Stable and Metastable Pyroxene Crystallization in Tayered Komatiite Lava Flows

American Mineralogist, Yolume64, pages E56-864, 1979

Stable and metastable pyroxene crystallization in tayered komatiite lava flows

Nrcnorns T. ARNDT Deportmentof Geological Sciences,University of Saskatchewan Saskatoon,Saskatchewan S7N 0W0

eNo Mrcnenl- E. FLEET Department of Geology, University of WesternOntario London, Ontario N6A 3K7

Abstract

Pyroxenesin the rapidly-cooled upper portions of thich layered komatiite lava flows have significantly different compositions and structures from pyroxencs in the cumuliatelower portions of the same flows. Rocks in the flow tops contain composite pyroxene mcgacrystsmade up of needleswith coresof pigeonite and margins of subcalcic, aluminous augite. The pigeonite is more magnesian [Mgl(Mg + Fe) = 0.841than any previously reported in terrestrial rocks. In the cumulate zon€s,the pyroxenes are bronzite and augite. The pigeonite probably formed under conditions of strong undercooling developed during the solidifcation of the upper parts of the flows, whereasthe cumulus pyrox€nes formed under conditions approaching equilibrium in the more slowly-cooled ccntral parts of the flows.

Introduction nald Township about 70 km west of Munro. It has In thick, layered komatiite lava flows, the composi- been discussedvery briefly by Arndt (1977x). tions and the types of pyroxenes in flow top zones Fred's Flow can be divided into four units: a brec- difer considerably from those in lower cumulate liay- ciated or spinifex-textured uppermost unit; a gab- ers of the same flows. Magnesian pigeonite and sub- broic unit; a cumulate unit; and a basal, olivine-de- calcic augite are present in the flow top zones, pleted marginal unit (Fig. la). The flowtop breccia is whereasbronzite coexistswith augite in the cumulate made up of fragments of porphyritic lava containing Layers. Furthermore, the pigeonite has crystallized olivine phenocrystsand skeletal olivine in a matrix of from magmatic liquids, which" under experinental fne-grained acicular augrte and devitrffied glass. equilibrium conditions (Arndt, 1977a), would have Spinifex is the name given to textures dominated by crystallized olivine. In this paper, the crystallogra- skeletal olivine on pyroxene grains, which form when phy, mineral habit, and chemistry of pyroxenes from ultramafic s1 highly mafic magmas crystallize rela- the upper and central parts of two layered komatiite tively rapidly, as 1o the upper sections of lava flows lava flows are described and contrasted, and the con- (Nesbitt, l97l; Donaldson, 1976;Amdt et al.,1977). ditions that controlled their formation are investi- In Fred's Flow, a thin layer of olivine spinifex-tex- gated. tured rock containing thin wafers of olivine in a pyroxene-glass matrix overlies a thicker pyroxene spin- Thick, layered komatiite lava flows ifex-textured layer, in which large composite The principal features of the two lava flows are il- pigeonite-augite needles (described below) are ori- lustrated in Figure la,b. One of them, Fred's Flow, ented perp€ndicular to the flow margins, and lie from Munro Township in northern Ontario, has been within a pyroxene-plagioclase matrix. The gabbroic described in detail in other publications (Arndt, unit, which has a normal, medium-grained, sub- 1976, 1977b: Arndt et al., 1977). The other, which hedral-granular texture, is underlain by a ^hin inter- will be referred to as the Alexo Flow, is in Dundo- val of alternating layers of augite plus bronzite 00n.3-{04.)'./ 79 / U 0E4856$02.00 856 ARNDT AND FLEET: PYROXENE CRYSTALLIZATION 857

