Uranium, thorium and potassium contents of possible mantle materials
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Neutron activation analysis of U, Th and K has been made on some possible mantle materials, mainly peridotite nodules from continental and oceanic areas. The paper first describes the analytical procedures of U and Th determinations. Results of analysis showed that oceanic nodules have more uniform concentrations of U, Th and K and Th/U, K/U ratios than continental nodules. Average values for 7 oceanic lherzolites are: (1.87±0.27)×10-8 gU/g, (5.48±0.87)×10-8 gTh/g, and (54.2±7.5)×10-6 gK/g, giving a rate of heat production, 26×10-16 cal/cm3·sec, which is about 70% of that for the average chondrite. There is no indication that oceanic lherzolites are more enriched in these elements than the continental ones. These results suggest that, from the thermal point of view, peridotite nodules may not be the representative constituent of the upper mantle, unless the oceanic mantle is capable of transferring heat exceedingly well. The average values of the ratios are Th/U=2.93±0.61, and K/U=(0.29±0.06)×104. The latter is significantly smaller than the crustal value (1×104), which is already smaller than the chondritic value (5×104).Keywords:
Peridotite
Neutron Activation Analysis
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This article reviews studies on fundamental question, whether oceanic peridotite can be the source of oceanic crust or the residue left after formation of oceanic crust, using a radiogenic isotopic composition from the 1990s. The Sr-Nd isotopic compositions of the oceanic peridotite are highly heterogeneous compared to those of the oceanic crust. The wide isotopic variation cannot be explained by simple partial melting or interaction between melt and peridotite. It requires several stages of ancient partial melting and interaction with melt. It has been suggested that the Sr-Nd isotopic compositions of the oceanic crust are homogenized by partial melting of the heterogeneous mantle peridotite. Accumulation of Sr-Nd isotopic data of the oceanic peridotites will be useful to further decipher the processes of partial melting and/or melt-peridotite interaction which occurred through the Earth's history.
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The stretching and thinning of the continental crust, which occurs during the formation of passive continental margins, may cause important changes in the velocity structure of such crust. Further, crust attenuated to a few kilometres' thickness, can be found underlying 'oceanic' water depths. This paper poses the question of whether thinned continental crust can be distinguished seismically from normal oceanic crust of about the same thickness. A single seismic refraction line shot over thinned continental crust as part of the North Biscay margin transect in 1979 was studied in detail. Tau-p inversion suggested that there are differences between oceanic and continental crust in the lower crustal structure. This was confirmed when synthetic seismograms were calculated. The thinned continental crust (α ≥ 7.0) exhibits a two-gradient structure in the non-sedimentary crust with velocities between 5.9 and 7.4 km s-1; an upper 0.8 s-1 layer overlies a 0.4 s-1 layer. No layer comparable to oceanic layer 3 was detected. The uppermost mantle also contains a low-velocity zone.
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We investigate how subduction may be triggered by continental crust extension at a continental margin. The large topography contrast between continental and oceanic domains drives the spreading of continental crust over oceanic basement. Subduction requires the oceanic plate to get submerged in mantle, so that negative buoyancy forces may take over and drive further descent. This is promoted by two mechanisms. Loading by continental crust bends the oceanic plate downwards. Extension in the continental domain induces crustal thinning, which acts to raise mantle above the oceanic plate. In this model, the width of the continental region undergoing extension is an important control parameter. The main physical controls are illustrated by laboratory experiments and simple theory for elastic flexure coupled to viscous crustal spreading. Three governing dimensionless parameters are identified. One involves the poorly constrained oceanic plate buoyancy. We find that the oceanic plate can be thrust to depths larger than 40 km even if it is buoyant, enabling metamorphic reactions and density increase in the oceanic crust. Another parameter is the ratio between the width of the continental extension region and the flexural parameter for the oceanic plate. Initiating subduction is easier if the continent thins over a short lateral distance or if the oceanic plate is strong. The third important parameter is the ratio of oceanic plate thickness to initial continental crust thickness, such that a weak plate and a thick crust do not favour subduction. Thus, the change from a passive to an active margin depends on the local characteristics of the continental crust and is not determined solely by the age and properties of the oceanic lithosphere. It is shown that the spreading of continental crust induces uplift of the margin as the adjacent seafloor subsides. Evidence for the emplacement of continental crust over oceanic basement at passive margins is reviewed.
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(1980). Transition zones from continent to ocean in areas of development of young continental and oceanic crust. International Geology Review: Vol. 22, No. 7, pp. 759-768.
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