An Aenigmatite-Richterite-Olivine Trachyte from Puu Koae, West Maui, Hawaii

American Mineralogist, Volwne 63, pages 771-778,1978

An aenigmatite-richterite-olivinetrachyte from Puu Koae, WestMaui, Hawaii

DeNtnlln Vnlon Laboratoirede Pbtrographie,Uniuersitb Paris VI 4 placeJussieu, 75230 Paris Cedex05, France

Abstract

The Puu Koae trachyteforms a conspicuousbulbous dome on the north shoreof West Maui. This trachyte has a peraluminouscomposition, but containsalkali amphibole,aenigmatite,iron-rich olivine, and sodicclinopyroxene. This mineralogicalassociation indicates that the trachytecrystallized from a peralkalinemelt, and that its compositionchanged at the time of crystallizationby lossof sodium.The conclusionis that peralkalinesalic liquids have beenproduced at one stageof the evolutionof the islandof Maui.

Introduction Description The geologyof West Maui has beendescribed by The Puu Koae trachyteforms an obviousbulbous Stearns and Macdonald (1942). West Maui is a domewhich is a well-knownlandmark on thenorth- mainly basalticvolcano, the rocksof which,known westerncoast of the island of Maui. The sample as the Wailuku volcanicseries, are almostentirely examinedwas taken from the southeasternside of the tholeiitic.The youngestflows, however, unquestion- dome.In hand specimenthe lava has a chalky un- ablybelong to the alkali suite.The Wailukuseries is inviting aspectwhich certainlyjustifies the fact that "overlain by a thin cap of mugeariteand trachyte the outcropis singledout as a good opportunityfor namedthe Honoluavolcanic series." Postdating the the observationof erosionalfeatures, in particular Honolua seriesare a few basaniteeruptions, the La- the etchingof the rocksby oceanspray (Stearns and haina volcanic series(Macdonald and Katsura, Macdonald, 1942). The rock appears completely 1964).The Puu Koae trachyte,the subjectof this crystallized,with obvious white feldspargrains in a note, belongsto the Honolua series.A samplecol- brownishspotted groundmass. lectedat Puu Koae containsaenigmatite, alkali am- Under the microscope,it showsa typicaltrachytic phibole,olivine, and a sodium-richpyroxene, a type texture, most alkali feldsparlaths being orientedin of mineralogicalassociation not hitherto reported the samegeneral direction. There is an obvioussize from the Hawaiianarchipelago. Mentions of alkali- discontinuitybetween the largest(2mm X 0.3mm) rich mineralscan be found in the literature.Washing- and the smallerfeldspar crystals. All the phenocrysts ton (1923,p. 107),for example,noted the possible havelamellar polysynthetic twinning (Fig. l), as do presenceof colorless(?) acmitein the Puu Anahulu most of the groundmasscrystals. Feldspar is, of trachytefrom the islandof Hawaii, though this find- course, the major constituentof the lava. In any ing hasnever been confirmed. Macdonald (in Stearns petrographicthin section there are usually a few and Macdonald, 1942) noticed the presenceof a ratherlarge (0.5mm X 0.3mm)olivine crystals (Fig. "riebeckite-like"alkali amphibolein some trachyte 2), freshbut for a narrowand continuousiron-oxide and "andesites"from WestMaui, and in particularin rim. More numerousare sections of alkaliamphibole the Puu Koae trachyte, where he found it accom- that canreach 0.7mm in length(Fig. 3);these amphi- panied by abundant acmite.In spite of thesebrief bole crystalsare usuallypartly moldedaround large observations,no definiteperalkaline lava is knownin feldsparlaths, but can also show regularcrystal out- Hawaii, and a recent survey of the occurrencesof lines.Their polychroismis quitevariable, in coloras peralkalinesilicic rocks in the oceanicislands (Baker, well as in intensity,both within a singlecrystal and 1975)understandably does not evenmention the ar- from crystal to crystal. Aenigmatite is abundant, chipelago. forming mostlysmall (0.l5mm X 0.05mm)crystals; 'l'7 ffio3-o0/'x / 7I /0708-077l $02.00 | VELDE: TRACHYTE FROM HAWAII

