American Mineralogist, Volume 69, pages 660-676, 1984

Probable relations between plagioclase zoning andmagma dynamics, Fuego Volcano, Guatemala

ArpRe,o T. Annp,nsoN, Jn.

Department of the Geophysical SciencesThe University of Chicago, Chicago, Illinois 60637

Abstract

Millimeter-sized plagioclase phenocrysts were explosively erupted in high-aluminabasaftic ash from Fuego volcano, Guatemala on October 14, 1974. The crystals haveunzoned, patchy-zoned and oscillatory-zoned parts. Unzoned, inclusion-poor cores, up to2 mm thick have round edges and compositions between AneT and An6s. Patchy-zonedcores and regions are rich in inclusions of glass and gas, about 50 pm thick and Anes toAq3. Oscillatory-zoned regions surround the cores and occur between patchy-zonedregions; compositions are Ane6 to Ans3 except for thin rims (Anzs to Anzo).

Patchy-zoned, inclusion-rich regions probably crystallized in gas-saturated environ-ments during episodes of relatively large supersaturation because: (l) such regions areeither cores (homogeneous nucleation?) or have flat inner terminations suggestive of aconstructive rather than resorptive origin, (2) most such regions are relatively sodic(relatively large concentration gradients in the nearby melt?), (3) such regions terminateoutward with irregular, convolute oscillatory zones suggestive of morphological instability,and (4) such regions are rich in inclusions of gas many of which are large and flat against theinner boundary suggesting onset of patchy-zoned growth together with effervescentdecompression. Crystallization with effervescence is supported by decreasing Cl/KzO withincreasing K2O for inclusions of glass. Patchy-zoned regions alternating with oscillatory-zoned regions suggest episodes of accelerated decompression.

Oscillatory zones are 0.5 to 8 pm thick; average internal compositional variation is aboutl% An, with the most calcic portion being earliest. Although individual zones are thinneston {010}, many thin zones on some forms correspond to non-oscillatory-zoned growth on{010} and other slow-growing forms, not cessation of growth. Consequently, zones aregenerally thinnest and most numerous on the fastest growing forms: {001}, {201}, {l l0}, and{110}. Discontinuous zones suggest dominant sideways rather than radial growth forindividual zones. Thickening of zones on irrational or curved surfaces and in reentrantssuggest dominant surface-attachment control of growth rate. Most of the zones correlatebetween opposite faces of the same crystal form on individual crystals. Some zonescorrelate between different crystals. The number ofzones from the outside ofthe crystalsto patchy-zoned regions or other disturbances cluster around 25 and 50. The varioustextural and compositional features of the zones are consistent with modulation of growthby hypothetical pulses of upward motion of a gas-saturated magma which sheared theboundary layer of melt near the growing crystals, thereby renewing the level of supersatu-ration at the crystal face. Tidal triggering of the motion seems likely. The oscillatory zoneson the fast growing faces of the Fuego plagioclases probably are twice daily "ticks" of avolcanological "clock".

Probably the Fuego magma body inherited initial unzoned crystals (cores). Subsequentgrowth probably accompanied ascent and was alternately patchy or oscillatory, possiblyassociated with various distances from the walls of the body: a crystal temporarily near awall might ascend relatively slowly, experience only moderate decompression and super-saturation, and consequently grow only oscillatory zones; a crystal near the center ofthebody might ascend rapidly, experience greater decompression and supersaturation, andconsequently grow some patchy-zoned regions. Explosive eruption could coextrudecrystals from the various environments.

0003-004xi84/0708-0660$02. 00 660

Introduction

It would be petrologically useful to know when andhow fast crystals grow because such knowledge couldlead to an increased understanding about the eruptivebehavior of volcanoes and their connections, if any, withplutons.

The estimation of time and rate in igneous processes ismore difficult than the estimation of temperature andpressure. Even in lava flows, dikes, and sills where heatflow theory is useful, a generally unknowable amount ofcrystallization took place before the magma stoppedmoving. Volumetric and linear rates of migration ofmagma are plausibly estimated from surface deformation,rates of extrusion of magma or gas, heights of lavafountains, migration of seismicity, and the dimensions ofvents and dikes. Probably crystallization commonly ac-companies movement because of cooling along walls andefervescence of gas rich in H2O with decompression.Consequently, it is reasonable to associate textural rec-ords of crystal growth with magma movement as wasdone by Ewart (1963) for zoned phenocrysts of plagio-clase in dated eruptions of ignimbrite. The present studyalso associates zones on phenocrysts of plagioclase withlikely times of movement of magma.

The origin of zoning in plagioclase, particularly oscilla-tory zoning, is uncertain and controversial (see review bySmith, 1974, and recent contributions by Sibley et al.,1976; Haase et al., 1980; Allegre et al., l98l; Loomis,l98l; and Lasaga, 1981, 1982). The ideas may be dividedinto dynamical and static categories, according to wheth-er the idea implicates motion of the plagioclase crystal,either absolutely or relative to the surrounding melt.Static models (Harloff, 1927;HLlls,l936; Bottinga et al.,1966; Sibley etal.,1976; Haase et al., 1980, and Allegre etal., l98l) are proposed only for continuous oscillatoryzoning without major abrupt jumps and resorption. Dy-namical models are proposed for most kinds of zoningincluding continuous oscillatory zoning. A dynamicalmechanism is inferred in this work.

Table l. Proportions of various plagioclase crystals hand-picked from ash, Fuego 1974 (VF74-53)

P e r c e n t b y N o . o f S u b e q u a n t P l a t yPort ionr Mass (q) l , , |e iqht crains Prisms Rhmbs Other2

661

In this paper there are two related parts: First, Idescribe certain textural and compositional features ofzones in plagioclase phenocrysts from Fuego volcano,Guatemala. Second, I use the featqres to correlate zonesand arrive at a working hypothesis for the origin andsignificance of oscillatory zones.

Selection and preparation of crystals

Euhedral plagioclase phenocrysts are abundant in air-winnowed basaltic ash (sample VF74-53 of W. I. Rose)which fell on October 14, 1974, about 7 km from Fuego'ssummit vent (Rose et al., 1978). Most particles in the ashhave diameters between 0.3 and 3 mm. Most of theplagioclase phenocrysts are individual, euhedral to sub-hedral platy rhombs and sub-equant prisms. Partly glassyto devitrified groundmass coats most of the phenocrysts(Table 1). Accidental particles from the vent walls proba-bly comprise less than about 20 wt.%o of the ash becausethe eruptive jet was intense and incandescent in daylight(Rose, pers. comm., 1982). Some broken fragments ofplagioclase with partial coatings of dense, devitrifiedmatrix may be accidential, but the euhedral crystalscoated with partly glassy to devitrified groundmass prob-ably were free-swimming crystals immersed in magmabefore eruption.

