gsa bulletin: origin of broken phenocrysts in ash-flow...

ABSTRACT

Surprisingly little attention has been devoted to the textural natureof phenocrysts of feldspar and quartz in tuff. Although many geolo-gists have briefly alluded to “broken” phenocrysts, none have ad-dressed their origin in any detail. Petrographic study of 117 coolingunits in the middle Tertiary ash-flow province of the Great Basin,United States, provides a basis for characterization of the shapes andfor interpretation of the origin of felsic phenocrysts in ash-flow tuffs.Although not proven to be wholly ineffective, breakage of phenocrystsby mutual impact in the erupting magma and pyroclastic flow isdoubtful for at least four reasons. First, the statistical probability ofmutual collision between phenocrysts diminishes exponentially astheir proportion to vitroclasts diminishes (e.g., only 1% probability for10% phenocrysts); collision is less likely if pyroclasts move by laminarrather than turbulent flow. Second, the coating of glass and/or melt onthe phenocrysts provides a cushion that absorbs the impact force.Third, plagioclases broken by impact in the laboratory have unusualshapes unlike those seen in Great Basin tuffs. Fourth, euhedral phe-nocrysts of feldspar are commonplace in many Great Basin tuffs, andin some they constitute a significant proportion of the phenocrysts, in-dicating that mutual impact does not modify all intratelluric crystalsduring explosive eruption.

The two most populated categories of phenocryst shape in GreatBasin tuffs probably correspond to what has been previously called“broken” phenocrysts. Somewhat less than half of the plagioclase andmany sanidine phenocrysts are subhedral to anhedral. These are sim-ilar in shape, size, and composition to grains in polycrystalline aggre-gates within the same thin section. Kindred aggregates and discretephenocrysts could have been derived from holocrystalline to partlycrystalline material in the magma chamber that was disaggregated tovarying extents during explosive eruption. More than half of the pla-gioclase and all of the quartz phenocrysts in Great Basin tuffs consistof irregularly shaped fragments with cuspate, embayed outlines, re-sembling pieces of a jigsaw puzzle, which we call phenoclasts. Inclu-sions of glass are common and are especially evident in larger, more orless whole crystals. Textural features of some phenocrysts in cognatepumice clasts in the tuffs reveal that they broke apart while still in thevesiculating but unfragmented magma. As the erupting magma de-compressed, vesiculation of the melt that was entrapped at higherpressures as inclusions within the phenocrysts blew them apart, form-ing the phenoclasts.

Shapes of felsic phenocrysts in volcanic rocks provide insight intotheir mode of emplacement. Euhedral phenocrysts are common inash-flow tuffs as well as lava flows. Phenoclasts, however, are diagnos-

tic of ash-flow tuffs, because they do not occur in Plinian ash-fall de-posits and are rare in lava flows. These textural contrasts are useful forinterpretation of generally older, but in any case altered and recrys-tallized, volcanic rocks. In such rocks, critical groundmass featuresand field relations that could provide clues to their origin have beenobscured, but the shapes of relict phenocrysts are commonly wellpreserved.

INTRODUCTION

Although ash-flow deposits have been the subject of numerous investi-gations over the past three decades, especially since the classic work ofRoss and Smith (1961), no study has directly addressed in any detail thetextural character and origin of their included “broken” phenocrysts. Vir-tually all of the textural descriptions of pyroclasts deal only with the vitro-clasts (pumice fragments and glass shards). However, broken phenocrysts,together with the associated vitroclasts, have been considered for 200 yr tobe the hallmark of deposits derived from explosive volcanic eruption (e.g.,Cas and Wright, 1987; see Williams, 1926, for early references). Fisher andSchmincke (1984, p. 105) noted “The rapid decline of pressure during ex-plosive eruption commonly causes breakage upon ejection or impact. . . .Broken crystals are especially characteristic of ash-flow tuffs.” Henry andWolff (1992) concluded that broken phenocrysts in tuff range widely insize, but breakage is probably minor in low-explosivity deposits.

Some investigators explicitly noted that not all phenocrysts in tuffs arebroken. Pirsson (1915, p. 200) wrote that, in addition to broken accidentalcrystalline material, tuffs contain juvenile crystals which can be “inplaces . . . strikingly perfect,” or which “may also be corroded, with deepembayments, the latter perhaps filled by glass.” Although he associated thecuspate form of glass shards with expansion of gas bubbles in the melt,Pirsson did not explain the cause of “shattering of solid substances” intuffs. Ross and Smith (1961) dealt at length with the glassy material intuffs, but in only one brief paragraph (p. 35) did they consider phenocrystshapes, recognizing four types (our numbers in brackets): “The essentialfeldspar grains are [1] sharply euhedral in outline in a few tuffs, but morecommonly they are [2] subhedral. Some show one side with a crystal face,and elsewhere show irregular or fractured edges. Other feldspar grains are[3] rounded or irregularly embayed . . . In some tuffs . . . there has been[4] extreme fracturing, so that most of the grains are sharply angular in out-line.” Noble (1970) briefly noted the presence of euhedral phenocrysts insome Great Basin tuffs. These four investigators recognized the fact thatbroken phenocrysts are not always present in tuffs and that different phe-nocryst shapes exist which, by implication, originated by different, but un-specified, processes.

Over the past several years we have examined hundreds of thin sectionsof 117 cooling units and included cognate pumice clasts in the middle Ter-tiary ash-flow tuff province of the Great Basin, western United States (Best

63

Origin of broken phenocrysts in ash-flow tuffs

Myron G. Best*Eric H. Christiansen } Department of Geology, Brigham Young University, Provo, Utah 84602

GSA Bulletin; January 1997; v. 109; no. 1; p. 63–73; 13 figures.