Mineralogicalcomposition All mineralshave been analyzed on the Geophysi- cal Laboratoryautomated electron microprobe (Fin- ger and Hadidiacos,1972), with natural or synthetic mineralsas standards. Feldspars Both largephenocrysts and groundmassfeldspars wereanalyzed, and Figure4 showsthe rangeof compositionsfound in terms of albite, anorthite,and orthoclaseend-members. Particular emphasis was put on the unusualCa-rich phenocryst composition, so that the numberof pointsin the diagramis in no way relatedto the modal proportionsof the two varietiesof feldsparsin the rock. Table I contains analysesof the two typesof compositions. The phenocrystscorrespond to an oligoclasewith 5 to 6 percentOr in solid solution.They are consis- tentlyrimmed by alkali feldsparwhose composition is similarto that of groundmassfeldspars. They do not give any evidenceof being xenocrysts,nor do

Fig. l. Oligoclase phenocrysts, with regular crystal outline and polysynthetictwinning Length of the bar is 0.15mm.

clinopyroxene is very rare, forming minute prisms (lessthan 0.02 mm), usually developed around un- identified rusty grains. Patchesof interstitial brown isotropic material are present and may perhaps be altered glass. Magnetite forms small- or medium- sized (0.2mm) crystals with regular outlines. The order of crystallizationcannot be unequivo- cally determined.Textural relationshipsnevertheless indicate that the large feldspar phenocrysts(oligoclase),olivine, and magnetitewere followed by alkali amphiboles; among these richterite, with weak pleochroism,appeared before arfvedsonite. Sodic pyroxene was, after alkali feldsparand aenigmatite,the last phase to crystallize. Mention should also be made of two former phenocrysts,one hexagonal in shape, the other an elongated rectangle, that have been entirely altered to finely-granulariron ore, and could be pseudomorphs Fig 2. Olivinecrystal, rimmed by iron oxides.Length of the bar after biotite. is0.l0mm. VELDE: TRACHYTE FROM HAWAII I lJ

opticalvariations in birefringencewere observed, and the differencesin compositionsbetween the center and the edgesof the crystalsare very small.There is a positivecorrelation betweenFeO, MnO, and CaO, theseelements being inverselycorrelated with MgO. The composition varies from Fos.Fa.rT% to ForrFa.uTg,the high tephroitecontent being some- what unusual.The MnO contentof olivinesfrom the Mount Suswatrachytes and phonolites,for instance, variesbetween 3.1 and 3.8 percent (Nash et al.,1969), for similar fayalite values. However, Barker and Hodges(1977) have recently found in nephelinesye- nites similar or higher MnO percentagesin olivines containing77 to87 percentfayalite in solidsolution. Aenigmatite Aenigmatitehas frequentlybeen reported in oversaturatedperalkaline rocks. Analysestabulated by Sutherland(1975) show very little compositionalvar- iation in this mineralin extrusiverocks. Analyses reported in Table 3 appearto be higher in MnO, Al2Os,and MgO, and lower in TiO, than most re- cordedso far in volcanicrocks. The highmanganese valuesare in keepingwith the high MnO contentof all the other iron-bearingphases analyzed in the Puu Koae trachyte.Variations between crystals are small, but it wouldappear (Fig. 5) that low TiOzvalues are associatedwith high MnO, and perhapshigh total iron. New analyseslisted by Barker and Hodges Fig. 3. Richterite(grey) and alkali feldsparlaths. Length of the (1977;appendix Table 3) showvariable Ti and Mn bar is 0.lOmm. contentsbut do not conform to the pattern found in Puu Koae. It appearsthat analysesare still not nu- they show any indicationof corrosion(Fig. l). The merousenough to supporta particularsubstitution groundmassfeldspar, on the other hand,is a potas- schemefor volcanicaenigmatites. sium-bearingalbite. The last-formed crystals, the However, a considerationof published analyses richestin potassium,contain virtually no Ca and (Sutherland,1975; Yagi and Souther,1974; Borley, cluster around OrzuAbruto Ors.Ab'.. They do not 1976) seemsto indicate a higher TiO, content in reach the most frequentcomposition found in per- aenigmatitesfrom rocks with a high contentof nor- alkalinerocks (Or3uAbru; Bailey, 1975). The spreadof mative quartz,the highestTiO, content(10.23 per- compositionsfound is strikinglysimilar to that recent) being recordedin a peralkalinegranite with portedby Keil et al. (1972)for feldsparsin a trachyte 33.56percent normative quartz (Borley, 1976;Jacob- from theUkumehame dome, also from WestMaui. It sone/ al.,1958). appearsfrom Macdonaldand Katsura'sreported analysesthat their sampleC 128(from Puu Koae) R i cht eri t e-arfu ed s onit e amphibo le s and sampleC 116 (from Ukumehame)have very Alkali amphiboleis the mostabundant iron-bear- similar bulk compositions.However, the alumina. ing phasein the Puu Koae trachyteand showsthe deficientphases described here have not been re- greatestchemical variation. The polychroismvisible cordedin the Ukumehametrachyte. in thin sectionis much deeperin the ircin-richlast- formed arfvedsonitethan in the richterite:richterite Oliuine forms large crystals,whereas arfvedsonite forms ei- Table2 givesanalyses of the two crystalsof iron- ther small crystalsor rims the earlier-formedrichte- rich olivine found in the Puu Koae trachyte.No rites. Colors vary from pale yellow to purplish-pink '174 VELDE. TRACHYTE FROM HAWAI|