FLOWTOPBRECCIA groundmass.The upper 3-4 meters of the olivine-en- Mun 5Ol OLIVINESPINIFEX.TEXTURED LAVA Mun 5O2 riched layer contain skeletal, hopper olivine grains Mun 164 PYROXENESPINIFEX-TEXTURED LAVA (Donaldson, 1976) n an augite-devitrified glass matrix. A thin [a16 of bronzite cumulate separatesthis Mun 224 from the lower olivine cumulate which llun 5O5 type of rock GABBRO contains solid, polyhedral olivine grains. 3 tle lava flows are believed to have been erupted very high temperatures(1300-1350"C, the liq- ! "t PYROXENECUMULATE ; uidus of olivine spinifex-textured lavas; Arndt, l977a,b) as highly magnesian(15-20 percent MgO) 'i liquids containing a small propoftion of olivine phenocrysts.Following eruption, the lavas differenti- OLIVINECUMULATE ated to form the olivine and pyroxene cumulates at the base of each flow. At the same time, spinifex-textured lava grew downwards from the top of each flow. In Fred's Flow, the quartz-normative basaltic BORDERZONE a liquid that resulted from this differentiation solidffied as the gabbroiclayer. The calcium-poor pyroxenes in the pyroxene spinifex and cumulate layers of Fred's Flow have completely altered to chlorite; olivine is partially serpen-

PYROXENESPINIFEX.TEXTURED LAVA Ar 12 tinized in the cumulatelayers and completelyaltered Ar llo lod Ar l3 and serpentine in the flow top layers. In PORPHYRITICZONE to chlorite Ar 2O mafic minerals have escaped SKELETALOLIVINE ZONE Ar 26 the Alexo Flow, most complete alteration. The exception is olivine in the BRONZITECUMULATE d spinifex layers, which is completely altered to chlo- e= rite. Augite rarely is altered, and bronzite in the OLIVINE CUMULATE F cumulate layer also is quite fresh. The state of preser- vation of pigeonite, however, varies from 5amFle to

,ab Fig. l. Diagramrnatic section through (a) Fred's Flow and (b) the Alexo Flow. Sample locations are shown on the right of the columnar section. cumulates, and by a thicker interval of olivine cumulate. The uppermost part of the Alexo Flow appears to have been eroded away by younger lava flows (Fig. lb). The present top is pyroxene spinifex-textured like the lower spinifex layer in Fred's Flow. Olivine (now completely chloritized) appears sporadically in generally the spinifex layer, not at the top, but about Fig. 2. (a) Photograph of a hand sample of clinopyroxene spin- a meter below the presentsurface. No gabbro is pres- ifex-textured lava from Fred's Flow showing pigeonite-augite ent in the Alexo Flow,'and the transition from spin- megacrysts.Note the pronounced curvatute of needlesin the bot- ifex-textured lava to underlying olivine-rich rock tom left of the picture. The surface shown was polished, then usually is abrupt. In some areas, however, there is a etched in HF, and illuminated with highly oblique liChtinC. (b) of a composite pigeonite-augite needle and ia- porphyritic Photomicrograph narrow interval of rock 66alaining 10-60 terstitial splaying sheaths of augite and plagioclase. Pigeonite in percent of solid or skeletal, equant olivine pheno- the core ofthe needle has altered to chlorite. The augite mantle is crysts in a fine-grained basaltic (augite + plagioclase) unaltered. A RNDT AN D FLEET: PYROXEN E CRYSTALLI ZATI ON

sample. In some samplesit is qurte unafected by alteration, and in others it has been pseudomorphically replaced by chlorite.

