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ORIGINAL PAPER

Intermittent mixing processes occurring before Plinian eruptionsof Popocatepetl volcano, Mexico: insights from textural–compositional variations in plagioclase and Sr–Nd–Pb isotopes

G. Sosa-Ceballos • J. E. Gardner • J. C. Lassiter

Received: 14 January 2012/ Accepted: 9 January 2014/ Published online: 29 January 2014 Springer-Verlag Berlin Heidelberg 2014

Abstract Volcá n Popocate´petl has explosively erupted inPlinian style at least ve times in the last 23,000 years.Extreme deviations in composition and the occurrence of dissolution features in plagioclase and pyroxene, and theoccasional presence of xenocrysts of Cr-rich Fe–Ti oxidesand Mg-rich olivines and pyroxenes indicate that magmamixing has been a major process affecting the magmaticsystem. The nearly invariant composition of the eruptedproducts (andesitic–dacitic) suggests, however, that mixingis not acting alone and must be balanced by assimilationand/or crystallization. To investigate the magmatic pro-cesses that have modied the Plinian magmas, textural andcompositional variations and growth rates in plagioclasewere used to approximate the frequency of mixing eventsaffecting each magma. Systematic analysis of Sr, Nd, andPb isotopes was carried out on plagioclase, pyroxene, andpumice matrix glass to constrain the extent of assimilationof upper crustal rocks. Additionally, a series of phaseequilibrium experiments were carried out to constrain thedepth where such mixing and assimilation occurred. We

nd that magma was stored at one of two different depthsbeneath Popocate ´petl with magma mixing acting in bothreservoirs. Mixing frequency and the relative impact onmineral compositions and textures has varied with time.Assimilation of calcareous rocks underneath Popocate ´petlhas not been pervasive and does not contribute signicantlyto the evolution of the Plinian magmas. The similar com-positions of magmas with diverse mixing histories suggestthat fractional crystallization, and possibly assimilation of deep crust, takes place at depth and that intermediatemagmas ascend into the upper crust already differentiated.

Keywords Popocaté petl Magma mixing Plagioclase Phase equilibrium experiments Radiogenic isotopes

Introduction

Fractional crystallization of basaltic magma contributesgreatly to the generation of calc-alkaline magma in sub-duction zones (e.g., Heiken and Eichelberger 1980 ; Singeret al. 1995 ). Mixing of magmas and assimilation of crustalrocks must also contribute, however, in order to explain thewide diversity of bulk isotopic compositions of interme-diate-silicic magmas and the disequilibrium textures intheir minerals (e.g., De Paolo 1981 ; Sisson and Grove1993 ; Gardner et al. 1995a , b; Clynne 1999 ; Tepley et al.2000 ; Couch et al. 2001 ; Annen et al. 2006 ; Bohrson andSpera 2007 ; Brophy 2009 ). The occurrence of mixing andassimilation is indisputable, but how often such eventsoccur and how much they affect the composition and tex-ture of crystals in any single magma body are poorlyconstrained. In addition, because mixing has been linked asa potential trigger for eruptions worldwide, it is of interestto know whether such events occur similarly before Plinian

Communicated by G. Moore.

Electronic supplementary material The online version of thisarticle (doi: 10.1007/s00410-014-0966-x ) contains supplementary

material, which is available to authorized users.

Present Address:G. Sosa-Ceballos ( & ) J. E. Gardner J. C. LassiterDepartment of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin,Austin, TX, USAe-mail: [email protected];[email protected]

G. Sosa-CeballosInstituto de Geofı ´sica, Campus Morelia, Universidad NacionalAutó noma de Me´xico, Morelia, Michoaca ´n, Mexico

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Contrib Mineral Petrol (2014) 167:966DOI 10.1007/s00410-014-0966-x

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eruptions, especially at volcanoes that do not erupt fre-quently in Plinian fashion.

One way to determine those frequencies and intensitiesis to track systematic records of those processes in asequence of magmas tapped by multiple Plinian eruptions.Two common methods used to evaluate such recordsinvolve measuring variations in Sr–Nd–Pb isotopic com-positions and variations in plagioclase composition (e.g.,De Paolo 1981 ; Tepley et al. 2000 ; Davidson et al. 2001 ;Bohrson and Spera 2007 ; Andrews et al. 2008 ). Plagioclaseis particularly suited for investigating magmatic processesbecause of its extensive thermal stability, its slow internaldiffusive re-equilibration (Morse 1984 ; Zellmer et al. 1999 ;Costa et al. 2003 ), and its composition reects variations inintensive parameters (e.g., Tepley et al. 2000 ; Couch et al.2001 ; Humphreys et al. 2006 ; Berlo et al. 2007 ; Andrewset al. 2008 ).

Magmas erupted at Popocate ´petl volcano, Me´xico,exhibit evidence that mixing between silicic and moremac magmas is common (e.g., Straub and Martin Del-Pozzo 2001 ; Witter et al. 2005 ; Schaaf et al. 2005 ; Atlaset al. 2006 ; Roberge et al. 2009 ; Arana-Salinas et al. 2010 ;Sosa-Ceballos et al. 2012 ). If such mixing were volumet-rically signicant, however, erupted magmas should pro-gressively become more mac with time. But, at least formagmas erupted in Plinian events over the past 23 ky, thedominant type is intermediate in composition, with noapparent long-term shift to more mac compositions (e.g.,Schaaf et al. 2005 ; Sosa-Ceballos 2006 ; Arana-Salinaset al. 2010 ). The question thus becomes, does crustalassimilation or fractional crystallization (or both) tend tobalance injections of mac magma, with small variations incomposition reecting differences in their frequencythrough time, or does mixing act alone, but is insufcient inmagnitude to signicantly alter bulk composition throughtime?

In this study, we investigate the magmatic processesresponsible for the evolution of magmas tapped during thePlinian eruptions at Popocate ´petl volcano. In particular, weseek to know whether the record of the magmatic processesis similar before each Plinian event. Although such erup-tions are relatively rare, they invariably tap magmas withbulk compositions similar to recent ash emissions (Strauband Martin Del-Pozzo 2001 ), as well as lavas that built thepresent-day cone (Sosa-Ceballos 2006 ). The record of magmatic processes is inferred from textural and compo-sitional analyses of plagioclase and analyses of Sr, Nd, andPb isotopes in plagioclase, pyroxene, and pumice matrixglass (silicate melt). In addition, the relative pre-eruptivestorage conditions under which those processes occur areconstrained through phase equilibrium experiments.Results show that over the last 23 ky, magmas erupted inPlinian fashion were variably modied by replenishment of

mac magma at one of two depths. The magnitude of thosereplenishments appears relatively constant, but their fre-quency varies signicantly. Assimilation of local calcare-ous rocks is negligible, leaving fractional crystallizationand possibly assimilation of lower crust as the processesmost likely balancing mixing to maintain the relativelyinvariant bulk composition.