Ab 10 20 30 An

Fig. 4. Electron microprobe analyses of feldspars represented in terms of their Ab, An, and Or contents (mole percent). in richteritesto dark greenish-bluein the arfvedson- dicatingthat the Ukumehametrachyte is not per- ite. Table 4 lists representativeanalyses of the range alkaline. of compositionsfound, and Figure 6 illustratesthe variationsfound, which spanthe greaterpart of the Iron-titanium oxides richterite-arfvedsoniteseries of compositions.Partic- Iron-titaniumoxides are relatively abundant in the ular effort was made to find out if therewas a com- trachyte,sometimes forming euhedralcrystals of var- pleteseries between the two end-members;it appears iablesize. They are rich in TiO, and MnO (Table5). from the 29 analysesmade that thereare indeed two In spite of their high ulvbspinelcontent (above 60 clusters,corresponding to richteriteand to an amphi- mole percent),they are free of exsolutiontextures. bole containing60 to 70 percentof the arfvedsonite They are similarin this respectto someof the mag- component.Fluorine has been found in similaram- netitesfound in the peralkalinelavas from Mount phibolesfrom oversaturatedperalkaline lavas (Nich- Suswa(Nash et al., 1969).Small magnetitecrystals olls and Carmichael,1969), but wasnot determined are sometimesincluded in.aenigmatite, but thereap- in the courseof the presentstudy. pearsto be no specialor necessaryspatial relation- shipbetween these two phases,and no evidenceof a Clinopyroxenes reactionrelationship. Clinopyroxeneis rare and forms very small eu- hedral crystals.The minute size of the grains pre- Chemicalcomposition of the Puu Koaetrachyte vented a preciseanalysis; however, compositions Table 6 lists a new analysisof the Puu Koae found would indicatethe presenceof hedenbergitetrachyte,along with theanalysis previously published with 50 percentacmite in solidsolution as well asthe by Macdonaldand Katsura(1964; specimen C 128). existenceof pureacmite. Fodor et al. (1975)reported The striking peculiarityof theseanalyses is their per- analysis of clinopyroxenesin the Ukumehame aluminouscharacter. From all previousknowledge, trachyte.They are very differentfrom thosefound in and as forcefullystated by Macdonald(1975), "the Puu Koae, and are solidsolutions between diopside mostobvious indication of peralkalinityis themodal and hedenbergite.This could be interpretedas in- presenceof certainindex minerals,of which alkali VELDE: TRACHYTE FROM HAWAII 775

Table 1. Electronmicroprobe analyses of feldspars Table3. Electronmicroprobe analyses of aenigmatite

