18. BASALT DRILLED AT THE VEMA FRACTURE ZONE, DSDP LEG 39

Transcription

1 18. BASALT DRILLED AT THE VEMA FRACTURE ZONE, DSDP LEG 39 Enrco Bonatt and Keth Hartman, LamontDoherty Geologcal Observatory of Columba Unversty, Palsades, New York Fabrzo Innocent, Insttuto d Petrografa, Unversta d Psa, Psa, Italy, and Robert Kay, Department of Geology, Cornell Unversty, Ithaca, New York ABSTRACT An attempt to drll nto lower crust at the foot of the northern wall of the Vema fracture valley resulted n recovery of basalt cobbles beneath about 170 meters of sedment. Maxmum penetraton nto the basalt cobble zone was 13 meters. Data obtaned by optcal mcroscopy, electron probe, and major and rareearth element analyss ndcate that the basalts are olvne tholetes and represent a magma type commonly erupted along mdoceanc rdges. The basalts were probably erupted orgnally at the axal segment of the MdAtlantc Rdge. The basaltc cobble zone probably represents talus materal channeled nto the valley from the upper levels of the valley wall. INTRODUCTION The man objectve of drllng close to the foot of the north wall of the Vema fracture valley was to attempt to sample a secton of the lower oceanc crust. Ths attempt was based on the followng two assumptons: (a) a partal secton of more or less "normal" oceanc crust s exposed at the north wall of the Vema Fracture Zone valley (where by "normal" crust we mean crust orgnally emplaced n the actve, axal zone of the Md Atlantc Rdge); (b) by drllng close to the foot of the north wall one would bypass a thckness of crust roughly equal to the heght of the wall (about 2 km), thus allowng one to drll drectly nto lower crust. Assumpton (a) s supported by several lnes of evdence whch have been dscussed by Bonatt and Honnorez (1976). Assumpton (b) may also be vald, wth some lmtatons, as suggested by the fact that a varety of gabbros and metagabbros, thought to represent lower crustal materal, have been dredged from the Vema north wall (Bonatt and Honnorez, 1976). The drllng attempt was done about 30 to 40 klometers west of the northern axal MdAtlantc Rdge segment, close to the foot of the north wall (Fgure 1), allowng for a sedment column above basement thck enough to permt drllng nto hard rock. No prevous detaled ste surveys were avalable, and the lmted tme allotted to ths ste dd not permt any extensve survey from the Glomar Challenger. Fgure 2 shows the locaton of the drllng stes relatve to the north wall of the Vema Fracture Zone. Three attempts were made. The frst one (Hole 353, 'N, 'W) had to be nterrupted after drllng nto about 380 meters of sedment and before reachng basement, because of a mechancal falure. In the second attempt (Hole 353A) basement was reached after gong through a sedment secton 168 meters thck. A thrd attempt (Hole 353B) was made a few hundred meters from Hole 353B (Fgure 2); basement was reached below about 170 meters of sedment. In both Holes 353A and 353B the basement conssted of cobbles of fresh basalt. Ths basalt rubble layer was penetrated for 13 meters at Hole 353A, and for about 10 meters at Hole 353B. No serous attempt at deeper penetraton was made due to tme lmtatons. DESCRIPTION OF THE BASALT The cobbles recovered at Holes 353A and 353B range n dameter from fve to about 15 cm; some smaller fragments were also recovered. Some of the cobbles appear to have been partally smoothed by the acton of the drll and were probably part of larger cobbles. The cobbles consst exclusvely of a dark, fresh, mcrocrystallne, and glassy basaltc rock. Thn sectons of several samples from both holes were mcroscopcally studed. The texture of the rocks vares from aphyrc to sparsely phyrc. Typcally, the samples consst of clusters of elongated platelets and needles of calcc Plagoclase (frequently wth skeletal habt), and abundant solated partly euhedral crystals of olvne, partally altered to smecttes and chlortes, n a very fne groundmass contanng secondary phases, opaques, and glass. The varous samples from both holes dsplay a range of textures and degree of crystallnty, but the essental mneralogc characterstcs appear to be smlar n all the samples. One sample (DSDP 39353B 11, 8592 cm) was studed n more detal. Ths sample has partly euhedral and skeletal mcrophenocrysts of olvne (rangng from F07585) scattered wthn accular or felty aggregates of Plagoclase (rangng from AΠÓO65) 507

