4. PETROLOGY AND CHEMISTRY OF BASALTIC ROCKS FROM … · petrology and chemistry of basaltic rocks from hole 396b, ... chemical (7 units), and ... petrology and chemistry of basaltic

4. PETROLOGY AND CHEMISTRY OF BASALTIC ROCKSFROM HOLE 396B, IPOD/DSDP LEG 46

Hiroaki Sato, Department of Earth Sciences, Faculty of Sciences, Kanazawa University, Kanazawa, JapanKen-ichiro Aoki, Department of Petrology, Mineralogy, and Economic Geology, Tohoku University, Sendai, Japan

Kenji Okamoto, Department of Geological Sciences, Faculty of Liberal Arts and SciencesUniversity of Osaka Prefecture, Osaka, Japan

andBun-ya Fujita, Department of Earth Sciences, Faculty of Sciences, Kanazawa University, Kanazawa, Japan

INTRODUCTION

Leg 46 of the International Phase of Ocean Drilling(IPOD) successfully penetrated 255 meters of basalticbasement at Hole 396B, 150 kilometers east of the axis ofthe Mid-Atlantic Ridge at 22°59.14' North (Figure 1). Werecovered basalts, typical of the ocean floor, that consistprimarily of sparsely-phyric olivine-plagioclase basalt (totalphenocryst content, <1%) and phyric Cr-spinel-bearingolivine-plagioclase basalt (total phenocryst content, 15 to25%). As shown in Figure 2, these rocks may be dividedaccording to the following three methods: lithologic (4 un-its; although the shipboard party defined 8, site chapter ofthis volume), chemical (7 units), and magnetic (4 units). Inthis paper, we present petrographic and chemical data onthese rocks to show the diversity of basalts within andamong chemical units; additionally, we discuss an estima-tion of temperature of magmas, the crystallization history ofphenocrysts, and the degree of fractional crystallization.

PETROGRAPHY

Basaltic rocks recovered in Hole 396B are either glassy tofine-grained pillow lava or fine to medium-grained massivelava. The textural features of groundmass of pillows aresimilar among all the pillow lavas of Hole 396B. A massivelava of lithologic Unit 2 exhibits systematic textural andgrain-size variations of groundmass against the depth of thecore specimen. In the following section, textural and grain-size variations of the groundmass of these pillow and mas-sive lavas are described first. Then, other petrographic fea-tures (such as mode, grain-size distribution, and textures ofphenocrysts) are described for each chemical unit.

Groundmass

Pillow LavaThe size of pillow is 40 to 80 cm in diameter, as deduced

from the complete section of pillows obtained in Section20-1. Each pillow is divided into four zones according to thetexture of groundmass. From the rim to the interior, they aresideromelane glass zone, cryptocrystalline variolite zone,microcrystalline spherulite zone, and intersertal interiorzone. The textural features of these zones are summarized inTable 1. All the sideromelane glass zone of Hole 396Bpillows contain minute skeletal olivine and thin fork-shapedplagioclase crystals, indicating that olivine and plagioclase

are the near-liquidus phases of these basaltic magmas. Crys-tallinity increases rapidly from the variolite zone to thespherulite zone, and the primary constituents of the interiorof the pillows are olivine, clinopyroxene, plagioclase,titanomagnetite, and mesostasis. The alteration products arepalagonite after sideromelane glass, Fe-Mn oxides andhydro-oxides, smectites, mica, zeolites, and carbonates.

Vesicles are found throughout the pillows. Most of vesi-cles are spherical and their diameter ranges from 0.1 to 1mm. Vesicle contents are 0 to 5 per cent. The low vesiclecontents are indicative of the greater water depth (more than1000 m) of the formation of pillows (Moore and Schilling,1973). Some of the vesicles are filled by either residualmaterial similar to the intersticies of groundmass (Satb, thisvolume) or by smectite, zeolite, or carbonate.

Massive Lava

The textural features and the grain size of the groundmassconstituents of the massive lava are shown in Table 2. Theunit can be divided into three zones according to thegroundmass texture; the upper (235.92 to 238.75 m sub-bottom depth), the middle (238.78 to 240.32 m), and thelower zones (240.34 to 242.00 m). Within the upper zone,groundmass texture varies from intersertal to intergranular,and the grain size of every primary constituent mineral in-creases downwards. The middle zone of the lava unit ischaracterized by patchy structure. Fine-grained varioliticpatches, average diameter of 5 mm and constituting one-third to one-half of the rock, are surrounded by coarse-grained intergranular part. The intergranular part consistsmainly of thick plagioclase laths and stout idiomorphic crys-tals of olivine and clinopyroxene, while the variolitic patchis mainly composed of feather-like or dendritic crystals ofclinopyroxene with interstitial minute plagioclase laths andopaque minerals. The width of clinopyroxene andplagioclase of the patch increases downwards within themiddle zone, leading to the gradual disappearance of thepatchy structure from the middle to the lower zones.Through the massive lava unit, olivine and plagioclase showsimple grain size variation; they attain maximum size in themiddle zone, suggesting that the unit is a single coolingunit. The presence of fine-grained patches in the middlezone may be interpreted as the result of either an increasedcooling rate at the advanced stage of crystallization of thelava caused by the penetration of water through cracks, or

115

H. SATO, K. AOKI, K. OKAMOTO, B. FUJITA

4101 ^ I I t i l 1 f I I ' I

, B 1 6 3 53 38 / / / / / /

\

45 30° 15°

Figure 1. Location of Hole 396, IPOD/DSDP Leg 46 in the Atlantic Ocean. Dashed lines are the magnetic anomaly linea-ments, the attached number denotes the age of their formation in m.y. B.P.

two crystallization stages during simple cooling of a lavacaused by the supercooling effect as demonstrated for lunarbasalt (Dowty et al., 1974; Walker et al., 1976).

PhenocrystSubunit Ai: Chemical Subunit Ai (150.5 to 178.5 m)

consists of pillow lavas of sparsely-phyric basalt. Pheno-crysts constitute less than 1 volume per cent of the rock, andare composed of olivine and plagioclase. The grain size ofall the phenocrysts in thin sections of Subunit Ai basalts isshown in Figure 3. The grain size represents the geometricmean diameter of each crystal at thin-section surface, andthe frequency of occurrence is summed for each logarithmicscale interval of the factor of 1.5. The deviation of theseapparent size distributions from the true size distribution isvery small (about 75% of the apparent diameter fall withintwo-thirds of the true diameter). Figure 3 illustrates thatolivine phenocryst is much more abundant than plagioclasephenocryst in Subunit Ai. The grain size of olivinephenocrysts ranges from 0.1 to 2.5 mm and its mode is 0.2mm. Larger olivine phenocryst tends to be corroded, whilesome of the smaller olivine phenocryst show skeletaltexture. Plagioclase phenocrysts are either rounded ortabular idiomorphic. The rounded plagioclase phenocrystdisplays fine polysynthetic albite twinning. Plagioclasephenocrysts of Subunit Ai show little zoning.

Subunit A2: Mineral assemblage and the mode of oc-currences of phenocrysts in chemical Subunit A2 are similarto those in chemical Subunit Ai except for much more fre-quent occurrence of large and corroded phenocrysts of botholivine and plagioclase (Figure 3).

Subunit A3: Chemical Subunit A3 is also composed ofsparsely phyric basalt, but has slightly different phenocrystoccurrences than Ai and A2 (Figure 3). Olivine phenocrystis idiomorphic and occurs only in one sample (in Core 14).Rounded plagioclase phenocrysts have calcic cores with asharply defined sodic rim. A Ca-rich clinopyroxene-plagioclase crystal clot, 15 mm by 8 mm in size, was foundin Sample 15-5, 70-77 cm (Plate 1); this is the onlyclinopyroxene phenocryst found to date in Hole 396B. Theclot shows ophitic texture; a few large clinopyroxene crys-tals (3 to 10 mm) contain smaller plagioclase laths (1 to 3mm long). Clinopyroxene occupies 66 volume per cent ofthe clot. Clinopyroxene is evidently corroded againstgroundmass. Tabular plagioclase lath projects intogroundmass, in which it is slightly rounded and has a sharpreverse-zoned rim (Plate 1). The projection of plagioclasecrystal from the clot indicates the selective corrosion ofclinopyroxene phenocryst. Plagioclase within the clotexhibits slight concentric and sector zoning. Along theboundary with clinopyroxene, plagioclase has minute fluidinclusions and a calcic rim.

116

PETROLOGY AND CHEMISTRY OF BASALTIC ROCKS

150

2 0 0 -

- 250IQ_LUQ

OCQ

OQ

CO

300 -

350 -

ccOo

>-

EC

OV

ER

cc

zDOtn

ITH

OLO

<

zDO

IAG

NE

TI

Z

_ l

HE

MIC

A

o45

6

7

8

9

10

mmmmmm

-

1

13 I 1

15 • U 2

"17""*=

19 |

20 |

21

22

23

24

25

262728

29303132

33

• •

3

4

I

II

...

IV

A1

A2

A 3

B1

B2

C

D400 -

Figure 2. Stratigraphic division of the basalticbasement of Hole 396B.

Subunit Bi: This subunit is composed of pillow lava ofphyric basalt, containing phenocrysts of plagioclase,olivine, and chromian spinel. Mode of phenocrysts inphyric basalt of Subunit Bi is presented in Table 3 andplagioclase-olivine-matrix ratios are plotted in Figure 4.The total phenocryst content of Subunit Bi is 15 to 20volume per cent, and the plagioclase/olivine modal ratio isabout 6. Mode of chromian spinel phenocryst averages 0.02per cent. Grain-size distributions of these phenocrysts areshown in Figures 5 and 6. All of these phenocrysts show

unimodal grain size distribution; hence, there is no reason toseparate phenocryst from microphenocryst for these phyricbasalts of Hole 396B, though some of the ocean-floor basaltmay display bimodal grain-size distribution for phenocrysts(Shido et al., 1973). Most of the olivine phenocrysts inSubunit Bi are idiomorphic. Glomerophyric crystal aggre-gates of olivine and plagioclase, up to 7 mm in diameter, arefound occasionally. The aggregate consists of 1 or 2 largeolivine crystals and abundant smaller plagioclase lathsshowing ophitic texture. Olivine phenocryst often containsmatrix, plagioclase, and chromian spinel as inclusions.Plagioclase phenocryst is tabular idiomorphic or rounded,or skeletal. Sometimes, a large plagioclase phenocryst is acomposite grain. Matrix inclusion occupies an average of 2per cent of host plagioclase, but it sometimes occupies up to50 per cent. Matrix inclusions occur only in the core of theplagioclase phenocryst, or only in the marginal zone, orevenly throughout the crystal. Plagioclase phenocryst withabundant matrix inclusion tends to be skeletal, and oftendevelops fine poly synthetic albite twinning. Some of thetabular plagioclase phenocrysts show sector zoning. Chro-mian spinel occurs as inclusions in olivine or plagioclase, oras isolated grains in groundmass. It is euhedral, or rounded,or skeletal, and is 10 to 660 µm in diameter (Figure 5). Thecolor of chromian spinel varies from brown to reddishbrown to dark red.

Subunit B2: Mineral assemblage and the mode of oc-currences of phenocryst in chemical Subunit B2 are similarto those in chemical Subunit Bi, except for the slightlyhigher content of phenocrysts olivine and plagioclase (Fi-gure 4).

