Thorium-232, Igneous rocks

An alternative to the bridge technique was recently reported for thorium analysis in silicate rocks for which both Th and Th are measured on a single lon-counting detector (Rubin 2001). With careful chemistry and mass spectrometry, °Th/ Th ratios of igneous rocks can be measured with this technique with a precision that is similar to the bridge method. The disadvantage of this technique is that °Th ion-count rates are extremely low (around 10 cps) with normal silicate thorium ratios and are therefore subject to perturbations from background variation and low-level isobaric interferences in normal samples. 

Thorium is widely but rather sparsely distributed its only commercial sources are monazite (together with the rare earths) and uranothorite (a mixed Th, U silicate). Uranium is surprisingly common and more abundant than mercury, silver or cadmium in the earth s crust. It is widely distributed and it is found scattered in the faults of old igneous rocks. Concentration by leaching followed by re-precipitation has produced a number of oxide minerals of which the most important are uranite (also called pitchblende) U308 and carnotite, K UC HVO -SF O. 

Brown H. and Silver L. T. (1955) The possibility of obtaining long-term supplies of uranium, thorium, and other substances of uranium from igneous rocks. In Proceedings of Conference on Peaceful Use of Atomic Energy, Geneva, IAEA, pp. 91-95. 

The types of substance that are thermoluminescent, either in their natural state or after radiation bombardment, include (112) the alkali metal halides, calcite, dolomite, fluorite, aluminum oxide, magnesium oxide, gypsum, quartz, glass, feldspars, feldspathoids, certain dried clays, and ceramic materials. Of over 3000 rock samples examined for thermoluminescence, some 75% showed visible fight emission (112). Nearly all limestones and acid igneous rocks are naturally thermoluminescent, due mainly to the presence of trace elements of uranium, thorium, and so on. Calcium and magnesium... 

Dating sulfide minerals is complicated because they lack radioactive elements such as uranium, thorium, and rubidium. However, some contain potassium, e.g., the mineral rasvumite, KFe2S3. Samples of this mineral were collected from an alkali-rich igneous rock at Coyote Peak near Areata, northern CaKfomia, and they gave a date of 26.5 0.5 Ma, while associated phlogopite mica yielded a total release date from Ar/ Ar measurements of 28.3 0.4 Ma. This confirms that potassium-bearing sulfide minerals can be dated on the basis of the radioactive decay scheme... 

In sedimentary rocks, the relative mean abundances are less predictable than in igneous rocks. On the average, potassium is lower in effective concentrations than uranium or thorium, and thorium contributes about the same level of activity as uranium. As a class, carbonates are the lowest in natural radioactivity of the sedimentary rocks. Generally, shales will have a higher level of natural radioactivity than other sediments consequently, the gamma-ray sonde is used to distinguish between shales and other sediments". (Hearst and Nelson, 1985). 

In igneous rocks, the general tendency is an increase of radiation from ultra-basic to acid rocks. This is attributed to the higher uranium, thorium and potassium content of mica and alkali feldspars. Alteration can change the radioactivity. 

In most igneous rocks, uranium and thorium contribute in a comparable amount, whereas potassium always contributes a substantially smaller amotmt to total heat production, in proportions of approximately 40% (U) 45% (Th) and 15% (K) (Rybach and Cermak, 1982). Table 5.10 gives some data. 

Uranium, thorium and potassium content varies with rock type and shows increasing radioactive heat generation from basic to acid igneous rocks. This tendency is also reflected in empirical equations correlating heat generation and density and seismic velocity, because both parameters increase from acid to basic t5q>es. Examples are given in Table 5.12 ... 

If one assumes, as Lord Kelvin did, that the internal heat is still part of the primordial heat, now emerging as a consequence of the cooling process from a once molten state, the age of the Earth would be only 30 x 10 years, which is inconsistent with all other evidence. Now we know the Earth has, for all practical purposes, completely cooled from the primordial state and the internal heat presently originates from radioactive decay of uranium, thorium, and potassium, all elements found in igneous rock such as granite and basalt. 

Today, the more external part of the crust or lithosphere constitutes the superficial covering of the earth. Two kinds of crust are easily distinguished by composition, thickness and consistency continental crust and oceanic crust. Continental crust has a thickness that, in mountain chains, may reach 40 kilometers. It is composed mainly of metamorphic rock and igneous blocks enriched with potassium, uranium, thorium and silicon. This forms the diffuse granitic bedrock of 45 % of the land surface of the earth. The oceanic crust has a more modest thickness, in the order of 5-6 kilometers, and is made up of basaltic blocks composed of silicates enriched with aluminium, iron and manganese. It is continuously renewed along mid-ocean ridges (cf Table 2.2). 

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