OCEANOLOGICA ACTA 1980 - VOL. 3 - N° 4

omogeneous basalts from e East Pacifie Rise at 21 0 N:

East Paciflc Rise Petrology of submarine basalts

Pillows, sheet flows, pillars Generations of crystals

Sulphide globules

Dorsale Est-PacifIque Pétrologie des basaltes sous-marins

eady state magma reservoirs . couss~:~~ii~~r:gâ:~~~t~~~

t moderately fast spreading centers T. Juteau * a, J. P. Eissen a, J. Francheteau * b, D. Needham * b, P. Choukroune * C, C. Rangin *d, M. Séguret *e, R. D. Ballard *f, P. J. Fox *g, W. R. Normark *h,

A. Carranza * i, D. COl'doba *j, J. Guerrero *j, * Cyamex ScientifIc Team. a Université de Strasbourg, Laboratoire de Minéralogie et Pétrographie, 1, rue Blessig, 67084 Strasbourg Cedex. b Centre Océanologique de Bretagne, B. P. n° 337, 29273 Brest Cedex, C Université de Rennes, Laboratoire de Géologie structurale, B. P. n° 25 A, 35031 Rennes Cedex. d Université de Paris-VI, Laboratoire de Géologie structurale, 4, place Jussieu, 75230 Paris Cedex 05. e Université des Sciences et Techniques du Languedoc, Laboratoire de Géologie structurale, place Eugène-Bataillon, 34060 Montpellier Cedex. f Woods Hole Oceanographic Institution, Woods Hole, Massachussetts, USA. g State University of New York at Albany, Albany, New York 12222, USA. h United States Geological Survey, Paciflc-Arctic Branch of Marine Geology, Menlo Park, California 94025, USA. i Centro de Ciencias deI Mar y Limnologia, Ciudad Universitaria, Mexico 20 DF. j Instituto de Geologia UNAM, Ciudad Universitaria, Mexico 20 DF.

Received 13 /2/80, in revised form 30/4/80, accepted 23/5/80.

Fort y basaltic rocks collected by submersible during the "Cyamex" expedition (1978) on the East PacifIc Rise at 21 oN, a moderately fast spreading segment (6 cm/year opening rate) of the mid-ocean ridge, consist of angular pillow fragments and glass buds, sheet-flow slabs and samples of columnar pillars standing in collapsed fossillava pools. Most of the rocks are from the crestal are a of the Rise. The collection shows a striking petrographic homogeneity wh en compared with the range of basalts found on other segments of mid­ocean ridges: olivine-phyric, or highly plagioclase-phyric rocks, so common in the slow­spreading "Famous" are a in the Atlantic, are absent. All samples are typical low­potassium oceanic tholeiites with a limited fractionation trend. Pillow-lavas, thin and thick sheet-flows cannot be distinguished by their major element compositions, as in the Galapagos rift which has the same spreading rate as the EPR at 21°N. Further, ferrobasalts have been described from the Galapagos rift, but do not appear in the Cyamex rocks. In the Cyamex area, olivine and plagioclase are the main silicate phases, and clinopyroxene is absent. In the pillows and sheet-flow samples, four generations of olivine and plagioclase crystals are distinguished. Samples from the fossillava pools are aphyric. The corresponding magma batches are presumed to have migrated rapidly through the magma chamber, and to have been extruded in large volumes, possibly during episodes ofhigh instantaneous opening rate. Fe-Ni and Fe-Cu-rich sulphide phases are common in an lava types as massive globules scatterred through the glass, or as microglobules decorating the walls of empty vesicles. Palagonite and Fe-Mn oxide thicknesses across the strike of the Rise indicate relative ages compatible with successive extrusions at the Rise axis. The few basaltic samples collected in the Western Brunhes­Matuyama reversaI area and the Tamayo transform fault zone are not signiflcantly different from those described in the crestal area, except that they are more altered and have .thicker palagonite and manganese coats.

Oceanol. Acta, 1980, 3, 4, 487-503.

1$ 5.001 © Gauthier-Villars 487

T. J UTEAU et al.

RÉSUMÉ

INTRODUCTION

Homogénéité des basaltes de la Dorsale Est-PacifIque à 21°N réservoirs magmatiques permanents sous les zones d'accrétion moyennement rapide.

Quarante échantillons basaltiques ont été récoltés par submersible l'expédition «Cyamex» (1978) sur la Dorsale Est-PacifIque par 21 ° d'ouverture moyen sur ce segment est de l'ordre de 6 cm par an. La r-Alllo".h~_ des fragments anguleux et des protubérances vitreuses de pillow-Iavas, des minces coulées fluides et des morceaux de piliers basaltiques provenant d' lave. La collection « Cyamex » montre dans son ensemble une frappante pétrographique, comparée aux basaltes d'autres dorsales océaniques: décrits dans la zone Famous, dont le taux d'expansion est trois fois plus basaltes très riches en phénocristaux d'olivine ou de plagioclase, sont h"d·nl~. __ _

Toutes les laves ont des compositions de tholéites océaniques pauvres en montrant une faible évolution par cristallisation fractionnée. Les pillow-l coulées fluides et laves des lacs fossiles, ne peuvent être distingués par sitions chimiques, comme dans le rift des Galapagos, dont le taux d' identique. Les ferro basaltes décrits dans le rift des Galapagos sont collection Cyamex. L'olivine et le plagioclase sont les phases silicatées tous les types de laves, le clinopyroxène étant presque totalement générations de cristaux d'olivine et de plagioclases sont décrites dans les pillow-Iavas et de minces coulées fluides. Les laves des lacs fossiles sont aphyriques. On suppose ici qu'elles correspondent à de volumineuses montées n'ayant pas stationné dans les chambres magmatiques, et délivrées pendant des taux d'ouverture « instantanée» élevés. Des sulfures riches en Fe-Ni et en rencontrent dans tous les types de laves sous la forme de globules massifs rh'" ""''''''''n!

verre, ou de microglobules décorant les parois des vacuoles. Les d'encroûtement de palagonite et de manganèse fournissent des âges relatifs avec l'idée d'injections successives à l'axe de la dorsale. Les quelques basaltiques récoltés dans la zone d'inversion Brunhes-Matuyama à l'ouest de la dans la zone transformante Tamayo, ne montrent pas de différences signifIca . basaltes de la zone axiale, en dehors d'une altération plus poussée et d'une épaisseur de palagonite et de manganèse.

Oceanol. Acta, 1980, 3, 4, 487-503.

The fIrst diving phase of the French-American-Mexican project Rita, devoted to detailed studies of a small part of the East PacifIc Rise, took place in February and March 1978. The expedition, known as Cyamex, led to a collection of rock samples relatively positioned, with respect to the fme-scaled structural setting, with a precision that only work with submersibles has so far been able to achieve (Hekinian et al., 1976; Arcyana, 1977; Bryan, Moore, 1977). It is the purpose ofthis paper to provide a general description of the petrology and major element chemistry of these unique samples, and to comment on their petrogenesis. The structure and general geology of the 21 ON area are discussed in detail elsewhere (Cyamex Team, 1980).

contribution towards establishing the character of basalts associated with different rates along the mid-ocean ridge system. rocks include the fIrst set of numerous, closely well-Iocated samples collected from any part of PacifIc Rise. AlI the rocks come from near 21 ° N, off Mexico the sea-floor opening rate is about 6 cm. year they thus represent a type suite for a spreading mid-ocean ridge segment. They can be compared, for example, with a similar collection Famous area in the North Atlantic where the rate is only one third as rapid.

Three are as were visited during the " expedition (Fig. 1 A). The main target (12 dives) crestal zone near 21°N 109°W, 90 km north Rivera fracture zone and 250 km south of the T fracture zone. In addition, there were two reco dive areas: in the Brunhes-Matuyama reversaI 21 km to the west of the East PacifIc Rise crest (2 the reversaI area itself and one at an intermediate and the other in the Tamayo transform fault area at (6 dives).

Interest in the tholeiitic rocks of the ocean floor is linked to such questions as the pattern of differentiation and partial fusion at the accreting plate boundary, and the meaning of petrological diversity linked to the fIlling and tapping of magma sources within the dynamic structural regime. There is also continuing curiosity about the heterogeneity of the magma sources themselves as revealed by sorne trace element studies. As far as the present, limited study is concerned, this is essentially a

488

B

on of the "dive areas explored during the Cyamex expedition. al area; 2, Bnll1hes-i\1atuyama reversai area and intermediate . 3, Tamayo tral1.~form zone (qfter Cyamex Tea111 et al., 1978,

Cyamex Team, 1980).

'V,","'H"«UVn des zones de plongées explorées au cours de l'expédition 1, crête axiale; 2, zone d'inversion magnétique Brunhes­

yama et station intermédiaire; 3, faille transformante Tamayo 'après Cyamex Team et al., 1978, 1979, et Cyamex Team, 1980).

tion of the Cyamex dive samples collected at the Brunhes­~nt"Il.'rJ'l11n reversai area (NI / B), dives CY 78-15 and 16, and at an

ate station between the crestal and reversai areas (C y 78-17). sI, II, III, IV indicate the main tectonic zones (see text), (after

ex Team et al., 1980).

lisation des échantillons récoltés au cours des plongées Cyamex la zone d'inversion magnétique Brunhes-Matuyama (M / B),

CY 78-15 et 16, et à la station intermédiaire entre la crête et la 'inversion (CY 78-17). Les chiffres l, II, III et IV indiquent les

pales zones tectoniques (voir texte), (d'après Cyamex Team et al.,

used in Figures 1, 2 and 3: 0 pil/ow Fagment; • glass buds; t-flow; (> pillars; 0 al1desitic pumice;. pumice;T hydro­

al deposits; V Mn crust; 0 sediment; Il volcano-sediment: position of acoustic transponders. Samp/e nUll1ber in bo/d face type dive number underlined.

boles utilisés dans les fIgures 1, 2 et 3 : 0 fragments de pillow; prC1tut)enll1ces vitreuses; L,. coulées massives; (> piliers; 0 ponces

llnrlp~itinllP~; Il ponces; T dépôts hydrothermaux; V croûtes de maJ[}ga.ne~>e; 0 sédiments; II1II dépôts volcano-sédimentaires; * ; posi­

des balises acoustiques. Numéros d'échantillons en caractère gras, éros des plongées soulignés.

rty rock samples were collected in the crestal area: 33 basaltic lavas, including 17 glass buds and pillow gments, 8 samples of sheet-flows , and 8 samples of

fragments from fossil lava lakes, 6 samples of hydrothermal deposits, and one sample of sediment (Table 1 and Fig. 2). The account given here is restricted to the setting and petrology of the tholeiitic rocks, which are the majority of the samples collected. The details of the sulphide ore deposits are presented in separate papers (Cyamex Team et al., 1978, 1979).

