INTRODUCTIONSilicate magmas form by partial melting of silicate rocks pri-marily in the mantle. These melts may collect into magmareservoirs within the crust, where they cool and start tocrystallize, before ascending towards the surface and erupt-ing. Decompression and degassing further promote thecrystallization of phenocrysts. Because the crystal composi-tion in a magma depends on crystallization conditions,such crystals provide us with a record of the processes andconditions (pressure, temperature and volatile content) inthe magma chamber and may also contain clues about themechanisms triggering eruption. The study of phenocrystsby optical microscopy has documented compositional zon-ing with respect to major elements. Close inspection hasrevealed complex growth patterns. Microbeam techniquesnow allow the analysis of major and trace elements in thesecrystals 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 thecompositional evolution of magmas and growth histories ofphenocrysts. Today we are beginning to decipher the mean-ing of the variations in these crystals. We have abandonedour simple concept of phenocrysts that float (sink or rise)while they grow in a magma chamber. New research hasdocumented multiple recharge events in magma chambersbefore phenocrysts erupt. Redistribution and dispersion ofgrowing crystals among different portions of a magmareservoir appear to be common processes. Therefore, grow-ing crystals record different environments (e.g. magmachamber 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 backgroundon compositional zoning studies. Then we detail the electronmicroprobe methods (and some SEM techniques) available

and compare them with othermethods. We show how these tech-niques can be applied to studymagmatic systems in terms ofprocesses and the pre-eruptive his-tory of the magmas. Finally, we dis-cuss some recent advances andremaining challenges in the inves-tigation of magma systems usingzoned phenocrysts.

HISTORY OF ZONINGSTUDIES

Studies of zoning patterns involve both the characterizationof growth-zone morphology and the acquisition of quanti-tative information on compositional variations. Most zoningstudies in magmatic systems to date have been performedon plagioclase (sodium-calcium feldspar) because the chem-ical variations in this mineral can readily be inferred fromits optical properties seen in polarized light using an opticalmicroscope. 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 crystalmorphologies. “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 compositionat the rim. This compositional change mimics cooling andchemical 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 L E M E N T S , V O L . 3 , P P . 2 6 1 – 2 6 6 AUGUST 2007

Catherine Ginibre1, Gerhard Wörner2 and Andreas Kronz2

1 Département de Minéralogie, Université de Genève 13 rue des Maraîchers, 1205 Genève, SwitzerlandE-mail : catherine.ginibre@terre.unige.ch

2 Geowissenschaftliches Zentrum Göttinger. Abt. GeochemieGoldschmidtstr. 1, 37077 Göttingen, Germany

Crystal Zoning as an Archivefor Magma Evolution

261

Spatial compositional variations in magmatic minerals record chemicaland physical changes in the magma from which they grew. Electron-beamtechniques allow high-resolution imaging and quantitative analysis of

this compositional archive for major, minor and some trace elements. In thisway, magmatic processes such as crystallization, recharge in a magma chamber,decompression during ascent, and convection in the magma chamber can beidentified 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 fromTaapaca volcano (northern Chile). The variable concen-

tration is shown by the colouring and highlights complex growth zonesand irregular resorption surfaces (arrows). Glass and mineral inclusionshave the lowest Ba concentration (black). Width of the image = 9.8 mm

FIGURE 1

microns in width (FIG. 1). “Patchy” zoning is characterizedby irregular areas of contrasting compositions. Morphologiesof crystals and growth zones reflect the kinetics of growth(Lofgren 1974) or dissolution (Tsuchiyama 1985).

Quantitative analysis of major elements at a resolution of afew microns was made possible in the 1960s by the devel-opment of the electron microprobe, now a routinely usedtool. Simple core and rim spot analyses and quantitativeprofiles through crystals were used to characterize zoningpatterns in petrological studies. Later, 2D compositionalmaps were made using laser interferometry (e.g. Pearce andKolisnik 1990), a technique based on the perturbation ofinterference patterns by the variation of crystal composition.Nomarsky differential interference contrast (NDIC) pro-vides high-resolution imaging (but no quantitative compo-sitional information) of zoning patterns by enhancing thecomposition-related microrelief on the surface of etchedcrystals (e.g. Pearce and Kolisnik 1990). It soon became evidentthat trace element variations during crystal growth bearimportant information on magma compositions and con-ditions during crystallization. Several studies in the 1990scombined NDIC imaging with electron microprobe and ionprobe (SIMS) analysis of trace elements to identify crystalrecycling (e.g. Blundy and Shimizu 1991) and convection(Singer et al. 1995). The study of minor-element (Mg, Fe)distribution in plagioclase using the electron microprobewas pioneered by Kuritani (1998). More recently LA–ICP–MShas also been used. Modern studies combine quantitativeand imaging methods.

