34. PETROGRAPHY, MINERALOGY, AND GEOCHEMISTRY OF BASALTS FROM HOLE 534A,LEG 761

John Logothetis, Department of Geology, Dalhousie University, Halifax, N. S., Canada

ABSTRACT

Leg 76 sampled 31.5 m of basaltic basement at Deep Sea Drilling Project Hole 534A in the Blake-Bahama Basin.The basalts represent a short section of mineralogically uniform, sparsely plagioclase-phyric pillow flows, composedmainly of plagioclase, augitic clinopyroxene, iron-titanium oxides with variable amounts of alteration products (smec-tite ± carbonate ± quartz). Their major element chemistry is typical of mid-ocean ridge tholeiites and has normativecompositions of olivine tholeiites. Mg/(Mg + Fe2"1") ratios range from 0.58 to 0.60, which suggests that these basaltsare evolved compared to primitive mantle melts.

INTRODUCTION

Hole 534A of the Deep Sea Drilling Project pene-trated 31.5 m into the basaltic basement at 28°20.6/N,75°22.9'W in the Blake-Bahama Basin. The basal sedi-mentary section overlying the basement is dated as mid-dle Callovian (154 m.y.) (Site 534 report).

The Leg 76 shipboard party undertook a visual andgeneral petrographic description of the basaltic rocks.On the basis of texture, occurrence of glassy margins,and breccia zones, 29 cooling units of a dominantlydark gray to greenish gray aphyric basalt were iden-tified. This study describes the petrography of the ba-salts and gives the results of a reconnaissance micro-probe study of plagioclase and pyroxene groundmassand microphenocryst phases. Major oxide whole-rockchemistry of selected sections from the core is presentedand is related to the mineralogy and crystallization his-tory of the basalts.

ANALYTICAL TECHNIQUESPlagioclases and pyroxenes were analyzed by electron microprobe

using the energy-dispersive system (Cambridge Microscan). Naturalmineral standards were used for both phases. All microprobe datawere corrected according to the program EMPADR VII of Rucklidgeand Gasparrini (1969).

Whole-rock determination of four samples was done by energy-dispersive microprobe analysis as described by MacKay (1981) onwhole-rock powders fused for 15 to 20 s on molybdenum strips. TheDPL standard (Dalhousie Mid-Ocean Pillow Lava Standard 56-2) wasused as a control. Each analysis represents an average of five spotanalyses, having a precision that is better than 2.0% of the amountpresent for all major oxides except Na2O. Ferrous iron was deter-mined by visual titration, according to the modified cold acid decom-position method of Wilson (Maxwell, 1968), and ferric iron was deter-mined by difference. P2O5 was analyzed spectrophotometrically afterformation of a phosphomolybdate complex and its reduction to mo-lybdenum blue. HQJJO ~ and H2O

+ were determined by oven dryingat 110°C and by modified Penfield method, respectively (Maxwell,1968). Carbon dioxide was determined by a nonaqueous automatic ti-tration by sodium methylate solution, following absorption in a solu-tion of 2% tri-ethanolamine in acetone and methanol (Sen Gupta,

Sheridan, R. E., Gradstein, F. M., et al., Init. Repts. DSDP, 76: Washington (U.S.Govt. Printing Office).

1970). Analyzed whole-rock samples were selected from sections thatshowed the least alteration in thin section. Glass was not identified inthin section and chilled margins were completely altered to clay.

PETROGRAPHY AND MINERALOGYThe basalts of Hole 534A are aphyric to sparsely phy-

ric with up to 5% microphenocrysts of plagioclase. Thegroundmass is comprised of calcic plagioclase, augiticclinopyroxene, titanomagnetite, and sulfides. Olivinewas not identified in any of the thin sections examined,although shipboard studies reported the presence of sev-eral completely clay pseudomorphs after olivine. Allspecimens examined exhibit some degree of alteration.Smectite, carbonate, and minor quartz replace micro-phenocrysts and interstitial groundmass material andfill veins and vesicles. Hematite and pyrite also occur insome veins and vesicles.

TEXTUREWith increasing distance from the outer margins of

pillow flows, the basalts exhibit a textural sequence con-sisting of an altered chilled margin, a variolitic-subvari-olitic (sheaf-textured) zone, and finally a granular inter-sertal zone over several centimeters into the pillow inte-riors. Plate 1 illustrates the main textural types observedin the basalts of Hole 534A.

