Recent Work on Oscillatory Zoning in Plagioclase

Abstract

Growth of crystals from magma is one of the fundamental processes of geology. Changes in composition during growth produce a zonation resembling “tree rings” which is interpreted in a somewhat similar manner (although the periods of growth resulting in zones are not necessarily annual). In principle, zoned crystals have immense potential as tracers of magmatic processes. Major and trace elements have, at least in theory, the potential to record different aspects of the growth phenomenon. Complications result from the fact that crystallization is inherently a non-equilibrium process and the chemistry of the precipitating phase may therefore be influenced by kinetic factors. This zoning is present in a wide variety of rocks and has been studied for well over 100 years, yet it remains a challenging and perplexing subject.

Modern methods of microanalysis and imaging techniques are providing a new view of zoning. The application of interference imaging including Nomarski Differential Interference Contrast (NDIC) has provided hither-to-unknown information regarding the extreme complexity of magmatic zoning. Dissolution and reaction are extremely common features in magmatic phenocrysts. The complexity of typical zoning appears to indicate that phenocrysts move around a magma chamber and may alternatively experience (cycle through) different liquids, experiencing different temperatures and pressures during growth - as was posited by Homma in his classic work of 1932.

It is generally accepted,a priori, that crystal growth is a deterministic process and that zoning results from a complex interaction of growth and diffusion processes. New work indicates that, in many instances, zonation is further complicated by dissolution and reaction phenomena. In spite of the complexity of the processes, it should be possible to model plagioclase growth using continuous differential equations; yet, oscillatory zoning of magmatic plagioclase has, so far, defied detailed analysis or explanation. Published models at the present time typically assume a relatively simple growth mechanism and do not yield realistic zoning patterns or profiles. A possible exception is the constitutional supercooling model, but one key assumption of this model - that composition is more important than temperature in determining the initial supercooled crystal composition - has yet to be proven.

Utilizing new techniques recently developed for the analysis of dynamical systems (“chaos” theory), along with computer experiments, it is possible to extract previously unrecognizable kinetic information contained in zoning patterns. Detailed analysis of oscillatory zoning in magmatic plagioclase from several volcanoes indicates that the plagioclase- magma system is a deterministic nonlinear system whose attractor has a relatively low dimension. Patterns of zonation are not consistent with random or stochastic processes.

The ultimate objective of current studies of plagioclase is to find a method of decoding the zonation so that we may read the life history of a crystal treating the zones like ”pages” of a book.