Distribution of Elements in a Spherulitic Andesite

TIIE AMERICAN MINERALOGTST. VOL 53. NOVEMBER_DECBMBI'R. 1968

DISTRIBUTION OF ELEMENTS IN A SPHERULITIC ANDESITE

SrnpnBN E. Knsron, DepartmentoJ Geology, Louisiana State LInivers,ity,Balon Rowge,Louisiana 70803 AND Paur W. WtrrnroN, Departmentof Geologyand Geophysics, Llniversilg oJ Minnesola, Minneapolis, Minnesttta 5515 5.

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Electron microprobe and bulk chemical analyses of a spherulitic andesite indicate that notassium is concentrated in the spherulites,whereas sodium and magnesium are depleted. Calcium, silicon and titanium are concentrated, and aluminum and potassium are depleted in a 10pm-wide band of brittle mica(?) which is peripheral to the spherulites The spherulites appear to consist of an open aggregate of 1Opm-wide,radially-oriented plagioclase fibers with abundant potassium feldspar and minor biotite in the interfibrillar areas The groundmass contains unoriented plagioclasefibers, some potassium feldspar, hornblende(?) and hematite Both the groundmass and spherulite plagioclase have a composition of approximately Anro and have potassium-rich rims. This distribution of elements conforms to that predicted from observations of spherulite formation in polymer melts. The growth rate of the spherulites was approximately 10-6 cm/sec.

hqrnooucrrox During a study of the Colombier volcanic sequencein the Terre- Neuve Mountains of Haiti, an unusual distribution of potassium and calcium was discoveredin a spheruliticpyroxene ande3ite when staining for potassium and calcium-bearingfeldspars indicated a strong partitioning of potassium into the spherulites and a concentration of calcium at the spherulite borders. This distribution has been verified by bulk and electron microprobe analysesand appearsto agree with predictionsbased on theoriesof spherr.rlitegrowth in pclvmers. The geology of the Terre-Neuve Mountains has been discussedb1' Kesler (1966, 1968). The Upper Cretaceous(?) Colombier volcanic sequencefrom which the spherulitic andesitewas collected,is exposed along the axis and periphery of the Terre-Neuve Mountains (Fig. 1). The maximum thicknessof the unit is 500 m although the basalunit is not exposedand the upper contact is an erosionsurface. The seque;rce is dominated by terrestrial flows of pvroxene andesiteand trachyande- site with some basalt, hornblendeandesite, and dacite. The spherulitesare found in a 5 m-thick pyroxeneandesite flow from the middle of the exposedvolcanic sequence.The flow containspheno- crysts of plagioclase,clinopyroxene (Woas, Fs26, Ena2), and orthopyroxene (Woa, Fs36,En66) in a microcrvstalline groundmass which retains a vague perlitic structure. As the modal analysesof Table 1 indicate, the 2025 2026 STP,PH]iN IJ. KESLI'R AND PAUL W. WDIBLEN

Frc. 1. Location of the Colombier volcanic sequence and the spherulitic andesite describedhere. spherulitic andesite is somewhat more mafic than most flows in the sequence.The spherulitesare found as bands, apparently concentrated along lines of flow, and isolated individuals in all parts of the flow' The diameter of the spherulitesranges from 1.0 to 2.3 mm, but most are about 1.3 mm in diameter.

Dpscnrprron oF THE Spspnutrrns AND GRoUNDMASS Spherulitesexamined in thin section appear to have a crude radial structure definedby feldsparlaths and interfibrillar material (Fig. 2a), although the feldspar cannot be positively identified because of its small grain size. Under high magnification, the fibrous nature of the spherulitecan be seenbest (Fig. 2b), and it is alsopossible to distinguish anhedral massesand subhedralto euhedralgrains of biotite up to 5 p in

T,lrrn 1. Moo.lr Axlrvsns ol Spnnnutrrtc Anotsrtr (M-180) 'lno Orrnn Axorsrrns lRoM TrrE Cor,oMernn VoLcANlc SrQunNcr

