ANOTHER LOOK AT THE CONSTANT SUM PROBLEM IN GEOCHEMISTRY

Compositional data-that is data where concentrations are expressed as proportions of a whole, such as percentages or parts per million-have a number of peculiar mathematical properties which make standard statistical tests unworkable. In particular correlation analysis can produce geologically meaningless results. Aitchison (1986) proposed a log-ratio transformation of compositional data which allows inter-element relationships to be investigated. This method was applied to two sets of geochemical data-basalts from Kilauea Iki lava lake and grantic gneisses from the Limpopo Belt - and geologically 'sensible' results were obtained. Geochemists are encouraged to adopt the Aitchison method of data analysis in preference to the traditional but invalid approach which uses compositional data.