Of 133 euhedral phenocrysts of plagioclase mountedand sectioned, 2l were optimally oriented for seeingzones with thicknesses down to about 0.5 pm and internalcompositional amplitude as small as about lVo An(Ander-son, 1983). This study is primarily based on the 2loptimally oriented crystals. They include both platyrhombs and subequant prisms, but the platy rhombs areunderrepresented because of an inadvertent orientationbias.

Definition of zone

For the purposes of this report a zone is a repeat unit ofmicroscopic crystal thickness. Like a waveform its begin-ning is arbitrary. The Nomarski interference contrastviews ofetched polished sections reveal pairs oflight anddark bands corresponding to the alternating ridges andvalleys produced by differential etching of the polishedsurface. Each zone thus consists of two parts: an etch-resistant (sodic) part and a less etch-resistant (calcic)part. The average composition of successive zones maybe identical.

Zoning textures of the plagioclases

Three varieties of zoning are present: (l) no zoning(Ans6-sr); (2) patchy-zoning (Ane2-62); (3) oscillatory zon-ing (Ane2-s3 and about Anzs-zo). There may be somecontinuous compositional variation within unzoned re-gions. Some unzoned regions contain inclusions of olivineor glass and in one case hornblende. Patchy-zoned re-gions are rich in inclusions of gas and glass. Someoscillatory-zoned regions contain a few inclusions of

ANDERSAN: PLAGIOCLASE ZONING, FUEGO VOLCANO

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62

olivine and/or glass. Although unzoned regions are re-stricted to cores ofcrystals, the patchy-zoned and oscilla-tory-zoned regions occur in various sequences from coreto rim. The thicknesses of rims of oscillatory and patchy-zoned regions in large, centimeter-sized phenocrystsfrom an unsorted ash flow are similar to the dimensions ofsuch rims and regions in the smaller, millimeter-sizedcrystals from the sample of winnowed ash. Two or moresub-regions with different average compositions comprisemany oscillatory zoned regions. Faint, thin, and relative-ly sodic (Anzs-zo) oscillatory zones characterize the mi-crophenocrysts and the outermost few tens of microns ofthe phenocrysts.

There is a correlation between crystal shape and inter-nal texture (Table 2), suggesting two separate modes ofcrystal growth.

Features of oscillatory zones

Zone continuity

Some zones are discontinuous (Figs. l-3). Most dis-continuous zones have prominent bright bands. Somediscontinuous zones end in a step perpendicular to theface. It is common for discontinuous zones with particu-larly bright bands to be absent on or near crystal edges(Fig. 2). Other discontinuous zones with bright bandshave irregular interruptions (Fig. 3). A few discontinuouszones are succeeded outward by patchy-zoned regions.

Some zones are truncated near crystal edges. Sometruncations are smooth, round edges (Fig. 2) and alongsmooth, round reentrants. Irregular interruptions arerepetitious on rare plagioclase crystals which are en-closed by glass free of crystallites and devitrificationproducts.

Zone discontinuity commonly occurs at the edge be-tween adjoining faces where a zone on one face is not

Table 2. Shape and patterns ofzoning ofsectioned phenocrystsof plagioclase: Fuego 1974 (VF74-53)

Zon inqpaffern

core * r im lSubequantpri sms uncertai n r o f , a l

ANDERSON: PLAGIOCLASE ZONING. FUEGO VOLCANO

P la tyrhmbs

Fig. l. Discontinuous zones on plagioclase phenocryst 2-1.The outer margin of the crystal is along the right margin of thephotograph. Bright bands end at (a), (b), (c), (d), and (e). At (c)two bright bands end. Bright bands at (d) and (e) project out overunderlying dull bands and probably are skeletal protrusions.

This and all following photographical illustrations are reflectedlight, Nomarski interference contrast views of etched, polishedthin sections. See text and Anderson (1983) for details. The scaledivisions are l0 pm apart in all cases. The attitudes of somezones and crystallographical axes are shown with conventionalstrike, dip, and plunge symbols referenced to the plane ofpolishas horizontal. Compositions [l00 Cal(Ca + Na) atomic]determined by microprobe analysis are indicated by numbers inwhite circles at the position of the analysis.

Fig. 2. Truncated, irregular and discontinuous zones onplagioclase phenocryst 4-lA. At (C) 5 inward lying zones aretruncated and overlain by irregular zones which are thickestwhere curvy (irrational). Along the dark band (B) numerouszones are truncated on the grain edge. Near (A) several brightlybanded zones are discontinuous on {001}. Zones are generallythicker toward the grain interior at the bottom of the photograph.

Fig. 3. Curvy zones and an irregular zone on crystal 2-9. Therim ofthe crystal is along the right margin ofthe photograph. Theirregularly spaced reentrants into zone (A) are confined to thatzone. Zone (C) and some outward lying neighboring zonesthicken toward the top of the photograph. About 8 zones near (B)are curvy. The outermost zones are truncated at the outer sodicnm.

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Fig. 4. Groups of bright (W, X, Y, Z) and dull zones on (110).Between (W) and (X), (X) and (Y), and (Y) and (Z),25 to 30individual zones may be counted. Fewer zones can be countedon other faces. Groups (X) and (Y) contain some zones whichappear to thicken toward the crystal edge as can be seen bysighting subparallel to the surface of the picture.

Fig. 5. Oscillatory zones (near e and between a and b) on(201) continue as single zones on 1l I l;. Because the edges of thezones on (201) extend beyond the trace of the zones on (l 1 l), itmay be concluded that unzoned growth on (lll) accompaniedzoned growth on (201). See text for further discussion.

663

evident on the adjoining face. Consideration of thisimportant phenomenon is taken up below under the topicof zone thickness.

Zone flatness

Most zones are flat (no curvature apparent to the eye ininterference contrast views), but many are curvy, particu-larly on the {110} and {l I l} forms. Zones in patchy-zonedregions are curvy as are many zones filling reentrants.Curvy zones make up uncommon protrusions particularlyon {010}. Rare curvy zones are wavy (Fig. 3).

Zone thickness

There are two ways to consider zone thickness. First,the thickness of an individual zone may vary as it istraced along one face, around an edge, and into theadjoining face. Second, the average thickness of zonesmay vary from one face to another and from the interiorof the crystal toward its rim. There is no necessaryconnection between the two because not all zones corre-late from face-to-face.