ELECTRON MICROBEAM TECHNIQUES

MethodsElectron microbeam techniques (EPMA, SEM) use the emis-sion of electrons, visible light and X-rays from a sampleunder a high-energy electron beam, as described below.

Electrons. Most of the incident electrons are conducted awayfrom the carbon-coated sample, but some are emitted fromthe surface. Low-energy secondary electrons come directlyfrom the surface of the sample and give an image of its 3Dmorphology (SEM images) at high resolution. Back-scat-tered electrons (BSE) are emitted after elastic interaction ofelectron-beam electrons with the atomic nuclei of the sample.Their intensity depends directly on average atomic number.Scans of BSE produce compositional images with a spatialresolution down to 20 nm under ideal conditions (depend-ing on the beam current and at high atomic number contrast).For low atomic number contrast, the spatial resolution islimited to ~0.1 µm using state-of-the-art detectors.

Light. Under the electron beam, some minerals emit visiblelight of variable wavelength. This cathodoluminescence(CL) is caused by crystal defects or by trace elements thatact as activators in the crystal lattice. The CL signal can beused quantitatively in some minerals, such as carbonates

(Gillhaus et al. 2001) and quartz (Müller et al. 2003). CL isless well understood in other minerals, but produces quali-tative images showing details of growth or resorption tex-tures that cannot be obtained by other methods (FIG. 2).

X-rays. The energy and wavelength of emitted X-rays areelement specific and allow quantitative chemical analysisby spectrometry. Spatial resolution is a few microns at thesurface and in depth. Energy dispersive spectrometry (EDS)measures all energies simultaneously but has limited spectralresolution. Wavelength dispersive spectrometry (WDS) usesa diffracting crystal to separate the X-rays by wavelength.Spectral resolution and sensitivity are an order of magnitudebetter than with EDS, but each element must be analyzedseparately.

There are three main applications for these X-rays: (1) forqualitative analysis, a scan of all energies (or wavelengths)allows the identification of elements present in a mineral(EDS or WDS); (2) by moving the beam or the stage, whilemeasuring a given energy or wavelength, the variation in agiven element can be imaged in a profile or as a map; (3)quantitative analysis of major and minor elements withWDS involves a comparison with a standard material ofknown composition.

Recent advances in microbeam techniques allow more spe-cific applications. Trace elements can be analysed on theelectron microprobe by using a longer counting time and ahigher beam current to increase the sensitivity. Detectionlimits are in the range of a few tens of ppm (e.g. Müller etal. 2003). Although BSE intensities give no direct informa-tion about specific element concentrations, they can be cal-ibrated in binary systems where only two componentsexchange, such as Na and Ca contents in feldspars (Ginibreet al. 2002). A high-quality BSE image combined with a fewquantitative WDS analyses can thus produce a quantitativeimage (or compositional profile) with a spatial resolutionbetter than 1 µm (FIG. 3).

Comparison with Other TechniquesOther microbeam techniques that allow in situ quantitativeanalysis of minor and trace elements include incident X-raymethods (synchrotron XRF, micro-XRF), the proton micro-probe (PIXE), transmitted electron beam methods (analyticaltransmission electron microscopy [TEM], electron energyloss spectrometry [EELS]) and mass spectrometry. InLA–ICP–MS, the sample is vaporized by a laser beam andionized in a plasma. In secondary ion mass spectrometry(SIMS), secondary ions are produced by a primary ion beam.FIGURE 4 shows the sensitivity and spatial resolution of thesevarious techniques. Detection limits are higher for electronmicrobeam techniques, but their striking advantage is goodspatial resolution (<5 µm). The analytical sensitivity of theelectron microprobe, however, is sufficient for some keytrace elements if the instrument is equipped with sensitiveanalyzer crystals and specifically “tuned” for trace elements.

