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Oxides of low pressure origin from alkalibasaltic rocks, southern highlands,N.S.W., and their bearing on thepetrogenesis of alkali basaltic magmasSuzanne Y. Wass aa School of Earth Sciences , Macquarie University , North Ryde, NewSouth Wales, 2113Published online: 01 Aug 2007.

To cite this article: Suzanne Y. Wass (1973) Oxides of low pressure origin from alkali basaltic rocks,southern highlands, N.S.W., and their bearing on the petrogenesis of alkali basaltic magmas, Journalof the Geological Society of Australia: An International Geoscience Journal of the Geological Societyof Australia, 20:4, 427-447, DOI: 10.1080/00167617308728827

To link to this article: http://dx.doi.org/10.1080/00167617308728827

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OXIDES OF LOW PRESSURE ORIGIN FROM ALKALIBASALTIC ROCKS, SOUTHERN HIGHLANDS, N.S.W.,AND THEIR BEARING ON THE PETROGENESIS OF

ALKALI BASALTIC MAGMASby SUZANNE Y. WASS

(With 5 Tables and 13 Figures)(Received 29 May 1973; revised MS received 24 August 1973)

ABSTRACT

Alkali basaltic rocks from the Southern Highlands, N.S.W., contain oxide phasesof both high and low pressure origin. The two phases are readily distinguished usingchemical and textural criteria.

Chemical data for low-pressure Fe-Ti oxides indicate that oxygen fugacities ofthe host lavas range from 10-12.8 to 10-8 atm at 950° to 1110°C. In most cases,the oxygen fugacities of the individual lava flows appear to be principally a functionof temperature and intrinsic chemical equilibria existing at the time of formationof the basaltic liquid. However, some relatively differentiated flows shows a highdegree of oxidation due to volatile enrichment with fractionation. Rare glassy flowsshow dendritic crystallization of Fe-Ti oxides. Most flows in which abundant olivinewas the first phase to be precipitated also contain Cr-rich spinels associated, andapparently coeval, with the earliest-crystallizing olivine.

INTRODUCTIONIron-titanium oxides are ubiquitous in the

alkali basaltic rocks of the Southern Highlandsof New South Wales. With a few exceptionsthese minerals have not until recently beengiven equal emphasis with silicate phases inthe study of volcanic rocks. This probablyreflects the difficulty of separation andanalysis of Fe-Ti oxides, especially as small-scale intergrowths of different Fe-Ti oxidephases are common. The electron microprobehas provided a means of surmounting thesedifficulties to obtain quantitative data.

Coexisting ilmenite and titanomagnetite canyield valuable data on the physico-chemicalenvironment of the melt at the time of theirfinal equilibration and, together with coevalsilicates, on the evolution of a basaltic magmaat various levels of its ascent. Buddington &Lindsley (1964) demonstrated that the com-position of co-equilibrated ilmenite andmagnetite can provide an estimate of tempera-ture and oxygen fugacity at the time ofequilibration. Application of this principle hasresulted in quantitative physico-chemical datafor numerous volcanic series. These includesalic volcanic rocks (Carmichael, 1967b),trachybasalts, trachyandesites, and trachytes(Anderson, 1968b) and tholeiitic basalts (e.g.Anderson & Wright, 1972). Direct measure-ments of f (O,) and T have also been made on

volcanic gases (Sato & Wright, 1966; Healdet ah, 1963). However, as yet there are fewpublished detailed chemical data on the Fe-Tioxides of the more mafic, undersaturated line-ages of alkali basaltic rocks.

In the Southern Highlands, a nephelinebasanite — felspathoidal hawaiite series, analkali olivine basalt — hawaiite series, and anepheline trachybasalt — felspathoidal trachy-andesitc — phonolite series occur. These lavascontain oxide phases of three main origins:(1) oxides of low-pressure origin, (2) oxidesprecipitated at high to moderate pressures, and(3) oxides which are crystal debris from dis-aggregated lherzolite nodules.

Compositional and textural criteria readilydistinguish them. Compositional criteria in-clude high MgO and A12O3 contents, with orwithout significant Cr2O3, for spinels of high-pressure origin. In addition, where high-pressure oxides occur as cumulates, thechemical composition of the associated phases(e.g. clinopyroxene, orthopyroxene, olivine) isdistinct from the analogous low-pressurephases. Textural criteria include the marginalresorption or marked reaction rims character-istic of the high-pressure oxides (Wass, 1971).These oxides also occur as large discrete grains(up to 2 cm long), whereas the oxides oflow-pressure origin are generally less than0.5 mm. High-pressure oxides occurring in

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428 SUZANNE Y. WASS

cumulates usually show either intercumulousgrowth patterns or grain-boundary adjustmentshapes producing rounded included grains andpolygonal grains with good triple junctions.Low-pressure oxide grains are generally dis-crete and euhedral.

This paper is concerned with the oxides oflow-pressure origin and aims at (a) assessingthe significance of their chemistry and texture;(b) estimating the temperature and oxygenfugacity of the host basaltic magma at thetime of equilibration of the Fe-Ti oxide pairs;(c) determining the factors affecting oxygenfugacity in alkali basalts; and (d) inferring theeffect of crystallization of the Fe-Ti oxides,along with coexisting silicate phases, on theevolution of these alkali basaltic magmas. Fourdistinct groups of oxides are discussed inseparate sections below: (I) the common low-pressure Fe-Ti oxides, (II) olivine-oxideintergrowths, (III) dendritic opaques, and(IV) Cr-spinel euhedra in olivine-phenocrysts.Method of Recalculation of Analyses

Two main problems arise in recalculating oxidemicroprobe analyses into the spinel or rhombo-hedral series when only the total iron value isknown. The first is to select a distribution of com-ponents so that an estimate of Fe2O3:FeO isobtained. The second is to provide some check onanalytical accuracy by deriving totals for theanalyses. As evident in Tables I, II, and III, thesums of recalculated end members consistentlyadd up to reasonable totals, implying an accept-able accuracy in analysis and supporting themethod of recalculation used.

Spinel and rhombohedral phases (members ofthe magnetite-ulvospinel series) were recalculatedby Anderson's (1968b) method. Analyses wererecast into the end-members as shown in Tables

•I, II, and III. This method appears to overcomeproblems of estimating oxygen fugacity whichoccur when MgO is present as a significant com-ponent (Speidel, 1970) as in the oxides of thisstudy.

THE COMMON LOW-PRESSUREFE-TI OXIDESPetrography

Most of the basaltic rocks from the South-ern Highlands contain coexisting spinel andrhombohedral phases, a few show evidence oftwo generations of Fe-Ti oxide precipitation,and rare flows show exclusive precipitation ofeither a rhombohedral or a spinel phase. Tenrocks containing coexisting spinel and rhom-bohedral phases, four with a spinel phase only,and two with two generations of oxideprecipitation were selected for detailed investi-gation.

Fig. 1. Ilmenite-titanomagnetite intergrowth, withwide ilmenite lamellae (highest reflec-tivity) predominantly parallel to (111)planes of the associated titanomagnetitegrain. These wide lamellae are in placescurved and are therefore not strictlyaligned on rational planes. Reflected light.

Euhedral titanomagnetite, commonly with atendency to develop skeletal peripheries, occursas discrete grains or as grains intergrown withilmenite (Figs. 1, 2). The fabric of suchintergrowths is important in determiningwhether this is due to primary intergrowth ofphases or to subsolidus oxidation. (Buddington& Lindsley (1964) show that the spinelstructure cannot accommodate a rhombo-hedral component as such and that apparent'exsolution lamellae' of ilmenite in titano-magnetite are due to subsolidus oxidation ofspinel components). No evidence of post-crystallization oxidation is found in the opaquephases of fresh basaltic specimens selected foranalysis in this study.

Several patterns of ilmenite intergrowth areevident: (1) Ilmenite forms intergrowths ofnarrow lamellae parallel to (111) planes inthe spinel. These lamellae (Fig. 2) aregenerally even throughout their length but maytaper and rarely terminate abruptly within thetitanomagnetite hosts. (2) Intergrowths ofthick ilmenite lamellae predominantly parallelto one of the (111) planes but not alwaysstrictly rational. These lamellae thicken, thin,and terminate abruptly within the titano-magnetite grains. (3) Blebs of ilmenite whichare generally elongated sub-parallel to one ofthe (111) planes within the spinel.