Individual zones vary in thickness on a single face. Ona particular face, an individual zone may show edgethickening, reentrant thinning, and reentrant thickening.Edge thickening and reentrant thinning are mostly re-stricted to the outer sodic rims on the phenocrysts. Anexample of edge thickening within the interior of aphenocryst can be seen for some ofthe more contrastingzones in Figure 4 by holding the figure nearly parallel tothe line of sight and by sighting along the zones. It is notpossible to be certain, however, that the thickeningsapparent in Figure 4 are thickenings ofindividual zones orslightly curvy zones overlying discontinuous zones pre-sent only near the edge. Thickening of zones on a singleface is mostly associated with underlying irrational sur-faces (Fig. 2).

Individual zones vary in thickness from one face toanother (Fig. 5). Relative thicknesses ofindividual zonesvary by less than a factor of about three from form-to-form. Zones on {010} have the same or smaller thick-nesses as their continuations have on other faces includ-ing the typically curvy faces {l l0} and {111}.

Termination of a zone at a crystal edge may occur eventhough growth continues on both faces meeting at theedge. Figure 5 reveals several zones which are present on(2ot) Uut absent on (TTt). The ends of several zonesbetween a and b on Figure 5 define a line which makes anangle (ofabout l5') with the trace ofthe (l l l) zones. Thisangle reveals that growth on (TTl) accompanied growth ofthe zones on (Z0l). However, the growth on (l l l) wasunzoned, or imperceptably zoned. Evidently non-oscilla-tory-zoned growth occurs relatively slowly on some facesconcomitantly with oscillatory-zoned growth on otherfaces.

The above observation is important because someideas for the origin of oscillatory zoning postulate stop-

ANDERSON: PLAGIOCLASE ZONING. FUEGO VOLCANO

664

t5FREQUENCY

0246Z0NE THICKNESS pm

Fig. 6. Frequency distribution of zone thicknesses. Thethicknesses shown are for the outer 32 consecutive zones of{021}ofcrystal 50-l-l (see Fig. 4). The zones dip at a shallow angle of44' facilitating accurate measurement of individual thicknesses.The average thickness of the outer I 3 zones is I .4 pm and of theinner 19 zones is 2.25 p.m if two thick zones are included or 1.9pm ifthe two thick zones are excluded.

page of growth to account for missing zones (Bottinga etaI.,1966; Sibley et al.,1976). The above facts reveal tharthere is no necessary connection between the absence ofzones and stoppage of growth. Close scrutiny suggeststhat in regions of missing zones, growth occurs but at aslow rate.

Successive zones on individual faces vary in thicknessby afactor ofabout 10, but the thicknesses ofmost zoneson a particular face vary by less than a factor of 3 (Fig. 6).The average thicknesses of zones vary with crystal form(Fig. 7) in a different way than predicted from variationsof equivalent zones on diferent forms. For examplezones on {010} are thicker on the average than zones onother forms (Fig. 7). However, equivalent zones tracedfrom {010} to other forms are thinnest on {010}. Some thinzones on other forms are not evident on {010}, where theyappear to be represented by unzoned growth. Thus, thethicker, fast growing forms have more zones which grewfaster than zones on thin forms such as {010}.

Zones generally decrease in thickness toward the outermargins of individual grains (Figs. 7 and 8), but thedecreases roughly correspond to constant volume perzone (Fig. 8). There are some reversals to the generaltrend on most ofthe large grains. The average thicknessesof zones in the outer 60 ;r.m of the 21 crystals areindependent of grain size dnd range from about 2 g,m to0.7 Wr with one exception at about 3 pm per zone.

Average zone thickness is independent of averagecomposition. Thick zones tend to lie toward the inside ofexceptionally bright (sodic) bands. However, I have notbeen able to adequately resolve the compositional varia-tion within zones; consequently, the present data are notsumcient to demonstrate a relation between composition-al structure within zones and their relative thicknesses.

ANDERSON: PLAGIOCLASE ZONING, FUEGO VOLCANO

r0

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r60t2040

Dis tonce pnFig. 7. The relation between zone number and perpendicular

distance from the rim for various crystal forms on crystal 2-9.Dashed lines connect the positions of correlated marker bandson the various forms. The numbers at the ends of the linesindicate the distance divided by zone number for the final pointon the lines, and equal the mean width of zones. The ordinaterecords the accumulated number of zones inward from the rim(0).

I

Zone eoNumber

t6t4t2t0

Distonce CuUeA (r7O7pm3)Fig. 8. Relation between zone volume and distance. Distance

(r) is dip corrected from the central axis of the crystal. The valueof r at the "center" of the crystal is taken as zero. The center ofvolume of the crystal cannot be determined for sectionedcrystals. The cube of the distance is plotted. The crystal numbersare given together with the mean zone width (in parentheses).Measurements were on the {20U form. Note that the inverses ofthe slopes of lines on the graph have units of volume per zone.The dashed lines indicate how zone number would relate todistance if the zones had uniform thickness equal to the meanthickness. Approximate linearity of the curves indicates thatzones are approximately equal volumetric increments onindividual crystals. Curves for larger crystals have gentler slopes(greater volumes per zone).

{rTo}

ANDERSON: PLAGIOCLASE ZONING, FUEGO VOLCANO 665

Fig. 9. Patchy-zoned regions on crystal 3-8. A slightly zonedcore (C) is rimmed by successive oscillatory zoned and patchy-zoned (PZ) regions. Groundmass (GM) is attached to the rim ofthe crystal. Two inclusions of gas appear as black circles about30 pm in diameter and are next to the inner boundary of the outerpatchy-zoned region. The inner boundaries ofboth patchy-zonedregions are flat, but the outer boundaries are curvy on someforms.

Features of patchy-zoned regions

Most of the crystals of plagioclase in the ash samplehave one or more patchy-zoned regions. Such regionshave irregular zones and inclusions of glass and gas (Fig.

Fig. 10. A patchy-zoned region (PZ) on {ll0} continues intoan irregular zone (lZ) on {010}. The inner limit of the patchy-zoned region is flat, whereas the outer limit is overlain by curvyoscillatory zones. Three discontinuous zones lie inward from thepatchy-zoned region.

9). Most patchy-zoned regions have a flat contact next tothe interior of the crystal (Fig. 9). The largest inclusionsofgas are preferentially located adjacent to the inner, flatcontact (Fig. 9). About one-third of the patchy-zonedregions are associated with discontinuous zones. Typical-ly one or more discontinuous zones lie inward of oradjacent to a patchy-zoned region (Fig. 10). The outer-parts of patchy-zoned regions have protruding edges andare capped by irregular, curvy zones (Fig. l0).

