Pyroxene chemistry and evolution of alkali basaltic rocks from Burgenland and Styria, Austria

/ r t l - l i I L1 K RAe] l t r_, . t l

Mon uskr. N'" """""4"9-.{5--....pp- (_/.*)

Mineralogy and Petrology (1991) 43:275-292Mineralggy

anoPetrology

@ by Springer-Verlag 1991Printed in Austria

Pvroxene Chemistrv and Evolution of AlkaliBasaltic Rocks from Burgenland and Styria, Austria

G. Dobosil, R. Schultz-Giitt ler2, G. Kurat3, and A. Krachera

l Hungarian Academy of Sciences, Laboratory for Geochemical Research,Budapest, Hungary2Instituto de Geociencias, Universidade de Sao Paulo, Sao Paulo, Brazil3 Naturhistorisches Museum. Wien. AustriaaDepartment of Earth Sciences, Iowa State University, Ames, Iowa, U.S.A.

With 8 Figures

Received July 2,1990;accepted December 28, I99O

Summary

The mineral chemistry of several Pliocene alkali basaltic rocks from Burgenland and

Styria (Eastern Austria) have been investigated in order to determine the evolution path

of the basalt magmas prior to eruption. With their wide range of substitutions, clino-pyroxenes provide the best records of the evolution history of rocks. Pyroxene pheno-

crysts ofthe investigated basalts show both concentric and sector zoning. The investiga-tion of sector zoned crystals shows, that not only Ti, Al and Fe contents are different in

different sectors but there can be signihcant differences also in their Cr content. This fact

apparently suggests that the distribution of Cr between clinopyroxene and melt could beinfluenced by crystallization kinetics.

The depth of crystallization and differentiation of the basalts can be estimated fromTi and Al contents of clinopyroxene phenocrysts. From a combination of data onclinopyroxene composition, compatible trace element contents and mg-values of therocks, it is concluded, that the alkali basalts of Pauliberg and Steinberg underwent slightolivine and clinopyroxene fractionation in shallow magma chambers prior to eruption,while the nephelinite of Stradnerkogel evolved mainly through clinopyroxene fractiona-

tion under high pressure conditions, probably in the upper mantle.

Zusammenfassung

Chemie der Pyroxene und Entwicklung uon Alkalibasalten aus dem Burgenland und derSteiermqrk, 6 sterreich

Einige plioziine alkalibasaltische Gesteine aus dem Burgenland und der Steiermarkwurden mineral-chemisch untersucht, um AufschluB iiber ihre Evolution vor der Erup-

276 G. Dobosi et al.

tion zu erhalten. Klinopyroxene mit ihren vielfiltigen Subtitutionsmtiglichkeiten erlau-ben am besten eine Abschritzung der Evolution der Basalte. Pyroxen-Einsprenglinge derBasalte zeigen sowohl konzentrischen als auch sektoralen Zonarbau. Die von unsuntersuchten Einsprenglinge zeigen in den verschiedenen Sektoren nicht nur unter-schiedliche Gehalte an Ti. Al und Fe. sondern vielfach auch unterschiedliche Cr-Gehalte.Dies macht es wahrscheinlich, daB die Verteilung von Cr zwischen Klinopyroxen undSchmelze von der Kristallisations-Kinetik beeinfluBt wird.

Die Tiefe in der die Basalte kristallisierten und differenzierten kann von den Ti- undAl-Gehalten der Klinopyroxen- Einsprenglinge abgeschdtzt werden. Die Zusammen-setzung der Klinopyroxene im Verein mit den Gehalten an kompatiblen Spurenele-menten und den mg-Werten der Gesteine erlauben den SchluB, daB die Alkaliba-salte von Pauliber! und Steinberg vor ihrer Eruption eine geringfiigige Olivin-und Klinopyroxen-Fraktionierung in einer seichten Magmakammer erlebten. DerNephelinit vom Stradnerkogel hingegen erfuhr hauptsdchlich eine Klinopyroxen-Fraktionierung unter Hochdruck-Bedingungen, mciglicherweise im oberen Erdmantel.

Introduction

Several small eruptive centers of alkali basaltic rocks, mainly of Pliocene age,dominate the geomorphology of the southeastern part of Styria and Burgenland,Austria, near the Yugoslavian and Hungarian border (Fig. 1); they form flows,necks and tuff cones and are part and border facies of the recently defined Trans-danubian Volcanic Region (Embey-Isztin et al,l989) extending into Yugoslavia andcentered in Hungary (Balaton Area). The suite of these alkaline mafic rocks com-prises alkali basalts, basanites and nephelinites. The petrology, geochemistry andgeological setting of these rocks have been discussed in details by Heritsch (1967),Piso (1970), Kurat et al. (1980), Poultidis (1980) and Poultidis and Scharbert (1986).The eruption of the alkali basalts in Austria is connected with the Pliocene-Pleistocene alkali basaltic volcanism of the Pannonian Basin (Jugouics, 1976;Embey-Isztin, I98I).

..i -\_.^..! '''t I

i

i l'---ll.ji.1i^ -.-.-./- .1 I J Ii-.-..,.-.-.Austria | . i I..^._._._--,L::j l

trWien e r

Neustadt -. ' j

' t . - .

Paut iberg o ' i

,t)

q

a.Y

q,\

qn

Y

i f-1

', Szom ba i'I ' hetY

Steinberg a . , !Ka pfensteino i

Stra d ne rkoge I o, . - ' - " - ' '

\(_.oio,'r. C

_i., J

j>-

Kti jch oi

. ._ - .r-- ^

' ' - )

Yugostavia0 50km

Fig. L Schematic map of the investigated alkali basalt occurrences in Eastern Austria

Pvroxene Chemistrv and Evolution of Alkali Basaltic Rocks 277

This paper deals mainly with the chemical composition and zoning of rockforming minerals, especially clinopyroxenes of this rock suite. Minerals have beenanalysed with an automated ARL.SEMQ (in Vienna)and a JEOL Superprobe 733(in Budapest) electron microprobe. Microprobes were operated at conventionalconditions and data were corrected by ZAF corrcction programs. Feldspars andinterstitial glass were analysed with broad electron beam in order to avoid volatiliza-tion of alkalies.