Composite clinopyroxene megacrysts in the spinifex- textured layers The pyroxene spinifex-textured lava is composed of 25 to 40 percent composite pigeonite-augite needles set in a matrix of dendritic augite and plagioclase. These needles are oriented roughly parallel to one another, perpendicular to the flow top (Fig. 2). They are 0.1 to 1.0 mm in diameter,and most are at least 2 cm in length. Their maximum length is diffi- cult to estim.atebecause they are usually inclined at a low angle to thin sections or fractured hand-sample surfaces.In many cases,however, individual needles exceed 5 cm in length. There is a tendency for their size to be fairly uniform i1 3 single hand sample and to increasedownward toward the middle of the spinifex layer. In thin section,the needlescan be seento be orga- nized into colonies of individuals in near-optical con- tinuity. In fact, they form highly complicated, skeletal megacryststhat are coarse-grained equivalents of Fig. 3. Typical appearanceofindividual composite clinopyrox- the feathery dendritic or spherulitic grains interstitial ene megacrystsfrom pyroxene spinifex rock sectionednormal and to olivine in peridotitic komatiite lavas (Fleet, 1975). subnormal to c axis in (a) and (d) Ax I ld; (b) and (c) Ax 13; (e) Ax Each megacrystconsists of one or more parallel rows I lc: clear, pigeonite; stippled, augite. of needles,the whole appearing in three dimensions like the stemsof a bunch of flowers(Fig. 2). tor zoning. Thus, these composite clinopyroxene The megacrysts, in turn, are arranged to form crystals are morphologically similar, and clearly ge- sheathsor columns that extend for distancesof up to netically equivalent, to the composite pyroxene several meters. Although they maintain an average phenocrysts in Apollo 12 basalts (Hollister et al., orientation perpendicular to the flow top, the col- l97l). However, becausethey are much more elon- urrns are gently curved, kinksd, and warped (Fig. 2). gated than their lunar counterparts, they are charac- This structure appearsto be equivalent to that of the teristically tubular in appearance.A few grains are "folded columnar spinifex" of Nisbet et at. (1977). markedly elongated along [00]* and poly- However, the curvature of the pyroxene columns is synthetically twinned on (100). In the megacrysts, sirnilar to that observed in comb-layered rocks (Lof- parallel rows of composite clinopyroxene needles ex- gren and Donaldson, 1975)and probably has a simi- tend in the [00]* direction (Fleet, 1975). lar origin: r.e.individual grains probably are slightly Fresh pigeonite in the composite needles is opti- curved as a result of rapid growth from super- cally homogeneous:all the sectionsexamined have saturatedliquids. the optic axial plane parallel to (010) and a 2V near The individual clinopyroxene needles that make 5o. X-ray diffraction precessionanalysis of fragments up the megacrystsare elongated parallel to the c axis of the composite needlesremoved from thin sections and are bounded predominantly by {ll0} prism Gig. a) confirmed that the Ca-poor clinopyroxene faces,with minor developmentof {010}. They con- cores are pigeonite, with Y2,/c symmetry. Measure- sist of hollow cores of pigeonite, generally box- ments on precession photographs of several grains shaped in cross section, mantled and partly lined by from Ax lld give B parametersof 108"15'for pi- augite (Fig. 3). A few megacrystsshow well-devel- geonite and 106o35'for augite. Pigeonite xrrgite oped augite {l l0} growth sectors(Frgure 3e), and reciprocal lattices have a preferred orientation"o6 which others show birefringence differences indicating sec- is equivalent to'aon inclined to a,u", with (aooA a",J ARNDT AN D FLEET: PYROXENE CRYSTALLIZATION 859

tion to these diffraction patterns. In the composite needlesofAx 13 the pigeonite and augite c axes are slightly misaligned, and a single grain from Ax 12, in which most of the core pigeonite has been altered, gave the 'opposite' preferred orientation to that observed in Ax I ld with coo approximately parallel to e,,"1(conA c""")= AB. Finely laminated striations, subparallel to (001), are observed in the mantle augite of Ax I lc clinopyroxene megacrysts.These could be (001) pigeonite exsolution lamellae, since the latter were identified Fig.4. X-ray diffraction precessionphotograph ofa fragrnent of optically in one grain of a double-polishedthin sec- compositeclinopyroxene needle from lld, showingprefcrred ori- tion. However, pigeonite reflections were not de- pigeonite (a*p, entation of c*p) and epitaxially overgrown sub- precessionphotographs of calcic augite (ata, c*a); zero level, b axis, p : 25o, MoKa radia- tected in long-exposure tion, 35kV, 20mA,3-day exposur€. thesepyroxenes, and it is possiblethat someof these striations may be crystal growth features. = A,B; brr" approximately parallel to b"o"; ceis approxi- The pigeonite in the composite needleshas unusu- mately parallel to c..s (Fig. a). This orientation rela- ally high magnesium-iron ratios [Table I, Mg/(Mg tionship is different from that of the composite py- * Fe) : 0.81to 0.851and high alumina contents(2.0 roxene phenocrystsof Apollo 12 basalts(Ross et aL, to 2.7 weight percent). The composition is fairly uni- 1973),which have (ao,*A a,"") = (co."= c",e)= LP/2, form, although there is, from a point close to the in- and in fact is similar to that for (100) pigeonite and ner margin to the outer margins of the pigeonite augite exsolution lamellae in, respectively,augite and areas,a slight increase in CaO, a gradual decreasein pigeonite. However, for each fragment X-rayed, the MgO/FeO, and a marked decreasein Cr (Fig' 5). intensities of the two superimposed diffraction pat- The clinopyroxene surrounding the pigeonite cores terns were consistent with the observed proportions of the composite needlesis aluminous, subcalcic aug- of core pigeonite and mantle augite, and exsolution ite (Table l). In many grains, CaO remains essen- lamellaecould not have made a significantcontribu- tially constant from the inside to the outside of the