Unit C: Chemical Unit C is composed of variable basal-tic clastic rocks, such as basaltic sand and sandstone,hyaloclastite, and pillow breccia. They are sparsely phyricbasalt containing 1 to 3 per cent of phenocryst of olivine andplagioclase. Pillow fragments of Unit C are characterized bylarger varioles (0.5 to 1.0 mm in diameter) in thegroundmass than those of Units A and B (the size of variolesis 0.1 to 0.3 mm).

Unit D: Chemical Unit D is also composed of basalticclastic rocks. The petrographical feature of basalt of Unit D,however, varies from sparsely phyric basalt (total phenoc-ryst content < 1%) to phyric olivine-plagioclase basalt (to-tal phenocryst content up to 15%).

MINERAL CHEMISTRY

Olivine

The chemical composition of olivine is shown in Table 4and Figures 7, 8, 9, and 10. The following three types ofchemical zoning of olivine are recognized: (1) normal zon-ing of phenocryst core, (2) reverse zoning of phenocrystcore,and (3) normal zoning of phenocryst rim andgroundmass. In normal zoning of the phenocryst core (Type1), the homogeneous Mg-rich inner zone (Fo = forsteritecontent up to 89%) is surrounded by an Fe-increasing mar-ginal zone (Figure 7 [b]). Reverse zoning (Type 2) has ahomogeneous Fe-rich inner zone (Fo, 83%) surrounded by aMg-increasing marginal zone (Figure 7 [a]). The composi-tional range of zoned core of the olivine phenocryst (Types1 and 2) is up to 3 mole per cent in Fo content (Figures 7 and

117


TABLE 1Zonal Variation of Textures of Pillow in Hole 396B

ZoneThickness

(mm) Primary Constituents

I

II

III

IV

Sideromelaneglass zone

Cryptocrystallinevariolite zone

Microcrystallinespherulite zone

Intersertalinterior zone

avg. 8(3-16)

avg. 8(4-12)

10-30

500-1000

glass (>99%): palagonitized at surface or along cracks (1-2 mm, thick)plagioclase (<1%): hopper (0.1X0.01 mm)olivine (<1%): skeletal (0.01-0.03 mm)sulfides (<1%): rounded, irregular, or lining in vesicles (0.001-0.02 mm)variole (>99%): cryptocrystalline, occasionally with incipient retardationplagioclase (<1%): hopper (ß = 0.1 mm)olivine (<1%): skeletal (0.02-0.05 mm)sulfides (<l%): rounded or irregular (0.001-0.02 mm)plagioclase (10-50%): hopper, radially arranged forming the spherulitictexture with clinopyroxene (0.1-0.2 mm)

olivine (~3%): skeletal, euhedral (0.03-0.1 mm)clinopyroxene (0-40%): variolitic, very finesulfide (<1%): rounded or irregular (0.001-0.02 mm)titanomagnetite: very fine, dustyplagioclase (40-60%): dense hopper, random orientation (0.1-0.5 mm)olivine (1-5%): euhedral, skeletal (0.05-0.1 mm)clinopyroxene (40-50%): variolitic, dendritic in the intersiticiessulfide (<1%): rounded or irregular (0.001-0.02 mm)titanomagnetite (1-5%): cube or skeletal (0.001-0.01 mm)

8). Olivine phenocrysts with Types 1 or 2 zoning occur inpillow margin, indicating that they were formed in the in-tratelluric stage of crystallization. They co-exist in onespecimen. The rims of these olivine phenocrysts have thesame Fo content (Figure 8). Type 3 normal zoning ofolivine is only observed in the well-crystallized interior ofpillows and in the massive lava. It is observed only at thenarrow periphery of olivine phenocryst (Figure 7 [a]),suggesting that diffusion homogenization after the extrusionof magma (Moore and Evans, 1967) is minimal in the olivineof pillow lava.

The compositional variation of olivine in relation to thestratigraphic units is shown in Figure 9. The analyses werecarried out on the rim of olivine crystal. For each chemicalunit, mean olivine composition is nearly constant. Forsteritecontent of the analyzed olivine is well correlated with theMg/(Mg+Fe) ratio of the host glass. Supposing the oxida-tion index (OX = Fe 3+ /total Fe) of 0.1, we calculatedMg-Fe partition coefficient between olivine and glass foreach unit. The partition coefficient is nearly constant andthe average value is 0.285 which is in close agreement withthe equilibrium experimental value of Roeder and Emslie(1970) (0.30). Thus, the olivine and glass may have re-tained the equilibrium partition relation at the time of extru-sion. Among trace elements of olivine, nickel is of particu-lar interest in evaluating the role of olivine in the fractionalcrystallization of basaltic magmas (Sato, 1977). As isshown in Figure 10, olivines of sparsely phyric (chemicalSubunits Ai and A2) and phyric (Bi and B2) basalts show alittle different trends in the NiO versus forsterite contentdiagram. Olivine in phyric basalt is more Mg-rich than thatin sparsely phyric basalt for the same NiO content. Bothtrends show positive correlation between the forsterite andNiO contents, and are nearly parallel to the curve of themodel fractional crystallization trend of olivine (Figure 10).

Ca-rich Clinopyroxene

The only one Ca-rich clinopyroxene phenocryst in thedrilled basalt of Holes 396 and 396B occurs as crystal clot in

the lower portion of the massive lava of chemical SubunitA3. Chemical compositions of minerals of both the crystalclot and the surrounding groundmass are shown in Table 5,and are plotted in Figures 11 through 14. Clinopyroxene ofthe clot is rather homogeneous with Ca36-4oMg5o-53Feio-n andis distinct from the groundmass clinopyroxene (Figure 11).The Mg/(Mg+Fe) ratio of the clot clinopyroxene is 0.83,while that of the groundmass clinopyroxene is 0.74 to 0.65.The clot clinopyroxene is much lower in Ti, Al, and Nacontents as well as in Ti/Al ratio than those in thegroundmass (Figures 13 and 14). The difference of chemis-try between the clot and the groundmass clinopyroxenessuggests a discontinuous crystallization history ofclinopyroxene. Mg/(Mg+Fe) ratio of the clot clinopyroxeneis similar to that of the groundmass olivine, and it is consi-dered that clot clinopyroxene was in close partition equilib-rium with the host magma. One atmosphere fusion experi-ment (Fukuyama and Hamuro, this volume) shows that Ca-rich clinopyroxene is unstable at the liquidus temperature ofbasalt of chemical Subunit A3. The corrosion of the clotclinopyroxene may have taken place at shallow depth beforethe extrusion of the magma. Kushiro (1973) has shown thatthe stability of Ca-rich clinopyroxene increases with in-creasing pressure, and clinopyroxene appears as a liquidusphase above 7 to 8 kilobars for ocean-floor olivine tholeiite.It is conceivable that the clinopyroxene crystal clot in Sub-unit A3 of Hole 396B represents relatively high-pressurephenocryst when compared with most of the other euhedralphenocrysts of olivine and plagioclase. The sodic nature ofthe clot plagioclase (Ca/[Ca + Na] = 0.64) may beinterpreted by the shift of partition equilibrium due topressure.

Plagioclase

Representative chemical analyses of plagioclase areshown in Table 6, and they are plotted in Figures 15 through20. The plotted data include the partial analyses ofplagioclase in which only Na and Ca are analyzed. Thestandard error of the obtained Ca/(Ca+Na) ratio of partial

118


TABLE 2Grain Size of Groundmass Constituents of the Massive Lava (µ)

Massive lavaSub-zone Sample

Depth(m) Texture

Olivinef o

Plagioclase(1)

lightdark

σa

Plagioclase(2)

lightdark Granulitic Variolitica

ClinopyroxeneMesostasis

Dendritic (f)Ilmenite

(d)Sulfide

wUpper

Middle

15-1,6

15-1, 12

51-2, 2(B)

15-3, 2(B)

15-3, 3(A)

15-3,7

15-4, 2(C)

15^,4

15-4, 7(A)

15-5,3

15-5,9

135.4

135.95

237.55

238.20

238.80

239.35

239.90

240.20

240.65

241.30

241.75

glass variolitic

intersertal

intergranular

intergranular

intergranular-variolitic




intergranular

intersertal

intersertal

57

50

17

86 18

149 99

115 23

134 38

103 28

173 118

97 20

65

57

10

13

14213

66749

1345122

1070209

1356149

1789297

1193192

1317190495

7734645

330

7322

7633

40575119899

34782110472

85706

66299

20168

18125

17

-

-

30-1005-30

80-20010-5080-20010^0

100-40010-80

-

-

2-6

30-120

30-150

30-100

30-150

30-150

30-150

20-80

30-150

30-100G5-20 P

30-150 G10-30 P30-150 G10-40 P30-150 G20^0 P20-80

3-10

3-10

1-4

5-20

5-20

3-15

5-10

3-10

5-30

4-10

1-3

1-3

n.d.b

1-3

1-3

1-2

1-2

1-3

1-2

1-3

n.d.

n.d.

1-2

n.d.

2-10

2-10

1-6

2-15

2-10

2-10

2-5

n.d.

n.d.

aG = groundmass; P = phenocryst.''n.d. = not detectable.

log

- 6 - 5 - 4 - 3 - 2 - 1 0 1 2 3 4 5

5 -

T I I I I I I I

5 -

5 -

1 I I I I I

Pi.

ol.

Pi.

A., (9)

A9 (13

A3(15)

cpx.

I I I I0.09 0.13 0.20 0.30 0.44 0.67 1.0 1.5 2.25 3.38 5.06 7.59

<t> (mm)

cpx-pl. clot in 15-5, #9

rounded crystal

angular crystal

Figure 3. Mode of occurrence of phenocryst in the sparselyphyritic basalt of chemical Units Aj, A2, and A3.

analyses is about 0.005. Plagioclase phenocrysts in thephyric basalt usually show normal zoning at the margin.Calcic core is surrounded by a broad sodic inner rim, whichis then surrounded by a more sodic outer rim. The inner rimis 50 to 300 µm thick and is homogeneous. The boundariesbetween the core and the inner rim and between the innerand the outer rims are sharp. The core of the plagioclasephenocryst in phyric basalt is either euhedral or rounded andshows variable zoning pattern, flat, oscillatory, gradualnormal and reverse, sharp normal and reverse, patchy, andother complex zoning. It is difficult to find regularity of thecore zoning pattern among plagioclase phenocrysts in a rockspecimen. Figures 15 through 18 illustrate thecompositional range of plagioclase phenocrysts for phyricbasalts. Compositional range of the core is read from thecontinuous scanning profiles of phenocryst grains on whichseveral partial analyses are made for calibration. Scanningprofiles on 15 to 23 grains are taken for each sample. Thesefigures show that the compositional range of the core differsfrom grain to grain in a sample, irrespective of the grainsize. The Ca/(Ca+Na) ratio of the core of plagioclasephenocryst ranges from 0.72 to 0.89, The composition ofthe inner rim of phenocryst is 0.70 to 0.75. The occurrenceof the inner rim of plagioclase phenocryst does not dependon the crystallinity of groundmass of pillow lava, indicatingthat the inner rim was formed before the eruption of magmaon the sea floor. The outer thin rim of plagioclasephenocryst is absent in the outer sideromelane and variolitezones of pillow lava, and occurs only in its interior. Thecompositional range of the outer rim of plagioclasephenocryst coincides with that of groundmass plagioclasecore (Figures 15 through 18 and 20), showing that the outerrim has grown after the eruption of magmas.