Petrography of Samples

Six representative specimens from hve localities were selected for detailed investiga-tion (Fig. 1); the samples are described below and their petrographic and somegeochemical characteristics are summarized in Table 1.

Stradnerkogel (sample Stk 307). The nephelinite of Stradnerkogel contains clino-pyroxene, nepheline, Fe-Ti-oxides and small amounts of olivine; the latter is alwaysenclosed in clinopyroxenes. Clinopyroxene occurs as phenocryst,400 to 800 pminsize, as needle shaped microphenocryst (80 to 400 ,r.rm) and as very small (5 to 20pm) euhedral grains in the groundmass. Nepheline (up to 150 pm) encloses tinygrains of hematite and the later two generations of clinopyroxenes. Nepheline is alsosurrounded by very small grains of clinopyroxenes and Fe-Ti-oxides. Apparentlythese phases crystallized simultaneously during the last stage of cooling.

Steinberg near Feldbach (sample St 287). The basanite of Steinberg is composedmainly of olivine, clinopyroxene, plagioclase and glass. Olivines are euhedral tosubhedral and vary in size between 1000 and 30 ,um. Clinopyroxenes displayhourglass and sometimes oscillatory zoning and their size varies within the samerange as that of the olivines. Plagioclase is present as needle shaped crystals, 300 to500 pm long with diameters ranging from 30-60 prm. The brownish glass containsrhonites (?) developed as frne long needles, with an intense yellowgreen-brownpleochroism (Heritsch, 1986). Fe-Ti-oxides form small grains of up to 80 pmdiameter.

Table 1. Some Important Petrographic and Geochemical Features of the Inuestigated Rocks. Bulk rockchemical data (mg-numbers, Ni and Cr contents) aie taken from Poultidis and Scharbert (1986) forStradnerkogel, Steinberg, Kkich and Pauliberg, and from Kurat et al. (1980) for Kapfenstein; mg-numbers were calculated supposing Fe3*/Fero, :0.15. The number of x-es gives the relative amountof the phase present

S lradnerkogel Stei ' rberg Kl i jch Kaplenstei n "auI iberg Paul ibergl ighter basal t darker basal t

Stk 107 St 281 K 109 Ka 1708nephel in i te basani te basani te basani te

P 178 P 185alkal i basal t a lkal i basal t

01 iv ineC I i nopyroxeneFe-T i -ox idesPl cni ncl eee

Glass

X XX

XXXX XXX

xx

XXX

- XX

XX

XXX

XXX

XXX

XX

XXX

XX

XX

XXX

XX

1*

mg-nurnber 0.58 a.6J o.61 o.64 o .65 o,65

Ni (ppm)Cr (ppm)

240316

190lB0

70127

965B

t46-t ]gIlI-L7B

r15432


Ktdch (sample K 109). The basanite of Kloch contains mainly olivine, clino-pyroxene, plagioclase and Fe-Ti-oxides. Olivines are euhedral to subhedral (800 to50 pm); they are often marginally altered to iddingsite and some crystals are

corroded. Clinopyroxene phenocrysts (up to 1000 pm) are euhedral, zoned crys-tals, with stubby, corroded cores. The needle shaped euhedral groundmass clino-pyroxenes are fresh. Fe-Ti-oxides are common and form euhedral crystals of severalhundred pm across; plagioclase is restricted to the groundmass and forms 100 pm

long, 10-20 pm wide laths.

Kapfenstein (sample Ka 1708). The basanite of Kapfenstein (Kurat, l97I; Kuratet al., 1980) contains olivine, clinopyroxene, glass and Fe-Ti-oxides. Olivines arepartly fragmented and partly euhedral crystals, 1-2 mm across, some of them withundulous extinction. Two generations of clinopyroxenes are present: micropheno-cryst of 40-10 pm in size and needle shaped microlites (about 1 pm wide and30 pm long)in isotropic, brownish groundmass.

Pauliberg (samples P 178 and P 185 of Piso (1970); they correspond to the lighterand darker gray basalts, respectively). The alkali basalts of Pauliberg are holocry-stalline and contain olivine, clinopyroxene, plagioclase and Fe-Ti-oxides. These twosamples can be grouped together because of the similarity in their constituentphases. Olivines form phenocrysts (1000 to 150 pm); the size of the clinopyroxenesranges frorn about 150 pm (phenocrysts) to about 20 pm in the groundmass'Plagioclase, one of the main constituent in these samples, varies within the samerange in size as the clinopyroxenes. The Fe-Ti-oxides form large grains and theyenclose olivine, clinopyroxene and marginally also plagioclase. Apparently olivine,clinopyroxene and the Fe-Ti-oxides crystallized contemporaneously with plagio-

clase following slightly later. This crystallization sequence is different from that ofthe previously decribed rocks.

Mineral Compositions

In this study the main emphasis lies on the chemistry of clinopyroxenes and thecompositional features of other phases will be discussed only briefly.

Oliuines

Table 2 gives the compositional variation of olivines in the investigated specimenswith representative analyses of the most Mg-rich and the most Fe-rich compositionsencountered.

Mg-numbers of the olivines in our basalts vary between 0.91 to 0.76 (Table 2);the most Mg-rich olivines having mg-numbers between 0.89-0.91 are considered tobe xenocrysts of upper mantle origin. This is supported by their anhedral shape andcompositional similarity to olivines of upper mantle xenoliths found in the samelocalities. Xenocrysts of olivine can be found in the basanite of Kapfenstein and lessfrequently in the nepheline basanite of Kloch; from both localities upper mantlexenoliths have been described (Kurat et al., 1980; Dietrich and Poultidis, 1985).