2 E. BONATTI, K. HARTMAN, F. INNOCENTI, R. KAY 12 1 T VEMA F.Z km AXIAL VALLEY Mn TRANSVERSE RIDGE DSDP 39 VALLEY AB W basalt + metabasalt only serpentnte + basalt Q = serpentnte + gabbro + basalt = serpentnte only D = serpentnte + gabbro Fgure 1. Smplfed morphology of the Vema Fracture Zone (after Van Andel et al, 1967), showng the approxmate locaton of drllng stes, Leg 39 and other stes where basalts were dredged. slope here = 21 h northern wall 10 55' VEMA FRACTURE VALLEY X DR fx satellte fx Locaton of DSDP Leg 39 holes Vema Fracture Zone (F. McCoy) 10 50' depths n fathoms 44 05' 44 00' 43 55' Fgure 2. Locaton of Holes 353, 353A, and 353B relatve to the north wall of the Vema fracture valley. Complaton by F. McCoy. 508

3 BASALT DRILLED AT VEMA FRACTURE ZONE n a groundmass of fne skeletal Plagoclase crystals, opaques, and partly devtrfed glass. A dstnct olvneplagoclase parageness was noted; namely, olvne crystals were observed as nucle for the radatng Plagoclase varoltc aggregates (Fgure 3), and n some cases, crosscuttng the Plagoclase crystals. These observatons suggest that crystallzaton of olvne preceded that of Plagoclase. Electron probe data for some olvne and Plagoclase crystals are reported n Table 1. Table 2 shows the results of major and rareearth elemental analyss and CIPW norms of ths sample. Major elements were determned by classcal methods, rareearth elements by mass spectrometry, as descrbed n Kay (1970). Shown are also major element data for two other samples from Hole 353B, obtaned n the U.S.S.R. by G.C. Kazpe (personal communcaton). ^ t ^j p *,/ '».. < \ j j f» 5 f f I \ '.#; "y 0.2 mm Fgure 3. Olvne crystal as nucleus for radatng Plagoclase laths. Sample 35311, 8593 cm. DISCUSSION The mneralogy and chemstry of the basalts drlled at Holes 353A and 353B are very smlar to that of several other basalts obtaned on the northern wall of the Vema fracture valley, at varous dstances from the axal segment of the MdAtlantc Rdge, and to basalts obtaned wthn the axal rft valley tself (Bonatt, et al., n preparaton). All of these basalts would fall n the olvne tholete or quartz tholete feld of Yoder and TABLE 1 Major element composton and QPW norms for basalt Sample 353B11, 8593 cm, Concentraton of REE also shown. SO 2 A1 2 O 3 Fe 2 O 3 Na 2 O K 2 O H 2 O + TO 2 p 2 o 5 MnO Oq Or Ab An D Hy 01 Ne Mt 11 Ap La Ce Nd Sm Eu Dy Er Yb Lu Ba Ba/Ce 353B αpw 0! Composton (n 353B5 a a Analyses performed by G. C. Kazpe %) 353B12 a U.S.S.R. Tlley (1964) normatve classfcaton (Fgure 4). The chondrtenormalzed rareearth elements pattern from Hole 353B shows lght REE depleton typcal of oceanrdge basalts (Fgure 5). The petrochemstry of basalts from the northern wall of the Vema fracture valley (ncludng those drlled n DSDP Leg 39), s consstent wth the assumpton that these basalts were orgnally emplaced at the Md Atlantc Rdge axal segment. Basalts wth alkalne affntes and/or a more dfferentated chemstry would be expected n the case of wthnfracturezone, "leaky" volcansm (Melson et al., 1967; Honnorez and Bonatt, 1970; Barber et al., 1975). The hypothess that the basalts from the Vema fracture north wall were orgnally emplaced at the MdAtlantc Rdge axal segment s supported by ndcatons of an ncrease n age of these basalts wth dstance from the Md Atlantc Rdge axal rft valley (Bonatt and Honnorez, 509

4 E. BONATTI, K. HARTMAN, F. INNOCENTI, R. KAY TABLE 2 Electron probe analyses of Plagoclase and olvne crystals n basalt Sample 353B11, 8593 cm. Elements Probe Analyss Results (n %) Plagoclase SO 2 TO 2 A1 2 O 3 MnO Na 2 O K 2 O Olvn< SO 2 A1 2 O 3 Cr ). Followng ths assumpton, and gven a spreadng rate from the northern MdAtlantc Rdge axal segment of about cm/year (Van Andel et al., 1971; Bonatt and Honnorez, 1976), the age of the basalt drlled at Holes 353A and 353B should be close to 3 m.y. We nterpret the basalt cobbles sampled at Ne D RIDGE AXIS BASALTS OFFSET ZONE BASALTS (NORTH WALL) TRANSVERSE RIDGE BASALTS ALKALI FIELD OL Fgure 4. QDHyOlNe normatve dagram for basalts from the Vema Fracture Zone ncludng sample 35311, 8593 cm. Norms were obtaned assumng Fe 2 O 3 = 1.5%. (from Bonatt et al, n preparaton). Hy 510

