23. PETROLOGY OF BASALTIC ROCKS COLLECTED FROM LEG 14

Thomas L. Wright, U. S. Geological SurveyW. E. Benson, National Science Foundation

W. G. Melson U. S. National MuseumS. R. Hart, Carnegie Institute of Washington

Basalt was recovered from four sites (136, 137, 138 and141) drilled on Leg 14 of the Deep Sea Drilling Project.Recovered samples were originally glassy to holocrystallinebasalt, now altered to varying degrees. Four of the freshersamples have been chemically analyzed. The analyses aregiven in Tables 1 and 2, and modes in Table 3. Table 4summarizes the X-ray study of alteration products. Ageneral petrographic description of each analyzed sample,based on hand specimen, optical and X-ray examination, isgiven below.

The results show a greater diversity of basalt types thanmight have been expected, ranging from low-K2O tholeiite(Site 136) through alkalic basalt (Site 138) to highly alteredPlagioclase porphyritic basalt of uncertain affinity (Site137). Basalt from Site 141 is so altered that we could notdetermine its magmatic parentage.

Site 136

Core 9 from Site 136 consists entirely of basalt andincludes one piece of solid core some 15 centimeters longand several other large core fragments that can be fittedtogether. A large number of smaller fragments range from afew millimeters to several centimeters across.

Most of the basalt is barely phaneritic as seen with ahand lens; grains of Plagioclase and pyroxene can bedistinguished. Secondary veinlets of calcite cut the core atvarying angles to the long axis. Calcite also fills some of thesparse vesicles. Pyrite is associated with calcite in bothveinlets and the vesicles and seems to be slightly later inorigin. Pyrite is present also as disseminated grains,especially near the veinlets. Chalcopyrite is present in lesseramounts in a few of the veinlets, associated with the pyriteand calcite. Other vesicles are filled with two secondaryminerals—an outer rim of dark greenish gray material and acore of light green material. None of these were seen on thethin sections and they remain unidentified. They areprobably smectite judging from an X-ray study of thewhole rock powder. The filled vesicles suggest that the rockis a flow rather than a sill.

A few fragments contain micropegmatite areas. Plagio-clase laths are as much as 2 millimeters long, and smallveinlet-like areas are coated with specular hematite.Commonly, a thin patchy film of pyrite coats the hematite.

Three thin-sections were cut from the uniform "fresh-looking basalt." Two pieces of the same basalt weresubmitted for chemical analyses (9A and 9B, Table 1). The

TABLE 1Chemical Analyses of Basalt, DSDP Leg 14, and Average Composition of Mid-Ocean Ridge Abyssal Basalts

(Melson et al., 1968); (Column A)

SiO2

A1 2O 3

F e 2 O 3

FeO

(Total iron)as " F e 2 θ 3 "

MgOCaONa 2 OK 2 OH 2 O +

H 2 O -(Ignition Loss)TiO 2

P 2 O S

MnOCO 2

Total

Analyst

14-136-9A

47.3013.70

(13.30)6.40

10.902.590.14

3.751.680.150.19

100.10

HJR-RPC

6.016.55

2.560.16

EJ

0.161.24

0.95

SH

14-136-9B

46.6014.10

(13.50)7.149.682.750.10

4.501.830.170.14

100.51

HJR-RPC

6.446.36

2.710.10

EJ

0.101.38

0.71

SH

14-137-17

48.3018.00

(6.65)

6.782.982.152.43

9.251.410.080.22

98.25

HJR-RPC

3.872.50

2.402.63

EJ

2.624.39

0.20

SH

14-138-7

44.0016.80

(11.80)6.592.673.862.90

7.501.910.510.16

99.70

HJR-RPC

4.246.81

3.903.00

EJ

3.305.18

0.34

SH

A

49.2115.812.217.19

-

8.5311.142.710.26————

1.390.15-

98.74

aAnalyses as follows: HJR-RPC X-ray fluorescence analyses by H. J. Rose, Jr. and Ralph P. Christian, U.S. Geological Survey; EJ. EugeneJarosewich, Smithsonian Institution-alkalis determined by flame photometry, FeO by wet chemical methods, and Fe 2 θ3 by difference.SH. Isotope dilution analyses by Stanley Hart, Carnegie Institution Department of Terrestrial Magnetism (see Table 2).

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T. L. WRIGHT, W. E. BENSON, W. G. MELSON, S. R. HART

TABLE 2Concentrations of Selected Elements in Leg 14 Igneous Rocksa

K,%Rb, ppmCs, ppmSr, ppmBa, ppmH2O+,%

c o 2 , %

136-9A

0.1341.4200.008

189.00031.800

1.2400.950

136-9B

0.08540.86000.0080

198.000027.3000

1.38000.7100

137-17

2.18011.5000.095

96.00098.800

4.3900.200

138-7

2.7463.80

0.58367.00

1242.005.180.34

Concentration determined by isotope dilution and gas chromato-graphy, generally following the techniques of Hart (1971).

rock in thin section has an intergranular to subophitictexture and consists of Plagioclase, pyroxene, and minoropaque oxides, set in a microcrystalline groundmass. Mostof the Plagioclase is in long twinned laths 0.1 to 0.75

millimeters long. A few blocky crystals show intensezoning. Average composition determined by the maximumextinction angle on (010) is An 45 (andesine) withextremes of An 60 at center and An 30 at the edge.Oscillatory zoning is common in specimen 9B.

The dominant pyroxene is augite, with most magnesiancomposition around FS1 4 Wo38 En49 and zoned towardferro-augite and ferrohedenbergite (Figure 2). Pigeonite was

Figure 1. Basalt from Site 136 (Sample 136-9) partly albitized Plagioclase in nearlyopaque altered groundmass with relict pyroxene granules, oxides and other min-erals. Upper photo plane light, lower photo crossed nicols (courtesy of Ulrichvon Rad).

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OlivineClinopyroxene3

PlagioclaseOpaqueBiotiteChloriteEpidoteApatiteCalciteZeoliteAltered glasse

—15.132.04.3-

-—1.10.1

47.5

0.110.424.2

4.8—_——1.2_

60.3

TABLE 3Modal Analyses (Transmitted Light) of Basalt from DSDP Leg 14

14-136-9A 14-136-9B 14-137-17 14-138-7

1.2C

18.24.6

12.27.41.40.40.2

18.4d

36.0

a14-136-9A and 9B contain a pyroxene with low birefrigence (pig-eonite ?) in addition to clinopyroxene of moderate birefrigence(augite). The ratio of pigeonite (?) to augite is about 1:4 in 9Aand 1:9 in 9B.

"Mode not counted. The rock consists of altered Plagioclase pheno-crysts in devitrified glass.

cChlorite pseudomorphs after olivine and pyroxene."Colorless sheaves-probably altered Plagioclase laths. Identified as

natrolite on the whole rock X-ray pattern.eMainly smectite—see Table 4.

seen in thin section but was not revealed during theelectron probe survey. The pigeonite appears clearer andless altered than the augite. The groundmass (originallyglass?) is now a micro-crystalline complex of fibrous"serpentine", chlorophaeite (?) and chlorite. Some of thechlorophaeite (?) areas have outlines that suggest replace-ment of original microlites of pyroxene or olivine. Some of

Wo

PETROLOGY OF BASALTIC ROCKS

TABLE 4Results of an X-ray Diffraction Examination in the

20 Range 4°-12°, of Five DSDP Rock Powders,Using CuK Radiation and Oriented Slidesa

Sample Number

14-136-9A14-136-9B14-136-9A14-138-7

14-137-17 (bottom)

dÅ°

BeforeGlycolation

13.59812.45013.59814.02910.04812.627

AfterGlycolation

16.67316.67316.67316.36510.04816.518

Mineral

SmectiteSmectiteSmectiteSmectiteBiotite?Smectite

aExamined by Harold Banks, Smithsonian Institution.

the "serpentine" is fibrous and extends from the ground-mass into grains of augite and feldspar. Other alterationproducts include minor saussurite and sericite along•cleavage cracks of feldspar, late albite in some of theserpentinized areas, and quartz of deuteric origin. Someserpentine areas contain patches of small fibrous crystals ofmoderate birefringence that are probably secondaryhornblende.

The major and minor element chemistry of two samplesfrom Hole 136 (9A and 9B, Tables 1 and 2) is typical ofspreading ridge tholeiites (for example, see Hart, 1971).The low alkali concentrations are preserved even though theH2O+ contents are high. As the alkali elements, particularlycesium, are commonly strongly enriched during submarineweathering of basalt (Hart, 1971), it is likely that the high

• Centers

• MarginsO Interstitial

EN FS

Figure 2. Pyroxene compositions in basalt from Site 136 (Sample 14-136-9) projected in the pyroxene trenary enstatite(EN), - ferrosilite (Fs) - wollastonite (Wo). Solid line is fractionation trend of calcic pyroxenes in the Skaergaardintrusion. The pyroxenes fall close to this trend and more toward ferrozugite and hedenbergite. Some interstitial, latestage pyroxenes show a reversal to more magnesian compositions, probably reflecting extraction of Fe° from the meltaccompanying late-stage oxidation and crystallization of magnetite.

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T. L. WRIGHT, W. E. BENSON, W. G. MELSON, S. R. HART

H 2 0 + and carbon-dioxide contents are related more todeuteric alteration or low-grade metamorphism than tosubmarine weathering.

Site 137

Core 17 at Site 137 recovered 1.4 meters of a dark-graybrecciated igneous rock. Filling the spaces between frag-ments and cementing them together are veins of a dark-green serpentine-like material that contains patches ofbright red material. Thinner veins of white calcite and of asoft creamy white material also cut the rock.

In thin section the rock is seen to be highly altered; itappears to have originally been porphyritic with pheno-crysts of Plagioclase and minor augite in a glassy (?) base.An X-ray pattern of the whole rock shows reflections forPlagioclase (with a composition near albite), natrolite,stilbite (?), and a 14A phase (smectite—see Table 4).

The major element chemistry (Table 1) also indicates ahigh degree of alteration. Neither the major element nortrace element chemistry (Table 2) suggest what the originalbasalt might have been.

Site 138

Two beds of alkalic basalt, about 8 meters apart, werecored at Site 138. The upper bed (Core 6, Section 2) is 50centimeters thick. The hole bottomed in the lower bed andits thickness is unknown. A total of 120 centimeters wasrecovered—95 centimeters from Core 7 and 25 centimetersfrom the drill bit.

Upper Bed

The upper bed is a dark aphanitic greenstone with littlemegascopic texture. In thin section it appears as a highlyaltered basalt with intergranular texture. Some laths oflabradorite (An 50-55) are moderately fresh, but most ofthe feldspar has been altered to saussurite (epidote andclinozoisite), calcite, albite and natrolite. The pyroxeneshave been completely replaced by chlorite, biotite andhornblende. The groundmass, some 15 per cent of the rock,may have been glass; it is now a fine-grained intergrowth ofchlorite and zeolite. Although no baked sediments wererecovered either above or below the basalt, the moderategrains size (feldspars up to 0.5 millimeter), the smallamount of glassy groundmass, and the 50-centimeterthickness combine combine to suggest that the bed may bea sill.

Lower Bed

The lower bed is petrographically similar to the upper,except that the grain size is slightly coarser, especially inthe lower part of the cored section, and alteration is lesscomplete. In thin section (Figure 3) the essential mineralsare: altered Plagioclase (labraderite), augite (largely pseudo-morphed by chlorite) biotite and minor olivine. Accessoryeuhedral apatite is common. Secondary (alteration)minerals are: radiating sheaves of natrolite (positivelyidentified by X-ray), epidote, calcite, chlorite, stilbite (?),and a potash feldspar. The groundmass is a mat of zeolite,smectite and chlorite.

The major element chemistry (Table 1) shows a definiteaffinity to that of alkalic basalts found on seamounts,although the alteration may have affected the originalchemistry to some degree (for example, low CaO). Thetrace element content (particularly the high barium) is alsoconsistent with that of an alkalic basalt. Neither thepetrography nor the chemistry indicates whether the lowerbed is a sill or a flow.

Site 141

The rock that comprises the hard core of the "diapir" atSite 141 is a highly altered and fractured basic igneousrock, nearly black in color and largely aphanitic. Chloriteand serpentine group minerals are present but were notidentified by X-ray diffraction, The rock is cut by darkgreen to black serpentinous veins, and, below 90 centi-meters by while calcite veins. Local discrete areas of a darkgreen serpentinous mineral are present, especially in thelower part of the core. Spherulites are locally abundant,especially in the altered areas.

Alteration of the rock is virtually complete in all but afew slightly coarser-grained areas. In these, the rock consistsof ragged laths of Plagioclase (labradorite), 0.1 to 0.6millimeter long, partly to completely saussuritized andalbitized, between which are tabular areas of "serpentine"(probably chlorite plus some serpentine) that occupy theposition of former pyroxene (and olivene?) grains. Sur-rounding both the feldspar and "serpentine" is a micro-crystalline groundmass of chlorite or "serpentine",commonly dotted with "dust" of iron oxides, some ofwhich is recognizable as hematite. In addition, there aresmall patches of an antigorite-like mineral with cross-cutting veinlets of "serpentine" (commonly fibrous"chrysotile" sandwiching flaky "antigorite") and a fewpatches of chlorite. Despite the intense alteration theoriginal sub-ophitic to intersertal texture has been preservedin many areas.

In the finer-grained areas alteration of the fledspars isnearly complete, and there is less chlorite and "serpentine".The altered rock consists largely of a fibrous mixture ofalbite and saussurite-prehnite with some areas of nearlypure prehnite and some of albite (Figure 4). Some of theprehnite is segregated in small spherulitic masses. "Serpen-tine" or chlorite is present as veinlets, small patches ofmicro-crystalline groundmass, and small green spherulites.Other small spherulites are filled with a white fibrousmineral, probably saponite. As in the coarser-grained areas,the "serpentine" veinlets commonly consist of fibrous"chrysotile" sandwiching a central filling of an antigorite-like mineral.

The white veins in the lower 50 centimeters of the coreare nearly pure calcite with a few small veinlets of"serpentine", patches of microcrystalline "serpentine" orchlorophaeite, and a few grains of partly altered biotite andchlorite.

The rock is probably of the gabbro clan (probablybasalt), nearly completely altered to a saussurite-prehnitegreenstone. The alteration is probably deuteric or hydro-thermal, consistent with the interpretation that this rock isthe top of a volcanic plug that did not break through to thesea floor.

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PETROLOGY OF BASALTIC ROCKS

REFERENCESMelson, W. G., Thompson, G. and van Andel, Tj., 1968.

Volcanism and metamorphism in the mid-Atlantic ridge./ . Geophys. Res. 73, 5925.

Hart, S. R., 1971. K, Rb, Cs, Sr, and Bacontents and Sr-isotope ratios of ocean floorbasalts. Roy. Soc. London Phil. Trans., A. 268,573.

Figure 3. Highly altered "basalt" from Site 138 (Sample 138-7-1-g). Magnetite andbiotite in a highly altered zeolite-rich matrix. Upper photo plane light, lowerphoto crossed nicols (courtesy of Ulrich von Rad).

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T. L. WRIGHT, W. E. BENSON, W. G. MELSON, S. R. HART

Figure 4. Highly chloritized "basalt" from Site 141 (Sample 141-10-1-50). Relictradial clusters of quenched Plagioclase, largely replaced by albite and chlorite,in a highly altered matrix.

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