The first part of this programme is about the strontium isotope method for distinguishing between the earth's crust and the mantle as sources of igneous rocks. Dr. R.S. Thorpe shows how the theory ...is applied, before introducing Dr. S. Moorbath (University of Oxford), who describes how he and his colleagues resolved a controversy about the origin of the granites from the Isle of Skye. Dr. Moorbath continues by showing how the strontium isotope measurements from acid and basic rocks from Iceland point to the absence of continental crust under this volcanic island. Finally Dr. Thorpe considers how the strontium ratio has varied with time to show in more detail what the theory involves. In the second part of the programme John Wright (Reader in Earth Sciences) discusses some of the difficulties of finding a comprehensive theory to account for the origin of metal ores. Magmatic fractionation and the role of ground water are discussed, together with the value of oxygen isotope data for determining the source of ore bearing solutions tapped near the Salton Pea in California.
|First transmission date:
|Restrictions on use:
|+ Show more...
|S Moorbath; Richard Thorpe; John Wright
|BBC Open University
|Crust and mantle; Igneous rock origins; Isle of Skye granite; Magmatic fractionation; Metal ore origins; Oxygen isotope data; Salton Sea California; Strontium isotope method
|R.S.Thorpe introduces the programme. Shot of a drawing showing a thin section of granite. Crystals of minerals can be seen. Cross section diagram of the earth showing the crust and mantle. Thorpe discusses two ways in which granite may have been formed. Shot of the granite section drawing again. Thorp explains that among its trace elements, the granite contains strontium and Rubidium. (Sr and Rb). He then explains why 86 Sr. and 87 Sr isotopes are particularly important for determining the origins and dates of a granite specimen. Graph shows concentration/time for 87Rb-87Sr isotopes. Commentary by Thorpe explains how origins and dates for rocks can be determined from an analysis of ratios of 87Rb and 87 Sr isotopes and 86Sr-87Sr isotopes. The ratios are plotted on a graph. Thorpe introduces the next sequence. Several shots of the Isle of Skye. Commentary gives its geological background. Both sides of the controversy over the origins of the granite there are given. S. Moorbath explains how the study of strontium isotope values aided in clearing up the above controversy. He uses diagrams and charts as aids Moorbath turns to an examination of Iceland's geological background. He again uses diagrams and charts as aids. "Ratio of strontium isotope" technique is used to show the place and method of igneous rock formation in Ireland. Thorpe introduces a sequence which explains how strontium 87 isotope changes to strontium 86 isotope. The ratios over time are plotted on a graph. The sequence covers rocks formed both in the crust and in the mantle. Thorpe introduces J.B.Wright who will discuss methods for determining the origins of metal. Wright begins by discussing "What is a metal ore" He shows samples of metal and non-metal ores. Shot of photograph showing a vein of copper ore. Wright explains where metal ores are found and how veins of metals in rocks are formed. Animated diagram of granite magma shows the formation of crystals, mainly anhydrous minerals but also metal crystals. The diagram shows metal ores escaping in hydrothermal solutions and solidifying in veins on their way to the surface. Wright uses the diagram of granite magma and hydrothermal fissures to broaden the picture of the actual process of metal ore formation. Wright gives an alternative theory, that of hydrothermal leaching, to explain the origins of metal ore veins. Wright discusses some of the evidence for the two theories of metal ore formation above. Wright briefly explains a technique for distinguishing magmatic water from ground water. He uses a diagram as an aid. Wright places the result of the Salton Sea (California) sample on his diagram. This shows that a great deal of ground water is involved in the Salton Sea system Wright shows a map of Britain on which ore and granite locations are marked. He explains that the models are by no means perfect for forecasting the location of metals.
|Master spool number:
|Available to public: