Abundance Of Radioactive Elements Research Articles

Precipitation correction of airborne gamma-ray spectrometry data using monitoring profiles: methodology and case study

Variations of soil moisture content caused by precipitation often complicate the interpretation of airborne gamma-ray spectrometry data. This is particularly the case in repeated surveys designed to monitor the change of near surface abundances of radioactive elements or in large and time-consuming surveys.To counter this precipitation effect we propose a correction method based on repeated survey flights over a monitoring profile. Assuming that the weather and the soil conditions at the monitoring profile are representative for the survey area, the weather dependent effect of soil moisture can be observed and sufficiently corrected.

Stellar ages and their cosmological context

Stellar ages have traditionally been used to constrain cosmological models by setting lower limits to the age of the universe. We quickly review the basic cosmological concepts that underpin the derivation of a finite age, and the constraints given by the measure of cosmological parameters at a redshift z ∼ 1000. The most direct way of measuring stellar ages through the abundance of radioactive elements is shown to suffer from systematics in the evaluation of the production ratios, to the extent that the constrains are less stringent that previously thought. The discrepancy between the ages of the oldest stars and the cosmologically inferred age of the universe may well be dominated by systematc effects in the predicted evolution of low-mass stars.

Crustal Composition of China Continent Constrained from Heat Flow Data and Helium Isotope Ratio of Underground Fluid

Abstract:Based on conservation of energy principle and heat flow data in China continent, the upper limit of 1.3 μW/m3 heat production is obtained for continental crust in China. Furthermore, using the data of heat flow and helium isotope ratio of underground fluid, the heat productions of different tectonic units in China continent are estimated in range of 0.58–1.12 μW/m3 with a median of 0.85 μW/m3. Accordingly, the contents of U, Th and K2O in China crust are in ranges of 0.83–1.76 μg/g, 3.16–6.69 μg/g, and 1.0%–2.12%, respectively. These results indicate that the abundance of radioactive elements in the crust of China continent is much higher than that of Archean crust; and this fact implies China's continental crust is much evolved in chemical composition. Meanwhile, significant lateral variation of crustal composition is also exhibited among different tectonic units in China continent. The crust of eastern China is much enriched in incompatible elements such as U, Th and K than that of western China; and the crust of orogenic belts is more enriched than that of platform regions. It can also be inferred that the crusts of eastern China and orogenic belts are much felsic than those of western China and platform regions, respectively, derived from the positive correlation between the heat production and SiO2 content of bulk crust. This deduction is consistent with the results derived from the crustal seismic velocity data in China. According to the facts of the lower seismic velocity of China than the average value of global crust, and the higher heat production of China continent compared with global crust composition models published by previous studies, it is deduced that the average composition models of global continent crust by Rudnick and Fountain (1995), Rudnick and Gao (2003), Weaver and Tarney (1984), Shaw et al. (1986), and Wedepohl (1995) overestimate the abundance of incompatible elements such as U, Th and K of continental crust.

Mantle convection controls the observed lateral variations in lithospheric thickness on present‐day Mars

Sounding radar observations of the north polar layered deposits on Mars suggest an elastic lithosphere thickness of at least 300 km in that region, significantly larger than any prior estimate of lithospheric thickness on Mars. Previous studies interpreted this result as requiring either that Mars is significantly sub‐chondritic in its abundance of radioactive elements, or alternatively that there are large lateral variations in lithospheric thickness. We find that the required depletion of radioactivity is geochemically unlikely. On the other hand, stagnant lid convection naturally produces factor of two or more lateral variations in both mantle heat flux and lithospheric thickness and is the preferred explanation for the polar elastic thickness measurement. Our mantle plume models, which have previously been shown to explain observations of present‐day magma production on Mars, can also explain the new elastic thickness observations.

Radiolytic Hydrogen and Microbial Respiration in Subsurface Sediments

Radiolysis of water may provide a continuous flux of an electron donor (molecular hydrogen) to subsurface microbial communities. We assessed the significance of this process in anoxic marine sediments by comparing calculated radiolytic H(2) production rates to estimates of net (organic-fueled) respiration at several Ocean Drilling Program (ODP) Leg 201 sites. Radiolytic H(2) yield calculations are based on abundances of radioactive elements (uranium, thorium, and potassium), porosity, grain density, and a model of water radiolysis. Net respiration estimates are based on fluxes of dissolved electron acceptors and their products. Comparison of radiolytic H(2) yields and respiration at multiple sites suggests that radiolysis gains importance as an electron donor source as net respiration and organic carbon content decrease. Our results suggest that radiolytic production of H(2) may fuel 10% of the metabolic respiration at the Leg 201 site where organic-fueled respiration is lowest (ODP Site 1231). In sediments with even lower rates of organic-fueled respiration, water radiolysis may be the principal source of electron donors. Marine sedimentary ecosystems may be useful models for non-photosynthetic ecosystems on early Earth and on other planets and moons, such as Mars and Europa.