GEOLOGY OF THE CRESTAL AREA

The are a of the East PacifIc Rise (EPR) crest explored near 21°N is a rectangle about 10 km long and 7 km

HOMOGENEOUS BASALTS FROM THE EAST PACIFIC RISE AT 21° N

489

109'05W 109'03W

Figure 2

Location of the samples collected in the cresta/ area along the 12 corresponding dive tracks (symbols as in Figure 1).

Localisation des échantillons récoltés dans la zone axiale, et des 12 plongées correspondantes (symboles: voir figure 1).

wide (Fig. 2). Previous studies of the area by deep-tow surveys have been made by Larson (1971), Larson and Spiess (1969), Klitgord (1976), Normark (1975, 1976), and Normark et al. (1978). In 1976, four basalt samples were dredged from the same study are a (Moore et al., 1977).

Tectonic zonation

The crestal area is marked by a narrow, 5 km wide axial block standing about 80 m above the adjacent flanks of the Rise, and formed of crust with a magnetic age of less than 100000 years. The 12 dives of Cyamex, coupled with the previous deep-tow data (Normark, 1976), le ad to the def1l1ition of the following zones (Cyamex Team, 1978 and 1980), Figure 1 B: Zone 1 (extrusion zone): innermost zone, characterized by numerous, isolated topographic hills and ridges 20 to 90 m high, and limited to the central area of the EPR crest, where there is little or no visible flssuring and faulting. Its width ranges from 0.6 to 1.2 km and the zone is subject to offsets along strike.

Zone 2 (horst and graben zone): two asymmetric zones of extension, one on each side of the central extrusion zone, and characterized by f.aulted and fIssured terrain with markedly linear relief aligned parallel to the EPR axis. Large scarps, generally inward-facing, are common. The combined width of zones 1 and 2 is approximatively 4 km.

Zone 3: zone characterized by large tilted blocks and fewer faults and fIssures. The scarps are more continuous along strike than in zone 2, and have greater vertical offsets.

Lithology

The extrusion zone (zone 1) can be divided into two sub­zones associated with different volcanic features:

Sub-zone 1 A incorporates the youngest and freshest

T. J UTEAU et al.

Table 1

Crestal area: list ofsamples coUected by diving saucer Cyana during Cyamex program, indicating dive number, tectonic setting, rock depth. (GE, glass bud; PF, pillow-fragment; PiF, pillarfragment; SFPi, sheet-flow associated with pillars).

Liste des échantillons récoltés par le submersible Cyana au cours de la campagne Cyamex dans la zone axiale, avec indication du la zone te~tonique, .~u type d~ .roche, de la taille et de la profondeur (GB, protubérance vitreuse; PF, fragment de pillow; PiF, fragment lave massive assoclee aux pIliers). .

Basalts

Pillow Pillar Tectonic . Sheet Massive Size

Dive No. Sample No. zone GB PF flow PiF SFPi sulphides Sediment (cm)

CY -78-02 CYP-78-02-0 1 2 02-02 1 02-03 1

CY -78-03 CYP-78-03-04 1 03-05 2

CY-78-04 CYP-78-04-06 2 04-07 1

CY -78-05 CYP-78-05-08 2 CY-78-06 CYP-78-06-09 1

06-10 1 06-11 3

CY-78-07 CYP-78-07-12 2 07-13 3

CY-78-08 CYP-78-08-14 2 CY-78-09 CYP-78-09-15 2 CY-78-1O CYP-78-1O-16 2

10-17 2 10-18 3 10-19 3 10-20 3

CY-78-12 CYP-78-12-33A 1 12-33B 1 12-34 1 12-35 2 12-36 2 12-37 2 12-38 2 12-39 2 12-40 2

CY-78-18 CYP-78-18-62 2 18-63 2 18-64 1 18-65 2 18-66 2 18-68 1

CY-78-19 CYP-78-19-69 2 19-70 2 19-71 2 19-72 2 19-73 2

Total

General total

x

x

x x

x x

x

x

8

x

x x

x

x x x

x

x

9

x

x

x x

x

x

x

x

8

lavas devoid of any sediment dusting. They build constructional volcanics hills (30 to 80 m high) made entirely of pillows. On the slopes, the pillows are elongated, black and shiny, being ornamented with abundant glass buds up to 30 cm long (Plate 1, Fig. 1). On the summits, both bulbous and fiattened pillows are typical, many of them collapsed and hollow (Plate 1, Fig. 2).

Sub-zone 1 B though neither faulted nor fIssured, con tains slightly older lavas. The pillows are less brilliant, prominent glass buds have been broken and the pillows are dusted with sediment. The area includes sorne sheet-fiows and fossillava lakes. Sheet-fiow units consist of piles of several individual sheets, several centimetres to 30 cm thick and displaying commonly a fiat and glassy top-surface that is quite commonly wrinkled or undulating (Plate 1, Fig. 3). Collapsed fossil lava pools and associated pillars were discovered during the

490

x

x x

x

13x6x5 . 22 x 20 x4 8x7x4 2xlx.5 17x14x8 21x14x4.5 24 x 8 x4 2x.5x.1 3x2x1 20 x 3 x 3 22 x 10 x4 9x6xl

27 x 7 x 6 11 x 10 x 5 14 x 6 x 3 lOxlOx.7 20 x 15 x 5

x x

9x5x5 6x4x3 28 x 15 x4 16 x 9 x 4 13 x 10 x 6

x x x x

x x x x

17x8x5 16x14x6 15 x 8 x 3 17x9x4 11x5x5 10 x 8 x 7 17x13x5 llx5x4 16xll x7 18x10x8 8x8x3

6

x

2 6

40

Cyamex dives at two or three localities, at the zones 1 and 2. They al ways occur in topogr and cover surfaces of modest area (of the several square kilometres); their total between 10 and 15 m. Numerous cylindrical 10 to 15 m high and 1 to 5 m wide, stand up abundant angular and coarse rubble, spectacular "ruiniform " landscape (Plate 1, Fig. pillars exhibit a very regular apparent stratifIca centimetre scale, with alternating black, glassy basaltic layers (Plate 1, Fig. 5). Examination of pillars in the field, and a study of samples of shown that it is a pseudo-Iayering restricted to the surface of the pillars (Francheteau et al., 1979).

Zone 2 (the horst and graben zone) is sediment-covered except locally on steep surfaces sediments thicken from sever al millimetres to decimetres as one moves away from zone 1. Pillo

t Ji'ee pil/ow-Iavas Ji'0111 the youngest extrusion zone (zone 1), g numerous black and brilliant glass buds. Dive C Y-78-09,

Im.41 min. 10 sec. coussins dépourvues de sédiment affleurant dans la zone

ex tr u:s 10 n la plus jeune (zone 1), montrant de nombreuses vitreuses noires et brillantes. Plongée CY-78-09,

h 41 mn 10 s.

us collapsed hollow pillow at the summit of an axial volcanic hill el). Dive C Y-78-10, 9 hrs. 38 min. 10 sec.

bulbeux et creux, dont le toit s'est effondré, affleurant au sommet des collines axiales de la zone 1. Plongée CY-78-10,

h 38 mn 10 s.

sheetflow with a typical wrinkled sU/face (zone 2). Dive CY-78-06, . 20 min. 10 sec.

fluide et massive, à surface typiquement plissée (zone 2). CY-78-06, 14 h 20 mn 10 s.

- .... r~- ... ,-~~.ï lava lake and associated basaltic pil/ars (zone 1): general . Dive CY-78-06, 13 lm. 25 min. 20 sec.

de lave fossile effondré, et piliers basaltiques associés (zone 1) : vue ale. Plongée CY-78-06, 13 h 25 mn 20 s.

lof upper part of a pillaI' showing the de/icate centime tric pseudo­ng, with th in and dm'ker salient glassy ledges stuck onto the basal tic

of the pillaI'. Notice stalactite-/ike glass shards hanging at the side of the ledges. Dive CY-78-19, 13 lm. 33 min. il de la partie supérieure d'un pilier de lave, montrant le pseudo­

centimétrique dû à de minces couches de verre accrochées à la basaltique externe du pilier. Remarquer les« stalactites» de verre

t à la surface inférieure des anneaux vitreux. Plongée CY -78-19, h 33 mn.

ted pillow-Iavas on the scarp of a normalfault (zone 2). Dive C Y-14 hrs. 20 min. 10 sec. en coussins tronquées sur un miroir de faille normale (zone 2).

ngée CY-78-06, 14 h 20 mn 10 s.

HOMOGENEOUS BASALTS FROM THE EAST PACIFIC RISE AT 21° N

491

2

3

4

5 6

PLATE 2.

Figure 1

Massive piece of aphyric and somewhat vesicular pillow-Iava. Fragment massif de lave en coussin, aphyrique et quelque peu amygdalaire.

Figure 2

Piece of moderatelyplagioclase phyric basait, one of the most phyric samples (around 12 % phenocrysts). Fragment de basalte à plagioclase modérément porphyrique (lave en coussin), un des échantillons les plus riches en phénocristaux (12 % environ).

Figure 3 Section across a sub-aphyric glass, bud takenJi'om a pillow-Iava of zone 1 (extrusion zone), showing an entirely glassy outer rim 2 cm thicl<, and a fine grained basaltic core. Section à travers une protubérance vitreuse sub-aphanitique d'une lave en coussin de la zone 1, montrant une bordure vitreuse de 2 cm d'épaisseur, et un cœur basaltique finement cristallisé.

Figure 4

Slabs of thin sheet flows. (a) internai sU/face of the top glassy crust, showing delicate "septa" supposed to be perpendicular to the lava CUITent under the glassy crust during cooling oftheflow (sample CYP-78-12-34). (b) section showing the 1 cm thick glassy crust (sample C YP-78-07-12).

Plaquettes de fines coulées fluides. (a) face interne de la croûte vitreuse supérieure, montrant de délicats « septa » développés perpendiculaire­ment au courant de lave sous la croûte vitreuse au cours du refroidissement de la coulée (Ech. CYP-78-12-34). (b) section montrant dans la tranche la croûte vitreuse épaisse de 1 cm environ (Ech. CYP-78-07-12).

Figure 5

Piece of the paleosurface of afossillava pool, showing a totally aphyric basait capped by a t1ün glassy crust. Échantillon de la paléo-surface d'un lac de lave fossile, constituée d'un basalte totalement aphanitique et recouverte d'une mince couche vitreuse.

Figure 6

Massive piece ofpseudo-layered basal tic pillaI', collected in afossile lava pool. Glass layas are stuc!< onto the pillaI' sU/face (fi'ont face of the picture) and do not ex tend inside (see left side of the picture). Sample CYP-78-19-72. Échantillon d'un pilier basaltique pseudo-lité, prélevé dans un lac de lave fossile. Les couches de verre brillant Sont attachées à la surface externe du pilier (côté de face) et ne passent pas dans la masse finement basaltique du pilier (côté gauche de la photo). Échantillon CYP-78-19-72.

T. J UTEAU et al.

Table 2 Modal analysis 0123 samples coltected in the crestal area du ring Cyamex program. Megacrysts are >2 mm, phenocrysts between 0.5 microphenocrysts between 0.1 and 0.5 mm. Corrected volume per cents are recalculated palagonite, F e-M n oxides, vesicles and cracks

Lava type

Sample No.

~ Megacrysts CI! () Phenocrysts 0 Microphenocrysts 50 CI!

li: Microlites ~ Phenocrysts c:

"E ::€ Microphenocrysts ~ Ô Microlites u 1-< ~ ~ 0- c: ~ ~

El x Phenocrysts 0

;::1 1-<

'0 :> Microphenocrysts 0-:> 0 Microlites '"0 .S ~ 0 0 ~ 1-< 1-<

<Il Fresh glass 0 U

<Il-

Plumose groundmass CI!

Ô· Varioles Total

<Il Spinel 1-< ~

',.c

6 Oxydes

"E Palagonite ~ Ox Fe-Mn u 1-< Vesicles ~ 0- Cracks ~

El Sulphide globules ;::1 Sul phi des in vesicles 0 >- Number of points

02-02B

8.80 2.80 1.63

1.03

0.44 0.88

66.89

17.53 84.42

0.07

1. 58 1.22 + + 1392

04-07A

5.36 1.13

1.28

0.06 0.49

59.83

31.85 92.68

0.96 1. 79 + + 1445

06-10

0.68 4.01 0.88

1.63

0.13 0.88 1.49

82.40

7.90 90.30

0.06

1.12 1.45 + + 1509

are smoother and inter-pillow spaces are fIlIed with sediment and hyaloclastic material. The thickness of manganese oxide coating seems to increase in parallei with the thickness of sediments, and becomes notable in the outer part of zone 2. The numerous open fIssures and fauIt scarps in this zone ex hi bit truncated pillow sections, with typical radial-joint patterns (Plate 1, Fig. 6). Sheet­flows are estimated to make up at least 20 per cent of the flows in zone 2 and may commonly be interbedded with pillow flows judging from exposures along sorne of the biggest fauIt scarps. Zone 2 is also characterized by the fIrst appearance of piles of large angular talus, and by many signs of hydrothermal activity: colonies of fossil giant clams, colour staining on pillow-surfaces, coloured deposits on scarps, and massive polymetallic sulphide deposits (Cyamex Team, 1978, 1980; Cyamex Team et al., 1978, 1979). Zone 3 is highly sedimented. Small fauIts and pillow­lavas become completely covered by sediments and crop out only in small areas or along the scarps of the larger faults. No clear evidence of sheet-flows were seen in this zone. Zone 4 shown in Figure 1 B on the Western flank of the ridge is tectonically inactive.

BASALTS FROM THE CRESTAL AREA

The basalts are most typically aphyric to subaphyric, and

Pillow-lavas

10-17 10-18 12-33B 18-62 18-64

0.69 0.70 0.24 0.52 0.51 2.29 1.94 0.16 1.46 1.01 1.41 1.41

0.90 1.29 1. 33 1.11 0.75

1.97 0.10 0.29 0.15 0.07 0.25 0.41 0.64 0.54 0.07 0.25 0.61 0.35 1.02

66.05 1.05

30.87 98.56

93.57

93.57

89.49

6.54 96.03

66.16

27.05 93.21

90.24

3.85 94.09

0.10

1.96 2.11 0.05 0.27 0.36 1. 33 1.30 t 0.77 0.32 1.60 1.15 0.52 0.78 2.03 0.55 4.74 2.36 + + + + + + 1270 1229 1997 1812 1905

492

ev en the most phyric pillow fragments contain than 10 per cent phenocrysts, with plagioclase dominant over olivine. Clinopyroxene DheUCJCrv: totally absent. Modal analyses of 23 samples (10 lavas, 7 sheet-flows and 6 pillars; Table 2) megascopic impressions that the rocks - regar'dle:s~ morphological characteristics of the parent represent a relatively very homogeneous tholeiites, certainly much more homogeneous associated, for example, with the Famous Area 1977; Bryan, Moore, 1977). AlI the Cyamex would fall either in the "plagioclase-olivine b the "moderately phyric plagioclase basaIt" defIned by Arcyana (1977). Other basait types in the Famous collection such as picritic basaIts, basaIts, plagioclase-pyroxene basaIts, and highly plagioclase basaIts are remarkable for their a the Cyamex collection.

Pillow-lavas, sheet-flows and pillars

Pillow samples include angular pillow fragments by radial joints and with a glassy top layer 0.5 to 2 thick (Plate 2, Fig. 1) and buds, which are generally glassy (Plate 2, Fig. 3). The proportion of phen (0.5 to 2 mm) ranges from less than 1 per cent ( samples, Plate 2, Fig. 1) to 12 per cent (mode phyric samples, Plate 2, Fig. 2). Megacrysts crysts longer than 2 mm), always of plagioclase, are

HOMOGENEOUS BASALTS FROM THE EAST PACIFIC RISE AT 21°N

modales de 23 échantillons prélevés dans la zone axiale. Les mégacristaux sont > 2 mm, les phénocristaux entre 0,5 et 2 mm, les cristaux entre 0,1 et 0,5 mm. Les pourcentages en vohlme corrigés ont été calculés en excluant la palagonite, les oxydes de Fe-Mn, les vacuoles

craquelures.

Sheet-flows

07-12B 07-12b 09-15 18-63A 18-63B

3.62 3.15 0.74 1.17 1. 84 0.48 1.10 3.03 2.72 1. 55

0.96 1. 85 2.22 2.23 3.09

0.48 0.13 0.04 0.74 0.56 0.40

0.80 1. 38 1.41 0.80 1. 36

80.85 43.34 68.73 86.52 91.59

. 12.81 49.18 23.00 5.95 0.09 93.66 92.52 91. 73 92.52 91.68

0.05 0.04

0.62 1.06 1. 65 1.19 t 0.17 0.10 0.32

0.77 1.15 0.20 0.96 0.23 2.02 1. 51 0.80 Il.26 3.22

+ + + + + +

1285 1122 1498 1873 2170

maximum content being 8 per cent (sample CYP-78-02; Table 2). The ratio of plagioclase to olivine ranges

4 to 15. Pyroxene phenocrysts are apparently t. Rare, tiny spinel crystals occur included within,

lying close to, olivine phenocrysts. The structure is . cally hyalopilitic, with a progressive evolution from gin to core (Plate 3).

Sheet flow samples are typically sub-aphyric (less th an 5 per cent phenocrysts, Plate 2, Fig. 4); one sample contains about 9.5 per cent phenocrysts. Plagioclase phenocrysts dominate over olivine phenocrysts, to the same extent as in the pillow samples (Table 2). Neither c1inopyroxene nor spinel crystals have been seen. The structure is hyalopilitic, with a very high (about 90 per cent) ratio of glass to crystals.

Pillal' samples are either aphyric or subaphyric, with rare scattered plagioclase phenocrysts in a more or less opaque (devitrifled) groundmass. Olivine phenocrysts are absent. One sample (CYP 78 19-70) exhibits the only clinopyroxene phenocrysts observed in the collection. Samples from dive CY 78-12, which are aphyric slabs of the former roof of a fossillava lake, are almost entirely glass (> 99 per cent) and quite devoid of micro lites (Plate 2, Fig. 5). Samples from dive CY 78-19 are massive pieces of pseudo-Iayered pillars (Plate 2, Fig. 6), and are microlitic, with fluidal arrangemcnts in somc zones. The apparent layering associated with glassy ledges on the outer sides of the pillars is only an external feature, the inner mass of the pillars being unstratifled.

Pillars

18-66B 12-34 12-35 19-69 19-70 b 19-70h 19-71

4.54 0.46 1.98 1: 78 2.65 0.13 0.07 0.09 0.9X

0.75 . 0.13 0.14 0.06 0.53 0.57 1. 34 2.05 18.02

1.45 1.05 0.35 0.20 0.60 1.19 0.22 0.09 5.99

2.36

87.07 60A3 57.95 77.57 71.88 66.58 50.60 10.79 10.61 2.23 3.99 2.98 Il.37

1. 94 . 28.32 30.70 16.51 22.85 25.21 9.65 89.01 99.54 99.26 96.31 98.72 94.77 72.62

0.20 0.39 0.27

1.14 0.44 0.48 2.06 0.22 0.60 1.09 t 0: 14 0.96 t 0.90 0.13 0.37 1.22 1. 05 0.65 1.12 1. 61 0.34 0.14 0.70 0.75 0.R3 1. 30

+ 2104

493

+ + + + + +

1951 1344 1556 1327 1352 1153

Groundmass textures

AH the lavas, either phyric or aphyric, exhibit the same general evolution of groundmass textures from glassy margins to microlitic cores, with intervening variolitic and spherulitic zones (Lofgren, 1971; Marshall, Cox, 1971; Lofgren, 1974; Natland, 1978; Juteau et al., 1978, 1980). Plate 3 describes this evolution.

Crystal genesis

Pillow-lavas and sheet-flows

We distinguish in these rocks four kinds of plagioclase and four kinds of olivine crystals, using morphological features as the main criteria;

• Olivine: the four generations of olivine crystals are represented respectively by: (a) scattered rounded and / or deeply embayed phenocrysts (Plate 4, Fig. 1), containing very few glassy inclusions, with a high Mg / Fe ratio: Fo 89 (Fig. 4 a, 2 microprobe analyses); (b) euhedral olivine phenocrysts(0,5 to 2 mm) contai­ning few glassy inclusions (Plate 4, Fig. 2). These phenocrysts are the majority; they are slighty less Mg­rich than those classed under (a), having a mode at F0 88

(Fig. 4 b, 7 analyses); (c) skeletal, sub-euhedral micro­phenocrysts (Plate 4, Fig. 3), forming clusters with plagioclase crystals of the same generation. The Mg / Fe ratio is lower, with a mode at F0 85 - 86 (Fig.4c,

T. J UTEAU et al.

PLATE 3. Groundmass textures

Textures de la mésostase

This plate illustra tes the evolutiol1 ofgroundmass texturesfi'om mal'gin to core of the basait jlows in any type ofjlows.