In sections with variolitic texture the variolites arepale brown, up to 0.5 mm in diameter, often bow-tie orellipsoidal-shaped, and are composed of extremely fineintergrowths of crystalline plagioclase and pyroxene de-scribed by Bryan (1972) as typical of quenched basaltssuch as those from the Mid-Atlantic Ridge. Fine plagio-clase crystals and microlites occur as randomly orientedcrystals crosscutting variolites and as oriented acicularcrystals parallel to internal variolite crystal growth. Swal-low-tail plagioclase commonly merges with varioliticfiber overgrowths and appears to serve as points of nu-cleation (Plate 1, Fig. 1). Titanomagnetite grains aredistributed along variolite boundaries and along thelong direction of crystal fiber growth. With increasingcrystallinity, variolitic texture is succeeded by coarser-grained, sheaf-textured, fan-shaped intergrowths of py-

713

J. LOGOTHETIS

roxene and plagioclase (Plate 1, Fig. 2). This texture, inturn, is transitional to microcrystalline, intersertal tex-tured sections with characteristic subophitic intergrowthsof plagioclase laths and pyroxene in a finer-grainedgroundmass of plagioclase, pyroxene, iron-titanium ox-ides, as well as variable amounts of altered palagoniticmesostasis.

PLAGIOCLASEPlagioclase occurs as needlelike, acicular microlites

and laths (0.1-0.5 mm) with simple albite twinning. Inspecimens with variolitic subvariolitic textures, swallow-tail and skeletal plagioclase are common, whereas incoarser-grained sections plagioclase laths are intergrownand overgrown by clinopyroxene in subophitic fashion(Plate 1, Fig. 4). Microphenocrysts of plagioclase (Plate1, Fig. 5) compose up to 5% of the examined sectionsand occur as equant to lath-shaped crystals in interlock-ing and reticulate clusters up to 2 to 3 mm in size, com-monly with thin sodic rims.

Representative plagioclase analyses are reported inTable 1. On the basis of the anorthite component it ispossible to distinguish groundmass plagioclase with uni-form An70 to An75 content from the more calcic cores(An80 to Anço) of microphenocrysts. The micropheno-cryst rims are An75, approximating the composition ofgroundmass plagioclase. In addition, the thin sodic rimsshow a slight uptake of potassium with K2O = 0.8%.The composition of phenocryst cores and groundmasspairs (Table 1) shows an increase of FeO and MgO withdecreasing An content.

CLINOPYROXENEClinopyroxene of augitic composition (2V° = 40-45°)

is much less abundant than plagioclase and appears tobe absent in some sections. Clinopyroxene occurs asanhedral, granular-textured, groundmass grains and assubophitic intergrowths, with plagioclase in the coarser-grained sections. It is commonly present as anhedral toeuhedral crystals up to 0.3 mm associated with plagio-clase glomerocrysts.

Clinopyroxenes from Hole 534A lie within the com-positional fields of Fe-rich endiopside and Fe-poor au-

Table 1. Representative groundmass and microphenocrystic plagioclasecomposition.

Component

SiO2A12O3MgOFeOCaONa2OK 2 O

Total

OrAbAn

534A-128-1(Piece

g

50.9030.21

0.601.16

14.542.970.00

100.38

2773

10)

mΦ

47.4333.36

0.150.62

17.221.490.00

100.27

1486

Sample

534A-128-3(Piece 1C)

g

51.7528.60

0.681.29

13.523.210.13

99.18

13069

mΦ

47.0033.02

0.120.44

17.251.250.00

99.08

1288

534A-129-5(Piece 7)

g

51.5229.91

0.300.87

14.313.170.00

100.08

2971

mΦ

48.3032.75

0.080.69

16.711.760.00

100.29

1684

534A-130-2(Piece 6)

g

51.6330.19

0.240.76

14.133.020.00

99.97

2872

mΦ

46.9833.41

0.180.44

17.481.130.00

99.62

1090

gite as defined by Poldervaart and Hess (1951) (Fig. 1and Table 2). These analyses represent only groundmassand subophitic pyroxenes, as no phenocrysts were ob-served. The most iron-rich specimen analyzed in theserocks is Wo36En25Fs39 (Analysis 4, Table 2) of ferro-augite composition from a minute subhedral ground-mass grain. This grain is noticeably enriched in iron andaluminum, although an iron enrichment trend (as is of-ten observed in the groundmass pyroxenes of ocean-floor basalts) is not well defined in Figure 1. Its dis-tinctly different chemistry may be accounted for in termsof crystal-melt partitioning of elements. During crystalgrowth of pyroxenes of endiopside to augitic composi-tion the immediately adjacent liquid becomes enrichedin Al, Ti, and Fe and diluted in Ca and Mg (Huebner,1980). Subsequent crystallization of a pyroxene fromsuch a liquid will have the observed aluminous ferroau-gite composition of Analysis 4, Table 2, and be relative-ly depleted in Ca and Mg. The interstitial setting of thisminute ferroaugite grain relative to immediately adja-cent subophitic clinopyroxenes of endiopside to augitecomposition supports this interpretation.

OPAQUE OXIDES

Iron-titanium oxides occur only as groundmass phas-es, as euhedral crystals of skeletal and cruciform habitdistributed along the outer boundary or outlines of vari-olites, and as inclusions in brown, altered mesostasis. Inreflected light, individual crystals are commonly corrod-ed with an outer brownish alteration rim.

VESICLES

Vesicles of up to 1 mm in size comprise 1 to 5% of allexamined sections and are filled with smectite, ± cal-cium carbonate, ± quartz. Vesicle and vein-filling claymineralogy consistently grade from a bright green outerrim to a yellowish brown inner core. Carbonate is a latervesicle phase and partly replaces earlier-formed smec-tites. Segregation vesicles are relatively common in thecoarser-grained intersertal textured segments with well-defined lunes of finely crystalline residual melt (Plate 1,Fig. 6).

En Fs

Note: g = groundmass plagioclase; m<£ = microphenocrystic plagioclase.

Key• 128-1 (Piece 10) ° 130-2 (Piece 6)+ 128-3 (Piece 1C) Y 130-4 (Piece 6)X 128-5 (Piece 7A) v 130-6 (Piece 3B)0 129-4 (Piece 2B)

Figure 1. Pyroxene quadrilateral showing representative groundmassclinopyroxene compositions from basalts of Hole 534A by core-section (piece).

714

PETROGRAPHY, MINERALOGY, AND GEOCHEMISTRY OF BASALTS

Table 2. Representative clinopyroxene compositions analyzed by microprobe.

Analysis No.

Sample

SiO2

TiO2

A12O3

Cr2O3

FeOMnOMgOCaONa2OK2O

Total

SiAliv

Alvi

TiCrFeMnMgCaNaK

WoEnFs

1

534A-128-1(Piece 10)

53.240.284.200.366.610.21

17.8316.630.370.04

99.77

1.9330.0670.1130.0080.0100.2010.0060.9650.6470.0260.002

35.753.211.1

2

534A-128-3

52.440.223.000.675.810.17

18.5118.230.000.00

99.05

1.9250.0750.0550.0060.0190.1780.0051.0130.7170.0000.000

37.653.1

9.3

3

(Piece 1C)

52.950.182.910.586.360.16

20.1716.180.000.00

99.49

1.9270.0730.0520.0050.0170.1940.0051.0940.6310.000.000

32.957.010.1

4

534A-128-5

48.771.25

11.200.00

17.290.236.23

12.611.690.4

99.31

1.8480.1520.3480.0360.0000.5480.0070.3520.5120.1240.002

36.324.938.8

5

(Piece 7A)

53.110.422.340.00

10.130.21

17.1417.380.000.00

100.73

1.9450.0550.0460.0120.0000.3100.0070.9360.6820.0000.000

35.448.516.1

6

534A-129-4

53.560.172.500.188.460.24

18.5117.010.000.00

100.63

1.9460.0540.0530.0050.0050.2570.0071.0030.6620.000.000

34.552.213.3

7

(Piece 2B)

52.480.363.290.188.520.23

18.1017.300.000.00

100.46

1.9160.0840.0570.0100.0050.2600.0070.9850.6770.000.000

35.251.313.5

8

534A-130-6

51.400.454.280.129.530.27

17.0917.420.000.00

100.56

1.8860.1140.0710.0120.0030.2920.0080.9350.6850.0000.000

35.848.915.3

9

(Piece 3B)

53.410.262.550.477.810.17

19.3616.480.000.00

100.51

1.9370.0630.0460.0070.0130.2370.0051.0460.6400.0000.000

33.354.412.3

ALTERATION

Smectites, carbonate, and quartz are the main altera-tion phases replacing the fine groundmass and micro-phenocryst phases, and extensively altering the origi-nally glassy mesostasis. Groundmass plagioclase andclinopyroxene crystals are fractured, resulting in frac-ture-filling and replacement by clays. In contrast, pla-gioclase microphenocrysts are usually quite fresh andunfractured.