Ilmenite occurs also as discrete elongateneedles or rods, commonly with indented

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TABLE I

Representative Chemical Data for Titanomagnetites co-existing with Jlmenite, Corresponding USP and HEM Values, and resultant f(O2) and T Values

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1745028 45008 45008 33631 33631 45087 45097 45097 45100 45001 45065 45065 45065 45065 45065 45065 45055

(I) (II) (I) (II) (I) (II) (I) (II) (III) (IV) (V) (VI)

SiO2TiO2A12O3Cr2O3*FeOMgOCaO

0.14 0.1426.5 26.02.21 2.82

— 0.09 0.04 0.14 0.14 0.54 0.2467.4 66.8 66.3 65.7 69.6 65.1 69.9

1.53 2.76 2.56 1.06 1.14 4.37 2.130.12 0.04 0.04 0.01

0.35 0.07 0.30 3.26 0.19 0.1226.1 26.4 26.3 20.1 23.8 24.62.70 2.53 1.21 4.88 1.36 1.240.04 0.14 0.14 0.54 0.24 0.39

68.42.780.070.04 — 0.10

0.22 0.04 0.26 0.04 0.44 0.00 0.14 0.00 0.0424.9 27.4 29.7 27.4 28.4 27.5 27.9 27.1 27.0

5.15 1.39 1.34 1.16 1.30 1.89 1.52 1.89 2.410.04 — — — — — — — —

66.1 69.2 65.0 68.0 64.3 65.2 67.0 64.7 67.12.60 1.21 2.74 2.24 3.62 3.25 1.93 3.47 1.790.02 0.20 0.37 0.35 0.36 0.35 0.36 0.35 0.40

50

73raCOCO

JOW

gs

Total 97.9 98.7 98.1 95.9 98.7 98.3 97.7 97.6 99.0 99.4 99.4 99.2 98.4 98.2 98.9 97.5 98.7

•Total Fe as FeOCalculated End-MembersMg.7Fe.3AloO4 3.3 4.3 4.1 3.8 1.8 7.1 0.4 0.7 7.7 2.1 4.0 1.7 2.0 2.7 2.3 2.9 3.6Mg.>TiO4 1.9 4.0 3.7 0.7 1.6 6.1 2.1 1.8 2.2 1.6 4.0 3.9 6.4 5.5 3.1 5.9 2.3Fe2Si04 0.4 0.4 1.2 0.2 1.0 0.8 2.7 4.5 0.1 0.9 0.8 0.2 1.4 0.0 0.4 0.0 1.4CaFe2O4 0.4 0.2 0.2 0.1 0.2 — 0.6 0.4 0.1 0.1 1.3 1.3 1.3 1.3 1.3 1:3 1.5FeCr2O4 — 0.2 0.1 0.2 0.2 0.4 0.4 0.2 0.1 — — — — — — — —Fe2TiO4 71.6 67.2 68.2 73.3 71.6 47.6 58.8 62.2 66.6 74.5 77.8 71.2 70.4 69.5 73.8 67.5 72.4Fe3O4 21.9 24.2 23.2 22.5 24.2 38.5 34.4 29.6 23.2 22.1 14.0 22.4 18.1 20.2 19.3 21.4 19.5

CO

7*oo

Total 99.5 100.5 100.7 100.8 100.1 100.5 99.4 99.4 100.0 101.3 101.9 100.7 99.6 99.2 100.2 99.0 100.7

CorrespondingUSPHEMLog 1 0 f(O2)T°C

76.5 73.5 74.6 76.5 74.8 55.3 63.16.3 4.4 3.3 5.2 5.2 11.6 24.3

-10.2 -11.7 -11.9 -10.2 -10.8 -10.9 -8.01100 1000 990 1070 1075 1000 1200

67.77.2

-10.81025

74.27.7

-10.21080

77.12.0

-12.3980

84.02.4

-10.11140

76.01.6

-13.0940

79.54.5

-10.01100

77.54.7

-10.01090

79.32.2

-11.01030

76.09.6

-9.81140

78.87.4

-10.01100

All specimen numbers refer to rock specimens housed in the collection of the Dspartment of Geology, University of Sydney. Roman numerals in brackets belowspecimen numbers distinguish analyses from single flows.45028: alkali olivine basalt.45008: analcime trachybasalt.33631: ankaramitic dolerite.45087, 45097, 45100. nepheline hawaiites from a mildly differentiated flow sequence. 45087 is probably close to primitive composition, 45100 is most fractionated.45001: alkali olivine basalt.

nepheline basanite.basanite.

45065:45055:NOTE: USP = 100 Fe2TiO4/(Fe2TiO4 + Fe 3O 4) .

HEM = 100 Fe2O3 /(Fe>O3 + FeTiO3).

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TABLE II

Representative Chemical Data for llmenites Co-existing with Titanomagnelites

SiO2TiO2AUO,Cr2O*FeOMgOCaO

Total

•Total Fe as FeO

Calculated End-Members

A12OQC r , O ,CaTiOaMgTiO3

FeSiO3FeTiO,Fe 2 O 3

Total

HEM

145028

0.8349.4

0.45

46.02.030.88

99.6

0.5

2.26.02.4

83.95.6

100.6

6.3

245008

(I)

0.1451.6

—45.12.700.06

99.6

0.18.20.3

87.44.0

100.0

4.4

345008

(ID

0.0552.5

0.04—43.5

3.500.04

99.6

0.110.50.1

S6.42.9

100.0

3.3

445008(III)

0.0252.4

—44.03.290.04

99.8

0.19.9

—87.03.2

100.0

5.2

533631

0.2249.9

0.110.15

47.80.930.08

99.2

0.10.20.22.80.5

91.15.0

99.9

5.2

645087

0.2349.1

0.330.03

43.15.520.00

98.3

0.3

16.50.5

72.49.5

99.2

11.6

745097

(1)

0.7652.2

0.770.05

42.63.300.10

99.8

0.80.10.39.70.4

68.321.9

101.5

24.3

845097

(II)

0.1850.5

0.070.08

44.30.123.95

99.2

0.10.10.3

11.70.4

80.96.3

99.8

7.2

945100

51.90.020.05

42.25.76

—

99.9

,0.1

_17.2—76.96.4

100.6

7.7

1045001

0.2053.10.26

—41.14.320.35

99.3

0.3_—13.5—83.9

1.7

99.4

2.0

1145065

(I)

52.90.15

—42.4

3.670.40

99.5

0.2—

1.010.9—85.62.1

99.8

2.4

1245065

(II)

52.90.15

—43.1

3.100.40

99.7

0.2_

0.99.1

—88.3

1.4

99.9

1.6

1345065(III)

52.40.26

40.75.460.36

99.2

0.3—

0.816.2—78.0

3.7

99.0

4.5

1445065(IV)

0.0352.10.26

—40.5

5.270.35

98.5

0.3—

0.815.80.1

78.23.8

99.0

4.7

1545065(V)

53.60.15

—42.8

1.930.36

98.8

0.2—

0.811.1—86.8

1.9

100.8

2.2

1645065(VI)

49.90.36

—41.8

5.210.43

97.7

0.4—

1.015.5—74.17.8

99.3

9.6

1745055

50.40.25

—45.8

1.790.40

98.2

0.3—

1.08.30.1

84.16.7

100.5

7.4

r/iCN

zm

See explanation for Table I.

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LOW-PRESSURE OXIDES IN BASALTIC ROCKS

TABLE III

431

Chemical Data for 7itanomagnetites from Alkali Basaltic Rocks with no Corresponding llmenite and for oneTholeiitic Rock

SiO.,TiO2AI._,O3

Cr.,O,*FeOMgONiOCaO

Total

•Total Fe as FeO

10.38

19.15.500.61

67.02.240.450.25

95.5

Calculated End-Members

Mg.fFe.3Al.O4Mg.,TiO4Fe.,SiO4CaFe.)O4FeCr.,O4

Fe.,NiO4Fe.,TiO4Fe 3O 4

Total

USP

8.25.51.21.50.91.5

46.133.6

98.5

50.3

20.46

17.07.034.20

62.35.850.400.14

97.4

10.57.91.40.66.30.9

36.935.7

100.2

50.6

31.23

22.25.29

—66.3

2.66

0.37

98.1

7.92.44.31.6

——58.625.2

100.0

70.0

40.48

23.87.44

—62.5

3.35

0.15

97.7

11.12.71.60.6

——63.120.2

99.3

75.7

50.26

15.91.911.84

72.83.31

—

96.0

2.95.60.8

2.7—36.750.9

99.6

41.9

60.24

23.45.28

—66.0

3.34——

98.3

7.93.80.8

——60.427.3

100.2

68.8

70.12

20.24.23

—70.5

1.70—

0.88

97.6

6.21.60.40.4

——55.036.1

99.7

60.4

Groundmass titanomagnetites from (1) nepheline basanite, (2) basanite, (3) nepheline basanite, (4) basanite,(5) nepheline hawaiite, (6) nepheline hawaiite, (7) tholeiitic dolerite.

margins analogous to the semi-skeletal marginsof the titanomagnetite.

Optically, the pinkish-brown titanomagnetitesare characterized by some reflection pleo-chroism and may be weakly anisotropic.However, they are quite distinct from theilmenites as the latter show marked anisotropy,strong reflection pleochroism, and deepercolour.