*Email address: [email protected]

BEST AND CHRISTIANSEN

64 Geological Society of America Bulletin, January 1997

et al., 1989). Significant differences in shapes of omnipresent feldspar phe-nocrysts can be categorized, following Ross and Smith (1961, p. 35), as(1) euhedral, bounded by planar crystal faces, (2) subhedral, possessingpartial crystal faces or a somewhat irregular outline that is rectangular totriangular in overall shape, but ranging to anhedral, or lacking any regularshape, and (3) irregularly embayed, cuspate, resembling pieces of a jigsawpuzzle and some containing glass inclusions. Category (4), angular, ofRoss and Smith (1961) was interpreted by them to be produced by “ex-treme fracturing.” We propose that the irregular shapes (3) represent crys-tals blown apart by expansion of melt within inclusions, whereas subhedralto anhedral feldspars (2), as well as angular grains (4), probably were de-rived by break-up of polygranular aggregates.

Our intent in this textural study is twofold. We plan to document shapesof felsic phenocrysts in Great Basin tuffs (e.g., Page and Dixon, 1994) andtheir cognate pumice clasts and to propose an origin for these contrastingshapes, beginning first with shapes whose origin is more certain followedby the more problematic and ambiguous phenocryst shapes. We aim toshed light on the general processes of “breakage,” “impact,” and “fractur-ing” of phenocrysts alluded to in published accounts of ash-flow tuffs.Such processes can provide insight into dynamics of volcanic eruptionsthat bear upon such widely ranging applications as climate change, vol-canic hazards, magma genesis, and recognition and characterization of an-cient ash-flow deposits and their related calderas and ore deposits.

EUHEDRAL PHENOCRYSTS

Some cooling units of ash-flow tuff in the Great Basin (Best et al., 1989)contain significant proportions of euhedral feldspar phenocrysts similar tothose commonly seen in lava flows. Such tuff units include the rhyoliticShingle Pass Tuff, Ryan Spring Formation, Escalante Desert Formation,and especially the Condor Canyon Formation (Fig. 1), many trachydaciticcooling units, and the crystal-rich dacitic Three Creeks Tuff Member of theBullion Canyon Volcanics (Best and Grant, 1987). Euhedral plagioclasesalso occur in the crystal-rich Nugent Tuff Member of the Hu-pwi Rhyo-dacite (Fig. 2) in which the “plagioclase phenocrysts . . . are . . . surpris-ingly . . . little shattered and broken” (Ekren et al., 1980, p. 38). Many othercooling units contain euhedral phenocrysts mixed with other shapes.

There is no doubt that euhedral feldspar phenocrysts in a tuff representwell-formed intratelluric crystals that existed in the magma body prior toits explosive eruption which were not broken during eruption and em-placement of the pyroclastic flow. These euhedra presumably existed asfree-floating, isolated crystals as well as grains in crystal-rich mush in mar-ginal parts of the magma body.

EXPLOSION OF MELT-CONTAINING PHENOCRYSTS

Phenocrysts in all volcanic rocks—tuffs as well as lava flows—com-monly contain glass inclusions. Sieve-textured plagioclase in intermediate-composition lava flows and embayed quartz in silicic rocks are particularlycommon and have been described and illustrated in numerous publications(e.g., Williams et al., 1982, Figs. 4-2A and 9-3C). Since 1989, laboratoryinvestigations and theoretical analyses of melt inclusions have shown thatthey can fragment their host crystal during explosive eruptions, thus pro-viding a mechanism for the breakage of phenocrysts. We first describe meltinclusions, then explain how they blow their host crystal apart, and finallyshow examples from Great Basin tuffs.

Melt Inclusions in Phenocrysts

Inclusions can form in at least two ways. First, skeletal crystals can en-trap pockets of melt (Roedder, 1979) during growth under high degrees of

undercooling or supersaturation in the magma (e.g., Lofgren, 1980). Sec-ond, more slowly grown euhedral-faceted crystals can subsequently un-dergo partial resorption or corrosion under changing magmatic conditions,such as mixing with a compositionally different magma (Hibbard, 1981;Glazner et al., 1990) or decompression of the magma (Nelson and Mon-tana, 1992). In thin sections of standard thickness it is difficult, if not im-possible, to distinguish between a true inclusion, which is completely en-closed within the host crystal and most probably formed during skeletalgrowth or growth following partial resorption, and an apparent inclusion,which is an embayment or corrosion channel extending inward from themargin of the crystal into the plane of the section from the third dimension.

Sparse, more or less symmetric skeletal phenocrysts of quartz (e.g.,

A

B

Figure 1. Bauers TuffMember of the CondorCanyon Formation, a low-silica rhyolite. Sample CND-1TL from east end of Con-dor Canyon, southeast ofPioche, Nevada. (A) Photo-micrograph showing manyeuhedral and subhedralfeldspars together withsparse inconspicuous maficphenocrysts in a eutaxitic,vitroclastic matrix. Note afew polygranular clots.(B) Drawing of all sanidine,plagioclase (lightly stippled),and sparse attached maficcrystals (dense stippling)randomly arranged fromentire 32 mm × 20 mm thinsection. Scale bar at top ofphotomicrograph is 0.5 mm.

BROKEN PHENOCRYSTS IN ASH-FLOW TUFFS

Geological Society of America Bulletin, January 1997 65

Roedder, 1979, Fig. 3) have been found in only a few Great Basin tuffs.Quartz phenocrysts in virtually all quartz-bearing tuffs possess, to varyingdegrees, rounded external corners and smooth embayments. Glass inclu-sion outlines range from equant ovoids to boomerang and slender hour-glass shapes to negative-crystal, or euhedral-faceted, shapes. Whetherthese morphologies are due to growth or partial resorption, or both, is con-troversial (for a sample of the growing literature on melt inclusions inquartz see Whitney, 1988; Donaldson and Henderson, 1988; Anderson,1991; Wark, 1993; Manley, 1996).

All of the 117 tuff cooling units examined in the Great Basin containplagioclase phenocrysts and, of these, 59% have at least some plagioclasewith glass inclusions. Because inclusions commonly transect composi-tional zones and are generally irregular in outline and in arrangementwithin the crystal (Fig. 3), they apparently formed by partial resorption af-ter initial growth, but entrapment as melt inclusions during skeletal growthcannot be ruled out with certainty.

Among the Great Basin cooling units that contain sanidine phenocrysts,only 16% have at least some sanidine with inclusions and small marginalembayments.

Blown Apart Phenocrysts: Concepts

Regardless of their origin, melt inclusions in felsic phenocrysts providea means of fragmentation during explosive eruption and decompression ofgas-charged magma. In relatively small, true-melt inclusions, the host crys-tal can be an effective, unfailing pressure vessel during even nearly isother-mal explosive eruption as the pressure differential increases between themelt in the inclusion and the melt surrounding the host crystal (Lowen-stern, 1994a). However, in a study of the Bishop Tuff, Anderson et al.(1989) and Anderson (1991, p. 536) found that some melt inclusions hadburst their quartz phenocryst hosts while melt was still available to coatconchoidal fracture surfaces. In the conclusion of their study of tuffs atCrater Lake, Bacon et al. (1992, p. 1029) briefly referred to phenocrystsblown apart during eruption by vapor overpressure in melt trapped as in-clusions. Tait (1992) calculated the coupled effects during magma ascentof the evolution of vapor pressure within a melt inclusion and the elasticdeformation that the melt exerts on the host crystal. Melt inclusions are es-sentially unable to decompress significantly by expansion of the host crys-tal because of its relatively large elastic moduli, regardless of the chemicalcomposition of the magma and its volatile species. So, as the ascendingmagma surrounding the crystal decompresses, the stress within the crystalin the neighborhood of the melt inclusion can exceed the tensile strength ofthe crystal, and it will crack. The cracking threshold decreases with in-creasing relative inclusion size because stresses are greatest in a narrowzone along the wall of the inclusion. Crystals containing many inclusionsseparated by relatively thin walls would be especially prone to breakage.Inclusions trapped at greater depth are more likely to blow apart upon de-compression of the magma. Once the host crystal is cracked, the meltwithin the inclusion decompresses, volatiles exsolve, bubbles nucleate, ex-panding the inclusion further, and the crystal blows apart. Melt withinhourglass inclusions connected through a narrow constricting neck to themelt surrounding the host crystal can partially decompress during eruption,but the crystal can still burst (Anderson, 1991).

Tait (1992, p. 154) also suggested that, because of the sluggish rate ofexsolution of dissolved volatiles, melt inclusions in rapidly decompressedPlinian ejecta that rapidly quench in the turbulent cool atmospheric columnmight have insufficient time to nucleate bubbles in the melt; crystals wouldnot rupture. This suggestion was confirmed by Bacon et al. (1992, p. 1029),who found that only some of the inclusions in Crater Lake plinian depositshad ruptured their host, but all inclusion-filled crystals in subsequent co-genetic ash-flow deposits from the same magma system are broken. Thesame contrasts between Plinian and ash-flow deposits were recorded inother locales by Dunbar and Hervig (1992a, 1992b), Lowenstern (1994b,1995), and Skirius et al. (1990). Moreover, it has been observed that bub-bles in melt inclusions are smaller in plinian than ash-flow deposits (Dun-bar and Hervig, 1992a; Lowenstern, 1993, 1995). As the ash-flow magmadecompresses in its chamber during precursory Plinian eruptive activity,overpressured melt inclusions had more time after cracking of the hostcrystal to nucleate bubbles, expand, and blow it apart.

A

B

Figure 2. Nugent TuffMember of the Hu-pwi Rhy-odacite from southeast ofCopper Mountain in Nu-gent Wash, north-northeastof Hawthorne,Nevada, sam-ple COPPER-1A. (A) Pho-tomicrograph of abundantplagioclase, some euhedraland many subhedral, pluslesser amounts of biotite, cli-nopyroxene, and Fe-Ti ox-ides, in a devitrified vitro-clastic matrix. (B) Drawingof all plagioclases, includingclotted ones, randomly ar-ranged in one-half of thethin section. Note clottedplagioclases, the individualshapes and sizes of which re-semble the isolated subhe-dral phenocrysts. Scale barat top of photomicrograph is0.5 mm.



Blown-Apart Phenocrysts: Phenoclasts

Blown apart phenocryst fragments, or phenoclasts, of plagioclase andquartz were found in more than 100 thin sections of the ash-flow tuff andhosted cognate pumice clasts of the Pahranagat Formation in the southernGreat Basin (Best et al., 1995). Plagioclases in the pumice clasts containnumerous inclusions of vesicular glass. These significantly weakened crys-tals ruptured while still encased within the nonfragmented vesicular melt,because it is molded tightly around the disaggregated phenocrysts (Figs. 3and 4). In phenocrysts that retain considerable regular external crystalform, phenoclasts shaped like pieces of a jigsaw puzzle have been dis-placed or jostled to create uneven optical extinction and faulted crystalmargins surrounded by vesicular glass. Such phenocrysts were not rup-tured by shear stresses in a flowing frothy melt because neither the crystalnor the host pumice show any indication of local flow, such as tubular vesi-cles. Rather, it appears conclusive that expansion of gas bubbles within theincluded pockets of melt ruptured the host crystals during decompressionthat accompanied eruption. Some plagioclase phenocrysts are so disruptedthat it is impossible to reconstruct phenoclasts into an originally regular

crystal form (Fig. 3C). In some pumice clasts, complete disaggregation oc-curred within a shearing froth so that the phenoclasts have been drawn outinto lenticular aggregates within flow-foliated pumice.