Geologic background

Popocate´petl volcano, located in the central portion of theTrans-Mexican Volcanic Belt and related to subduction of the Cocos plate beneath North America, has had at leastthree construction–destruction periods (Siebe et al. 1995 ;Sosa-Ceballos et al. 2012 ). The age of the rst collapse thatdestroyed the oldest paleostructure, as described by Mooseret al. ( 1958 ), is unknown. The second edice was destroyedby a massive sector collapse * 23,000 years ago, as datedby the associated White pumice Plinian deposit (WP)(Fig. 1; Siebe et al. 1995 ). The third destructive event(caldera collapse) occurred * 14,100 years ago during theTutti Frutti Plinian Eruption (TFPE). This eruption, com-posed of the Milky Tephra (MT) and Gray Tephra (GT),resulted in the formation of a small caldera on the NWank of the volcano (Sosa-Ceballos et al. 2012 ). Sincethen, there have been at least three other Plinian eruptions.Siebe et al. ( 1996 ) named these deposits as Upper Pre-Ceramic, Lower Ceramic, and Upper Ceramic. Here, weuse their respective informal names: Ochre Pumice (OP)(Espinasa-Peren ˜a and Martı´n-Del Pozzo 2006 ; Arana-Salinas et al. 2010 ); Lorenzo Pumice (LP); and Pink Pumice (PP) (Fig. 1).

Previous studies have related to the full spectrum of magmas erupted at Popocate ´petl to generation in themantle wedge, resulting from dehydration of the subduct-ing slab, followed by fractional crystallization duringascent and mixing–assimilation during stagnation in shal-low magma reservoirs (e.g., Straub and Martin Del-Pozzo2001 ; Schaaf et al. 2005 ). Emphasis has been placed onrecharge and pre-eruptive mixing between mac and silicicmagmas to produce andesites (Nixon and Pearce 1987 ;Pearce and Kolisnik 1990 ; Straub and Martin Del-Pozzo2001 ; Schaaf et al. 2005 ; Witter et al. 2005 ; Atlas et al.2006 ) and on the assimilation of local Cretaceous lime-stones and Tertiary terrigenous rocks (Goff et al. 2001 ;Siebe et al. 2004 ).

Samples and analytical methods

Large pumice fragments collected from the six Pliniandeposits (WP, MT, GT, OP, LP, and PP) were broken intothree pieces, one of which was used for whole-rock

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compositional analysis, one for thin sectioning, and the lastfor mineral separation. Mounts of 132 plagioclase pheno-crysts (an average of 20 per deposit) were made. Parts of the OP, MT, and WP fragments used for bulk analysis werealso crushed and ground for use in phase equilibriumexperiments. Whole-rock compositions were analyzed atthe GeoAnalytical Laboratory at Washington State Uni-versity using x-ray uorescence (XRF).

Major elements in plagioclase, clinopyroxene, orthopy-roxene, hornblende, Fe–Ti oxides, and groundmass glass

were analyzed using a JEOL JXA-8600 Electron Micro-probe (EPMA) housed in the Department of GeologicalSciences at The University of Texas at Austin. Analyseswere made using 15 keV accelerating voltage and 10 nAbeam current. The beam was focused to analyze plagio-clase, pyroxenes, and Fe–Ti oxides, but was defocused to5 l m to analyze hornblende and to 10 l m to analyzegroundmass glass, in order to avoid water loss and sodiummigration. Detailed textural analyses and core-to-rimcompositional transects across plagioclase were made in

steps of 10 l m with the electron microprobe, except forthose in GT, whose internal sieved texture made itimpossible to analyze uninterrupted transects. Errors(± 2r ) on plagioclase analyses during the course of thisstudy average 2 mol% An content.

Sr, Nd, and Pb isotopic compositions of plagioclase,clinopyroxene, and groundmass glass concentrates weremeasured by thermal ionization mass spectrometry (TIMS)at the University of Texas at Austin, Department of Geo-logical Sciences, using a ThermoFisher TRITON. Allsamples were rst leached with hot 6 N HCl to removesupercial contamination. Crystals coated by a ne lm of

matrix glass were slightly etched with HF and then cleanedwith water in a sonic bath. Chemical separations of Sr, Nd,and Pb were accomplished using standard ion-exchangecolumn techniques, similar to those outlined in Lassiteret al. ( 2003 ), with the exception that: (1) Sr was separatedby using Sr-Spec resin; and (2) rare earth elements (REEs)were separated using RE-Spec resin, followed by Nd sep-arations using glass columns lled with hydrogen di-eth-ylhexyl-phosphate resin (HDEHP). Pb separations wereperformed using Teon columns and anion exchange resin.

Experimental methods

Ten grams each of OP, MT, and WP pumice clasts wereground to ne powders for use in phase equilibriumexperiments. All experiments were run in Ag 70 Pd30 cap-sules (2 or 5 mm in diameter) in cold-seal pressure vesselsmade of either Waspaloy (experiments B 900 C) or TZMmetal (experiments [ 900 C).

For those runs in Waspaloy pressure vessels, capsuleswere welded at one end and starting material and distilledwater were added. Sufcient water was added to ensure

Fig. 1 Composite stratigraphic column of Plinian deposits of Pop-ocaté petl volcano: White Pumice ( WP ), Tutti Frutti Plinian Eruption(Gray Tephra ( GT ) and Main Tephra ( MT ), Ocher Pumice ( OP ),Lorenzo Pumice ( LP), and Pink Pumice ( PP )). Radiocarbon ages arefrom Siebe et al. ( 1995 , 1996 ), Arana-Salinas et al. ( 2010 ), and Sosa-Ceballos et al. ( 2012 ). Mineralogies are pl plagioclase, px pyroxene,ox Fe–Ti oxides, and amp amphibole. Pre-eruptive temperature andoxygen fugacity (given as the relative difference from the Ni–NiObuffer curve) are derived from magnetite–ilmenite and/or amphibolecompositions. Oxygen fugacity was not determined for LP

Table 1 Bulk composition of magmas in wt%

WP GT MT OP LP PP

SiO 2 64.81 60.55 61.36 62.55 62.07 62.70

TiO 2 0.81 0.82 0.81 0.82 0.81 0.81

Al2O3 16.63 16.82 17.04 17.28 16.53 16.16

FeO* 4.16 5.20 5.07 4.84 4.87 5.28

MnO 0.10 0.10 0.10 0.10 0.10 0.10MgO 2.33 4.59 3.96 2.88 4.46 3.46

CaO 4.36 5.91 5.88 5.04 5.07 4.88

Na 2O 3.96 4.08 3.96 4.42 4.16 4.37

K 2O 2.64 1.73 1.62 1.85 1.72 2.03

P2O5 0.20 0.20 0.20 0.21 0.20 0.20

Total 100 100 100 100 100 100

Analytical precision (2 SD) average 0.38 wt% SiO 2 , 0.03 wt% TiO 2 ,0.33 wt% Al 2O3 , 0.13 wt% FeO, 0.37 wt% MgO, 0.12 wt% CaO, 0.09wt% Na 2O, 0.04 wt% K 2O, 0.02 wt% P 2O5

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Table 2 Average composition of most common glass, and phenocrysts rims tapped by the Plinian eruptions at Popocate ´petl

SiO 2(wt%)

TiO 2(wt%)

Al2O3(wt%)

FeO*(wt%)

MnO(wt%)

MgO(wt%)

CaO(wt%)

Na 2O(wt%)

K 2O(wt %)

Cr2O3(wt%)

Total(wt%)

n

WP

Gl 69.10 0.42 14.78 2.79 0.05 0.73 2.98 5.07 3.22 n.a. 99.10 15

Plg 55.94 n.a. 28.05 0.34 n.a. n.a. 9.88 5.33 0.28 n.a. 99.82 10

Am 44.90 2.50 9.50 12.90 0.20 14.10 11.00 2.20 0.70 n.a. 98.00 20Cpx 52.20 0.30 1.70 12.30 0.40 13.40 19.90 0.60 0.00 n.a. 100.80 10