The stratigraphic variations of the composition of bothphenocryst and groundmass plagigclases are shown in Fig-ures 19 and 20, respectively. The compositional range of

119


TABLE 3Modal Compositions of Phyric Basalt of Hole 396B, Leg 46

Sample(Interval in cm) Piece No.

PhenocrystInclusion Inclusion

Olivine in Olivine Plagioclase in Plagioclase Spinel Groundmass Vesicle Vein Point

16-2, 35-4416-2, 124-13216-3,131-13717-1, 19-2517-1, 131-13717-2, 17-25

132-14180-88

4-1182-9095-102

114-12220-1,122-12620-2, 4-1020-2, 37-4520-2, 136-14120-3, 20-28

69-7544-5370-89

18-118-219-120-120-120-1

20-4,20-6,21-1,21-2, 43-5022-1, 105-12122-2, 125-145

Average

# 4(A)#11 (A)#15# 2(A)#11(B)# 2(A)# 7(F)# 4# 1# 4(F)# 4(G)# 4(1)# 4(J)# 1# 4# 9(G)# 3#11 (A)# 6# 9# 4#13#70

4.261.271.923.553.422.506.892.352.211.982.102.382.911.713.382.853.723.403.434.635.932.201.79

0.000.000.000.000.110.050.060.000.000.000.000.110.050.000.030.050.040.000.050.090.080.000.03

15.6615.2014.3015.2313.5714.6613.5114.8714.1517.2216.4015.3013.6514.0718.1513.3018.6319.3421.3220.2316.0217.9618.81

0.470.120.130.230.280.230.420.430.070.270.000.420.790.220.290.220.560.350.690.420.350.100.39

0.000.000.000.030.000.010.000.000.000.000.000.000.000.000.070.000.000.000.000.030.000.000.05

79.2382.2482.6680.1682.1181.9578.7381.6883.2279.5080.4079.2979.9683.1976.7183.1475.8575.5272.9573.1477.5079.1378.70

0.381.160.960.810.500.960.380.670.351.021.091.981.260.811.370.441.191.391.541.880.120.600.24

0.000.000.000.000.000.000.000.000.000.000.000.501.390.000.000.000.000.000.000.000.000.000.00

340481593022355235671053330842555289063082561357438183213307436534460373941933347256538193850

Groundmass

Mode ofPhyric Basalt

Plagioclase

Figure 4. Modal ratio groundmass-phenocryst olivine-phenocryst plagioclase ofphyric basaltsof chemical Units Bj and B2

120


30

20 -

10 -

-

-

-

olivine

plagioclase

ε 40 -

30 -

20 -

10 -

.04 .06 .09 .13 .20 .30 .44 .67 1.0 1.5 2.25 3.38 5.06 (mm)

_8 - 7 - 6 - 5 - 4 - 3 - 2 - 1 0 1 2 3 4

Log 1 5 </>

Figure 5. Grain-size distribution of phenocryst olivine andplagioclase in phyric basalts.

20 -

10

Spinel grain size(avg. 0.020 vol %)

16-1, 75-80 cm (#10C)

(0.035 %)

17-1, 20-25 cm (#2 A)

(0.013)

20-1,92-97 cm (#4G)

(0.037)

i—i—r

-I T

22-1, 105-110 cm (#13)

(0.006)

22-2, 125-145 cm (#7D)

(0.009)

8 12 17 26 39 59 88 132 198 296 444 667 1000

Φ (µm)

Figure 6. Grain-size distribution of phenocryst spinel inphyric basalts.

the phenocryst core is nearly constant among analyzedphyric basalts of Subunits Bi and B2. Plagioclase phenoc-rysts in the sparsely phyric basalts of Units Ai and A2 areless calcic than those of Subunits A3, Bi, and B2, and theydo not show normal zoning at the rim. The composition ofthe groundmass plagioclase is nearly constant except for theplagioclase of Subunit A3, which are lower in Ca/(Ca+Na)ratio in accord with the low normative An/(An+Ab) ratio ofhost A3 basalt as compared with other basalts of Hole 396B.Figure 20 also illustrates that composition of groundmassplagioclase in both variolitic and intersertal interior zones ofpillows is nearly the same. The element partition relationsbetween glass and groundmass plagioclase core are verysimilar to those in the 1-atmosphere equilibrium meltingexperiments of Fukuyama and Hamuro (this volume). Theelement partition findings of this study differ from the parti-tioning models of Kudo and Weill (1970) and of Drake(1975), i.e., about 10 mole per cent higher An content foranalyzed plagioclase than the models at 1 atmosphere and1200°C).

Chromian Spinel

Chromian spinels are homogeneous within a grain, butthe composition may vary from grain to grain. Representa-tive analyses of chromian spinel are shown in Table 7 andplotted in the Mg/(Mg+Fe2+) versus Cr/(Cr+Al) andFe3 + +Cr+Al) diagrams (Figure 21). The overall composi-tional range of chromian spinel of Hole 396B is narrow andis included within the compositional range of spinels fromthe Mid-Atlantic Ridge of 30°-40° North latitude(Sigurdsson and Schilling, 1975). There is a positive corre-lation between Cr/(Cr+Al) and F e 2 + /(Mg+Fe2+) ratios. InFigure 21, spinel compositions are shown to be related tothe mode of occurrences. Spinels included in plagioclase areusually more magnesian than those in groundmass and inolivine phenocryst.

Chemistry of Rocks and GlassesMajor element chemical analyses of rocks and glasses

were carried out by three methods: XRF analyses by Cam-bon (Leg 46 Scientific Party this volume), wet chemicalanalyses (gravimetric, flame photometric, and colorimetric)by Aoki, and electron microprobe analyses by Sato. Theanalytical results of the latter two are shown in Tables 8 and9, and the average bulk rock and glass compositions of eachchemical unit are shown in Tables 10 and 11. The neutronactivation analyses of Cs by Okamoto on fresh glass areshown in Table 12. Because of the common occurrences ofsecondary minerals, the alteration effect is expected for thevariolitic to more crystalline basaltic rocks. Some of theanalyses of palagonite, smectites, mica, cryptocrystallinevarioles, and altered interstitial materials are shown in Table13. By comparing the analyses of fresh sideromelane glassand crystalline rocks of the sparsely-phyric basalt, it is seenthat the composition of the crystalline rocks show muchlarger scatter than those of the glasses for FeO* (total Fe asFeO), MgO, K2O, and H2O, thereby suggesting that thesecomponents have migrated during the sea-floor alteration.The alteration reduced MgO and FeO*, and increased K2Oand H2O contents. Oxidation index (Fe 3 + /total Fe) is muchhigher for crystalline rocks than for the glass sample ofSection 10-1 (Table 8). These chemical changes of the

121

toro

TABLE 4Representative Analysis of Olivine

Analysis No.

Sample(Interval in cm)

Piece No.

Grain No.

Position

1

5-2,26-32

4

Ph-2

core

2

Ph-2

rim

3

7-35-19

IB

1*a

4

2*a

5

10-1,49-57

9(A)

Ph-4

rim

6

Ph-8

core

7

Ph-8

rim

8

11-1,110-117

12

1*a

9

2*a

10

13-2,34-50

5(A)

Ph-3

core

11

Ph-3

rim

12

15-1,92-96

12 (A)

1*a

13

2*a

14

3*a

15

16-1,73-82

10(C)

1*a

16

2*a

17

3*a

18

22-1.104-121

13

Ph-2

core

19

Ph-2

rim

20

Ph-3

core

21

Ph-3

rim

SiO 2

A12O3

FeO

MnO

Mg()

CaOC r2°3NiO

Total

Oxygen = 4

Si

Al

Fe

Mn

Mg

Ca

Cr

Ni

Total

Mg/(Mg+Fe)

40.3

14.3

0.22

45.1

0.04

0.16

100.0

1.006

0.299

0.005

1.679

0.001

0.003

2.993

0.848

40.2

12.8

0.20

45.9

0.04

0.21

99.4

1.006

0.268

0.004

1.711

0.001

0.004

2.994

0.864

39.8

0.07

13.3

0.22

45.6

0.28

0.05

0.19

99.5

0.998

0.002

0.278

0.005

1.706

0.008

0.000

0.004

3.001

0.859

38.4

0.05

18.6

0.2841 .8

0.25

0.04

0.11

99.4

0.989

0.001

0.401

0.006

1.604

0.007

0.000

0.002

3.010

0.800

40.3

13.5

0.20

45.2

0.05

0.23

99.5

1.010

0.283

0.004

1.688

0.001

0.005

2.990

0.856

40.3

14.9

0.22

44.3

0.03

0.17

100.0

1.011

0.313

0.005

1.657

0.001

0.003

2.989

0.840

40.0

14.2

0.20

45.4

0.03

0.18

100.0

1.001

0.2970.004

0.004

1.693

0.001

2.999

0.850

40.0

0.04

14.5

0.21

45.1

0.24

0.05

0.18

100.3

1.000

0.001

0.3030.004

0.004

1.681

0.007

0.000

2.999

0.847

40.5

0.08

10.9

0.16

48.0

0.29

0.06

0.23

100.3

0.997

0.002

0.2250.003

1.762

0.008

0.000

0.004

3.001

0.886

40.0

14.0

0.25

44.7

0.06

0.23

99.3

1.007

0.2950.005

1.678

0.001

0.005

2.992

0.850

39.9

13.6

0.19

46.1

0.07

0.23

100.1

0.995

0.284

0.004

1.714

0.001

0.005

3.004

0.857

39.9

0.06

12.4

0.21

46.4

0.25

0.07

0.18

99.4

0.997

0.002

0.259

0.004

1.729

0.007

0.001

0.003

3.002

0.869

39.80.07

14.8

0.23

44.8

0.26

0.06

0.19

100.2

0.998

0.002

0.310

0.005

1.675

0.007

0.000

0.004

3.001

0.843

39.4

0.05

16.8

0.25

43.6

0.25

0.02

0.13

100.5

0.994

0.002

0.355

0.005

1.640

0.007

0.000

0.002

3.005

0.821

40.50.06

12.7

0.20

47.3

0.27

0.07

0.23

101.3

0.995

0.002

0.260

0.004

1.732

0.007

0.001

0.004

3.005

0.869

40.3

0.09

10.2

0.18

47.8

0.29

0.09

0.25

99.1

0.999

0.003

0.212

0.004

1.767

0.008

0.001

0.005

2.999

0.892

39.8

0.07

13.5

0.21

45.6

0.30

0.08

0.16

99.6

0.997

0.002

0.282

0.004

1.703

0.008

0.001

0.003

3.000

0.857

41.3

1 1.1

0.15

47.6

0.06

0.2 1

100.4

1.012

0.228

0.003

1.739

0.001

0.004

2.987

0.884

40.3

12.3

0.22

45.9

0.06

0.19

99.0

1.009

0.258

0.005

1.714

0.001

0.004

2.990

0.869

40.1

13.0

0.24

45.8

0.06

0.17

99.3

1.004

0.272

0.005

1.710

0.001

0.003

2.995

0.863

40.9

12.1

0.23

46.7

0.07

0.20

100.2

1.010

0.250

0.005

1.719

0.001

0.004

2.989

0.872

a* = Separated olivine crystals on epoxy mount are analyzed to avoid the contamination effect for Al and Ca.


0 0.2 mm 0.2 mm

83. 86.

Mg

Fe

7-3, #1B

Figure 7. Electron microprobe scanning profiles of olivine phenocrysts showing bothreverse (a) and normal (b) zonings.

