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Model for the evolution of the chemical systems of the earth’s crust and mantle defined by radiogenic strontium distribution, and the rubidium-strontium geochemistry of the Shulaps Range and other ultramafic bodies in and near southwestern British Columbia Athaide, Dileep Joseph Anthony

Abstract

Initial strontium isotopic ratios have contributed much to our understanding of the chemical, and related tectonic, evolution of the earth. Improved analytical techniques have recently provided a flood of precise strontium analyses which are very useful in the determination of the genesis of various rocks. The theories of sea-floor spreading and plate tectonics have prompted a new interpretation of the earth's chemical evolution, based on the tracer properties of radiogenic strontium. Strontium isotopic ratios are considered for rocks of various composition, environment and age: stony meteorites, oceanic and continental basalts, island arc and andesitic volcanics, anorthosites, carbonatites and alkaline intrusives, and granitic and sedimentary rocks. Special emphasis is placed on ultramafic rocks, believed in some circumstances to provide direct samples of the mantle regions of the earth. Here the rubidium-strontium geochemistry is studied for the different petrogenetic categories: oceanic ultramafics, alpine-type intrusions, concentrically-zoned bodies, nodules in alkali basalts and in kimberlites, and the layered ultramafic zones in major intrusions. Patterns revealed by the strontium distribution lead to a unique definition of the major chemical systems of the earth. These systems, and the ranges of their characteristic present-day Sr87/Sr86 ratios are: the lower (deep) mantle (0.701 to 0.703), the oceanic upper mantle (0.707 to 0.715), the continental upper mantle (0.703 to 0.706), and the continental crust (0.701 to 0.703+). The source of all surface magmatic rocks can be attributed to one or a combination of these reservoirs. Several mechanisms are proposed for the transfer of material from the internal to surface systems. A model, together with a computer-plot, is presented for radiogenic strontium evolution in the earth's major chemical systems. It includes the following present-day conditions: (1) an alpine-type ultramafic zone constituting primarily the oceanic, rather than the continental, upper mantle; (2) a common deep-mantle source for both oceanic tholeiitic and alkaline basalts; and (3) the possibility of at least two, and perhaps three, distinct reservoirs contributing to igneous activity from directly below the continental crust. A rubidium-strontium geochemical study was undertaken for the Shulaps Range and other ultramafic bodies in and near southwestern British Columbia. The Shulaps rocks, predominantly serpentinized harzburgites, yield Rb and Sr concentrations averaging 0.2 ppm and 4.2 ppm respectively, as determined by x-ray fluorescence analysis. The corresponding Rb/Sr ratio of 0.05 is fairly typical of alpine-type intrusions. Mass spectrometer analysis gives an average Sr87/Sr86 ratio of 0.7064 for these whole-rocks. This is just slightly below the range which is normally observed for alpine-type ultramafic bodies and which is believed to represent the oceanic upper mantle system.

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