Exceptions to these descriptions are flowswhich contain quench dendritic crystals in aglassy mesostasis and those rocks containingtwo generations of oxide precipitation. In thelatter, the first generation comprises large,discrete euhedral grains and the second smallskeletal to subhedral quench crystals withmutual intergrowths of ilmenite and titano-magnetite.

Fig. 2. A subhedral titanomagnetite gram con-tains both broad, discontinuous blebs andnarrow lamellae parallel to its (111)planes. The reflection pleochroism of thebroad ilmenite lamellae is evident in thetwo different orientations of the ilmenite.Reflected light.

All chemical data for the common Fe-Tio x ; d summarized in Tables I, II, and III,a n d l n F l S u r e 3-

Journal of the Geological Society of Australia, Vol. 20, Pt. 4, pp. 427-448, December, 1973.

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432 SUZANNE Y. WASS

Fig. 3. Plot of chemical composition of selectedcoexisting titaniferous magnetites (recal-culated to magnetite-ulvospinel end-members) and ilmenite (recalculated toilmenite-hematite end-members), with tie-lines joining coeval oxide pairs. Progres-sion away from the FeTiO3-Fe2TiO< joinindicates increased oxidation.

Method of Analysis. To assess compositionalvariation of the Fe-Ti oxides within a givenbasaltic flow of average thickness, one alkalibasaltic flow exhibiting both discrete spineland rhombohedral phases as well as mutualintergrowths was studied in detail. Severalpolished thin sections were made of specimensfrom successive levels in the 3 m thick flow,and electron microprobe analyses of bothdiscrete ilmenite and titanomagnetite, as wellas lamellar intergrowths of both phases, werecarried out. Representative analyses (TablesI and II, Analyses 11-16) show that theoxides vary little in composition and appear tobe in equilibrium throughout this flow. Tem-perature and oxygen fugacity values deter-mined plot extremely close to the lineindicating that f(O2) is a function oftemperature only (Figs. 4, 5). In all otherflows examined, a representative sample ofspecimens throughout the flow was used tocheck whether such equilibrium conditionsexisted throughout each flow, as Watkins &Haggerty (1967) demonstrated considerablevariations in oxidation throughout a single lavaflow and Sato & Wright (1966) measured widevariations of f(O2) within a crystallizing lavalake. In both of these examples the flows beingstudied were much thicker than any of theSouthern Highlands flows and therefore muchless likely to approach homogeneity. Anderson

(1968b) showed large variations in the calcu-lated f(O2) and T values, depending onwhether data from discrete or intergrownphases were considered, and also that equili-brium occurred at the interface betweencontiguous spinel and rhombohedral phaseswhere mutual saturation and reaction havetaken place. Therefore to minimize inconsis-tencies due to non-equilibrium effects, adjacenttitanomagnetite and ilmenite from mutualintergrowths were analysed where possible inthis study.Phenocryst-Groundmass Fe-Ti Oxide Pairs.In flows which display two distinct generationsof Fe-Ti oxide crystallization, the phenocrystspinel phases show compositional differencesfrom groundmass phases (Table IV).Titanomagnetite phenocrysts contain signifi-cantly more of the ulvospinel component andare also richer in A12O3.

The higher A12O3 content of phenocrysttitanomagnetites suggests that Al substitutionin the ulvospinel-titanomagnetite series may beextremely pressure-sensitive. This is stronglysupported by the high A12O3 content (up to10%) of megacryst titanomagnetites ofmoderate to high-pressure origin (Wass,1971). Aoki (1966) recorded 'spineliferoustitanomagnetites' containing up to 11.5%A12O3 as phenocrysts in trachyandesites fromJapan. Lewis (1970, 1973) investigated largetitanomagnetite crystals containing up to6% A12O3 from ejected blocks from theSoufriere volcano. Both these types oftitanomagnetite are inferred to be products ofdeep intratelluric crystallization, allowingconsiderable amounts of A12O3 to be incor-porated in the titanomagnetite lattice.

The greater ulvospinel content of the pheno-cryst spinel phases accords well with Wilkin-son's (1965) data on the differentiation trendof titanomagnetites within a sill of alkalibasaltic affinities. Here the ulvospinel contentof the titanomagnetites decreases markedlywith fractionation in an undersaturated alkalibasaltic magma. As Wilkinson notes, this trendis 'opposite to the slope of the liquidus surfacein the spinel field in the syntheticFeO.Fe2O3.TiO2 system (Taylor, 1963)', butthis natural basaltic liquid is a much morecomplex system incorporating alkali enrich-ment. Anderson's (1968a) results, however,show that groundmass magnetites in ananorthositic complex contain highest ulvospinelcomponents. These data emphasize that the

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LOW-PRESSURE OXIDES IN BASALTIC ROCKS 433

25

Eugsler and Wones(l962]

TOO 800 900 1000•C

MOO 1200 1300

Fig. 4. Oxygen fugacities (-login atm.) of basalticrocks from the Southern Highlands,plotted against temperature (°C). Pointswith the same symbols refer to rocks froma single flow or differentiation sequence.FMQ is the quartz-magnetite-fayalitebuffer curve (Eugster & Wones, 1962).The long dashed curve is the calculatedequilibrium oxygen fugacity of analysedvolcanic gas (Heald, et al., 1963). Thearea partly enclosed by small-dashed linerepresents the field for basalts deduced byCarmichael & Nicholls (1967). PW repre-sents the oxygen fugacity and temperaturemeasured in a Kilauean lava lake (Peck &Wright, 1966). F, and F2 show themaximum range of oxygen fugacity at1200°C in basaltic rocks as determinedexperimentally by Fudali (1965).

chemistry of Fe-Ti oxides in igneous rocks iscomplex and influenced by both physical andchemical parameters of the host melt.Minor Element Distribution. Several patternsof minor element distribution between co-equilibrated titanomagnetite and ilmenite areevident. Ilmenite is always richer in MgO thanthe coexisting spinel phases (Tables I, II, IV).However, A12O3 and Cr2O3 are accommodatedpreferentially in the spinel phase. Lovering &Widdowson (1968) show that the MgOcontent of ilmenite for a variety of rocks(excluding volcanic rocks) is related to theMgO/FeO ratio of the rock. Thus ilmeniteswith a high MgO content were formed from-ocks with a high MgO/FeO ratio. Thisrelationship holds for ilmenites of low-pressureorigin from the Southern Highlands basalts.The MgO/FeO ratio of these basalts is gener-ally approximately unity and the MgO contentof ilmenites varies between 2% and 6%. In

the low-pressure spinel phases the MgOcontent may be simply restricted by apartitioning factor with coexisting ilmeniteand thus also reflect melt MgO content.However, the high MgO content of megacrystspinel phases of high-pressure origin (Wass,1971) suggests that pressure favours intro-duction of Mg into the spinel lattice.Similarly, A12O3 content of spinel phases maybe pressure dependent. The A12O3 content ofsome titanomagnetites exceeds 1% (Tables Iand III). The A1,O3 content of spinel phaseslacking a coexisting rhombohedral phase isgenerally greater than that of spinels associa-ted with a coexisting rhombohedral phase(Table III).

Comparison of Iron-Titanium Oxides fromTholeiitic and Alkali Basalts. Complete dataon Fe-Ti oxides from alkali basalts are rare.Wilkinson (1965) gives analyses for titano-magnetites and Smith & Carmichael (1969)and Anderson (1968b) record the onlyanalyses of coexisting rhombohedral and spinelphases for alkali basaltic rocks. However, inthe past few years a substantial record of suchcoexisting phases has been reported fortholeiitic and high-alumina basalts (e.g.Carmichael, 1967a; Smith & Carmichael,1969; Wright & Weiblin, 1967; Peck et al,1966; Anderson & Wright, 1972).

Oxide equilibrium data for Fe-Ti oxidesfrom all of these basaltic rocks plot near, butslightly displaced downwards from, the curveof the synthetic system fayalite-magnetite-quartz (Eugster & Wones, 1962; Wones &Gilbert, 1969 (extrapolated curve)).

Tholeiitic basalts typically precipitate eithera rhombohedral phase first, or rhombohedraland spinel phases together, whereas alkalibasalts precipitate a spinel phase first or spineland rhombohedral phases together. Fromavailable chemical data it appears that thespinel phases in alkali basalts contain signifi-cantly more Al and Mg than those oftholeiites.

f(O,) and Fe sVFe2 t Controls in Basalts.