Some olivines from the Stradnerkogel nephelinite have mg-numbers between0.88 and 0.85. It cannot be resolved unambiguously whether they are xenocrysts or

Pyroxene Chemistry and Evolution of Alkali Basaltic Rocks

Table 2. Compositional Range of Oliuines in the Austrian BasaltsShown by the Most Mg-rich and Fe-rich Compositions (in wtft);mg-nvmber: Mg/Mg + Fe (atomic)

sample

279

Stradnerkogelstk 107 st 281

Klt ichK 109

xenocryst

s i02Fe0Ni0Mn0l'4s0Ca0

40.6 JB.911.9 20.0o .24 0.140.18 l .J9

41 .2 39.3o .2t 0. 71

40.1 11.717.0 22.80.15 0.11o.2B 0.44

42.1 J7.1o.22 0.55

4r.3 40,3 39.J10.4 15.1 20.30.35 0.11 0.090.11 o.21 0.49

48.3 4J.8 J9 .60.04 0.20 0.49

Sum IOO.J3 100.44

mg-nunber 0. BB 4.71

100. 45 99 .30

0.81 0.15

100.52 100.78 IDO.21

0.89 0.Bl 4.11

samFjle KapfensteinKa 1708

xenocryst

Paul i bergP I7B

Pau I ibergP 185

s102Fe0Ni0lr4n0MgoCa0

41.1 40.2 40. B9.t ) .3.9 L5.50.31 0.23 0.220.15 0.19 0.J2

49.3 45.4 4J.70.08 0.14 0,21

40.1 31 .r 40.4 39.8rJ.3 21 .5 r t . ] I7. lo.35 0.r2 0.29 0.150. 15 0.50 0.29 0.46

46.L 33.9 45.J 42.6o .o2 0 .2t 0 .01 8.22

Sum 100 . 10 100 .05 100.01

mg-number 0.90 0.85 0.81

100.02 99.J3 100.01 100.11

0 . 85 0.68 0.85 0.81

Mn0

Sym bots : o

+

Ni0

e Stradnerkoget (Sik 307)a Steinber '9 (Si 287)o Ktoch (K 109)

0,9 0,8 0,7

<- mg -numbef

Kapfenstein (Ka 170B)Paut iberg, l ighfer basal i (P 178)Paut iberg, darker basatt (P185)

0, I

1,4

1,2

1,0

0,8

0,6

o' l+

0,2

j

*.,giffi.

. t'-. .-- Is. :igryo o

-{ .o *

axA

?f;o.. ^ *

"o-18-"fff "qas

A

0,9 0,8 0,7

<_ mg_number

F

Fig. 2. Variation of MnO and NiO vs. mg-nvmber in olivines of Austrian alkali basalts


cognate phenocrysts. There are two facts against their xenocrystic origin: 1. theolivines of Stradnerkogel cover an almost continuous compositional range from mg0.88 to 0.70 and there is no separate compositional population of olivines as in theKlcich and Kapfenstein basanites (see Fig. 2), and 2. their composition (Ni and Cacontents) differs from that of the olivines found in the upper mantle dunite xenolithsat the same locality (Dietrich and Poultidis, 1985).

The cognate olivine phenocrysts are generally zoned; they began to crystallizewith about mg : 0.86 - 0.82. In the course of crystallization their mg-numberdecreases to 0.78-0.75 at the rims of phenocrysts and in the groundmass. Thegreatest variation in olivine composition can be observed in the lighter basalt ofPauliberg (sample P 178, see Table 2). During the course of crystallization the NiOof the olivines decreases with decreasing MgO content, while CaO and MnOincrease (Fig.2). The most iron-rich olivines contain generally 0.5-0.6 wt/"MnO,but the nephelinite of Stradnerkogel is a remarkable exception. Here the iron-richolivine rims may contain MnO up to 1.37 wtlo, which is a significantly highercontent than that of the similarly iron-rich olivines from other samples (Fig. 2).One reason of this can be the MnO contents of the rocks: while the MnO con-tents of the investigated basalts varies between 0.15-0.19 wt/", the nephelinite ofStradnerkogel contains MnO 0.27 wt\(see Poultidis and Scharbert,19S6). However,this cannot explain that the MnO content of the early, Mg-rich olivines is compar-able to that of olivines from the other rocks (Fig. 2) which lie on a fractionation linewith an approximately constant Fe/Mn ratio. The Fe/Mn ratio of the Stradnerkogelnephelinite, however, continuously decreases with increasing Fe content. Since thedistribution coeffrcient for Mn between olivine and melt is approximately unity (seeIruing,1978), the Fe/Mn ratio should stay constant during olivine crystallization.The very strong increase in Mn contents of olivines with increasing Fe thereforemost likely reflects a change of Fe/Mn ratio in the melt. Such a change can beachieved by either adding Mn to or extracting Fe from the system. Of the twopossibilities the extraction of Fe from the evolving liquid seems to be the more likely

Fig. 3. Composition of feldspars in the An-Ab-Or ternary. Symbols for the alkali basalts ofPauliberg: solid circles darker basalt (P 185) and open circles-lighter basalt (P 178)

0rAbPaul. iberg Ste inberg

t


process. The oxidation of Fe2* to Fe3* can be such an extraction, since Fe3+not enter olivine. However, the solution of this problem needs further studies.

Fe-Ti-oxides

Apart from the basalts of Pauliberg, the Fe-Ti-oxides in the investigated samplesare predominantly Ti-magnetites. Their TiO, content varies between ll to 2l wt/"(the lowest TiO, content was found in the basanite of Kapfenstein, the highestin Steinberg). Ti-magnetites always contain CrrO. (l-3 wt'/,), A12O3 Q-7 wt%)and MgO (about 3-5 wt/"). It is interesting to note that Ti-magnetites in theStradnerkogel nephelinite contain more MnO (about 1.20-2.19 wt/") than theTi-magnetites of other basalts investigated (0.44-0.80 wt\).Thebasalts of Paulibergcontain both Ti-magnetite and ilmenite. The larger grains (50-60 prm across) are allTi-magnetites, while the small grains (about 10 pm) in the groundmass are bothTi-magnetites and ilmenites. Unfortunately, their size is not sufficient for precisequantitative analysis, thus the temperature and oxygen fugacity cannot be estimatedfrom the composition of magnetite and ilmenite.

Plagioclases and Alkali Feldspars

Plagioclase was investigated in the rocks of Steinberg and Pauliberg; their composi-tional range is displayed in the An-Ab-Or ternary (Fig. 3).

