Uranium Disequilibrium Research Articles

Developing a correlation index and U disequilibrium factor for the exploratory boreholes in Wahkut block of West Khasi Hills district, Meghalaya (India)

With the objective to affirm the apparent uranium ore zone grade/thickness with good confidence for the non-cored as well as those boreholes with poor core recovery in the sedimentary environment of Wahkut block, West Khasi Hills district of Meghalaya, a systematic study between the in situ measured γ-ray logs under field conditions and laboratory analyzed core assay under controlled conditions has been carried out.The study area spans 2sqkm having 33 cored and 39 non-cored boreholes with a cumulative drilled depth of about 11,000m. On this available cored borehole database, simultaneous uranium ore zone continuity, both for in situ γ-ray logs and radiometric core assay, was evaluated with the experimental parameters of 1.0m ore zone thickness at 0.010% eU3O8 cut off grade. A population of 18 such simultaneous qualifying zone (borehole log and core assay) across the 15 cored boreholes gave a grade thickness continuity index of 0.92 with a moderately strong relationship between the radiometric core assay and the in situ γ-ray logs.After validation of an in situ uranium ore zone, study boreholes were examined for their uranium disequilibrium status by fitting regression equations for the radiometric U3O8(β/γ) and spectrometric radium measurements, done on the borehole core samples. A parent favoring uranium disequilibrium was observed in the sedimentary environment of Wahkut block, with a log normal distribution. An average uranium disequilibrium factor 1.46±0.245 was estimated for the 19 study boreholes, across the block.

Uranium disequilibrium as a hydrological aid in studying the salinization processes in the Southeastern Coastal Plain of Israel

Saline waters, of unknown origin, have been encountered in the basal portions of the phreatic Coastal Plain aquifer of Israel. 234U/ 238U disequilibrium was used to trace their origin to the evaporate layers within the Saqiye aquiclude, the most saline, and the warmest (up to 42 °C) waters are also those that have the highest uranium concentrations combined with low 234U/ 238U activity ratios, derived as the ascending brines traverse underlying uranium-rich Senonian phosphorites, in secular equilibrium along fault conduits.

Reply to discussion by D. I. Cole

We strongly disagree with D.I. Cole’s contention that our use of strontium isotopes has not lead to any meaningful improvement in the chronostratigraphy of the Prospect Hill Formation exposed in the Tabakbaai Quarry (Franceschini and Compton, 2004). Interpretation of the complex stratigraphy of the West Coast has suffered from a lack of chronostratigraphy. The integration of radiocarbon, uranium disequilibrium and luminescent dating has greatly improved our understanding of upper Pleistocene and Holocene deposits of the West Coast (Roberts and Berger, 1997; Felix-Henningsen et al. , 2003; Compton and Franceschini, 2005). The challenge of the beach and aeolian deposits of the Prospect Hill Formation at Tabakbaai Quarry is that they are too old to date using these methods and so we applied the Sr isotope dating method because it has proved to be extremely useful in sorting out the complex late Cenozoic depositional history of the outer shelf offshore of Cape Columbine (Compton et al. , 2004; Wigley, 2005). Unfortunately the resolution of the Sr-derived ages we obtained is relatively poor mostly because the marine Sr isotope curve flattens out in the late Miocene (Farrell et al. , 1995) and …

Pleistocene deposits at Stoke Goldington, in the valley of the Great Ouse, UK

At Stoke Goldington in the valley of the Great Ouse in Buckinghamshire a river terrace at a height of about 7 m above the floodplain is underlain by fluvial sediments representing climatic fluctuations in the late Middle Pleistocene. Near the base of the succession, at a level only 1 m above the modern floodplain, a fossil assemblage, including pollen, plant macrofossils, molluscs, insects and ostracods, provides evidence for the local development of herb-rich grassland under temperate climatic conditions. The fossil record, amino-acid racemisation ratios and uranium disequilibrium dating all suggest deposition of this material during Oxygen Isotope Stage 7. The deposits containing the temperate assemblage are immediately overlain by typical cold-climate gravels of the Great Ouse. These have been subjected to a later cut-and-fill episode, with the fill accumulating in cool climatic conditions. The cut-and-fill episode was succeeded by aggradation, forming the overlying terrace surface. Amino-acid racemisation ratios indicate that the fill was emplaced, and the terrace surface created, during or after Oxygen Isotope Stage 5. © 1996 John Wiley & Sons, Ltd.

