31. PETROGRAPHY OF OPAQUE MINERALS IN BASALTS DRILLED ON DSDP LEG 45

P. Eisenach, Institut fur Allgemeine und Angewandte Geologie, Ludwig-Maximilians Universitàt, München,Federal Republic of Germany

INTRODUCTION

Basalts have been studied for centuries, but inten-sive studies of the opaque mineral phases they contain,especially of the iron-titanium oxides, have only re-cently been undertaken. These, even if they constituteless than 1 volume per cent of the whole, are the carri-ers of the natural remanent magnetization (NRM).The contribution of ilmenite and sulfides to the mag-netization can be neglected. Reference to the ternarysystem FeO-Fe2O3-TiO2 is necessary for interpretationof palaeomagnetism data. Different oxidation states,high temperature, deuteric oxidation (classified in sixstages of increasing oxidation [Watkins, 1967]), low-temperature oxidation (T

P. EISENACH

TABLE 1Opaque Minerals of Holes 395, 395A, and Site 396 Basalt

Sample(Interval in

cm)

11-2, 100-106

18-2,31-33

18-2, 35-37

8-1,127-135

11-1,56-66

14-1,87-99

14-2, 125-134

17-1,46-55

21-1, 112-123

22-2, 125-130

23-1,137-142

28-1,4448

31-1,125-128

32-1, 140-150

Lithology

fine-grainedaphy. basalt

pl.ol.phyric basalt

pl.ol.cpx.phyric basalt

aph.

aph.

pl.ol.ph.

pl.ol.ph.

pl.ol.cp.ph.

pl.ol.cpx.phyric

pl.ol.cpx.phyric

pl.ol.cpx.phyric

pl.ol.phyric

pl.ol.(cpx)phyric

volcanicbreccia

Titanomagnetite

Two generations; tiny skeletal to anhedralforms, partly as a rim round the plag.laths.The larger skeletal ones contain expansioncracks and are fringed by secondary titan-omagnetite; weak anisotropic;smaller: 1 µm-sub.larger: 50 µm-10 µm

Only tiny skeletal grains (snowstars)showing reddish internal reflections underhighest magnification;10 µm-sub.

Like Sample 18-2, 31-33 cm.

Subhedral to anhedral forms; grain sizedown to the limit of visibility; somefringed by a fine titanomag., some show-ing red staining around the grains; 1 µm-sub.

Subhedral to skeletal shapes, which arecontained particular in the roundishareas of the groundmass; 20 µm-sub.

Two size generations of skeletal to sub-hedral forms; expansion crack andmottling; partly larger ones fringed bytitanomag.; tiny white reflecting parti-cles may be hematite;larger: 60 µmsmaller: 1 µm

Two size generations of skeletal forms;larger ones displaying mottling and ex-pansion cracks; some fringed by a sec.extra fine grained titanomag.larger: 63 µmsmaller: 5 µm

Small skeletal forms; larger displayingmaghemitization and sometimes tinyfringes of hematite;12 µm-sub.

Subhedral to skeletal forms; larger onesshow mottling by maghemitization;15 µm

Two generations; larger one anhedral,smaller one skeletal; former are mottled;near the skeletal margins lie tiny hema-tite grains. Partly red-stained surround-ing groundmass;larger: 12µmsmaller: 5 µm

Small skeletal shapes; weak mottling andexpansion cracks, some fringed by fine-granuled hematite; 6 µm

Small skeletal grains, altered, mottled like23-1; 3 µm

Small skeletal grains; partly mottled bymaghemitization, partly fringed by fine-grained hematite; 5 µm

Two skeletal generations from differentgrain size; smaller range from 15 µmdown to the limit of visibility, largerfrom 150 to 30 µm; latter show mottlingand expan. cracks. Others contain iso-lated coarse lamellae of ilmenite. High-temperature oxidation followed by low-temp, oxidation;larger: 130 µmsmaller: 10 µm

Ilmenite

Small primary laths often mantledby titanomagnetite; 10-5 µm.

Rare small primary laths;10 µm-sub.

Some tiny laths and rare largergrains mantled by titanomagne-tite; 10 µm-sub.

Small primary laths sometimesmantled by titanomag.; 5 µm-sub.

Increased content of small pri-mary laths, partly arranged in aparallel direction; 30 µm

Primary laths, partly formed liketitanomag. skeletals, some man-tled by titanomag.; 35 µm

Usually small prim, laths; 15 µm

Small prim, laths and two largerilmenohematite; 20 µm

Larger grains, habit like titano-mag. and small laths; all prim,phases; 10 µm

Small prim, laths; 3 µm

Small prim, laths; 3 µm

Larger subhedral grains; anothermore common phase composedof small laths; 7 µm

Prim, and secondary (lamellae)phases; former are laths, mostlymantled by titanomag., replacedby iron hydroxide again. Smallerlaths displaying twin lam. andpartly parallel arrangement;60 µm

Sulfides

Masses of small grains of pyrrhotite,especially near the iron-titaniumoxides; 10 µm-sub.