The oxygen fugacity of a given basaltic com-position is an intrinsic function of the originalmelt unless additions and losses occur in themagma. If closed system conditions apply,there are three main factors determiningf(O2). The first depends on the crystal-liquidrelationships and chemistry of phases existingat the site of magma generation. The second

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TABLE IV

Representative Analyses of Groundmass and Phenocryst Phases from a Nepheline Hawaiite

T1O2A12O,Cr2O3*FeOMgOCaO

Total

T o t a l Fe as FeO

Calculated End-Members

Mg.7Fe.3Al2O4Mg2TiO4

Fe2SiO4CaFe2O4

FeCr2O4Fe2TiO4Fe3O4

Total

USP

0.1923.5

1.480.26

70.02.240.07

97.7

2.33.70.60.20.4

55.038.2

100.4

59.0

0.3325.94.750.00

65.91.980.27

99.1

7.82.20.10.10.0

66.623.2

100.0

74.2

AfeO,Cr2O3

CaTiO,MgTiO3

FeSiO,FeTiO3

Total

HEM

0.2551.70.270.06

43.24.220.12

99.8

0.30.10.3

12.60.5

82.14.5

100.4

5.2

0.051.9

0.020.05

42.25.760.00

99.9

0.00.10.0

17.20.0

76.96.4

100.6

7.7

1 and 3 co-equilibrated titanomagnetite and ilmenite groundmass phases respectively. Calculated logx0 f(O2)= -12.8, T°C = 920.

2 and 4 co-equilibrated titanomagnetite and ilmenite phenocryst phases respectively. Calculated log10 f(O2)= -10.9, T°C = 1050.

is change in the physical environment (mainlytemperature and pressure) during the move-ment of the magma to the surface. Thethird is compositional change in the magmaduring its ascent — involving the extent ofdifferentiation and the chemistry of phasesprecipitated before eruption. Each is discussedbelow.

External Factors. Lack of data makes it im-possible to assess, quantitatively, effects of gasloss or gain of erupted rocks. However, it ispossible that intratelluric crystallization mayproceed at a somewhat lower f(O2) thancrystallization in environments such as theHawaiian lava lakes. Initial seething of the lavain a lake must trap atmospheric oxygen, andvents at later stages of cooling allow gas ex-change. Osborn (1959) proposed two diver-gent paths of basaltic f ractionation: onerequires crystallization with constant totalcompositions and consequent iron enrichmentin residual liquids; the other involves crystal-lization along an oxygen isobaric line on theliquidus surface, resulting in silica enrichmentbut restricted Fe enrichment. This latter modelrequires a steady increase in oxygen content,implying constant or increasing f(O2). Here,

any internal buffer reaction between crystal-lized phases and the liquid must be insignificantcompared to the imposed O2 pressure. In con-trast to fractionation controlling the f (O2) thef(O2) in this case controls the fractionationpath.

Chemistry of the Melt. The chemistry of themelt is vital in determining the f(O2) prevail-ing, in the following ways: (a) the presenceof crystal-liquid relationships and (b) theeffect of fractional crystallization anddifferentiation.

In the system MgO-FeO-Fe2O3-SiO2 (Muan& Osborn, 1956; Presnall, 1966) it can beseen that Fe2O3 concentration determines theappearance of a spinel phase. If the Fe2O3concentration is low, i.e. if the Fe2O3/FeOratio of the liquid is low, a ferromagnesiansilicate will crystallize before spinel. However,in natural basalts, the presence of Ti allowsthe precipitation of a spinel phase withsignificantly lower Fe2O3/FeO ratio (TableI) . Thus spinel crystallization in a naturalbasalt will neither affect, nor be affected by,the melt's Fe3+/Fe2+ ratio as much as in thesimplified Ti-free system. In fact, in thesystem MgO — iron oxide — TiO2 in air

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LOW-PRESSURE OXIDES IN BASALTIC ROCKS 435

- C - IO3x 30

and Gilbert (1969)

800 900 1000 1100 1200 1300 1400_]_C*

Fig. 5. Oxygen fugacities (—logio atm.) plottedagainst temperature (10VK) for basalticrocks from the Southern Highlands.Points with the same symbol refer torocks from a single flow or differentiationsequence as in Fig. 4. The unbroken linerepresents the theoretical slope ofC= -103 X 30, calculated using the dataof Shibata (1967) and Darken & Gurry(1946). The dashed line is the experimen-tally determined curve of Wones &Gilbert (1969) for the system magnetite-quartz-fayalite.

(Woermann et ah, 1969) large primary-phaseregions of spinel and pseudobrookite(MgTi2O5) occur on the liquidus surface.These areas are continuous across the triangu-lar MgO — TiO2 — iron oxide diagram ofWoermann et ah, (1969, fig. 5, p.470). Theprimary phase areas of rhombohedral phases,however occur only in two small regions

adjacent to the MgO-TiO2 and iron oxide —TiO2 boundary lines. Again, this system cannotbe assumed to be completely analogous tonatural basaltic melts as f(O2) is extremelyhigh under equilibration with unlimited accessof air. It does suggest, though, that rhombo-hedral precipitation is somewhat more limitedthan spinel precipitation. The distribution ofopaque oxides in the Southern Highlandsalkali basaltic rocks is in complete accordwith this result, as many flows contain a spinelphase only and rhombohedral phases arealways associated with a spinel phase exceptin two basaltic rocks which have been heavilyoxidized at a late stage.

As the Fe3* /Fe2+ ratio in a melt is import-ant in determining oxide phase chemistry thefactors determining it are now considered.Melt composition, temperature, and f(O2)again are crucial. The effect of melt chemistrymay be assessed by analogy with experimentson alkali-silicate melts (Paul & Douglas, 1965)wherein for constant T and f(O2) theFe3VFe2+ ratios depend on the particularalkali ion: it is directly proportional to bothNa+K and K:Na. Following this, as alkalibasaltic liquids become enriched in alkaliswith differentiation, the Fe3+/Fe2+ ratio wouldincrease as crystallization proceeds, in theabsence of any other control. However,Anderson (1968b) notes that in the systemCaO+SiO2+FeO+Fe2O3> a decrease in CaOconcentration increases f(O2) by about twoorders of magnitude, if the bulk compositioncontains 20 mole % FeO + Fe2O3 and iftemperature and Fe3+/Fe2+ remain constant.He contends that because basaltic magmais first depleted in CaO and MgO in crystal-lization, thus giving passive enrichment in Naand K, 'these two results will have opposinginfluence on the ratio of (FeOa.5)/(FeO)'However, both these factors would actuallyincrease f(O2) or Fe3VFe2+ at constanttemperature. Thus it is possible that particu-larly with alkali basalts, early depletion in Caand Mg accompanied by increase in alkalicontent may increase f(O2) or Fe37Fe2+

ratio at constant T. Such a trend is in factshown in Fudali's (1965) experiments.

This highly theoretical discussion assumesthat it is valid to extrapolate qualitativelyresults of these simple artificial systems tobasalt melts. Consideration of actual earlycrystallization phases of natural basalts pro-vides a sounder basis.

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In the Southern Highlands, the first low-pressure phases to crystallize are olivine andopaque oxides with or without clinopyroxene.The Fe2O3/FeO ratio for olivines is extremelylow, less than 0.01, and for clinopyroxenes inalkali basalts is about 0.1 (using clinopyroxenecompositions in Wilkinson, 1966). The Fe2O3/FeO ratios of Fe-Ti oxides in basalts so farrecorded (Buddington & Lindsley, 1964; Car-michael, 1967b) range from 0.1 to 0.35 forilmenites.

Thus where olivine-opaque crystallization ismost significant, the Fe2O3/FeO ratio of theliquid will increase unless much more oxidethan olivine is precipitated. Crystallization ofclinopyroxenes with Fe2O3/FeO comparableto that of oxide phases will have a small effectowing to low total Fe content, but this effectwill parallel that of the oxides under the sameconditions. Assuming a melt of alkali basaltwith pristine Fe2O3/FeO ratio of 0.15 (anupper limit based on Coombs' 1963 discussion)and 1.5% Fe2O3 and 10% FeO, removal of10% by weight of olivine with 20% FeO, thenthe Fe2O3/FeO remaining in the melt is en-hanced by a factor of about 1.3. Olivineprecipitation therefore increases Fe3+/Fe2+ inthe melt, and if temperature remains constantwould increase f(O2). Fe-Ti oxide and clino-pyroxene precipitation could maintain, in-crease, or decrease this ratio depending ontheir particular Fe3+/Fe2+ partition andamount of crystallization.

