INTRODUCTION Silicate magmas form by partial melting of silicate rocks pri- marily in the mantle. These melts may collect into magma reservoirs within the crust, where they cool and start to crystallize, before ascending towards the surface and erupt- ing. Decompression and degassing further promote the crystallization of phenocrysts. Because the crystal composi- tion in a magma depends on crystallization conditions, such crystals provide us with a record of the processes and conditions (pressure, temperature and volatile content) in the magma chamber and may also contain clues about the mechanisms triggering eruption. The study of phenocrysts by optical microscopy has documented compositional zon- ing with respect to major elements. Close inspection has revealed complex growth patterns. Microbeam techniques now allow the analysis of major and trace elements in these crystals at high spatial resolution. Such information – espe- cially if combined with 2D mapping of zoning patterns (e.g. FIG.1) – has provided us with unprecedented detail on the compositional evolution of magmas and growth histories of phenocrysts. Today we are beginning to decipher the mean- ing of the variations in these crystals. We have abandoned our simple concept of phenocrysts that float (sink or rise) while they grow in a magma chamber. New research has documented multiple recharge events in magma chambers before phenocrysts erupt. Redistribution and dispersion of growing crystals among different portions of a magma reservoir appear to be common processes. Therefore, grow- ing crystals record different environments (e.g. magma chamber boundary layers, cool magma cupolas, hot interi- ors) and crystallization conditions (e.g. mixing of magmas, degassing, assimilation). In this contribution we present some historical background on compositional zoning studies. Then we detail the electron microprobe methods (and some SEM techniques) available and compare them with other methods. We show how these tech- niques can be applied to study magmatic systems in terms of processes and the pre-eruptive his- tory of the magmas. Finally, we dis- cuss some recent advances and remaining challenges in the inves- tigation of magma systems using zoned phenocrysts. HISTORY OF ZONING STUDIES Studies of zoning patterns involve both the characterization of growth-zone morphology and the acquisition of quanti- tative information on compositional variations. Most zoning studies in magmatic systems to date have been performed on plagioclase (sodium-calcium feldspar) because the chem- ical variations in this mineral can readily be inferred from its optical properties seen in polarized light using an optical microscope. Homma (1932), for example, proposed convec- tion in the magma chamber as an explanation for oscilla- tory plagioclase zoning. Since then, numerous optical studies of natural igneous pla- gioclase have shown a variety of zoning patterns and crystal morphologies. “Normal” zoning in plagioclase is a monot- onous change from a high-temperature Ca-rich composi- tion in the core to a lower-temperature Na-rich composition at the rim. This compositional change mimics cooling and chemical differentiation of the host magma. “Reverse” zon- ing indicates disequilibrium conditions and a return to less- evolved compositions. “Oscillatory” zoning is a repetitive, more or less periodic variation in plagioclase composition, resulting in concentric growth zones from a few to tens of E LEMENTS ,V OL . 3, PP . 261–266 AUGUST 2007 Catherine Ginibre 1 , Gerhard Wörner 2 and Andreas Kronz 2 1 Département de Minéralogie, Université de Genève 13 rue des Maraîchers, 1205 Genève, Switzerland E-mail : catherine.ginibre@terre.unige.ch 2 Geowissenschaftliches Zentrum Göttinger. Abt. Geochemie Goldschmidtstr. 1, 37077 Göttingen, Germany Crystal Zoning as an Archive for Magma Evolution 261 S patial compositional variations in magmatic minerals record chemical and physical changes in the magma from which they grew. Electron-beam techniques allow high-resolution imaging and quantitative analysis of this compositional archive for major, minor and some trace elements. In this way, magmatic processes such as crystallization, recharge in a magma chamber, decompression during ascent, and convection in the magma chamber can be identified and the history of magmatic systems prior to eruption reconstructed. KEYWORDS: zoning, igneous petrology, magma chamber, electron microprobe, scanning electron microscope Parinacota volcano, northern Chile Barium X-ray distribution map of a sanidine crystal from Taapaca volcano (northern Chile). The variable concen- tration is shown by the colouring and highlights complex growth zones and irregular resorption surfaces (arrows). Glass and mineral inclusions have the lowest Ba concentration (black). Width of the image = 9.8 mm FIGURE 1