262262 AUGUST 2007

Cathodoluminescence(CL) image of a sanidine

crystal from Laacher See phonolite(Germany) compared with a backscat-tered electron (BSE) image. Only theCL image reveals complex dissolutionand regrowth in the core and roundedoscillatory zones, whereas the BSE imageshows Ba zonation, mainly related tothe chemical composition of the melt(dark core: low Ba; lighter zone: higherBa, more mafic melt).

FIGURE 2

E L E M E N T S

The optimal combination of techniques in the study of zon-ing patterns is first optical microscopy, then BSE and CLimaging, followed by quantitative major and trace elementanalysis, and possibly complemented by element mappingwith an electron microprobe. For trace element concentra-tions lower than some tens of ppm, LA–ICP–MS or SIMSanalyses should be added.

APPLICATIONS TO MAGMATIC PROCESSES

Process IdentificationUsing experimentally determined phase relationships, i.e.compositions of minerals in equilibrium with a melt as a func-tion of pressure, temperature and water content, the chemicalcomposition of mineral assemblages allows the reconstructionof the conditions of crystallization. Conversely, crystal pop-ulations out of equilibrium can be identified. In the case ofbinary systems such as plagioclase, major elements are notsufficient for distinguishing among the numerous parametersthat may control its composition. It is therefore useful to useminor or trace elements (e.g. Fe, Mg, Ba, Ti, Sr in plagioclase)that will mostly reflect melt composition (see below).

Trace element contents of crystals are related to melt com-position by partition coefficients that can be experimen-tally measured, calculated or empirically determined forequilibrium conditions (see Blundy and Wood 2003 for adiscussion). For example, the partitioning of Sr and Babetween plagioclase and silicate melt strongly depends onthe Ca content (i.e. the amount of the anorthite end mem-ber, An) of plagioclase. Using partition coefficients, traceelement analyses of zoned crystals allow the reconstructionof melt composition during their growth. This is becausesuch elements are “captured” almost accidentally by thegrowing crystal from the adjacent melt. If an An-rich spikein plagioclase is caused by a major change in melt compo-sition (e.g. recharge), this growth zone will also show dif-ferent Sr, Mg and Fe contents compared to the adjacent,older zone. By contrast, changes in temperature, pressureand H2O content will affect the An content but will causelittle change in Sr, Mg and Fe. This is because the bulk com-positional change of the host magma would have a strongerimpact on Sr, Mg and Fe in the plagioclase than the effectof changing partition coefficients due to changing An inthe crystal. Kinetic effects at the crystal–melt interface mayalso influence the minor and trace element contents.

The interpretation of crystal or zone morphologies benefitsfrom numerous experimental and theoretical investigationsof growth and dissolution kinetics. For example, skeletalgrowth results from high growth rates (Lofgren 1974), andthe degree of disequilibrium (in temperature and composi-tion) determines the morphology of resorption surfaces(Tsuchiyama 1985). Examples of crystal or resorption zonemorphologies caused by dissolution under various condi-tions are given in FIGURES 5, 6 AND 7. The rim of a sanidinefrom Parinacota volcano (Chile) is resorbed at the contactwith the andesitic host magma (FIG. 5). The combinedFe–Ca–Ba map shows that the infiltrating melt reacted withthe sanidine to form plagioclase, which is the only feldsparin equilibrium with the melt. This contrasts with thesmooth and rounded resorption surfaces of Laacher Seesanidine (FIG. 2), where sanidine remains a liquidus phasein the phonolitic melt and continues to crystallize at simi-lar compositions after the dissolution event. In plagioclase,rounded resorption surfaces may correlate with a strongcompositional change in the host magma, as indicated bythe minor elements (e.g. Andagua plagioclase, see below;FIG. 7). By contrast, some plagioclase crystals show highlycomplex resorption and growth morphologies (FIG. 6), butminor elements (Sr, Mg, Fe) remain constant (Ginibre andWörner 2007). In such cases, the resorption event is notlinked to the chemical effects of magma mixing but rathermay indicate thermal disturbance and/or decompressionand degassing. Faint resorption surfaces are also observed inmany cases of oscillatory zoning (FIG. 3); these surfacesreflect small changes in P, T or melt composition. Linkingof morphology to growth conditions remains difficult. To“calibrate” our observations, we need more experimentalwork simulating various growth and resorption processes.