2.25

1.50

1.00

.67

.44

.ü .30

.20

.13

.09

.06

.04 i-22-1, 105-110 cm (#13)

Figure 8. Compositional range of olivine phenocryst inSample 22-1, #13 plotted against grain size. Note theuniformity of the rim composition.

ocean-floor basalts are consistent with the data of Shido etal. (1973).

The compositional range of all the analyzed glasses ofHole 396B (23°N) is shown in Figure 22, together with

1500.90

Mg (Mg+Fe)0.85 0.80

200 -

250 -

300 -

1 1

i—ni

i—1 i i—i

r - j

,-TT~I-T-~ '

r-T~ 1 ,—.1 1

p 1 0F the number crystalEo

1

2

3 -

4

5

6

7

8

9 ~

A 1

A 2

A 3

B 1

B2

C

Figure 9. Stratigraphic variation of olivine composition ateither phenocryst rim or groundmass core.

those of the glasses from the Atlantic Ocean (Melson et al.,in press), and the FAMOUS region (37°N), (Bryan andMoore, 1977). Figure 22 shows that the compositionalrange of Hole 396B glasses is generally very much re-stricted as compared with those of the FAMOUS glasses.Figure 23 shows that Hole 396B glasses are higher in T1O2content for the same FeO*/MgO ratio than are the FAM-OUS glasses. Bulk AI2O3 content of the phyric basalts of

123


0.4

~ 0.3

Io2 0.2

0.1 12%

0.90 0.85

Mg/(Mg+Fe)

0.80

Figure 10. Nickel versus Mg/(Mg+Fe) ratio of olivine. Solidcircle: chemical Unit Aj, plus: A2, cross: A3, opencircle: Bj, open square: B2. Solid line represents thecompositional variation of olivine during fractional crys-tallization of olivine, and the attached number denotesthe degree of fractionation.

Hole 396B ranges from 16.8 to 18.0 weight per cent and ismuch higher than the average AI2O3 content of ocean-floorbasalt glasses (Figure 22). On the other hand, electron mic-roprobe analyses of glasses as well as wet chemical analysesof separated groundmass (Table 8) show that the matrix ofthese phyric basalt contains 15.2 to 15.8 weight per cent ofAI2O3. The high AI2O3 content of bulk phyric basalts maybe attributed to the accumulation of plagioclase phenocryst.

In Figure 24, FeO-MgO relations are shown for analyzedglasses. The figure demonstrates that each chemical unit hasits own compositional range, and is very uniform as to themajor element chemistry. The compositional gaps amongchemical units suggest that these basalts are not derived bythe successive eruption from a single magma chamber,which had gradual compositional gradient in it. Because ofthe supposed short time interval between the eruptions ofthe chemical units which belong to the same magnetic unit(the constant magnetic inclination within a unit suggests thetime interval is short enough not to cause the secular varia-tion of magnetic field), close petrogentic consanguinitywould be expected for these chemical units. Chemical Sub-units Ai and A2 belong to magnetic Unit 1, and Bi and B2belong to magnetic Unit 3 (Figure 2). Both of these pairs ofchemical units show similar compositional relations, suchthat as MgO decreases from Ai to A2, or FeO, Tiθ2, andNa2θ increase from B2 to Bi, and CaO and AI2O3 decrease.These compositional variations agree with the general frac-tionation trends of ocean-floor basalt (Kay et al., 1970;Miyashiro et al., 1969). The crystal fractionation model forthese Leg 46 basalts will be discussed in the following chap-ter.

DISCUSSIONS

Fractional Crystallization of Leg 46 Basalts

As shown in the previous section, chemical units within amagnetic unit display compositional variations common in

ocean-floor basalts. MgO, AI2O3, and CaO decrease andFeO, TiOβ, and Na2 increase from the chemical Subunit Aito A2, and from B2 to Bi. These compositional relations areexamined in light of the fractional crystallization model,involving olivine, plagioclase, and Ca-rich clinopyroxeneas fractionating phases. Because the compositional range ofbasalt is narrow, compositions of the fractionating phaseswere kept constant except for plagioclase. Plagioclasephenocrysts in hole 396B basalt show rather widecompositional range, so that plagioclase compositions areadjusted to give the best fit of the calculation. Table 14compares the calculated compositions with the analyzedcompositions, showing both of these compositions tocoincide within error of analyses. The result indicates thatplagioclase and Ca-rich clinopyroxene are the major phasesparticipating in the fractional crystallization.

Because normative components of plagioclase, Ca-richpyroxene, hypersthene, and olivine or quartz compriseabout 95 per cent of ocean-floor basalt composition,quaternary system olivine-plagioclase-Ca pyroxene-silicawell illustrates their compositional relations. Figure 25illustrates the projections of glass compositions of Leg 46,as well as those of FAMOUS region and East Pacific, on theternary diagrams olivine-Ca pyroxene-silica andplagioclase-olivine-Ca pyroxene, which terminate thequaternary system. In this figure, compositions ofsparsely-phyric basalts of Subunits Ai and A2 and phyricbasalts of Bi and B2 show different trends, the former beingmore enriched in olivine component than the latter. Both ofthese trends are nearly parallel to the projectedolivine-plagioclase-Ca-rich clinopyroxene "quaternary"cotectic line drawn by Shibata (1976), suggesting that thetrends were controlled by these three crystalline phases.Figure 26 shows that Mg/(Mg+Fe) ratio decreases as silicacomponent increases in the quaternary system, supportingthe fractional crystallization model in the formation of thecompositional trends. The compositions of Subunit A3 andUnit C do not lie on these trends in Figures 25 and 26;hence, they have no direct genetic relationships to theothers.

Basaltic glasses of Leg 46, as a whole, have a ratherrestricted compositional range and are lower inMg/(Mg+Fe) ratio (0.60 to 0.65, assuming the oxidationindex of 0.1) than the most magnesian basaltic glass fromthe FAMOUS region (the ratio is 0.68 to 0.70). The lowMg/(Mg+Fe) ratio of Leg 46 glasses indicates that they areevolved from the more primitive magmas throughcrystallization differentiation. Assuming the mostmagnesian basaltic glass from the FAMOUS region as astarting material, the composition of Leg 46 basalts weretried to fit by subtracting olivine, plagioclase, and Ca-richclinopyroxene, but it was revealed that Leg 46 basalticcompositions cannot be derived from the most magnesianFAMOUS glass by this crystal fractionation model. Thecalculation always gives higher FeO contents than theobserved value. The difference of the composition ofprimary magmas from both the Leg 46 and FAMOUSregions may reflect the differences of the depth and thedegree of partial melting and the different compositions ofthe source peridotites between these regions.

Meanwhile, the degree of fractionation of olivine can beestimated approximately by NiO content and Mg/(Mg+Fe)

124


TABLE 5Compositions of the Ca-rich Clinopyroxene Phenocryst and the Coexisting Minerals in Sample 396B-15-5, #9

No.

SiO2

TiO2

A12O3

FeOMnOMgOCaO

Na2OK 2 ONiOCr2O3

TotalOxygen = 6.0

SiAl

TiCr

FeMnNi

MgCaNaK

Total

Mg/(Mg+Fe)

CaMg

Fe

No.

SiO2

TiO2

A12O3

FeOMnO

MgOCaO

Na2OK 2ONiO

Cr2O3

TotalOxygen = 6.0

SiAl

TiCr

FeMnNi

Mg

CaNaK

Total

Mg/(Mg+Fe)

Ca

MgFe

01

53.30.42

2.606.44

0.1718.219.30.24

0.000.04

0.22100.87

1.932

0.111

0.0110.006

0.195

0.0050.0010.982

0.7480.017

0.000

4.008

0.835388511101

02

53.3

0.49

2.796.34

0.1718.019.3

0.230.000.03

0.23100.86

1.929

0.1190.013

0.006

0.192

0.0050.0010.974

0.7480.016

0.0004.003

0.836390510100

Phenocryst Ca-rich Clinopyroxene

03

52.9

0.49

2.99

6.260.18

17.119.4

0.250.010.03

0.2599.85

1.934

0.1290.013

0.0060.191

0.006

0.0010.934

0.7620.018

0.001

3.995

0.830404

495101

04

53.8

0.452.73

6.68

0.1818.219.7

0.290.000.04

0.22

102.25

1.926

0.1150.012

0.005

0.2000.006

0.0010.9700.757

0.020

0.0004.012

0.832

390507103

Groundmass Ca-rich Clinopyroxene

11

48.61.844.64

10.60.22

13.319.3

0.43

0.010.01

0.2599.2

1.8390.2070.052

0.007

0.3350.0070.000

0.753

0.7850.031

0.0014.017

0.691

420

401179

12

49.4

1.785.519.11

0.1913.7

20.10.44

0.000.02

0.35100.6

1.8330.241

0.050

0.0100.283

0.0060.0000.757

0.798

0.032

0.0004.010

0.729

434

413153

13

49.2

2.055.59

11.21

0.2312.319.9

0.45

0.010.02

0.20

101.1

1.830

0.245

0.0580.005

0.3490.0070.000

0.682

0.7930.033

0.0014.003

0.664

434

375190

14

48.7

1.97

6.208.47

0.1813.521.2

0.450.020.020.24

100.8

1.8030.271

0.055

0.0060.262

0.0060.0000.744

0.8409,9330.0014.021

0.741

454

404

142

05

52.90.50

3.18

6.980.19

18.317.90.32

0.010.030.24

100.46

1.9210.1360.014

0.006

0.212

0.006

0.0010.9900.6960.023

0.001

4.006

0.824

366522111

15

50.51.64

2.9112.4

0.32

13.219.0

0.500.050.020.04

100.5

1.896

0.129

0.0460.000

0.3890.0100.000

0.7380.764

0.0370.0024.011

0.656

404

391205

06

52.80.513.02

6.360.21

17.818.60.310.000.04

0.24

99.87

1.929

0.1300.014

0.0060.194

0.0060.001

0.9690.7290.022

0.000

4.000

0.833

386512103

16

48.71.523.529.23

0.2513.520.2

0.450.010.03

0.1297.4

1.8700.1590.044

0.002

0.2970.0080.001

0.7750.8320.034

0.0014.023

0.719

440

403158

07

53.10.49

3.136.79

0.18

18.117.7

0.350.000.03

0.25100.14

1.9320.134

0.013

0.006

0.207

0.006

0.0010.9830.6910.024

0.000

3.997

0.826

368523110

08

53.10.42

2.844.57

17.220.4

0.230.04

0.030.18

99.01

1.951

0.1230.012

0.006

0.140

0.0010.9400.801

0.0160.002

3.992

0.870

426500075

09*

49.31.66

6.607.63

0.2014.4

20.50.370.000.02

0.29

100.85

1.810

0.2860.046

0.0080.234

0.0060.000

0.7880.8050.027

0.000

4.010

0.777

440432

128

125


TABLE 5 - Continued

Plagioclase

Phenocryst associated with the clinopyroxene phenocryst

No.Grain No.

Position

SiO2

TiO2

A12O3

FeOMgO

CaONa2OK 2 O

TotalOxygen = 8SiAlTiFe

Mg

CaNaK

TotalCa/(Ca+Na)

Groundmass

No.