Opx 53.73 0.09 0.97 19.47 0.74 24.83 0.98 0.09 n.d. n.a. 100.90 10

Mt n.a. 8.66 3.27 80.78 0.33 2.35 n.a. n.a. n.a. 0.35 95.74 10

Il n.a. 35.30 0.51 59.15 0.22 2.32 n.a. n.a. n.a. 0.25 97.76 10

MT

Gl 63.10 0.62 17.21 4.28 0.07 2.24 5.44 4.49 1.74 n.a. 99.21 12

Plg 58.10 n.a. 25.81 0.29 n.a. n.a. 8.39 6.41 0.44 n.a. 99.40 10

Cpx 52.24 0.47 1.78 8.03 0.30 15.26 21.34 0.43 n.d. n.a. 99.84 10

Opx 55.18 0.07 0.94 13.01 0.30 28.43 1.31 0.00 n.d. n.a. 99.25 12

Mt n.a. 8.10 3.90 83.70 0.50 2.50 n.a. n.a. n.a. n.a. 98.70 10

Il n.a. 44.40 0.90 47.40 0.30 5.30 n.a. n.a. n.a. 0.80 99.10 10

GT

Gl 62.20 0.60 18.95 4.85 0.02 1.17 4.83 5.08 2.20 n.a. 99.90 15

Plg 48.30 n.a. 32.20 0.50 n.a. n.a. 15.30 2.80 0.10 n.a. 99.20 13

Am 43.00 2.90 12.10 10.50 0.10 15.30 11.00 2.70 0.50 n.a. 98.10 15

Cpx 51.40 0.83 3.56 7.68 0.18 14.98 20.21 0.50 n.d. n.a. 99.34 10

Opx 53.60 0.27 1.58 15.55 0.29 27.39 1.31 0.04 n.d. n.a. 100.02 10

Mt n.a. 10.70 3.60 80.30 0.40 3.50 n.a. n.a. n.a. 0.50 99.00 10

Il n.a. 44.10 0.80 49.70 0.50 4.30 n.a. n.a. n.a. 0.00 99.40 10

OP

Gl 68.64 0.56 15.75 3.12 0.06 0.83 2.84 4.67 2.57 n.a. 99.03 13

Plg 54.98 n.a. 26.82 0.48 n.a. n.a. 10.11 5.42 0.25 n.a. 98.05 15

Cpx 52.14 0.47 2.31 9.17 0.34 14.58 21.00 0.56 n.d. n.a. 100.56 10

Opx 53.56 0.24 1.31 18.86 0.45 24.98 1.47 0.00 n.d. n.a. 100.87 10

vMt n.a. 13.11 1.84 78.54 0.39 2.24 n.a. n.a. n.a. 0.29 96.40 10

Il n.a. 44.32 0.25 51.52 0.44 3.19 n.a. n.a. n.a. 0.10 99.82 10

LP

Gl 65.59 0.99 15.80 5.06 0.07 1.62 4.08 4.25 2.33 n.a. 99.78 10

Plg 57.67 n.a. 25.69 0.48 n.a. n.a. 10.21 4.57 0.18 n.a. 98.78 9

Cpx 52.08 0.43 2.82 5.27 0.09 16.06 20.51 0.46 n.d. n.a. 97.73 10

Opx 53.49 0.32 1.26 19.79 0.45 23.95 1.39 0.04 n.d. n.a. 100.69 10

Mt n.a. 6.73 2.81 84.06 0.29 1.96 n.a. n.a. n.a. 0.35 96.20 10

Il n.a. 47.35 0.43 45.18 0.30 5.30 n.a. n.a. n.a. 0.60 99.15 10

PP

Gl 67.07 0.75 15.85 3.11 0.08 1.13 3.41 4.38 2.83 n.a. 98.61 9Plg 55.85 n.a. 26.31 0.44 n.a. n.a. 8.93 7.13 0.40 n.a. 99.09 11

Cpx 52.06 0.54 1.77 10.55 0.34 14.45 20.18 0.48 n.d. n.a. 100.38 10

Opx 53.12 0.32 1.34 18.60 0.46 24.45 1.30 0.02 n.d. n.a. 99.61 10

Mt n.a. 9.67 2.63 79.80 0.32 3.40 n.a. n.a. n.a. 0.83 96.67 11

Il n.a. 38.00 0.57 55.17 0.23 3.80 n.a. n.a. n.a. 0.45 98.22 11

Gl matrix glass, Plg plagioclase, Am amphibole, Cpx clinopyroxene, Opx orthopyroxene, Mt titanomagnetite, Il ilmenite, n.a . not analyzed, n.d.not detected, n number of analysis. 2 standard deviations for standards measured during the course of this study average 0.4 wt% Na 2O, 0.1 wt%Al2O3 , 1 wt% SiO 2 , 0.1 wt% K 2O, 0.2 wt% CaO, 0.6 wt% FeO, 0.5 wt% TiO 2 , 0.7 wt% MnO, 0.6 wt% MgO


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that a separate uid was present throughout the run. Werst estimated the minimum amount of water needed fromthe model of Papale et al. ( 2006 ), and then added more thanthat to the capsule. The capsule was then weighed, weldedshut, heated on a hot plate, and weighed again to check forleaks and ensure that no water was lost during welding. To

ensure that a uid remained present throughout the run, thecapsule was weighted after quenching. In addition, whenthe capsule was split open a slight hiss is heard if uid ispresent. Finally, uid-saturated samples contain bubbles.Equilibrium was approached by verifying the occurrence of experimental phases and comparing the compositions of experimental plagioclase within aliquots of previously runmaterial, one of which had run at a higher temperature andthe other at a lower temperature. MT, OP, and WP powderswere placed in separate, 2-mm-diameter Ag 70 Pd30 cap-sules, which were only crimped shut and placed inside a5-mm-diameter Ag 70 Pd 30 capsule to ensure previously run

materials equilibrated at the same pressure and tempera-ture. Oxygen fugacity was buffered near that of the Ni–NiObuffer reaction by using a Ni ller rod and water as thepressurizing medium (Geschwind and Rutherford 1992 ;Gardner et al. 1995a , b).

For experiments in TZM pressure vessels, 2-mm-diam-eter Ag 70 Pd30 capsules were welded at one end, powderwas added, and the capsule was crimped shut. Another2-mm-diameter Ag 70 Pd 30 capsule was welded at one end

Fig. 2 Total alkalis versus silica diagram for bulk-rock ( solid symbols ) and groundmass glass ( open symbols ) compositions of thePlinian deposits (Tables 1, 2). The shaded region outlines the elddened by older lavas and tephra erupted since 1996 (Sosa-Ceballos2006 ; Schaaf et al. 2005 ). Symbols are diamond WP, triangle GT, star MT, pentagon OP, square LP, and circle PP

cFig. 3 Relative percentages of compositions of ca. 20 plagioclasecrystals in each Popocate ´petl Plinian magma ( n = number of analyses). For comparison, shaded area represents the distributionof OP plagioclase


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and loaded with Ni metal and NiO powder and crimpedshut. Both capsules were then placed inside a 5-mm-diameter Ag 70 Pd30 capsule that contained enough distilledwater to both saturate the sample and facilitate the reactionbetween the buffer materials. The 5-mm capsule waswelded shut and checked for leaks. The pressurizingmedium in the TZM pressure vessels was argon, to whichseveral bars of methane were added to maintain hydrogenfugacity during the experiment to discourage hydrogen lossfrom the capsule (Sisson and Grove 1993 ). At the end of each run, the buffer and water were checked to ensure that

both Ni metal and NiO powder were still present and thatthe experiment was water saturated. If one was absent, thesample was discarded.