Extremely fine roundish indet.grains; 1 µm

Extremely fine grains, too small fordet. 1 µm

Rare extremely fine indet. grains;1 µm

Rare small pyrrhotite and finerindet. grains; 1 µm

Rare small grains of pyrrhotite;1 µm

Small isometric pyrrhotite andpyrite; some spherical structures oforiginal pyrite replaced by ironhydroxide

Considerable content of pyrrhotite

Small roundish pyrrhotite, partlyenclosed by plag. phenocryst;1 µm

Tiny roundish forms of indet.grains; 1 µm

Too small for det.; 1 µm

Fine pyrrhotite grains surroundedby plag.ph.

Abundant pyrrhotite and pyrite,partly as separated grains, partly asmultiphase of both

Other Phases

A large euhedral porous chromite

A large euhedral porous chromitegrain, slightly mottled andrimmed by a tiny seam of titano-magnetite; 50 µm

A large euhedral porous grain,partly rimmed by titanomagnet-ite and homogeneous ones sur-rounded by pyroxene and/orplag.phenocrysts; 70 µm

Rare content of iron hydroxide

Increasing content of ironhydroxide; ilmenite is partlyreplaced by iron hydrox. andpyrite

Rare large euhedral, homo-geneous chromites, 10-20 µm,show a small seam of titanomag.;seam is absent in the part lying inplag. phenocrysts; iron hydroxidein and around the displacedolivines replaces titanomag.

One large porous chromite, 60 µm,partly rimmed by a fine seam oftitanomag; increasing content ofiron hydroxide in veinlets andaround the pyroxenes and olivines

Isometric, homogeneous chromitegrain, 50 µm, sometimes rimmedby a fine seam of titanomag.;abundant iron hydroxide, partlyreplacing titanomag.

Large anhedral chromite grainshow a fine seam of titanomag.;50 µm

Great content of iron hydroxide,on basis of decomposition ofolivine, red staining of the adja-cent groundmass; euhedralchromite enclosed by plag.phenocrysts; 30 µm

Abundant content of ironhydroxide lining veinlets andvesicles; large homogeneous andporous chromites; 30 µm

Some euhedral grains of chromite;homogeneous but slight mottling;50 µm

558

OPAQUE MINERALS

TABLE 1 - Continued

Sample(Interval in

cm)

33-1,70-77

49-1, 77-84

52-1,45-50

57-1,47-53

58-2, 88-94

61-1, 140-150

62-1,40-50

63-1,0-10

64-4,115-122

64-1, 137-142

64-2, 67-73

64-2, 120-132

644, 5-10

66-2,4248

14-6,3743

15-1, 115-123

Lithology

pl.ol.phyric

volcanicbreccia

aphyricfine gr.

aphyricfine gr.

hyaloclastit.

aphyric

aphyric,fine-grained

pl.ol.cpx.doleritic

pl.ol.cpx.doler.

pl.ol.cpx.doler.

pl.ol.cpx.doler.

pl.ol.cpx.doler.

aph. finegrained

aph. fine-grained

f. gr. ph.basalt

f gr. pi.ph. bas.

Titanomagnetite

Skeletal shapes of very fine grain size;expan. cracks and mottling by maghemi-tization; lOµm-sub.

Two generations; smaller consists ofskeletal forms (in the glassy matrix),larger are skelet. and fragments in the re-crystallized areas. Expan. cracks.larger: 15 µmsmaller: 5 µm

Extremely fine grained skeletal toanhedral forms, displaying mottling morereddish, partly colored int. reflections,and tiny rims of maghemitization1 µm

Very fine skeletal to anhedral shapes, yetwithout the reddish tints, as in 52-1;1 µm

Small skeletals, partly fringed by extreme-ly fine grained hematite maghemiteappearances on the straits of the skeletals(for example the axes or spurs of thecenters of the planes). Expan. cracks;20 µm-sub.

Very fine grained skeletal shapes withslight reddish tints; expan. cracks andfringes of fine hematite; 10 µm-sub.

Two generations of skeletals from differ-ent grain size; smaller ones around thepyroxene boundaries; the larger ones con-tain a network of sec. ilmenite (lamelae)displaying hematite exsolutions to thephase boundaries of magnetite; occasionalfine fringes of ilemenite around thegrains. Some cracks filled by a red-brownphase (less distinct anisotropy asilmenite); all grains show strong corrosionand numerous expan. cracks. High-temp.oxidation;larger: 150-30 µmsmaller: 15 µm-sub.

Two generations like 63-1;larger: 150-40 µm-sub.smaller: 5 µm-sub.

Two generations like 63-1; only singleex solution lamellae of ilmenite;larger: 80-30 µmsmaller: 3 µm

Two generations like 63-1; curiously onlyfew grains contain single ilmenitelamellae.

Very fine grained skeletal shapes showcolored and a red-stained groundmassabout the grains, indications of strongalteration.3 µm-sub.

Small anhedral to skeletal forms, mostlyarranged between the plag. laths of thegroundmass; expan. cracks and mottling;10 µm-sub.

Very fine grained skeletals; expan. cracksand tiny white reflecting particles nearthe margins; 15 µm-sub.

Very small grains show skeletal to sub-hedral forms, partly rimmed by a finegranuled hematite. Appear to be twotypes of grains with a different color anda diff. magnetic behavior. The gray-blueones have less magnetization than thered-brown ones. The former are moreoxidized (maghemite) than the other;3 µm

Extremely fine grained subhedral forms;otherwise like 14-6; 2 µm

Ilmenite

Very fine prim, laths; 10 µm-sub.

Relatively high content; largershow a rim of titanomag., anotherare small laths and show texturelike titanomag.15 µmtabular: 40 µm

Tiny prim, laths and larger withseams of titanomag.

Some prim, laths; 1 µm

Small prim, laths, fringed likewiseby hematite; 3 µm-sub.

Many small prim, laths, somemantled by titanomag.; 5 µm

Large prim, laths, sometimesmantled by titanomag. andsecondary phase in the titano-mag.; 70 µm-sub.

Like 63-1; some of the grainspseudomorph. to skeletaltitanomag.

Like 63-1; rare larger ones oftabular habit; 60-15 µm

Like 63-1

Some small prim, laths; 5 µm-sub.

Many prim, laths contain exsolu-tions of hematite; 10 µm-sub.

Rare tiny prim, laths; 3 µm

No record

No record

Sulfides

Small pyrrhotite and less pyrite;5 µm

Rare, very small indet. grains

Extremely fine roundish indet.grains; 1 µm

Rare small indet. grains; 1 µm

Some grains of pyrite enclosed byplag. phenocr., partly intergrownwith pyrrhotite

Some larger pyrrhotite and in themajority, small, round, ind.grains; 5 µm

Fine grains of pyrite and finer onesof pyrrhotite; 1 µm

Masses of small pyrite, pryyhotite,and polyphases of these; 20 µm

Like 63-1; some spherical shapesrimmed by small magnetite grainscontain pyrite-pryyhotite inter-grown. 15 µm

Content strongly decreased; rareindet. grains

Tiny indet. grains, 1 µm

Extremely fine isometric grains; nodet.; 1 µm

Some small grains of intergrown py-rite. Chalcopyrite sometimes re-placed by iron hydroxide frommargin; 1 µm

Other Phases

Rare, homogeneous chromitegrains, partly rimmed by a tinyseam of titanomag.; 15 µm

Abundant content of ironhydroxide in or around theolivines and vesicles

Iron hydroxide fills veinlets andvesicles.

Rare iron hydroxide, mostly invesicles

Iron hydroxide lining vesicles andreplacing titanomag. under redstaining of the adjacent ground-mass

Homogeneous, euhedral chromite;15 µm

Iron hydroxide fills sphericalstructures

Titanomag. partly replaced byiron hydroxide

Some larger clusters of ironhydroxide in amygdales, usuallylining veinlets and vesicles

Like 64-4, lower content

Rare large euhedral chromitegrains, partly porous and alwaysrimmed by a small seam of titano-mag.; 140 µm

Iron hydroxide partly replacestitanomag.; red staining of thesurrounding groundmass; rarelarge homogeneous chromite;43 µm

559

P. EISENACH

TABLE 1 - Continued

Sample(Interval in

cm)

15-2,8-14

154, 72-78

154,53-111

16-2,113-117

164, 76-85

18-2, 18-26

19-1,94-102

22-3, 95-107

22-3,106-117

22-3,117-118

224, 77-87

24-2, 87-98

24-3, 16-24

24-3, 75-80

25-1,84-93

Lithology

f. gr. pi.ph. bas.

f. gr. pi.ph. bas.

f. gr. pi. px.ph. basalt

f. pi. ph.basalt

f. pi. gr.ph. bas.

extr. f. pi.phi basalt

f. gr. pi.ph. bas.

f. gr. pi. px.ph. basalt

fine gr. pi.ph. basalt

f. gr. pi.px. ph. bas.

f. gr. pi.ph. basalt

f. gr. pi.ph. basalt

fine gr. pi.ol. ph. basalt

extr. f. gr. pi.px. phy. bas.

fine gr. pi.px. phy. bas.

Titanomagnetite

Finer grain size than 15-1; 2 µm

Very small skeletal to subhedral shapesshow different stages of oxidation. Somecontain expansion cracks, some fringedby fine granuled hematite. Red stainingof the groundmass near the crystals;3 µm

Small skeletal grains of different oxida-tion stages; 15 µm-sub.

Skeletal to subhedral forms; larger onesmottled by low-temperature oxidation;5 µm

Small skeletal to subhedral shapes; partlyrimmed by a fine granuled hematite,partly containing expan. cracks; 5 µm

Extremely fine grained; 1 µm

Small skeletal forms, mottled but onlylittle expan. cracks; some fringed by fine-grained hematite; 30 µm-sub.