5 BASALT DRILLED AT VEMA FRACTURE ZONE 10 78A THOLEIITIC MAGMA SERIES Juan de Fuca Rdge THOLEIITIC BASALT Gorda Rdge FERRO BASALT foot and the lower part of the slope but that t s thcker beneath the channels, and thn or absent away from the channels. If ths nterpretaton s correct, the relatvely thck talus encountered at Ste 353, contanng no serpentnte or other plutonc rocks, coupled wth the presence of serpentnte closeby on the wall, mght ndcate that the drllng was done unforeseeably n an area close to the ext of one of the channels feedng talus to the Vema transverse valley. Postste evaluaton of topographc lnes ndeed suggests that the drllng was done n one such area, as shown n Fgure 1. These consderatons suggest that the orgnal objectve of drllng at Ste 353, namely, to obtan a secton of the lower oceanc crust, bypassng the upper crustal basaltc layer n the Vema Fracture Zone, s stll a vable objectve, despte our falure to accomplsh t durng Leg 39. A successful accomplshment of drllng nto secton of oceanc lower crust at the Vema Fracture Zone wll requre, n addton to detaled prelmnary surveys of the area, a tme expendture at the ste larger than that afforded n Leg 39. Sm Eu Fgure 5. Chondrtenormalzed rare earth pattern for basalt , 8593 cm, compared wth pattern for Gorda and Juan de Fuca rdge basalts (from Bonatt et al, n preparaton). Holes 353 A and 353B as part of a zone of talus materal whch was fed nto the Vema fracture valley from the upper part of the northern wall. Dredge hauls collected from the slopes of the northern wall n the vcnty of the drllng stes ndcate that serpentnzed perdottes are exposed on the wall. These dredge hauls nclude Staton AII208 from Melson and Thompson (1971), located about 5 klometers west of the drll stes, and staton P7OO3O9 from Bonatt and Honnorez (1976) located about 30 klometers west of the stes. Gabbros and metagabbros were recovered at several stes on the north wall (Bonatt and Honnorez, 1976). Gven the presence of serpentnte on the wall, t s surprsng that no fragments of serpentnte or of other plutonc rocks were found n the talus materal drlled at Ste 353. A possble explanaton of these fndngs s that the talus from the upper basaltc porton of the northern wall s fed to the transverse valley preferentally along southfacng shallow channels or "canyons." Some ndcaton of the exstence of such shallow channels can bee seen n van Andel et al. (1971) topographc map of the area under dscusson. It s possble therefore that the talus materal s not homogeneously dstrbuted along the Dy ACKNOWLEDGMENTS Fgure 2 was compled by F. McCoy. Research was supported by NSF/IDOE and ONR. REFERENCES Barbed, F., Bonatt, E., Marnell, G., and Varet, J., Transverse tectoncs durng the splt of a contnent: data from the Afar Rft: Tectonophyscs, v. 23, p Bonatt, E. and Honnorez, J., Sectons of the Earth's crust n the equatoral Atlantc: J. Geophys. Res., v. 81, p Bonatt, E., Honnorez, J., Innocent, F., and Kay, R., n preparaton. Basalts from the Vema Fracture Zone (North Atlantc). Honnorez, J. and Bonatt, E., Nephelne gabbro from the MdAtlantc Rdge: Nature, v. 228, p Kay, R., The rare earth geochemstry of alkalne basaltc volcancs: Ph.D. Thess, Columba Unv., 179 p. Melson, W.G., Jarosewch, E., Cfell, R., and Thompson, G., Alkal olvne basalt dredged near St. Paul's Rocks, MdAtlantc Rdge: Nature, v. 215, p Melson, W.G., and Thompson, G., Petrology of a transform fault zone and adjacent rdge segments: Phl. Trans. Roy. Soc. London, v. 268, p Van Andel, T.H., Corlss, J.B., and Bowen, V.T., The ntersecton between the MdAtlantc Rdge and the Vema Fracture Zone n the north Atlantc: J. Marne Res., v. 25, p Van Andel, T.H., Von Herzen, R.P., and Phllps, J.D., The Vema fracture zone and the tectoncs of transverse shear zones n oceanc crustal plates: Marne Geophys. Res., v. 1, p Yoder, H.S., and Tlley, C.E., Orgn of basaltc magmas: an expermental study of natural and synthetc rock systems: J. Petrol., v. 3, p