Results

All Plinian magmas are calk-alkaline, high-silica andesites,except WP, which is a dacite (Table 1). Phenocryst contentsrange from 13 to 36 vol% and consist of plagioclase, orth-opyroxene, clinopyroxene, titanomagnetite, ilmenite, and

Table 3 Textures and compositional variability in plagioclase

Texture 1 (%) Texture 2 (%) Texture 3 (%) Texture 4 (%) ? D An (mol%) - D An (mol%) Plag affected (%)

WP 41 59 0 0 14 14 24

MT 0 0 86 14 32 30 46

GT 0 0 0 100 n.a. n.a. n.a.

OP 31 65 4 0 14 16 27

LP 19 31 50 0 22 25 36

Texture 1 = undisturbed zoned plagioclase; texture 2 = patchy sieved cores overgrown by undisturbed zoned plagioclase; texture 3 = zonedplagioclase with sieved zones near the rim; texture 4 = thoroughly sieved crystals. DAn = maximum compositional variation ( ? positive,- negative). Plag affected = percentage of crystal affected by compositional variations greater than 10 An mol% (taking the length of allcrystals from each eruption as 100 %)

Fig. 4 Back-scattered electronimages of representativeplagioclase crystals.a oscillatory growth withnormal compositional zonation(Group I), b patchy cores andoscillatory growths (Group II),c sieved zones near the rims(Group III), and d thoroughlysieved crystals (Group IV).Scale bar is 250 l m in allphotos


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trace amounts of apatite and pyrrhotite, all set in vesicularglass with abundant microlites of plagioclase and pyroxene.Amphibole also occurs as phenocrysts and microlites in WPand GT pumices. Matrix glass ranges in composition fromandesitic (MT) to rhyolitic (WP) (Fig. 2).

Plagioclase

Plagioclase phenocrysts range in size from about100–2,000 l m, and in composition from * An 30 to * An 60with modes differing between magmas (Fig. 3). Plagio-clase is unimodal in composition in OP and WP andbimodal in LP, PP, and MT. The outer rims of plagioclasein OP have compositions that match that of the mode,whereas rims in each of the other magmas are diverse andencompass the entire compositional range found in thatmagma.

Overall, plagioclase can be grouped into four maintextural groups (Table 3). The rst (Group I) consists of crystals characterized by variably zoned compositions,with only minor internal resorption boundaries and small

glass inclusions (Fig. 4a). Most Group I crystals in WP andOP are normally zoned from * An 55 to * An 40 . A few inWP are reversely zoned near their rims, increasing to* An 50 . Group I crystals in LP and PP are more variable incomposition and are not consistently zoned between coreand rim.

The second group of plagioclase (Group II) consists of crystals that have prominent patchy sieved cores that areovergrown by plagioclase with only minor internal distur-bances (Fig. 4b). The compositional difference betweencores and overgrowths is generally less than 15 mol%anorthite, with cores being more anorthitic, except in PP inwhich cores are more albitic (Table 3). Overgrowths onGroup II crystals in WP and OP are normally zoned withcompositions that match those from Group I. Most over-growths in LP are unzoned, although a few are normallyzoned, decreasing from * An 47 to * An 37 . Overgrowths onPP crystals are reversely zoned from * An 38 to * An 52 .

Group III plagioclase is similar to Group II, except thatthere are prominent sieved zones near their rims (Fig. 4c).Those outer sieve zones in MT, LP, and PP consist of intricate networks of micron-sized glass inclusions that aregenerally overgrown by up to 15 l m of texturally undis-turbed plagioclase. Those outer growths are always morealbitic by 15 and 30 mol%. Two sieve zones occur close tothe rims of OP crystals, with the outer one characterized bymultiple melt inclusions (tens of microns in size) that areoften connected to the adjacent matrix glass.

The fourth group (Group IV) is thoroughly sievedcrystals with compositions of * An 75 (Fig. 4d). A few of those are mantled by micron-thick rims of unaltered pla-gioclase with compositions of * An 50 . Group IV crystalsare found only in GT.

Mac minerals

Orthopyroxene and clinopyroxene occur in all deposits aseuhedral to subhedral phenocrysts up to 3,000 l m in size.Orthopyroxene varies in composition between En 65 andEn 70 , with Mg# = 54–60 (where Mg# ¼ Mgð Mgþ Fe total Þ 100).Clinopyroxene varies between En 39 Fs20 Wo 41 andEn 48 Fs8Wo 44 , with Mg# = 52–64. Both pyroxene types inWP and OP lack internal disequilibrium textures and arecompositionally homogeneous. Pyroxenes in LP and PP aretexturally more diverse, with some having internal disso-lution features. Amphibole occurs as phenocrysts in WP andGT. Crystals in these magmas are compositionally homo-geneous pargasite (Table 2). All are euhedral and lack reaction rims.

Ilmenite and titanomagnetite, typically 5–25 l m in size,occur in all magmas as compositionally unzoned crystals.Ilmenite is more abundant, except in MT, and varies in

Fig. 5 Distribution of Mg# ((Mg/Mg ? Fe tot ) 9 100) in orthopy-roxene (opx) and clinopyroxene (cpx) from all Plinian magmas. Solid bars are relatively Mg-rich pyroxenes in MT, LP, and PP, thought tobe most likely xenocrysts. Open bars are relatively Fe-rich pyroxenesfrom all Plinian magmas


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composition from Ilm 63 to Ilm 86 . Titanomagnetite variesfrom Usp 20 to Usp 70 . Most magmas contain a singlecompositional population of each (Table 2). Pre-eruptivetemperatures and oxygen fugacities of magmas were cal-culated using compositions of coexisting titanomagnetiteand ilmenite (Ghiorso and Evans 2008 ). Equilibriumbetween titanomagnetite and ilmenite was conrmed usingthe model of Bacon and Hirschmann ( 1988 ). In addition,temperatures were estimated for WP and GT from com-positions of amphibole phenocrysts (Ridol et al. 2010 ).Differences between temperatures estimated from Fe–Tioxides and amphibole are within uncertainties of themodels. Overall, temperature ranges from 875 ± 15 to960 ± 20 C, and oxygen fugacities range from 0.7 to 1.3log units above the NiNiO oxygen (NNO) buffer (Fig. 1).There are no systematic changes in temperature with eitherbulk composition or age.