Extremely fine skeletal shapes; 1 µm

Extremely small skeletal shapes; 1 µm

Small skeletal forms; mottled and partlyfringed by fine granuled hematite; 5 µm

Extremely small skeletal forms; 1 µm

Few large euhedral grains and masses ofsmall skeletal forms; some show a titano-mag.-richer core; 5 µm

Small skeletal to subhedral forms showvarying oxidation stages and expan.cracks, some rimmed by hematite; 2 µm

Extremely small skeletal forms, some-times fringed by fine-granuled hematite;2µm

Small euhedral to subhedral shapes,sometimes fringed by tiny sulfidegrains, mottled; 10 µm

llmenite

No record

Some primary laths, 2 µm

Small laths and grains of a dis-tinct tabular habit; some are re-placed by hematite from margin.5 µm-sub.

Rare fine primary laths; 5 µm

Few small laths of primary phase;3µm

Extr. fine gr.; 1 µm

Some prim, laths; 5 µm

No record

Some prim, laths; 3 µm

Rare prim, laths; 5 µm

No record

Small prim, laths; 5 µm

Rare prim, laths, some of whichare mantled by titanomag.

No record

Prim, laths; 10 µm

Sulfides

Finer than 15-1

Roundish, drop-like grains; notdet.; 1 µm

Small roundish indet. grains; 3 µm

Tiny grains of pyrite and pyrrho-tite; 2 µm

No det. fine grains; 1 µm

Fine isometric grains; 3 µm

Masses of tiny grains between theplag. laths of the groundmass; 1 µm

Large grains enclosed by plag.phenocrysts; corrosion of pyrr-hotite by pyrite produces a kind ofnetwork.

Tiny roundish grans; 1 µm

Increasing content pyrrh. ingrownwith pyrite

Very fine grain; 1 µm

Small pyrrh., partly enclosed byplag. phenocr.; 1 µm

Tiny roundish grains and a largergrain of pyrrhotite

Small grains and clusters ofmarcasite

Other Phases

Some euhedral microphenocrystsof dense, homogeneous chromite;45 µm

A large euhedral homogeneouschromite; content of iron hydrox-ide slightly increasing; 25 µm

Iron hydroxide replaces titano-mag., whereby the surroundinggroundmass is red-stained.

Larger, homogeneous euhedralchromites show fine seams oftitanomag.; decreasing content ofiron hydroxide; 110 µm

Chromite like 16-2; 65 µm

Microphenocrysts of chromite;30 µm

Large euhedral, porous chromite;150 µm

Large euhedral, partly porouschromite; 50 µm

Many large euhedral chromiteslie mainly near the pyiox.phenocrysts; some contain inclus-ions of groundmass; 70 µm

A large euhedral porous chromiteenclosed by pyrox.; homogeneousone lies in plag. phenocr.; 100 µm

Large euhedral chromites, partlyporous, partly homogeneous, butall rimmed by a fine seam of titanomag.; 80 µm

Mostly small, homogeneous and alarge porous grain of chromite; in-creasing content of iron hyd. nearthe margins of pyroxenes and thedecomposed spinels; 80-10 µm

Iron hydroxide fills vesicles andcracks of pyroxene

Large subhedral chromite with atiny fringe of titanomag.;150-30 µm

Note: sub = submicroscopic grain size; px. = pyroxene: phy. = phyric: bas = Dasait: ol. = oiivme: apn. = apnyπc

breccia zone within phyric basalt (Unit P-5) and in thedoleritic intrusion (Unit P-4), larger titanomagnetitesoccur with exsolved ilmenite lamellae (secondary il-menite) of oxidation class 3 or 4 (Plate 1, Figures 3, 4;Plate 2, Figure 1).

Myrmekitic intergrowths of titanomagnetite with il-menite and with the silicate groundmass (Sample395A-32-1, 140-150 cm) demonstrate partial leachingof titanomagnetites by hydrothermal solutions (Plate 2,Figure 2).

Samples 395A-63-1, 0-10 cm shows a more distinctleaching of titanomagnetite from the crystal margin tothe first exsolution lamellae of ilmenite, so that the tita-nomagnetite grains now are also fringed by ilmenite.The more resistant ilmenite lamellae have been left be-yond the original grain borders, and now protrude intothe groundmass (Plate 1, Figure 4; Plate 2, Figure 1).

Ilmenite

Both primary and secondary ilmenite can be distin-guished texturally. The former, commonly lath-shaped,occurs in all samples. In a few cases, ilmenite has atabular habit and somewhat larger grain size. Fre-quently, grains are developed similar to the skeletalforms of titanomagnetite (Plate 2, Figures 3, 4, 5),making identification difficult, especially in unfavorablecross-sections. Ilmenite mantled by titanomagnetite iscommon. Occasionally, more altered laths containminute lamellae of exsolved hematite. Secondary il-menite intergrown with titanomagnetite (subsolidus ex-solution lamellae) is a product of high-temperature oxi-dation. It occurs in Samples 395A-32-1, 140-150 cm;395A-63-1, 0-10 cm; 395A-63-4, 115-122 cm; 395A-64-1, 137-142 cm; and 395A-64-2, 67-73 cm.