Table l. Represcntativeelectron microprobe analysesfor pyroxenesin layered flows

ql (1 0 (1 ? sio, 55.2 )J.O 0 50.6 51.3 57.L 52.5 Tin- n 11 n.d. .37 n.d. n.d. .51 .46 0. 04 0.20 A1,0. 2.02 2.32 4.6 5.1 3.r4 2.67 z,)o 0.91 2.53 crioi 0.63 0.76 .23 .L6 10 1? 0.04 0. 50 0 .81 reO (tot) 9.4 7.7 8.4 t.6 10.6 13.8 70 5.91 MnO 0.26 n.d. .25 n.d. n.d. .26 o.2r 0.15 MgO 28.9 26.7 r7.8 rt.6 18.3 l4.8 L5.2 32,O 18.4 CaO 4.Oz 6.76 r7 .3 r1.o 17.8 r9.4 15.1 I .18 19.7 NarO 0.04 n.d. .L4 n.d. n.d. .L7 0.14 0.01- 0.11

Toral 100.6 99.8 100.2 98.9 99.r 99.8 100.4 99.8 r00,5

Ca 7.7 t2.9 36.0 35.4 JO.L 40.2 31,2 )) 39.5 Ms 78.1 72.3 5r.5 50.9 51.8 42.7 46.6 86.0 51.3 te Lq. z 14.8 tz .4 13.6 L2.O L] .L 22.2 l-1.8 9.2 Mg/(Mg+Fe) 0.85 0.83 .81 0.79 0.81 o.7r 0.68 0 .88 0.85

1. Pigeonite from core of needle in pyroxene spinifex-textured lava (Ax fld) 2. Pigeonite from core of needle in pyroxene spinifex-textured lava (Ax f4) 3. Augite adjacent to pigeonite in composite needle (Ax lfd) 4. Augite at margin of composite needle (Ax 11d) 5. Augit.e fron centre of mantle surrounding pigeonite; composite needle in sPinifex-Eextured lava (Ax 14) 6. Augite intersEitial to composite needles in spinifex-textured lava (FC-l) 7. Augite interstitial to composite needles in spinifex-texEured lava (FC-l) 8. Bronzite from cumulate layer, Alexo I'low 9: Augite from cumulate layer, Fredrs Flow 860 ARNDT AND FLEET: PYROXENE CRYSTALLIZATION

E.D' OUTER AUGITEMANTLE

D.C PIGEONITECORE

i c-B TNNERAUGTTE LtNtNG

B.A HOLLOW CENTER Mg/(M9 +Fe)

1-a-o-,4-ra-a-a

Mg/(Mg+Fe) '-.-.. \^-^{zor{,ror -;i:.7/ -^-"jl -.-'-' -\"-' _"_"/-"-.-.--t-a^r^r- )= DD.

oISTANCE(pm)

Fig. 5. Chemical profrles acrosscomposite pigeonite-augite needlesfrom (a) Fred's Flow and @) the Alexo Flow. subcalcic augite mantle, although in others it varies geonite grains has a subcalcic augite composition in an irregular manner, usually to produce a CaO-de- with somewhat lower CaO and AlrO, contents, much pleted outer rim (Figs. 5 and 6). Magnesium-iron ra- lower CrrO3 contents, and conspicuously higher tios and Cr contents decreasefrom the inside to the FeO(tot) contents than the subcalcic augite in the outside of the mantles, and AlrO, increasesmarkedly outer mantles.The inner linings are too thin to per- in the same intervaf continuing trends present in the mit detailed measurementof their chemical zonation. pigeonite cores. The composition of the augite at the outermost rims resemblesthat of smaller interstitial Clinopyroxenesin gabbro and ctmulate layers augrte grains. The clinopyroxenes in the gabbroic and cumulate The pyroxels lining the hollow cores of the pi- layen of Fred's Flow have been described previously ARNDT AND FLEET: PYROXENE CRYSTALLIZATION 861