Xenocrysts

Xenocrysts are found in most Plinian magmas and includeolivine (in MT, OP, LP, and PP), Cr-rich magnetite andilmenite (MT, LP), enstatite (MT, GT, LP, PP), barite (PP),amphibole (MT), and calcic plagioclase (MT). Olivine inMT, OP, LP, and PP contains reaction rims. Schaaf et al.(2005 ) report olivine in the Popocate ´petl magmas withchromite inclusions, compositions between Fo 70 and Fo 90 ,and orthopyroxene rims on Fo-rich olivine (MT and PPmagmas). They explain the reaction rims and the compo-sitional range of olivine as a result of re-equilibration atvariable temperatures produced by mixing between macand silicic magmas. Given the composition of the MTgroundmass (Table 2) and the Fo 90 reported in Schaaf et al.(2005 ), that would imply an iron–magnesium partitioncoefcient ( K Dolivine-melt

Fe–Mg ) of * 0.10. Such a small value

Table 4 Sr and Nd isotopic compositions

Eruption 87 Sr/ 86 Sr 143 Nd/ 144 Nd eNd 206 Pb/ 204 Pb 207 Pb/ 204 Pb 208 Pb/ 204 Pb

WP

Gl 0.70458 0.51281 3.3 18.69 15.60 38.48

Plg 0.70444 0.51283 3.8 18.68 15.60 38.46

Cpx 0.70455 0.51282 3.5 18.67 15.58 38.41

MTGl 0.70420 0.51285 4.1 18.65 15.61 38.44

Plg 0.70440 0.51282 3.5 18.68 15.56 38.30

Cpx 0.70421 0.51285 4.2 18.63 15.58 38.36

GT

Gl 0.70445 0.51282 3.5 18.63 15.57 38.33

Plg 0.70448 0.51282 3.5 18.63 15.56 38.32

Cpx 0.70449 0.51283 3.8 18.65 15.59 38.41

OP

Gl 0.70416 0.51286 4.4 18.65 15.60 38.43

Plg 0.70409 0.51286 4.3 18.63 15.58 38.35

Cpx 0.70415 0.51287 4.5 18.65 15.61 38.44

LP

Gl 0.70430 0.51284 3.9 18.66 15.59 38.42

Plg 0.70429 0.51282 3.6 18.66 15.60 38.45

Cpx 0.70429 0.51284 3.9 18.63 15.56 38.31

PP

Gl 0.70454 0.51283 3.8 18.68 15.61 38.48

Plg 0.70452 0.51282 3.6 18.66 15.60 38.45

Cpx 0.70452 0.51284 3.9 18.66 15.58 38.40

Gl matrix glass, Plg plagioclase, Cpx clinopyroxene. Total blanks for these procedures were 40 pg for Sr, 60 pg for Nd, and 16 pg for Pb. Sr dataare fractionation corrected to 88 Sr/ 86 Sr = 8.3752. 87 Sr/ 86 Sr ratios for NBS 987 measured during the course of this study average0.710256 ± 0.000005 (2 SD, n = 600). The Nd data are fractionation corrected to 146 Nd/ 144 Nd = 0.7219. 143 Nd/ 144 Nd ratios for La Jollastandard measured during the course of this study average 0.511835 ± 0.000005 (2 SD, n = 600)

The Pb data were corrected for * 1.1 % amu - 1 mass fractionation determined from the difference between the average measured values of NBS981 Pb and the values 206 Pb/ 207 Pb = 0.9149, 206 Pb/ 204 Pb = 16.9418, 207 Pb/ 204 Pb = 15.5000, 208 Pb/ 204 Pb = 36.7265 reported for the samestandard in Baker et al. ( 2004 ). 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, 208 Pb/ 204 Pb ratios and 2 standard deviations for NBS 981 measured during the course of this study average 16.9031 ± 0.006, 15.4466 ± 0.008, and 36.5582 ± 0.024, respectively


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indicates that those olivines are xenocrystic, and not inequilibrium with the MT magma (e.g., Takahashi 1987 ).Water-saturated experiments using magmas with similar

bulk composition to MT and LP, at similar fO 2 conditions(* log

- 10 fO2) but with greater concentrations of Cr andNi, grow olivine at 200 MPa and 1,000 C (Blatter and

Table 5 Experiments with MT, OP, and WP powders

Experiment Material Duration (h) P (Mpa) T (Celsius) Run product phases

MT-5 Powder 96 50 900 G, Plg, Pyx, Ox

MT-9 From MT-2 312 75 800 G, Plg, Pyx, Ox

MT-16 Powder 146 75 1,000 G, Plg, Pyx, Ox


MT-22 From MT-16 171 75 850 G, Plg, Pyx, OxMT-6 From MT-2 84 100 900 G, Plg, Pyx, Ox


MT-11 From MT-2 218 100 800 G, Plg, Pyx, Ox, A

MT-18 From MT-16 96 100 1,000 G, Plg, Pyx, Ox

MT-19 From MT-17 96 100 1,000 G, Plg, Pyx, Ox

MT-31 Powder 96 100 1,150 G



MT-12 From MT-2 219 125 800 G, Plg, Pyx, Ox, A

MT-17 Powder 96 125 1,000 G, Pyx, Ox

MT-27a From MT-2 168 125 850 G, Plg, Pyx, Ox, A

MT-27b From MT-1 168 125 850 G, Plg, Pyx, Ox, A

MT-1 Powder 96 150 900 G, Plg, Pyx, Ox, A

MT-2 Powder 168 150 800 G, Plg, Pyx, Ox, A

MT-28a From MT-2 168 150 850 G, Plg, Pyx, Ox, A

MT-28b From MT-1 168 150 850 G, Plg, Pyx, Ox, A

MT-26 From MT-2 168 175 825 G, Plg, Pyx, Ox, A, Bi

MT-3 From MT-1 96 200 875 G, Pyx, Ox, A

MT-4 From MT-2 169 200 825 G, Plg, Pyx, Ox, A, Bi

MT-23 Powder 72 200 1,000 G, Pyx,

MT-24 From MT-23 96 200 975 G, Pyx,

MT-25 From MT-23 72 200 925 G, Plg, Pyx, A

MT-29 From MT-25 144 200 850 G, Plg, Pyx, Ox, A, BiMT-14 From MT-13 98 225 850 G, Pyx, Ox, A, Bi

MT-13 Powder 96 250 850 G, Pyx, Ox, A, Bi

OP-7 From OP-6 168 75 875 G, Plg, Pyx, Ox

OP-9 Powder 96 75 950 G, Plg, Pyx, Ox

OP-3 From OP-1 96 125 900 G, Plg, Pyx, Ox

OP-10 From OP-1 96 150 950 G, Pyx, Ox

OP-2 From OP-1 96 175 825 G, Plg, Pyx, Ox, A, Bi

OP-4 From OP-1 168 175 825 G, Plg, Pyx, Ox, A, Bi

OP-6 From OP-4 85 175 900 G, Pyx, Ox, A

OP-8 From OP-1 144 225 850 G, Pyx, Ox, A, Bi

OP-1 Powder 144 250 850 G, Pyx, Ox, A, Bi

WP-3 From WP-1 144 75 875 G, Plg, Pyx, OxWP-2 From WP-1 144 125 915 G, Plg, Pyx, Ox, A

WP-5 From WP-1 144 175 900 G, Pyx, Ox, A, Bi

WP-4 From WP-1 144 225 850 G, Pyx, Ox, A, Bi

WP-1 Powder 169 250 850 G, Pyx, Ox, A, Bi

G glass, Plg plagioclase, Pyx pyroxene, Ox opaque, Fe–Ti oxides, A amphibole, Bi biotite, Powder initial material


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Carmichael 2001 ). We found, however, that experimentsusing MT, OP, and WP pumice do not grow olivine atthose P–T conditions.