Constraints on Numerical ModelsElectron microbeam techniques can help constrain modelsof magmatic processes. We give two examples below.

Oscillatory ZoningModels explaining oscillatory zoning involve kinetic effectsat the crystal–melt interface or repeated, large-scale changesin growth conditions controlled by external parameters (seePearce 1994 for a review of such zoning in plagioclase). The

263E L E M E N T S AUGUST 2007

(A) High-resolution BSE image of oscillatory zoning in aplagioclase from Parinacota volcano (the black oblique

line is a crack in the crystal). Irregular zone boundaries (arrows) areinterpreted as resorption surfaces. (B) Compositional profile throughthe crystal along the white line of the greyscale image, calibrated for An-content using selected electron microprobe point analyses. Variations of0.5% An amplitude over only 1–2 µm can be quantified by this method.

FIGURE 3

Detection limits (sensitivity) as a function of ablation orexcitation volume (spatial resolution) for several microbeam

analytical techniques. Colours show the nature of the incident beam:red = photons (light); yellow = photons (X-ray), green = electrons,blue = protons and other ions. The detection limits for ablation meth-ods such as secondary ion mass spectrometry (SIMS) and laser-ablationinductively coupled plasma mass spectrometry (LA–ICP–MS) stronglyvary with the total amount of ablated material. SEM: secondary electronmicroscopy; EDS: energy dispersive system; EPMA: electron probemicro-analysis; WDS: wavelength dispersive system; µXRF: micro-X-rayfluorescence, SYXRF: synchotron-X-ray fluorescence; PIXE: proton-induced X-ray emissions.

FIGURE 4

B

A

264E L E M E N T S AUGUST 2007

distinction between the two types of processes has directimplications for the dynamics of the growth environment.High-resolution imaging and quantitative profiles can helpchoose among the various models. For example, BSE imagingof narrow (less than 10 µm) oscillatory zones in plagioclasecrystals (Ginibre et al. 2002) suggests a high-frequencygrowth–dissolution process, probably caused by minor fluc-tuations in pressure, temperature and chemistry of the hostmelt (convective movement or cyclic degassing or both).Recently, Perugini et al. (2005), using BSE images of plagio-clase, modelled oscillatory zoning by magma mixing. Intheir model, compositional oscillations in the crystals reflectgrowth from a mixed magma that has large-amplitude com-positional heterogeneities over a small (mm) spatial scale.Such a scenario along with repeated recharge events is themost likely explanation for oscillations showing dissolutionsurfaces. On the other hand, kinetic effects might explainbetter planar oscillatory zoning in other examples.

DiffusionIn some cases, compositional gradients in crystals are mod-ified by diffusion. High-resolution chemical profiles acrosssuch gradients can be used in diffusion modelling to infertime scales of magmatic processes (Turner and Costa 2007this issue). One element is usually sufficient, but combiningseveral elements (including minor elements) with differentdiffusion rates adds constraints to the model (Costa andDungan 2005; Morgan and Blake 2006). Electron micro-probe analyses provide the necessary spatial resolution andprecision for major and minor elements, whereas BSEimages have sufficiently high spatial resolution for quanti-fying steep diffusion profiles (Morgan et al. 2004).

Case StudiesOnce processes such as resorption, growth and chemicalvariation are identified by detailed compositional imagingand major and trace element analysis, we can learn aboutthe evolution of a magmatic system (recharge, differentiation,eruption, assimilation). To illustrate the technique, we presentbelow some of our cases studies on plagioclase.

FIGURE 7 shows examples of different styles of zonation insamples from the El Misti stratovolcano (southern Peru)and the Pico Viejo on Teide volcano (Tenerife Island).Plagioclase in andesites from El Misti shows an extremelylarge variety of zoning patterns, and we record here only afew simple examples.

Ruprecht and Wörner (2007) found that in El Misti plagio-clase (FIG. 7A–D) the minor elements Fe and Mg remain con-stant across the resorption surface. This suggests that thezoning does not represent the chemical effect of magmarecharge, but rather is the result of thermal perturbations.The variable wavelength of compositionally variablegrowth zones and resorption zones in the crystals illustratesthe variability of magma dynamics (convection, local mix-ing, heat transfer) in the El Misti system.