SiO2

FeO

MnO

MgOCaONiOTotalOxygen = 4SiFeMn

Mg

CaNi

TotalMg/(Mg+Fe)

211

core

53.00.10

29.30.530.38

13.04.060.06

100.5

2.3971.5620.0030.0200.0260.6300.3560.0034.9980.638

221

core

52.90.09

29.70.540.32

13.33.830.07

100.7

2.3861.5800.0030.0200.0220.6410.3350.0044.9900.657

Olivine

31

40.015.10.26

43.50.430.18

99.5

1.0110.3190.0060.6390.0120.0042.9910.837

32

39.718.50.31

41.00.520.11

100.1

1.0120.3940.0071.5580.0140.0022.9870.798

231

rim/cpx

50.80.05

30.60.560.41

14.93.130.05

100.4

2.3121.6400.0020.0210.0280.7250.2760.0035.0060.724

33

39.817.40.32

41.90.510.12

99.8

1.0110.3700.0071.5860.0140.0022.9900.811

246

core

53.50.11

29.30.590.24

13.04.180.07

101.0

2.4051.5550.0040.0220.0160.6270.3640.0044.998

0.632

256

rim/g.m.

52.90.14

29.60.730.33

13.43.910.07

101.1

2.3841.5690.0050.0280.0220.6540.3410.0044.9990.654

2612

core

52.40.11

30.00.700.30

13.83.610.05

100.9

2.3641.5950.0040.0260.0200.6650.3160.0034.9940.678

Groundmass

2713

core

51.70.12

30.10.700.31

13.83.590.05

100.3

2.3481.6120.0040.0270.0210.6700.3170.0035.0020.679

2813

rim

54.50.17

27.71.000.36

11.94.660.07

100.4

2.4641.4780.0060.0380.0240.5760.4090.0044.9980.585

aRim of phenocryst (ca. 10 mm from the periphery).

ratio of magma, because olivine effectively removes nickeland magnesium from magma. The effects of olivinefractionation on MgO-FeO and NiO-MgO compositionalrelations are shown in Figures 27. The initial compositionswere chosen to be in partition equilibrium with the supposedmantle olivine (NiO = 0.40 wt. %, Fo = 90%), and haveMgO = 10.0 wt. per cent, FeO2 = 6.6 per cent, and NiO= 0.038 per cent (Sato, 1977). These figures demonstratethat about 5 to 8 per cent of olivine was removed from theprimary magma in the formation of Leg 46 basalts. NiOcontent of olivine phenocryst is around 0.2 weight per cent(Figure 10), which also supports the value of the degree ofolivine fractionation. Though olivine is expected to be one

of the major phases to crystallize in the early stage of thecrystallization of ascending basaltic magmas, plagioclaseand Ca-rich clinopyroxene, and also minor amount ofchromian spinel may be incorporated in the fractionalcrystallization. Primitive basaltic glasses (Mg/(Mg+Fe) =0.68-0.70) from ocean floor generally have higher CaO andAI2O3 contents than Leg 46 basalt, suggesting that bothplagioclase and Ca-rich clinopyroxene in addition to olivineare the major fractionating phases in the derivation of Leg46 basalts.

The fractionation model discussed above shows theimportant role of Ca-rich clinopyroxene, which isdiscordant with the very rare occurrence of Ca-rich

126

CaMg


CaFeΛ A

olivine Fe

Figure 11. Composition of Ca-rich clinopyroxene phenocryst (open circle), groundmassclinopyroxene (solid circle), and groundmass olivine (bar) in the lower part of themassive lava, lithologic Unit 2. Sample 15-5, 70-78 cm.

CaMg CaFe

Mg olivine Fe

Figure 12. Composition of groundmass clinopyroxene and olivine in the central part ofthe massive lava. Sample 15-3, 15-23 cm.

clinopyroxene phenocryst in these basaltic rocks. Thisapparent discrepancy may indicate that there were twostages of intratelluric crystallization for Leg 46 basalts. Thecrystallization of the first stage determined the maincompositional trends and involved olivine, plagioclase,Ca-rich clinopyroxene, and minor amounts of chromianspinel. It took place at a deep level of magma reservoir,while, the second stage is represented by most of thephenocrysts of olivine and plagioclase. It is possible tointerpret the large rounded phenocrysts of olivine,plagioclase, and Ca-rich clinopyroxene in the sparselyphyric basalts to represent the first stage of crystallization.

Temperature of Magmas

The eruption temperatures of Hole 396B basaltic magmaswere calculated using the method of Roeder and Emslie(1970), which is based on the observation that MgO andFeO contents of magma in equilibrium with olivine at 1atmosphere are related to the temperature. The glass

analyses are used for calculation, because thesesideromelane glasses are saturated with olivine, and hardlyaffected by the crystallization during quenching. Thetemperature was calculated for each chemical unit as shownin Table 15. The absolute error of the obtained temperatureis about 15°C (Roeder and Emslie, 1970), while its relativeerror may be within 5°C. The calculated temperature israther uniform among chemical units and ranges from 1202°to 1217°C, which accords well with the result of1-atmosphere melting experiments of Fukuyama andHamuro (this volume). The liquidus temperature of olivine,however, is affected by the dry pressure as well as volatilecomponents of magma. The dry pressure of 0.5 kilobars,corresponding to the water depth of 5000 meters, mayincrease the olivine liquidus temperature by 4°C (Nichollsand Ringwood, 1973). On the other hand, H2O content ofocean-floor basalt is estimated to be 0.30 weight per cent(Moore and Schilling, 1973). The depression of liquidustemperature of olivine is calculated by Burnham's (1975)

127


0.4

0.2

0.4

0.2

0.1

0.05

\ 0.4 -

0.2 -

6.0

4.0

2.0

2.0

1.0

0.¥

CCP

88o

-

i i

+

+

o

O ,

o

1 1

1

+

+

+

1

1

1 1

1 1

1 I

1

1

0.10

0.8 0.7

Mg/(Mg+Fe)

Nao0

MnO

NiO

Cr2°3

-J A I2°3

TiO-

0.6

Figure 13. Minor element content of both phenocryst(open circle) and groundmass (cross) slinopyroxene.Sample 15-5, 70-77 cm (#9).

0.05

o phenocryst core

• phenocryst rim

+ groundmass

0.1 0.2 0.3

AI/6 oxygens

Figure 14. 77 and Al relation of phenocryst and ground-mass Ca-rich clinopyroxene.

model of the water solubility in silicate melts and using theexperimental data of Nicholls and Ring wood (1973). Watercontent of 0.3 weight per cent, corresponding to 5.3 moleper cent of basaltic liquid, reduces the liquidus temperatureby 13°C. Though there remains uncertainty as to the effectsof minor volatile components, such as C02 and SO2 in theocean-floor basaltic magmas (Anderson, 1975), theextrusion temperature of Hole 396B basalt may be1190-1210°C (Table 15).

ACKNOWLEDGMENTS

Critical comments on the manuscript by Drs. S. Banno and M.Yamasaki of Kanazawa University were most helpful. Dr. H.

TABLE 6Representative Analyses of Plagioclase

Analysis No.


Piece No.

Grain No.

Position

SiO2

TiO 2

A1 2O 3

FeO

MnO

MgO

CaO

Na 2 O

K 2 O

Total

Oxygen = 8

Si

Al

Ti

Fe

Mn

Mg

Ca

Na

K

Total

Ca/(Ca+Na)

1

13-234-50

5A

PH-1

core

49.3

0.03

32.3

0.52

0.00

0.29

15.48

2.77

0.03

99.7

2.266

1.693

0.001

0.020

0.000

0.020

0.762

0.247

0.003

5.011

0.755

PH-1

50.9

0.03

31.0

0.48

0.00

0.24

14.92

3.14

0.03

100.7

2.308

1.655

0.001

0.018

0.000

0.016

0.725

0.276

0.002

5.002

0.724

3

PH-1

πm

51.8

0.01

30.6

0.55

0.00

0.27

14.54

3.36

0.03

101.2

2.334

1.627

0.002

0.021

0.000

0.018

0.702

0.294

0.002

4.999

0.705

4

Gra-5

51.5

0.01

30.2

0.73

0.00

0.32

14.36

3.41

0.02

100.6

2.336

1.617

0.002

0.028

0.000

0.021

0.698

0.300

0.001

5.004

0.699

5

16-1,73-82

10C

PH-1

core

47.3

0.09

32.9

0.35

0.09

0.26

17.43

1.65

0.03

100.1

2.175

1.784

0.003

0.013

0.004

0.018

0.859

0.147

0.002

5.004

0.854

6

PH-1

core

48.3

0.10

32.7

0.37

0.11

0.38

16.94

1.86

0.04

100.8

2.202

1.756

0.003

0.014

0.004

0.026

0.827

0.165

0.002

5.000

0.834

7

PH-1

innerrim

50.4

0.08

31.0

0.45

0.12

0.44

15.01

2.93

0.05

100.5

2.292

1.664

0.003

0.017

0.005

0.030

0.732

0.259

0.003

5.004

0.739

8

Ph-1

outerrim

54.8

0.12

27.7

0.59

0.11

0.53

13.41

3.86

0.06

101.2

2.459

1.465

0.004

0.022

0.004

0.035

0.644

0.336

0.003

4.973

0.657

9

Gm-21

core

52.5

0.13

29.1

0.84

0.10

0.75

13.67

3.67

0.05

100.9

2.375

1.553

0.005

0.032

0.004

0.050

0.662

0.322

0.003

5.006

0.673

10

22-1,104-121

13

Ph-8

47.6

0.03

33.4

0.29

0.00

0.32

17.80

1.62

0.02

101.1

2.168

1.792

0.001

0.011

0.000

0.021

0.868

0.143

0.001

5.006

0.859

11

Ph-1

49.3

0.03

32.4

0.36

0.00

0.35

16.11

2.32

0.03

100.9

2.238

1.732

0.001

0.013

0.000

0.024

0.783

0.204

0.002

4.998

0.793

12

Ph-1

core

48.4

0.00

32.9

0.37

0.01

0.33

17.09

1.94

0.02

101.1

2.201

1.761

0.001

0.014

0.000

0.023

0.832

0.171

0.001

5.003

0.830

13

Ph-1

innerrim

50.4

0.00

31.8

0.44

0.00

0.37

15.54

2.65

0.02

101.3

2.275

1.692

0.001

0.017

0.000

0.025

0.752

0.232

0.001

4.995

0.764

14

Gm-17

51.2

0.05

31.1

0.66

0.01

0.43

14.08

3.53

0.02

101.1

2.3131

1.653

0.002

0.025

0.000

0.029

0.681

0.309

0.001

5.014

0.688

15 a

22-1,

104-121

13

Bulk

48.25

0.06

32.41

0.88

0.01

0.70

15.79

2.12

0.04

100.26

2.206

1.747

0.002

0.030 b

0.000

0.048

0.774

0.188

0.002

4.997

0.800

Wet chemical analyses of separated plagioclase phenocryst.' Iron as ferric.

128


1 core composition range

O broad inner rim

• thin outer rim

5.06

3.38

2.25

1.5

E 1.0

0.67

0.44

0.30

0.20

o i-

0.60 0.70 0.80 0.90

Ca/(Ca+Na)

Figure 15. Composition of plagioclase phenocryst in phyricbasalt Sample 16-1, 75-80 cm (#10C).

Kurasawa of the Geological Survey of Japan kindlyaccommodated us with facilities for sample preparation and withthe polished thin sections. Messrs. S. Kasajima and K. Nakamuraof Kanazawa University made the polished thin sections, anddrafted the figures, respectively. The typing of the manuscript wasdone by Mrs. Y. Fukuda and Miss Y. Shibuya of KanazawaUniversity. We gratefully acknowledge the contributions of allthese individuals.