Correlations of oscillatory zones in thesame crystal

Most oscillatory zones on opposite faces of the {110}ana {2Ot} forms correlate, but some do not (Figs. ll and12). The features which correlate between opposite facesare: (l) the relative brightness (probably relative Naenrichment) of bands; (2) the relative thicknesses ofzones; (3) groups of dully or brightly banded zones; (4)

irregular or incomplete zones.The (201) and (Tl0) faces share a common edge as do

(20T) and (lf-O) at the opposite end of the crystal. Theends of the crystals are up to about 0.7 mm apart andseparated from each other by extensive faces (mostly

{010} and {001}). Growth on {l0l} appears comparable tothat on {2Ot} and {Tl0}; consequently it is possible for azone which might initiate on (2Ot) to run around thecrysral via (TI0) and (001) to (TTl), (20T) and (l l0).

Alternatively the correlated zones on opposite ends of acrystal reflect independent growth under environmentalconditions which were similar up to 0.7 mm distant' Thecorrelations of incomplete zones suggest the secondalternative because their incompleteness makes lateralpropagation around edges questionable.

Correlations of patchy-zoned regionsin the same crystal

Patchy-zoned regions vary in character from face-to-face and form-to-form. One face may have a thick patchy-zoned region and the opposite face of the same form mayhave only a thin irregular zone. However, most patchyzoned regions can be traced around at least half of theperimeter of a section through an individual crystal. Mostcrystals have fewer than three patchy-zoned regions:consequently, patchy-zoned regions may facilitate corre-lations between grains.

Correlations between crystals

Some representative correlations between crystals areshown in Figures 13 and 14. On twelve crystals patchy-zoned regions occur near zone numbers 25 and/or 50.

In terms of total numbers of zones the grains fall intotwo groups (Fig. l5): most crystals have fewer than about100 zones, but some larger crystals have 140 to 200 zones.About a third of the crystals have 40 to 60 zones withpatchy-zoned cores or regions. Most large crystals haveunzoned calcic cores (mostly greater than about Ane2).

66 ANDERSON; PLAGIOCLASE ZONING, FUEGO VOLCANO

Groups of dully and brightly banded oscillatory zonescorrelate between grains and are respectively relativelycalcic and sodic. The patchy-zoned regions are relativelysodic but overlap the compositions of the brightly bandedregions. Thus the average composition of various seg-ments may be of some correlative value, but the details ofthe compositional structure are too fine to be resolvedwith the microprobe.

A smaller proportion ofzones correlates between crys-tals than correlates between opposite or similar faces on asingle crystal. For some groups ofcrystals about one outof five zones is sufficiently similar in thickness, brightnessand morphology to justify correlation between the crys-tals. Such correlations are subjective, but comparable tocorrelations of equivalent zones on individual crystals.No individual zones on different crystals appear identical,and it is likely that lack of identity is real and not anartifact of various angular relationships, etching and soon.

Compositions of glassy inclusions

Inclusions of glass in plagioclase phenocrysts havevariable and mostly low ratios of Cl/KzO compared toinclusions in olivine (Rose et al., 1978). These factssuggest some efervescence of Cl during the crystalliza-tion of plagioclase.

A plagioclase crystallization model

Two groups of phenocrysts

My interpretation of the mechanism of growth andorigin of zoning of the Fuego plagioclases adopts adynamical body of magma in which crystals can havevariable physical/chemical histories, based on the follow-ing facts and ideas.

Two groups of phenocrysts are defined on the basis ofthe numbers of zones on grains (Fig. l5). The divisionmade on this basis is supported by different correlationgroups (group I = Fie. 14, group 2: Fig. l3). Each groupincludes both platy crystals with patchy-zoned cores andprismatic crystals with non-oscillatory zoned subroundcores. The Fuego magma body may have received twoinjections of magma which contained non-oscillatoryzoned plagioclase crystals. Because there are no regionson the group I crystals (more than 140 zones, Fig. la)which are comparable to the prominent patchy-zonedregions and cores ofthe group 2 crystals at zone numbersnear 25 and 50 (Fig. l3), the two groups of phenocrystsmay have grown in different parts of the magma body.

N on-o s cillat ory zo ne d c o r e s

The non-oscillatory zoned cores of many plagioclasephenocrysts are relatively calcic, and commonly subhe-dral, subequant and subround in shape. The lack ofoscillatory zoning in the cores may reflect growth in an

Fig. I l. Correlation of bright bands between alternate facesof the {l l0} form on crystal 2-9. The rim of the crystal is near thetop of both photographs (2-9a and 2-9b). The correlated bandsare numbered inward from the sodic rim (about An75) of thecrystal. About half of the zones can be easily correlated.

environment which was either physically more stable orchemically more uniform than for the zoned exteriors.The subround shapes of most of the cores probablyreflect partial resolution before the oscillatory zoned rimsbegan to grow. Partial resorption could be caused bydecompression of a system undersaturated with H2Ovapor, addition of H2O or heating (as by falling into ormixing with hot magma).

2*9a ,2-,9b

Patchy-zoned regions

The features of patchy-zoned regions suggestive of orconsistent with relatively large supersaturation of plagio-clase are: (l) the composition of patchy-zoned regionscommonly is more sodic than the adjacent oscillatoryzoned regions. (2) Most patchy-zoned regions have anirregular outer margin suggestive of cellular growth. (3)Most patchy-zoned regions are thicker at the cornersbetween principal crystal forms than inwardlying oscilla-tory zones. (4) Patchy-zoned regions are rich in inclusionsof gas and glass. (5) Patchy-zoned regions form sub-euhedral platy cores of most phenocrysts suggesting thathomogeneous nucleation is associated with the develop-

67

ment of patchy-zoned regions. These features are inter-preted more fully below.

For a given melt composition, supercooling correlatespositively with sodium content of the plagioclase (Lof-gren, I974a,b). Cellular growth morphology is suggestedby (2) and (4) above and is typical of rapid growth ratesand larger supercooling than faceted growth (Lofgren,1974b; Kirkpatrick, 1975). Corner growth ((3) above) isfavored during rapid growth because sharp corners "see"more liquid within a small distance than do flat faces.Inclusions of glass are consistent with resorption, as wellas cellular growth (Vance, 1965). Resorption is not con-sistent. however, with either the euhedral inner boundaryof many patchy-zoned regions or the paucity of flat

ANDERSON: PLAGIOCLASE ZONING. FUEGO VOLCANO

2-9A 2-98Fig. 12. Correlat ionsofbandsbetween{110}and{201}formsonoppositeendsofcrystal 2-g.Therimsofthecrystal arenearthe

upper margins ofthe photographs. Some bands are more curvy on {ll0} than on {201}.

i41

ANDERSON: PLAGIOCLASE ZONING, FUEGO VOLCANO66E

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670 ANDERSON: PLAGIOCLASE ZONING, FUEGO VOLCANO

oscillatory zones within the patchy-zoned region. Resorp-tion, like etching, proceeds preferentially along latticedefects and tends to produce an irregular, vermicularborder on a subround rather than euhedral core. Thezones which are present within patchy-zoned regionshave sinuous outlines subparallel to the walls of inclu-sions indicating a constructional origin related to theinclusions rather than a relic of destructive resorption. Iconclude that the patchy-zoned regions grew during epi-sodes of greater supersaturation and faster crystal growththan the oscillatory zoned portions of the phenocrysts.