Quartz phenocrysts in pumice clasts within the Pahranagat tuff showfragmentation similar to that of plagioclase, but the quartz phenoclasts aredifferent because the shape, size, and spacing of the inclusions and em-bayments in quartz are different (Fig. 4). The photomicrographs show thatthe quartz phenocrysts ruptured while enclosed within the vesicular meltand not during subsequent fragmentation and flow of the vesiculatingmagma. However, because the inclusions and/or embayments tend to berelatively larger and more widely spaced than in plagioclase, as few as twoor three phenoclasts were created from one crystal.

Prior to eruption, the Pahranagat magma decompressed by as much as4–5 kbar, on the basis of Al concentrations in zoned amphibole phe-nocrysts (temperature-corrected pressures; Christiansen and Best, unpub-lished data). This 15–19 km ascent in the magma system may have in-tensely corroded both the plagioclase and the quartz phenocrysts, creatingabundant melt-filled embayments. Fragmentation of the vesiculating meltand of phenocrysts of plagioclase and quartz, like those illustrated in Fig-

Figure 3. Photomicrographs of inclusion-filled, blown-apart plagio-clase phenocrysts in cognate pumice blocks in the Pahranagat tuff.Note fragments shaped like pieces of a jigsaw puzzle and glass inclu-sions. (A) Plane- and cross-polarized light images of a phenocryst insample LTTLFSH-2EPF from southern Little Fish Lake Valley north-east of Tonopah, Nevada. Pumice is molded tightly around the vari-ably disoriented fragments separated by abundant vesicular glass in-clusions. (B) and (C) Sample MUDSP-3-42P from southern MonitorRange, Nevada. Matrix is devitrified so that crystal fragments wereheld in place during preparation of thin section. (B) Crossed-polarizedlight image of twinned phenocryst in which vesicular inclusions, somelocalizing intracrystal faults, transect oscillatory compositional zoningnear margin. Note uneven optical extinction between adjacent frag-ments that indicate slight rotation. (C) Cross-polarized light images ofalmost completely disaggregated phenocryst, or possibly glomero-cryst, into small phenoclasts. Scale bars in all photomicrographs are0.5 mm.

A

C

B



ures 3 and 4, during eruption and dispersal of the Pahranagat ash flowwould have produced ash-size vitroclasts and phenoclasts. Ash-flow tuffsof the Pahranagat Formation contain such phenoclasts (Fig. 5).

At least some of the plagioclase grains in 68 of the other 116 middle Ter-tiary tuff cooling units in the Great Basin have jigsaw-puzzle shapes simi-lar to those in the Pahranagat tuff. One of these cooling units, the BlueSphinx Tuff, was noted by Ekren et al. (1980) to contain intensely resorbedquartz phenocrysts; like the Pahranagat, the Blue Sphinx also containsabundant small phenoclasts of plagioclase and quartz (Fig. 6), togetherwith zoned amphiboles, the Al concentrations of which record 3–4 kbar ofmagma decompression prior to emplacement. Original phenocrysts of pla-gioclase and quartz in the Pahranagat and Blue Sphinx tuffs are among themost inclusion-rich of any cooling units in the Great Basin; they also con-tain amphiboles known to be strongly zoned in Al concentration. Furtherstudy is needed to determine what role, if any, the apparently large decom-pressions in silicic magmas play in the development of abundant melt in-clusions in plagioclases that are subsequently blown apart. Dissolved wa-ter concentrations in the melt inclusion are certainly important and, at thepossibly greater depths of entrapment implied for the Pahranagat and BlueSphinx magmas, concentrations could have been greater, allowing formore efficient rupturing of hosts’ crystals.

Many tuff cooling units in the Great Basin (e.g., Fig. 7) contain largerphenoclasts of quartz and smaller of plagioclase in which inclusions aremore closely spaced, as is the case in the Pahranagat tuff. Contrasts inshapes, as well as sizes, of plagioclase phenoclasts are evident in sometuffs; where inclusions are elongate as well as closely spaced (Fig. 8),many corresponding phenoclasts are elongate, compared to the more com-monly equant phenoclasts in Figures 5–7. Many tuff cooling units that havephenoclasts of plagioclase also host cognate pumice lapilli which containvariably disaggregated, inclusion-filled plagioclase phenocrysts similar tothe phenoclasts (Fig. 9). Such paired samples of the initial, unfragmentedmagma and the erupted pyroclastic material lend strong support to the con-cept that inclusion-filled phenocrysts blow apart during explosive eruption.Several other examples of felsic phenoclasts are obvious in the photomi-crographs in Ross and Smith (1961, Figs. 17, 21, 34, 36, and 38).

Phenoclasts of sanidine are rare in Great Basin tuffs, reflecting the factthat sanidine phenocrysts contain few melt inclusions.