REFERENCES

Anderson, A.T., 1975. Some basaltic and andesitic gases, Rev.Geophys., Space Phys., v. 13, p. 37-55.

Bryan, W.B. and Moore, J.G., 1977. Compositional variations ofyoung basalts in the Mid-Atlantic Ridge rift valley near lat36°49'N, Geol. Soc. Am. Bull., v. 88, p. 556-570.

Burnham, C.W., 1975. Water and magmas, a mixing madel,Geochim, Cosmochim, Ada, v. 39, p. 1077-1085.

Clague, D.A. and Bunch, T.E., 1976. Formation of ferrobasalt atEast Pacific Midocean Spreading centers, J. Geophy Res., v.81, p. 4247-4256.

Drake, M.J., 1976. Plagioclase-melt equilibria, Geochim,Cosmochim. Ada, v. 40, p. 457-465.

core composition range

broad inner rim

thin outer rim

1.5

1.0

EE 0.67

ë 0.44E

CJ

Φ

g 0.30

0.20

0.13

0.09

oO h

• 1 o -•-Jl

• o o iO I

r> i—i 1

O I

O I

I 1

0.60 0.70 0.80 0.90

Ca/(Ca+Na)

Figure 16. Composition of plagioclase phenocryst in phyric

basalt Sample 17-1, 20-25 cm (#2).

Dowty, E., Keil, K., and Prinz, M., 1974. Lunar pyroxene-phyricbasalts: crystallazation under super cooled conditions, J.Petrol, v. 15, p, 415-453. ;

Hodges, F.N. and Papike, J.J., 1976. DSDP Site 334: magmaticcumulates from oceanic layer 3, J. Geophy. Res. v. 81, p.4135-4151.

Kay, R., Hubbard, N.J. and Gast, P., 1970. Chemicalcharacteristics and origin of oceanic ridge volcanic rocks, J.Geophy. Res., v. 75, p. 1585-1613.

Kudo, A.M. and Weill, D.F., 1970. An igneous plagioclasethermometer, Contrib. Mineral. Petrol., v. 25, p. 52-65.

Kushiro, I., 1973. Origin of some magmas in oceanic andcircum-oceanic regions, Tectonophysics, v. 17, p. 211-222.

Melson, W.G., Byerly, G., Nelen, J., O'Hearn, T., Wright, T.,and Valuer, T., in press. A catalog of the major elementchemistry of abyssal volcanic glasses.

Miyashiro, A., Shido, F., and Ewing, M., 1969. Diversity andorigin of abyssal tholeiite from the Mid-Atlantic Ridge near 24°and 30° north latitude, Contrib. Mineral. Petrol., v. 23, p.38-52.

, 1970. Crystallization and differentiation in abyssaltholeiites and gabbros from Mid-oceanic ridges, Earth Planet.Sci. Lett., v. 7, p. 361-365.

129



broad inner rim

thin outer rim

2.25

1.5

1.0

0.67

0.44

0.30

0.20

•

0.6 0.7 0.8Ca/(Ca+Na)

0.9


basalt Sample 18-1,132-142 cm (#7F).

Moore, J.G. and Evans, B.W., 1967. The role of olivine in thecrystallization of the prehistoric Makaopuhi tholeiitic lava lake,Contrib. Mineral. Petrol., v. 15, p. 202-223.

Moore, J.G. and Shilling, J.G., 1973. Vesicles, water, and sulferin Reykjanes ridge basalts, Contrib. Mineral. Petrol., v. 41, p.105-118.

Nicholls, I. and Ringwood, A.E., 1973. Effect of water on olivinestability in tholeiites and the production of silica-saturatedmagmas in the island-arc environment, J. Geol., v. 81, p.285-300.

Roeder, P.L. and Emslie, R.F., 1970. Olivine-liquid equilibrium,Contrib. Mineral. Petrol., v. 29, p. 304-325.


broad inner rim

thin outer rim

1.5

1.0

§ 0.67ε

0.44

oO

0.30

0 . 2 0 -

0.130.6 0.7 0.8 0.9

Ca/(Ca+Na)


basalt Sample 22-1,104-106 cm (#13).

Sato, H., 1970. Nickel content of basaltic magmas: identificationof primary magmas and a measure of the degree of olivinefractionation, Lithos, v. 10, p. 113-120.

Shibata, T., 1976. Phenocryst-bulk rock composition relations ofabyssal tholeiites and their petrogenetic significance, Geochim,Cosmochim. Acta., v. 40, p. 1407-1417.

Shido, F., Miyashiro, A., and Ewing, M., 1972. Compositionalvariation in pillow lavas from the Mid-Atlantic Ridge, Mar.Geol., v. 16, p. 177-190.

Sigurdsson, H. and Schilling, J.G., 1975. Spinels in Mid-AtlanticRidge basalts: chemistry and occurrence, Earth Planet. Sci.Lett., v. 29, p. 7-20.

Walker, D., Kirkpatrick, R.J., Longhi, J., and Hays, J.F., 1976.Crystallization history of lunar picritic basalt sample 12002:Phase-equilibria and cooling-rate studies, Geol. Soc. Am.Bull., v. 87, p. 646-656.

130


o groundmass plagioclase core (max. An)

—— Compositional range of phenocrysts plagioclase core

150

0.8Ca/(Ca+Na)

Figure 19. Stratigraphic variation of the composition ofplagioclase phenocryst in Hole 396B.

[ I variolite zone

J intersertal interior

5-2, 27-32 cm (#4) A1

•—|_J 7-1, 20-36 cm (#11) A.

H~7-3, 6-19 cm (#18) A1

8-2, 22-28 cm (#3) A2

10-1, 49-56 cm (#9A) A2

15-3, 15-24 cm (#2B) A3

16-1, 73-82 cm (#10C)B1

18-1 (#7F) B1

20-1, 95-100 cm (#4G)

20-1, 120-122 cm (#1)

22-1,103-121 cm (#13) B2

60 65 70Ca/(Ca+Na)

75

Figure 20. Stratigraphic variation of the composition ofgroundmass plagioclase in Hole 396B.

131


TABLE 7Chromian Spinel Composition


Piece No.Grain No.

Occurrence

AnalyzedPosition

SiO2

TiO2A12°3FeOMnOMgOCaONiOC r2°3TotalOxygen = 4.0SiAlTiCrFe 3 +

Fe2 +

MnMgNiCaTotalMg/(Mg+Fe2+)Cr/(A1+Cr)AlFe3 +

Cr


Piece No.Grain No.

Occurrence

AnalyzedPosition

TiO2

FeOMnOMgOCr2O3

TotalOxygen = 4.0AlTiCrFe 2 +

Fe 3 +

MnMgTotalMg/(Mg+Fe2+)Cr/(A1+Cr)AlFe 3 +

Cr

16-1,1307

g.m.

core

0.290.45

29.319.80.23

14.80.070.13

35.7100.8

0.0081.0280.0100.8400.1260.3660.0060.6570.0030.0023.0460.6430.450

515063422

22-1,1306

ol/g.m.

core

23.9519.130.19

14.3639.5597.84

0.8830.0160.9790.3590.1420.0050.6702.0540.6510.526

441071489

121-126 cm

rim

0.260.54

28.020.90.20

14.20.110.11

34.198.4

0.0081.0130.0120.8280.1560.3800.0050.6490.0030.0033.0570.6310.450

507078415

105-121 cm

07

ol/g.m.

core

27.3518.670.22

15.5037.6199.88

0.9720.0120.8970.3270.1440.0060.6973.0550.6800.480

483072446

01

plag.

core

0.310.52

26.817.50.24

15.40.110.15

38.699.6

0.0090.9540.0120.9200.111

0.3310.0060.6900.0030.0043.0400.6760.491

480056463

01

plag.

core

31.4316.970.13

16.6335.32

100.95

1.0790.0100.8140.2980.1160.0030.7233.0430.7080.430

537058405

rim

0.350.49

26.817.60.24

15.60.210.15

37.999.4

0.0100.9560.0110.9050.1270.3160.0060.7040.0040.0073.0460.6900.486

481064455

rim

31.4916.520.15

16.5934.96

100.21

1.0870.0110.8100.2960.1090.0040.7253.0420.7100.427

542054404

06

ol/g.m.

core

0.230.53

28.217.80.21

15.30.040.08

38.0100.5

0.0070.9910.0120.8960.1000.3240.0050.6800.0020.0013.0180.6650.475

499051451

05

pl/g.m.

core

29.8617.820.18

15.0835.0598.45

1.0620.0110.8360.3430.107OJ0040.6783.0410.6640.440

530053417

rim

0.330.56

28.519.70.21

14.30.100.15

35.198.9

0.0101.0190.0130.8440.1220.3790.0050.6470.0040.0033.0460.6310.453

513061425

rim

27.5918.090.18

14.0836.0996.59

1.0110.0130.8870.3680.1020.0050.6533.0390.6400.467

506051444

08

pl/g.m.

core

0.430.85

21.822.50.24

12.90.250.08

40.099.1

0.0130.8110.0200.9980.1650.4300.0060.6070.0020.0083.0600.5850.552

411083506

02

g.m.

core

26.8618.710.19

14.9537.3998.68

0.9680.0130.9040.3430.1360.0050.6823.0510.6650.483

482068450

03

pl/ol

pairrim

0.270.60

24.921.10.23

14.00.040.11

37.799.0

0.0080.9100.0140.9240.1610.3870.0060.6450.0030.0023.0600.6250.504

456081464

rim

26.1718.930.14

14.9237.1197.88

0.9540.0140.9070.3410.1490.0040.6883.0570.6690.487

475074451

olivine

rim

41.5

13.70.20

45.90.320.17

101.8

1.010

0.2790.0041.6650.0030.0082.9690.857

132


TABLE 7 - Continued


Piece No.Grain No.

Occurrence

AnalyzedPosition

TiO2

A12O3

FeOMnOMgOCr2O3

TotalOxygen = 4.0AlTiCrFe 2 +

MnMgFe 3 +

TotalMg/(Mg+Fe2+)Cr/(Cr+Al)AlFe 3 +

Cr

16-1,7D01

g m.

core

0.6026.9818.780.25

15.2340.57

102.41

0.9400.0130.9480.3500.0060.6710.1143.0420.6570.502

470057473

121-126 cm

g m.

rim 1

0.6427.2919.240.22

14.7739.60

101.77

0.9570.0140.9310.3670.0060.6550.1123.0420.6410.493

479056466

g.m.

rim 2

0.6527.3219.000.22

14.9439.28

101.40

0.9600.0150.9260.3610.0050.6640.1133.0440.6480.491

480057463

n

02

ol

core

0.6028.2119.030.23

15.0336.4999.59

1.0030.0140.8710.3490.0060.6760.1313.0500.6600.495

500065434

I MAR

03

g.m.

core

0.4932.3818.020.18

15.8434.59

101.49

1.1080.0110.7940.3350.0040.6850.1023.0590.6720.417

553051396

30-40° N

04

pl/g.m.

core

0.5331.5116.930.18

15.6936.45

101.29

1.0810.0120.8390.3360.0050.6810.0763.0300.6700.437

536038426

pl/g.m.

rim

0.6030.3217.990.20

14.9236.86

100.88

1.0530.0130.8590.3620.0050.6560.0823.0300.6440.449

528041431

05

ol/g.m.

core

0.5330.8618.710.22

15.1933.9399.45

1.0850.0120.8000.3450.0060.6760.1223.0460.6620.424

541061399

06

pi

core

0.5032.2416.860.21

16.1334.10

100.04

1.1130.0110.7900.3140.0050.7040.0993.0360.6920.415

556049395

• in matrixD included in plagioclase

+ included in olivine

1.0 0.9 0.8 0.7

Mg/(Mg+Fe")

0.6 0.5

Figure 21. Composition of chromian spinel in phyricbasalts of Samples 16-1, 75-80 cm (#10C); 22-1, 104-106 cm (#13); and 22-2, 125-146 cm (#7D).