Cette planche illustre l'évolution des textures de la mésostase de la bordure au cœur des coulées basaltiques, quel que soit leur type.

Figure 1

General view of the evolution of groundmass texturesfrom mm'gin to core ( x 4), with a Glassy Zone (0,5 to 2 cm), a variolitic zone (1 to 2 cm), a spherolitic zone (0 to 1,5 cm) and a microlitic zone.

Vue générale de l'évolution des textures de la bordure (en haut) vers le cœur des coulées ( x 4), avec une zone vitreuse (0,5 à 2 cm), une zone variolitique (1 à 2 cm), une zone sphérolitique (0 à 1,5 cm) et une zone microlitique.

16 analyses); and (d) quenched micro lites appearing as sub-euhedral grains, "linked-chain" microlites (Donald­son, 1976), or "lantern and chain" olivine (Bryan, 1972). The characteristic "swallowtail" morphology of olivine crystallites appears in the glassy margins of the pillows and sheet-flows (Plate 4, Fig. 4). The microlites are the least Mg-rich olivines, with a mode at F0 83 (Fig. 4d, 8 analyses). A histogram drawn from the various microprobe analyses (Fig. 4 e) shows clearly only three distinct modes corresponding to the chemical evolution from phenocrysts (generations 1 and 2) to micropheno­crysts (generation 3) and microlites (generation 4), respectively. Phenocrysts from generations 1 and 2 cannot be distinguished by chemistry alone.

• Plagioclase: the four generations of plagioclase crystals that we distinguish comprise: (a) rounded phenocrysts, remaining commonly as relicts in the cores of younger plagioclase phenocrysts (Plate 4, Fig. 5) and having a high Ca INa ratio, ranging from An 82 to An 85 (Fig. 4.1; 5 analyses); and (b) euhedral crystals containing nume-

494

Figure 2

Glassy zone ( x 40), with isolated varioles, made of ,,'U,'.'(J'fJ"· , with a honey yellow colour under microscope, usually scattered crystallites of plagioclase and olivine. Zone vitreuse ( x 40) contenant des varioles isolées, frais isotrope de couleur jaune miel sous le mllcroscO'De. fréquemment des cristallites dispersés de plagioclase et

Figure 3

Variolitic zone (x40), where dark brownfibrous and develop around isolated crystal/ites or phenocrysts. coalescent inwards, grow slightly in size and become because of the crystallization of very tiny Fe-Ti oxides.

Zone variolitique (x 40), où des varioles brunes fIbroradiées se développent autour des microlites ou des Les varioles deviennent progressivement coalescentes vers augmentent légèrement de taille et deviennent plus probablement à cause de la cristallisation de ml'llluscut~ d'oxydes de Fe-Ti.

Figure 4

Spherolitic zone (x 40), where amber-coloured fibres fan plagioclase microlites. Tiny opaque minerais outline the the fan spherolites.

Zone sphérolitique ( x 40), où des fIbres de couleur ambrée ~ amas radiés en bordure des microlites de plagioclase. opaques soulignent la forme externe des sphérolites.

Figure 5

Microlitic zone (x 40), where the spherolitefibres grade to thin plagioclase laths, enclosing tiny grains of clinopyroxene minerais.

Zone microlitique ( x 40) : dans ce cas, les fIbres des spJb.érolites, de fmes lattes divergentes de plagioclase, enserrant de petits clinopyroxene et de minéraux opaques.

Figure 6

Microlitic zone (x 40), a case different from Figure 5, spherolites become grey and loose their shape as they arborescent and fibrous plagioclase microlites, centered pyroxene grains. Opaque minerais grow in size and COI1'C"'I1'trn. mm'gins of the arborescent features.

Zone microlitique ( x 40) : un cas un peu différent, où les deviennent gris et perdent leur forme en donnant arborescents de plagioclase fIbreux, centrés sur des clinopyroxène. Les minéraux opaques sont plus gros et se en bordure des structures arborescentes.

Figure 3

Location of the Cyamex dive samples and dive tracks in the transform zone (after Cyamex Team et al., 1980; symbols as in

Localisation des échantillons récoltés dans la faille Tamayo, et des plongées correspondantes, d'après Cyamex et al., 1980 (symboles: voir fIgure 1).

rous glassy inclusions, generally concentrated core totally devoid of inclusions (Plate 4, Fig. 6). are the most abundant of the phenocrysts. A concentrically zoned, but in the general case margin is zoned. In the more porphyritic satrWl'I;:>, phenocrysts form large glomerocrysts. M

ologie des cristaux

olivine phenocryst (lst generation). Sample CYP-78-66B

pnlf'nr\rrl<!ral résorbé d'olivine (1'e génération). Ech. CYP-78-18-66B

Figure 2 Euhedral olivine phenocrysts (2nd generation). Sample CYP-78-18-66B ( x40).

analyses so far made of the cores of the phenocrysts show three groups of composition (Fig.4g): An 83 -84 (7 analyses), An 8o- 81 (11 analyses) and An 76-78 (6 ana­lyses). Analyses ofzoned rims give a range between An66 to An 68; (c) euhedral and elongated microphenocrysts, with skeletal margins, forming clusters with olivine crystals of the same generation (Plate 4, Fig. 7) and ranging from An68 to An74 (Fig. 4 h, 24 aÎialyses), except for a small group with a composition of An 76 (3 analyses); and (d) quenched bifId acicular microlites, generally with a hollow core (Plate 4, Fig. 8) and ranging from An60 to An 70 , with a small secondary mode at An66 (Fig. 4 i, 22 analyses). In summary, the plagioclase microprobe analyses (Fig. 4 j) show a clear difference between the phenocryst group (generations 1 and 2) with 3 modes reflecting a rather complex evolution, and a group including the microphenocrysts and microlites, with considerable compositional overlap.

Diagrammes de fréquence des compositions des cristaux d'olivine (Fo %) et de plagioclase (An %) dans les pillow-lavas et laves massives, déterminées par microsonde électronique (Camebax, Universités de Brest et de Nancy). Olivine: (a) 1 re génération, phénocristaux résorbés; (b) 2e génération, phénocristaux sub-automorphes; (c) 3e génération, microphénocristaux; (d) 4e génération, micro lites trempés; (e) diagramme de fréquence cumulatif pour les 4 générations d'olivine. Plagioclase: (f) Fe génération, phénocristaux résorbés; (g) 2e génération, phénocristaux sub-automorphes; (h) 3e génération, microphénocristaux allongés; (i) 4e génération, microlites trempés; (j) diagramme de fréquence cumulatif pour les 4 générations de plagioclase.

HOMOGENEOUS BASALTS

495

Phénocristaux automorphes 18-66B ( x 40).

Figure 3 Sub-euhedral skelettal 78-18-68 (x 80). Phénocristaux sub-automorphes et squelettiques d'olivine (3 e géné­ration). Ech. CYP-78-18-68 (x 80).

Figure 4

Quenched olivine micro lites in fi'esh glass (4th generation), showing the classical "comb", "belt-buckle" and "swallow-tail" morphologies. Sample CYP-78-18-68 (x 200). Microlites et cristallites d'olivine «trempés» dans le verre frais (4e génération), montrant les formes morphologiques classiques en « peigne », en« boucle de ceinture» ou en « queue d'hirondelle ». Ech. CYP-78-18-68 (x 200).

Figure 5 Reliet of rounded resorbed plagioclase phenocryst (1st generation), included in a plagioclase phenocryst of the second generation. Sample CYP-78-02-03 (x40). Relique d'un phénocristal résorbé et arrondi de plagioclase (1'e génération) inclus dans un phénocristal de plagioclase de la seconde génération. Ech. CYP-78-02-03 (x 40).

Figure 6 Euhedral plagioclase phenocrysts containing numerous glassy inclusions (2nd generation). Sample CYP-78-18-66B (x40). Phénocristaux automorphes de plagioclase à nombreuses inclusions vitreuses (2 e génération). Ech. CYP-78-18-66B (x40).

Figure 7 Euhedral and elongated plagioclase microphenocrysts, with skeletal mm'gins, forming clusters with olivine crystals of the same generation. Sample CYP-78-18-68 (x40). Microphénocristaux automorphes et allongés de plagioclase, à extrémités squelettiques, associés à des microphénocristaux d'olivine de la m.ême génération. Ech. CYP-78-18-68B (x40). .

Figure 8 Quenched plagioclase microlites in fi'esh glass. Sample CYP-78-18-68 (x 200). Microlites de plagioclase « trempés» dans le verre frais. Ech. CYP-78-18-68 ( x 200).

Figure 4

~ Olivines a Plagioclases

2 ~L 2

BO 85 90 Fo '-r::c60~~6cr5 ~::C70~~~~~!'h:-

~1 n bL . 9

t",pdb" " : .. o",~...uJll ~LL

80 85 90Fo

h

4lnl~.;d 4~~_. i 2 2

B 85 9QFo 60 5 70 5 "-,--r-~~

~e~dA~~i 80 85 90Fo 60 65 70 75 8Q 85 An

Frequency diagrams for the compOSl tlOns of olivine (F 0 %) and plagioclase (An %), crystals in pillow-lavas and sheet-flows, based on microprobe analyses (Camebax, Brest and Nancy Universities). Olivine: (a) 1st generation, resorbed phenocrysts; (b) 2nd generation, sub­euhedral phenocrysts; (c) 3rd generation, skeletal microphenocrysts; (d) 4th generation, quenched micro lites; . (e) cumulative frequency diagramfor the 4 generations of olivine. Plagioclase: (f) 1st generation, resorbed phenocrysts; (g) 2nd generation, sub-euhedral phenocrysts; (h) 3rd generation, elongate microphenocrysts; (i) 4th generation, quenched microlite; (j) cumulative frequen'cy diagram for the 4 genera­tions of plagioclase.

T. J UTEAU et al.

The oldest crystals in the pillow lavas and sheet-flows are the rounded olivine and plagioclase phenocrysts characterized by high Mg/Fe and Ca/Na ratios, respectively, and that were partially resorbed in the magma chamber before crystallization of the second generation of crystals. The resorption process may have resulted from either a rise of temperature after introduction of hot magma into the chamber or the lowering of the pressure by the tapping of magma at the surface. The second generation of euhedral phenocrysts has crystallized with very homogeneous compositions for olivine, and more complex compositions for plagioclase. In the third generation, plagioclase and olivine are closely associated in clusters and are clearly contempora­neous. The skeletal morphology of the olivine crystal, and the elongated shape of the plagioclase crystals, indicate a rather high rate of over-cooling. The crystals may weIl have formed at high levels during the ascent of magma.

The skeletal morphologies of the crystals of the fourth generation imply strong chilling conditions with quenching of the lava, and correspond evidently to crystallization after eruption on the ocean floor.

Pillar basal ts

The samples of pillars are considered separately: fIrst because these highly aphyric lavas are totally devoid of phenocrysts and second, because they contain the only pyroxene phenocrysts so far observed in the whole collection;

• Olivine: phenocrysts and microphenocrysts are totally absent. Scattered olivine microlites (about 1 per cent; Table 2) have a composition of Fo 86 (3 analyses). In one sample the proportion present rises to 6 per cent in the glassy margin (CYP 78-19-71) along with up to 19 per cent of plagioclase microlites. This sample cornes from one of the numerous horizontal glassy ledges decorating the external face of the pillars; the ledges probably represent the temporary, chilled, upper surface of a fossil lava lake, and this could explain the higher degree of crystallization of the glass.

• Plagioclase: samples from the upper glassy surface of pillar samples collected during dive CY 78-12 are totally aphyric, but rare and scattered phenocrysts and microphenocrysts (0.5 to 2.8 per cent) are observed in the massive parts ofpillars sampled during dive 19. Their compositions range from An 76 (3 analyses) to An 70 -73 (7 analyses). Plagioclase micro lites (0.1- 2.1 per cent in aIl samples except CYP 78-19-71) have a lower Ca /Na ratio, ranging from An 60 to An 67 (5 analyses), except for a few with An 53 (1 analysis) and An 40 -42 (3 analyses).

• Pyroxene: a single cluster of three clinopyroxene phenocrysts associated with one lath of plagioclase has been observed (sample CYP 78-19-70). The average composition of the pyroxene, based on fIve micro probe analyses, is En 52 W0 40 Fs 8. Plumose pyroxene crystals in the groundmass have a slightly different composition: En48 W0 42 Fs 10'

As we shall see, these highly aphyric lavas cannot be distinguished from the pillows and sheet -flows on the basis of major element chemistry, and settling of silicate

496

phenocrysts cannot be advocated to explain character. It has been suggested that the correspond to the delivery of large ",.,."".,..,--<

and fluid magma at the axis of the Rise 1979; Francheteau et al., 1979; Cyamex This hypothesis is compatible with the maintaining very high temperatures until extrusion in order to prevent signifIcant taking place. The implication is therefore could not have had more th an a very short in a magma chamber.

Vesicles and globules

Glassy margins in samples of aIl three lava about 1 per cent void vesicles 20-30 Jim in results, which extend those of Moore

a

b

c Figure 5

Decorated vesicles and slilphide globules. (a) SEM pho 2 decorated vesicles; that on the right is 30 /lm wide. The shows a reglilar arrangemenl (J/ tinl' sulphide /11

3 distinct size groups. Sam pie C YP 78-04-06 (x 1 scanning X -ray intensity displays of sulphur i/1 a decora Sample C YP 78-04-06 (x 1 500). (c) displays of coppel' vesicle. (d) SEM photograph of a polished section massive sulphide globule (180 /lm in diameter). Sample CYP ( x 300). (e) beam scanning X-ray intensity displays ofnickel in globule. (f) displays of coppel' in the same globule. Vacuoles décorées et globules de sulfures; (a) photographie au deux vacuoles décorées; celle de droite fait 30 /lm de diamètre. interne montre un arrangement régulier de petits globules montrant nettement trois groupes de tailles différentes. Cyp 78-04-06, (x 1500); (b) image par balayage de la soufre dans une vacuole décorée. Échantillon CYP 78-04-06, ( (c) image de la répartition du cuivre dans la même (d) photographie au MEB d'une section polie montrant globule massif de sulfure (180 ~lm de diamètre). Échantillon 02-02B, ( x 300); (e) image par balayage de la répartition dans le même globule; (f) image de la répartition du cuivre même globule.

te that the vesicularity of the East PacifIcRise rocks 21°N is qui te comparable with that of samples from

r depths on the Juan de Fuca Rise at 46°N. from the Mid-Atlantic Ridge (Famous Area) are

more vesicular (1-3 per cent).

of the vesicles, especially those < 100 !lm in , are delicately decorated with sulphide

globules on their internaI walls (Moore, Calk, 1). The microglobules are regularly arranged in three

size groups (Fig. 5 a): the larger globules, 1-3 !lm , are surrounded by others O. 1 !lm wide, which in are surrounded by very tiny sulphide spots. Beam-

.. «H~H~"I'"> X-ray intensity displays of sulphur (Fig. 5 b), (Fig. 5 c), nickel and iron give qualitative

rmation on the composition of these microglobules. and cobalt have not been detected. These data are

able with those obtained by Mathez and Yeats 76) on DSDP samples from Hole 319 A and Site 320.

art from decorated vesicles, massive sulphide globules in approximatively one third of the samples so far

A«JHH~"'U. Perfectly spherical, they range in diameter 50-150 !lm (Fig. 5 d), and are made up of Fe-Ni and

sulphide phases with a pattern supporting olution (Fig. 5 e and f). The unmixed phases corres­

nd probably to pentlandite and chalcopyrite, pectively (Mathez, Yeats, 1976; Czamanske, Moore,

977). The presence of the globules reflects the saturation the basaltic magma with respect to sulphide phases.

her, the existence of an immiscible sulphide liquid in the magma prior to eruption has been proposed

,lY~"tLU\ .. L, Yeats, 1976; Czamanske, Moore, 1977). There important chemical differences between the globules hydrothermal sulphide deposits from the same area. globules are rich in nickel (up to 33 per cent after

'-"L..'_UHUH,J~"'" and Moore, 1977) and very po or in zinc, as the hydrothermal deposits are rich in zinc (up to

per cent) and very po or in nickel (Cyamex Team et al., 1979).

-'-J1!;,Il\:o1 al chemistry and normative compositions

Major element chemical analyses re-inforce the petrogra­hic evidence showing that the collection as a whole is

strikingly homogeneous (Table 3). AIl samples have the composition of typicallow potassium oceanic tholeites. The pillow fragments, sheet flows and pillars cannot be distinguished on chemical criteria except in that the most plagioclase-phyric pillow samples are relatively ri ch in A1 2 0 3 (up to 16-18 per cent) and poorer in total iron as Fe 2 0 3 « 10.5 per cent) and Ti0 2 « 1. 3 per cent). It appears also that aIl the lavas collected are remarkably fresh, since they show a negative loss on ignition, due to oxidization of Fe + + .

The narrowness of the range of compositional variation among the samples is illustrated by the increase of FeO * /MgO with respect to TiO 2, FeO * /MgO varying from 1-1,5 versus 1,2-2 for Ti0 2 (Fig. 6). At a given FeO * /MgO ratio, the rather wide range of TiO 2 values (more or less 0.5 %) can be interpreted as the effect of magma mixing in a shallow magma chamber.

Samples showing less th an 1 per cent TiO 2 and low FeO * /MgO ratio, common in the Famous collection

HOMOGENEOUS BASALTS FROM THE EAST PACIFIC RISE AT 21° N

F:sr.- 2.5 MgO

497

2

1.5

0.5

Figure 6 0.5 1.5 2 2.5

Weight pel" cent ofFeO* / MgO in relation to weight per cent ofTiOJor bulk rocks (XRF analyses) and fresh glasses (electron microprobe analyses)fi"om Cyamex collection. Symbols used in Figures 6 and 7: 0 pillow-lavas;. glass buds; 0 sheet­jlows; L. pillars; 0 pillow and sheet-flow glasses;. pillar glasses; .... glassesfrom dredged EPR basalts, after Moore et al. (1977). Fields of bulk-rock analyses of other basal tic collections are shown in the same diagram. (GR, Galapagos Ridge; CRR, Costa Rica Ridge; TJ, Triple Junction (GR-EPR); datafrom Anderson et al., 1975, and Byerly et al., 1977. Leg 34 (DSDP), Nazca Plate, datafi"om Rhodes et al., 1976; and Bunch and Laborde, 1976. Leg 37 (DSDP), Mid-Atlantic Ridge, data fi'om Bryan et al., 1977.

Relation entre le rapport FeO*/MgO (en poids %) et Ti0 2 (en poids %) dans les basaltes (analyses par fluorescence X) et les verres frais (analyses par microsonde électronique) de la collection Cyamex. Symboles utilisés dans les figures 6 et 7 : 0 pillow-Iavas; • protubé­rance vitreuse; <) coulées massives; D,. piliers; 0 verres de pillows et coulées massives; • verres de piliers; .... verres de basalte dragués sur la dorsale du Pacifique oriental à 21 ON, d'après Moore et al., 1977. Les champs des analyses de roches totales d'autres collections de basaltes sont indiqués dans le diagramme (GR, Galapagos Ridge; CRR, Costa Rica Ridge; Tl, Triple lunction (GR-EPR) : d'après Anderson et al., 1975, et Byerly et al., 1977. Leg 34 (DSDP) plaque Nazca, d'après Rhodes et al., 1976, et Bunch et Laborde, 1976. Leg 37 (DSDP), Dorsale médio-atlantique, d'après Bryan et al., 1977.

(Arcyana, 1977; Bryan, Moore, 1977), are absent. Furthermore, ferrobasalts (more th an 12 per cent FeO * and 2 per cent TiO 2), such as have been described froin the Galapagos rift zone (Anderson et (fI .• 1975: Byerly et al., 1976; Ballard el al., 1979) are not round in the Cyamex collection, although Moore et al., 1977, report one dredged sample of ferrobasalt from 21°N. The cluster of representative points in the plagioclase­pyroxene-olivine normative diagram (Fig. 7) is remarka­bly concentrated and lies close to the middle of the plagioclase-pyroxene side. The olivine component in this diagram varies between 0 and 20 per cent, and three samples which have a slight excess of free silica should probably lie on the plagioclase-pyroxene line. Most ofthe representative points are located in the plagioclase­tholeiite fIeld as defmed by Muir and Tilley (1964), Shido et al. (1971) and Bougault and Hekinian (1974). Shido et al. (1971) propose that the boundary curve separating the primary fIelds of plagioclase tholeiites and olivine tholeiites (Fig. 7) could represent a cotectic line. It appears that the cluster of the Cyamex samples in this diagram is very close to that boundary, which would be in accordance with there being clusters of plagioclase and olivine microphenocrysts (generation 3) in the pillows and sheet flows. The data suggest that the two silicate phases crystallized simultaneously during the evolution of the basaltic liquids.