Plagioclases in the basanite of Steinberg have the highest An-contents of therocks studied, starting with Anru and covering the range Anru-Anou. The range ofplagioclase composition is similar in the two basalts from Pauliberg (both P 178and P 185). The plagioclase crystallization starts at Anu, and shows a range ofcompositions till about Anro (Fig. 3).

Alkali feldspars can be found only in the basanite of Steinberg with a range fromAbuoOroo to AbroOrro. h this rock the alkali feldspars form the intergranulargroundmass.

Table 3. Auerage Compositiort of ResidualGlasses in the Basalts of Steinberg and Kapfen-stein (in wt"/,). Standard deviations are givenin parenthesis

281

will

Steinberg

No. of anal . I

l { rnf pna+pi n

L2

si02T i02A].28JFe0l'4S0Ca0Na 20K20

55.r (2.6J) 55.3 (1.70)0.50 (8.50) 0.62 (O.32)

21.e (0.85) 2t . .2 (0.56)2.65 (1.99) J.4r (1.55)o. lo rc.22) o.9o (0.58)r .05 (0.95) J.5r (1.70)8.56 (0.78) 6.09 (0.18)5.9r (0.7e) 5.26 (D.99)

96.98 96.29


Glass Phases

The basanites of Kapfenstein and Steinberg contain glass in the groundmass.

Analyses of this phase with broad beam techniques revealed SiOr, AlrO. and

alkali-rich compositions with low amounts of CaO, FeO and MgO (Table 3). The

amount of FeO usually greatly exceeds the amount of MgO, which means that a

considerable iron enrichment has taken place.

The composition of the glass in the Steinberg basanite is more sodic and less

calcic than that of Kapfenstein (Table 3). Apparently the beginning of the precipita-

tion of plagioclase with high An content in the Steinberg nephelinite lead to an

enrichment of NarO in the residual melt and further depletion of CaO. The deple-

tion of Ca, Mg, Fe and Ti can be explained by the precipitation of olivine and

clinopyroxene.

Clinopyroxenes

Zoning

All investigated samples contain zoned clinopyroxene phenocrysts except for the

Kapfenstein basanite. The clinopyroxenes of the latter are microphenocrysts or rare

anhedral xenocrysts with a narrow cognate rim which is compositionally similar to

the microphenocrysts. These xenocrysts are Cr-diopsides and apparently are xeno-

crysts from disaggregated upper mantle lherzolites. The phenocrysts of the remain-

ing rocks exhibit concentric and sector zoning and the combination of both:

1 . Concentric zoning. Almost every phenocryst exhibits concentri c zoning under

the optical microscope with a euhedral colourless core and a yellowish-brown or

brown rim. The concentric zoning can also be observed in the backscattered electron

scanning images (Fig. a.). The compositional difference between cores and rims

follows the normal pyroxene fractionation trend: the cores are richer in Mg, Si and

Cr and poorer in Fe, Ti and Al, than the rims. The transition between the core

and rim is sharp, suggesting a rapid change in the P-T-x parameters during crystal-

lization.2. Sector zoning. Sector zoning can be observed in almost all phenocrysts; it is

especially conspicuous in the darker gray cores of the backscattered images (see Fig.

4). Though in some cores (Fig.44 and C) the sector zoning exhibits the well known

"hour-glass" structure, in several phenocrysts it is not so easy to recognize it as

sector zoning, because it looks like a concentric zoning (Fig. 4B and D). In this

latter case, the chemical differences between the questionable zones provide evidence

for the type of zoning (e.g. the Cr distribution, see later). Since detailed optical

investigation of the clinopyroxenes has not been carried out, the chemically different

sectors will be termed Ti-poor and Ti-rich sectors, without specifying the crystal

faces forming the bases of the pyramids.In some samples, however, the cores of clinopyroxene phenocrysts are partly

resorbed. The backscattered electron images show, that there are lighter gray

"patches" in the cores, generally surrounding the groundmass-filled cavities, which

are similar in composition to the rims. During ascent, the earlier crystallized cores

probably became unstable because of the changing P-T conditions, and they were

partly resorbed to produce spongy grains containing a network of irregular cavities.

Subsequently late clinopyroxene precipitated on the inside walls of these cavities.


'.*\ir, i-.

Fig. 4. Back-scattered scanning electron images of zoned clinopyroxene phenocrysts. The

crosses within the crystals indicate the points where microprobe analysis were made; num-

bers refer to the analyses as given in Table 4. The lengths of the bars are 100 pm. A) Pyroxene

phenocryst from the nephelinite of Stradnerkogel with darker gray sector zoned core. B)

Zoned pyroxene phenocryst from Stradnerkogel. The sector zoning in the darker gray core

looks like concentric zoning. C) Zoned phenocryst from the basanite of Steinberg with

resorbed and sector zoned core. The lighter gray "patches" in the core are compositionally

similar to the rims. D) Sector zoned phenocryst from Steinberg; sector zoning looks like

concentric zoning. E) Sector zoned phenocryst from the basanite of Kloch. The Ti-rich sector

is completely resorbed and replaced by late clinopyroxene, compositionally similar to the

rim. F) Sector zoned phenocryst from the darker alkali basalt of Pauliberg (P 185); sector

zoning in the core looks like concentric zoning


Such resorptions are typical for the clinopyroxene cores of the basanite of Klcich.In this sample the preferential dissolution of the Ti-rich sector can be observed (Fig.4E). Similar resorption phenomena were observed and investigated in detail byO'Brien et al. (1988).