560

OPAQUE MINERALS

TABLE 2Micropiobe Analyses of Iron-Titanium Oxides (wt %)

Unit

Lithology

Sample(Interval in

cm)

MnOA12O3FeOTiθ2MgOCr 2 O 3Sum

MnAlFe2+Fe3+TiMgCr

F e 2 θ 3FeOTotal

F e 2 O 3FeOTotalMol% Usp.

PI. olph. Basalt

14-1,87-99

0.331.47

70.0923.91

0.570.02

96.39

0.0110.0681.6660.5110.7100.0330.001

36.3737.36

100.03

20.4551.6998.4468.4

PI. olph. Basalt

14-2,125-134

0.321.19

72.1423.40

0.550.02

97.62

0.0110.0561.6560.5450.6990.0320.001

38.1937.78

101.45

22.6051.8199.8966.1

VolcanicBreccia

32-1140-150 F

Spinel Phase

0.30 0.281.46 7.95

70.33 68.8423.09 19.74

0.81 0.580.03

96.02 90.42

Spinel Formula

0.0100.0681.6300.5560.6880.0480.001

Recalculated AlalysesIlmenite Basis

37.7836.3499.81

Ulvospinel Basis

22.3950.1898.2665.2

Beginningof Dolerite

61-1140-150

0.331.70

69.6421.96

0.47_

94.10

0.0110.0801.6200.6020.6590.028

-

36.9336.4197.80

22.3149.5796.3464.7

Dolerite

63-1,0-10

0.291.41

70.0321.34

0.520.03

93.62

0.0100.0671.6040.6420.6450.0310.001

37.8335.9997.41

23.6248.7895.9962.9

E

0.301.47

69.6521.36

0.39_

93.17

0.0100.0691.6130.6400.6450.023

-

37.3436.0596.91

23.1248.8595.4963.5

MnOA1 2 O 3FeOTiO2MgOCr 2 O 3Sum

MnAlFe2+Fe3+

TiMgCr

F e 2 O 3FeOTotalMol%K 2 U 3Temp. fC)f θ 2

0.380.20

46.2351.13

0.500.04

98.48

0.0080.0060.9480.0430.9750.0190.001

1.7044.7098.65

1.9

81010-16-0

0.350.16

50.6649.13

0.610.01

100.92

0.0070.0030.9120.1120.9420.0230.001

8.7842.76

101.808.6

102010-10-7

Rhombohedral Phase

0.330.16

48.1349.86

0.93-

99.41

Mineral Formula

0.0070.0050.9180.0750.9600.035

-

Recalculated Analyses

5.8742.8499.99

5.9

95510-12-0

0.280.26

43.0249.67

0.40—

93.63

0.0060.0080.9290.0920.9500.015

-

___-

_

-

0.26_

47.1548.77

0.32-

96.50

0.003_

0.9340.1100.9450.008

-

4.5943.0296.96

4.6

91010-12.9

0.470.38

53.5041.67

0.69_

96.71

0.0100.0720.8000.3730.8370.027

-

16.6935.7898.6820.1

114010-8.2

Note: F = fringe grain of a spherule; E = exsolved grain; S = secondary ilmenite; Recalculationprocedure according to Carmichael (1967).

Iron Hydroxides

These are abundant, but always in altered areas,veins and vesicles. They partly replace iron-titaniumoxides.

Sulfides

Sulfides make up a widespread but volumetricallysubordinate mineral phase in these rocks. Pyrite andpyrrhotite are the most common sulfides, and showdifferent shapes. Globules are abundant (a result of im-miscibility of silica and sulfide melt) and are usuallyfringed by a fine-grained, strongly magnetic cubicphase (Plate 3, Figures 1,2). Some of these globulescontain a core of pyrite, pyrrhotite, or both; others-better characterized as spherules—contain extremelyfine grained iron hydroxide which has replaced theoriginal sulfide. A microprobe analysis of a fringe grainof such a spherule (Sample 32-1, 140-150 cm) gives atitanomagnetite with approximately 20 wt. per cent

TiO2. This titanium content is not so greatly differentfrom that of the bulk titanomagnetites that one candemonstrate two different origins of the iron-titaniumoxides. As results from Leg 34 (Ade-Hall and Johnson,1976) illustrate, extensive analyses are necessary to re-solve such problems. Only in the dolerite sample (63-1,0-10 cm) do occasional marcasite nodules (Plate 3,Figure 3) occur. In this sample, a two-phased sulfidegrain, consisting of exsolution lamellae of chalcopyritein a pyrrhotite host, was formed.

Chromium Spinel

This mineral is rare, and appears as isolated mi-crophenocrysts in the rocks. Two textural varieties canbe discerned: a homogeneous type and a porous type.Both kinds are normally rimmed by titanomagnetitecrystals. However, when completely surrounded by py-roxene or by Plagioclase phenocrysts, the homogeneousgrains have no such rim.