Modeling the Volcanism on Mars

The total amount of melt produced in Mars during its evolution is estimated by means of a parameterized, one-dimensional, analytic mantle convection model that assumes a stagnant lid and whole mantle convection. The fertility of the mantle—defined as the potential to create basalt—and its variation with time are taken into account. The model is composed of core, mantle, and lithosphere, with two boundary layers separating them. The contributions to volcanism by pressure release melting (PRM), and by plumes from the core–mantle boundary layer, are compared and discussed. We show that such models tend to produce considerable melting during the early evolution of the planet, and that the amount of melting depends strongly on the abundances of radioactive elements. Although the model's assumptions may not be valid for the early evolution of the planet, the model is relevant to the later history, which is insensitive to initial conditions. We find that PRM volcanism should have ceased between 1 and 2.5 Byr ago and any recent volcanic activity must have originated in plumes.

Deep temperatures and lithospheric thickness along the European Geotraverse

One-dimensional, steady-state heat conduction models in the crust and upper mantle for 24 areas along the European Geotraverse (EGT) show that temperatures are higher beneath young geological regions; at the Moho they range from 450° to 825°C. Heat flowing out from the mantle varies from 18 to 72 mW/m 2 and the contribution of crustal heat-flow density, evaluated by the correlation between seismic velocity and the abundance of radioactive elements, accounts for about 48% of the surface heat-flow density. The temperature gradient below Moho (from about 5 to 23 mK/m) is low where the surface heat-flow density reaches its minimum values. The depth at which the geotherms reach 0.85 of the mantle melting temperature is greater in the Precambrian and Caledonian areas, with a maximum value of about 195 km under the Early Proterozoic Scandinavian Shield. In the Hercynian and Alpine areas, this depth is generally lower. Higher values (maximum 150 km) coincide with the orogenic belts and the continental block of Sardinia and Corsica; under the Ligurian Sea and the Sardinian Channel, the lithospheric thickness decreases remarkably to about 50 km. Globally, the lithosphere thickness is controlled by the mantle heat-flow density which decreases, according to a power law, with an increase in the lithosphere-asthenosphere depth.

Two‐layer mantle convection and the depletion of radioactive elements in the lower mantle

Only a small percentage of the mantle’s total content of radioactive elements can be in the lower mantle if this region convects steadily and separately from the upper mantle. Recent geochemical models of a depleted upper mantle and an undepleted lower mantle are thus inconsistent with a steady thermal model involving distinct upper and lower mantle convection cells. Steady two‐layer mantle convection requires a depleted lower mantle and an undepleted upper mantle to match the observations of surface heat flow and mantle viscosity. The constraint on the abundance of radioactive elements in the lower mantle depends on the existence of at least one thermal boundary layer at the upper mantle‐lower mantle interface with thickness comparable to that of the surface thermal boundary layer. Since the temperature drop across an interface boundary layer is limited by the relatively small difference between the solidus temperature and the upper mantle temperature, the amount of heat which can be transferred across this layer is small compared with the surface heat flow. The upper limit on the concentration of radioactive elements in the lower mantle can be relaxed only if both the lower boundary layer of the upper mantle cell and the upper boundary layer of the lower mantle cell are extremely thin or absent altogether, which we consider, however, highly unlikely.

Heat flow from a differentiated Earth

Calculations are made of the heat flow from an initially cold earth with radioactivity distributed uniformly through a surface shell, with account taken of radioactive decay. It is found that the heat flow exceeds the heat produced by about 10 per cent if the abundances of radioactive elements are the same as in chondrites and the radioactive shell is less than about 300 km thick. The heat flow calculated in this way exceeds the observed value, but neither figure is considered known accurately enough to warrant rejection of the chondrite model of the earth.

Some aspects of the thermal evolution of the earth

Empirical data relating to the thermal history of the earth are examined. Recent astronomic and geochemical evidence strongly suggests that the earth formed by accretion from an initially low-temperature gas-dust cloud of solar composition. The distribution of U, Pb, Th and K within the earth imply that it passed through a melting or partial melting process about 4.5 × 10 9 years ago. This conclusion is confirmed if the core is assumed to consist dominantly of iron-nickel. Formation of the core, which likewise occurred about 4.5 × 10 9 years ago would liberate sufficient gravitational energy to cause melting. Evidence in favour of melting is also provided by analogy with meteorites. An examination is made of possible causes of this early melting stage and it is concluded that gravitational energy is chiefly responsible. Radioactive heating does not appear to be important. A critical factor in the early heating and chemical evolution is the interaction of accreting dust falling with high velocity into the primitive reducing atmosphere surrounding the earth. Because of this interaction, a metallic phase is produced by reduction. The distribution of temperature within the earth 4.5 × 10 9 years ago will be given by the melting-point gradient. Recent data on the electrical conductivity of the mantle and the melting point of metals under high pressures suggest that the present temperature distribution is much less than the melting point gradient. This implies that the earth has cooled considerably. The inferred cooling is consistent with present data on the abundance of radioactive elements in meteorites and in the earth, and also with possible modes of internal heat transfer—particularly convection and radiation.