BASALT CHEMISTRY

Four whole-rock analyses of basalts from Hole 534Aare presented in Table 3. The samples are composition-ally similar, and significant chemical differences werenot observed with depth. The basalts are typical mid-ocean ridge tholeiites with low TiO2 Cess than 1%); lowtotal alkalies (less than 3.0%); low K2O/Na2O ratios;and low P2O5 (less than 0.1%). However, extensive al-teration is indicated by high total water (4.5-6.5%) andCO2 (up to 1.1%), which significantly reduce the othermajor oxides by dilution. Consequently, H2O"~ is notincluded in the totals and is reported separately. HighFe2O3/FeO ratios of 0.6 or greater indicate extensive ox-idation of the rocks during alteration. The effects of lowtemperature alteration may also include leaching ofCa+2, Mg+2, and addition of SiO2, Fe, Al, and K.

CIPW calculations (H2O, CO2-free) suggest that thebasalts are saturated tholeiites with normative diopside,hypersthene, and quartz of up to 1.2% in analyses ofSamples 534A-128-5, 73-81 cm and 534A-130-2, 90-96cm. Because CIPW calculations are sensitive to changesin Fe2O3/FeO ratio, the analyses have been recalculatedwith the reduction of Fe2O3 to give values of 1.50%,which is in the range of fresh tholeiitic rocks (Shido andMiyashiro, 1971). Following the application of this cor-

Table 3. Chemical composition and CIPW norm of basalt from Hole534A.

Analysis No.

Sample

SiO2

TiO2A12O3

Fe2O3

FeOMnOMgOCaONa2OK2OP2O5V2O5Cr2O5

NiO

co2H2O

1

534A-128-1121-131 cm

47.510.93

15.134.397.310.198.909.702.100.310.080.060.080.00.332.53

2

534A-128-521-26 cm

47.730.87

14.634.187.060.228.17

12.162.020.130.090.00.010.021.061.66

3

534A-128-573-81 cm

48.390.96

14.605.576.400.148.78

11.532.050.230.090.00.00.00.541.51

4

534A-130-290-96 cm

48.990.90

14.874.487.480.128.36

10.812.020.520.080.00.00.00.241.42

Total

H 2 O"

99.55

4.09

100.10

2.84

100.79

4.01

100.29

3.25

QOrAbAnDiHyOlCrMt11Ap

Total

01.90

18.3932.0213.8625.080.030.126.591.830.19

100.00

00.79

17.5731.3224.2417.790.140.026.231.700.21

100.00

1.161.38

17.5730.3421.5317.79008.181.850.21

100.00

0.863.12

17.3330.3918.9020.89006.591.730.19

100.00

Note: — indicates no data available.

715

J. LOGOTHETIS

rection, CIPW calculations yield normative olivine inthe range of 7.7 to 10.5%, which is indicative of theoriginal undersaturated tholeiitic composition of the ba-salts and plots in the olivine tholeiite field of the basalttetrahedron (Yoder and Tilley, 1962). Figure 2 illus-trates the normative olivine, plagioclase, pyroxene rela-tionships in relation to the inferred olivine-plagioclasecotectic drawn by Shido and Miyashiro (1971). After re-duction of their ferric iron to 1.50% and recalculationof normative minerals, the basalts plot in a tight field tothe right of the cotectic within the olivine-tholeiite field.The Mg/(Mg + Fe2+) ratios can be applied to indicatewhether the basalts represent evolved or primitive liq-uids that have ratios of approximately 0.70 in equilib-rium with their peridotitic source material (Ayuso et al.,1976). Because these basalts have Mg/(Mg + Fe2+) ra-tios between 0.58 and 0.60, after reduction of Fe2O3 to1.50% as discussed earlier, they do not represent primi-tive liquids but have undergone fractionation of ferro-magnesium phases such as olivine and/or pyroxene re-sulting in higher Mg/(Mg + Fe2+) ratios.

SUMMARY

The basalts of Hole 534A represent a short sectionof pillow lava flows of one textural and chemical type.The textural range exhibited is characteristic of basaltserupted on the seafloor at present median valley envi-

70Key

Uncorrected analyses

Fe2<D3 corrected as discussed in text

60Normative plagioclase

50

Figure 2. Normative olivine-plagioclase-pyroxene relationships in ba-salts from Hole 534A. (The empirically determined olivine-plagio-clase cotectic is from Shido and Miyashiro [1971]. Samples 1-4 arefrom Table 3.)

ronments. In agreement with the shipboard description,the basalts are sparsely phyric with minor amounts ofplagioclase microphenocrysts in a groundmass of calcicplagioclase, clinopyroxene, iron-titanium oxides, andvariable amounts of extensively altered palagonitic meso-stasis. Variable degrees of alteration, mainly by greenand brown smectite, carbonate, and trace amounts ofquartz have affected these basalts with an observed or-der of preference for palagonitic mesostasis, fine-grainedpyroxene and plagioclase intergrowths, clinopyroxene,and plagioclase.