133


TABLE 8Bulk Rock Analyses of IPOD Leg 46, Hole 396B Basalt


Piece No.

SiO2

TiO2

A 12°3r e 2 o 3

FeO

MnO

MgO

CaO

Na2O

K2Op 2 θ 5

H2O(+)

H2O(-)

Total

Qz

Or

Ah

An

Wo

EnFsFo

Fa

Mt '

Dm

Apat

Oxidation Index

Mg-value

Mg-valuea

4-1,126-132

12

49.74

1.49

15.38

2.24

7.17

0.17

7.67

11.36

2.61

0.16

0.15

1.03

0.44

99.61

0.96

22.50

30.34

10.90

19.32

9.27

0.11

0.06

3.31

2.88

0.35

0.219

0.656

0.623

6-1,5 2-60

7

49.22

1.49

15.32

3.11

5.40

0.15

7.38

11.90

2.68

0.24

0.15

1.45

1.36

99.85

0.56

1.46

23.37

29.95

12.48

18.94

5.32

4.65

2.92

0.36

0.341

0.709

0.641

7-1,50-54

7

48.91

1.41

15.49

3.81

5.30

0.16

7.86

11.10

2.59

0.23

0.16

1.39

1.47

99.88

1.06

1.40

22.59

30.88

10.36

20.18

4.69

5.69

2.76

0.38

0.393

0.726

0.641

7-1,116-130

11

48.98

1.50

15.47

3.11

6.54

0.17

7.71

11.76

2.41

0.24

0.15

1.25

0.65

99.94

0.22

1.45

20.80

31.30

11.37

19.59

7.42

4.60

2.91

0.35

0.300

0.678

0.621

7-3,6-20

18

48.57

1.45

15.46

2.83

6.24

0.16

8.25

11.64

2.61

0.22

0.15

1.37

1.10

100.05

1.33

22.63

30.56

11.54

16.56

5.66

3.15

1.19

4.20

2.82

0.36

0.290

0.702

0.650

8-1,60-6718A

49.15

1.58

14.64

3.77

6.1 1

0.17

7.90

11.03

2.57

0.28

0.16

1.31

1.31

99.98

1.18

1.70

22.34

28.33

11.20

20.21

5.97

5.61

3.08

0.38

0.357

0.697

0.622

9-1118-124

15B

48.78

1.63

14.69

3.74

5.54

0.19

7.26

11.49

2.65

0.28

0.16

1.36

1.57

99.34

1.36

1.72

23.26

28.38

12.39

18.76

4.92

5.62

3.21

0.38

0.378

0.700

0.618

10-1,48-53

9A

49.46

1.67

15.40

3.12

7.04

0.17

7.46

11.15

2.69

0.19

0.16

0.92

0.59

100.02

0.30

1.14

23.11

29.83

10.55

18.86

8.03.

4.59

3.22

0.38

0.285

0.654

0.600

10-1,48-539Aa

50.41

1.60

15.34

1.09

8.79

0.18

8.20

11.06

2.71

0.13

99.51

0.77

23.04

29.45

10.73

15.65

9.91

3.41

2.38

1.59

3.05

0.100

0.625

0.624

11-1,110-117

12

48.62

1.62

15.18

3.67

6.46

0.18

7.79

11.12

2.71

0.22

0.16

1.30

1.00

100.03

1.33

23.46

29.27

10.91

18.70

6.25

0.81

0.30

5.44

3.15

0.38

0.338

0.683

0.613

12-1,126-133

8

48.06

1.58

15.42

4.44

5.29

0.19

7.46

10.89

2.74

0.31

0.16

1.94

1.33

99.81

0.42

1.90

24.01

29.89

10.44

19.25

3.93

6.67

3.11

0.38

0.430

0.715

0.614

13-2,3745

5A

49.16

1.64

15.33

2.81

6.97

0.17

7.75

11.08

2.73

0.22

0.15

1.26

0.66

99.93

1.33

23.57

29.51

10.68

17.86

7.48

1.28

0.59

4.16

3.18

0.35

0.266

0.665

0.618

13-3,7-15

1

48.75

1.72

15.11

4.49

5.12

0.15

6.90

10.66

2.89

0.30

0.16

2.08

1.86

100.19

2.05

1.84

25.41

28.44

10.62

17.86

3.25

6.76

3.39

0.39

0.441

0.706

0.599

14-3,58-60

5A

48.66

1.69

15.07

3.04

6.48

0.17

7.38

11.61

2.89

0.33

0.16

1.89

0.57

99.94

2.00

25.09

27.87

12.59

14.46

5.44

3.08

1.28

4.52

3.29

0.38

0.297

0.670

0.613

12-1,120-133

2C a

48.86

1.75

15.11

2.75

7.26

0.16

7.88

10.49

2.82

0.18

0.18

1.49

1.18

110.11

1.09

24.49

28.77

9.79

17.87

7.71

1.59

0.76

4.09

3.41

0.43

0.259

0.659

0.616

1 Analyses of separated fresh glass.3Analyses of separated groundmass.

TABLE 9Glass Analyses of IPOD Leg 46, Hole 396B Basalt


Piece No.

SiO2

TiO2

A12O3

FeOMnOMgOCaONa2OK2O

Total

OrAbAnWoEnFsFoFa

nMt

Mg-value

4-1126-132

12

50.61.35

15.69.100.188.45

11.432.700.10

99.5

0.5922.9430.2111.1615.519.073.942.542.571.47

0.648

5-1,85-91

9

51.31.43

15.69.180.178.57

11.402.770.10

100.5

0.5923.2929.5711.1316.18

9.313.532.242.701.47

0.649

5-2,27-33

4

51.21.40

15.59.270.158.48

11.402.760.10

100.3

0.5923.2629.5611.1816.11

9.483.452.242.651.49

0.644

7-2,93-102

9

50.61.40

15.48.990.138.47

11.432.830.10

99.4

0.5924.0629.2711.5714.56

8.254.652.912.671.45

0.651

8-2,22-23

3

50.71.51

15.59.510.168.06

11.292.850.10

99.7

0.5924.1729.2411.2314.609.203.872.692.871.53

0.627

9-1,23-26

5

51.31.51

16.09.450.168.20

11.022.750.11

100.4

0.6523.1430.74

9.8716.7510.29

2.501.692.851.51

0.632

10-1,48-53

9

50.61.54

15.59.590.178.08

11.272.820.12

99.7

0.7123.9129.2311.1914.57

9.253.912.732.931.55

0.625

11-1,55-59

6

51.71.49

15.49.570.168.02

11.322.770.12

100.5

0.7023.3028.9711.2217.4111.13

1.711.212.811.53

0.624

11-2,37-39

4

51.01.51

15.69.610.188.15

11.152.810.12

100.1

0.7123.7229.4510.7515.539.823.312.312.861.54

0.627

13-1,35-40

4

51.01.49

15.59.580.168.14

11.132.830.12

99.9

0.7123.9429.2010.8615.42

9.733.402.362.83]1.54

0.627

13-2,3745

5

50.61.44

15.49.600.148.01

11.142.840.11

99.3

0.6524.1729.2011.0214.779.523.712.642.751.55

0.623

134


TABLE 8 - Continued

15-3,15-232B

48.811.73

15.172.707.220.167.89

10.492.790.150.131.621.12

99.98

0.9124.2829.23

9.7818.117.801.470.704.033.380.310.2520.661

0.618

15-4,112-116

7A

48.941.68

14.822.567.580.147.44

10.582.790.180.171.211.37

99.46

1.1024.3728.2610.3518.349.200.550.303.833.290.410.2330.6360.599

15-5,29-31

3

48.911.71

15.233.536.300.137.70

10.532.780.210.141.501.43

100.100.471.28

24.2129.29

9.8319.746.25

5.273.340.330.3350.6850.617

16-1,72-81IOC

48.531.30

16.833.794.990.207.02

11.832.510.200.121.291.33

99.940.921.21

21.8235.0010.2317.974.37

5.652.540.290.4060.7150.623

17-1,18-262A

48.281.25

16.993.555.040.117.44

11.772.550.220.111.261.50

100.07

1.3422.1735.2110.0517.954.320.770.205.292.440.260.3880.7250.642

18-1,132-142

7F

49.011.24

16.612.506.030.147.67

11.642.500.100.121.211.37

100.14

0.6121.6834.65

9.9218.937.160.460.193.722.410.280.2720.6940.47

20-1,90-1014G

48.531.20

17.003.434.890.137.31

11.782.490.210.121.401.41

99.900.421.28

21.7035.62

9.9318.754.54

5.12

2.350.290.3710.7510.645

20-6,44-53

6

48.591.01

18.022.944.490.137.58

12.262.450.180.100.911.45

100.11

1.0921.2138.50

9.6317.063.971.580.414.361.960.240.3870.7270.678

21-1,69-79

9

48.331.04

17.423.474.400.137.59

12.172.520.200.101.071.60

100.04

1.2121.9036.5910.3417.303.421.480.325.172.030.240.4150.7550.667

22-1,111-122

13

47.411.06

17.263.494.56

7.9711.612.340.120.132.411.65

100.01

0.7421.6437.77

8.9319.683.710.710.155.272.100.310.4080.7570.672

22-1,111-1,22

13°

47.941.33

15.763.785.63

7.3611.722.430.120.132.611.39

100.200.890.74

21.3732.9911.1019.065.22

5.702.630.310.3770.7000.617

22-3,28-37

2A

48.271.65

17.303.204.080.147.81

11.992.350.230.141.581.09

99.830.591.40

20.4737.039.72

20.022.45

4.783.230.330.4140.7730.690

32-1,77-81

11

49.041.59

16.263.446.120.166.83

11.452.910.390.160.860.82

100.03

2.3425.0430.6610.8814.945.331.650.655.073.070.380.3360.6660.595

TABLE 9 - Continued

14-1,

3-6

1

50.21.64

15.4

10.200.177.79

10.603.120.11

99.2

0.6526.5827.8510.4811.868.275.374.133.141.65

0.602

15-1,37-43

6

50.91.66

15.79.970.177.70

10.873.120.11

100.2

0.6526.3228.3110.6312.848.804.403.323.141.60

0.605

16-2,125-132

4

50.81.34

15.29.430.168.01

11.562.800.10

99.3

0.5923.8328.7212.1014.999.623.562.522.561.53

0.627

16-3,130-137

15

51.01.44

15.4

9.560.167.90

11.582.770.12

99.9

0.1723.4329.1711.8115.4310.07

2.972.142.731.54

0.621

17-2,17-25

2

50.71.47

15.39.590.177.87

11.572.650.12

99.4

0.722.529.711.716.210.62.51.82.81.6

0.619

18-2,81-90

4

51.11.37

15.39.440.237.92

11.662.780.10

99.9

0.623.528.912.115.610.22.92.12.61.5

0.624

21-1,80-90

4F

50.61.35

15.39.390.167.89

11.502.790.10

99.1

0.623.829.311.815.09.73.42.42.61.5

0.625

20-2,3-10

1

51.51.32

15.59.230.17

8.0011.69

2.840.09

100.3

0.523.929.112.015.79.92.92.02.51.5

0.632

20-2,136-140

9

51.11.25

15.49.040.198.23

11.942.700.11

100.0

0.722.829.612.415.79.53.42.22.41.5

0.643

21-2,37-44

4

50.61.28

15.58.950.168.31

12.062.650.09

99.6

0.522.530.212.514.98.74.12.72.41.5

0.648

22-1,111-122

13

51.01.23

15.88.530.188.32

12.4

2.710.11

100.2

0.722.930.413.014.17.94.62.92.31.4

0.659

24-119-24

4

51.11.50

16.59.030.148.22

11.313.030.17

100.9

1.025.430.510.512.06.95.83.72.81.4

0.643

32-177-81

11

50.61.52

16.29.130.178.06

11.122.970.15

100.0

0.925.130.510.312.77.65.23.42.91.5

0.636

135


Unit

No. ofAnalyses

SiO2

TiO2

A12O3

F e 2 O 3

FeOMnO

MgOCaONa 2 OK 2OP2°5H2O(+)H2O(-)TotalorabanwoenfsfofamtilapMg/(Mg+Fe)