Empirical observations on time of crystal-lization of Fe-Ti oxide species in alkali basalts(present study) and tholeiitic basalts (e.g.Peck et al., 1966) show that titaniferous mag-netite and ferrian ilmenite are respectively thefirst Fe-Ti oxides to appear. Thus Fe-Ti oxidesin alkali basalts should have a higherFe3+ /Fe2+ ratio than those in tholeiitic basalts,and based on this alone, the f(O2) of alkalibasalts would decrease with fractionation morerapidly than that of tholeiites. However, theusual greater amount of olivine in alkalibasalts would counter this effect, as calculatedabove. Thus there is no a priori way of pre-dicting relative or absolute f(O2) of basalticliquids without consideration of the chemistryof crystallizing phases in a given basalt. If anytrend does emerge it is that a highly fraction-ated alkali basalt may show slight increases inf(O2) while a moderately fractionated alkalibasalt may show a very slight decrease inf(O2) (due mainly to olivine crystallization),

assuming isothermal and isobaric conditions ina closed system.Temperature. As well as these chemicalinfluences, temperature may play an import-ant role. This may be estimated qualitativelyby considering the equilibrium constant for thereaction representing oxidation. Anderson(1968b) uses the equation

4FeO+O2=4FeO1.5However, from the work of Paul & Douglas

(1965) and Carmichael & Nicholls (1967) abetter representation of the same reaction,implying dependence on melt composition, offerric-ferrous equilibrium in alkali-silicatemelts is the equation:

Fe2+ + iO2 = Fe3+ + iO2-Thus, the temperature-dependent equili-

brium constant KT is:

K _T

* x (aO2-)i (a = activity) ( I)K

T aFe2* x (£O-2)iThis condition shows that fO2 depends onT and aO2- (i.e. melt composition and Fe3*^

IP7'now a = Y

(V= activity^ ctration)

now a = YX^ coefficient; X = molar concen-

ation)

K = X F e 3 'T XFe2*

_ C.

(2)

f(O2)

now K =•$••;+B (thermodynamic conditions)T B = constant

and C =H C = constant

2.3O3R

d(log K T ) = C(3)

dO)(T)

from (3) f(O..)» =•£• " XFe3*XFe2* (X O2-) •

Also from (3), assuming that O2-: Fe3VFe2*remains constant, then

d log K = -J d log f(O2)

=

.'.(t log fO2)

i.e. (log fO,)

cd illc ( T )i, - a log fo2), = c(-l- -

, - (logfO,)2 = Cj (^ •

1~T2

- —

( C , = 4C)Thus by assuming that O2- x Fe3* remains constant

Fe2*

it is shown that f(O2) varies as the reciprocalof the absolute temperature. Using thermo-dynamic data of Darken & Gurry (1946) andShibata (1967), as in Anderson (1968a), an

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LOW-PRESSURE OXIDES IN BASALTIC ROCKS 437

approximate value of Cr = 30 x -103 isderived. This curve is shown on Figure 5 tofacilitate comparison with plotted data fromthe present study. Also shown on Figure 5 isthe curve derived from the experimental sys-tem magnetite-quartz-fayalite (Wones &Gilbert); the slope of this line is -25 x 10". Thishas a slightly shallower slope than thetheoretically derived line.

Figure 5 shows that lines joining f(O2) andT points for a given basalt or related sequenceof flows (i.e. flows from a common vent)have a slope approximately parallel to thetheoretical line and almost exactly parallel tothe quartz-magnetite-fayalite line. This indi-cates that for the alkali basalts of the SouthernHighlands, change in Fe' i+/Fe2+ ratio and O2-activity during crystallization or fractionationhas only a small influence on f(O2) and thattemperature is the most significant factor.The basalts studied have not undergoneextreme differentiation. It would be interestingto study a sequence of flows representingsuccessive differentiates from a commonparent liquid • and which show marked com-positional changes, to observe the slopedivergence of this f(O,)/T plot against thetheoretical line of slope -30 x 10s. Nash &Wilkinson (1970) used silicate-oxide relation-ships in a similar way to show that in theShonkin Sag laccolith, an extremely alkalinemass showing fractionation, there is a regularvariation of f(O2) and T, indicating that themagma was internally buffered.

Results and DiscussionFor the Southern Highlands basalts, the

temperatures inferred on the basis of coexistingtitanomagnetite-ilmenite pairs, using Budding-ton & Lindsley's (1964) curves, are thoseexpected for alkali basalts; i.e. they range from1150°C to 950°C (Table I) . However, theestimates of f(O2) obtained in the same wayare lower than those predicted (Fudali, 1965),varying from 10-10 to 10-ls atm.

Fudali (1965) experimentally equilibratedselected basaltic rocks at known oxygenfugacities at constant temperature so that theoriginal Fe,O3/FeO ratio of the rock remainedunchanged. Fudali's experiments show amaximum range in f(O2) of 10-° to 10-fi-5

atm at 1200°C. His results also indicate that'there is a strong tendency for the f(O2) toincrease with increasing acidity of the rocksstudied'.

One source of uncertainty in Fudali's experi-mental method appears to be whether or notthe rock has been oxidized since crystallization.Fudali did not describe any polished specimensof the rocks used, so that one cannot assesswhether or not the oxides themselves show anyoxidation. Coombs (1963) suggests that allbasalt analyses where Fe2O3>1.5% indicatethat secondary oxidation may have taken place.Any secondary oxidation would result invalues of f(O2) higher than those at the timeof crystallization, as the Fe2O:, of the pristinelava would be lower than the determined value.

The alkali basalts from the Southern High-lands are much more undersaturated thanthose investigated experimentally by Fudali.Therefore, considering Fudali's conclusionthat f(O,) increases with increasing SiO?content, the rocks of the present study shouldhave somewhat lower oxygen fugacities. Thiseffect, coupled with the possibly inflatedexperimental f(O2) values due to secondaryoxidation, makes it reasonable to infer that theSouthern Highlands alkali basalts would havef(O2) lower than 10'8-2 at 1200°C (Fudali'slowest calculated value). Fudali's experimentswere carried out at 1200°C, up to 250°Chigher than inferred temperatures at the timeof crystallization of the Fe-Ti oxides in thealkali basalts of the present study. Extrapola-tion of the curve of the -log f(O2)/(103 / abs.temp.) (Fig. 3) to 1200°C gives a value off(O2) of approximately 10-° for theSouthern Highlands basalts. Sato & Wright's(1966) direct measurements show that f(O2)increases with increasing T and, in fact,extrapolation of their curve of -logf(O2)/(10:!/abs. temp.) for the Makaopuhilava lake agrees closely with Fudali's experi-mental determination of f(O2) for theKilauean basalt. Once again, the effect ofcrystallization environment of the Fe-Ti oxidesassumes importance.

For example, crystallization within a lavalake cannot be considered to take placewithout change in oxygen fugacity, owing tochanging composition of the liquid phase,temperature changes, and external (atmos-pheric) contamination. However, basalts whichhave almost wholly crystallized in a 'closed'system (e.g. a shallow magma chamber) orquickly chilled in eruption, will more closelyreflect a primary f(O2).

In the Southern Highlands, three flowsfrom one volcanic eruptive centre, built up by

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successive extrusions of restricted amounts oflava, exhibit serial variation with upwardprogression in the sequence. All flows arehighly undersaturated nepheline hawaiites andshow a slight differentation trend towardsdecrease in silica content and increase inalkali content. The lowermost glassy flowcontains an ilmenite-titanomagnetite assem-blage which equilibrated at f(O2) 10-10-9 atm(1000°C). The small grainsize of all phasesincluding the Fe-Ti oxides indicates rapidquenching. The next flow, towards the middleof the sequence, was determined to have anf(O2) range of 10"80 to 10"10-8 atm (attemperatures from 1200°C down to 920°C)at the time of equilibration of the oxidephases. The lower temperature is expected asthe grainsize of this flow suggests a slowerrate of cooling and possible subsequent lowertemperature of equilibration than the lower-most flow. The uppermost flow is a relativelycoarse-grained late differentiate which waserupted as a crystal mush after considerableintratelluric crystallization (Wass, 1973).Phenocryst ilmenite-magnetite pairs equili-brated at f(O2) 10-10-9 at about 1080°C, whilesmaller groundmass oxide pairs equilibratedat the higher f(O2) of 10"10-2 atm at 1000°C.This effect is shown graphically in Figure 5,where representative values of f(O2) and T forthese flows (denoted by crosses) show a trendtowards displacement from just below to justabove the curve with increasing fractionation.Such a tendency towards increasing f(O2)would be due to a combination of factors,including internal build up of volatiles, changein temperature, and change in the chemistryof the liquid, without any necessary additionfrom atmospheric oxygen. These factors arediscussed in detail below.

The oxygen fugacity and temperature ofequilibration of co-existing magnetite andilmenite for all the basalts studied have beenplotted in Figures 4 and 5. It is obvious fromthese diagrams that some oxygen buffersystem certainly operates for the alkali basaltcompositions studied and that this creates anenvironment in the melt close to that resultingfrom buffering by the quartz-magnetite-fayalite system. Most points lie within thefield which Carmichael & Nicholls (1967)regard as representing limits of f(O2) andtemperature for basaltic rocks (Fig. 4). Thedata plotting outside this field are those whichindicate equilibration at temperatures below

1000°C, which Carmichael & Nicholls take asthe lower limit of temperature of crystallizationof basaltic melts. This field should probablybe extended to 900°C in the light of directevidence that crystallization in basaltic meltsmay continue to this temperature (Peck et ah,1966; Moore & Evans, 1967).