Sosa-Ceballos et al. ( 2012 ) identied two populations of pyroxenes in GT and MT magmas that resulted from mixing.Very similar populations are found in LP and PP, with thedistinctly magnesian-rich pyroxenes considered xenocrystic(Fig. 5). Amphibole and calcic plagioclase are also presentin MT, but Sosa-Ceballos et al. ( 2012 ) demonstrated thatthose were inherited from GT and mac magma, respec-tively, just before eruption. All Cr-rich ilmenites and ti-tanomagnetites and those with exsolution textures in GT andMT are related either to more mac magma or to rockshosting the reservoir (Sosa-Ceballos et al. 2012 ). LP alsocontains two compositional groups of Fe–Ti oxides, one of which consists of Cr-rich ilmenite and titanomagnetite thatmay have equilibrated at * 1,000 C and are consideredxenocrystic (Table 2). Barite was found in PP, which has notbeen reported before in Popocate ´petl magmas, and did notgrow experimentally; barite is thus considered xenocrystic.

Sr, Nd, and Pb isotopes

Matrix glasses vary in 87 Sr/ 86 Sr from 0.70416 to 0.70458, ineNd from ? 3.3 to ? 4.4, in

206 Pb/ 204 Pb from 18.63 to 18.69,in 207 Pb/ 204 Pb from 15.56 to 15.62, and in 208 Pb/ 204 Pb from38.33 to 38.49 (Table 4). WP has the most radiogenic glassin terms of Sr, whereas OP glass is least radiogenic. Themost radiogenic for Pb is MT, and the least is GT.

Plagioclase and clinopyroxene have variable Sr, Nd, andPb isotopic compositions (Table 4). In the case of pla-gioclases, the most radiogenic occur in PP, whereas themost radiogenic clinopyroxenes occur in WP. The leastradiogenic crystals are both from OP. Clinopyroxenes arein Sr, Nd, and Pb isotopic equilibrium with matrix glassesfor all eruptions, whereas plagioclases from MT and WPare in isotopic disequilibrium with corresponding matrixglasses and clinopyroxenes. The Pb isotopes for pyroxenesin WP, GT, LP, and PP are in disequilibrium with pla-gioclase and groundmass, OP contains plagioclase in dis-equilibrium with pyroxene and groundmass, and in MT, all3 phases are in disequilibrium. Overall, there is isotopicdisequilibrium between phases in all magmas.

Experimental results

Experiments were carried out using OP, MT, and WPwhole-rock samples between 800 and 1,050 C and 50 and250 MPa. Although groundmass glass and many crystals inMT are not in equilibrium, we used it as a proxy formagmas of similar bulk composition with the assumptionthat imparting a specic pressure and temperature on its

bulk composition will yield a given mineral assemblageand compositions (e.g., Gardner et al. 1995a , b). In fact,compositions of experimentally produced plagioclase andglass using MT coincide closely with those using OP,which is similar in bulk composition to MT (Fig. 2),demonstrating that the presence of disequilibrium pheno-crysts, which comprises [ 30 vol% in MT, made littledifference. All experiments were run at pressures andtemperatures where melt and uid (bubbles) were present(Table 5). At 125 MPa, pyroxene and Fe–Ti oxides are theliquidus phases at * 1,025 C, plagioclase appears at* 975 C, and amphibole at * 900 C (Fig. 6). At200 MPa, pyroxene crystallizes rst at * 1,000 C, thenamphibole at * 950 C, Fe–Ti oxides and biotite at* 875 C, and plagioclase at * 800 C (Fig. 6).

Glass and crystals in experiments using MT and OP varysimilarly in composition with changes in water experi-mental pressure (75–250 MPa) and temperature(825–950 C), although compositions in OP are slightlymore evolved at any given temperature and water pressure(Table 6). In general, lower temperature and lower waterpressure result in more silicic melts that are enriched inK 2O and depleted in FeO, MgO, and CaO. Plagioclase alsovaries in composition with water pressure and temperatureand, in general, becomes more albitic with decreasing

Fig. 6 Experimental results using OP ( solid half symbols ), MT ( opensymbols ), and WP ( asterisk ) samples. All experiments were watersaturated (P H2O = P total ) and run at an oxygen fugacity equal to orslightly more oxidized than the Ni–NiO buffer curve. Glass (melt) andvapor are present at all conditions. For MT runs, left-pointing arrowsrepresent crystallization experiments; right-pointing arrows , melting.Squares represent natural powder run directly at those conditions.Curves represent the upper stability limits for mineral phases ( dashed where inferred). Experimental plagioclase compositions are shown bydotted curves , based on results from the MT runs. Results using OPand WP samples generally match those of the MT runs


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temperature and decreasing water pressure (Fig. 6). Over-all, compositional variations of amphibole with changes of pressure and temperature are more scattered, but in general,

Na 2O, FeO, and MnO contents tend to be greater at lowertemperature. Pyroxene and biotite were identied bymorphology only and are too small to analyze.

Table 6 Composition of experimental products

Experiment P (Mpa) T Celsius Na 2O(wt%)

MgO(wt%)

Al2O3(wt%)

SiO 2(wt%)

K 2O(wt%)

CaO(wt%)

TiO 2(wt%)

MnO(wt%)

FeO(wt%)

OxideTotals(wt%)

MT-5 500 900 G 4.77 1.32 18.05 63.96 2.15 4.85 0.54 0.06 2.29 98.00

Plg 5.56 23.50 58.31 0.80 8.08 0.96 97.21

MT-9 750 800 G 3.26 0.37 13.70 68.53 3.85 1.68 0.23 0.02 1.20 92.83MT-16 750 1,000 G 4.45 0.92 16.75 64.96 2.28 4.18 0.66 0.06 2.09 96.35

Am 0.93 14.12 5.52 54.51 0.66 15.52 0.67 0.27 5.31 97.51

MT-22 750 850 G 3.28 0.29 12.42 69.93 3.50 1.27 0.31 0.03 1.31 92.34

MT-18 1,000 1,000 G 5.97 0.76 15.44 67.15 2.75 2.41 0.65 0.10 1.83 97.08

Plg 3.88 30.21 51.99 0.14 13.20 0.31 99.74

MT-31 100 1,150 G 4.14 3.55 16.34 57.19 1.76 5.72 0.73 0.11 5.00 94.54

MT-8 1,250 900 G 3.76 0.66 14.34 64.09 2.69 2.55 0.46 0.06 2.29 90.90

Plg 4.54 25.09 60.20 0.65 8.49 0.89 99.86

MT-12 1,250 800 G 2.60 0.24 12.57 69.87 3.01 1.29 0.25 0.04 1.42 91.30

MT-17 1,250 1,000 G 5.04 2.23 17.34 59.54 1.62 5.51 0.77 0.09 4.63 96.76

MT-27a 1,250 850 G 3.57 0.30 13.86 70.18 3.08 1.47 0.30 0.03 1.52 94.33

MT-1 1,500 900 G 4.03 0.57 15.05 64.50 2.55 2.60 0.44 0.07 2.38 92.18Plg 4.54 25.68 57.28 0.57 9.53 0.74 98.34