Hole 396

This hole cored a fairly uniform pillow basalt series.Variation in the textures of the iron-titanium oxides iscorrespondingly small. Mottled color, occasional vol-ume-change cracks, and fringes of hematite indicateprogressive alteration of the grains, whose last stage istypified by the diffusion of Fe into the surroundinggroundmass. All these phenomena point to strong al-teration during low-temperature oxidation. Ilmenite,here developed only as primary laths, is not present inall samples, and can be overlooked, because of thevery small grain size (

P. EISENACH

prefer secondary ilmenite, and Ti prefers primary il-menite.

Total iron, determined originally as FeO in the mi-croprobe analyses, is expressed as FeO and Fe2O3, ac-cording to the recalculation procedure of Carmichael(1967). The temperature and oxygen fugacities corre-sponding to these compositions were determined fromthe f(O2)-T curves of Buddington and Lindsley (1964).The average total for the spinel phase, computed onthe basis of ulvospinel and ilmenite, respectively,should be close to 100 per cent, thus allowing one tocheck the quality of the analyses. But this check is re-stricted to compositions lying in the field between theulvospinel-magnetite and the magnetite-ilmenite join.The higher the oxidation state of titanomagnetite, thegreater a deviation from a total of 100 per cent shouldbe expected. This is in good agreement with recalcu-lated analyses of minerals seen under the petrographicmicroscope to be progressively more altered. Sample395A-14-2, 125-134 cm is the least altered of all sam-ples, and yields, from the Buddington and Lindsleycurves, T = 1020°C and f(O2) = 10-

10 7atm-the crys-tallization conditions of titanomagnetite and primary il-menite in equilibrium with each other and with themagma.

These f(O2)-T conditions suggest that cooling of thebasalt was too quick for equilibrium to develop. Op-posed to this, the lower temperature and f(O2) datafrom samples 395A-32-1, 140-150, 395A-63-1, 0-10,and 395A-14-1, 87-99, demonstrate a much slowercooling of basalt here, such that the iron-titaniumphase was always in a state of equilibrium. Abovethese low temperatures, fast cooling rates preventedequilibrium. I infer high-temperature oxidation forSample 395A-63-1, 0-10, which has a relatively hema-tite-rich ilmenite phase.

Optical and Magnetic Zoning

The widespread halmyrolytic alteration of titano-magnetite in submarine basalts is of more than meremineralogical interest. Ade-Hall and Johnson (1976)show that the cation deficiency, z, of titanomagnetite isstrongly correlated with a variation of magnetic prop-erties. One of these properties, the magnetization of theiron-titanium oxides, can be observed very well usingthe magnetic colloid (Plate 3, Figures 4, 5, 6; Plate 4,Figures 1,2). Under reflected light, Sample 395A-14-2,125-134 cm appears to have only homogeneous titano-magnetite, but the colloid test demonstrates magneticzoning, a result of low-temperature oxidation. By con-trast, in Sample 395A-61-1, 140-150 cm, there is asharp separation between a light, nonmagnetic phaseand a darker, strongly magnetic phase. Plate 4, Figure2 shows a microprobe scan over the different magneticareas of an oxidized titanomagnetite (titano-maghemite). The microprobe analyses of these differ-ent portions are given in Table 3. The data demon-strate a slight preference of Al, Mg, and Fe for thestrongly magnetic phase, and Ti and Mn to the lighter,nonmagnetic phase. All these investigated grains are

strongly maghemitized, as the cation deficiency of theborders and the cores indicate. The cation deficiency isreal, and is not a result of error in the microprobe anal-yses.

SUMMARY AND DISCUSSIONIn all the investigated samples, titanomagnetite is

the dominant phase; ilmenite is subordinate and sul-fides much less abundant. Microscopic observationsshow that the fine-grained, rapidly cooled basalts arecharacterized by fine skeletal to anhedral forms of tita-nomagnetite. The frequent volume-change cracks andof mottling indicate low-temperature oxidation of therocks. This is a common appearance of titanomagneti-ties in submarine basalts in ocean floor basalts (Ade-Hall and Johnson, 1976; Petersen et al., in press). Incontrast to subaerial basalts, high-temperature oxida-tion of titanomagnetite seems to be rare in ocean floorbasalts, and appears to occur only in the centers ofmassive flows and sills (Watkins et al., 1967; Grommeet al., 1969). In good agreement with this inference,high-temperature oxidation of titanomagnetite (class 3or 4) was found in the dolerite intrusion. Microprobeanalyses of an "exsolved" titanomagnetite from thedolerite give f(O2)-T values which are too high to rep-resent crystallization conditions. The values of T =1020°C and f(O2) = 10

natm obtained from Sample395A-14-2, 125-134 cm, are similar to results from Leg34 (Mazullo et al, 1976), and may demonstrate moretypical equilibrium conditions during the crystallizationof iron-titanium oxides.

The preliminary results of optical and magnetic zon-ing of low-temperature-oxidized titanomagnetites aresimilar to those of Prevot (1968).

