Selenium in certain igneous rocks

Abstract

The selenium contents of the igneous rocks from Finland and Iceland and a few from West-Greenland studied in this paper can be tentatively summed as follows: The selenium content shows a general tendency to decrease when passing from a basic to a more siliceous rock. Excluding ultrabasic rocks, this statement can also be expressed by saying that the selenium content exhibits a positive correlation with the colour index of the rock. Selenium very probably does not enter the structures of the common rock-forming minerals but is largely contained in the accessory sulphide phase. The distribution of selenium between the various calc-alkalic igneous rocks mainly reflects the abundance of the sulphide phase in the rocks. The volcanic rocks of Iceland are generally more rich in selenium than their abyssal equivalents in the Finnish Precambrian. This fact could possibly result from the slowly advancing crystallisation of the igneous rock masses under deep-seated conditions, thus offering a better opportunity for the sulphide-bearing emanations to escape from the magma and enter the surrounding bedrock. Adopting the standard section constituting the upper continental earth's crust, tentatively estimated by Wedepohl (1969, 244), the data of Table 1 yields a value of 47 ppb as the average selenium content of abyssal igneous rocks. Using the average of two parts acidic (granite) to one part basic (gabbro) rock, the average selenium content of abyssal igneous rocks amounts to 58 ppb. Both these average estimations are near the value 50 ppb presented by Turekian and Wedopohl (1961) and by Vinogradov (1962).