OLIVINE-OXIDE INTERGROWTHSIN OXIDIZED LAVASPetrography

Two flows contain olivine phenocrystspacked with opaque inclusions. The olivinesare euhedral and occur as single phenocrystsor in glomeroporphyritic aggregates with otherolivine grains with or without zoned titan-augite. Individual olivine grains range from0.2 to 2.5 mm in length; glomeroporphyriticaggregates of up to eight grains may reach 0.8cm across. Determination of the compositionof least altered olivine grains by X-raydiffraction and refractive index determinationindicates an average composition of aboutFa25 in both basalts.

In transmitted light, the olivine shows asponge-like texture due to dense distributionof opaque granules. Racking through the planeof focus indicates these granules have a3-dimensional vermicular morphology. Theapparent dimensions of these opaques are amedian length of 0.01 mm and width of0.002 mm. Rare opaque 'globules' about0.05 mm in mean diameter occur in someolivines.

Reflected-light examination reveals a largevariety of textures ranging from olivines withfew opaque inclusions to those which appearto be over 50% opaque. The actual surfaceexpression of the opaque is much less thanthat suggested in transmitted light (Figs. 6-8)because of their highly dendritic or symplec-titic nature, and the mode varies from grainto grain. At very high magnification (Fig. 7)this feature is clearly displayed. The vermiculesappear to consist solely of magnetite. Almostinvariably there is a nearly continuous rim oftitanomagnetite and rare hematite granulesaround the margins of the olivine grain,succeeded immediately by a zone of olivinerelatively free of opaque granules. Where theolivines show cracks these are densely crowdedwith magnetite (sometimes with hematite).Some olivines contain large subhedral oranhedral blebs up to 0.1 mm across. These aresurrounded by a rim up to 0.01 mm across

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LOW-PRESSURE OXIDES IN BASALTIC ROCKS 439

Fig. 6. An oxidized olivine grain showing aspongy texture due to 'exsolution' ofdensely distributed magnetite. A zone ofrelatively inclusion-free olivine occursbefore the outer rim of granular magne-tite. Transmitted plane polarized light.

of a silicate phase (Figs. 7 and 8).Transmitted-light examination shows this to becolourless, of low birefringence, and with arefractive index considerably less than thesurrounding olivine. It is tentatively identifiedas quartz.

Results and DiscussionThis intergrowth of vermicular magnetite

and rare hematite blebs with olivine is inter-preted as resulting from deuteric* oxidationof originally homogeneous olivine grains. Thereaction may be represented by the followingequation:3(Fe2SiO4 + Mg2SiO4) + O, ±? 2Fe3O,+6MgSiO:).

The tentative identification of quartz rim-ming hematite blebs suggests silica may be ametastable product in patches of extremeoxidation. No other definite silicate phase wasidentified. Powder diffraction photographsshow lines consistent with olivine and mag-

netite phases but not unequivocably attribu-table either to pyroxene or silica phases.Broad diffuse olivine lines with no separationin back-reflection regions suggest either zoningof the original olivine (highly likely),possible production of a forsterite phase onoxidation, or both.

That oxidation caused this intergrowth issupported both by the nature of the hostbasalts and two experimental studies on heat-ing of natural olivines. Both flows showexcessively large percentages of Fe2O3(8.03% and 8.15%). One flow contains ovoidpatches up to 1.0 mm across which appear tobe gas bubbles or segregation vesicles infilledlate in crystallization by hydrous phases suchas analcime, amphibole, and zeolites, and theother is abundant in zeolitic patches. How-ever, neither shows a large proportion ofrhombohedral opaque phases in the ground-mass.

Fig. 7. Magnetite in oxidized olivine. Theelongate opaque segregations show a pre-ferred orientation parallel to the c axis ofthe host olivine. The large globule in thebottom left-hand sector is rimmed bysilica. Transmitted plane polarized light.

* Deuteric is used in the sense of late-stage magmatic phenomena in the presence of high volatile orvapour concentration.

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Fig. 8. The same view as in Fig. 7, in reflectedlight. The actual surface expression ofopaques is less than that suggested intransmitted light. The vermicular natureof the opaque segregations is evident.Reflected light.

Haggerty & Baker (1967), in a series ofheating experiments on natural olivine, foundthat magnetite-enstatite is the first-formedassemblage. This is metastable, and subsequentoxidation produced hematite-forsterite. Champ-ness & Gay (1968) used X-ray single-crystaland powder methods, infrared spectroscopy,and electron optical and diffraction techniques.They found evidence of silica identifiable asquartz, tridymite, and cristobalite with a'pyroxene-like' phase appearing on subsequentreaction.

These latter results accord well with obser-vations in the present study except that nopyroxene-like phase could be definitelyidentified. It is possible that silica is firstformed as a metastable reaction product whichthen reacts with olivine to form a pyroxene.

Frisch (1970) describes a gabbro nodulefrom Lanzarote (Canary Islands) in which theolivine has undergone complex oxidation and

hydrolysis after inclusion in the host basalt.Hydrated iron oxides, forsteritic olivine (Fo9,),and free (?) silica were produced. As in theolivines in the present study, there is no evi-dence for a pyroxene phase. Silica was notidentified optically but it was shown to bepresent by microprobe traverses in areas ofgoethite. This supports the interpretation thatrims around hematite are some form of silica(Figs. 7 and 8).

The rarity of hematite in these oxidizedflows is attributable to two factors: (a)insufficient availability of O2 or (b) the opaquerims formed around the olivine which mayprovide an effective barrier to diffusion of O,.

EXTREME DENDRITICCRYSTALLIZATION OF FE-TI OXIDESPetrography

Dendritic1 growth of titanomagnetite andskeletal1 growth of ilmenite are common inflows from the Southern Highlands whichcrystallized rapidly in the last stages and whichcarry a crypto-crystalline or glassy mesostasis.One flow shows good development of thesecrystal forms in all stages.

Field and petrographic evidence indicatethat this particular flow was marginally chilledon extrusion, but cooled more slowly incentral regions. A gradational textural changeexists from a glassy to a finely crystallinemesostasis, the most vitreous groundmasscontaining best examples of the crystal formsdiscussed here.

The habit of titanomagnetite is spectacular(Figs. 9, 10, and 11) with dendritic crystalsdisplaying multi-branched arms all in apparentcrystallographic continuity. Major branches upto 0.2 mm long give rise to lateral arrow- andstar-shaped protuberances generally terminatedby growth points (morphologically analogousto swallow-tail terminations in skeletalcrystals). Cubic symmetry is reflected in themutual orientation of arms and the smallcomponent octahedra generally at 90° or 45°within a single crystallographically continuouscrystal.

These titanomagnetite crystals are accom-panied by skeletal ilmenite grains (Fig. 11)with hollow centres and swallow-tail extremi-ties. The crystallization of these skeletal and

1 The term dendrite is here restricted to crystals exhibiting arborescent forms of a continuous crystallo-graphic lattice; skeletal refers to crystals with a basically euhedral shape, often discontinuous or'hollow' owing to central voids or swallow-tail terminations and lacking branching typical of dendrites.

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Fig. 9. Dendritic crystal of titanomagnetite. Thecurvature of the upper left marginsuggests continual interference by theadjacent clinopyroxene crystal duringcrystallization. This may indicate con-temporaneous growth of the pyroxene andtitanomagnetite. Note ilmenite needle,lower centre. Reflected light.

Fig. 10. This dendritic crystal of titanomagnetite,from a coarser part of the flow than thetitanomagnetite crystals in Fig. 11, islarger and more 'filled in' than the den-dritic titanomagnetite crystals from moreglassy parts of the flow. Reflected light.

dendritic opaque phases in the mesostasisappears to have been contemporaneous withthat of clinopyroxene skeletal and subspheru-litic forms. The clinopyroxene is titanaugite,strong zoning being indicated by markedincrease in depth of colour towards the rim.In this particular flow at least, texturalevidence (dendritic and skeletal habit, as well

Fig. 11. Dendritic crystal of titanomagnetite fromthe most glassy part of the flow.Reflected light.

as occurrence confined to mesostatis) suggeststhat the opaque phases crystallized at a verylate stage compared with the majority of thesilicates.

DiscussionDendritic crystallization implies a certain

physico-chemical environment of precipitation,as the crystal morphology with numerousbranches and protuberances represents a high-energy form (Horvay & Cahn, 1961). Tem-perature gradients due to differential thermalconductivity in the growing crystal, slowdiffusion rates of critical ions, and supersatura-tion or supercooling of the melt, coupled witha limited number of nuclei, may all result indendritic or skeletal growth. Spry (1969)suggests that continued crystallization will leadto infilled interstices between branches ofdendrites, culminating in a complete, euhedralcrystal. The observable gradation exhibited inthe described flow, from extreme dendriticmorphology in the glassy sections to euhedralgrains with a small number of dendritic out-growths in the more crystalline variants, (e.g.Figs. 11 and 9) suggests that rate of growthcontrols dendrite formation. The euhedraltitanomagnetite grains are formed by slower,more uniform crystallization rather than bysubsequent infilling. Chemical data on zoningof the titanomagnetite could resolve this (anindication of zoning is optically detectable intitanomagnetite by slight reflectivity differen-ces), but some data are provided by analogywith clinopyroxene skeletal and euhedralcrystals of similar occurrence in the mesostasis.