By contrast in the last eruptive product of El Misti, weobserve plagioclase crystals with rims richer in An, Fe andMg. These rims have grown over a resorption surface (lightgrey rim, FIGS. 7E and F). This suggests that a new maficmagma entered the system and completely mixed with theresident magma just prior to eruption. A similar effect isdocumented by the distinctly An-rich overgrowth (lightgrey, FIG. 7G) in plagioclase erupted from small-volume“monogenetic“ andesite centres in the “Valley of theVolcanoes” (Andagua, Peru). In small systems, eruptionstend to immediately follow recharging of the magma cham-ber with a more mafic liquid, while larger magma reservoirsappear to be better buffered against such disturbances andoften document only the thermal effects of recharge, with-out mixing immediately triggering an eruption.

Plagioclase phenocrysts in phonolites (highly evolved, alka-line volcanic rocks) from the Pico Viejo centre at Teide volcanoon Tenerife show an outer, more evolved margin (i.e. An-poor and darker grey, FIG. 7H). Triebold et al. (2006) arguedthat these crystals resided in a magma chamber that over-turned due to the gravitational collapse of a more evolvedmagma layer near the roof of the chamber. In this case,mixing took place with a more evolved (rather than a moremafic) magma, and eruption ensued shortly after. Such gravi-tational instabilities could be a trigger for volcanic eruptions.

The Pleistocene to Holocene Parinacota stratovolcano inthe Central Andes erupted magmas that ranged frombasaltic andesite to rhyolite. Trace element concentrationsvary widely at the mafic end of the range. Two basalticandesites from flank eruptions show contrasting Sr contentsof 900 and 1700 ppm. FIGURE 8 shows a zoned plagioclase

Detail of the rim of a sanidine crystal (core outside of thepicture to the bottom left, host melt to the top right)

showing reactions with the host andesite magma (from Parinacota vol-cano, northern Chile). Greyscale X-ray maps (right) are for Ba, Fe andCa: light grey represents higher, dark grey lower concentrations. Con-verting greyscales in each image into intensities of different colour andcombining these into one single multi-coloured image results in a high-resolution image of the resorption process. The original sanidine is richin Ba (red). The plagioclase that forms in the reaction has high Ca con-centrations (green). The melt is richer in Fe (blue) than either sanidineor plagioclase. New plagioclase forms in narrow melt channels at thecontact with sanidine.

FIGURE 5

BSE image of com-plex resorption and

regrowth patterns in Parinacotaplagioclase. Light grey: Ca-richzones, dark grey: Na-rich zones.(1) sodic remnant of the initialplagioclase; (2) calcic plagioclasegrown after dissolution of the initialplagioclase; (3) late oscillatory-zoned plagioclase grown frommelt inclusions; CMI: crystallizedmelt inclusion left after the disso-lution and regrowth process.

FIGURE 6

phenocryst from a main-cone andesite, with multiplerounded, irregular boundaries between growth zones.Repeated dissolution events are shown by irregular resorp-tion surfaces truncating earlier growth zones. An increase inMg and Fe from core to rim suggests that growth andresorption occurred during repeated, increasingly maficrecharge events (FIG. 8B). Sr concentrations also vary fromcore to rim, from high Sr to low Sr and back to high Sr; thevariations are greater than can be explained by the controlexercised by major element concentrations on Sr partition-ing. These zoning patterns reflect changes in melt composi-tion and Sr content, and record the involvement of twodistinct recharge magmas similar in composition to theerupted mafic end-members. Zoning patterns in other sam-ples from Parinacota show that both Sr-rich and Sr-poorrecharge magmas were present throughout the volcano’shistory. Plagioclase crystals in younger eruptive productstend to show a higher frequency of recharge events and thusdocument a secular change in the evolving magma-recharge system (Ginibre and Wörner 2007).