The following attributes suggest or are consistent withthe presence of bubbles of vapor in the melt duringgrowth of the patchy-zoned regions: (1) inclusions of gasare present within the patchy-zoned regions. Commonlythe largest inclusions of gas are located adjacent to theinner boundary of the p4tchy-zoned region (Fie. 9). (2)The greater modal abundance of plagioclase in the firstpulse of the eruption (Rose et al., 1978) can be explainedby rafting ofgrains by attached bubbles. (3) The variationin CUK2O ratio for inclusions of glass in plagioclase isconsistent with degassing (Rose et al., 1978).

Probably the preferred location of gas bubbles againstthe inner margin of patchy-zoned regions is best ex-plained by initial nucleation on the surface ofthe rapidlygrowing crystal of plagioclase because of an enhancedconcentration of H2O in the melt next to plagioclase fromwhich it is rejected. Possibly the growth of the patchy-zoned regions was accompanied by accelerated efferves-cence of the magma, perhaps caused by comparativelyrapid decompression.

Discontinuous zones, many of which lie inward andadjacent to some patchy-zoned regions (Fig. 10, and seep. 1l), possibly mark an intermediate condition of super-saturation whereby zones start growth faster than theycomplete growth. The bright bands typical of discontinu-ous zones are consistent with greater than average super-saturation if supersaturation and composition are relatedas found by Lofgren (1947b).

O s cillat ory -zo ne d r e gions

The evidence bearing on the mechanism of oscillatoryzoning can be put in three categories: (1) textural evi-dence for growth influenced by surface attachment kinet-ics; (2) textural and compositional evidence for growthduring vapor-saturated decompression and therefore as-cending motion of magma; and (3) textural evidence forperiodic changes in supersaturation. The evidence foreach of the above is presented in sequence below.

Surface attachment kinetics. Growth influenced bysurface attachment kinetics is suggested by three fea-tures. Individual zones are thicker where curvy or irratio-nal as expected according to the relations between sur-face roughness and the rate of interface attachment(Hartman, 1973; Kirkpatrick, 1975). Individual zones arethinnest on (010) consistent with Dowty's (1976) predic-tion based on surface morphological considerations.

Curvy zones are uncommon and generally succeededoutwards by within five or fewer zones by flat zonesconsistent with a stable interface without runaway bumpsprojecting into a strongly supersaturated region beyond aboundary layer (Sekerka, 1973).

Vapor saturation. The textural position of oscillatoryzoned regions outside of and between patchy-zoned re-gions and the continuous decline in CllKzO with increas-ing K2O in melt inclusions are consistent with the pres-ence of gas during growth of both kinds of regions,because once vapor saturation is attained an increase inpressure is required ifit is to be redissolved. Probably therate of generation of gas was less during growth ofoscillatory zones than during growth of patchy-zonedregions; however, repeated increases as well as decreasesin pressure cannot be ruled out for crystals within dynam-ical bodies of magma.

Periodic changes in supersaturation, Regions ofpatchy-zoning (including correlative incomplete zonesand irregular zones) occur preferentially at or near zonenumbers 25 and 50 (Fig. 13). The supersaturation of manycrystals apparently went through a maximum at aboutzone 50 and again at about zone 25. Shortly after zone Iformed the crystals were extruded. Thus major events inthe growth of the plagioclase phenocrysts occurred atsubequal intervals corresponding to about 25 oscillatoryzones.

The origin of periodic changes in supersaturation isspeculative and rests on auxilliary geological and volca-nological data. The outer sodic rim may be related in timeto the onset of eruptive outgassing and quenching whichprobably occurred on October 10, when the first eruptionof steam occurred (Rose et aI., 1978\. October 10, 1974was close to the fortnightly minimun (neap tide). Theeruption times of Fuego corresponded to semidiurnalminima during the 1974 eruption (Martin and Rose, l98l).The eruptive behavior of Ngaruhoe is similar to Fuegoand correlates with fortnightly minima (Michael andChristoffel, 1975). The descent ofthe Fernandina calderafloor occurred at 6 hour tidally correlated intervals (Fil-son et al., 1973). Tidal oscillations of Halemaumau lavalake occurred in l9l9 (Shimozuru and Nakagawa, 1969).Extrusion and seismicity correlate with tides for the IslasQuemadas dome in 1879-1880 (Golombeck and Carr,1978). Other similar correlations have been noted in otherregions by Mauk and Johnston (1973) and Mauk andKienle (1973). In short, tidal stresses appear to play a rolein the rise and fall of material in volcanic regions. It isreasonable therefore to associate the 25 zone intervalbetween the outer rim and the first major patchy-zoned ordisturbed region with the 28 tides in a fortnight. Possiblyeach twice daily low tide triggers increments of magmaascent leading to definition of a zone. On or near theforthnightly minimum solid-earth tide, some magma maysprint upwards resulting in relatively large supersatura-tion and consequent transition to patchy-zoned growthand nucleation of patchy-zoned cores of crystals.

IVE (N,+Nr)

| | \ /cuMULATrvE(N,)_ _ - - - t - - r - - - - ! - - - - - { t -

ANDERSON: PLAGIOCLASE ZONING, FUEGO VOLCANO 671

20 40 60 80 100 120 140 160 180 200TOTAL NUMBER OF ZONES ON GRAIN

Fig. 15. Frequency of grains according to number ofoscillatory zones. The 21 best etched grains are shown withpattern. Total distribution of 52 best and fairly etched grains isshown without pattern. Two groups are present: grains with 140to 200 zones (group l) and grains with 100 or fewer zones (group2). The curves show the cumulative numbers ofgrains in the twogroups with decreasing numbers of zones: N; : group I, Nz :group 2. The scale for the histogram is at left and the scale for thecumulative curves is at right.

