Magmatic crystal stratigraphy and constraints on magma chamber dynamics: Laser interference results on individual phenocrysts

This work reports progress in current studies of zoning of magmatic crystals from basalts and andesites. A simple law of normal growth is tested using zoning profiles of olivine and plagioclase crystals and is shown to be compatible with the zoning profiles of some olivines and parts of complexly zoned plagioclases. Using a convenient linearizing technique (in which the ratio of compositional end-members is plotted against volume), we can identify those parts of the zoning profile which fit a simple normal growth law. This technique permits easy identification of those parts of a complexly zoned crystal which require a more complex law of growth. Calculations used in this modeling are similar to those previously used for olivine (Pearce, 1984c, 1987) and are now extended to plagioclase. It is concluded that it is both theoretically possible and practically feasible to make inferences about the liquid in the local system from which a crystal grew by examining the detailed zoning pattern of the crystal.