ADVANCES AND CHALLENGES IN ZONING STUDIES Recent studies using microprobe and other techniques(LA–ICP–MS, SIMS, NDIC) suggest that a large range ofprocesses occur in magmatic systems. The study of zoningpatterns allows a detailed reconstruction of the fractional

crystallization path in closed systems (Kinman and Neal2006), but crystallization in an open system is in fact morecommon. Open-system processes include recharge events,some with complete chemical mixing (Singer et al. 1995)and involving variable recharge rates (Ginibre and Wörner2007), and some characterised only by thermal effects (e.g.Couch et al. 2001; Ginibre et al. 2004; Ruprecht andWörner 2007). Ascending magmas decompress under eitherwater-saturated or water-undersaturated conditions, whichmay result in resorption and decompression-driven crystal-lization respectively (Pietranik et al. 2005; Humphreys et al.2006; Blundy et al. 2006). Various dynamic states andprocesses within a magma reservoir have been recognised,including heterogeneities and wall boundary layers(Kuritani 1998), crystal dispersion within a zoned magmachamber (Ginibre et al. 2004) and convection in a magmachamber (Singer et al. 1995; Ginibre et al. 2002). By con-trast, well-correlated zoning patterns between crystals,implying a less dynamic environment, have been found inplutonic rocks (Pietranik et al. 2005) and volcanic rocks(Triebold et al. 2006). Extraction of magma from crystal-richmush zones has also been proposed (Brophy et al. 1996).

Some of the processes mentioned above have been welldocumented in zonation studies and are widely accepted.However, some zonation features are interpreted differentlyby different authors. For example, temperature effects havebeen proposed to result from crystals settling into deeper,hotter magma (Anderson et al. 2000) or from reheating dueto magma recharge (Ruprecht and Wörner 2007). It has also

265E L E M E N T S AUGUST 2007

Contrasting zoning patterns in plagioclase. Increasing fre-quency of oscillations in feldspars from the andesitic El

Misti stratovolcano (southern Peru) (A–D) and abrupt Ca-rich over-growths due to recharge of the magma chamber by a more maficmagma (light-coloured rims, E, F). Some of the numerous resorptionsurfaces are indicated by arrows. The effect of a single mafic recharge inandesites from the Andagua monogenetic centres in southern Peru(light grey Ca-rich overgrowth in (G) contrasts with the effect of magmachamber overturn and growth of a Na-rich outer rim (dark zone in H)from a more evolved magma (Teide/PicoViejo phonolite, Canary Islands).

FIGURE 7

(A) BSE image of a zoned plagioclase crystal from Parina-cota volcano. (B) Fe–Sr plot of growth zones of many

similar crystals shows the effect of recharge (higher Fe than in the pre-vious zone) with two distinct magmas: one high in Sr and one low in Sr.

FIGURE 8

A

A B C

D E F

G H

B

266E L E M E N T S AUGUST 2007

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Turner S, Costa F (2007) Measuringtimescales of magmatic evolution.Elements 3: 267-272 !

been argued that crystallization of sodic plagioclase iscaused by magma-chamber overturn (Triebold et al. 2006)or by decompression-driven degassing (Blundy et al. 2006).Such differences in interpretation come partly from the dif-ficulty in interpreting complex zoning patterns in terms ofprocesses – for example, the distinction of diffusion-con-trolled skeletal growth processes from those of dissolution.The way forward includes constraining possible processesby experimental studies and applying a combination ofhigh-resolution compositional imaging, major elementpoint analysis and trace element analysis of zoning patternsat high spatial resolution (<5 µm). The latter approach willbenefit from future developments in techniques such asnano-SIMS and PIXE, allowing better resolutions and detec-tion limits. Advances will come from a combination ofmethods, including in situ chemical analysis as describedabove, Sr isotope microsampling (Davidson et al. 2007 thisissue) and crystal size distribution (CSD) measurements(Jerram and Higgins 2007 this issue). Although zoning pat-

terns in plagioclase are the best studied, other mineralshave been shown to be useful, e.g. quartz (Müller et al.2003), sanidine (Anderson et al. 2000; Ginibre et al. 2004),amphibole and pyroxene. The detailed investigation of zon-ing patterns in the various minerals found in an igneousrock may reveal different histories, and thus may lead to amore comprehensive understanding of magma evolution.In any case, even studies using the most advanced high-res-olution in situ analytical tools must always be combinedwith careful petrographic work and quantitative imaging ofgrowth and zoning patterns.

ACKNOWLEDGMENTSWe thank P. Ruprecht and W. Wegner for their unpub-lished images acquired at the GZG Göttingen. Constructivecomments by F. Tepley, N. Halden and I. Parsons are greatlyappreciated. !