Sodic rims

Virtually all of the phenocrysts of plagioclase havesimilarly thick rims (on {201} about 20 pm) with about 10percent more albite component than the adjacent interior(An75 compared to Anes). Assuming that the effect of non-equilibrium undercooling on composition found by Lof-gren (1974b) for plagioclase melts can be generalized tobasaltic melts a supersaturation equivalent to an under-cooling of about 40"C is implied. At such undercoolingsLofgren (1974b) found a transition to skeletal morphologyconsistent with the edge thickening and reentrant thinningevident for the sodic rims. Johannes (1978, 1980) hasshown that the equilibrium plagioclase composition issensitive to the SiO2 content of the melt. Inasmuch as theresidual liquid from which the sodic rims crystallizeddiffers from the pure plagioclase plus H2O system (theliquid has substantial normative qz, for example), it maybe anticipated that the relations between plagioclasecomposition and undercooling of the actual melt willdiffer from those found by Lofgren. Although it is notpossible to predict the actual relations in the naturalmagma, it is worthwhile to pursue the implications ofLofgren's results in order to show how such knowledgecould be petrologically useful. The key idea is that anabrupt change in plagioclase composition can be taken asa measure of an impulse of supersaturation whether bysudden cooling or loss of H2O. The latter process ispressure dependent: the same loss ofH2O at low pressureresults in a larger supersaturation than if it occurred athigher pressure. Undercooling (supersaturation) causedby loss of H2O seems more likely in the case of Fuegothan a sudden and uniform thermal chilling of the whole

body of magma. If the magma had negligible H2O dis-solved in it after the event of supersaturation, then adecompression of about 100 atmospheres would sumce toprovide the undercooling (compare the effect of Ps,6 onplagioclase-out curves for andesitic liquids by Eggler'1972 and Sekine etal.,1979). If300 atmospheres ofHzOpressure remain after decompressive supersaturationthen a decompression of about 200 atmospheres would berequired. These figures are close to seismic estimates ofthe short term strength of rocks and plausibly relate toinitial rupturing of the conduit to the surface on October10, 1974 when the first major steam blast of the 1974eruption occurred. A hypothetical pulse of effervescentdecompression that possibly led to the abrupt decrease inAn recorded by the sodic rims may have been comparableto those which possibly led to much sinaller decreases inAn recorded by the patchy-zoned regions.

A dynamical model for the originof oscillatory zones

Oscillatory variations of composition evident in the

phenocrysts of plagioclase from Fuego seem to rule out

iquilibrium causes of oscillatory growth, because the

equilibrium composition is so insensitive to both pressure

(0.5% Anll000 atm, Lindsley, 1968, and see discussion by

Pringle et al.,1974),H2O (l% An/1000 atm-Yoder et al'

1957; Johannes, 1978), and temperature (l%/l0"C-Kudo

and Weill, 1970; Drake, 1976)- Consequently' unlikely

changes in these parameters are implied by the more than

200 zones with an average of about l%o An or more

change of composition within zones (see Anderson,

1983).Although composition is not sensitive to supercooling

either (about 2-37o Anll}'C, Lofgren' 1974b; Loomis'personal communication, 1982) concentration gradients

may amplify the effects. Variations in supersaturation are

indicated by the variable growth modes: oscillatory and

patchy zoning. Furthermore, some instances of oscilla-

tory zoning seem related to motion of magma (Anderson'

1981; Anderson et al., 1982). The presence ofroughly 25

zones between patchy-zoned regions suggests that oscil-

latory zones on Fuego's phenocrysts may be tidally

triggered. Consequently, it seems plausible to seek an

explanation of oscillatory zones connected with variable

motion and supersaturation in the melt'The idea advanced here is that pulses of motion within

a body a magma result in a rearrangement of nearbycrystals as a consequence ofviscous shear past a station-ary wall such as the wall of a dike. As a result of the sheareach crystal is brought into a slightly new environmentwhere it must compete with newly arranged crystalneighbors. During the shear pulse preexisting gradients ofcomposition associated with the individual crystals aredistorted and on the average the melt near each crystal iscompositionally replenished. After the shear-pulse accel-erated growth of relatively Ca-rich plagioclase occurs

50

40

30

2020

az.E r cd , J

Lo l 0UJ

= )J

z

672 ANDERSON: PLAGIOCLASE ZONING. FUEGO VOLCANO

because the replenished melt is richer in Ca and moresupersaturated.

The above model introduces an extrinsic dynamicalmechanism to account for changes in either the rate ofgrowth or the composition of new growth or both. Themechanism consists of pulses of shearing motion through_out the magma as suggested by precursory seismicity,eruptive episodicity and repetitious and partially correla-tive zoning textures of coextruded phenocrysts. Theconcept of concentration gradients in the melt next togrowing crystals is adopted from earlier workers (seeBottinga et al., 1966; and Smith, 1974for reviews; Hasseet al., 1980; Allegre et al., l98l; Lasaga, l9gl, l9g2 forrecent treatments). The shear pulse mechanism differsboth from the extrinsic dynamical mechanisms proposedby early workers (e.g., paliuc, 1932) andfrom the intrin_sic dynamical mechanism proposed recently by Loomis(1981). The early dynamical mechanisms which reliedupon changes in pressure (or water pressure) and tem_perature in an equilibrium context (for example seePringle et al., 1974) seem not feasible for micron-thickoscillatory zones in view of their large number andcompositional amplitude. Loomis' (l9gl) mechanism ofintrinsic face-specific fluid motion and the various staticdiffusional mechanisms are grain-specific; consequently,grain-to-grain correlations of zones would be largelyaccidental. In view of the imperfect grain-to-grain corre_lations a process with both a grain specific as well as ageneral or common aspect seems required. The shear_pulse mechanism is general but has a grain-specific aspectwhich arises because non-uniformly distributed crystalswill develop geometrically distinct concentration gradi_ents around individual crystals.

Loomis (1981) considered it possible that melt nexr to agrowing crystal face might move in response to localcomposition-related gradients of density. He followed thetreatment of this problem by Shaw (1974) who consideredthe velocity of fluid flow in a boundary layer next to avertical wall of solidifying magma. In exploring the sameapproach I adopted the following values of various prop-erties: length of crystal edge (l mm), viscosity (710poise), density (2.39 gmlcm3), effect of crystallization ofplagioclase on melt density

/ t a p \I o.oo :\ n AAnT

and diffusivity of anorthite (9 x 10-6 cm3/sec) for thecrystallizing Fuego basaltic melt with 3 wt.Vo H2O at1020'C. Two notable results are: (l) as also found byLoomis, a significant maximum velocity of melt move-ment is predicted to occur near a growing crystal edge. Iestimated this to be 30 pm per day if a 2 pm thick layer ofplagioclase is extracted from the intergranular melt atl0Vo cry stallization, because the appropriate formulationof the non-dimensional density change term in the Ray-leigh number is

where f refers to the fraction of liquid remaining. In thecalculation a 2 p,m layer of new plagioclase correspondsto a Af of 6 x l0-5. Thus the liquid near to the crystal faceis predicted to move faster than the crystal face isadvancing. (2) The boundary layer in which movement iscalculated to occur is predicted to be thicker than thedistance between crystals, however. This is a conse-quence of the Rayleigh number being less than unity(about 0.06). The physical significance of this resultseems to be that motion of the liquid near a growingcrystal cannot occur without cooperation from neighbor-ing crystals. In this case the viscosity is inappropriate,because suspensions have larger viscosity and probablyare non-Newtonian. Thus a yield stress would need to beconsidered. I do not know how to handle such a complexproblem but consider it likely that boundary layer meltnext to growing crystals in a Fuego-like magma will movemuch slower than the 30 p.mlday predicted rate becauseofthe distributed drag related to the neighboring crystals.