CoMg Si2O5 5Omole % CoSiO3

FtRiE[a)"SIFLOW ALEXO FILOW

,:.,;J-*u'ouo'"

+ CUMULUSBRONZITE

M9Si O3 MgSrO3 40 mole % Fe Si05

o CLINOPYROXENE FROM GAB8RO e'noxe'e,se'Hrrex CUMULATE : :L:';,i'r''J;l^-"' " PYROXENE I ''^''"' . . INNERAUGITE LINING | o n OLIVINECUMULATE e ORTHOPYROXENEFROM PYRoXENE CUMULATE - CUMULUS OLIVINE BCDE Fig. 6. Compositions and principal chemical variations of spinifex and cumulus pyroxenes in both layered flows. The lne variation shows the compositional variation in the spinifex needle from the Alexo Flow illustrated in profile b, Fig. 5; thc line IZ shows within 6 spiaif6x needle from Fred's Flow.

(Arndt, 1977a),and only aspectspertinent to the fol- tion, augite is found as epitaxial overgrowths on cu- lowing discussionwill be reviewed here. mulus bronzite grains, in the form of {100} platelets Toward the base of the spinifex layers, the pyrox- and more irregular protrusions on [l0l] faces. ene grains lose their skeletal habit and become Where present in two-pyroxene cumulates, augite shorter and stubbier. They characteristically have grains are smaller (0.3-0.5 mm) than pseudomorphs unusual curved outlines [bent about the D crystal- after bronzite grains (0.5-0.8 mm). Augite in cumu- lographic axis, (Figure 10, Arndt, 1977a)1,are twin- liate rocks is more magnesian than that in gabbro, ned on (100), and show evidenceof (ft0l) and (100) and its compositional variation within the pyroxene laminated growth features. A few grains have chlo- quadrilateral follows the high-temperature end of the rite cores,presumably after Ca-poor pyroxene. In the normal "plutonid'trend (Fig. 6). gabbroic layer, augite makes up 30-40 percent of the Bronzitein the cumulatelayer of theAlexoflow rock, and @curs as subhedral prismatic grains, and as intergrowths with plagioclase.Sector 2sning pro- The bronzite cumulate is composed of 70 to 80 duces characteristic hourglass patterns in some sam- modal percent prismatic bronzite grains and scat- ples, particularly those from coarser-grained pegma- tered euhedral grains of olivine and chromite in a toid patches. Throughout the gabbroic layer, the fine-grained (devitrified glass) groundmass. The pyroxene is augite. Zonation in individual grains and bronzite grains are elongated parallel to c axis with compositional variation between samplesresults in a length: width ratios of about 2 : I to 6 : I (FrC.7) and normal "plutonic" trend of Fe-enrichment with are bounded by the crystallographic forms {010}' slightly decreasingCaO content(Fig. 6). {100}, [01] and minor {210}. Although the grains The augite in augite cumulate layers occurs as eu- are euhedral in outline, the grain bound4ries are hedral or more or less equant grains, although in markedly crenellate (Fig. 7); this texture may be some cases adcumulus growth has produced anhe- characteristic of Mg-rich orthopyroxene grown under dral outlines and the augite partially or completely a high cooling rate. The bronzite grains have a dis- enclosesgrains of other cumulus minerals. In addi- continuous rim of epitaxially overgrown augite. ARNDT AND FLEET: PYROXENE CRYSTALLIZATION

which is principally in the form of {100} platelets, and contain ragged augite exsolution lamellae oriented parallel to (100). Long-exposureX-ray precession photographs confirmed the common orthopyroxene spaoegroup Pbca. Compositional data arc reported in Table I and Figure 5.