Note: In the

TABLE 10Average Bulk Rock Composition

A l

5

49.08

1.4715.42

3.026.130.167.77

11.552.580.220.151.301.00

99.851.3

22.430.711.314.5

8.83.82.51.52.90.40.635

A 2

7

49.09

1.6215.14

3.236.600.187.69

11.122.690.230.161.160.92

99.831.4

23.329.311.014.0

9.23.92.81.63.20.40.616

A 3

6

48.82

1.7115.09

3.186.660.157.53

10.732.830.220.161.63

1.2699.97

1.324.728.710.513.4

8.84.23.01.63.40.40.610

calculation of C.I.P.W. norm

B l

4

48.591.25

16.86

3.325.24

0.157.36

11.762.510.180.121.291.40

100.031.1

21.935.210.112.9

7.84.22.81.42.4

0.30.639

B 2

4

48.151.03

17.75

3.214.38

0.137.74

12.012.420.180.121.491.45

100.061.1

21.138.2

9.411.6

5.95.83.31.22.00.30.678

and Mg/(Mg+Fe) ratio,

C

1

49.04

1.5916.26

3.446.12

0.166.83

11.452.910.390.160.860.82

100.032.4

25.130.710.9

9.46.65.64.41.53.10.4

0.595

oxidation

Unit

No. ofAnalyses

SiO 2

TiO

T i O 2

A 1 2°3FeO*

MnO

MgO

CaO

Na2OK 2 OTotalorabanwoenfsfofa

mt

ilMg/(Mg+Fe)

mol. ratio

olcpx

pi

A l

4

50.9

1.40

15.5

9.14

0.16

8.49

11.42

2.770.10

99.9

0.623.429.611.3

15.59.04.02.6

1.5

2.70.648

0.2250.213

0.136

TABLE 11Average Glass Composition

A 2

7

51.0

1.54

15.6

9.56

0.16

8.09

11.19

2.810.11

100.1

0.723.729.610.8

15.69.83.22.2

1.5

2.9

0.626

0.221

0.202

0.149

A 3

2

50.6

1.6515.6

10.09

0.16

7.75

10.74

3.120.11

99.8

0.726.428.310.5

12.38.54.93.81.6

3.1

0.603

0.2300.204

0.109

B l

6

50.8

1.39

15.3

9.48

0.187.92

11.57

2.760.11

99.5

0.723.429.111.9

15.310.0

3.12.31.5

2.7

0.623

0.211

0.225Λ n o

U . l i o

B 2

4

50.9

1.25

15,68.84

0.18

8.29

12.13

2.690.16

100.0

0.622.730.212.5

14.88.74.12.61.42.4

0.650

0.210

0.239n 1 | 7

0.117

C

1

50.9

1.5116.4

9.08

0.16.14

11.22

3.000.16

100.6

0.925.230.610.3

12.37.35.53.61.52.9

0.640

0.229

0.202π i ΛIU.iUl

index is assumed to be 0.1. Note: *total iron as ferrous. In calculating C.I.P.W. norm and Mg/(Mg+Fe) ratio, the oxidation index is assumed to be 0.1.

TABLE 12

Cs Contents of Hole 396B Basalts

Sample

(Interval in cm) Piece No. Cs (ppm)

4-1,10-1,

14-1,

126-12749-56

3-5

129A

1

0.031

0.051

0.051

0.004

0.001

0.008

Note: Data represent average of two measure-

ments by neutron activation method.

The analyzed samples are fresh siderome-

lane glass.

TABLE 13

Miscellaneous Analyses


Phase

SiO2

TiO2

A12O3

FeO*

MnO

MgO

CaO

Na2O

K,0

Total

Si

Al

Ti

Fe

Mn

Mg

Ca

Na

K

Total

20-1, 82-92 (#4F)

01sideromelane

550.9

1.34

15.6

9.22

0.15

8.44

11.44

2.80

0.11

100.0

02variole

50.8

1.42

16.8

9.89

0.15

6.11

11.42

3.12

0.12

99.8

20-1, 128(#4K)

03palagonite

36.4

1.87

17.2

14.2

0.00

1.68

0.80

1.11

2.32

75.5

15-3

04smectitevesiclefill

49.5

0.00

3.33

12.17

0.02

20.4

0.56

0.29

0.68

87.0

Oxygen =

7.420

0.587

0.000

1.524

0.003

4.556

0.090

0.083

0.131

14.394

, 15-23 (#:

05smectiteafterglass

43.2

1.04

5.96

14.65

0.03

15.86

1.95

0.53

1.22

84.4

22.0

6.832

1.112

0.124

1.938

0.005

3.737

0.330

0.163

0.247

14.488

06micaafterolivine

56.1

0.02

14.67

9.72

0.00

0.75

0.38

9.15

0.53

91.3

7.859

2.442

0.003

1.140

0.000

0.156

0.057

2.486

0.094

14.237

136


latitude

8 0 V

60°

40°

20c

0°

20°

(Total 331)

V

(34)

J IV

(80)

(105)

II

(70)

(22)

V

(N)

20

100

IV

III

40

20

0

80

60

40

20

040

20

010

' 0

-i—P

=q ^

-T-p=P48 50 52

SiOo

t L

π-r-π—r~r~t-•

1.0 2.0

TiO2

-i—r

rΦ>m14 16 18wt%

AI 2 O 3

Figure 22. Composition of glass from the floor of the Atlantic Ocean. Open-bar data from Melson etal, in press; stippled-bardata of the FAMOUS Region from Bryan and Moore, 1977; and hatched-bar data represents Hole 396B findings.

1.0 1.1

FeO /MgO

Figure 23. T1O2 versus FeO*/MgO ratio of glass from Hole396B and FAMOUS region.

137


(N)

20

V 10

40

IV 20

0

80

60

40

20

40

II 20

10

tLfL -TL

I

k 1m• «=*-

rf

H i8 10 12

FeO

Figure 22. Continued.

8

MgO

10-i—r i8 10 12

CaO

14 2 3 4

Na2O

0.2 0.4 0.6 0.7

K2O

0.2 wt%

10 -

9 -

8

A

A

vo +

ow

a

D

1

ΦAO

D

T

•

*

A 1A 2

A 3B 1B 2

C

8MgO (wt %)

Figure 24. FeO versws A/̂ O contents of Hole 396Bglass.

138


TABLE 14Fractionation Model of Leg 46 Basalts

SiO2

TiO2

A12O3

FeO

MnO

MgO

CaO

Na2OK2°

Crystal Composition

oliv.a

40.45

0.00

0.00

11.94

0.24

47.07

0.30

0.00

0.00

plag.a

48.17

0.06

32.35

0.79

0.01

0.70

15.76

2.12

» 0.04

(Fo89) (An 80)

albite

68.70

0.00

19.43

0.00

0.00

0.00

0.00

11.81

0.06

cpx.

52.84

0.13

2.95

5.05

0.15

19.45

19.33

0.10

0.00

subtractedcrystal (%)

oliv.

plag.

albite

cpx.

glass (Aj

A l

50.96

1.40

15.52

9.15

0.16

8.40

11.44

2.77

0.10

L A2>

analyt.

50.96

1.54

15.59

9.56

0.16

8.09

11.18

2.81

0.11

Result of Calculation

A,

calc.

50.88

1.52

15.54

9.66

0.17

8.09

11.22

2.82

0.11

0.8

3.0

1.0

3.5

bulk (B 2

B 2

49.64

1.06

18.30

7.49

0.13

7.98

12.38

2.49

0.19

B lanalyt.

50.15

1.29

17.43

8.49

0.15

7.60

12.14

2.59

0.19

B lcalc.

49.76

1.24

17.55

8.32

0.14

7.67

12.09

2.60

0.22

1.8

10.0

0.6

2.9

glass (B 2

B 2

50.93

1.25

15.60

8.84

0.18

8.29

12.13

2.69

0.10

B lanalyt.

51.05

1.40

15.38

9.53

0.18

7.96

11.63

2.77

0.11

B lcalc.

50.82

1.40

15.43

9.58

0.19

7.95

11.73

2.78

0.11

0.5

4.9

1.0

5.0

Analyzed phenocrysts in 22-1-13, Hole 396B.1 Hodges and Papike (1976).

Plagioclase Ca-Pyroxene

Oliviπe Ca-f ree pyroxene

Figure 25. Glass composition of Hole 396B and FAMOUS region and bulk rock composition of East Pacific (Clague andBunch, 1976) in the normative tetrahedron plagioclase-Ca pyroxeneolivine-silica projected from silica apex (left triangle)and plagioclase apex (right triangle), respectively. The methods of calculation and projection are the same as those ofShibata's (1976) except for assuming the oxidation index of 0.1 for the present calculation. Note that compositionaltrends extends toward silica apex in the tetrahedron, which may suggest that these trends are controlled by olivine, Ca-richclinopyroxene, and plagioclase.

139


Mg/(Mg+Fe)

Figure 26. Composition of glass of Hole 396B andFAMOUS region, showing the positive correlation be-tween Mgj(Mg+Fe) ratio and the normative "quartz"content in the tetrahedron (Figure 25).

10

Equilibration linewith mantle olivineFo 90. NiO = 0.4 wt

100 200NiO (ppm)

300 400

Figure 27. MgO versus NiO contents of Hole 396B basalt (Leg 46 Shipboard Party, thisvolume). Dotted line represents the compositional variation of basalt during fractionalcrystallization of olivine (see text).

TABLE 15Estimated Temperature of Leg 46

Basaltic Magmas (°C)

Chemical Unit

1-atm, dry0.5 kilobars,0.3 wt.% H2O

A l

1217

1208

A 2

1208

1199

A 3

1202

1193

B l

1205

1196

B 2

1210

1201

C

1207

1198

140


-–

5 mm

PLATE 1

Ca-rich clinopyroxene-plagioclase crystal clot in Sample 396B-15-5,70-76 cm.

141

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4. PETROLOGY AND CHEMISTRY OF BASALTIC ROCKS FROM … · petrology and chemistry of basaltic rocks from hole 396b, ... chemical (7 units), and ... petrology and chemistry of basaltic