ESR dating of the Rotoiti Breccia in the Taupo volcanic zone, New Zealand

Electron spin resonance (ESR) can be used to determine the time of formation of quartz in volcanic rocks, based on the trapping of changes in the quartz resulting from environmental radiation. The method has been used on quartz from a rhyolitic pyroclastic flow, the Rotoiti Breccia, deposited NE of Rotorua, New Zealand. It yields a date of 45.2 ± 8.2 ka BP, the mean of ages of 48.6 and 41.8 ka BP obtained from the Al and Ti centers, respectively. This result is in good agreement with previous 14C ages of >41.6 ka BP and 44.2 ± 4.3 ka BP, but does not agree with a date of 71 ± 6 ka BP, based on uranium disequilibrium. We suggest that this discrepancy reflects the time difference between attainment of equilibrium at depth, and the emergence time of the pyroclastic flow on the surface where radiation bombardment commenced.

Uranium disequilibrium dating of phosphate deposits from the Lau Group, Fiji

Uranium-series methods of absolute age determinations were first applied to fossil coral reefs1 and more recently to fossil bone2, marine apatite3–6 and insular apatite7,8. Past sea level stands, climatic conditions, primary productivity, and other factors are thought to be influential in the deposition of avian guano and subsequent insular phosphorite formation9. Some deposits appear to have formed during high stands of the sea while others may be related to glacial epochs8. We report here an attempt to determine the absolute age of two phosphate deposits in the Lau Group, Fiji by uranium-series disequilibrium techniques, in order to assist in the evaluation of their origin in terms of known palaeoclimatic information.

Apatite-glauconite associations off Peru and Chile: palaeo-oceanographic implications

Authigenic apatite and glauconite occur within organic-rich diatomaceous sediment from the coastal upwelling zone off Peru and Chile. Both minerals are found as unconsolidated pellets and within indurated phosphate nodules. Uranium disequilibrium dating has shown that the apatite may range from present-day to +100,000 yr (Quaternary) in age. Episodes of phosphorite formation appear to be related to major climatic variations. The occurrence and progressive alteration of glauconite within some of these phosphate nodules also appears to be related to large scale oceanographic phenomena which fluctuated during the Quaternary. A lateral shifting or contraction of the oxygen minimum zone and locus of maximum upwelling in response to climate variations best explains the data.

A flow pattern deduced from uranium disequilibrium studies for the Cenomanian carbonate aquifer of the Beersheva region, Israel

Changes in the uranium isotopic activity ratio and uranium content are correlated to changes in the calcium and sulphate content in waters from wells in various locations in the Cenomanian aquifer in the Beersheva region. These changes are related to an isotopically non-selective leaching of uranium from the overlying uranium- and sulphate-enriched Senonian rocks as the waters flow. A flow pattern can therefore be produced by plotting these systematic changes on a regional map.

Racemization of isoleucine in fossil molluscs from Indian middens and interglacial terraces in southern California

The amino acid racemization dating technique is applied to the West Coast mollusc Chione from interglacial terrace deposits and Indian midden sites. The racemization kinetics of isoleucine were studied in Total hydrolysates and in the non-dialyzable Protein fraction from Chione shell. The reliability of the isoleucine dates was evaluated by (1) comparison with ages determined by radiocarbon and uranium disequilibrium dating, (2) species effects, and (3) reproducibility. Isoleucine dates for the interglacial terrace shell were found to be more reliable than the midden shell dates. A radiocarbon analysis of amino acids extracted from Del Mar midden shell was performed in order to resolve the discrepancies between the racemization ages and the 14 C dates on shell carbonate.

Studies on thorium and uranium disequilibrium on the surface layers of the coastal sediments and their absolute concentrations in the sediments

Measurements of thorium and uranium isotopic activity ratios have been carried out with top layers of the sediments of the West coast of India in order to understand the behaviour and distribution of these isotopes in the coastal region. The southern part of the West coast contains a large amount of placer deposits while the northern region is free from such deposits.

Uranium series dating of stalagmites from Blanchard Springs Caverns, U.S.A.

Because stalagmites are enriched in 234U beyond the equilibrium value at the time of deposition, they are potentially datable by measuring their progress toward equilibrium. Procedures for uranium-thorium and uranium disequilibrium dating of speleothems were developed, making use of alpha spectrometry for thorium determinations and mass spectrometry for uranium determinations. The stalagmites tested were generally beyond the age range of the U-Th age dating method and, as a result, 234U/ 238U ratios provided the most useful dating technique. The estimated age range for a stalagmite from Blanchard Springs is 135,000 to 785,000 years BP with an interval of slow or suspended growth from about 320,000 to 570,000 years BP.

Uranium disequilibrium in groundwater: an isotope dilution approach in hydrologic investigations.

The distribution and environmental disequilibrium patterns of naturally occurring uranium isotopes (U(234) and U(238)) in waters of the Floridan aquifer suggest that variations in the ratios of isotopic activity and concentrations can be used quantitatively to evaluate mixing proportions of waters from differing sources. Uranium is probably unique in its potential for this approach, which seems to have general usefulness in hydrologic investigations.