ACKNOWLEDGMENTSThe author wishes to acknowledge Prof. Dr. D. D. Klemm

and Dr. N. Petersen for stimulating discussion during thecourse of this research. J. Burn provided helpful assistance inthe microprobe laboratory of the University of Lausanne-Dorigny. The financial support of the Deutsche Forschungs-gemeinschaft is gratefully acknowledged.

REFERENCESAde-Hall, J. M. and Johnson, H. P., 1976. Petrography of

opaque minerals, Leg 34. In Hart, S. R., Yeats, R. S., etal., Initial Reports of the Deep Sea Drilling Project, v. 34:Washington (U.S. Government Printing Office), p. 349-362.

Buddington, A. F. and Lindsley, D. H., 1964. Iron-titaniumoxide minerals and synthetic equivalents, J. Petrol, v. 5, p.310-375.

Carmichael, I. S. E., 1967. The iron-titanium oxides of salicvolcanic rocks and their associated ferromagnesian sili-cates, Contrib. Mineral. Petrol, v. 14, p. 36-44.

Grommé, C. S., Wright, T. L., and Peck, D. L., 1969. Mag-netic properties and oxidation of iron-titanium oxide min-erals in Alae and Makaopuhi lava lakes, Hawaii, /. Geo-phys. Res., v. 74, p. 5277-5293.

Mathison, C. I., 1975. Magnetites and ilmenites in the Somer-set Dam layered basic intrusion, southeastern Queensland,Lithos, v. 8, p. 93-111.

562

TABLE 3Microprobe Analyses of Iron-Titanium Oxides Showing Optical and Magnetic Zoning (wt. %)

Sample(Interval in cm)

Analysis-pointSpinel Phase

MnOA12O3FeOTiOMgθ2C r 2 O 3Sum

Spinel Formula

MnA12+Fe3+FeTiMgCr

Recalculated Analyses

Ilmenite Basis

F e 2 O 3FeOTotal

Ulvospinel Basis

F e 2 O 3FeOTotalMol % Usp.

1

0.311.31

70.9222.42

0.55

95.51

0.0100.0621.6300.5920.6730.033-

37.8836.8499.31

22.9450.2897.8164.7

14-2,125-134

2

0.301.81

76.6021.55

0.60-

100.86

0.0100.0851.6010.6210.6470.036

42.4638.39

105.10

28.1151.31

103.6858.8

3

0.301.51

75.9121.21

0.66_

99.59

0.0100.0711.5910.6490.6400.039-

42.4237.74

103.84

28.2850.46

102.4258.4

61-1,140-150

1

0.351.81

67.3222.60

0.52-

92.60

0.0120.0851.6320.5650.6750.031-

34.8235.9996.02

19.7349.5794.5867.9

2

0.361.69

68.7921.96

0.54-

93.34

0.0120.0791.6150.6030.6590.032-

36.4336.0196.99

21.8049.1795.5265.1

61-1,140-150

1*

0.301.38

68.0322.46

0.40_

92.56

0.0100.0651.6410.5850.6750.024-

35.4636.1396.12

20.5149.5894.6267.3

2*

0.301.91

70.4120.80

0.400.01

93.83

0.0100.0901.5940.6530.6280.047-

38.0636.1797.65

24.2048.6396.2561.6

1

0.301.44

67.5622.65

0.91_

92.86

0.0100.0671.6130.5790.6770.054-

35.6435.4996.43

20.5549.0794.9266.9

2

0.261.56

69.9723.05

0.790.08

95.70

0.0090.0731.6300.5530.6860.0470.002

36.8736.7999.39

21.5250.6197.8666.4

3

0.271.47

71.7722.57

0.85_

96.93

0.0090.0691.6160.5810.6750.050-

38.6936.95

100.80

23.6550.4999.3063.8

4

0.301.65

72.6222.420.75_

97.74

0.0100.0771.6160.5830.6700.044-

39.2237.33

101.67

24.2850.77

100.1763.1

5

0.281.56

72.3722.420.80_

97.43

0.0090.0731.6140.5870.6700.047-

39.1337.16

101.35

24.1950.6099.8563.2

32-1,140-150

6

0.261.64

72.6322.370.82_

97.72

0.0090.0771.6110.5850.6690.049-

39.3237.24

101.65

24.4250.66

100.1762.9

7

0.311.66

71.9322.760.880.07

97.60

0.0100.0771.6110.5650.6780.0520.002

38.6337.17

101.47

23.4650.8299.9564.0

8

0.321.73

71.3722.390.840.06

96.72

0.0110.0811.6070.5810.6680.0500.002

38.3836.63

100.56

23.4750.2699.0863.7

9

0.281.70

70.0122.890.78_

95.66

0.0090.0791.6270.5570.6820.046-

36.9736.7599.37

21.7250.4797.8466.0

10

0.291.72

69.1522.930.82_

94.90

0.0100.0801.6210.5620.6790.048-

36.3636.4498.55

21.0850.1897.0166.6

11

0.311.74

68.8523.000.76_

94.66

0.0100.0811.6290.5510.6840.045-

36.0136.4598.27

20.6950.2396.7367.1

ü>

d

P. EISENACH

Mazullo, L. J. and Bence, A. E., 1976. Abyssal tholeiites from Prévot, M., Rémond, G. and Caye, R., 1968. Etude de laDSDP Leg 34: The Nazca Plate, J. Geophys. Res., v. 81, transformation d'une titanomagnetite en titanomaghemitep. 4327-4351. dans une roche volcanique, Bull. Soc. France Mineral.