Chemical Composition: Ca, Fe, Mg and Na

The general compositional range of the clinopyroxenes is shown by the conventionalpyroxene quadrilateral (Fig. 5). The core and rim compositions are more or lessseparated. They all plot within the field of alkali basaltic clinopyroxenes in thediagrams of Fodor et aL. (1975). The simultaneously crystallized Ti-poor and Ti-richsectors ofthe cores are also separated from each other with the latter projecting athigher Ca cation content (a consequence of their higher Al and Ti contents). Thereare, however, significant differences in the relative positions of pyroxene plotsbetween the various basalts in the quadrilateral; these differences are especially wellresolved by the rim compositions. The phenocryst rims, microphenocrysts andgroundmass grains in the rocks of Stradnerkogel, Steinberg and Kloch project athigher Ca, while the same clinopyroxene generation in the basalts of Pauliberg plotsat lower Ca. The relationship between the relative positions of the clinopyroxenetrends in the quadrilateral and the undersaturation (silica activity) of the host rocks

o phenorryst cone, Ti-poor sector

o phenocrysi core, Ti-r ' ich sector

A x eno crysl

+ phenocfyst r im, microphenocrYst

and gnoundmass

Fig. 5. Clinopyroxene compositions of the Austrian basalts plotted in the atomic Mg-Fe-Caternary (pyroxene quadrilateral). Abbreviations: Srk Stradnerkogel, Sr Steinberg, K Klcich,Ka Kapfenstein and P Pauliberg

FeFeFe

stk 307 st 287

Ka 170B

Pyroxene Chemistry and Evolution of Alkali Basaltic Rocks 285

has been established by several authors (see Gibb,1973; Fodor et al.,1975; Larsen,1976).In our samples this order can also be correlated with the host rock chemistry:the most undersaturated rocks are the nephelinites and basanites of Stradnerkogel,Steinberg and Kloch, while the alkali olivine basalts of Pauliberg are the leastundersaturated (see Heritsch, 1967; Piso,1970; Poultidis and Scharbert,1986).

The NarO content of the clinopyroxenes is low, less than I wt/". Apparently Nadoes not play an important role in their chemistry. The variation of Na withcrystallization generally follows the normal fractionation trend with a slight increasewith decreasingmg. The only exception is the nephelinite of Stradnerkogel, wherethe Mg-rich cores are richer in Na than the more evolved rims. This can be relatedto the influence of pressure on the clinopyroxene chemistry.

The Contents of Al and Ti

Titanium and Al show the greatest variation of all elements in the investigatedclinopyroxenes. TiO, and AlrO. vary between 0.68-5.23'/"and 1.65-11.53%, respec-tively. Titanium enters the clinopyroxene lattice together with two Al ions, in formof the hypothetical Ti-Tschermak's molecule, TiTs (CaTiAlrOu; Yagi and Onuma,1967), which is expressed by their correlation in Fig. 6. However, Al can enterthe clinopyroxene solid solution also in other forms such as CaAlrSiOu (CaTs),CaFe3+AlSiOu (FATs), and NaAlSi2O6 (Jd). This is the reason why the Ti/Al ratiois usually less than 0.5. The incorporation of CaTs and Jd is favoured by highpressures (Clark et al., 1962; Kushiro, 1969), while elevated pressure conditions are

0,02 0,06 0, '10 0,1/- 0,02 0,06 0,10 0,11 0,02 0,06 0,10 0,11

Ti Ti TiFig. 6. Variation of Ti and Al (cations per formula unit based on 6 O) in clinopyroxenes ofAustrian alkali basalts. Symbols and abbreviations as in Fis. 5

lvS' /

srk 307


not favourable for the incorporation of TiTs (Yagi and Onuma, 1967). Thus, at firstapproximation, the Ti/Al ratio can be used as a qualitative indicator of the pressureprevailing during crystallization: the Ti/Al ratios of pyroxenes crystallized underelevated pressures must be low, 0.1-0.2 or less, while for the low pressure pyroxenes(phenocryst rims, microphenocrysts and groundmass pyroxenes) a Ti/Al ratio closeto 0.5 can be expected. However, the so called "high-pressure" CaTs component canbe present in small amounts also in groundmass clinopyroxenes (Wilkinson,1974),and its amount depends on the chemistry of the co-existing liquid and growth rate.The accomodation of Fe3+ (the amount of which is not known) in form of FATs(CaFe3+AlSiOu) and Cr in form of CaCrAlSiOu also needs the incorporation of anequal amount of Al. This explains why the Ti/Al ratios of the low pressure pheno-cryst rims, microphenocrysts and groundmass pyroxenes are generally significantlybelow 0.5 (Fig. 6); only the late crystallized pyroxenes in the Pauliberg basaltsand the outermost rims and groundmass grains in the Stradnerkogel nepheliniteapproach the 0.5 Ti/Al ratio. However, we cannot make a direct comparisonbetween the Ti/Al ratios of the different samples, but some estimation can bemade within the same sample between the Ti/Al ratios of cores and rims. The coresof the clinopyroxene phenocrysts have lower Ti/Al ratio than the rims. However,significant differences between the cores and rims can be observed only in theStradnerkogel nephelinite. The general trend implies, that the cores crystallizedunder slightly higher pressure than the rims and high pressure crystallization can beassumed only for the Stradnerkogel nephelinite. The high AlvI content and highAlvI/AlIv ratio which varies between 0.63 and 1.00 also is in favour of a high pressureorigin (Aoki and Kushiro, 1968; Wass,1979).

The variation of Ti in the course of crystallization is displayed by Fig. 7. Titaniumcontents of pyroxenes usually increase continuously with decreasing mg-numberexcept for the Stradnerkogel nephelinite. The behaviour of Al (not shown)is similar.The variation of Ti follows the normal pyroxene fractionation trend, documentedfrom several alkali basalts (Tracy and Robinson,I976; Bedard et a1., 1988). Experi-ments of Yagi and Onuma (1967) and Sack and Carmichael (1984) suggest that theaccommodation of Ti in pyroxene as a CaTiAlrO6 component is controlled bytemperature and pressure: low pressure and temperature are favourable for Tiaccommodation. In addition, sector zoning also has an effect on the Ti variation ofpyroxenes: from all cores, Ti-rich sectors plot at higher Ti and lower mg-numberthan the Ti-poor sectors.