The feasibility of extrinsically forced motions remainsto be evaluated. There are two parts to the problem: (l)what will be the distance range of crystallization-relatedgradients of composition? (2) Will externally forced mo-tions sufficiently reorganize the gradients? The range ofcomposition gradients is at least a millimeter if the timebetween successive growth zones is of order 40,000 sec(12 hours) and ifthe gradient develops or relaxes approxi-mately in accordance with the model of diffusion from asemi-infinite sheet with a boundary composition fixed byequilibrium with the crystal face (following Burton et al.,1953, and Lasaga, l98l). If there is an initially largeconcentration change for melt near the growing crystal,then the crystal grows fast for a short time and subse-quently the concentration gradient largely relaxes beforenew growth begins. The distance out to which the gradi-ent reaches depends mainly on the time between growthsprints and the diffusivity. In roughly 12 hours time thegradient reaches almost halfway to the neighboring crys-tal. As a consequence of the dimension of the gradientapproximating the length of the crystals and the half-distance between them, a shearing motion of about

t : A lI

will easily deform the region in which the gradient hashalf or more of its rnaximum value after 40,000 seconds.

The above analysis does not validate the shear pulsenotion, however. This is because it is not known whetherthe distortion ofthe concenrration gradients by the hypo-thetical shear pulse is a significant perturbation of thesystem. If growth is initially rapid (5 to l0 times theaverage rate), then the initial gradients associated withgrowth will be much larger than those remaining after

l d o- - - a fp d f

ANDERSON: PLAGIOCLASE ZONING. FUEGO VOLCANO 673

40,(X)0 seconds of relaxation. Further destruction of thegradients by shearing motion would then seem to be oflittle consequence. Ifthe growth rate varies only by afewpercent as zones develop, then the shearing motionswould be the major perturbations of the concentrationgradients. The small variations in zone thickness fromface-to-face, unzoned growth and the rarity of incompletezones seem to me to be more consistent with the latterview. Accordingly, I think it is reasonable to suggest thattidally-triggered pulses of shearing motion in an ascend-ing magma modulate oscillatory growth by periodicallyreorganizing weak gradients of composition associatedwith the growing crystals.

Finally we may ask: Is the accumulated shearing mo-tion implied by 200 zones on some crystals consistentwith the probable ascent of Fuego's magma? The thick-ness of Fuego's near surface feeding dike or conduit isprobably less than about 20 meters (Rose et al., 1978).The total upward transport probably is at least 5 kilome-ters (Rose et al., 1978, and Harris, 1979). The total shearthus may be of the order of 5000/20 or 250. If there aresome 2fi) pulses then the shear per pulse is of order unity.Because the crystals are about as far apart as they arelarge (Fig. 16), an average pulse with unit shear wouldplace each crystal in the vicinity of a new neighbor, asimplied by the model. If the shear were only 0.1 per pulse,it would be difficult to see how the motion would signifi-cantly perturb the boundary layer around an individualcrystal.

There are several ways in which tidal stresses mighttrigger the ascent of magma. Tidal stresses could acceler-

Zone Number

Fig. 16. Summary model of crystallization of plagioclase in theFuego high alumina basalt of October 14, 1974. Plotted withrespect to zone number are the temperature (7), water pressure(Pn.o), average distance between the surfaces of neighboringcrystals (/), the number of crystals per cm (n) and the volumefraction of plagioclase in the magma (f).

The curves for l, n and f are schematic but based on data givenin Fig. 15. See text for details.

ate the propagation ofcracks. Tidal stresses could triggerthe fall of calderas (Filson et al., 1973\ and other largeblocks of crust into chambers of magma thereby displac-ing magma upward along conduits. Tidal stresses couldtrigger the accumulation or redistribution of tectonicstress in the vicinity of magma reservoirs thereby forcingmagma toward the surface. Thus, the source of energy tomove the magma upwards against gravity could be itsown buoyancy, the potential energy offalling blocks, orstored tectonic strain.

Summary of plagioclas e crystallization

It is possible to infer a crude history of the nucleationand growth of the plagioclase phenocrysts, if it is as-sumed that the outermost zone (zone l) on each crystal iscontemporaneous and the other zones measure timesimilarly on all crystals. Thus the crystals with the largestnumber of zones are assumed to have been in the magmalongest and the crystals with patchy-zoned cores andsmaller numbers of zones are assumed to have nucleatedlater, closer in time to extrusion. The relative number ofcrystals with various total numbers of zones is taken toreveal the increase in the accumulated number (n) ofcrystals per cm3 of magma as a function of time (decreas-ing zone number).

It is important to stress that time is measured backwardwith zone number. This was the only effective way toproceed, because the most justifiable assumption is thatthe rims of all crystals are coeval.

A simple model was adopted to compute relationsbetween the crystal size (a), spacing (l), and numberdensity (n) and the total crystal fraction (f). According tothe model the magma contains regularly spaced cubicalgrains which vary in size and proportion with time (zonenumber) as indicated by observed proportions of crystalswith various numbers ofzones and sizes. Zones are takento be equally thick on all grains. The equations are:

f = n a 3

The results were obtained by combining increments ofcrystallization due to the large crystals with more than140 zones and increments due to crystals with fewer than100 zones. The results rely on the data presented inFigure 15 for 52 grains of plagioclase and on a finalfraction of plagioclase in the magma at extrusion of 0.3 asmeasured in large tapilli, a crystal edge of 0.02 cm forboth the non-oscillatory zoned cores ofthe large crystalsand the patchy-zoned cores ofthe other crystals, and 0.06cm and 0.05 cm for the final cubical edges of big crystalsand the average crystal.