Discussion Magnesianpigeonite Pigeonite in the spinifex units of the layered komatiite flows is signfficantly more magnesian than any pigeonite that has been reported from terrestrial rocks (for example, Kuno and Nagashima, 1952; Ishii, 1975).ln tholeiitic lavas and differentiatedin- trusions, magnesian pigeonite has Mgl(Mg + Fe) of about 0.7, which is considerablylower than the val- Fig.7. (010) rouoo orilffi frombronzite cumulate ues of 0.81 to 0.85 for komatiite pigeonite. pigeonite rock (Ax 27), showing crenellate outline and cpitaxial augite over- of similar composition to komatiite pigeonite may growth (stippled). exist in Apollo l7 spinel troctolites (Smith and Steele, 1976). Smith and Steele(1976) also suggest that Ca-poor pyroxene from an Apollo 17 dunitic mafic rocks and appears to be characteristically de- rock with composition En'Fs,,Wo, (Albee et al., veloped in volcanic (rapidly-cooled) rocks in which 1974)mry be a clinopyroxene. However, there are no Ca-poor pyroxeneappears early in the crystallization supporting crystallographic data for any of these py- sequence; examples include zoned clinoenstatite roxenes. from Cape Vogel, Papua (Dallwitz et aI., 1966), Highly magnesian pigeonites have been produced zoned pyroxene phenocrysts from Apollo 12 basalts experimentally. When Huebner et al. (1973) parrially (Hollister et al., l97l). and zoned phenocrysts and melted pigeonite from a lunar basalt, certain samples groundmassgrains from Oshima,Japan (Ishii, l9Z5). lost iron to their platinum-iron containers, resulting Although the compositions of the liquids that crystal- in pigeonites with compositions that extended from lized these composite clinopyroxene grains vary natural compositions to the Di-En join. markedly, the only essential difference in the resulting zoning trends is a displacement Compositionalzoning and crystal growth to higher Fs values. The stratigraphic position of the pyroxene spinifex The composition of the clinopyroxene in the rocks suggests that the clinopyroxene megacrysts Fred's Flow gabbro defines a trend of FeO enrich- grew downwards with continued crystallization of ment largely at the expenseof MgO. The trend fol- the komatiite flows. The morphology and chemical lows the 900+"C isotherm for the augite limb of the zonation of the clinopyroxene needles (Figs. 5, 6) is pyroxen€ solvus (Fleet, 1974,Fig. l). However, this consistent with initial crystallization of the pigeonite clinopyroxene does not coexist with Ca-poor pyrox- cores as hollow skeletal crystals and later epitaxial ene and probably crystallized within the augite sta- overgrowth of subcalcic augite. The subcalcic augite bility field, above tle corresponding solvus temper- lining tle interior of the needlescrystallized from the ature. In contrast, the composition of the cumulus residual liquid that was trapped within the hollow augite, which frequently coexists with bronzite, is cores and effectively isolated from the bulk ofthe re- more subcalcic and correspondsto an apparent crys- maining liquid. Chromium and MglFe zonation tallization temperature near I l00oC relative to the (Fig. 5b) indicate that crystal growth proceeded out- simple geothermometerof Fleet (1974). Correction wards and inwards from a point close to the inner for the high CaTs content would raise the calculated margin of the present pigeonite core. crystallization temperature somewhat, and it appears The compositional zoning trend of the composite that the cumulus augite has probably crystallized on clinopyroxene needleshas many counterparts in less or near the pyroxene solvus. ARNDT AND FLEET: PYROXENE CRYSTALLIZATION