Chemically the basalts of Hole 5 34A are typical mid-ocean ridge tholeiites with normative olivine and hyper-sthene, and plot in the olivine tholeiite field of the basalttetrahedron of Yoder and Tilley (1962). The Mg/(Mg +Fe2+) ratios range from 0.60 to 0.58, which indicate thatthese basalts are evolved compared to primitive mantlemelts. The presence of only plagioclase micropheno-crysts is indicative of plagioclase tholeiites as defined byShido and Miyashiro (1971) on the basis of being thefirst phase to precipitate. However, after partial reduc-tion of Fe2O3, all analyses plot within the olivine tholei-ite field of the normative olivine, plagioclase, pyroxenetriangle (Figure 2), which more accurately characterizesthe composition of the basalts.

ACKNOWLEDGMENTS

The author is grateful for the helpful discussions with Dr. J. M.Hall, and for his financial support of this research. I wish to expressmy thanks to Drs. J. M. Hall and P. T. Robinson for their helpfulcriticism in reviewing this paper.

REFERENCES

Ayuso, R. A., Bence, A. E., and Taylor, S. R., 1976. Upper Jurassictholeiitic basalts from DSDP Leg 11. / . Geophys. Res., 81:4305-4325.

Bryan, W. B., 1972. Morphology of quench crystals in submarine ba-salts. J. Geophys. Res., 77:5812-5819.

Engel, A. E. J., Engel, C. G., and Havens, R. G., 1965. Chemical char-acteristics of oceanic basalts and the upper mantle. Geol. Soc. Am.Bull., 76:719-734.

Huebner, J. S., 1980. Pyroxene phase equilibria at low pressure. InPrewitt, C. T. (Ed.), Pyroxenes: Reviews in Mineralogy, 7:213-288.

MacKay, R. M., 1981. Whole rock analysis using the electron micro-probe (B. Sc. thesis). Dalhousie University, Halifax, Nova Scotia,Canada.

Maxwell, J. A., 1968. Rock and Mineral Analysis: New York (Inter-science).

Poldervaart, A., and Hess, H. H., 1951. Pyroxenes in the crystalliza-tion of basaltic magma. /. Geol., 59:472-489.

Rucklidge, J. C , and Gasparrini, E., 1969. EMPADR VII: Toronto(Department of Geology, University of Toronto).

Sen Gupta, J. G., 1970. Determination of carbon by non-aqueous ti-tration after combustion in a high-frequency induction furnace.Anal. Chim. Acta, 51:437-447.

Shido, F., and Miyashiro, A., 1971. Crystallization of abyssal tholei-ites. Contrib. Mineral. Petrol., 31:251-266.

Yoder, H. S., Jr., and Tilley, C. E., 1962. Origin of basalt magmas:an experimental study of natural and synthetic rock systems. / .Petrol., 3(pt. 3):342-532.

Date of Initial Acceptance: April 15, 1982

716

PETROGRAPHY, MINERALOGY, AND GEOCHEMISTRY OF BASALTS

:-• • ' .. j ' • ; :

i•

BIB* •

Plate 1. 1. Chilled margin of pillow flow, showing common coalesced variolites, and plagioclase microlites, a number of which occupy centralpositions within individual varioles. Sample 534A-128-4, 39-45 cm. Length of field—0.9 mm. 2. Coarser textured groundmass of sheaf- andplumose-textured pyroxene intergrown with fine acicular plagioclase, and interstitial iron-titanium oxide grains. Sample 534A-129-4, 37-43 cm.Length of field—3.4 mm. 3. Detailed view of Fig. 2; field of view—0.9 mm. 4. Subophitic intergrowths of plagioclase laths and augitic clino-pyroxene, with interstitial dark brown and layered palagonite containing inclusions of skeletal iron-titanium oxides. Sample 534A-128-5, 73-81cm. Length of field—0.9 mm. 5. Glomerocrystic cluster of plagioclase microphenocrysts, set in a vesicular variolitic groundmass. Sample534A-128-5, 21-26 cm. Length of field—3.4 mm. 6. Segregation vesicles with well developed lune, of residual ironrich melt. Sample 534A-130-5, 14-21 cm. Length of field—3.4 mm.

717