Pyroxene Pyroxene Hornblende Andesite Andesite Andesite M 180 M-175 M 186

Piagioclase 26 .tJ 20 Phenocrysts Orthopyroxene tr tr Spherulites t6 Clinopyroxene 8 .) tr Groundmass 4t Hornblende 6 Magnetite L z Groundmass 59 59 71 L,LEMENTS IN A SPHERULITIC ANDESITL 2027 diameter betweenthe fibers (Fig. 2c). Although this biotite is a minor componentof the spherulites,it is much more abundant in the spherulites than in the groundmass and gives the spherulites their character- istic light brown color. Observationsof the cathodoluminescenceduring electron microprobe analysis revealed that the spherulites Ittminesce light blue-greenand contain individual plagioclasefibers, which are about 10 p wide and up to 100pr in length, with bright blue luminescent borders.The compositionof thesefibers, as determinedwith the micro-

Ftc. 2. (a) Two spherulites with straight mutual boundary and unalligned plagioclase and pyroxene phenocrysts, some of which cross the spherulite boundary. (b) spherulite- groundmass boundary showing feldspar frbers(F; wirh interfibrillar biotite masses(arrow) in the spherulite and unalligned plagioclase and rods of hornblende in the groundmass. The border zone is delineated by dashed lines. (c) Euhedral biotite grain (arrow) in interfibrillar area of spherulite. probe rangesfrom a core of approximately Anasto a narrow potassium feldspar (anorthoclase:)rim. Less than 25 percent of the spherulitesis composed of the plagioclase fibers. The remainder of the spherulites consistsdominantly of a material with the composition of potassium feldspar,which appearsfibrous in cathodoluminescence.We have inter- preted these observations to indicate that the spherulites consist of an open aggregate of crudell' radiating plagioclase fibers with abundant interfibrillar potassiumfeldspar and somebiotite. Hematite, a few hornblende (?) needles,and apatite form small grains scatteredthrough the sphenrlites. The groundmass,in which therspherulites are contained, appearsin thin sectionto consistof randomlv oriented feldspar fiberswith a small 2028 STIiPI]DN I'. KESLIiR AND PAUL W. WJ'IBLEN

amount of interstitial hematite,apatite, and small needlesof hornblende. The hornblende needles,which are very scarcein the spherulitesbut abundant in the groundmass,appear to be the groundmassanalog of the biotite which is abundant in the spherulite and almost completely absent from the groundmass.The cathodoluminescenceof the groundmass differs from that of the sphenrlitesin that only the plagioclase fibers Iuminesceclearly. The remainingapproximately 75 percentof the groundmassis dominantlv a cryptocrl'stallinematerial which from re- connaissanceprobe anall'sescontains up to 2 percent total iron as FeO. Both the spherulitesand groundmasscontain phenocrystsof plagio- claseand ortho- and clinopyroxene.No differencebetween the primary magnetite of the spherulites and that of the groundmass was detected in polishedsection. Many of the phenocrystsare only partly included in the spherulites. In most places,the border between the spherulitesand groundmass is a band about 10 prwide. The band consistsdominantly of a weakly birefringent, pale greenish-yellowmineral with anomalousinterference colors,which forms plates oriented perpendicularto the spherulitesur- face. The optical propertiesand the high calcium content obtained in microprobeanalyses suggest that the mineral in theseperipheral bands is a calcic brittle mica or pumpellyite. fn some places,a very narrow band of a tangentially-oriented,greenish-brown, platy mineral, thought to be a low-potassium biotite, is found between the brittle mica (?) band and the spherulite.

Cupurcel Colrposrrrox or,THE Spsrnurrlns AND GRouNDMASS The compositionsof the spherulitesand groundmass,compared in Figure 3, were obtained by atomic absorption analysis of spherulite and groundmassseparates, both of which contained phenocrysts.Be- cause the phenocrystsare randomly distributed within the andesite, any difference in the two analyses can be assumedto represent a com- positional difference between the spherulite and groundmass. The results of theseanalyses indicate potassiumis partitioned strongly into the spherulite whereas sodium and magnesium are depleted in the spherulite. Caicium is not clearly partitioned into either part of the andesite. Electron microprobe analyses verified this partitioning of elements and clarified the nature of the border that surrounds the spherulites. Typical traversesshown in Figures 4 and 5 illustrate the sharp change in potassium content of the matrix from about 9 percent KzO in the spherulitesto about 2 percent K2O in the groundmass.The approximately constant calcium content of the matrix is also shown. The con- ]'LI'MENTS IN A SPHERULITIC ANDESITE 2029