Petersen N., Bleil, U. and Eisenach, P., in press. Rock- and Cristallogr., v. 91, p. 65-74.palaeomagnetism of Leg 43 basalts. In Tucholke, B.,Vogt, P., et al., Initial Reports of the Deep Sea Drilling Watkins, N. D. and Haggerty, S. E., 1967. Primary oxidationProject, v. 43, Washington (U.S. Government Printing variation and petrogenesis in a single lava, Contrib. Min-Office). eral Petrol, v. 15, p. 251-271.

564

P. EISENACH

PLATE 1

Figure 1 Microphenocryst of a large porous chromiumspinel shows limited development of a tinyseam of titanomagnetite.Sample 395-18-2, 35-37 cm.

Figure 2 Microphenocryst of a large homogeneous chro-mium spinel, partly fringed by titanomagnetite.Sample 395A-17-1, 69-75 cm.

Figure 3 Titanomagnetite of skeletal to anhedral formshows some secondary ilmenite lamellae andlimited development of volume-change cracks,partly filled with sulfide phase.Sample 395A-63-1, 0-10 cm.

Figure 4 High deuteric oxidation state of an exsolved ti-tanomagnetite grain; partial leaching of titano-magnetite produces a ilmenite fringe around thegrain (with colloid).Sample 395A-63-1, 0-10 cm.

566

OPAQUE MINERALS

PLATE 1

100µm 100µm

50µm 1OOµm

567

P. EISENACH

PLATE 2

Figure 1 High oxidation state of a titanomagnetite; themore resistant ilmenite lamellae spread beyondthe grain borders into the silica groundmass byprogressive leaching of the titanomagnetitephase. The white reflecting phase is a pyrrhotitegrain. Colloid.Sample 395A-63-1, 0-10 cm.

Figure 2 Myrmekitic intergrowth of ilmenite, silicagroundmass, and titanomagnetite; small relictsof titanomagnetite in the worm-shaped portionsdemonstrate the replacement of titanomagnetitephase by silica groundmass.Sample 395A-32-1, 140-150 cm.

Figure 3 Ilmenite fringed by tiny sulfide grains formedsimilar to titanomagnetite skeletals; colloidshows different magnetization in the skeletals oftitanomagnetite.Sample 395A-14-2, 125-134 cm.

Figure 4 Ilmenite formed as skeletal titanomagnetitehave been detected by magnetic colloid.Sample 395A-14-1, 87-99 cm.

Figure 5 Skeletal ilmenite, formed similar to titanomag-netite skeletals, is partly mantled by titano-magnetite.Sample 395A-14-2, 125-134 cm.

568

OPAQUE MINERALS

PLATE 2

100µm 50µm

100µm

50µm 50µm

569

P. EISENACH

PLATE 3

Figure 1 Small sulfide globules lie near the skeletaltitanomagnetite.Sample 395A-14-2, 125-134 cm.

Figure 2 Spherule of an original pyrite, now are replacedby iron hyroxide; spherule is rimmed by tiny ti-tanomagnetite grains (see microprobe analysis,Table 2 F).Sample 395A-63-4, 137-142 cm.

Figure 3 Markasite nodule, crossed nicols. Sample 395A-63-1, 0-10 cm.

Figure 4 An optical homogeneous titanomagnetite showsmagnetic zoning using the colloid test. Analysespoints 1-3, see Table 3.Sample 395A-14-2, 125-134 cm.

Figure 5 Shows a sharp separation into a light phase anda darker reddish brown phase.Sample 395A-61-1, 140-150 cm.

Figure 6 Colloid test demonstrates the strong magnetiza-tion in the darker phase of the same grain.Analyses points 1-2, see Table 3.Sample 395A-61-1, 140-150 cm.

570

OPAQUE MINERALS

PLATE 3

50 µm 50µm

100µm 50µm

50µm 50µm

571

P. EISENACH

PLATE 4

Figure 1 Colloid shows different magnetization in a skel-etal titanomagnetite (snowstar). Analyses points1*, 2*, see Table 3.Sample 395A-61-1, 140-150 cm.

Figure 2 Colloid demonstrates magnetic zoning resultingfrom low-temperature oxidation. A microprobescan (analyses points 1-11) gives the variationin the chemical composition from the moremaghemitized margin to the less maghemitizedand also strong magnetic core. A spherule filledby iron hydroxide is partially rimmed by tiny ti-tanomagnetite grains.Sample 395A-32-1, 140-150 cm.

572

OPAQUE MINERALS

PLATE 4

50µm

100µm

573