Am 2.75 5.42 14.41 57.24 1.57 5.94 0.80 0.17 5.85 94.14

MT-2 1,500 800 G 4.17 0.45 18.03 62.46 2.00 4.51 0.43 0.01 2.26 94.32

Plg 6.12 23.74 61.74 0.69 6.75 0.60 99.63

Am 3.00 3.29 15.51 60.88 1.63 4.98 0.38 0.09 2.93 92.69

MT-28a 1,500 850 G 3.47 0.47 14.21 69.06 2.95 1.76 0.27 0.03 1.36 93.57

MT-3 2,000 875 G 3.45 0.54 15.17 64.89 2.40 2.54 0.33 0.08 1.25 90.66

Am 2.38 10.08 12.29 51.25 1.18 7.56 1.29 0.20 7.05 93.27

MT-4 2,000 825 G 3.50 0.27 18.19 64.51 1.93 4.65 0.08 0.02 1.21 94.37

Plg 6.08 24.94 57.83 0.52 8.38 0.82 98.56

Am 2.05 11.64 11.18 52.03 0.78 8.68 1.23 0.25 9.27 97.11

MT-24 2,000 980 G 4.53 2.53 16.79 57.20 1.46 5.85 0.75 0.09 4.43 93.64

MT-25 2,000 925 G 4.30 0.94 16.49 61.04 2.02 3.70 0.38 0.09 3.40 92.37

MT-29 2,000 850 G 3.46 0.48 15.17 67.24 2.49 1.97 0.30 0.05 1.35 92.50

MT-14 2,250 850 G 3.07 0.25 15.35 62.60 2.22 2.33 0.24 0.05 1.19 87.30

MT-13 2,500 850 G 2.80 0.49 16.11 64.66 2.08 2.34 0.33 0.07 1.37 90.24

OP-7 750 875 G 3.81 0.63 14.32 71.02 3.38 0.94 0.32 0.04 1.24 95.71

OP-9 750 950 G 3.90 0.83 15.33 66.77 2.87 1.90 0.64 0.06 3.02 95.31

OP-3 1,250 900 G 4.14 1.03 15.71 66.69 2.84 1.96 0.54 0.09 1.95 94.95

OP-10 1,500 950 G 4.37 1.21 15.71 62.55 2.47 2.34 0.40 0.07 1.32 90.43

OP-2 1,750 825 G 3.13 0.46 14.13 69.05 2.72 1.31 0.25 0.07 1.33 92.46

Plg 6.22 26.36 57.81 0.58 8.77 0.60 100.41

OP-4 1,750 825 G 3.45 0.61 14.24 67.83 2.43 1.47 0.28 0.07 1.34 91.71Plg 6.30 24.67 60.24 0.57 7.10 0.45 99.33

OP-6 1,750 900 G 3.80 1.12 16.14 64.95 2.33 2.69 0.52 0.07 2.19 93.81

OP-8 2,250 850 G 3.57 0.89 15.23 66.01 2.20 2.34 0.29 0.07 1.52 92.12

OP-1 2,500 850 G 3.64 0.76 15.42 64.86 2.40 2.29 0.31 0.07 1.45 91.21

G glass, Plg plagioclase, Am amphibole, n.a. not analyzed


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Discussion

The occurrence of dissolution features and compositionaluctuations in plagioclase and pyroxenes, xenocrysts(olivine, Cr-rich Fe–Ti oxides, and Mg-rich pyroxenes), andisotopic disequilibria between coexisting phases all indicatethat magma mixing has occurred repeatedly before eachPlinian eruption over the past * 23 ky. But, how frequentand how intense have those mixing events been? Also, if mixing has repeatedly occurred, why have erupted magmasremained relatively constant in composition and not tendedto become more mac with time? In fact, the relativelyconstant bulk composition suggests that injections of macmagma were either balanced by other processes that drivebulk magma compositions back toward more silicic com-positions, or of small enough magnitude to have beeninsignicant relative to the pre-existing bulk magma.

Magma mixing

According to textural and compositional evidence in pla-gioclase, mixing has been uneven between Popocate ´petlPlinian magmas. For example, the lack of sieving texturesin WP crystals and the wider range in compositions in MTand PP plagioclase, which overall are more albitic than WPcrystals (Fig. 3), suggest magmas were modied by dif-ferent amounts of mixing through time.

In order to investigate the intensity and frequency of mixing events, we use the occurrence and magnitude of compositional variations along traverses in plagioclasefrom all magmas as proxies (an average of 20 crystalseach) (Table 3). We ignore GT plagioclase because theabundant glass inclusions prevented measurement of uninterrupted linear transects. Compositional variations areeither positive (increasing An mol%) or negative(decreasing An mol%) and were measured over the lengthof a crystal and normalized to the total length of that crystal

(Fig. 7). We focus on variations that occur within lengthsof less than 40 l m, because most sharp variations, wherecomposition changes by more than 10 An mol%, occur inless than 40 l m. Changes spread out across more than40 l m are almost always smooth decreases in An content.Fluctuations of less than 5 mol% are ignored because suchdifferences are near analytical precision. The largest vari-ations in An contents often exceed 10 mol% (Fig. 7).Based on our experimental results, if changes in plagio-clase composition by 10 mol% were to occur isothermally,

Fig. 7 Compositional transects of representative PP, LP, and MTplagioclases. Solid bars represent crystal lengths affected by positiveincrements of [ 5 mol%. Open bars represent lengths affected bynegative decrements [ 5 mol%. The ratio represents the total lengthaffected by positive increments divided by the total length of thecrystal

Fig. 8 Maximum incremental increase in An contents in plagioclasecrystals from each Plinian deposit as a function of the percentage of plagioclase affected by changes in An contents


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then water pressure would have had to change by morethan 125 MPa (Fig. 6). In other words, the crystal wouldneed to sink or oat by 4 to 5 km. That seems far forindividual crystals to travel inside a magma, especiallyconsidering many crystals have more than one of such

uctuation. Most of the compositional variability in pla-gioclase is thus assumed to result from changes in tem-perature and/or melt composition, that is from mixingevents.

Overall, the percentage of individual crystals affected bypositive and negative variations ranges from * 5 to 97 %,with a positive shift of 32 mol% An being the maximumvariation found. Individual crystals in MT have the greatestvariations in composition, whereas those in WP and OP arethe least variable. Overall, the greatest increases in Ancontent are found in Group III crystals (MT, LP, and PP),whereas the smallest increases are found in Group I crys-tals (WP, OP). When lengths of uctuations in compositionare summed for all crystals in a population and normalizedto the total length of all crystals measured in that popula-tion, it is found that MT plagioclase is, on average, themost variable (Fig. 8). In contrast, plagioclase from WPand OP is again the least variable, with only 25–29 % of the total measured length affected.

In order to investigate the frequency of signicant macinjections, as represented by increases in An content of C 10 mol%, we convert our analyzed plagioclase transectsto equivalent time spans. We estimate time using plagio-clase growth rates. Although plagioclase growth is unlikelyto be constant, because of changes in temperature andwater pressure, a general growth rate range has been esti-mated at 10

- 5 to 10- 7 l m/s (e.g., Dowty 1980 ; Cashman

and Marsh 1988 ; Izbekov et al. 2002 ). For each crystal, thetotal length of analyzed transect was converted into dura-tion using a growth rate of 2.5 9 10

- 6 l m/s. Analyzedtransects thus represent between 2 and 20 years of crys-tallization. Those times must be viewed as minima,because all crystals contain evidence of partial dissolution.The number of compositional positive variations thatexceed 10 mol% An in each transect was then divided bythe amount of time represented by the transect, yieldinghow frequent such large variations occur (Fig. 9). Notsurprisingly, the frequency of such events is variable. Morethan 50 % of crystals in MT, LP, and PP preserve morethan 10 mac injections every decade, whereas more than90 % of crystals in OP and WP record less than 8 injectionsevery decade, and some even lack any evidence of mixing(Fig. 9). Thus, some magmas had relatively long quiescentperiods (WP, OP), whereas others were modied fre-quently (MT, LP, PP). Most crystals have growth rims onthem, rather than dissolution rinds. If eruptions are trig-gered by mixing (e.g., Sparks et al. 1977 ), then the time-scale of mixing was too short for it to be recorded.