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Skeletal crystals exhibit normal zoning to adeeper pink margin, relatively enriched in Fe,Ti, and Al. Euhedral crystals exhibit similarzoning with no evidence of infilling of hollowcentres or swallow-tail indentations by laterprecipitation.

Rice et al. (1971) describe the occurrenceof skeletal pseudobrookite in a rapidly chilleddyke. Because of the association of ulvospineland pseudobrookite, a disequilibrium assem-blage in terms of phase equilibria in the systemFeO-Fe2O3-TiO2 (Taylor, 1964), they con-clude that skeletal crystallization 'is not onlyindicative of abnormal growth and temperatureconditions, but almost as surely of chemicaldisequilibrium as well'. Strong zoning ofskeletal titanaugites associated with thedendritic oxides supports this concept. Thedendritic growth of titanomagnetite appears tohave taken place contemporaneously withskeletal growth of associated ilmenite andclinopyroxene, difference in morphologyprobably being a result of different structuresof the crystallographic lattices of the variousphases. It is likely, therefore, that bothdendritic and skeletal forms reflect lack of

chemical equilibrium during crystallization,and oxygen fugacity and temperature estimatescannot be derived for the host basaltic liquidby Buddington & Lindsley's (1964) method.

CHROMIAN SPINEL EUHEDRAIN OLIVINEPetrography

Chromian spinel euhedra up to 0.2 mmacross are included in large olivines, many orwhich are interpreted on the basis of chemicalcomposition, morphology, and resorption tobe precipitated at moderate pressures (prob-ably less than 5 kb). Such olivine/Cr-spinelrelationships are evident in numerous flowswhere olivine appears to have been the firstmajor crystallizing phase. Usually the opaqueCr-spinels are completely enclosed within theolivine grains (Fig. 12), but where significantresorption has affected the olivines, the Cr-spinel euhedra may occur at new grain-boundary sites or isolated from the olivine inembayments and pseudo-inclusions resultingfrom the resorption (Fig. 13). Where incontact with the host basalt in this way theCr-spinels always show marginal reaction to a

Fig. 12. Cr-spinel inclusions in olivine. Note thegeneral euhedral nature of the Cr-spinelgrains. Reflected light.

SQfS'mroi

Fig. 13. Reaction rim on Cr-spinel. The opaquegrain in the centre is a Cr-spinel (innerpart with low reflectivity) surrounded bya titanomagnetite rim (relatively highreflectivity). The Cr-spinel is jutting intoa large embayment in a resorbed olivinegrain, two portions of which are evidentin the upper left and lower right of thefield of view. The edge of the Cr-spinelstill in contact with the olivine showsno reaction effects, but where in contactwith the host basalt in the embayed area,the Cr-spinel has reacted with the liquidto form the titanomagnetite rim.Reflected light.

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LOW-PRESSURE OXIDES IN BASALTIC ROCKS 443

TABLE V

Chemical Data for Chromian Spinels included in Olivine

1 2 3 4 5 6 7 10

SiO,TiCXA126,Cr2O3

*FeONiOMgOCaO

0.343.13

0.422.78

0.365.02

0.365.86

0.4311.5

0.4514.6

0.200.69

0.110.00

0.050.69

0.640.96

18.130.738.8—

7.400.07

17.333.636.1—

8.260.04

16.028.343.8—

5.640.08

14.526.146.4—

5.470.09

8.6517.455.7—

3.900.13

6.4510.163.1—

2.760.16

41.616.425.2

0.2114.30.04

20.835.732.40.178.930.00

21.529.834.3

11.8O.06

22.033.123.5

17.80.06

Total 98.5 98.5 99.2 98.8 97.7 97.6 98.6 98.1 98.2 98.1

* Total Fe as FeOCalculated End-Members

MgAl,O4MgCr2O4

FeAl2O4

FeCr2O4NiFe2O4

Fe.,SiO4

Fe2Ca04Fe-2TiO4

Fe3O4

25.21.50.0

43.9—

1.20.28.7

19.2

24.16.70.0

41.6—

1.40.27.8

17.7

19.40.02.8

41.6—

1.20.3

14.920.5

19.90.00.4

38.5—

1.20.4

16.323.1

12.12.30.0

23.1—

1.40.4

32.228.3

9.01.00.0

13.9—

1.40.7

40.733.3

50.40.09.4

24.20.70.60.22.0

12.4

29.03.30.0

48.80.70.40.00.0

17.7

30.015.40.0

26.0—

0.20.22.0

26.2

30.741.20.04.0

2.20.22.7

18.8

Total

HOST

FoFaLa

99.9 . 99.5

OLIVINE COMPOSITION

82.617.10.3

100.7 99.8 99.8 100.0 99.9

81.617.90.5

99.9

86.013.60.3

100.0

83.016.80.2

99.8

83.416.50.1

1 and 2. Cr-spinels in olivines from one flow.3 to 6: Progression from core to margin of a Cr-spinel now in contact with host basalt due to resorption

of host olivine (Fig. 13).7 to 10: Cr-spinels in olivines from different flows.

titanomagnetite compositionally equivalent togroundmass titanomagnetite. Cr-spinel is nota normal groundmass phase in the basalticrocks from the Southern Highlands and occursexclusively as early-formed inclusions inolivine.Chemistry

These spinels vary in composition fromflow to flow: e.g. A12O3 ranges from 20% to40%; iron (as FeO) from 23% to36%; MgO from 9% to 18%; and Cr2O3 from16% to 36% (Table V). TiO2 content israrely less than 1% and the variation in MgOis inversely proportional to the iron content.Microprobe traverses of one grain in contactwith the host basalt show the reaction rim isa result of gradational chemical change withCr2O3, A12O3, and MgO decreasing towardsthe margin while iron and TiO2 increase. Thisindicates miscibility of the Cr-spinel andulvospinel at temperatures below 1200°C.Recent work on spinels from lunar rocks (e.g.

Haggerty, 1971; Haggerty, 1972; and Reid,1971) also suggest that there is at least an in-complete solid solution series betweenFeCr2O4-FeAl2O4 and Fe2TiO4.

The Cr-spinels of this study and those dis-cussed by Evans & Moore (1968) and Evans& Wright (1972) are compositionally distin-guished from the Cr-spinels of stratiformultramafic bodies by the relatively low TiO2

content of the latter. The range of Cr2O3 inthe spinels of the present study also showsgreater variation than that recorded by Evans& Wright for the Kilauean lavas. This may bedue to the wider range of host basalticcompositions represented in the present study,or to a wider range of stages at which Cr-spinelwas precipitated, as relatively later-formedspinels will probably be enriched in theulvospinel component.Discussion

The significance and environment of forma-tion of the Cr-spinels in the Southern High-

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lands rocks is not clear. Their consistentassociation with olivine megacrysts containingrelatively low (0.25%) CaO contents indi-cates a moderate pressure origin (Simkin &Smith, 1970). Supporting this concept of somedegree of pressure dependence is the commonoccurrence of chromian spinels in intrusionssuch as Stillwater (Jackson, 1961). However,Evans & Moore (1968) record Cr-spinel as thefirst crystallizing phase in the Makaopuhi LavaLake, Hawaii; and D. H. Green (pers. comm.)has identified Cr-spinel as the first liquidusphase in experimental runs at atmosphericpressure on lunar samples. It is important tonote, however, that Evans & Moore (1968)detected a depth dependence for the composi-tion of the spinel, Cr2O3 increasing, and TiO2decreasing, with increasing depth. Evans &Moore also interpret the most Cr-rich spinelsin the chilled marginal basalt as beingpre-eruptive precipitates. Evans & Wright(1972) state that apparently primary composi-tional trends in liquidus chromite from the1965 Makaopuhi eruptions 'are observable inrapidly quenched basaltic pumices, but aresignificantly modified during subsequent crys-tallization of the host basalt'. In addition, inthese rapidly quenched rocks, mutual equili-brium appears to have been reached betweenco-precipitated olivine and chromite. Thequenched nature of the host pumice againsuggests intratelluric crystallization of the Cr-spinels and also eliminates the possibility ofsubsolidus re-equilibration of the olivine andCr-spinels.