In contrast to ash-flow tuffs, felsic phenoclasts in lava flows are rare. Inthin sections of 110 andesitic and rhyolitic Tertiary lava flows in the GreatBasin only a few phenoclasts were found among thousands of felsic phe-nocrysts, many of which contain melt inclusions. The relatively slow rateof extrusion of lava flows apparently allows diffusional reequilibration ofvolatiles, chiefly H2O, between the melt inclusion and melt surroundingthe crystal. Qin et al. (1992) found that less than 2 yr are required for H2Oin a 50-µm-diameter inclusion of rhyolitic melt at the center of a 2-mm-diameter quartz crystal to reach 95% reequilibration with external melt at800 °C. High-temperature annealing of cracks or ductile deformation ofthe host crystal during the slow extrusion may also play a role.

PHENOCRYSTS BROKEN BY IMPACT

Fragmentation of phenocrysts by collisions of juvenile crystals with oneanother, or mutual impact, in pyroclastic eruption columns and ensuing ashflows has been implied by some geologists (e.g., Fisher and Schmincke,1984, p. 105). Impact breakage of crystals rendered weaker because ofmelt inclusions could occur in the pyroclastic milieu; however, this cannotbe the only mechanism for creation of phenoclasts, because they are foundin pumice where impact did not happen. If mutual impact is a viable gen-eral mechanism, crystal-rich tuffs might be expected to contain more im-pact-fragmented phenocrysts than crystal-poor tuffs. However, this doesnot seem to be the case in Great Basin tuffs, because some crystal-rich tuffscontain abundant euhedral phenocrysts of feldspar. One can calculate thestatistical probability of one-time mutual impact between crystals in a mi-lieu of an infinitely large number of similarly sized crystals and vitroclasts.In an ash flow containing 50% vitroclasts and 50% crystals, the upper limitof phenocryst concentration in Great Basin tuffs, 25% of the mutual im-pacts are between crystals. For 30%–20% crystals, which is a more com-mon range of concentration, 9%–4% of the impacts are between crystals.In the case of 10% crystals, found in many tuffs, only 1% of the theoreticalimpacts are between crystals. If pyroclastic flows move by laminar rather

Figure 4. Photomicrographs of quartzphenocrysts in cognate pumice blocks in thePahranagat tuff. (A) Plane- and cross-po-larized light view of sample ALAMO-1E43PC2 from Pahranagat Range duesouth of Alamo, Nevada. Essentially threefragments, and several subfragments, alldisoriented to one another, represent theoriginal, inclusion-filled quartz bipyramid.Note more coarsely vesicular glass betweentwo main fragments compared to that inhost pumice. (B) Cross-polarized light im-age of sample MUDSP-3-42P from south-ern Monitor Range, Nevada. Original in-clusion-bearing bipyramid was blown intoabout 12 fragments, now encased in a devit-rified matrix. Scale bars are 0.5 mm.



than turbulent flow (e.g., Francis, 1994, p. 217), then neighboring particlesmove at about the same velocity and the probability of forceful collisionsbetween crystals is minimal. Another significant factor reducing the prob-ability of effective impact fracturing of phenocrysts is the fact that the crys-tals in expanding gas-charged magmas are coated with films of vesicularglass or melt (Fisher, 1963; Anderson, 1991). Such coatings would absorbto some degree the force of mutual impact.

Experiments

We have not attempted the difficult task of trying to experimentally sim-ulate fragmentation of phenocrysts by mutual impact in a fast-moving mix-ture of vitric particles and crystals. However, some insights into fragmentshapes were found by high-speed impact of plagioclase grains against astationary, polished slab of syenite at room temperature. The impactedgrains were diamond-sawed rectangular parallelepipeds of unfractured pla-gioclase that contained no glass inclusions. Two sizes of grains were im-pacted in 150 trials. The first were grains weighing 0.04–0.07 g, averaging0.05 g, and measuring 2–4 mm along an edge. The second were grains

weighing 0.02–0.04 g, averaging 0.025 g, and measuring 1–3 mm along anedge. Each grain before impact and all fragments recovered after impactwere examined with a binocular microscope to ascertain results.

Larger grains were impacted at speeds of 15 to 30 m/s by using a table-mounted slingshot. Such velocities lie in the lower range of published py-roclastic flow velocities (9–134 m/s or 32–482 km/hr according to Mooreand Melson, 1969; Fisher and Schminke, 1984, Table 8-1; Rowley et al.,1981; Taylor, 1958; Bullard, 1984; but 150–300 m/s according to Wilson,1985). For six grains impacted at 15 m/s, typical results were either that the

Figure 5. Pahranagat tuff, sample WATER-1K from the northernGolden Gate Range, Nevada. Two views in plane light of relativelylarge phenoclasts of quartz (Q), subhedral to euhedral unbroken phe-nocrysts of sanidine (S), and numerous, generally smaller plagioclasephenoclasts (all unlabeled), most of which have the characteristic jig-saw-puzzle shape like fragmented phenocrysts in Figure 3. Note inclu-sion-filled phenoclast of plagioclase in lower left of lower photomicro-graph. Scale bars are 0.5 mm.

Figure 6. Blue Sphinx Tuff, sample SRISEF-1D from the southeastend of the Gabbs Valley Range, Nevada, under cross-polarized light.All grains are plagioclase (unlabeled) except for one large irregularsanidine (S) and one rounded quartz (Q). Compare plagioclase phen-oclasts to fragments like those in Figure 3. Scale bar is 0.5 mm.