CoO 8 7 6 5 4 z 3 tll 2 Iu I rll 0 G

I I Mgo t! 8 3 7 6 5 4 3 2 I o

Frc. 3. Compositions of the spherulites (S) and groundmass (G)' Both analyses include phenocrysts. centration of potassium in the potassir-rmfeldspar and biotite-rich interfibrillar areasof the spherulite contrasts strongly with the concentration of calcium in the plagioclasefibers. The upper two potassium and calcium traverses illustrate the concentration of calcium and depletion of potassiumin the border zone,whereas the lower traverse illustrates the more unusual two-phase border zone consisting of a relatively potassium-poorbiotite inner zone and a calcium-rich brittle mica or pumpelll.iteouter zone.The border zoneis also enrichedin silicon,iron, and titanium and is clepletedin aluminum, as indicated by reconnais- sance microprobe traverses. A distinct enrichment of silicon in the spherulitesand titanium in the groundmasswas also detected. fron appeared to be present in approximately equal amounts in both the spherulitesand groundmass.

CnvsranrzerroN or rnB Sprtrtutttps By relating observationsof spheruliteformation in pol,"-mersto studies of crystallization in metals (Elbaum, 1959), Keith and Padden have STEPHEN D,. KESLER AND PAUL W. WEIBLEN

q 2 A. POTASSIUM o N 4 r o 4 (oo

SPHERUTITE ------>GROUNDMASS m-reo-r.r

------> GROUNDMASS M-r80-2 -2 to 8 4 EU az 6 o c{ 5R !a o 4 l{

2 o

SPHERUTITE+- :------) G ROUN DMA SS 00lmm

Frc. 4. Electron microprobe potassium traverses across spherulite-groundmass contact. The traverses are oblique to fibers and border zone and therefore do not sholr,' true dimensionr

proposed (1963), and partlv verified (1964a,b), a theory of spherulite formation. These investigators have pointed out that all spherulite- forming melts are multicomponent svstems which crystallize slowly and have low diffusionrates. They indicate that, as thesemelts crystallize, the componentsnot included in the growing phase accumulateat the crystal-liquid interface, thereby lowering the liquidus temperature of the melt in this region. Becausethe area near the growing crystal would be less supercooledthan the more distant melt with smaller amounts of impurities, protrusionson the surfaceof a growing crystal should develop more rapidly than planar surfaces,thereby causing a fibrous habit to develop, (Keith and Padden, 1963).Lateral crystallization and consequentwidening of the fibers is restrictedbv the high impurity content of the residualinterfibrillar melt which lowersboth the liquidus temperature and nucleation rates. Keith and Padden have shown that the thickness,d, of the impurity-rich layer is definedby IJLEMENTS IN A SPHTIRULITIC ANDESITL' 2031

e B. cAtctuM az 5R @

SPHERUIITE- - GROUNDMASS M'r8o-r'l r2O it. 812{f' lq I [6

:20 -t6 :a< G il2 tsq 9L ifs T :o LO SPHERU.TTE+ ,oo,--, ;,;..:ff,'I""1.t"t,.,.

Frc. 5. Electron microprobe calcium traverses across spherulite-groundmasscontact. 'fhe traverses are oblique to fibers and border zone and therefore do not sholv true di- mensions.