SiO2 tfi.25 4L.75 40.50 si02 63.39 66.63 66.44 66.76 A12o3 1.25 .56 1.11 A12O3 22.52 18.91 20.51 18.69 FeO * 39.74 tn.36 41.09 FerO, * .16 .63 .36 .50

a^6 a ?q Mgo .t7 1.41 .12 1.71 I.25 7.57

NarO 8.83 8.41 9.40 CaO .41 .18 .34

K20 1.OO 4.66 2.54 4.82 NarO 7.35 7.43 7.27

BaO .26 .o4 .21 .oo MnO 2.Oz 2.56 2.14

Tolal 99.9L 99.46 100.88 99.26 Tio2 6.78 5.O7 5.57 si 2.816 2.985 2.924 2.996 Total 99.51 99.t6 99.36 A1 t.178 .998 1.063 oe7 ?! Fe"' .OO5 .o2I .011 .016 5a 11.383 11.856 lL.458 Ca .778 .oo8 .065 .oo5 AI .4L7 .187 .370 Na .7 60 -3+ .737 .802 .728 te 1.963 2.027 2.331 K .056 .265 ,142 aa\ te 7.436 7.558 7.390 Ba .OO4 .oo0 .oo2 .000 Mo .721 .529 .662 .124 .o55 .103 * Total iron as Fe2O3 Na 4.030 4.O90 3.988 1 - Center of oligoclase phenocryst Mn .484 .616 .513 2 - Edge of same Ti 7.442 1.O83 1.185 3 - Center of microphenocryst

4 - Small groundmass crystal * Total iron as FeO K, V, and Cr below detection I imi rs.

Structural formulas calculaEed assuming Table 2. Electronmicroprobe analyses of olivines 28 cations and 40 oxygens.

pyroxene,alkali amphiboles, and aenigmatite are the mostcommon." As far asis known theseindex min- 33.08 32.54 sio2 si .995 .996 eralscan crystallizeonly from a liquid with a per- tL FeO* 49.07 51.25 Fe-' 1.234 r.313 alkalinechemistryl. Two possibilitieshave to be con- sidered:either the liquid was originallyperalkaline, MgO 14.25 11.83 Mg .639 . )qu or else the early crystallizationof a peraluminous CaO .47 Ca .Ot2 .ot7 phasegave it a secondaryperalkaline chemistry. This MnO 4.82 5 ?O Mn .123 .136 latter possibilitymust be entertainedwhen, as is the caseof the Puu Koae trachyte,a rock with a per- Total 1O1.63 101.43 alkaline mineralogy contains plagioclasepheno-

I An exceptionto this rule arethe pureacmite crystals deposited t* Total iron as FeO on the innersurface ofvugs, in somebasalts for example,that can be consideredas depositedby a vapor phase(e.9. Smith and - Center of crystal Lindsley,1971, p.285). But thetexture of the PuuKoae trachyte - Edge of cryscal does not support such an origin for its constituentalkali-iron- bearinsminerals. 776 VELDE, TRACHYTE FROM HAWAII

Ti Na+K

3.0

2.O

2.5 '.i, 1.5

2.O ,. (,^R 1.0 '*b

o.5 o.5 o.5 1.0 Mn 1.o Ca Fig. 5. Variation of Ti uerszsMn in analyzed aenigmatites from Fig. 6. Variation of (Na I K) uersus Ca in alkali amphibole Puu Koae. Open triangle is cossyrite from Pantelleria (in Zies, analysesfrom Puu Koae. Electron microprobe analyseshave been 1966). recalculated on the basis of 23 oxygen atoms; no cotrection was made to account for the obvious presence of ferric iron in crysts;the appearanceof fleldsparcontaining a cer- arfvedsonite. A = arfvedsonite; R : richterite. tain amount of the anorthitemolecule (the plagio- claseeffect) could inducea deficit in AlrO, with re- However, the observed modal abundance of oligo- spectto alkalis.Such a processhas been described in clasein the Maui trachyte (Table 6) is insufficientto a phonoliteby Carmichael(1964) and mentionedby account for the appearanceof the typical peralkaline Villari in a trachytefrom Pantelleria(1975, p.712). minerals. The unusual composition of the feldspar phenocrysts neverthelessdoes suggest that the per- Table 4. Electron microprobe analyses of richterite and alkalinecharacter ofthe initial liquid could havebeen rrfvedsonite