Figure 7. Lower member of tuff of Trail Canyon, sample JEFF-1,collected about 7 km north of Belmont, Nevada. Large to moderatesize phenoclasts of quartz (Q), subhedral sanidine (S), and moderateto small size phenoclasts of plagioclase (unlabeled) viewed undercross-polarized light. Scale bar is 0.5 mm.



grain was undamaged or that relatively minute pieces were broken off cor-ners and edges. For one grain, the thirteenth repeated impact finally yieldeda substantial fragment about one-quarter the volume of the parent grain.For another, 23 repeated impacts yielded only numerous minute fragmentsand a somewhat rounded parent grain. At 20 m/s, one grain cleaved intofour subequal pieces on the first impact, whereas four others required fromtwo to five repeated impacts to yield substantial fragments. The sixth grainwas impacted 10 times with only minute pieces breaking off edges and cor-ners. At 25 m/s, about half of the grains were broken into substantial frag-

ments on the first impact, whereas the others were broken after a few re-peat impacts. At 30 m/s, of 10 grains, 8 shattered on first impact and theother 2 yielded substantial fragments on the second impact. Smaller grainswere only impacted at 30 m/s and results were similar to those obtained forthe larger grains at 25 m/s. Of seven grains, three broke up on first impact;two to five repeat impacts were required to produce substantial fragmentsin the other four parent grains.

In summary, breakage by mutual impact of inclusion-free plagioclasesat low velocities of 15 m/s mostly produces dust-size pieces chipped off theedges and corners of the rounding parent grain, reminiscent of wind-blownsand grains. Slightly greater velocities can yield substantial breakage, butgenerally only after repeated impact, as could occur in a turbulent ash flow.Significant fragmentation occurs on first impact at 30 m/s for larger grainsbut only for about half the smaller grains. Impact breakage of still smallergrains, <1 mm, would presumably require greater velocities.

In other impact experiments, plagioclase grains were loaded into a shot-gun shell in lieu of “shot” and propelled with a speed of about 1000 m/sinto the stationary syenite slab. Recovery of impacted grains was less com-plete, but thin sections of fragments revealed shapes (see next section) sim-ilar to those produced at lower speeds from the slingshot.

If phenocryst shapes in ash-flow tuffs originated by impact fracturing,comparative size distribution analysis (e.g., Hartmann, 1969) of fragmentsgenerated in these experiments with phenocrysts in tuffs might provide ev-idence for phenocryst impact in the latter. However, because of differentialwinnowing, or elutriation, of finer particles, including small crystal frag-ments from a pyroclastic flow during its eruption and dispersal (e.g., Sparksand Walker, 1977), the validity of such a comparison would be limited. Elu-triation would also invalidate direct comparison of phenocrysts in a sampleof tuff and its included cognate pumice, in which no impact occurred.

Shapes of Plagioclase Fragments Produced by Impact

Plagioclase fragments produced by impact were embedded with randomorientations in epoxy cement on a glass slide. Thin sections allowed directcomparison with shapes of phenocrysts in tuffs. About half of the frag-ments are blocky cleaved pieces that appear as more or less regular rectan-gular or triangular shapes in thin section; these are similar to the shapes ofmany of the crystals in tuffs. Other fragments of impacted plagioclasecommonly have one or more stepped outlines governed by the (010) and(001) cleavages. More significant shapes are related to curved, stepped ribson conchoidal fractures. In the two dimensions of a randomly oriented thinsection, these ornamented fractures create bizarre shapes (Fig. 10); theseinclude deep linear reentrants grading into slender fragments, two or moreof which, though appearing separate and independent in the plane of thethin section, are actually ribs of one grain, as revealed by a common opti-cal orientation. Continued impact would tend to obliterate some of thesharply angular and more delicate features, which, however, are more ro-bust in three dimensions.

If impact fragmentation had generally occurred in Great Basin ashflows, the shapes displayed in Figure 10 would be more common. How-ever, of the hundreds of thin sections of Great Basin tuffs we have exam-ined, such shapes are virtually nonexistent in feldspars but have been notedin rare quartz; the upper member of the tuff of Ryecroft Canyon (Fig. 11)contains some stepped boundaries. Melt coatings on phenocrysts wouldcertainly inhibit breakage by mutual impact, and so too would laminarflow. The occurrence of euhedral, unbroken phenocrysts in some tuffs(Figs. 1 and 2) indicates that factors which produce breakage independentof rupturing related to melt inclusions have not been significant.

Aside from production of phenoclasts as detailed above, other processescould produce fragmented phenocrysts. Fractured xenocrysts might be

Figure 8. Unnamed trachydacitic tuff, sample ELYS-1G, from ElySprings Range west of Pioche, Nevada. Cross-polarized light imageshows that glass inclusions in plagioclases (only visible crystals) areelongate parallel to longest dimension of crystals and many pheno-clasts are also elongate. Scale bar is 0.5 mm.

Figure 9. Upper unit of Sierra Springs Tuff Member of the PintoPeak Rhyolite, sample 7L184 from the Fish Creek Range about 14 kmsouth of Eureka, Nevada. Plane-polarized light. Lower half of photo isa compacted pumice lapillus that contains a plagioclase phenocryst,the outer margin of which is charged with inclusions and is partiallydisaggregated. Host tuff above contains plagioclase phenoclasts likethose in the margin of the plagioclase phenocryst in the pumice. Scalebar is 0.5 mm.



broken from wall or roof rocks during explosive eruption. However, xeno-crysts appear to be rare in middle Tertiary Great Basin tuffs which we haveexamined. In contrast, for example, our few samples of the QuaternaryBishop Tuff, which vented partly through granitic rock, contain obviousfelsic xenocrysts marked by multiple sets of healed microfractures typicalof plutonic feldspars.