The variation of Ti in pyroxene phenocrysts of the Stradnerkogel nephelinite ismore difficult to explain. The increase in Ti with decreasing mg-numbers can beobserved in both cores and rims. However, there is significant overlapping in Ticontent between them. The change of melt composition between the crystallizationof clinopyroxene cores and rims may be one reason of this unusual trend. Thecores crystallized under elevated pressure where clinopyroxene could be a liquidusphase (e.g., Arculus, 1975), while the rims crystallized from a more differentiatedliquid at lower P and T together with other phases such as Ti-magnetite whichimpoverished the melt in Ti. The importance of the change of melt compositionbetween the crystallization of high- and lower-pressure pyroxenes was discussed byWass (1979). Non-equilibrium crystallization may be the other reason for this trend.The mg-Ti plot of clinopyroxene from Stradnerkogel (Fig. 7a) shows, that the higherTi contents in the cores (which can be higher than that in the rims) is achieved only


t++

+4+

ta

K 109

Ti0,14

0,10

0,0 6

0,02

<:- mg - number

Fig. 7. Plots of Ti (cations per formula unit based on 6 O) vs. r??g-number in clinopyroxL'nesof Austrian alkali basalts. Symbols and abbreviations as in Fig. 5

in the Ti-rich sectors, while the corresponding Ti-poor sectors contain about thesame or less Ti than the rims (e.g. anals. 2 and 3 in Table 4).

Cr Content

Chromium has a high cpx/melt distribution coeflicient. Consequently it is enrichedin clinopyroxenes during the early stages of crystallization. The contents sharplydecrease with decreasing mg-numbers (Fig. 8). The CrrO. content of cores canreach 1.19 wt/o,and the CrrO. content of the iron-enriched rims and groundmassgrains is generally near the detection limit. However, the variation of Cr is differentin the Ti-poor and Ti-rich sectors, the latter generally having higher Cr-content(Fig.8).

The role of Ti and A| and sometimes Fe in sector zoned clinopyroxenes hasbeen widely discussed: prism sectors are enriched in these elements with respect tothe basal sectors (Hollister and Gancarz, l97l; Leung, 1974; Dowty, 1976). Thebehaviour of Cr in sector zoned pyroxenes, however, has not been discussed so far.In our pyroxenes the Ti-poor sectors contain not only more Si, Mg, and less Fe, Tiand A| than the other sectors, but less Cr as well (Fig. 4A, B, C and D, and thecorresponding analyses in Table 4). And this explains the two descending trends inFig. 8a, b, e and f. Clearly, the distribution of Cr between pyroxene and melt isaffected by kinetic and crystallographic factors (e.g. sector zoning). It is interestingto note that Cr vaiiation in sector zoned pyroxenes sometimes contradicts normalclinopyroxene fractionation: Ti-rich sectors which are enriched also in Fe can

Ka 1708

E IEo-v F:Ea.ao!2o.u6

-v.u^twu6v

- - '<a\ iv 5

Y€m=cnEicY6<V

"Fco -r ie{VO^.^-R * i e

- i l& l<c'l . ', bD

d.E 1>N-P9rr-^Pn"i . iY.6hXFCX

) arA 3T

t g; Ptd

-T^god () =Ct

E 3,_V €, - tscddJ

E F€f iO.H !

<€f i0.

o$oNNN

OOA

n@rNNi

OOO

o6$

OOO

OOO

r@rNNi

OO-

OO-

9NN

OOO

6OJ

iOO

NN@NNN

OOO

H0

<E\d l

F <E

d. i cr l<F El

66

r

66

r66

I

o6

rN

6

N

c;O

@

-O

6

6

rr@h

)a

i@$6nnsoor6@+6=6On

@N+OhONNOs

-N

NS$6n;ndN6r60N-ro6

6+NQ6OSNO9 ;N

6r@NrNn@@

-669hd$@nno-o9o9io6

-N

6rNSi inro

- ino@=6Nr@N6O9ONNO$ JN

6rro6Ni6Qi€@ind@sQ

o-n+6o+io6 :N

sNn@no@99$oo96-N;S

n-NO$O$aOh JN

-rs69$s@$€66O$H@6n

6d6OTONNO< iN

no$Ns9r@<@6drnoo@r

-oro6Qn@-6

gn: 6-o6snNs +ins6

r t r l . t r . 'N6- TOONO$ d iN

NNOO OOONNOOOON.d. i ; k o c oo ooF<ou>>oz

@E q

L

!

6 . io@ o !+s; ! to\

I o. i ogL OF @

A

6 00@oafqr ! I O\

I o.a o+L OF O

s

N

N

NN

@rE4.4 |tq

c

cO!

r o ! fiq to I

I o. i oL OF O

o!@ ooroq++! ro IIo io

L OF @

o oPaca

ooo>lY- i ! q

o@oc> |YO!

XO

coo

O.H Aif l

@o . i@o o

G. Dobosi et al.

r +d+. +NN

rc6 ooh

. NNN

6 OOO

@

- 4d$

' @-;6. ! .6 OOO

Nr99

. NNNaO OOO

r6 th@. +i-6. . .O OO-

n €16' s+-

o!. t

. rh i6. . .6 OOO

n 6@6' N-d

6.. .6 OOO

. NNN@...

d s6r. s i r66 OOO

€n rs$

. NNNo..rO OOO

Os rnn.

-nNO OOO

€.6 6@n

. iNNo.. .

6 o+6. TJJ

o.. .

r$ @+n

' l \ ! -o' . .

rc6 NOd

. +dNO

r+ o6n

. Oid

O iOO

N$ ;n6

r r i -h. t .

>:

\ i lH <E

E > \ ! .a <F El

@-666dn6hNrinNd699

€N@OSOJNQS JN

or$ +:60r

-s+ rc in69

' r i t . , . , .€nr 9o+NO

$9Oi+Shn6so+N+o96S

o*9asQ6oo6 iN

dr6hnN@@6h$6oNr906

6n@oroino+ iN

N9O@NTNNT@$Nrnor<r

@_VO6ONNO$ :N

or6rNO+9iro6€rdsh9

iONO+ONNOh

-N

d$N66@66$

?qi ' ' : \eaa16NSO6QNNOs iN

N+6dn@6<rsosrro9-€OiNOSO6NO6

-N

h@@nN@os@NhTOOO6O6

€n@aro-no+ iN

€n906r+i6o€@ooQr6@

rd9-noAiO+ dN

r - i<nos@rO@ANTOdOS

SNTO€ONNO+ iN

6-SNO--r6N6€O@N6S6

OiNO€ON6Oh =N

oi€noi@N6

aal?1" ' :a:a6i$O9O6nOs iN

6@ioo90nNirnN$or66

6-@O$O+iOs

-N

€r696-66i

-1110:1' ' : i i iNO$O+OSOO6

-N

6S€ iSr@h9Nd nirn€

tt t t . . . . .od+ sonno6 iN

6NN6NOdO9sonN6door

Nig-$osio6 iN

Nh6NOTONSrs69no9$6

@+rosonio€ ;N

NNOO OOONNOOOON

OF<OL==OZ

!