Table 5. Electron microprobe analysis of titanomagnetite

sio2 51.01 si 7. s98 7.867 sio2 .2L AI2O 1.97 Al 107 3 1.11 .332 A12O3 .30 AL * )) aa FeO 16.60 Fe'' 2.067 2.862 FerOr* 26.10

q MgO 12.69 at Mg 2.815 1.896 FeO* 48.05

q, CaO 6. 50 1 Ca 1.036 .296 M9o .24

Naro 5.48 8.20 Na 1.583 2.4J5 TiO^ 2r-59

K2O 1.27 1.18 K .227 .229 MnO 2.77

MnO 1.O9 1.72 Mn .135 .220 'rota1 99.26

Ti02 1.48 r.45 Ti .t62 .166 Percentage ulvospinel = 62?

Total 97.96 9 t.54 Percentage magnetite = 388 *FeO and Fe2O3 calculated fron total iron as detemined *ToEal iron as FeO by electron microprobe analysis, and the structural

Richterite ; 2 - Arfvedsonite. formula. VELDE: TRACHYTE FROM HAWAII

Table6. Chemicalcomposition, CIPW normsand modal analysis Acknowledgments of the Puu Koae trachyte The samplewas collected during a vacationtrip to the Hawaiian Islandsin the summerof 1976,and I thank BruceVelde for his unfailingpatience. The mineralanalyses were made during a short sio2 59.82 60.85 1.74 1.04 Q stayat the GeophysicalLaboratory of the CarnegieInstitution of Ar2o3 t8.07 18.51 Or 20.15 21.2a Washington;I gratefullyacknowledge the hospitalityextended by helpgiven by Dr. D. Rumble Fe2O3 4.06 3.10 Ab 62.37 60.93 its Director,Dr. H. S. Yoder Jr., the III with the identificationofoxide phases,and the technicalassist- FeO 1.71 z,Oa An 3.16 6.89 anceprovided by C. Hadidiacosand D. George.The manuscript ugo 0.53 o.58 Hy 7.32 2.13 of this paperwas kindly reviewedby Dr. FelixChayes of theGeo- physicalLaboratory and Dr. R. Macdonaldof the Universityof CaO O.9O L.77 He 1.19 Lancaster(Great Britain). Na2O 7.37 7.20 ltr 4.16 4.49

KzO 3.47 3.60 11 1.48 1.23 References UnO O.28 o.27 Ap .46 .67 Bailey, D. K. (1975) Experimentalpetrology relating to over- rioz o.78 0.65 C 1.10 .24 saturatedperalkaline volcanics: a review.Bull. Volcanol.,38, P2A5 O.2O o.29 637-652. Baker, B. H. and L. F. Henage(1977) Compositional changes H2O+ 2.O7 o.47 during crystallizationof some peralkalinesilicic lavas of the H2O- O.55 o.42 Kenya rift valley.J. Volcanol.Geothermal Res., 2, 17-28. Baker, P. E. (1975) Peralkalineacid volcanicrocks of oceanic Totals 99.75 99.81 islands.8ul/. Volcanol.,38, 73'l -7 54.

Modal Analysis : Oligoclase (phenocrysts) 3.47. Barker,D. S.and F. N. Hodges(1977) Mineralogyof intrusions in

Alkali feldspars (groundmss) 88. 1% the Diablo Plateau,northern Trans-Pecos magmatic province. lvlagoetiEe Texasand New Mexico. Geol.Soc. Am. Bull., 88, 1428-1436.