PROBABLE ORIGIN OF SOME SUBHEDRAL TO ANHEDRALPHENOCRYSTS

Superficially, some shapes of subhedral grains resemble those producedby impact breakage. For example, phenocryst W in the upper center of Fig-ure 1B is a slender plagioclase grain, not unlike that shown in Figure 10(C and D), but lacking sharply angular steps. However, just below and tothe left is an aggregate of plagioclase that contains grain X, which is simi-

lar in shape and size to grain W. Also in Figure 1B, in the lower left, is asubhedral plagioclase grain Y that has perhaps three regular crystal facesand an irregular fourth side which, conceivably, could be the result of im-pact fracture. However, to its right is another aggregate of plagioclase thathas a grain Z, similar to Y. Additional examples of the similarity in shapeand size between discrete grains and grains in aggregates can be found bycareful examination of Figures 1B and 2B. Such similarities can also befound in numerous other Great Basin cooling units. Thus, disintegration ofaggregates can at least contribute to the population of isolated subhedral toanhedral grains in ash-flow tuffs, as Nakada et al. (1994) found in pre-Mazama lava flows at Crater Lake.

Some of the peripheral crystals in aggregates have euhedral externalmargins, whereas internal margins are generally not euhedral. All marginsof embedded grains in clots tend to be subhedral to anhedral, similar tocrystals in a holocrystalline plutonic rock. Aggregates can develop in crys-tallizing magma by nucleation of a crystal on the surface of another crys-tal, or as independent crystals grow and drift together, to become perma-nently attached. Continued growth of these attached crystals createmutually interfering, common grain margins that are not perfectly euhedral(Vance, 1969, p. 24; Bryon et al., 1994). Phenocrysts described by Rossand Smith (1961, p. 35) as “subhedral . . . one side with a crystal face, andelsewhere show irregular or fractured edges” may have originated by free-ing peripheral crystals from an aggregate, rather than by impact fragmen-tation of discrete phenocrysts.

It may be assumed that aggregates bear a cognate relationship to discreteanhedral to subhedral phenocrysts in the same tuff sample; that is, all crys-tals formed in the same magma. This was tested for the Nugent Tuff Mem-ber (Fig. 2) by electron microprobe analysis (Fig. 12) of the three texturaltypes of plagioclase-composite grains, discrete subhedral-anhedral pheno-crysts, and discrete euhedral phenocrysts. All three have the same composi-tional range, suggesting that free-floating discrete crystals and crystals sep-arated from aggregates during eruption were derived from the same magma.

Disintegration of aggregates could occur in three ways. First, interstitialmelt trapped between overgrowing grains could vesiculate and blow the ag-gregate apart during rapid decompression as the magma was explosivelyerupted, as is the case for isolated single phenocrysts that contain melt in-clusions. Nakada et al. (1994) found such glass-bearing aggregates to becommon in pre-Mazama rhyodacitic lava flows at Crater Lake. Aggregatesconsisting of as many as 12 or so crystals are widespread in Great Basin

Figure 10. Cross-polarized light photomicrographsof experimentally impacted plagioclase fragmentsproduced at a velocity of 30 m/s. Bizarre two-dimen-sional shapes are a consequence of curved fracturesurfaces ornamented with stepped ribs that are con-trolled in part by two intersecting cleavages. Appar-ently separate plagioclases in each photomicrographare all optically continuous and compose one intactfragment. Note intricate serrations and steps on frag-ments in (A) and (B) (ignore numerous air bubbles inmounting cement). Scale bars are 0.5 mm.

Figure 11. Upper member of the tuff of Ryecroft Canyon collectednear the mouth of Meadow Creek in the Toquima Range about 6 kmnorth of Belmont, Nevada, sample CORC-1B. Two labeled plagio-clases (P) show shapes not unlike those produced experimentally byimpact (see Fig. 10). Large subhedral sanidine (S) has parts pluckedout during processing of thin section and is not like impacted feldsparfragments. Scale bar is 0.5 mm.



tuffs, but only 2 have been found that contain glass (Fig. 13). Generally,melt-bearing aggregates hosted within tuff would be expected to have beenblown apart during explosive eruption, rather than commonly retaining theirintegrity as in more slowly extruded lava flows. Thus, only melt-free clotswould generally survive during the rapid decompression accompanying ex-plosive eruption. Second, some holocrystalline aggregates could break apartby impact in the pyroclastic flow. Although we find little evidence for frac-ture of discrete phenocrysts by impact, larger aggregate masses would havegreater momentum at a given velocity and could break apart on collision.Third, grains in aggregates might break up by differential expansion of therandomly oriented, low-symmetry grains during the rapid decompression ofthe exploding magma. It is not unlikely that two or more of these threemechanisms could conspire in explosively erupting magmas.

We have not attempted an analysis of crystal size distribution (Cashmanand Marsh, 1988) in these tuffs to ascertain whether the phenocrysts havea magmatic growth signature. Preferential elutriation of fine particles, in-cluding smaller phenocrysts and phenoclasts, from the pyroclastic materialduring eruption and dispersal would distort any pristine magmatic sizedistribution.

SUMMARY AND GEOLOGIC IMPLICATION

Shapes of felsic phenocrysts in Great Basin tuffs can be classified intothree basic categories, each created by a different process. The first cate-gory includes euhedral grains, bounded by planar crystal faces. These orig-

inated as unbroken, free floating crystals in the preeruption magma; theyindicate conclusively that not all intratelluric crystals are broken in ash-flow tuffs. The second catagory is made up of subhedral to anhedral grains,possessing only partial or no bounding crystal faces. The origin of thisshape is problematic. Many may be disaggregated polygranular crystallinematerial. Few were possibly formed by mutual impact. Some may origi-nate in the manner of the next category. The third category is the pheno-clasts, which are cuspate, embayed fragments of phenocrysts that con-tained melt inclusions. Textural observations presented here provideconvincing evidence that inclusion-filled phenocrysts rupture within vesic-ulating rhyolitic melt prior to fragmentation of magma. As decompressingmagma blows apart into pyroclasts during explosive eruption, phenoclastsbecome part of the ash-size ejecta along with vitroclasts.