6: D/G where D is the diffusion coemcient of the impuritv and G is the growth rate of the spherulite. They have also indicated that the diameter of the resulting fibers will be approximately equal to A. According to this interpretation of spherulitegrowth, potassiumand magnesium are impurities which were expelled into the interfibrillar areasof the spheruliteas the plagioclasefibers grew. The strong partitioning of sodium into the groundmassand the conccntrationof calcium in the border zone suggestthat sodium and some calcium were also expelled from the spherulite. Apparently, potassium was sufficiently concentrated in the interfibrillar melt to allow nucleation of potassium feldsparand biotite, thereby fixing most of the potassiumin the spherulite. The small amount of potassium that did not form these minerals was concentratedas the inner biotite border found around somespheru- lites, Sodium, magnesium,iron, titanium and the excesscalcium, which did not nucleatestable phaseswithin the interfibrillar areas,were con- 2032 STDPIIl'N E. KESLER ANI) PAUL W. WEIBLEN sequently expelled as potassium feldspar and biotite crystallized and were concentratedin the border zone of the spherulite. Calcium is strongly concentratedin the spherulite border zone (Fig. 4). This suggeststhat it was expelledfrom the spherulites;however it shows no preferential concentrationin the groundmass (Fig. 3). The explanationfor this might be that only a small amount of calcium was expelledfrom the spherulite and this was all retained in the border zones. The relatively low potassiumconcentration in the groundmassdoes not agreewell with the interpretation of spheruliteformation given above. The absenceof a negativeconcentration gradient around the spherulites makesit unlikely that the spherulitedepleted the groundmassin potassium. Instead, it is more plausible that the potassir-rmcontent of the rock was originally high and that most of this potassiumwas retained in the spheruliteswhereas it was lost from the groundmassafter spherulite formation. Although minor potassiumfeldspar and somepotassium- rich borders on plagioclasewere detected by microprobe analyses,no evidence could be found in the groundmassfor the past or present existenceof an abundant potassium-bearingphase. This suggeststhat the potassiumwas removed beforethe groundmasshad devitrified, and therefore relatively early in the cooling process.Thus, the potassium cor.rldhave been strongly partitioned into either a late escapingvolatile phaseor percolatinggroundwater through a processsimilar to that dis- cussedb1' Scott (1968). The possibilitv that. potassium escapedfrom the groundmassindicates that the distribution of elementsobserved in the sphenrlitic andesiteis influencedby secondar)'processes,as well as by the primarv processof spheruliteformation. The Agate Point obsidian, which was the subject of a controversy between Tanton (1925), Bain (1926), and Greig (1928), contains spheruliteswith borderssimilar to thosedescribed here. In an investiga- tion in which he concludedthat the spheruliteswere cognatexenoliths, Bain painstakingly analyzed the groundmass,spherulites, and border zone (including phenocrysts).The results of his analyses,calculated as water free,are shownin Figure 6. Calcium, potassium,iron, magnesium, and possibly aluminum were expelled during crystaliization of these spherulites,whereas sodium and silicon were apparently 1g1.ir.d. The relatively minor degreeof partitioning shown by these elementsmay be a function of the low diffusion rates in highly siliceousmelts. The distribution of the elements,however, agreeswith that predicted by Keith and Padden for a casein which the dominant phasesforming in the spherulite are qtartz and sodic plagioclase,rather than inter- LL]1M]JNTS IN A SPIIERULITIC ANDESITE 2033

FerOa (totol iron)

4 p3 z2 Sr ffo SB =F ogo t4 u70 l3 >;; l2 77 ll 76 lo 75 9 5 74 I 4 73 7 3 72 6 2 7l 5 I 70 4 o

Frc. 6. Compositions of spherulites (S), border zone (B), and groundmass (G) of the Agate Point vitrophyre. Analyses are calculated as water free from data of Bain (1926). mediate plagioclase,potassium feldspar, and biotite as in the andesite describedhere.

Gnowrn Rarr or rHE SPHERULTTES The width, 6, of the impurity-rich border zone in the spherulitic andesiteaverages about 10 p. The diffusion coemcient,D, of the impurities in the melt can be estimatedby extrapolatingthe data obtained by Towers and Chipman (1957) for the diffusion coefficientof calcium to the probabie temperature of formation of the spheruliteswhich can be estimatedby consideringmorphology and distribution of the spheru- Iites and the coolinghistory of the flow. The diameter, D,of the plagioclasefibers is approximately constant from center to rim of individual spherulitesand from spheruliteto spherulitethroughout the flow. Thus, the ratio D/G must have been constant during spherulite formation. This condition would have been fulfilled if D and G remainedindividu- ally constant during spherulite formation or if they varied equally with time. Keith and Padden (196aa)have shown that for an1,change in temperature, changesin D in polymer melts will be much greater than changesin G, and it seemslikell' that this will be the case in spherulite-formingsilicate melts. Therefore, D and G must have re- mained constant, which requiresthat spheruliteformation have taken place at approximately constant temperature. This would have been possibleonly during the earliestphase of cooling at about 900-1000'C, STEI'IIL,N E. KESLL.R AND PAUL V/, WEIBLEN