!ooq.a+ 0YO!O\

@ | o$o. i @

o!oooKQPO

Y O I O\o. i 0s

+@

EOA

o.i o

folo\@ o i o$

Ao!

ooo

I o I o\o€' i os

FO

okoo

EAPO+-d I Q\@ k.d os

F@

o!@oo

foro\@ o. i o$

FO

!O!

E !+ O+iIo\@ ! . i o€

FO

cO!

o. i o

POIO\@ o. i o{

F@

Ao o!kP o o

+ ot€\@ o o@+

a. ,9 -

@o! $

la

aa $

EOq

o.d oJ ! !P!foro\o o. i o+

-a!

ooo

foro\@ o. i os

-a

lE o

@a s

Aog

oooJqOP@f o I o\@ o.d 0g

F@

c

Y ! !+ A+o to\@ o. i o$

FO

coo

o. i a

o. i f

oo .Aoa o

i

N

O

288

q

Ut

\q)

\)\)xL\

q

L

L

q)

\a6\

3

q)

\

q)

L

L

q)

q)

q)L

jq;)

p


I r

0,0l

a.02

P 185Sco

0.9 0,8

<- mg-number

Fig. 8. Plots of Cr (cations per formula unit based on 6 O) vs. mg-number in clinopyroxenesof Austrian alkali basalts. Symbols and abbreviations as in Fig. 5

contain more Cr than the Mg enriched Ti-poor sectors (e.g. Table 4, anals. 22

and 23).

Basalt Evolution

Only few alkali basaltic magmas reach the surface without some modification byfractional crystallization, contamination or magma mixing. Many investigationshave shown that zoned clinopyroxenes are the best records of magma evolution(Duda and Schmincke, 1985; Bedard et al., 1988; Dobosi, 1989).

Zoned pyroxene phenocrysts from the basalts investigated suggest a relativelysimple evolutionary model. There is no evidence for sudden changes in melt com-position as would be caused by magma mixing during ascent. Clinopyroxenes followthe normal fractionation trend, core and rim crystallized essentially from the samemelt at different pressure and temperature, and the melt composition is changed onlyaccording to the fractionation by the phases crystallized.

Limited information can be obtained from clinopyroxenes from the basanites ofKl<ich and Kapfenstein. The composition of the Klcich basanite apparently ap-proaches a primary melt composition, having relatively high Mg, Cr and Ni contentsand mg (Table 1). Furthermore it contains small (few cm across) xenoliths of uppermantle lherzolite (Dietrich and Poultidis, 1985). During its ascent, only minor frac-tionation occurred probably before picking up the xenoliths. However, the strongresorption of the pyroxene cores does not allow us to determine the depth ofdifferentiation. The glassy basanite from Kapfenstein is slightly differentiated (see

289

0,01

f .6 *a ol

t t*

++.

0,9

oeo

aaao

foaood


mg1 and Cr content, Table 1) and it contains abundant olivine and clinopyroxenexenocrysts of upper mantle origin and cognate clinopyroxene microphenocrysts.Clinopyroxene apparently did not play an important role in the differentiation ofthis rock. The magma must have reached the surface relatively fast and the crystal-lization of pyroxene microphenocrysts began not far from the surface.

The composition of the basalts of Pauliberg also indicates minor differentiation(Table 1). According to the composition of clinopyroxene phenocryst cores, themagma partly crystallized in a shallow magma chamber. A similar conclusioncan be reached for the evolution of the basanite of Steinberg.

The nephelinite of Stradnerkogel is the most differentiated of the investigatedsamples. It is depleted in compatible elements, MgO and has a low mg (0.58)suggesting more extended fractionation. The clinopyroxene cores in this rock cry-stallized under elevated pressure conditions suggesting that differentiation tookplace at depth. The rock is more depleted in Cr than in Ni (Table l) which impliesthat clinopyroxene must have been the main fractionating phase. This view issupported by the scarcity of olivine phenocrysts compared with the abundance ofclinopyroxene. Under high pressure conditions clinopyroxene can be the liquidusphase instead of olivine in basalts (Arculus, I97 5). Unfortunately, quantitative datacannot be obtained from clinopyroxene composition for the crystallization pressure,but the presence of small (l-2 cm across) dunite inclusions from the upper mantle(Dietrich and Poultidis, 1985) suggests that the differentiation probably occurred inthe mantle at a depth greater than that at which the inclusions originated.

Acknowledgements

This work was frnancially supported by the Austrian "Fonds zur Forderung der wissen-schaftlichen Forschung" (Projekt P 4773, G.K., P.I.). The bilateral exchange agreementsbetween the Hungarian and Austrian Academies of Sciences enabled G. Dobosi to makeparts of the work in Vienna. Constructive comments by N. Boctor (Washington, D.C.) andtwo anonymous reviewers are gratefully acknowledged.