Stable and metastablepyroxene crystallization very similar to sequencesobserved in larger layered (e.9.Musko* Irvine, 1970). The petrologic importance of Archean komatiite intrusions crystallization sequenceof koma- flow rocks is not limited to their high MgO character. The equilibrium has been determined experinentally The rapid crystallization permitted by hrghly magne- tiitic liquids 1977b). Samples of peridotitic komatiite sian magma exaggeratesthe effect produced by the (Arndt, olivine as the sole silicate mineral for two contrasted thermal regimes,a slowly-cooled inte- crystallize crystallization interval at I atm. At tem- rior and a rapidly-cooled upper margin. Hence, the much of the peraturesof about I190"C, when the MgO content of chemical and mineralogical compositions of the cu- liquid has been reduced to about 9 mulus rocks and gabbros resemble those ordinarily the crystallizing percent, diopsidic pyroxene joins olivine, and associatedwith layered intrusions. Only the presenc€ weight ll80oC, plagioclase appears. Ortho- of hopper olivine and the skeletal and dendritic inter- at about pyroxene was not observed, but presumably appears stitial areas would indicate a different environment closeto the solidus (=ll70"C). This of formation on superficial examination. The chem- at temperatures di-ffersin some respectsfrom that observed istry of the pyroxenescrystallized in the two thermal sequence layers of the lava flows, where bron- regimes is clearly quite distinct and, in part, corre- in the cumulate before plagioclase and augite takes spondsto that observedfor pyroxene in a thick basalt zite crystallized place of diopside. Possibleexplanations for these flow from Picture Gorge, Oregon (Smith and Lind- the are sligbt alteration of the lavas following sley, l97l). The compositionof augite from the inte- di-fferences or differences between the conditions rior of the Picture Gorge flow, where it is presumed their eruption, which the experiments were run (equilibrium to have crystallized in stable equilibrium, is very sim- under volatile-free, a;tp, closeto the wiis- ilar to that ofthe Fred's Flow gabbro and definesa crystallization, buffer curve) and the conditions of "normal plutonic trend." [n contrast, the composi- tite-magnetite rapid cosling under which the lavas may have tion of augite from the chilled basal margin defines a more trend of FeO enrichment at the expenseof CaO, very crystallized. the crystallization sequencein the flow similar to the initial FeO enrichment of the zoned In contrast, little resemblance to the phenocrystsfrom Apollo 12 basalts. Smith and Lind- top spinifex layers bears sequence.Pigeonite first appearsin spin- sley have identifed this as a "quench trend" and as- equilibrium lavas that contain 12-14 percent MgO. cribe it to metastable crystal-liquid equilibration ifex-textured of these lavas have been interpreted to through rapid crystallization. The textures (a) pigeonite was the liquidus phase, In the spinifex layers of Fred's Flow, the observed indicate that (b) although there may have been some aocumu- crystallization sequenceis and lation of pyroxene during the growth of spinifex nee- olivine + pigeonite + subcalcic augrte + augite * dles, the compositions of the lavas are close to those plagioclase of the liquids from which they solidified. Thus pigeonite, followed by subcalcic augite, has crystallized Olivine appearedas the liquidus phase only briefly at from liquids that under equilibrium conditions the top of the spinifex layers, pigeonite and subcalcic would have crystallized olivine, followed by diopside augite form the composite megacrysts, and augite at much lower temperatures. Factors such as altera- and plagioclase crystallized as small prismatic grains tion of the lavas or accumulation of pyroxene during interstitial to the megacrysts. crystal growth may contribute to the discrepancy,but In the cumulate layers the crystallization sequenc€ these factors cannot be the complete answer: the dif- is ferences in crystallization order and in pyroxene olivine --->olivine + augite --+ augite * bronzite + minsmlegy appearto be too large. augite * plagioclase(Fred's Flow) It is suggested,therefore, that the pigeonite and subcalcic augi16 are metastable products whose ap- or pearance is related to the rapid cooling conditions have existed in the upper parts of the olivine --+ olivine + bronzite + augite + plagioclase that would explanation is tentatively pro- (Alexo Flow) flows. The following posed. (It will be presentedin more detail" and sup- The crystallization sequencesin the cumulus zones, ported by experimental data, in a forthcoming paper and the variation in pyroxene compositions, are thus by Arndt and I. H. Campbell, in preparation.) Dur- ARNDT AND FLEET: PYROXENE CRYSTALLIZATION