Precipitation of Cr-spinel is favoured bylow oxygen fugacity and high temperaturesaccording to Roeder & Emslie (1970). Aspreviously discussed, the oxygen fugacity inthe Southern Highlands basalts appears to bean intrinsic chemical parameter of each flow,changes in f(O2) being directly related totemperature changes. Therefore, at high tem-peratures, f(O2) is relatively low and viceversa, illustrated by a negative slope of thef(O2)/T curve (Fig. 4). It is possible thatsome intratelluric crystallization proceeded atlower f(O2) and higher temperature than thatat the time of precipitation of titanomagnetitesand ilmenites on final cooling of the basalticliquid.

A more likely explanation is that Cr-spinelis a liquidus phase in magmas of certaincompositions. In the Southern Highlands, theoccurrence of Cr-spinel is restricted to rocks

in which abundant olivine appears to have beenthe first crystallizing phase over a largeinterval. Its precipitation occurs at the earlieststage of moderate-pressure crystallization andits termination as a crystallizing phase may becontrolled by the incoming of a new liquidusphase. In this way it is possible that Cr-spinelswill be precipitated at various stages rangingfrom moderate to atmospheric pressures.Keith (1954) investigated liquidus crystalliza-tion in the system MgO-SiO2-Cr2O:l andshowed that Cr-spinel ('pichrochromite') isprecipitated together with forsterite (or amagnesian orthopyroxene) along cotecticboundaries close to the MgO-SiO2 join. ThusCr-spinel may be a liquidus phase within asmall range.

Irvine (1965, 1967) suggests that disappear-ance of Cr-spinel as a crystallization productmay be due to a 'peritectic (reaction) relationleading to formation of a pyroxene'. Relevantto this study are the instances cited whereincoming of clinopyroxene is accompanied bytermination of Cr-spinel crystallization. Ultra-mafic masses at Duke Island, Alaska, (Irvine,1963) and British Columbia (Findlay, 1963)contain abundant Cr-spinel in the dunites butnone in associated clinopyroxene-bearingrocks. Tholeiitic bodies such as Stillwater,Skaergaard, and Muskox show similar relation-ships with incoming of orthopyroxene but arenot directly analogous to the alkalic basalticmagmas of this study. In the basaltic rocks ofthe Southern Highlands, Cr-spinel is absent inany high-pressure cumulate or low-pressureassemblage where clinopyroxene was aprecipitating phase. Kushiro & Schairer (1963)in investigating liquidus phases in the systemMg2SiO4-CaMgSi;,OG-SiO2 demonstrated thatincoming of calcic clinopyroxene in a basalticmelt depends mainly on the CaO content ofthe melt. This then would also be a criticalparameter in determining the extent of Cr-spinel crystallization in a given liquid.

Irvine (1967) suggests a correspondence inMgO:FeO of spinel-silicate pairs. Howeverrather than the crystal-crystal equilibriumrelationship which Irvine implies, it isprobably a liquid-crystal equilibrium whicheffects this correlation.

In summary, the Cr-spinels from the South-ern Highlands basaltic rocks are interpretedas having precipitated at moderate pressures(<5 kb) together with olivine, from basalticmagmas of appropriate composition. Appear-

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LOW-PRESSURE OXIDES IN BASALTIC ROCKS 445

ance of clinopyroxene as a liquidus phaseterminated Cr-spinel crystallization. It ispossible that in other provinces Cr-spinel maycrystallize at atmospheric pressure under givenconditions, but in rocks of this study it isexclusively associated with intratelluricolivines, implying precipitation at moderatepressures.

CONCLUSIONSMost alkali basalts from the Southern

Highlands have f(O2) varying from 10-12-8 to10-* atm, at temperatures of 950°C and1100°C as inferred from coeval ilmenite-titanomagnetite pairs. Extrapolation (parallelto the theoretical curve) to temperatures of1200°C gives an f(O2) range of 10"9-5 to1O8-7 atm., which is about an order ofmagnitude lower than experimental values(Fudali, 1965) and directly measured valuesfor tholeiitic lava-lake basalts (Sato & Wright,1966). Because of their general intratelluriccrystallization, quenched nature, and lack ofevidence of post-crystallization oxidation ofthe opaque oxides of the alkali basalts studied,the lower f(O2) values are predictable. Withinthe range of alkali basaltic compositionsinvestigated, temperature is the most signifi-cant factor in controlling f(O2) after initialpartial melting, with minor influence fromchanging melt composition due to anyfractionation.

The ultimate origin of the f(O2) appears tobe closely linked with the genesis of the hostbasaltic rock. The close congruence ofdetermined values with the quartz-magnetite-fayalite buffer curve suggests a similar buffercontrol with minimum contamination eitherfrom, for example, H2O of surrounding rocksduring magma ascent, or atmospheric oxygen,implying effective internal buffering of thebasaltic magmas from the Southern Highlands.Results of the present study strongly supportthis; and it is in agreement with other datafrom co-equilibrated Fe-Ti oxides from basicand silicic lavas (Carmichael, 1967a; Car-michael & Nicholls, 1967; Anderson, 1968b),from volcanic gases (Heald et ah, 1963; Sato& Wright, 1966), and from silicate-oxiderelationships (Nash & Wilkinson, 1970).

If many of the Southern Highlands basaltsrepresent near-primitive liquids, their intrinsicf(O2) would result from original crystal/liquidequilibria at the source of partial melting inthe mantle (Green & Ringwood, 1967). In

such an environment of origin it is conceivablethat a buffer system closely approximating thequartz-magnetite-fayalite system may exist ina parent mantle rock similar to a lherzolite incomposition, despite the abundance of Mg.

' One such buffer system, suggested by Ander-son (1968b), may be:

3 Fe2Si04 + iO2 = Fe3O4 + 3 FeSiO3where participating phases Fe2SiO4, Fe3O4FeSiO3 represent solid solution components inolivine, chromian spinel, and pyroxene respect-ively.

In certain magmas, unusually high concen-tration of volatiles can create an environmentof abnormally high oxygen fugacity. Resultantoxidation of early-formed intratelluric pheno-crysts results in formation of magnetitehematite or both along with a more magnesianolivine and liberation of free silica.

Extreme dendritic crystallization of oxidestakes place in a supercooled lava. Such crystalsare characterized by inhomogeneity of chemi-cal composition as the result from non-equilibrium precipitation, and therefore cannotbe used to determine T and f(O2) at the timeof their crystallization.

Small euhedral Cr-rich spinels included inolivines were probably precipitated togetherwith olivine as the earliest phases in thesealkali basaltic rocks in which olivine was theexclusive silicate phase on the liquidus over aconsiderable interval. It is suggested that thecrystallization of Cr-spinels may be pressure-dependent to some degree and that composi-tional variations may derive from differencesin the host basaltic compositions and from thevarious stages in the magma's crystallizationhistory thai the Cr-spinel may have beenprecipitated.

ACKNOWLEDGMENTSResearch for this study was carried out in

the Department of Geology and Geophysics,University of Sydney, and in the Departmentof Mineralogy, British Museum (NaturalHistory), by courtesy of Professor C. E.Marshall and Dr A. A. Moss respectively.Financial assistance from the Edgeworth DavidTravelling Scholarship, awarded by the Univer-sity of Sydney, is gratefully acknowledged.Valuable assistance with electron microprobetechniques was given by Dr S. Reed and MrR. Symes. Associate Professors R. H. Vernonand T. G. Vallance critically read themanuscript.

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SMITH, A. L., & CARMICHAEL, I. S. E., 1969:Quaternary trachybasalts from southeasternCalifornia. Am. Miner., 54, 909-923.

SPEIDEL, D. H., 1970: Effect of magnesium on theiron-titanium oxides. Am. J. Sci., 268, 341-353.

SPRY, A., 1969: Metamorphic Textures. Per-gamon Press, Oxford.

TAYLOR, R. S., 1964: Phase equilibria in thesystem FeO-Fe2O3-TiO2 at 1300°C. Am.Miner., 49, 1016-1030.

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WASS, S. Y., 1971: Studies on some basaltic rocksin New South Wales. Unpub. Ph.D. thesis,University of Sydney, Sydney (1971).

WASS, S. Y., 1973: The origin and significance ofhour-glass zoning in titaniferous clinopyro-xenes. Miner. Mag., 38, 133-144.

WATKINS, N. D., & HAGGERTY, S. E., 1967:Primary oxidation variation and petrogenesisin a single lava flow. Beitr. Miner. Petrogr.,15, 249-269.

WILKINSON, J. F. G., 1965: Titanomagnetitesfrom a differentiation sequence, analcime-olivine theralite to analcime tinguaite. Miner.Mag., (Tilley vol.), 34, 528-541.

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WOERMANN, E., BRENZY, B, & MUAN, A., 1969:Phase equilibria in the system MgO-ironoxide-TiO2 in air. Am. J. Sci., 267A, 463-479.

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Suzanne Y. Wass,School of Earth Sciences,Macquarie University,North Ryde,New South Wales, 2113.

Journal of the Geological Society of Australia, Vol. 20, P t 4, pp. 427-448, December, 1973.

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