Geochemical evolution in an advancing, periodically replenished, periodically tapped, continuously fractionated magma chamber

Abstract

A summary of the development of ideas concerning geochemical evolution in periodically replenished, periodically tapped, continuously fractionated magma chambers lists 28 main conclusions or observations. The main body of the paper presents the derivations of the relationships which underpin these conclusions and lead on to a further 10 observations about the likely behaviour of such systems. If periodically replenished, periodically tapped, continuously fractionated magma chambers exist, they will evolve products whose phase petrology and trace element chemistry appear (in hitherto conventional interpretations) to require variable degrees of partial melting of in homogeneous source regions for their petrogenesis. When the additional effects arising from assimilation by the magma of the roof of the chamber and from variation of mineralogy at the solidus of even a chemically homogeneous peridotite mantle are added, the scope for confusion is greatly increased. Moreover, these relationships cannot be inverted in order to deduce uniquely the magma chamber parameters or mantle source compositions from a knowledge of the erupted products. Available evidence suggests that interpretation of many igneous provinces, especially that of the mid-ocean ridges, will be complicated by these considerations.