PH

f:'.,:.,(",-)'.(;)'( l )

(2)

674 ANDERSON: PLAGIOCLASE ZONING, FUEGO VOLCANO

The modeled values of the various parameters aredepicted in Figure 16. The curves are schematic in detail,because values were calculated for large intervals ofzones corresponding to Figure 15. Various kinks in thecurves reflect an attempt to portray bursts of nucleationand growth of patchy-zoned material. As crystallizationincreases from zone 200 to zone l, the volume fraction ofplagioclase (0 in the void-free magma increases from 0 to0.3 in a crudely exponential way reflecting both theincreased number (n) ofcrystals and their increased size.This is the expected result of the model which assumesequal thickness for successive zones. The area of crystalsurface increases exponentially with crystal growth and isfurther augmented by additional nucleation. Thus themodeled increment of total crystallization correspondingto zone 40 (for example) is more than twice that corre-sponding to zone 100. Average zone thickness doesdecrease slightly toward the rim of many crystals (Figs. 7and 8), but the increasing total surface area due to theincrease in the number density of crystals is so large thata general increase in total crystallization per zone isprobable with progressive crystallization, if the assump-tion of contemporaneity of zones is roughly correct.

The average size of the phenocrysts of plagioclaseincreases irregularly with crystallization from 0.02 cm to0.05 cm. It should be recognized that there is no require-ment that the average grain size increase with crystalliza-tion. A large proportion of small, late grown grains wouldresult in a decrease in average grain size. Many smallcrystals are indeed present but these are considered toconstitute a separate microphenocryst population coevalwith the sodic rims on the phenocrysts. It should berecalled, however, that the ash sample is air winnowed;consequently phenocrysts smaller than about 0.05 cmmay be underrepresented in the sample. It is neverthelessthe case that, for the population of 52 crystals, the grainsize increases with an'increasing proportion of crystalswith fewer than 100 zones. This relation is a consequenceof the initial growth of patchy-zoned cores which consti-tute the bulk of the grains. It seems likely that crystalswith relatively few zones have comparatively largepatchy-zoned cores; however, the possible efect of win-nowing on this relation also needs more study.

Accompanying crystallization of the Fuego plagio-clases were probable changes in temperature and waterpressure. The curves plotted on Figure 16 althoughconstrained by various facts are schematic and are in-tended primarily to convey the idea of pulses of nucle-ation and patchy-zoned crystallization (and temporarystorage of released latent heat) associated with episodesof rapid decompression (and H2O efervescence). Thecurves are roughly self-consistent. For example a burst ofcrystallization of 5 percent corresponds to a release oflatent heat of about

or 4 caVgm of magma yielding a warming of less thanabout

cal4 -102 callgm'

gm

or 20oC. (A small but uncertain fraction of the releasedlatent heat is consumed by the effervescence process[Harris, 1980, l98l]). With crystallization, transitory in-creases in temperature are assumed to associate withabrupt decreases in Ps.6 and increases in fraction ofplagioclase (0. The initial and final temperatures for theFuego magma are based on olivine/glass compositionaldata evaluated in the light of data by Roeder (1974),Anderson and Sans (1975), and Yoder and Tilley (1962)and explained by Rose et al. (1978). The maximum pH,sis based on Harris' (1980) results, and the minimum isassumed to be I atm. I assumed that a given decrease inPs"e would yield a greater fraction of crystallization andheating at lower pressures than at higher pressures, butmade no attempt to scale the relations in view of particu-lar experimental melting curves. The essential features ofFigure 16 are the general trends and the existence of atleast three separate kinks corresponding to bursts ofpatchy-zoned growth. These may be related to the zonedcrystal sketched in Figure 16. I think it is likely that thebursts of patchy-zoned plagioclase reflect tidally trig-gered acceleration of ascent, but this is not an essentialfeature of Figure 16.

For the Fuego phenocrysts the transition from non-oscillatory zoned cores with round corners to oscillatoryand patchy-zoned regions is plausibly related to the initialtransfer of a small portion of magma from a large bodywith suspended crystals of non-oscillatory zoned plagio-clase upwards into a thin conduit with concomittantrounding of corners by resorption caused by decompres-sion of a hydrous magma not saturated with a H2O-richgas (Fig. 16). Subsequently with pulsed ascent within thethin conduit crystals plausibly grew oscillatory zones aspulses of upward motion sheared the crystals past eachother and deformed their compositional boundary layer ofsurrounding melt. Finally the largely degassed Fuegomagma stiffened, fractured and lost a final burst of gas(probably on or about October 10,1974) prior to eruptionyielding a large supersaturation and consequent crystalli-zation of sodic rims and microphenocrysts which havebarely discernible oscillatory zones. Increase in gas pres-sure caused by the microphenocryst crystallizationopened the vent and blew out the upper part ofthe magmabody on October 14, 1978. In my model the oscillatoryzoning of the plagioclase phenocrysts is related to thedynamical ascent of the magma in a way that is suggestedby volcanological, textural, mineralogical and glass inclu-sion data.

Conclusions

My principal conclusion is as follows: the I /rm thickoscillatory zones on the Fuego plagioclase probably are

.05 gm

gm80 cal/gm x

ANDERSON: PLAGIOCLASE

like the ticks of a clock. Various textural and composi-tional features provide circumstantial evidence support-ing a triggered mechanism for the clock: pulses of motionmodulate growth. Growth is rate limited by interfaceattachment kinetics and yields concentration gradients inthe melt which result in decreasing An outward withineach zone. Reading the zones as ticks of a clock faces theproblem that not all forms of the crystal tick at the samerate, but several fast growing forms ({201}, {110}, {lT0})tick at about the same rate. Development of I pm thickzones on the Fuego plagioclase appears restricted togrowth rates less than those for patchy zoning (interface

instability) and greater than those for the non-oscillatory-zoned interiors of grains. The interpretation is that theFuego plagioclase grew at linear rates of0.2 to 25 pm per

day, consistent with the notion that most I pm scalezones on fast growing forms reflect tidally triggeredpulses of shearing motion within the magma.

Acknowledgments

I thank W. I. Rose for giving me the ash samples to study andfor encouragement and stimulating discussion. My student,David Harris, provided many helpful discussions. Victor Barci-lon, Frank Richter, Roscoe Braham, and Ramesh Srivastavahelped sharpen my thinking about tidal mechanisms, correlationsand growth of raindrops and crystals. The book by J. V. Smith(1974) was an invaluable source throughout my study. I thankJ. V. Smith for the loan of his universal stage and T. Peterson forhelp with the SEM. Part of this research was supported by NSFgrants EAR76-15016 and EAR79-26485. I thank Y. Bottinga forcareful and penetrative criticisms of earlier drafts of this manu-script, and T. Grove, T. Loomis, and W. Melson for helpfulsuggestions on the last draft.

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ZONING, FUEGO VOLCANO 675

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