Aenig@EiEe 2.4% Borley, G. D. (1976) Aenigmatitefrom an aegirine-riebeckite granite, Nigeria.Mineral. Mag., 40, 595-598. Alkali Amphiboles 2.8% Liruei Complex, I. S. E. (1964)Natural liquids and the phonolitic Cl inopyroxene Carmichael, minimum.Geol. J.,4, 55-60. Ol lvine <.r7" Finger,L. W. and C. G. Hadidiacos(1972) Electron microprobe I - Puu Koae trachyte. New analysls M. Lenoble and N. Vassard, automation. CarnegieInsl. Wash.Year Book, 71, 598-600. Paris,1977 Fodor.R. V.. K. Keil andT. E. Bunch(1975) Contributions to the 2 - Puu Koae trachyte; sample C. 128 (Macdonald and Katsura, mineral chemistryof Hawaiian rocks IV. Pyroxenesin rocks 1964) from Haleakalaand WestMaui volcanoes,Maui, Hawaii.Con- trib. Mineral.Petrol.. 50. l7f-196. Jacobson,R. R. E., W. N. Macleodand R. Black(1958) Ring- complexesin the youngergranite province ofnorthern Nigeria. lesspronouned than that indicatedby the bulk com- Geol.Soc Lond. Mem- I. positionof peralkalinerocks in whichplagioclase is Keil.K.. R. V. Fodorand T. E. Bunch(1972) Contributions to the typically absent. mineralchemistry of Hawaiianrocks II. Feldsparsand inter- in rocks from Haleakalaand West Maui volca- liquid from stitial material The probableconclusion is that the noes.Maui. Hawaii. Contrib.Mineral. Petrol., 37,253-2'16. which the trachytecrystallized was peralkaline,that Macdonald,G. A. andT. Katsura(l964) Chemicalcomposition of its compositionwas modified at the time of crystalli- Hawaiianlavas. "/. Petrol.,5, 82-133. zationand probably after, and that the presentbulk Macdonald,R. (1975)Nomenclature and petrochemistryof the rock compositiondoes not adequatelyrepresent this peralkalineoversaturated extrusive rocks. Bull. Volcanol, 38, 498-516. liquid. Macdonaldand Bailey(1973) and Bakerand - and D. K. Bailey(1973) Data of Geochemistry,sixth edi- Henage (1977), among others, have summarized tion, chapterN. Chemistryof igneousrocks. U. S. Geol.Suru. some of the effectsof crystallizationon peralkaline Prof. Pap.440-N-1. compositions,listing among the major changesthe Nash.W. P.. I. S. E. Carmichaeland R. W. Johnson(1969) The lossof sodium.The analyseslisted in Table 5 show mineralogyand petrologyof Mount Suswa."/. Petrol.,10,409- percent would 439. that additionof lessthan 1.5 NarO be Nicholls,J. andI. S.E. Carmichael(1969) Peralkaline acid liquids: enoughto give the Puu Koae trachytea mildly per- a petrologicalstudy. Contrib Mineral. Petrol.,20,268-294. alkalinecomposition. This amountis well within the Smith,D. and D. H. Lindsley(1971) Final products of thediffer- range of sodium lossesreported on samplesfrom entiation process.Carnegie Inst. V[/ash.Year Book,69,284-285. (1942) and Pantelleriaby Macdonaldand Bailey(1973, p. N2); Stearns,H. T. and G. A. Macdonald Geology groundwaterresources of the islandof Maui, Hawaii. Teritory the mechanismcould be expulsionof sodium-rich of Hawaii, Dioision of Hydrography. Bull. 7. fluids(Baker and Henage,1977). Sutherland,D. S. (1975)Petrography and mineralogyof the per- 77E VELDE: TRACHYTE FROM HAWAII

alkaline sili€ic rocks. Bnl/. Volcanol., 38. 5l'l-547. BritishColumbia, Canada. Am, Mineral., 59,820-829. Viltari,L. (1975)The islandof Pantelleria. Bull. Volcanol.,3S,6g0- Zil-s,E. G. (1966)A new analysisof cossyritefrom the islandof 724. Pantelleria.Am. M ineral., 5l, 200-205. Washington,H. S, (1923)Petrology of the HawaiianIslands, II. Huafalaiand MaunaLoa. Am. J, Sci.,6,100-126. Manusoipt receioed,December 29, 1977;accepied Yagi, K. and J. G. Souther(1974) Aenigmatite from Mt. Edziza, for publication,February 7, 1978.