References

Aoki K, Kushiro 1(1968) Some clinopyroxenes from ultramafic inclusions in Dreiser Weiher,Eifel. Contrib Mineral Petrol 18 326-337

Arculus RJ (1975) Melting behaviour of two basanites in the range 10 to 35 kbar and theeffect of TiO, on the olivine-diopside reactions at high pressures. Carnegie Inst Washing-ton Yearb 74:512 515

Bedard JHJ, Francis DM, Ludden J (1988) Petrology and pyroxene chemistry of Monteregiandykes: the origin of concentric zoning and green cores in clinopyroxenes from alkalibasalts and lamprophyres. Can J Earth Sci 25: 2041 2058

Clark SP, Schairer JF, de Neufuille J (1962) Phase relations in the system CaMgSirOu-CaAlrSiOu at low and high pressure. Carnegie Inst Washington Yearb 61: 59 68

Dietrich H, Poultidis 11 (1985) Petrology of ultramahc xenoliths in alkali basalts from Klcichand Stradner Kogel (Styria, Austria). N Jb Min Abh 151: 131 140

Dobosi G (1989) Clinopyroxene zoning patterns in the young alkali basalts of Hungary andtheir petrogenetic significance. Contrib Mineral Petrol l0l: l12-121

Dowty E (1976) Crystal structure and crystal growth. II. Sector zoning in minerals. AmerMineral 61,460-469

Pyroxene Chemistry and Evolution of Alkali Basaltic Rocks 291

Duda A, Schmincke If U (1985) Polybaric differentiation of alkali basaltic magmas: evidencefrom green-core clinopyroxenes (Eifel, FRG). Contrib Mineral Petrol 91: 340-353

Embey-Isztin A (1981) Statistical analysis of major element patterns in basaltic rocks ofHungary. Acta Geol Hung24:351-368Scharbert HG, Dietrich H, Poultidis 11 (1989) Petrology and geochemistry of peridotitexenoliths in alkali basalts from the Transdanubian Volcanic Region, West-Hungary.J Petrol 30: 79 105

Fodor RV, Keil K, Bunch TE (1975) Contribution to the mineral chemistry of Hawaiianrocks, IV. Pyroxenes in the rocks from Haleakala and West Mani volcanoes, Mani,Hawaii. Contrib Mineral Petrol50, 173-195

Gibb FGF (1973)The zoned clinopyroxenes of the Shiant Isles Sill, Scotland. J Petrol 14:333 344

Heritsch H (1967) Uber die Magmenentfaltung des steirischen Vulkanbogens. ContribMineral Petrol 15:330 344

- (1986) R.honit-Kristallite in basaltischen Gldsern des Steinberges bei Feldbach,Oststeiermark. Mitt naturwiss Ver Steiermark 116: 43 49

Hollister LS, Gancarz AJ (1971) Compositional sector zoning in clinopyroxene from theNarce area, Italy. Amer Mineral 56:959 9'79

Iruing AJ (1978)A review of experimental studies of crystafliquid trace element partitioning.Geochim Cosmochim Acta 42:743-770

Jugouics L (1976) Chemical features of the basalts in Hungary (in Hungarian with Englishabstract). Ann Rep of the Hungarian Geological Institute of l974,pp 431-470. MiiszakiKcinyvkiad6, Budapest

Kurat G (1971) Granat-Spinell-Websterit und Lherzolith aus dem Basalttuffvon Kapfenstein,Steiermark. Tschermaks Mineral Petrogr Mitt 16: 192-214

- Palme H, Spettel B, Baddenhausen H, Hofmeister H, Palme C, Wiinke 11 (1980) Geo-chemistry of ultramafic xenoliths from Kapfenstein, Austria: evidence for a varietyof upper mantle processes. Geochim Cosmochim Acta 44: 45 60

Kushiro 1(1969)Clinopyroxene solid solutions formed by reactions between diopside andplagioclase at high pressure. Mineral Soc Amer Spec Pap 2: 1,79-l9l

Larsen LM (1976) Clinopyroxenes and coexisting mafic minerals from the alkaline Ilimaus-saq Intrusion, South Greenland. J Petrol l7:258-290

Leung IS (1974\ Sector-zoned titanaugites: morphology, crystal chemistry and growth. AmerMineral 59:1,27 138

O'Brien HE, Iruing AJ, McCallurrz 15 (1988) Complex zoning and resorption of phenocrystsin mixed potassic mafic magmas of the Highwood Mountains, Montana. Amer Mineral73: L007 1024

Piso E (1970) Zusammensetzung und Genese der Basalte des Pauliberges und von Stoob-Oberpullendorf (Burgenland). Tschermaks Mineral Petrogr Mitt 14: 103-139

Poultidis H (1981) Petrologie und Geochemie basaltischer Gesteine des steirischen Vulkan-bogens in Steiermark und im Burgenland. Diss Univ Wien

- Schqrbert H (1986) Bericht iiber geochemisch-petrologische Untersuchungen an basal-tischen Gesteinen des cisterreichischen Teils der transdanubischen vulkanischen Region.Anz Osterr Akad Wiss math-naturwiss. Kl 123 65-76

SackRO,CarmichaelISE(1984)Fe2: Mg2andTiAlr: MgSizexchangereactionsbetweenclinopyroxene and silicate melts. Contrib Mineral Petrol 85: 103 115

Tracy RJ, Robinson NP (1977) Zoned titanium augite in alkali olivine basalt from Tahitiand the nature of titanium substitutions in augite. Amer Mineral62: 634-645

Wass SY (1979) Multiple origin of clinopyroxenes in alkali basaltic rocks. Lithos 12: l15-132Wilkinson JFG (1974) The mineralogy and petrology of alkali basaltic rocks. In:

Ssrensen H (ed) The alkaline rocks. John Wiley and Sons, London, pp 67-95

292 G. Dobosi et al.: Pyroxene Chemistry and Evolution of Alkali Basaltic Rocks

Yagi K, Onuma K (1967) The join CaMgSi2O6-CaTiAlrOu and its bearing on titanaugite.J Fac Sci Hokkaido Univ. Ser 4. Geol Mineral 13:463-483

Authors' addresses: Dr. G. Dobosi, Hungarian Academy of Sciences, Laboratory for Geo-chemical Research, Budacirsi ut 45. H-1112 Budapest, Hungary, Dr. R. Schultz-Gilttler,Instituto de Geociencias, Universidade de Sao Paulo, Caixa Postal 20.899, Sao Paulo, Brazll,Prof. Dr. G. Kurat, Naturhistorisches Museum, Postfach 417, 4-1014, Vienna, Austria,and Dr. A. Kracher, Department of Earth Sciences, Iowa State University, Ames, IA 50011U.S.A.

Documents

Pyroxene chemistry and evolution of alkali basaltic rocks from Burgenland and Styria, Austria