Restoration of silicic compositions

Our evaluation suggests that some magmas erupted inPlinian events experienced more numerous mixing events

Fig. 9 Frequency of mixing events recorded in plagioclase. Mixingevents (mac injections) are based on the assumption that an increasein An content of C 10 mol% is produced by a mixing event. Forexample, 5 % of crystals in MT and LP record more than 35 events ina decade, yet 55 % of crystals in WP record fewer than 5 of suchevents in the same time span. Years are estimated assuming aplagioclase growth rate of 2.5 9 10

- 6 l m/s (Izbekov et al. 2002 )


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although Plinian magmas erupted from Popocatepetl con-tain evidence of mixing, it seems that such energy input didnot precipitate assimilation of carbonates.

The lack of major carbonate assimilation does not pre-clude assimilation altogether. Indeed, the occurrence of partially digested carbonates found in some Popocate ´petllavas (Siebe, personal communication) shows clear

evidence of carbonate assimilation. But, the overall com-positional variations indicate that the extent of carbonateassimilation into Popocatepetl Plinian magmas was limited.This suggests that magmas erupted in Plinian events atPopocate´petl arrive into the upper crust differentiated byfractional crystallization, and if assimilation does occur,then crust different from the shallow calcareous basementwas involved. The Sr isotopic compositions of xenolithsfrom the upper mantle and lower crust below centralMexico range from 0.70309 to 0.7279 (Ruiz et al. 1988 ;Schaaf et al. 1994 ). Partial assimilation of such rocks couldexplain the observed variations in Popocate ´petl magmas.Alternatively, the compositional variability of Popocate ´petlPlinian magmas could result from mantle heterogeneity(e.g., Luhr 1997 ; Meriggi et al. 2008 ; Straub et al. 2008 ;Johnson et al. 2009 ). Indeed, the general trend of Pb iso-topes from NT, SCVF, and Popocate ´petl magmatic seriescould be explained by variable contribution of subductedsediments (Fig. 12 ). Generation of magmas from partialmelting of different portions of the mantle coupled withvariable degrees of fractional crystallization and assimila-tion of lower crust (e.g., Smith and Leeman 1987 ; Annenet al. 2006 ; Ownby et al. 2010 ) could be important factorscontributing to the compositional variability of the Plinianmagmas from Popocate ´petl.

Reconstruction of the magmatic plumbing system

Plinian eruptions of Popocate ´petl tapped magma that iseither amphibole-bearing (WP, GT) or amphibole-free(MT, OP, LP, PP). The presence of amphibole requiresrelatively high water pressures, and hence deeper storage(Rutherford et al. 1985 , Gardner et al. 1995a , b). Indeed,relatively high water pressures are required to stabilizeamphibole because pre-eruptive temperatures of amphi-bole-bearing WP and GT magmas exceed 900 C. Wefound that experiments with WP at 900 C stabilizeamphibole only at water pressures greater to 125 MPa.In addition, amphibole compositions in both GT and WPare consistent with water pressures in excess of 200 MPa (Ridol et al. 2010 ). Finally, Holtz et al.(2005 ) and Martel et al. ( 1999 ) performed experimentson bulk compositions similar to that of GT and foundthat water pressure must be at least 200 MPa to crys-tallize amphibole above 900 C. On the other hand,experiments at high water pressures using MT, OP, andWP grew biotite (Fig. 6), yet natural samples are biotitefree. The lack of biotite in natural samples constrainsstorage to be less than * 250 MPa.

For amphibole-free magmas, experimental resultsequivalent to pre-eruptive temperatures indicate that anassemblage of plagioclase ? two pyroxene ? Fe–Ti oxi-des is stable only at water pressures below 110–130 MPa

Fig. 10 Variation in major-element compositions as a function of Srisotopic composition. Solid symbols are data from this study, opensymbols are data from either lavas younger than 300 ky (Sosa-Ceballos 2006 ) or recent products (Schaaf et al. 2005 ). Curves arecalculated trends using AFC models (see Table 7), assuming end-member compositions of scoria collected from the pyroclastic owdeposit erupted in 2001 (Schaaf et al. 2005 ) and a coarse-grained

skarn (Sosa-Ceballos 2006 ). Values of used r in the models areshown. Tick marks along trends are the percentage crystallized


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(Fig. 6). Compositions of plagioclase, which generally

uctuate between An 40 and An 55 , are consistent with suchrelatively shallow storage (Fig. 6).Magma mixing recorded in plagioclase must have thus

occurred in two storage regions over the past 23 ky. One of those regions must have been within 200–250 MPa and theother shallower than 130 MPa. Magmas from both the deepand shallow reservoirs were tapped during the 14 ka Pli-nian eruption. This indicates that the pressures recordedreect different depths in the magmatic system, and notchanges resulting from edice collapse (Pinel et al. 2010 ).The deeper reservoir was tapped during the two olderPlinian eruptions. Mixing between magmas was diverse in

that region, as plagioclase in WP records only a few suchevents before that eruption. In contrast, plagioclase texturesand compositions suggest mixing was thorough before theGT magma was erupted. The shallower reservoir wastapped during the last four Plinian eruptions and, except forOP, appears to be a zone of frequent mixing events(Fig. 9).

The difference in storage depth also limits the likelyextent of carbonate assimilation. The two storage reservoirsare separated by * 70 MPa, or roughly about 2,600 m of

rock, assuming an average density of 2,500 kg/m 3. The

thickness of the carbonate platform underneath Popocate ´-petl has been well constrained to between 250 and 1,000 m(e.g., Fries 1960 ; Johnson 1990 ; Cabral-Cano et al. 2000 ).This suggests that only one of the reservoirs could havebeen hosted by limestone. The more likely candidate is theshallower one, because OP and MT magmas containfragments of calcareous rocks (Arana-Salinas et al. 2010 ;Sosa-Ceballos et al. 2012 ) and PP contains xenocrysts of barite. The absence of calcareous xenoliths in WP and GTmagmas from the deeper reservoir is important, becausetheir isotopic signatures are similar to, and in some casesmore enriched than, magmas from the shallow reservoir

(Fig. 11). This suggests that either the deeper reservoir washosted by carbonate rocks of unknown age or the isotopiccomposition of Popocate ´petl Plinian magmas reects pro-cesses unrelated to carbonate assimilation.

Conclusions

Plinian eruptions of Popocate ´petl over the past23,000 years have tapped magmas from two reservoirs, one

Fig. 11 a eNd , and b 206 Pb /

204 Pbversus 87 Sr/ 86 Sr isotopic datafor glass, plagioclase, andpyroxene separates fromPopocate´petl Plinian deposits.See Fig. 2 for symbols.Representation of 2 r standarddeviations are shown as crossbars . Solid symbols are usedwhen all three phases are inequilibrium, and open symbolsindicate one phase is out of equilibrium (plagioclase in bothcases). Inset shows AFC modelsusing different xenoliths asassimilants end members.Values of r 0.2 and 0.5 are usedto delineate the most likely AFCconditions observed in Fig. 11 .Model I = sandstone, ModelII = ne-grained skarn, ModelIII = coarse-grained skarn(Schaaf et al. 2005 ; Sosa-Ceballos 2006 ). Compositionsof Nevado de Toluca (NT)(Martı´nez-Serrano et al. 2004 )and cones from SierraChichinautzin volcanic eld(SCVF) (Agustı ´n-Flores et al.2011 ) are shown for comparison


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