Table 3 :-1

Chemical analysis by X -ray fluorescence spectrometry ofrock samples listed in Table 1 (cresta l area) . Analyses by H. Bougault and J. Etoubleau (Centre L

Océanologique de Bretagne, Brest) . Fe 2 0 3 represented total Fe. Symbols as in Table l. c :-1

Analyses chimiques par spectrométrie de fluorescence X des échantillons du tableau 1 (zone axiale). Analyses par H . Bougault et J. Etoubleau (Centre m » Océanologique de Bretagne, Brest). Fe2 0 3 représente le fer total. Mêmes symboles que dans le tableau l. c

~

Lava type Pillow-lavas ~

CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 Sample No. 02-02 02-03 03-04 03-05 04-07 06-09 06-10 06-10 10-1 6 10-17 10-18 12-33A 18-62 18-64 18-65 18-68

GB GB whole

Nat ure PF GB PF PF GB GB glass rock PF PF PF GB GB PF GB PF

(per cent) 49.17 Si0 2 48.61 49.39 49.54 49.94 50.42 50.09 49.24 48.86 49.23 49.77 49.89 50.43 48 .75 50 . 15 49.59

Al2 0 3 16.81 16.28 14.38 14.69 14.35 14.40 15.87 17.49 16.23 14.65 14 .66 14.71 16.23 16 . 12 15.27 15.72 Fe 2 0 3 9 . 57 9.80 Il.26 11.42 Il.20 11.11 9.96 9 .05 10.76 Il.26 10 .71 11.13 10.50 10.49 10.53 10.59 MnO 0 . 15 0.17 0.19 0.19 0.18 0 . 19 0.17 0 . 15 0.18 0.20 0.21 0.18 0.17 0.16 0 . 18 0.17 MgO 8.32 8.63 7.89 8.08 7.88 8.08 8 .50 7.79 7.70 7 . 51 8.30 7.78 · 8.32 8.28 8.54 8.37 CaO 12 .45 12 . 30 Il.69 Il.72 12.04 12.07 12.23 12.65 11. 95 11.91 12.40 11.83 11 .71 11.70 11.95 Il.77 Na 2 0 2.38 2.30 2 . 36 2 . 25 2.16 2.51 2.38 2.24 2.46 2.54 2.43 2.20 2.46 2 . 39 2.72 2.65 K 2 0 0.08 0.10 0.13 0.12 0.06 0.11 0.04 0.08 . 0.16 0.19 0.07 0.11 0.12 0.11 0.21 0.29

~I Ti0 2 1.17 1. 26 1. 61 1. 62 1.45 1.41 1. 26 1.11 1.40 1. 58 1. 37 1.48 1. 26 1. 29 1.43 1.60 P 2 0 S 0.15 0.18 0.20 0.20 0.17 0 . 17 0.12 0.10 0.19 0.21 0 . 17 0.18 . 0.18 0.17 0 ~ 21 0.21 H 2 Oll0° 0.10 0.08 0.12 0.13 0 .04 0 .04 0.05 0.08 0.11 0.05 0.05 0.06 0.03 0.16 0.03 0.13 H 2 0 1050° -0.36 - 0.66 - 0.48 - 0 .36 -0 .84 -0.76 -0.72 -0.55 -0.27 - 0.66 - 0.77 - 0.74 - 0.71 -0.27 -0 . 76 -0.28 Total 99.69 100.33 99.25 100 . 23· 99 .91 100 . 14 99.77 99.52 100 .26 99.82 100.21 100.03 99 .70 99.88 101.19 100.96

Lava type Sheet-flows Pillars

CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 ·CYP 7'8 CYP 78 CYP 78 CYP 78 CYP78 CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 CYP 78 Sample No . 02-01 ·04-06 06-11 07-12 ·09-15 10-19 18-63B 18-66B 18-66B 12-34 12-34 12-35 12-36 19-69 19-70 19-71 19-71 19-72 19-73

SF PiF SF whole SFPi whole PiF

Nature SF SF SF SF SF SF SF Glass rock slice SFPi PiF P iF PiF PiF rock Glass PiF SFPi

Si0 2 49 .32 49 .61 50.17 49.48 50.10 50.09 48.85 49 .53 49.44 ·50.39 49.95 50.06 49.80 50 . 33 50 . 11 49.64 49.85 49.20 49.18 Al 2 0 3 15.12 14 .74 15 . 15 15.41 14.87 14.86 16.50 14.27 16.27 14.61 14.67 14.54 14.54 14 . 18 14 . 13 15.21 15.19 15.11 15.08 Fe 2 0 3 11.16 Il .26 10.36 10.50 10 . 68 10 . 53 10.38 Il.24 9.90 11.10 Il.00 11.12 Il.08 11. 61 11.59 10.33 10.28 10.35 10.31 MnO 0.19 0.19 0.19 0.17 0.18 0.19 0 . 18 0.18 0.19 0.19 0 . 19 0.18 0.18 0 .21 0.19 0.17 0.17 0.17 0.17 MgO 8.11 7.88 8.46 7.98 8.37 8.33 8.29 8.83 8.67 8 . 19 8.06 8.06 8 .25 7 .83 7.39 8.43 8.64 8.80 8 . 74 CaO Il.79 12 . 05 12.37 12.03 12 . 31 12.35 Il.66 11.89 12.34 12.17 . 12.06 12.14 12 .14 11. 78 11.86 12 . 33 12.26 12.31 12.37 Na 2 0 2.46 2 . 39 2.36 2.68 2.29 2.20 2.54 2.33 2.70 2.21 2.27 2.26 2 .34 2.48 2.57 2.53 2.63 2.66 2.63 K 2 0 0.14 0.08 0.12 0.18 0.06 0.08 0.13 0.05 0.03 0.13 0.11 0.12 0.10 0.12 0.14 0.07 0.06 0.06 0.07 Ti0 2 1.62 1.50 1.34 1. 54 1. 30 1. 35 1.25 1.43 1.21 1.44 1.41 1.42 1.43 1. 65 1. 63 1.27 1.26 1.26 1.26 P 2 0 S 0.20 0.15 0 . 18 0 .21 0.16 0. 17 0 . 16 0 .14 0.12 .0. 19 0 . 16 0.1 8 0 . 14 0. 20 0. 22 0 . 17 0.15 0.1 6 0 .14 H 2 011O° 0.05 0.08 0 .04 0 .07 0.03 0.04 0.05 0 . 15 0 .09 0 .03 0 . 15 0.09 0.08 0 .05 0 . 09 0.06 0.05 0 .05 0.05 H 2 01050° -0 .78 - 0 . 54 -0.74 -0.53 -0.78 - 0.79 · -0 . 67 -0.41 -0 .55 -0 .56 -0.40 -0. 55 -0 .54 - 0 .82 -0. 51 -0.59 - 0 . 70 - 0.74 - 0 .57 T ot al J. \ 11 99 .85 100 .70 100 . 18 100.32 100 . 15 99 .94 99 .89 100.87 100. 62 99 .98 100 .08 100.00 100. 39 99. 83 100 .15 100 .49 100.08 99.9

HOMOGENEOUS BASALTS FROM THE '-'--'~'IIf)I","'-' RISE AT 21° N

Pyroxene Olivine

laI' normative diagramfor plagioclase-pyroxene-olit'Îl1c. \ho/f"ÎI1(/ tion ot samples fi'om the cresta/ area, calculated with the

value of Fe ZO}=1.6%. Dashed Une represenlS Ihe " line separating the "plagioclase tholeiites" and "olivine "fields after Shido et al. (1971). Symbols as in Figure 6.

/laiS'''"'cuU.<\, triangulaire normatif plagioclase-pyroxène-olivine, mon-composition normative des échantillons de la zone axiale avec une valeur arbitraire de Fe z 0 3 = 1,6 %). La ligne en

la ligne « cotectique» séparant les champs de à plagioclase» et de « tholéites à olivine» d'après Shido

al. (1971). Symboles: voir fig. 6.

'-'UllUllU~.· tion of fresh glasses'

n microprobe analyses of some fresh glassy ~"~'~M,'H~ from the different lava types show a very small

sitional range within the fIeld of low potassium V'-''-'U.UL'-' tholeiites (Table 4). The compositions are very

close to those of the bulk rock . ons, indicating restricted shallow-level cumulative fractionation during ascent of the magma. The FeO * /TiO 2 diagram (Fig. 6) confIrms that the bulk rocks and glasses have the same range of variation and that the whole collection of sampled basaIts are derived from the same magma type, with only a moderate evolution by crystal fractionation. A fIrst estimate of extrusion temperatures (Table 5), using the molecular FeO * and MgO content of the analysed glasses (Roeder, Emslie, 1970) is consistent with the temperatures inferred from the composition of the Famous glasses (Hekinian et al., 1976). We fmd a steady de crea se oftemperature from 1240°C for the least differentiated sample (sample 19-71) to 1194°C for the most differentiated (samples 02-01 and 10-16).

Palagonite and Fe-Mn oxide coating

Most of the samples from the crestal area have a fragile outer rim of palagonite, protected by a Fe-Mn oxide coating. Un der the microscope, the glass has a honey­yellow colour, and is perfectly isotropic. The palagonite is orange, with a red-brown to black colour under crossed nichols.

That the thicknesses of palagonite and Mn-oxide coating on basait apparently increase with time has been demonstrated by Moore (1966) and, using rocks from the Famous area, by Hekinian and Hoffert (1975) and Arcyana (1977). We have selected 15 samples in the Cyamex collection which are suitable for making such

Mù""n'1I'nl,P ana/ysis of fi'esh g/asses se/ected in the outer chilled mm'gins of the difJerent kinds of lava flows (C amebax, Brest). chimiques à la microsonde électronique de verres frais provenant de la bordure trempée des différents types de coulées (Camebax, Brest).

02-01 09-15 10-16 18-64 19-70 19-71

Sheet- Pillow Pillow Sheet-flow flow fragment fragment Pillar fragment Pillar fragment

50.73 50.33 50.55 50.16 50.94 49.84 50.19 49.31 51.16 50.03 50.53 51. 63 14.30 14.75 14.70 14.69 14.27 15.98 16.18 16.69 14.75 13.07 13.92 14.92 9.78 10.81 10.52 10.66 10.37 9.37 9.20 9.70 10.21 10.36 10.18 9.43 0.31 0.03 0.25 0.16 0.07 0.15 0.26 0.22 0.04 0.16 0.20 0.26 7.17 7.09 7.23 8.56 7.21 8.04 8.04 7.50 7.61 8.70 9.03 6.02

11.46 Il.23 Il.08 12.19 Il.24 Il.58 Il.01 Il.14 Il.30 12.54 12.00 12.04 3.03 2.86 2.92 2.42 3.15 2.56 2.82 2.74 3.08 2.53 2.58 3.10 0.16 0.10 0.09 0.03 0.08 0.16 0.08 0.09 0.12 0.07 0.03 0.05 1. 95 1. 87 1. 81 1.41 1. 91 1. 33 1. 31 1. 68 1. 66 1. 55 1.43 1. 65 0.44 0.57 0.40 0.30 0.31 0.42 0.29 0.36 0.46 0.32 0.25 0.34 0.03 0.00 0.04 0.01 0.08 0.12 0.05 0.02 0.00 0.06 0.12 0.00

99.36 99.44 99.59 100.61 99.64 99.55 99.42 99.46 99.61 99.38 100.27 99.43

* Ali Fe reported as FeO.

Mo/ecular percentages of FeO and MgO in coexisting glasses (outer chi/led mm'gins) and olivine microphenocrysts, and estimated temperatures of quenching (after the method of Roeder and Emslie, 1970). Pourcentages moléculaires de FeO et MgO dans des verres (bordures figées) et microphénocristaux d'olivine coexistants, et estimation des températures de trempe correspondantes, d'après Roeder et Emslie, 1970.

Glass Olivine

Estimated Sam pie No. Nature Mol°{, FeO Mol°{, MgO FeO/MgO TOC Fo 0;) FeO/MgO

02-01 SF 9.07 Il.27 0.80 1194° 83.18 0.20 09-15 SF 9.15 13.21 0.69 1225° 85.35 0.17 10-16 PF 9.06 Il.34 0.80 1194° 82.75 0.21 18-64 pp 8.19 12.66 0.65 1210° 85.59 0.17 19-70 PiF 8.69 Il.89 0.73 1202° Non analyzed 19-71 PiF 8.71 13.92 0.63 1240° 86.25 0.15

499

T. J UTEAU et al.

Thickness of Palagonite:

<10pm

10( <30l'm

l!!I!IlI 30pm<

Figure 8

Correlation between the structural setting of the basal tic samples and the thickness of their palagonite rim. Numbers 1,2, and 3 are the main tee tonie zones defined in the text. Dive numbers in italics. The lack of < 10 Ilm palagonite samples in the South may be an artefact ofsampling. Corrélation entre la position structurale des échantillons basaltiques et l'épaisseur de leur couche de palagonite. Les chiffres 1, 2 et 3 sont les principales zones tectoniques défillies dans le texte. Les numéros de plongées sont en italiques. L'absence d'échantillons avec une palagonite < 10 Ilm dans la partie Sud est sans doute un artéfact dû à l'échantillonnage.

Table 6

measurements. Average thicknesses for F coating rangefrom lOto 192 pm <lI1d averaget for palagonite coating from 5 to 64 pm, the arbitrarily assigned to three "thickness (0-10 pm, 10-30 pm. > 30 pm). The Fe-Mn oxide crust is too irregular and be used confIdently as a potential age . palagonite rim, where protected from spalling by the Fe-Mn oxide coating, "111\11""

give better information. If we take an a about 3 !lm/10 3 years for palagonite growth Hoffert, 1975), the age of the measured range between 1 700 and 21 000 years. Ifwe and Moore's estimates (1977), the pala could be as high as 20 !lm/10 3 years, and the measured samples would then range 3200 years. The association of greater thicknesses with increasing age of the samples on other grounds is encouraging (Fig. 8).

SAMPLES FROM WEST OF THE CREST AND FROM THE TAMAYO FRACTURE

Brunhes-Matuyama boundary reversaI area

During reconnaissance dives CY 78-15 and

List ofsamples collected during Cyamex program in Brunhes-Matuyama reversai area (dives Nos. 15 mid 16), in a station between crestal (//,('(/S (dive No. 17), and in the Tamayo transformfault zone, indicating dive l1umber, rock type, size and water depth.

I.i"tc des échantillons prélevés au cours de la campagne Cyamex dans la zone d'inversion Brunhes-Matuyama (plongées nOS 15 et 16); dans Intermédiaire entre la zone axiale et la zone d'inversion (plongée nO 17), et dans la zone transformante Tamayo, avec indication du numéro du type de roche, de la taille et de la profondeur.

Dive No.

CY-78-11

CY-78-13

CY-78-14

CY-78-20

Total General total

CY-78-15

CY-78-16

CY-78-17

Total General total

Sample No.

CYP-78-11-21 22 23 24 25 26 27 28 28B

CYP-78-13-42 13-43 13-44

Basait

without with Mn (pebble) Mn

Andesitic pumice +Mn

Sediment

Mn Pumice without with plate + Mn Mn Mn

Samples from the Tamayo fracture zone

x(A)

x x x

x x

x x x

x x

13-45 x 13-47

CYP-78-14-53 14-54

CYP-78-20-75 20-76

CYP-78-15-55 15-56

CYP-78-16-57 16-58

CYP-78-17-60 17-61

x x

x x

4

2

18

. Samples from west of the crestal area

6

x x

2

500

x x x

8

x

3

volcano- Size. sedim. (cm)

x

x(B)

2

15 x 5 x 5 8x6x3 6x5x3

20 x20 x6 15 x 15 xl 12 x 8 x 5 li x5 x4 (mud) 14x10x1 14xllx9 11x9x7 14x10xl

5 x4 x3 15xlOx5

10 x 5 x 2 lOx8x5 12 x 12 x 8 20 x 13 x 5 12x12x8 8x5x4

HOMOGENEOUS BASALTS FROM THE EAST PACIFIC RISE AT 21° N

were collected west of the aXIS, from the zone between crust emplaced during the

and Matuyama magnetic epochs, respectively age 0.7 M.Y.) (Fig. 1 B; Table 6). AlI four have a thick Fe-Mn oxide coating. The two from dive CY 78-15 are altered basalts; the two from dive CY 78-16 are very light and highly andesitic pumices, which are considered to be

onous. Thin sections (CYP 78 15-55 and 56) of the basalts show a sub-aphyric texture with scattered

ila~,lv\.'!aL'V phenocrysts (up to 1.5 mm), and a few quite olivine phenocrysts ( < 1 mm) associated in clusters plagioclase in a microlitic and glassy matrix, which

clinopyroxene microlites. The major element . tions of the two samples are similar to the typical tions of the basaIts from the crest al area.

.. t" ... ...., • .od"" te station

dive CY 78-17, two samples were taken in the a about half-way between the axial zone and the

Matuyama boundary to the West (Fig. 1 B; ble 6). Both are highly altered basalts with very thick Mn oxide coating. The predicted age of the crust in region is approximately 0.3 M.Y. Sample CYP 78-

7 -60 consists of two blocks with a coating 5-10 mm , and sample CYP 78-17-61 lS covered by a

om"-like Mn-encrustation up to 3 cm thick. major element compositions, like those of the basalts

m the Brunhes-Matuyama reversaI area, are similar to compositions of the crestal area basalts, although do show a slight enrichment in K 2 0, probably as a t of secondary alteration (Table 7).

ne of the samples (CYP 78-17-60A) is a sub-aphyric aIt with plagioclase laths and clinopyroxene grains,

a good sub-ophitic, microlitic texture and with vine micro lites and Ti-Fe oxides. The second sample is nearly aphyric basaIt, with scattered skeletal ·oclase and olivine micro lites in a ferruginous sub­

aque matrix (CYP 78-17-60B and 61).

Tamayo fracture zone

Eighteen samples were collected during dives CY 78-11, 13, 14 and 20 (Fig. 3). They include isolated Mn in plate form, pumice, and various de tric al or marly sediments as weIl as altered basalts with or whithout Mn crusts (Table 6). Of the 5 analyses of volcanic samples that have been made (Table 7), two of them are of rhyolitic pumices (CYP 78-13-43 and 13-44A),and the other three are of rocks with basaltic compositions, again very close to those of the basalts from the crestal area, except for a slight enrichment in K 2 O. Three thin-sections of the samples show the presence oftwo basaltic types. One has a sub-ophitic texture with olivine phenocrysts (0.5 to 1 mm) altered to celadonite and plagioclase phenocrysts (O. 5 to 1 mm), sometimes forming clusters in a microlitic, sub-ophitic matrix with skeletal plagioclase laths, tiny pyroxene grains, and Ti-Fe oxides. Rare vesicles less than 1 mm III diameter are filled with celadonite (CYP 78-11-26A and CYP 78-20-76). The second type (CYP 78-13-45) is moderately phyric, with

501

00 -r--i=O '0

0-.\0 >-' uS:

00 r--'00

O-.<n >-, u~

00 r--'r-­

O-.<n >-, u~

00 r--'\0

O-.<n >-, u~

00 r--'<n

O-.<n >-, u~

00 r--'\0

O-.r-­>-, u~

00 r--'<n

0-.'1" >-, u::

00 r--« '\c

0-. 0 1 >-, u:::

c Z <lJ

0.. 8 <Ii

r:/)

c ~ + i=O

r:/)

>

OO_\ON<nNNO\'1"<nO\N'1" <n\OONO\'1"OO<"'lOONNO-............. 0\'1"010\0-010-0000 '1"-- - 1 S

<n<"'l<"'lON\OOO'1" NO<n 01<"'1 '1" 0'1" 01<"'1\0 <n<"'l r--N<"'I-O\ ............. 0'1"010\0-010-0000\ <n-- _ 10\

r--r--r--<"'I<nN\Or--O-N<"'I<n oor--<nN-<n\O-r--NN<"'IOO

O\<"'INOr---NO-OOOo\ '1" __ - 10\

OO_<"'Ir--\O'1" 0\r--<"'I0<n r--oo OON __ \O'1"<nN'1"N-OO\ · ........... . O<nOOr--NNO-OOOO <n-- - IS

_\O'1"'1"<"'Ir--\OO<n-\ONr-­'1"Ooo<nO\\O\O<nN-OOO\ ONN<"'I __ <nNOONO\O

\0- - IS

<nO\N<"'IOONOO<nO\OO\'1"­O\O\r--\O<"'I-<"'Ir--<"'I-\OOO'1" _<"'Ir--_O_\O'1"OOONOO \0- 10\

_<"'1<"'10100'1"00010\0'1"01 '1" __ r--<"'I'1"OO'1"-\O--\ON · ........... . O\'1"_Or---NO-OOOo\ '1" __ - 10\

\O.n'1"O\\OOOOOr--<nOOO\O\O \Or--'1"-OO\<"'I-<n--<"'I<"'I · ........... . 0\'1"_000_010_0000 '1"-- - 1 S

\0 00 <"'1 000\ <"'1 00 <"'1 <n 0 '1" \0<"'1 _\O __ oo'1"N-<nN-O\O

O'1"_Or--NNO-OOOO <n-- - IS

\000 <n\O<noo <nN 0<"'1_0000 r--oO\_\Or--\O<"'I\ONN'1"-· ........... . 0000\0<"'101010-0-<"'10 <n_ - IS

r--N_\oN'1"'1"O\<n--N­<n\OooO_Oo\OO'1"-'1"-\O · ........... . <"'101'1"0_<"'101_000010 r--- 1 S

0\ <n 00 0\0 r--000\00\r--0\'1" <n\OO\'1"O\'1" 0\00<"'10<"'1'1" <"'1 · ........... . 01<"'10100-<"'10100-'1"0\ r-- _ 1 0\

_ <"'1 r-- \0 00 00 '1" - 0\ 01 If. 01 0\ r--<nO_\OO\O\<"'I-<"'I,..-·\OOO · ........ , .' _\OO\O<n-NONO--O t.n~..,.....j ~

T. J UTEAU et al.

altered olivine and plagioclase phenocrysts (up to 3 mm) and some plumose clinopyroxene in the glassy matrix.

SUMMARY AND CONCLUSIONS

To conclude, we want to stress the following points:

(1) The pillows and sheet flows are aphyric to moderately phyric, with a maximum of 12 per cent phenocrysts in some pillows. In aIl samples studied, megacrysts (> 2 mm), phenocrysts (0.5 to 2 mm) and micropheno­crysts (0.1 to 0.5 mm) are always olivine and plagioclase with a systematic preponderance of plagioclase over olivine (1: 4-1 : 15). The absence of pyroxene crystals requires that the lavas were quenched above the temperature of the ternary eutectic minimum relative to the three major silicate phases, and that clinopyroxene remained as a virtual glass phase. Slabs of the roof of the fossil lava pools are aphyric and glassy, and massive pillar samples are either aphyric or sub-aphyric, with plagioclase and olivine microlites showing a fluidal arrangement in some zones. Clinopyroxene crystals are absent, except in one sam pIe containing a cluster of pyroxene phenocrysts.

(2) The crystallization history of pillow and sheet-flow lavas is reflected in the succession of four generations of olivine and plagioclase crystals; the history predomi­nantly reflects a cotectic crystallization of these two silicate phases.

(3) Whole rock chemical analyses for major elements do not show distinctions between pillow-Iavas, sheet-flows and pillars, and microprobe analyses of fresh glasses in the three lava types show the same range of variations as the bulk rock analyses. The bulk rocks and glasses aIl fall within the compositional range of typicallow-potassium oceanic tholeiites, with a limited fractionation trend shown by FeO * /MgO versus TiO 2' Several conclusions can be drawn from these observations: (a) The fa ct that the fresh glasses cannot be chemically distinguished from the sub-aphyric to modenitely phyric bulk-rocks indicates that shallow-ievei cumulative fractionation was probably not an important process during crystallization of the various batches of magma. (h) The fa ct that the aphyric basaltic pillars cannot be chemically distinguished from the pillows and sheet-flows means that settling of silicate phenocrysts in the magma before extrusion cannot explain the markedly aphyric nature ofthese lavas. More probably, the corresponding magma batches moved quickly through the magma chambers and were extruded after a rapid and adiabatic ri se that prevented any signifIcant crystallization ta king place. This notion is consistent with the hypothesis that fluid lavas, and especially fossillava pools, correspond to the delivery ofvery large volumes of hot and fluid magma at or near the axis of the Rise (Ballard et al., 1979; Van Andel, Ballard, 1979; Francheteau et al., 1979; Cyamex Team, 1980). (c) The signifIcantly more phyric nature of the pillow­lavas compared with the sheet-flows suggests that the corresponding magma batches probably stayed longer in the magma chambers; at least long enough to permit crystallization of olivine and plagioclase phenocrysts to begin and to record one important event (magma

502

mixing?) affecting the chambers (resorption phenocrysts), but not long enough for differentiation to take place. Pillow-Iavas correspond simply to the delivery of magma at enough rate to permit rapid quenching by the water and thus the building of constructionaI highs.

(4) If we compare this collection of basalts medium-rate spreading ridge (around 6 cm/ the basalts of a slow spreading ridge such as area (around 2 cm/year), the following signiflcant: - there is a striking homogeneity of the collection wh en compared with that from F fIve to six lava types were described, 1· ... ,,,!t>rh ....

basalts, olivine basalts, highly phyric plul:;:.J.v\.,Jl<1;";[ and plagioclase-pyroxene basalts (Bougault, 1974; Hekinian, Moore, Bryan, 1976; Bryan, Moore, 1977); aIl these types are unknown in the Cyamex area;

- in the Famous area, there is an mineralogical and chemical zonation of the lavas reflecting a zoned distribution of the types on the rift-valley floor, with oliu1' r ""'_,~n1"l,,h.

near the axial zone and plagioclase-rich rocks valley walls, especially to the west. The relations the composition, geographic position, and age of the basaltic lavas of the rift valley suggest diverse lava types of the rift valley were shallow, zoned magma chamber about the inner floor (Hekinian et al., 1976). Differen accomplished by cooling and crystallization clase, olivine and clinopyroxene toward the the cham ber. The center of the magma enriched in primitive olivine-bearing lavas emplaced along the axis of the inner floor of the In the Cyamex area, aIl attempts to d\.dH.V.u . .)~.L"'" mineralogical or chemical zonation in the have failed. This homogeneity of the Cyamex evidence of weak differentiation pro cesses reflect a higher periodicity of eruption and of a shaIlow magma chamber. The higher i:HJJ.vUUU' which is three times that of the Famous rift reflected in the higher proportion of sheet-flows pools, which correspond to high "inst spreading rates concomitant with high ascent magma. (5) The fewbasaltic samples collected at the Brunhes-Matuyama reversaI boundary ( 0.7 M.Y.), at an intermediate station (0.3 M.Y.), the Tamayo fracture zone (1 M.Y.) are not . . different from the samples coIlected in the This suggests that the magma tic conditions which under the present-day EPR axis have not signifIcantly through time, a testimony to the nence of the magma chamber under ridges with a spreading rate. Finally, it must be clear that the in developped in this paper are based on the limited number of samples collected in a small area ridge axis. We are aware that some important outlined here, for instance the homogeneity

HOMOGENEOUS BASALTS FROM THE EAST PACIFIC RISE AT 21° N

tic collection, could be modifIed when the numerous. collected with sumersible and by dredging III

in a broader area will be studied.

thank F. Spiess, coordinator of Project Rita, , C. Riffaud, X. Le Pichon, J. Debyser and the ScientifIque des Submersibles for encouraging participation in the project and for sponsoring

Cyamex expedition. We thank C. Caillart, R. tzy, H. Leroux, G. Arnoux, J. M. Nivaggioli, D.

R. N., Clague D. A., Klitgord K. D., Marshall M., Nishimori K., 1975. Magnetic and petrologic variations along the Galapagos

ding center and their relation to the Galapagos melting anomaly, . Soc. Am. Bull., 86, 683-694. .

, 197~ Rocks collected by bathyscaph and diving saucer in the us area of the Mid-Atlantic Rift Valley: petrological diversity and

ructural setting, Deep-Sea Res., 24, 565-589.

R. D., Holcomb R. T., Van Andel T. H., 1979. The Galapagos rift at 86°W: sheet fIows, collapse pits and lava lakes of the rift valley, J. Geophys. Res., 84, B10, 5407-5422.

Bougault H., Hekinian R., 1974. Rift valley in the Atlantic Ocean near 36°50'N: petrology and geochemistry, Earth Planet. Sei. Lett., 24, 249-261. Bryan W. B., 1972. Morphology of quench crystals in submarine basalts, J. Geophys. Res., 77, 5812-5819.

Bryan W. B., Moore J. G., 1977. Compositional variations of young basalts in the Mid-Atlantic rift valley near 36°49'N, GeaI. Soc. Am.

Il., 88, 556-570.

W. B., Thompson G., Frey F. A., Dickey J. S., Roy Jr. and S., . Pet roI ogy and geochemistry of baserilent rocks recovered on

37 DSDP, Initial Rep. DSDP, 37, 695-703.

Bunch T. E., Laborde R., 1976. Mineralogy and composition of selected basaltsfrom DSDP Leg 34. Initial Rep. DSDP, 34, 263-275.

Byerly G. R., Melson W. G., Vogt P. R., 1976. Rhyodacites, andesites, ferrobasalts and ocean tholeiites from the Galapagos spreading center, Earth Planet. Sei. Lel/., 30, 215-221.

Cyamex Team, 1978. First submersible study of the East PacifIc Rise: Rita (Rivera-Tamayo) Project, 21 oN, Eos, Trans-A.G.U., 59, 1198.

Cyamex Team, 1980. First manned submersible dives on the East PacifIc Rise, 21 ON (Rita Project). General results, Mar. Geophys. Res. (in press). Cyamex Team, Bougault H., Camhon P., Hekinian R., 1978. Découverte par submersible de sulfures polymétalliques massifs sur la dorsale du PacifIque oriental, par 21 ON (projet «Rita »), C. R. Acad. Sei., Paris, sel'. D, 287, 1365-1368.

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503

Semac, W. Marquet, P. Plasseraud, J. Porteous, H. Lossouarn, the technical Team of Cyana, Captain ·Y. Langlois and the offIcers and crew of the MIS Nadir, who aIl contributed to the success of the dives. We thank also Janet Fox for her help in reviewing the manuscript, H. Bougault and J. Etoubleau for whole rock analyses by X Fluorescence, and M. Bohn for microprobe analyses.

The expedition was funded by CNEXO with contribu­tions from NSF and USGS, Woods Hole Oceanographic Institution, National Geographic Society and the Mexican Government.

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