Although some tuffs in the Great Basin contain felsic phenocrystsshaped by a single process, the majority are likely polygenetic. Most orig-inated by fragmentation of crystals containing melt inclusions and bybreaking apart aggregates.

Hence, both melt and many crystals in ash-flow eruptions are blownapart in response to decompression and vesiculation of melt, yielding, re-spectively, vitroclasts and phenoclasts. Vesiculation of melt in a magmabody and of melt inclusions in a crystal are analogous processes that oper-ate on vastly different scales. Vesiculation of a magma body accompaniesdecompression and communication with atmospheric pressure, locally bybreaking through the containing roof rock, leading to fragmentation of thebody and production of pyroclasts. Vesiculation of melt inclusions also ac-

Figure 12. Compositions ofthree different morphologic typesof plagioclase in the Nugent TuffMember, sample COPPER-1A(see Fig. 2). Each point representsan electron microprobe analysis.Little compositional difference isapparent, suggesting that all threetypes came from the same popu-lation of plagioclases in thepreeruption magma chamber.



companies decompression, causing the host crystal to fragment intophenoclasts.

The origin of crystal shape in ash-flow tuffs depends upon crystal phase.Thus, quartz virtually always has embayments and inclusions of melt so thatphenoclasts, both arcuate slivers and embayed anhedra, occur in all quartz-bearing tuffs. All tuff units contain plagioclase and in more than half of themsome plagioclase occurs as phenoclasts. Widespread feldspathic aggregatesare matched by equally widespread anhedral to subhedral phenocrysts offeldspar. Sanidine in most tuffs is the most free of inclusions of the three fel-sic phases, is usually subhedral to euhedral, and probably was derived frombreakup of aggregates and from free-floating euhedral crystals.

Obliteration of the diagnostic vitroclastic and eutaxitic matrix texturesin some ash-flow tuffs by secondary flow after emplacement (rheomor-phism) or by devitrification through hydrothermal alteration or metamor-phism has made positive recognition difficult or equivocal. For example,Henry and Wolff (1992) reviewed the difficulties of interpreting the mech-anism of emplacement of large bodies of high-temperature silicic volcanicrock in Africa, Australia, and North America. They indicated that “broken

phenocrysts ranging widely in size” is one of several characteristics of con-ventional ash-flow tuffs. However, our investigation of shapes of felsic in-tratelluric grains in Great Basin ash-flow tuffs has shown that phenoclastsare widespread. In contrast, felsic phenoclasts are rare in lava flows and ab-sent in Plinian fall deposits. These contrasts apparently are controlled bykinetic factors, as suggested by Tait (1992, p. 154). Rapidly erupted Plin-ian pyroclasts that form ash-fall deposits mix with cool atmosphere andthus generally quench before volatiles dissolved in melt inclusions can nu-cleate bubbles and blow their host crystals apart. Explosive eruption of py-roclastic flows commonly follows precursory Plinian activity, so melt in-clusions have some time to vesiculate in the magma chamber after itsinitial opening; eruption of the heat conserving flow allows additional op-portunity for bubble growth in melt inclusions and rupturing of the hostcrystal. Relatively slower extruded lava flows have few phenoclasts, forpossibly two reasons. Diffusive reequilibration of volatile pressure in meltinclusions with melt surrounding host crystals precludes rupturing over-pressures. Also, during slow extrusion cracked or ductily deformed crys-tals may anneal. Euhedral, subhedral, and anhedral phenocrysts can befound in any volcanic rock and are therefore not diagnostic of a particularmode of emplacement.

ACKNOWLEDGMENTS

We acknowledge the cooperation of our colleagues, Al Deino, ShermanGrommé, Lehi Hintze, and David Tingey, in the study of Great Basin tuffs.Keith Rigby took the photomicrographs shown in Figures 1–3. BartKowallis made available a photomicroscope for the other photomicro-graphs. Freeman Anderson of the Brigham Young University (BYU)Physics Department provided the laser timer for measuring speeds ofgrains from the slingshot and helped in its design. Peter Larson assistedwith some impact experiments. Bart Kowallis, Anita Grunder, Charlie Ba-con, Chris Henry, Frank Perry, and an anonymous reviewer provided help-ful and encouraging comments on earlier versions of the manuscript. Theeditorial and geologic insights of Bart Ekren, who has spent far more timestudying volcanic rocks in the Great Basin than we have, are especially ap-preciated. During the decade of the 1960s, seven U.S. Geological Surveygeologists, including Bart Ekren, recorded modal analyses and textural fea-tures of more than 300 thin sections of middle Tertiary ash-flow tuff in thesouthern Great Basin. Although these petrographic observations were notdesigned to systematically catalog phenocryst morphology, many com-ments on the original data sheets, which we were allowed to examinethrough the courtesy of Gary Dixon, reinforce our observations.

Financial support was provided by BYU and by National Science Foun-dation grants EAR-8604195, EAR-8618323, and EAR-8904245.

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Figure 13. Glass-bearing crystal aggregates in crystal-rich dacitictuff of the Lund Formation at the south end of the Egan Range, 12 kmsouth of Sunnyside, Nevada, sample SILVRWL-2P. (A) Inclusionabout 8 mm in diameter of vesicular glass and abundant plagioclaseswithin a pumice block hosted by the tuff. (B) Apparently holocrys-talline cognate(?) granodiorite inclusion in tuff has rare, small, iso-lated interstitial areas of vesicular glass (arrows). Scale bars are0.5 mm.



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