ing the initial stagesof rapid ccroling,olivine crystal- - (1977b) Thick layered peridotite-gabbro lava flows in lizes, but at L rate insufficient to prevent a progres- Munro Township, Ontario. Can. f. Earth Sci.,14,2620-2637. -, sive increase in the degree of supercooling of the A. J. Naldrett and D. R. Pyke (1977)Komatiitic and iron- lava. The stage is reached which rich tholeiitic lavas of Munro Township, northeast Ontario. J. at the temperature Petfol., 18,319-369. ofthe supercooled lava falls below the stable pyrox- Dailwit4 W. B., D. H. Green and J. E. Thompson (1966) Cli- ene liquidus. Even though the composition of the liq- noenstatite in a volcanic rock from the Cape Vogel Area, Papua. uid has changedonly slighlty (becauseof minsl 61i- J. Petrol.,7,375-403. vine crystallization),the drop ia lsmFeratureresults Donaldson, C. H. (1976) An experimental investigation of olivine morphology. -213. in an increasein the relative stability pyroxene. Contrib. Mineral. Petrol., 5 7, 187 of As Flect, M. E. (1974) Mg,Fe2+ site occupanciesin coexisting pyrox- a result, a pyroxene, pigeonite, crystallizes under enes.Contrib. M ineral. Petrcl., 47, 2O7-214. these conditions of high supercooling from liquids - (1975)Growth habis of clinopyroxene.Can Mineral., 13, which, under equilibrium conditions, would crystal- 33G34r. lize olivine. The crystallization of pigeonite would be Hollister, L. S., W. E. Trzcienski, Jr., R. R. Hargraves and C. C. Kulick (1971) Petrogenetic pyroxenes followed by crystallization of augite. significance of in two Apollo 12 samples. Proc. Second Lunar Sci. ConJ., Geochim If this explanation is correct, the following combi- Cosnochim. Acta, Suppl., 2, 529-557. nation of factors is required for the crystallization of Huebner,J. S., M. Rossand N. Hickling (1973)Partial mshing of highly magnesian pigeonite in nature: (a) highly pyroxcn€s and ttre origin ofmare basals (abstr.). ln Lunar Sci, nagnesian, high-temperature liquids; and (b) rapid 4, p.397-400..Lunar ScienceInstitute, Houston. T. N. (1970) 666ling of theseliquids high Irvine, Crystallization sequencein the Muskox intru- so that a degreeof su- sion and other Layeredintrusions: I---olivine-pyroxene-plagio- percooling is induced. If the liquid cools slowly, only clase relations. Geol. Soc. SouthAfrica, Spec.Publ., 1,4/.1476. olivine would crystallize over most of the solidi- Ishii, T. (1975) The relations between temp€ratures and composi- fication interval. In less magnesian liquids, meta- tion of pigeonite in some lavas and their application to geo- stable pyroxeneswith compositions falting within the thcrmometry.Mineral. f., E,48-57. Kuno, H. and K. Nagashima(l952) gap between stable Chemical compositions of hy- Ca-rich and Ca-poor pyroxenes pcrsthene and pigeonite in equilibrium with magms. Am. Min- might fe16 under rapid sooling conditions,but these eral.,37, tfiX)-1006. pyroxenes would have norrral Mg/Fe ratios. Lofgren, G. E. and C. H. Donaldson (1975) Curved branching crystals and differentiation in comb.layered rocks. Contrib. Min- Acknowledgnents eral. Petro1., 49, 3@-3 19. This researchwas supported by NsERc operating grants to both Nesbitt, R. W. (1971) Skeletal crystal forms in the ultramafic rocks authors. Electron microprobe analyses were carried out at the of the Yilgarn Block, Western Australia: evidence for an Ar- Geophysical Laboratory, Carnegie Institution of Washington, chaeanultramafic liquid. GeoL Soc.Aust. Spec.PubI., 3,331- where NTA had a PostdoctorateFellowship. Reviews by M. Ross, 34E. R. H. Hewins, I. H. Campbelt and L. C. Coleman wcre helpful. Nisbet, E. C., M. J. Bickle and A. Martin (1977)The mafic and ultramafic lavas of the Belingwe Greenstone Belt" Rhodesia. J. References Petrol., 18,521-566. Ross,M., J. S. Huebner and E. Dowty (1973)Delineation of thc Albee, A. L., A. A. Chodos,R. F. Dymek, A. J. Gancara D. F. gap pyroxenes Goldman" D. A. Papanastassiouand G. J. Wasserburg (1974) one atmosphere augite-pigeonite miscibility for from lunar basult l202l. Am. Mineral., 58, 619-635. Dunite from the lunar highlands: petrology, deformational his- Smith, D. and D. H. Lindslcy (1971) Stable and metastable augite tory, Rb-Sr age (abstr.). Lunar Sci.,t p. 3-5. Lunar ScienceIn- stitute,Houston. crystallization trends in a singls basalt flow. Am. Mineral., 56, 225-233. Arndt, N. T. (1976) Ulrramafic lavas in Munro Township; eco- Smith, J. V. and I. M. Stecle(1976) Lunar mineralogy: heavenly nonic and tectonic implications. In D. F. Stong, Ed. Metal- a detective story. Part ll. Am. Mineral., 61, 1059-I I 16. logeny and Plate Tectonics,p. 617-658. Geol. Assoc.Can Spec. Pap. 14. - (1977a) Mineralogical and chemical variation in two thick layered komatiitic lava flows. Carnegie Inst. Wash. year Book, Manuscript received,fuly 3, 1978; 76,49+501. acceptedforpublication, February 27, 1979.