Valley,southwestern Missouri

Isotopic fractionation as great as 1600% exists between ²³⁴U and ²³⁸U in spring waters, sediments, and fossils in the Pomme de Terre Valley, southwestern Missouri. The activity ratios of ${}^{234}\text{U}{}^{238}\text{U}$ in five springs range from 7.2 to 16 in water which has been discharged for at least the past 30,000 years. The anomalies in ${}^{234}\text{U}{}^{238}\text{U}$ ratio in deep water have potential usefulness in hydrologic investigations in southern Missouri. Clayey units overlying the spring bog sediments of Trolinger Spring are enriched in ²³⁰Th relative to their parent ²³⁴U by as much as 720%. The results indicate that both preferential displacement via alpha recoil ejection and the preferential emplacement via recoiling and physical entrapment are significant processes that are occurring in the geologic environment.     

Strontium isotopic studies of the volcanic rocks of the Saunda arc,Indonesia, and their petrogenetic implications

Pleistocene and Recent lavas from the Sunda arc range from those showing affinities with the island arc tholeiitic series, through a spectrum of calc-alkaline to high-K alkaline rocks. The tholeiitic rocks have relatively low ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios averaging 0–7043; the calc-alkaline rocks show a wide range (from 0.7038 to 0.7059, averaging 0.7048); the high-K alkaline rocks average 0.7045. A rhyolitic ignimbrite from Sumatra has an ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of 0.7139.The relationship between ${}^{87}\text{Sr}{}^{86}\text{Sr}$ and major and trace element geochemistry is variable and complex. Lavas from the same volcano sometimes show significant differences in ${}^{87}\text{Sr}{}^{86}\text{Sr}$ despite close geochemical relationships. Rocks of the calc-alkaline suite show a regular decrease in ${}^{87}\text{Sr}{}^{86}\text{Sr}$ from West Java to Bali and there is some evidence for increasing ${}^{87}\text{Sr}{}^{86}\text{Sr}$ with increasing depth to the Benioff zone. Calc-alkaline and tholeiitic rocks from the Sunda arc have significantly higher ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios than those from other island arcs, except from those arcs where continental crustal involvement has been inferred (e.g. New Zealand).A model of ⁸⁷Sr enrichment due to isotopic equilibration of oceanic crust with sea water and disequilibrium melting in the slab and/or mantle is favoured to explain the Sr isotopic composition of the tholeiitic and normal calc-alkaline lavas. Calc-alkaline lavas with high ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios are best explained by either sialic contamination, or the presence of alkali basalt as a component of the downgoing slab. The Sr isotopic data for the high-K alkaline lavas suggest a mantle origin. The high ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio in the Lake Toba rhyolite implies a crustal origin.     

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

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.     

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

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.     

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

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.     

Unscrambling the lead model ages

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.     

Radioelements,radiogenic helium and age relationships for groundwaters from the granites at Stripa,Sweden

The solution of radioelements and radiogenic ⁴He by groundwaters in fractured rocks is dependent upon the radioelement distribution in the rock matrix and the extent of the rock-water interface. The ${}^{234}\text{U}{}^{238}\text{U}$ activity ratio and the dissolved U, Rn and He contents of such groundwaters respond to changes in the flow regime with time. Although ${}^{234}\text{U}{}^{238}\text{U}$ activity ratios change with groundwater residence time as a consequence of ²³⁴Th-recoil induced solution of ²³⁴U, the activity ratio is strongly influenced by the U distribution within fractures, by the extent of the rock-water interface and by the amount of ²³⁸U in solution. A model for the quantitative evaluation of these effects is presented.Groundwaters from depths up to 880 m in the Stripa granite have variable dissolved uranium contents and ${}^{234}\text{U}{}^{238}\text{U}$ activity ratios. The uranium geochemistry is primarily determined by variations in flow path rather than by groundwater age.Dissolved radiogenic ⁴He in the groundwaters increases with their depth of origin, and is dependent upon the U content of the granite and upon its fracture porosity. It increases with groundwater residence time but movement of ⁴He by diffusion and transport processes make the actual groundwater age indeterminate.     

Paleoclimatic implications of U-series dates for lake sediments and travertines in the Arava Rift Valley,Israel

The Sayif travertines and lacustrine limestones cropping out in the northern Arava Valley, a region of extreme aridity, have been dated by the ${}^{230}\text{Th}{}^{234}\text{U}$ disequilibrium method. The consistency between the stratigraphic position and the apparent ages at each of the sampling sites strongly supports the reliability of the ages. Most of the ages fall within two clusters that are coincident with ages representative of oceanic oxygen-isotope stages 5 and 7. The colder intervening stage 6 was not a period of substantial travertine formation. During the warmer global episodes of isotopic stages 5 and 7 the arid south of Israel apparently was considerably wetter than at present.     

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

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.