or very late in the cooling history at 200-300"C.Because the diffusion rate in the andesiteglass would have decreasedwith temperature and reacheda very small value at 200-300'C,it is most likely that spherulite formation took place at 900-1000oC.The extrapolatedvalue of D for this temperature range is 2X10-s cm2/sec,which gives an estimated growth rale, G, of about 10-6 cm/sec, or a total growth time of 1 to 2 days. Greig (1928) has reported that spherulitesup to 3/4 in. in diameter formed in artificial glass in about two days which suggestsa slightly greater growth rate than that estimated here. In experimentson the devitrification of rhyolite glassat temperaturesof 240-650"Cand water pressuresup to 4 kbar, Lofgren (1967) reported devitrification rates of 10-7 cm/sec in the presenceof alkali-rich solutions in contrast to devitrification rates of 10-10cm/sec in the presenceof pure water. Al- though the conditionsof Lofgren'sexperiments differ greatly from those at which the spherulitesdescribed here are thought to have formed, his resultsindicate that an alkali-bearingfluid phasefacilitates spherulite growth. This fact, as well as the high temperaturesestimated for spheruliteformation in the andesiteemphasize the previously suggested possibility that the low potassiumcontent of the groundmasscould be the result of the escapeof a potassium-bearingfluid during and after spheruliteformation.

Concr-usroNs The resultsof this study indicate that spheruliteformation in silicate melts resultsin the partitioning of elementsbetween the spheruliteand groundmassas well as the concentrationof ttimpurity" elementsinto areasbetween fibers in the spherulite and in a peripheral zone around the spherulites.These features have been noted in polymer melts and have been predicted to be in other spherulite-formingsystems (Keith and Padden, 1963). The spherulites describedhere formed at about 900 to 1000"Cand grew at the rate of 10-Gcm/sec.

AcrNowr,rlclmNrs

G. K. Billings and R. E. Ferrell furnished helpful advice during the atomic absorption and X-ray difiraction analyses. G. E. Lofgren kindly called the writers'attention to the publications of Keith and Padden referred to in this paper and along with S. A. Heath and R. E. Ferrell, provided stimulating discussionand criticism of the ideas presented here.

RnlnnnNcrs

BarN, G. W. (1926) Difiusion in the Agate Point vitrophyres. Anter. I. Sci., 11,74 88. Er.eeuu, C. (1959) Substructures in crystals grown from the me1t. Prog. Metal Phys,ics, 8. New York. DLEMENTS IN A SPHERULITIC ANDESITE 2035

Gnorc, J. W. (1928) On the evidence which has been presented for liquid silicate immisci- bilityin the laboratory and in the rocks of Agate Point, Ontario. Amer. J.\ci.,265, 373-402. Krrrrr, H. D., ero F. J. Peonnx (1963) Aphenomenological theory of spherulitic crystallization. J. AppI. Pkys., 34, 2409-2421. - (196+a) Spherulitic crystallization from the melt: I. Fractionation and impurity segregation and their influence on crystalline morphology. I. Appl. Phys.,35, 1270- 1285. - (1964b) Spherulitic crystallization from the melt: II. Influence of fractionation and impurity segregation on the kinetics of crystallization. f . Appl,. Phys.,35, 1286- 1296. Krsrnn, S. E. (1966) Geology anil ore d.epositsoJ the Meme-Casseus d.'istrict, Haiti. Ph.D. diss, Stanford University. (1968) Mechanisms of magmatic assimilation at a marble contact, northern Haiti. Lithos, l, (in press). Lorcnrr, G. (1967) Effect of alkalirich solutions on the devitdfication rate of rhyolite glass (abstr.). Prog. Geol. Soc.Amer. Ann. Meet., 132. Scorr, R. B. (1968) Alkali exchange during hydration and devitrification of volcanic glass from Oligocene ignimbrites, central Nevada. Trans. Amer. Geophys.fInion,49, 350. TaNrox, T. L. (1925) Evidence of liquid immiscibility in a silicate magma) Agate Point, Ontario. J. Geol.. 33. 629-641. Towtns, H. aNo J. Cnrpuer (1957) Diffusion of calcium and silicon in a lime-alumina- silica slag. Trans. AIME 209, 769-773.

Manuscript receited, Febntary 19, 1968; accepteilfor publication, Jul,y 23, 1968.